Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m2) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.

•Polyamide films and membranes are fabricated by molecular layer-by-layer deposition.•Polyamide surface chemistry is enriched with amine or carboxyl functional groups.•Gypsum scaling is compared for homogeneous and heterogeneous nucleation conditions.•Long-term gypsum scaling is similar for polyamide with different surface chemistry.Mineral scaling of thin-film composite desalination membranes is affected by the surface chemistry and roughness of the membrane polyamide selective layer, but the relative contributions of these surface properties to scaling is unknown. We studied the influence of differences in polyamide surface chemistry on gypsum (calcium sulfate dihydrate) scaling of thin-film composite membranes, independent of surface roughness, with the goal of improving scaling resistance through changes to membrane surface chemistry. Smooth polyamide films and thin-film composite membranes were created using a molecular layer-by-layer deposition technique, and the surface chemistry of the polyamide films was enriched with amine or carboxyl functional groups by varying the final monomer deposition step in the layer-by-layer assembly process. Polyamide films and composite membranes with different surface chemistry were subjected to gypsum scaling by both homogeneous and heterogeneous nucleation mechanisms. Results from quartz crystal microbalance experiments and dynamic membrane scaling tests show that differences in the polyamide surface chemistry do not influence long-term gypsum scaling behavior. We conclude that hydrodynamic conditions have a greater effect than differences in surface chemistry on the gypsum scaling behavior of polyamide thin-film composite membranes.

•Graphene oxide–silver nanocomposites (GOAg) were synthesized in-situ.•GOAg sheets were chemically bound to the surface of thin-film composite (TFC) membranes.•TFC-GOAg membranes exhibited strong antimicrobial activity.•Biofilm formation was greatly suppressed by TFC-GOAg membranes.Innovative approaches to prevent bacterial attachment and biofilm growth on membranes are critically needed to avoid decreasing membrane performance due to biofouling. In this study, we propose the fabrication of anti-biofouling thin-film composite membranes functionalized with graphene oxide–silver nanocomposites. In our membrane modification strategy, carboxyl groups on the graphene oxide–silver nanosheets are covalently bonded to carboxyl groups on the surface of thin-film composite membranes via a crosslinking reaction. Further characterization, such as scanning electron microscopy and Raman spectroscopy, revealed the immobilization of graphene oxide–silver nanocomposites on the membrane surface. Graphene oxide–silver modified membranes exhibited an 80% inactivation rate against attached Pseudomonas aeruginosa cells. In addition to a static antimicrobial assay, our study also provided insights on the anti-biofouling property of forward osmosis membranes during dynamic operation in a cross-flow test cell. Functionalization with graphene oxide–silver nanocomposites resulted in a promising anti-biofouling property without sacrificing the membrane intrinsic transport properties. Our results demonstrated that the use of graphene oxide–silver nanocomposites is a feasible and attractive approach for the development of anti-biofouling thin-film composite membranes.

We demonstrate the functionalization of thin-film composite membranes with zwitterionic polymers and silver nanoparticles (AgNPs) for combating biofouling. Combining hydrophilic zwitterionic polymer brushes and biocidal AgNPs endows the membrane with dual functionality: antiadhesion and bacterial inactivation. An atom transfer radical polymerization (ATRP) reaction is used to graft zwitterionic poly(sulfobetaine methacrylate) (PSBMA) brushes to the membrane surface, while AgNPs are synthesized in situ through chemical reduction of silver. Two different membrane architectures (Ag-PSBMA and PSBMA-Ag TFC) are developed according to the sequence AgNPs, and PSBMA brushes are grafted on the membrane surface. A static adhesion assay shows that both modified membranes significantly reduced the adsorption of proteins, which served as a model organic foulant. However, improved antimicrobial activity is observed for PSBMA-Ag TFC (i.e., AgNPs on top of the polymer brush) in comparison to the Ag-PSBMA TFC membrane (i.e., polymer brush on top of AgNPs), indicating that architecture of the antifouling layer is an important factor in the design of zwitterion-silver membranes. Confocal laser scanning microscopy (CLSM) imaging indicated that PSBMA-Ag TFC membranes effectively inhibit biofilm formation under dynamic cross-flow membrane biofouling tests. Finally, we demonstrate the regeneration of AgNPs on the membrane after depletion of silver from the surface of the PSBMA-Ag TFC membrane.

In this study, we demonstrate a highly antifouling thin-film composite (TFC) membrane by grafting a zwitterionic polymer brush via atom-transfer radical-polymerization (ATRP), a controlled, environmentally benign chemical process. Initiator molecules for polymerization were immobilized on the membrane surface by bioinspired catechol chemistry, leading to the grafting of a dense zwitterionic polymer brush layer. Surface characterization revealed that the modified membrane exhibits reduced surface roughness, enhanced hydrophilicity, and lower surface charge. Chemical force microscopy demonstrated that the modified membrane displayed foulant-membrane interaction forces that were 1 order of magnitude smaller than those of the pristine TFC membrane. The excellent fouling resistance imparted by the zwitterionic brush layer was further demonstrated by significantly reduced adsorption of proteins and bacteria. In addition, forward osmosis fouling experiments with a feed solution containing a mixture of organic foulants (bovine-serum albumin, alginate, and natural organic matter) indicated that the modified membrane exhibited significantly lower water flux decline compared to the pristine TFC membrane. The controlled architecture of the zwitterionic polymer brush via ATRP has the potential for a facile antifouling modification of a wide range of water treatment membranes without compromising intrinsic transport properties.

•SiNPs or zwitterionic polymer brushes were grafted on TFC membrane surfaces.•Organic fouling resistance of modified membranes was systematically compared.•SiNP coating cannot prevent calcium-ion induced organic fouling mechanism.•Electrostatic attraction aggravates organic fouling of SiNP-modified membrane.•Zwitterionic polymer coating imparts excellent organic fouling resistance.We conducted a comparative study to investigate the efficacies of two different types of highly hydrophilic materials (i.e., silica nanoparticles (SiNPs) and zwitterionic polymers) for antifouling surface modification of polyamide thin-film composite (TFC) membranes. Dense layers of SiNPs and zwitterionic polymer brushes were grafted on the membrane surfaces via dip-coating with aminosilane-functionalized SiNPs (i.e., SiNP-TFC membrane) and surface-initiated atom-transfer radical-polymerization of sulfobetaine methacrylate (i.e., PSBMA-TFC membrane), respectively. With the same degree of enhancement of surface hydrophilicity and identical surface roughness, the PSBMA-TFC membrane exhibited significantly higher fouling resistance than the SiNP-TFC membrane in adsorption tests of proteins and bacteria as well as in forward osmosis (FO) dynamic fouling experiments using alginate as a model organic foulant. Chemical force microscopy measurements revealed that membrane-foulant electrostatic attraction aggravates organic fouling of the SiNP-TFC membrane to a certain degree, but the primary fouling mechanism is the complexation of organic foulants with carboxylic groups on the polyamide membrane surface. We attribute the lower fouling resistance of the SiNP-TFC membrane to the high density of surface carboxylic groups that may still be accessible to foulants as well as to membrane-foulant electrostatic interaction. On the contrary, the zwitterionic polymer brushes effectively shield the surface carboxylic groups and provide steric hindrance against foulant adsorption due to significant hydration of the zwitterionic brushes.Download high-res image (244KB)Download full-size image

•Acyl-chloride groups in nascent polyamide are quenched by amines and alcohols.•Quenching affects water permeability and selectivity, following a trade-off.•Quenching in ammonium hydroxide increased water permeability 2.8-fold.•Permeability effects largely stem from reactions to form esters and amides.•Quenching can decrease surface carboxyl density by over 50%.Fully aromatic polyamide thin-film composite (TFC) membranes are the industry standard for membrane-based desalination. Due to the extensive application of TFC membranes to treat feed waters of widely varying composition, new methods are needed to modulate the water permeability, water–solute selectivity, and surface charge of the polyamide selective layer. In this study, the quenching of residual acyl-chloride groups in nascent polyamide films is performed using amine, ammonia, and alcohol solutions, including common alcohol solvents such as methanol and ethanol. Membrane transport characterization in reverse osmosis demonstrates that quenching can substantially increase water permeability. For example, quenching in ammonium hydroxide resulted in water permeability of 2.50 L m−2 h−1 bar−1, which was 2.8 times greater than that for water-quenched control membranes, paired with a small decrease in sodium chloride rejection from 99.7% to 99.3%. By using different quenching solutions, quenching resulted in a range of water permeabilities (0.15–2.50 L m−2 h−1 bar−1) and water–salt selectivities (98.4–99.7% salt rejection), with performance falling along a previously proposed permeability–selectivity trade-off. Transport behavior and surface characterization indicate that chemical reactions occur to form amides and esters during quenching. Additionally, quenching decreased the carboxyl group density from 25.5 sites/nm2 for water-quenched control membranes to 11.8 sites/nm2 for membranes quenched in ammonium hydroxide. Taken together, the results demonstrate that acyl-chloride quenching provides a novel route to alter the surface charge in addition to water permeability and water–salt selectivity. Potential ways to further optimize the method are discussed.

•Further increasing membrane porosity greatly improves thermal efficiency.•Increasing permeability to thermal conduction coefficient ratio is essential.•Effective heat recovery is key to minimize entropic losses and energy consumption.•Single-stage membrane distillation has a maximum exergy efficiency of around 10%.•Reducing bulk pressure in membrane pores greatly improves exergetic performance.Direct contact membrane distillation (DCMD) is a thermal desalination process that is capable of treating high salinity waters using low-grade heat. As a water treatment process, DCMD has several advantages, including the utilization of waste heat (below 100℃), perfect rejection of nonvolatile solutes, low areal footprint, and high scalability. However, the energy efficiency of DCMD is relatively low compared to other work-based and thermal desalination processes. In this study, we aim to quantify how membrane properties and process conditions affect the exergy or second-law efficiency (ηII) of a DCMD desalination system with external heat recovery. In particular, we analyze how the membrane permeability coefficient (B) and thermal conduction coefficient (K̅) impact MD performance. We show that increasing the B value of a membrane by reducing its thickness, initially leads to an increase in ηII before conductive heat loss through the membrane causes ηII to fall. For a typical MD membrane with a porosity of 0.90, material thermal conductivity of 0.20Wm−1K−1, and a nominal pore diameter of 0.6μm, we find that the optimal permeability coefficient is 1.59×10−6kgm−2s−1Pa−1 (572kgm−2h−1bar−1). This value corresponds to an optimal membrane thickness of around 95μm. Our analysis stresses the importance of effective heat recovery in DCMD. We show that an external heat exchanger with a minimum approach temperature of 5℃ reduces energy consumption by 72%. Finally, we demonstrate that increasing the ratio B/K̅, rather than just the B value, is key to increasing the exergy efficiency of DCMD desalination. For example, increasing membrane porosity from 0.70 to 0.90, which yields a 160% increase in B/K̅, leads to a 42% increase in ηII from 5.3% to 7.6%. The advantages of reducing the bulk pressure (P) in the membrane pores are also explored. For a typical membrane, halving P from 1.0bar to 0.5⁢bar, results in a 21% increase in ηII from 7.0% to 9.2%. We conclude by identifying that the maximum exergy efficiency achievable as membrane porosity tends to unity is 10% for a bulk membrane pressure of 1.0bar and 12% for a bulk membrane pressure of 0.5bar, given perfect heat recovery.Download high-res image (250KB)Download full-size image

•A post-modification technique of electrospun nanofiber mats is developed.•Highly porous electrospun substrate was coated with polymeric materials.•Tunable membrane structure was achieved without compromising surface hydrophobicity.•Modified membranes exhibited stable desalination performance in membrane distillation.•The coating technique is suitable for large scale electrospun membrane fabrication.Post-treatment of electrospun nanofibers is a versatile and scalable approach for the fabrication of membranes with controlled pore size, porosity, and morphology. In this study, we demonstrate a novel solution-based approach for the fabrication of membrane distillation (MD) membranes with adjustable pore size and performance through non-solvent induced phase separation of a polymeric solution over an electrospun fiber mat. Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) was dissolved in a blend of acetone and dimethylacetamide and used to produce a highly porous electrospun fiber mat with an average pore diameter of ~1.2 µm. Surface coating of the PVDF-HFP nanofibers with polyvinylidene fluoride (PVDF) through phase separation enabled control of the membrane pore size by filling the empty domains between the fibers. The coated fiber mats were characterized for their surface hydrophobicity, porosity, and structure. The PVDF polymeric coating layer integrated within the electrospun mat decreased the average pore diameter to <0.6 µm without compromising the surface hydrophobicity. By controlling the depth of the PVDF coating layer within the substrate, we were able to fabricate robust membranes with near complete salt rejection (>99.9%) and a water flux of 30 L m−2 h−1 in direct contact MD experiments with 40 °C temperature difference between the feed and permeate solutions. This coating procedure is compatible with current roll-to-roll membrane fabrication processes, making it a viable approach for large-scale fabrication of electrospun membranes with exceptional performance for MD applications.

•Silver ions are bound, eluted, and measured by ICP-MS to quantify carboxyl density.•Six commercial RO membranes found to have densities of 7–37 carboxyl groups/nm2.•Ionization behavior shows no trends with permselectivity of membranes.•Dimethylformamide (DMF) decreases carboxyl density due to polyamide dissolution.•Method able to quantify absolute grafting density of charged functional materials.Polyamide thin-film composite (TFC) membranes are the industry standard for membrane-based desalination. Negatively-charged carboxyl groups in the polyamide selective layer play an important role in membrane performance, affecting ion permeation, fouling, and scaling. As such, simple and accurate quantitation of the carboxyl group density is needed. While several methods already exist, each has important drawbacks that limit its application. In this study, we develop a simple bind-and-elute method utilizing silver ion probes, with silver quantitation performed using inductively coupled plasma mass spectrometry. First, the efficacy of the binding, wash, and elution steps is verified, most notably using ion exchange resin with known binding capacity. The method is then used to characterize the carboxyl group density and ionization behavior of six commercial polyamide TFC membranes, with total densities ranging from 7.2 to 37 sites/nm2 and ionization behaviors best described using two acid dissociation constants (pKa values). Comparison with data for polyamide layers isolated using dimethylformamide (DMF) shows a 35%–65% decrease in carboxyl density, suggesting that DMF may dissolve uncrosslinked polyamide oligomers. Lastly, the method is used to characterize the effect of two membrane surface modifications, with the results supporting an alternative physical interpretation of the ionization behavior in which a carboxyl group's pKa is dependent on whether it is located on the surface or buried within the polyamide network. The simplicity, accuracy, and accessibility of the developed method should allow for widespread usage in future studies involving membrane surface charge, as well as studies involving other charged interfaces.Download high-res image (220KB)Download full-size image

•Ultrafiltration membrane with self-cleaning and anti-fouling properties is fabricated.•Silane coupling reaction enabled nanoscale TiO2 grafting in the polymer matrix.•Hybrid ultrafiltration membrane showed high reversibility to polyacrylamide fouling.•Hybrid ultrafiltration membrane was resistant to photocatalytic oxidation.We report the fabrication and characterization of a novel organic/inorganic hybrid ultrafiltration membrane with anti-fouling and self-cleaning properties. Nanoscale TiO2 clusters were grafted on the side chains of a poly(aryl ether sulfone) matrix containing trifluoromethyl and carboxyl groups (PES-F-COOH) using a silane coupling agent. Separation efficiency, fouling behavior, and self-cleaning property of the TiO2/PES-F-COOH hybrid ultrafiltration membrane were investigated by dead-end filtration experiments using a polyacrylamide foulant solution. Analysis of the membrane chemistry showed that grafting TiO2 on the side chain of the PES-F-COOH resulted in homogeneous dispersion of TiO2 clusters in the polymer matrix. The hybrid UF membrane exhibited significant self-cleaning efficiency. Specifically, water flux following polyacrylamide fouling was 53% recovered after membrane exposure to UV irradiation, which is attributable to photocatalytic degradation of the organic foulants by TiO2. We further demonstrated the anti-photocatalytic ageing property of the hybrid UF membrane, indicating resistance to decomposition of the membrane polymer matrix by photocatalytic oxidation. Our developed method can serve as a versatile platform for the development of anti-fouling and self-cleaning hybrid membranes or functional materials for a wide range of applications.Download high-res image (251KB)Download full-size image

•We grafted zwitterionic polymer brushes on UF membrane and pore surfaces via ATRP.•A large brush layer thickness (>100 nm) is required to prevent fouling.•Both surface and interior pore modified membranes showed similar fouling behavior.•Pore size limits the efficacy of grafting non-fouling brushes to UF interior surface.We studied the efficacy of grafting zwitterionic polymer brushes for the antifouling modification of ultrafiltration (UF) membranes. Poly(vinylidene fluoride) (PVDF) UF membranes were modified by surface initiated atom transfer radical polymerization (SI-ATRP) to graft poly(sulfobetaine methacrylate) (PSBMA) brushes to the membrane surface. Protein adsorption tests and chemical force microscopy show that a large brush layer thickness (greater than 100 nm) is necessary in order to impart effective fouling resistance. In dynamic fouling experiments with bovine serum albumin (BSA) as a model foulant, however, the modified membranes exhibited only a slight increase of flux recovery after fouling compared to pristine PVDF membranes. Despite the thick PSBMA brush layer, this low water flux recovery indicates that internal fouling takes place within the membrane pores. To prevent internal fouling, we grafted PSBMA brushes to both the surface and internal structure of the membranes. Nevertheless, dynamic fouling experiments again showed only a minute improvement in water flux recovery. While a thick brush layer is required for effective fouling resistance, we note that the small pore size of UF membranes imposes a fundamental limit on the brush layer thickness inside the pores. Finally, we discuss the challenges of polymer brush grafting for antifouling UF membrane modification and suggest possible alternative methods to create fouling resistant UF membranes.Download high-res image (123KB)Download full-size image

The enormous potential of harvesting energy from salinity gradients has been discussed for decades, and pressure-retarded osmosis (PRO) is being increasingly investigated as a method to extract this energy. Despite advancements in membranes and system components, questions still remain regarding the overall viability of the PRO process. Here, we review PRO focusing on the net energy extractable and the ultimate feasibility of the most widely explored configurations. We define the maximum energy that can be obtained from the process, quantify losses and energetic costs that will reduce the net extractable energy, and explain how membrane modules can be improved. We then explore the potential of three configurations of PRO: systems designed to control mixing where rivers meet the sea, power plants that utilize the high concentration gradients available from hypersaline solutions, and PRO systems incorporated into reverse osmosis desalination plants to reduce electricity requirements. We conclude by considering the overall outlook of the process and identifying the most pressing challenges for future research.

In this study, we present a facile and scalable approach to fabricate omniphobic nanofiber membranes by constructing multilevel re-entrant structures with low surface energy. We first prepared positively charged nanofiber mats by electrospinning a blend polymer–surfactant solution of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and cationic surfactant (benzyltriethylammonium). Negatively charged silica nanoparticles (SiNPs) were grafted on the positively charged electrospun nanofibers via dip-coating to achieve multilevel re-entrant structures. Grafted SiNPs were then coated with fluoroalkylsilane to lower the surface energy of the membrane. The fabricated membrane showed excellent omniphobicity, as demonstrated by its wetting resistance to various low surface tension liquids, including ethanol with a surface tension of 22.1 mN/m. As a promising application, the prepared omniphobic membrane was tested in direct contact membrane distillation to extract water from highly saline feed solutions containing low surface tension substances, mimicking emerging industrial wastewaters (e.g., from shale gas production). While a control hydrophobic PVDF-HFP nanofiber membrane failed in the desalination/separation process due to low wetting resistance, our fabricated omniphobic membrane exhibited a stable desalination performance for 8 h of operation, successfully demonstrating clean water production from the low surface tension feedwater.Keywords: desalination; electrospinning; membrane distillation; omniphobicity; saline wastewater

Current postcombustion CO2 capture technologies are energy intensive, require high-temperature heat sources, and dramatically increase the cost of power generation. In this work, we introduce a new carbon capture process requiring significantly lower temperatures and less energy, creating further impetus to reduce CO2 emissions from power generation. In this process, high-purity CO2 is generated through the addition of an organic solvent (acetone, dimethoxymethane, or acetaldehyde) to a CO2 rich, aqueous ammonia/carbon dioxide solution under room-temperature and -pressure conditions. The organic solvent and CO2-absorbing solution are then regenerated using low-temperature heat. When acetone, dimethoxymethane, or acetaldehyde was added at a concentration of 16.7% (v/v) to 2 M aqueous ammonium bicarbonate, 39.8, 48.6, or 86.5%, respectively, of the aqueous CO2 species transformed into high-purity CO2 gas over 3 h. Thermal energy and temperature requirements for recovering acetaldehyde, the best-performing organic solvent investigated, and the CO2-absorbing solution were 1.39 MJ/kg of CO2 generated and 68 °C, respectively, 75% less energy than the amount used in a pilot chilled ammonia process and a temperature 53 °C lower. Our findings exhibit the promise of economically viable carbon capture powered entirely by abundant low-temperature waste heat.

Microporous membranes fabricated from hydrophobic polymers such as polyvinylidene fluoride (PVDF) have been widely used for membrane distillation (MD). However, hydrophobic MD membranes are prone to wetting by low surface tension substances, thereby limiting their use in treating challenging industrial wastewaters, such as shale gas produced water. In this study, we present a facile and scalable approach for the fabrication of omniphobic polyvinylidene fluoride (PVDF) membranes that repel both water and oil. Positive surface charge was imparted to an alkaline-treated PVDF membrane by aminosilane functionalization, which enabled irreversible binding of negatively charged silica nanoparticles (SiNPs) to the membrane through electrostatic attraction. The membrane with grafted SiNPs was then coated with fluoroalkylsilane (perfluorodecyltrichlorosilane) to lower the membrane surface energy. Results from contact angle measurements with mineral oil and surfactant solution demonstrated that overlaying SiNPs with ultralow surface energy significantly enhanced the wetting resistance of the membrane against low surface tension liquids. We also evaluated desalination performance of the modified membrane in direct contact membrane distillation with a synthetic wastewater containing surfactant (sodium dodecyl sulfate) and mineral oil, as well as with shale gas produced water. The omniphobic membrane exhibited a stable MD performance, demonstrating its potential application for desalination of challenging industrial wastewaters containing diverse low surface tension contaminants.

Desalination membranes are essential for the treatment of unconventional water sources, such as seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel membrane materials may yield significant performance gains over state-of-the-art thin-film composite (TFC) membranes, which are constrained by the permeability–selectivity trade-off. However, little guidance currently exists on the practical impact of such performance gains, namely enhanced water permeability or enhanced water–solute selectivity. In this critical review, we first discuss the performance of current TFC membranes. We then highlight and provide context for recent module-scale modeling studies that have found limited impact of increased water permeability on the efficiency of desalination processes. Next we cover several important examples of water treatment processes in which inadequate membrane selectivity hinders process efficacy. We conclude with a brief discussion of how the need for enhanced selectivity may influence the design strategies of future membranes.

We investigated the factors that determine surface omniphobicity of microporous membranes and evaluated the potential application of these membranes in desalination of low surface tension wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and surface energy on membrane surface omniphobicity were systematically investigated by evaluating wetting resistance to low surface tension liquids. Single and multilevel re-entrant structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane increased with higher level of re-entrant structure and with lower surface energy. The secondary re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a critical role in enhancing the surface omniphobicity. Membranes coated with SiNPs and chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy) exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension liquids such as ethanol (22.1 mN m–1). We further evaluated performance of the membranes in desalination of saline feed solutions with varying surface tensions by membrane distillation (MD). The engineered membranes exhibited stable MD performance with low surface tension feed waters, demonstrating the potential application omniphobic membranes in desalinating complex, high salinity industrial wastewaters.

Zero liquid discharge (ZLD)—a wastewater management strategy that eliminates liquid waste and maximizes water usage efficiency — has attracted renewed interest worldwide in recent years. Although implementation of ZLD reduces water pollution and augments water supply, the technology is constrained by high cost and intensive energy consumption. In this critical review, we discuss the drivers, incentives, technologies, and environmental impacts of ZLD. Within this framework, the global applications of ZLD in the United States and emerging economies such as China and India are examined. We highlight the evolution of ZLD from thermal- to membrane-based processes, and analyze the advantages and limitations of existing and emerging ZLD technologies. The potential environmental impacts of ZLD, notably greenhouse gas emission and generation of solid waste, are discussed and the prospects of ZLD technologies and research needs are highlighted.

Graphene-based materials are gaining heightened attention as novel materials for environmental applications. The unique physicochemical properties of graphene, notably its exceptionally high surface area, electron mobility, thermal conductivity, and mechanical strength, can lead to novel or improved technologies to address the pressing global environmental challenges. This critical review assesses the recent developments in the use of graphene-based materials as sorbent or photocatalytic materials for environmental decontamination, as building blocks for next generation water treatment and desalination membranes, and as electrode materials for contaminant monitoring or removal. The most promising areas of research are highlighted, with a discussion of the main challenges that we need to overcome in order to fully realize the exceptional properties of graphene in environmental applications.

Functionalization of electrospun mats with antimicrobial nanomaterials is an attractive strategy to develop polymer coating materials to prevent bacterial colonization on surfaces. In this study we demonstrated a feasible approach to produce antimicrobial electrospun mats through a postfabrication binding of graphene-based nanocomposites to the nanofibers’ surface. A mixture of poly(lactide-co-glycolide) (PLGA) and chitosan was electrospun to yield cylindrical and narrow-diameter (356 nm) polymeric fibers. To achieve a robust antimicrobial property, the PLGA–chitosan mats were functionalized with graphene oxide decorated with silver nanoparticles (GO–Ag) via a chemical reaction between the carboxyl groups of graphene and the primary amine functional groups on the PLGA–chitosan fibers using 3-(dimethylamino)propyl-N′-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide as cross-linking agents. The attachment of GO–Ag sheets to the surface of PLGA–chitosan fibers was successfully revealed by scanning and transmission electron images. Upon direct contact with bacterial cells, the PLGA–chitosan mats functionalized with GO–Ag nanocomposites were able to effectively inactivate both Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria. Our results suggest that covalent binding of GO–Ag nanocomposites to the surface of PLGA–chitosan mats opens up new opportunities for the production of cost-effective, scalable, and biodegradable coating materials with the ability to hinder microbial proliferation on solid surfaces.Keywords: antimicrobial properties; electrospun fibers; graphene oxide; nanocomposites; silver nanoparticles;

We report a new macromolecular architecture of dual functional block copolymer brushes on commercial thin-film composite (TFC) membranes for integrated “defending” and “attacking” strategies against biofouling. Mussel-inspired catechol chemistry is used for a convenient immobilization of initiator molecules to the membrane surface with the aid of polydopamine (PDA). Zwitterionic polymer brushes with strong hydration capacity and quaternary ammonium salt (QAS) polymer brushes with bactericidal ability are sequentially grafted on TFC membranes via activators regenerated by electron transfer–atom transfer radical polymerization (ARGET-ATRP), an environmentally benign and controlled polymerization method. Measurement of membrane intrinsic transport properties in reverse osmosis experiments shows that the modified TFC membrane maintains the same water permeability and salt selectivity as the pristine TFC membrane. Chemical force microscopy and protein/bacterial adhesion studies are carried out for a comprehensive evaluation of the biofouling resistance and antimicrobial ability, demonstrating low biofouling propensity and excellent bacterial inactivation for the modified TFC membrane. We conclude that this polymer architecture, with complementary “defending” and “attacking” capabilities, can effectively prevent the attachment of biofoulants and formation of biofilms and thereby significantly mitigate biofouling on TFC membranes.Keywords: antifouling; antimicrobial; ARGET-ATRP; block copolymers; thin-film composite (TFC) membranes

In this study, we exploit the nitrogen–sulfur elemental contrast of thin-film composite (TFC) polyamide membranes and present, for the first time, the application of two elemental analysis techniques, scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy (STEM–EDX) and X-ray photoelectron spectroscopy (XPS) C60+ ion-beam sputtering, to elucidate the nanoscale structure and chemical composition of the polyamide–polysulfone interface. Although STEM–EDX elemental mapping depicts the presence of a dense polyamide layer at the interface, it is incapable of resolving the elemental contrast at nanoscale resolution at the interfacial zone. Depth-resolved XPS C60+ ion-beam sputtering enabled nanoscale characterization of the polyamide–polysulfone interface and revealed the presence of a heterogeneous layer that contains both polyamide and polysulfone signatures. Our results have important implications for future studies to elucidate the structure–property–performance relationship of TFC membranes.Keywords: elemental contrast; nanoscale characterization; polyamide−polysulfone interface; thin-film composite membrane

•Physicochemical / thermodynamic properties of trimethylamine (TMA) are presented.•TMA–CO2 is evaluated as a thermolytic draw solution for engineered osmosis.•TMA–CO2 produces relatively high water flux and reduced reverse draw permeation.•TMA–CO2 can be regenerated with low grade heat.•Potential challenges in the use of TMA–CO2 draw solution are discussed.We evaluated the performance of trimethylamine–carbon dioxide (TMA–CO2) as a potential thermolytic draw solution for engineered osmosis. Water flux and reverse solute flux with TMA–CO2 draw solution were measured in forward osmosis (FO) and pressure retarded osmosis (PRO) modes using thin-film composite (TFC) and cellulose triacetate (CTA) FO membranes. Water flux with the TMA–CO2 draw solution was comparable to that obtained with the more common ammonia–carbon dioxide (NH3–CO2) thermolytic draw solution at similar (1 M) concentration. Using a TFC–FO membrane, the water fluxes produced by 1 M TMA–CO2 and NH3–CO2 draw solutions with a DI water feed were, respectively, 33.4 and 35.6 L m−2 h−1 in PRO mode and 14.5 and 15.2 L m−2 h−1 in FO mode. Reverse draw permeation of TMA–CO2 was relatively low compared to NH3–CO2, ranging from 0.1 to 0.2 mol m−2 h−1 in all experiments, due to the larger molecular size of TMA. Thermal separation and recovery efficiency for TMA–CO2 was compared to NH3–CO2 by modeling low-temperature vacuum distillation utilizing low-grade heat sources. We also discuss possible challenges in the use TMA–CO2, including potential adverse impact on human health and environments.

We investigated the role of reverse divalent cation diffusion in forward osmosis (FO) biofouling. FO biofouling by Pseudomonas aeruginosa was simulated using pristine and chlorine-treated thin-film composite polyamide membranes with either MgCl2 or CaCl2 draw solution. We related FO biofouling behavior—water flux decline, biofilm architecture, and biofilm composition—to reverse cation diffusion. Experimental results demonstrated that reverse calcium diffusion led to significantly more severe water flux decline in comparison with reverse magnesium permeation. Unlike magnesium, reverse calcium permeation dramatically altered the biofilm architecture and composition, where extracellular polymeric substances (EPS) formed a thicker, denser, and more stable biofilm. We propose that FO biofouling was enhanced by complexation of calcium ions to bacterial EPS. This hypothesis was confirmed by dynamic and static light scattering measurements using extracted bacterial EPS with the addition of either MgCl2 or CaCl2 solution. We observed a dramatic increase in the hydrodynamic radius of bacterial EPS with the addition of CaCl2, but no change was observed after addition of MgCl2. Static light scattering revealed that the radius of gyration of bacterial EPS with addition of CaCl2 was 20 times larger than that with the addition of MgCl2. These observations were further confirmed by transmission electron microscopy imaging, where bacterial EPS in the presence of calcium ions was globular, while that with magnesium ions was rod-shaped.

Pressure-retarded osmosis (PRO) is a promising source of renewable energy when hypersaline brines and other high concentration solutions are used. However, membrane performance under conditions suitable for these solutions is poorly understood. In this work, we use a new method to characterize membranes under a variety of pressures and concentrations, including hydraulic pressures up to 48.3 bar and concentrations of up to 3 M NaCl. We find membrane selectivity decreases as the draw solution concentration is increased, with the salt permeability coefficient increasing by a factor of 2 when the draw concentration is changed from 0.6 to 3 M NaCl, even when the applied hydraulic pressure is maintained constant. Additionally, we find that significant pumping energy is required to overcome frictional pressure losses in the spacer-filled feed channel and achieve suitable mass transfer on the feed side of the membrane, especially at high operating pressures. For a meter-long module operating at 41 bar, we estimate feedwater will have to be pumped in at a pressure of at least 3 bar. Both the reduced selectivity and increased pumping energy requirements we observe in PRO will significantly diminish the obtainable net energy, highlighting important new challenges for development of systems utilizing hypersaline draw solutions.

We present a hybrid osmotic heat engine (OHE) system that uses draw solutions with an organic solvent for enhanced thermal separation efficiency. The hybrid OHE system produces sustainable energy by combining pressure-retarded osmosis (PRO) as a power generation stage and membrane distillation (MD) utilizing low-grade heat as a separation stage. While previous OHE systems employed aqueous electrolyte draw solutions, using methanol as a solvent is advantageous because methanol is highly volatile and has a lower heat capacity and enthalpy of vaporization than water. Hence, the thermal separation efficiency of a draw solution with methanol would be higher than that of an aqueous draw solution. In this study, we evaluated the performance of LiCl–methanol as a potential draw solution for a PRO–MD hybrid OHE system. The membrane transport properties as well as performance with LiCl–methanol draw solution were evaluated using thin-film composite (TFC) PRO membranes and compared to the results obtained with a LiCl–water draw solution. Experimental PRO methanol flux and maximum projected power density of 47.1 L m–2 h–1 and 72.1 W m–2, respectively, were achieved with a 3 M LiCl–methanol draw solution. The overall efficiency of the hybrid OHE system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages under conditions with and without heat recovery. The modeling results demonstrate higher OHE energy efficiency with the LiCl–methanol draw solution compared to that with the LiCl–water draw solution under practical operating conditions (i.e., heat recovery <90%). We discuss the implications of the results for converting low-grade heat to power.

In this study, we investigate the influence of surface structure on the fouling propensity of thin-film composite (TFC) forward osmosis (FO) membranes. Specifically, we compare membranes fabricated through identical procedures except for the use of different solvents (dimethylformamide, DMF and N-methyl-2-pyrrolidinone, NMP) during phase separation. FO fouling experiments were carried out with a feed solution containing a model organic foulant. The TFC membranes fabricated using NMP (NMP-TFC) had significantly less flux decline (7.47 ± 0.15%) when compared to the membranes fabricated using DMF (DMF-TFC, 12.70 ± 2.62% flux decline). Water flux was also more easily recovered through physical cleaning for the NMP-TFC membrane. To determine the fundamental cause of these differences in fouling propensity, the active and support layers of the membranes were extensively characterized for physical and chemical characteristics relevant to fouling behavior. Polyamide surface roughness was found to dominate all other investigated factors in determining the fouling propensities of our membranes relative to each other. The high roughness polyamide surface of the DMF-TFC membrane was also rich in larger leaf-like structures, whereas the lower roughness NMP-TFC membrane polyamide layer contained more nodular and smaller features. The support layers of the two membrane types were also characterized for their morphological properties, and the relation between support layer surface structure and polyamide active layer formation was discussed. Taken together, our findings indicate that support layer structure has a significant impact on the fouling propensity of the active layer, and this impact should be considered in the design of support layer structures for TFC membranes.

Graphene oxide (GO) is a promising material for the development of antimicrobial surfaces due to its contact-based antimicrobial activity. However, the relationship between GO physicochemical properties and its antimicrobial activity has yet to be elucidated. In this study, we investigated the size-dependency of GO antimicrobial activity using the Gram-negative bacteria Escherichia coli. GO suspensions of average sheet area ranging from 0.01 to 0.65 μm2 were produced and their antimicrobial activity evaluated in cell suspensions or as a model GO surface coating. The antimicrobial activity of GO surface coatings increased 4-fold when GO sheet area decreased from 0.65 to 0.01 μm2. The higher antimicrobial effect of smaller GO sheets is attributed to oxidative mechanisms associated with the higher defect density of smaller sheets. In contrast, in suspension assays, GO interacted with bacteria in a cell entrapment mechanism; in this case, the antimicrobial effect of GO increased with increasing sheet area, with apparent complete inactivation observed for the 0.65 μm2 sheets after a 3 h exposure. However, cell inactivation by GO entrapment was reversible and all initially viable cells could be recovered when separated from GO sheets by sonication. These findings provide useful guidelines for future development of graphene-based antimicrobial surface coatings, where smaller sheet sizes can increase the antimicrobial activity of the material. Our study further emphasizes the importance of an accurate assessment of the antimicrobial effect of nanomaterials when used for antimicrobial surface design.Keywords: antimicrobial surfaces; bacterial toxicity; glutathione; graphene oxide; oxidative damage;

Salinity gradient energy, which is released upon mixing two solutions of different concentrations, is considered to be a promising source of sustainable power. Of the methods available to harvest the salinity gradient energy, pressure retarded osmosis (PRO) has been one of the most widely investigated processes. In this study, we identify the thermodynamic limits of the PRO process by evaluating the obtainable specific energy, or extractable energy per total volume of the mixed solutions. Three distinct operation modes are analyzed: an ideal case for a reversible process, and constant-pressure operations with either co-current or counter-current flow in a membrane module. For module-scale operation, counter-current flow mode is shown to be more efficient than co-current flow mode. Additionally, two distinct thermodynamically limiting operation regimes are identified in counter-current flow mode—the draw limiting regime and the feed limiting regime. We derive analytical expressions to quantify the maximum specific energy extractable and the corresponding optimal feed flow rate fraction and applied pressure for each operation mode. Using the analytical expressions, we determine that maximum extractable energy in constant-pressure PRO with seawater (0.6 M NaCl) as a draw solution and river water (0.015 M NaCl) as a feed solution is 0.192 kW h per cubic meter of mixed solution (75% of the maximum specific Gibbs free energy of mixing). Considering that this is the theoretical upper bound of extractable energy by the PRO process, we discuss further efficiency losses and energy requirements (e.g., pretreatment and pumping) that may render it difficult to extract a sizable net specific energy from a seawater and river water solution pairing. We analyze alternative source waters that provide a higher salinity difference and hence greater extractable specific energy, such as reverse osmosis brine paired with treated wastewater effluent, which allow for a more immediately viable PRO process.

Membrane distillation (MD) is an emerging desalination technology that uses low-grade heat to drive water vapor across a microporous hydrophobic membrane. Currently, little is known about the biofilms that grow on MD membranes. In this study, we use estuarine water collected from Long Island Sound in a bench-scale direct contact MD system to investigate the initial stages of biofilm formation. For comparison, we studied biofilm formation in a bench-scale reverse osmosis (RO) system using the same feedwater. These two membrane desalination systems expose the natural microbial community to vastly different environmental conditions: high temperatures with no hydraulic pressure in MD and low temperature with hydraulic pressure in RO. Over the course of 4 days, we observed a steady decline in bacteria concentration (nearly 2 orders of magnitude) in the MD feed reservoir. Even with this drop in planktonic bacteria, significant biofilm formation was observed. Biofilm morphologies on MD and RO membranes were markedly different. MD membrane biofilms were heterogeneous and contained several colonies, while RO membrane biofilms, although thicker, were a homogeneous mat. Phylogenetic analysis using next-generation sequencing of 16S rDNA showed significant shifts in the microbial communities. Bacteria representing the orders Burkholderiales, Rhodobacterales, and Flavobacteriales were most abundant in the MD biofilms. On the basis of the results, we propose two different regimes for microbial community shifts and biofilm development in RO and MD systems.

We investigate the performance of pressure retarded osmosis (PRO) at the module scale, accounting for the detrimental effects of reverse salt flux, internal concentration polarization, and external concentration polarization. Our analysis offers insights on optimization of three critical operation and design parameters—applied hydraulic pressure, initial feed flow rate fraction, and membrane area—to maximize the specific energy and power density extractable in the system. For co- and counter-current flow modules, we determine that appropriate selection of the membrane area is critical to obtain a high specific energy. Furthermore, we find that the optimal operating conditions in a realistic module can be reasonably approximated using established optima for an ideal system (i.e., an applied hydraulic pressure equal to approximately half the osmotic pressure difference and an initial feed flow rate fraction that provides equal amounts of feed and draw solutions). For a system in counter-current operation with a river water (0.015 M NaCl) and seawater (0.6 M NaCl) solution pairing, the maximum specific energy obtainable using performance properties of commercially available membranes was determined to be 0.147 kWh per m3 of total mixed solution, which is 57% of the Gibbs free energy of mixing. Operating to obtain a high specific energy, however, results in very low power densities (less than 2 W/m2), indicating that the trade-off between power density and specific energy is an inherent challenge to full-scale PRO systems. Finally, we quantify additional losses and energetic costs in the PRO system, which further reduce the net specific energy and indicate serious challenges in extracting net energy in PRO with river water and seawater solution pairings.

Reverse electrodialysis (RED) can harness the Gibbs free energy of mixing when fresh river water flows into the sea for sustainable power generation. In this study, we carry out a thermodynamic and energy efficiency analysis of RED power generation, and assess the membrane power density. First, we present a reversible thermodynamic model for RED and verify that the theoretical maximum extractable work in a reversible RED process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible process with maximized power density using a constant-resistance load is then examined to assess the energy conversion efficiency and power density. With equal volumes of seawater and river water, energy conversion efficiency of ∼33–44% can be obtained in RED, while the rest is lost through dissipation in the internal resistance of the ion-exchange membrane stack. We show that imperfections in the selectivity of typical ion exchange membranes (namely, co-ion transport, osmosis, and electro-osmosis) can detrimentally lower efficiency by up to 26%, with co-ion leakage being the dominant effect. Further inspection of the power density profile during RED revealed inherent ineffectiveness toward the end of the process. By judicious early discontinuation of the controlled mixing process, the overall power density performance can be considerably enhanced by up to 7-fold, without significant compromise to the energy efficiency. Additionally, membrane resistance was found to be an important factor in determining the power densities attainable. Lastly, the performance of an RED stack was examined for different membrane conductivities and intermembrane distances simulating high performance membranes and stack design. By thoughtful selection of the operating parameters, an efficiency of ∼37% and an overall gross power density of 3.5 W/m2 represent the maximum performance that can potentially be achieved in a seawater-river water RED system with low-resistance ion exchange membranes (0.5 Ω cm2) at very small spacing intervals (50 μm).

We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal relative flow rate that maximizes the overall energy efficiency of the PRO-MD system for given working temperatures and concentration. In the case of unlimited mass and heat transfer kinetics, the energy efficiency of the system can be analytically determined based on thermodynamics. Our assessment indicates that the hybrid PRO-MD system can theoretically achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and a working solution of 1.0 M NaCl. When mass and heat transfer kinetics are limited, conditions that more closely represent actual operations, the practical energy efficiency will be lower than the theoretically achievable efficiency. In such practical operations, utilizing a higher working concentration will yield greater energy efficiency. Overall, our study demonstrates the theoretical viability of the PRO-MD system and identifies the key factors for performance optimization.

Biofouling is a major operational challenge in reverse osmosis (RO) desalination, motivating a search for improved biofouling control strategies. Copper, long known for its antibacterial activity and relatively low cost, is an attractive potential biocidal agent. In this paper, we present a method for loading copper nanoparticles (Cu-NPs) on the surface of a thin-film composite (TFC) polyamide RO membrane. Cu-NPs were synthesized using polyethyleneimine (PEI) as a capping agent, resulting in particles with an average radius of 34 nm and a copper content between 39 and 49 wt.%. The positive charge of the Cu-NPs imparted by the PEI allowed a simple electrostatic functionalization of the negatively charged RO membrane. We confirmed functionalization and irreversible binding of the Cu-NPs to the membrane surface with SEM and XPS after exposing the membrane to bath sonication. We also demonstrated that Cu-NP functionalization can be repeated after the Cu-NPs dissolve from the membrane surface. The Cu-NP functionalization had minimal impact on the intrinsic membrane transport parameters. Surface hydrophilicity and surface roughness were also maintained, and the membrane surface charge became positive after functionalization. The functionalized membrane exhibited significant antibacterial activity, leading to an 80–95% reduction in the number of attached live bacteria for three different model bacterial strains. Challenges associated with this functionalization method and its implementation in RO desalination are discussed.

Systematic fundamental understanding of mass transport in osmosis-driven membrane processes is important for further development of this emerging technology. In this work, we investigate the role of membrane surface chemistry and charge on bidirectional solute diffusion in forward osmosis (FO). In particular, bidirectional diffusion of ammonium (NH4+) and sodium (Na+) is examined using FO membranes with different materials and surface charge characteristics. Using an ammonium bicarbonate (NH4HCO3) draw solution, we observe dramatically enhanced cation fluxes with sodium chloride feed solution compared to that with deionized water feed solution for thin-film composite (TFC) FO membrane. However, the bidirectional diffusion of cations does not change, regardless of the type of feed solution, for cellulose triacetate (CTA) FO membrane. We relate this phenomenon to the membrane fixed surface charge by employing different feed solution pH to foster different protonation conditions for the carboxyl groups on the TFC membrane surface. Membrane surface modification is also carried out with the TFC membrane using ethylenediamine to alter carboxyl groups into amine groups. The modified TFC membrane, with less negatively charged groups, exhibits a significant decrease in the bidirectional diffusion of cations under the same conditions employed with the pristine TFC membrane. Based on our experimental observations, we propose Donnan dialysis as a mechanism responsible for enhanced bidirectional diffusion of cations in TFC membranes.

Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are emerging membrane-based technologies that can convert chemical energy in salinity gradients to useful work. The two processes have intrinsically different working principles: controlled mixing in PRO is achieved by water permeation across salt-rejecting membranes, whereas RED is driven by ion flux across charged membranes. This study compares the energy efficiency and power density performance of PRO and RED with simulated technologically available membranes for natural, anthropogenic, and engineered salinity gradients (seawater–river water, desalination brine–wastewater, and synthetic hypersaline solutions, respectively). The analysis shows that PRO can achieve both greater efficiencies (54–56%) and higher power densities (2.4–38 W/m2) than RED (18–38% and 0.77–1.2 W/m2). The superior efficiency is attributed to the ability of PRO membranes to more effectively utilize the salinity difference to drive water permeation and better suppress the detrimental leakage of salts. On the other hand, the low conductivity of currently available ion exchange membranes impedes RED ion flux and, thus, constrains the power density. Both technologies exhibit a trade-off between efficiency and power density: employing more permeable but less selective membranes can enhance the power density, but undesired entropy production due to uncontrolled mixing increases and some efficiency is sacrificed. When the concentration difference is increased (i.e., natural → anthropogenic → engineered salinity gradients), PRO osmotic pressure difference rises proportionally but not so for RED Nernst potential, which has logarithmic dependence on the solution concentration. Because of this inherently different characteristic, RED is unable to take advantage of larger salinity gradients, whereas PRO power density is considerably enhanced. Additionally, high solution concentrations suppress the Donnan exclusion effect of the charged RED membranes, severely reducing the permselectivity and diminishing the energy conversion efficiency. This study indicates that PRO is more suitable to extract energy from a range of salinity gradients, while significant advancements in ion exchange membranes are likely necessary for RED to be competitive with PRO.

Fouling of membranes by microorganisms is a major limiting factor in membrane separation processes. Novel strategies are therefore required to decrease the extent of bacterial growth on membranes. In this study, we confer strong antimicrobial properties to thin-film composite polyamide membranes by a simple graphene oxide surface functionalization. Using amide coupling between carboxyl groups of graphene oxide and carboxyl groups of the polyamide active layer, graphene oxide is irreversibly bound to the membrane. Surface binding of graphene oxide is demonstrated by scanning electron microscopy and Raman spectroscopy. Direct contact of bacteria with functionalized graphene oxide on the membrane surface results in 65% bacterial inactivation after 1 h of contact time. This bactericidal effect is imparted to the membrane without any detrimental effect to the intrinsic membrane transport properties. Our results suggest that functionalization of thin-film composite membranes with graphene oxide nanosheets is a promising approach for the development of novel antimicrobial membranes.

Pressure-retarded osmosis (PRO) has the potential to generate sustainable energy from salinity gradients. PRO is typically considered for operation with river water and seawater, but a far greater energy of mixing can be harnessed from hypersaline solutions. This study investigates the power density that can be obtained in PRO from such concentrated solutions. Thin-film composite membranes with an embedded woven mesh were supported by tricot fabric feed spacers in a specially designed crossflow cell to maximize the operating pressure of the system, reaching a stable applied hydraulic pressure of 48 bar (700 psi) for more than 10 h. Operation at this increased hydraulic pressure allowed unprecedented power densities, up to 60 W/m2 with a 3 M (180 g/L) NaCl draw solution. Experimental power densities demonstrate reasonable agreement with power densities modeled using measured membrane properties, indicating high-pressure operation does not drastically alter membrane performance. Our findings exhibit the promise of the generation of power from high-pressure PRO with concentrated solutions.

We have demonstrated the application of osmotic back-flushing (OBF) for the removal of biofilms from reverse osmosis (RO) membranes and proposed a new biofilm dispersal mechanism. OBF was conducted in a laboratory-scale RO test cell by introducing a sequence of hypersaline solution (1.5 M NaCl) flushes into the feedwater, while still maintaining the applied hydraulic pressure (13.8 bar). OBF resulted in significant biofilm detachment, leaving a thin, perforated bacterial film (24 μm thickness) with vertical cavities ranging from 15 to 50 μm in diameter. Application of OBF led to significant reductionin the biovolume (70–79%) and substantial removal of total organic carbon and proteins (78 and 66%, respectively), resulting in 63% permeate water flux recovery. Our findings demonstrate the potential of this chemical-free RO membrane cleaning method while highlighting the possible challenges of the technique.

In this work, we fabricate an omniphobic microporous membrane for membrane distillation (MD) by modifying a hydrophilic glass fiber membrane with silica nanoparticles followed by surface fluorination and polymer coating. The modified glass fiber membrane exhibits an anti-wetting property not only against water but also against low surface tension organic solvents that easily wet a hydrophobic polytetrafluoroethylene (PTFE) membrane that is commonly used in MD applications. By comparing the performance of the PTFE and omniphobic membranes in direct contact MD experiments in the presence of a surfactant (sodium dodecyl sulfate, SDS), we show that SDS wets the hydrophobic PTFE membrane but not the omniphobic membrane. Our results suggest that omniphobic membranes are critical for MD applications with feed waters containing surface active species, such as oil and gas produced water, to prevent membrane pore wetting.

Polyvinylidene fluoride (PVDF) has drawn much attention as a predominant ultrafiltration (UF) membrane material due to its outstanding mechanical and physicochemical properties. However, current applications suffer from the low fouling resistance of the PVDF membrane due to the intrinsic hydrophobic property of the membrane. The present study demonstrates a novel approach for the fabrication of a highly hydrophilic PVDF UF membrane via postfabrication tethering of superhydrophilic silica nanoparticles (NPs) to the membrane surface. The pristine PVDF membrane was grafted with poly(methacrylic acid) (PMAA) by plasma induced graft copolymerization, providing sufficient carboxyl groups as anchor sites for the binding of silica NPs, which were surface-tailored with amine-terminated cationic ligands. The NP binding was achieved through a remarkably simple and effective dip-coating technique in the presence or absence of the N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) cross-linking process. The properties of the membrane prepared from the modification without EDC/NHS cross-linking were comparable to those for the membrane prepared with the EDC/NHS cross-linking. Both modifications almost doubled the surface energy of the functionalized membranes, which significantly improved the wettability of the membrane and converted the membrane surface from hydrophobic to highly hydrophilic. The irreversibly bound layer of superhydrophilic silica NPs endowed the membranes with strong antifouling performance as demonstrated by three sequential fouling filtration runs using bovine serum albumin (BSA) as a model organic foulant. The results suggest promising applications of the postfabrication surface modification technique in various membrane separation areas.Keywords: antifouling; fouling; membrane functionalization; nanoparticles; PVDF; superhydrophilic; ultrafiltration;

Decentralized membrane-based water treatment and refill stations represent a viable and growing business model in Southeast Asia, which rely upon the purchase of water from refill stations by consumers. This feature article discusses these water treatment and refill stations, including the appropriateness of the technology, the suitability of the business models employed, and the long-term environmental and operational sustainability of these systems. We also provide an outlook for the sector, highlighting key technical challenges that need to be addressed in order to improve the capacity of these systems, such that they can become an effective and financially viable solution.

Forward osmosis (FO) is an emerging membrane-based water separation process with potential applications in a host of environmental and industrial processes. Nevertheless, membrane fouling remains a technical obstacle affecting this technology, increasing operating costs and decreasing membrane life. This work presents the first fabrication of an antifouling thin-film composite (TFC) FO membrane by an in situ technique without postfabrication treatment. The membrane was fabricated and modified in situ, grafting Jeffamine, an amine-terminated poly(ethylene glycol) derivative, to dangling acyl chloride surface groups on the nascent polyamide active layer. Surface characterization by contact angle, Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), zeta potential, atomic force microscopy (AFM), and fluorescence microscopy, confirms the presence of Jeffamine on the membrane surface. We demonstrate the improved fouling resistance of the in situ modified membranes through accelerated dynamic fouling FO experiments using a synthetic wastewater feed solution at high concentration (250 mg/L) of alginate, a model macromolecule for the hydrophilic fraction of wastewater effluent organic matter. Our results show a significantly lower flux decline for the in situ modified membranes compared to pristine polyamide (14.3 ± 2.7% vs 2.8 ± 1.4%, respectively). AFM adhesion force measurements between the membrane and a carboxylate-modified latex particle, a surrogate for the organic (alginate) foulant, show weaker foulant–membrane interactions, further confirming the enhanced fouling resistance of the in situ modified membranes.

Pressure retarded osmosis (PRO) has the potential to produce clean, renewable energy from natural salinity gradients. However, membrane fouling can lead to diminished water flux productivity, thus reducing the extractable energy. This study investigates organic fouling and osmotic backwash cleaning in PRO and the resulting impact on projected power generation. Fabricated thin-film composite membranes were fouled with model river water containing natural organic matter. The water permeation carried foulants from the feed river water into the membrane porous support layer and caused severe water flux decline of ∼46%. Analysis of the water flux behavior revealed three phases in membrane support layer fouling. Initial foulants of the first fouling phase quickly adsorbed at the active-support layer interface and caused a significantly greater increase in hydraulic resistance than the subsequent second and third phase foulants. The water permeability of the fouled membranes was lowered by ∼39%, causing ∼26% decrease in projected power density. A brief, chemical-free osmotic backwash was demonstrated to be effective in removing foulants from the porous support layer, achieving ∼44% recovery in projected power density. The substantial performance recovery after cleaning was attributed to the partial restoration of the membrane water permeability. This study shows that membrane fouling detrimentally impacts energy production, and highlights the potential strategies to mitigate fouling in PRO power generation with natural salinity gradients.

In the rapidly developing shale gas industry, managing produced water is a major challenge for maintaining the profitability of shale gas extraction while protecting public health and the environment. We review the current state of practice for produced water management across the United States and discuss the interrelated regulatory, infrastructure, and economic drivers for produced water reuse. Within this framework, we examine the Marcellus shale play, a region in the eastern United States where produced water is currently reused without desalination. In the Marcellus region, and in other shale plays worldwide with similar constraints, contraction of current reuse opportunities within the shale gas industry and growing restrictions on produced water disposal will provide strong incentives for produced water desalination for reuse outside the industry. The most challenging scenarios for the selection of desalination for reuse over other management strategies will be those involving high-salinity produced water, which must be desalinated with thermal separation processes. We explore desalination technologies for treatment of high-salinity shale gas produced water, and we critically review mechanical vapor compression (MVC), membrane distillation (MD), and forward osmosis (FO) as the technologies best suited for desalination of high-salinity produced water for reuse outside the shale gas industry. The advantages and challenges of applying MVC, MD, and FO technologies to produced water desalination are discussed, and directions for future research and development are identified. We find that desalination for reuse of produced water is technically feasible and can be economically relevant. However, because produced water management is primarily an economic decision, expanding desalination for reuse is dependent on process and material improvements to reduce capital and operating costs.

Sodium periodate (NaIO4) is shown to be a milder and more efficient terminal oxidant for C–H oxidation with Cp*Ir (Cp* = C5Me5) precatalysts than ceric(IV) ammonium nitrate. Synthetically useful yields, regioselectivities, and functional group tolerance were found for methylene oxidation of substrates bearing a phenyl, ketone, ester, or sulfonate group. Oxidation of the natural products (−)-ambroxide and sclareolide proceeded selectively, and retention of configuration was seen in cis-decalin hydroxylation. At 60 °C, even primary C–H bonds can be activated: whereas methane was overoxidized to CO2 in 39% yield without giving partially oxidized products, ethane was transformed into acetic acid in 25% yield based on total NaIO4. 18O labeling was demonstrated in cis-decalin hydroxylation with 18OH2 and NaIO4. A kinetic isotope effect of 3.0 ± 0.1 was found in cyclohexane oxidation at 23 °C, suggesting C–H bond cleavage as the rate-limiting step. Competition experiments between C–H and water oxidation show that C–H oxidation of sodium 4-ethylbenzene sulfonate is favored by 4 orders of magnitude. In operando time-resolved dynamic light scattering and kinetic analysis exclude the involvement of metal oxide nanoparticles and support our previously suggested homogeneous pathway.

Real-time monitoring of light scattering and UV–vis profiles of four different Cp*IrIII precursors under various conditions give insight into nanoparticle formation during oxidation catalysis with NaIO4 as primary oxidant. Complexes bearing chelate ligands such as 2,2′-bipyridine, 2-phenylpyridine, or 2-(2′-pyridyl)-2-propanolate were found to be highly resistant toward particle formation, and oxidation catalysis with these compounds is thus believed to be molecular in nature under our conditions. Even with the less stable hydroxo/aqua complex [Cp*2Ir2(μ-OH)3]OH, nanoparticle formation strongly depended on the exact conditions and elapsed time. Test experiments on the isolated particles and comparison of UV–vis data with light scattering profiles revealed that the formation of a deep purple-blue color (∼580 nm) is not indicative of particle formation during oxidation catalysis with molecular iridium precursors as suggested previously.

Thin-film composite polyamide membranes are state-of-the-art materials for membrane-based water purification and desalination processes, which require both high rejection of contaminants and high water permeabilities. However, these membranes are prone to fouling when processing natural waters and wastewaters, because of the inherent surface physicochemical properties of polyamides. The present work demonstrates the fabrication of forward osmosis polyamide membranes with optimized surface properties via facile and scalable functionalization with fine-tuned nanoparticles. Silica nanoparticles are coated with superhydrophilic ligands possessing functional groups that impart stability to the nanoparticles and bind irreversibly to the native carboxyl moieties on the membrane selective layer. The tightly tethered layer of nanoparticles tailors the surface chemistry of the novel composite membrane without altering the morphology or water/solute permeabilities of the membrane selective layer. Surface characterization and interfacial energy analysis confirm that highly hydrophilic and wettable membrane surfaces are successfully attained. Lower intermolecular adhesion forces are measured between the new membrane materials and model organic foulants, indicating the presence of a bound hydration layer at the polyamide membrane surface that creates a barrier for foulant adhesion.Keywords: antifouling; forward osmosis; fouling; membrane functionalization; nanocomposite membranes; superhydrophilic; thin-film composite membranes;

Seawater desalination for agricultural irrigation will be an important contributor to satisfying growing water demands in water scarce regions. Irrigated agriculture for food production drives global water demands, which are expected to increase while available supplies are further diminished. Implementation of reverse osmosis, the current leading technology for seawater desalination, has been limited in part because of high costs and energy consumption. Because of stringent boron and chloride standards for agricultural irrigation water, desalination for agriculture is more energy intensive than desalination for potable use, and additional post-treatment, such as a second pass reverse osmosis process, is required. In this perspective, we introduce the concept of an integrated forward osmosis and reverse osmosis process for seawater desalination. Process modeling results indicate that the integrated process can achieve boron and chloride water quality requirements for agricultural irrigation while consuming less energy than a conventional two-pass reverse osmosis process. The challenges to further development of an integrated forward and reverse osmosis desalination process and its potential benefits beyond energy savings are discussed.Graphical abstractHighlights▸ An analysis of an integrated forward and reverse osmosis desalination process is presented. ▸ Integrated process can achieve boron and chloride levels suitable for agricultural irrigation. ▸ Integrated forward and reverse osmosis process consumes less energy than a conventional two-pass RO process. ▸ Challenges to further development of the integrated process are discussed.

This article analyzes the influence of feed channel spacers on the performance of pressure retarded osmosis (PRO). Unlike forward osmosis (FO), an important feature of PRO is the application of hydraulic pressure on the high salinity (draw solution) side to retard the permeating flow for energy conversion. We report the first observation of membrane deformation under the action of the high hydraulic pressure on the feed channel spacer and the resulting impact on membrane performance. Because of this observation, reverse osmosis and FO tests that are commonly used for measuring membrane transport properties (water and salt permeability coefficients, A and B, respectively) and the structural parameter (S) can no longer be considered appropriate for use in PRO analysis. To accurately predict the water flux as a function of applied hydraulic pressure difference and the resulting power density in PRO, we introduced a new experimental protocol that accounts for membrane deformation in a spacer-filled channel to determine the membrane properties (A, B, and S). PRO performance model predictions based on these determined A, B, and S values closely matched experimental data over a range of draw solution concentrations (0.5 to 2 M NaCl). We also showed that at high pressures feed spacers block the permeation of water through the membrane area in contact with the spacer, a phenomenon that we term the shadow effect, thereby reducing overall water flux. The implications of the results for power generation by PRO are evaluated and discussed.

The Gibbs free energy of mixing dissipated when fresh river water flows into the sea can be harnessed for sustainable power generation. Pressure retarded osmosis (PRO) is one of the methods proposed to generate power from natural salinity gradients. In this study, we carry out a thermodynamic and energy efficiency analysis of PRO work extraction. First, we present a reversible thermodynamic model for PRO and verify that the theoretical maximum extractable work in a reversible PRO process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible constant-pressure PRO process is then examined. We derive an expression for the maximum extractable work in a constant-pressure PRO process and show that it is less than the ideal work (i.e., Gibbs free energy of mixing) due to inefficiencies intrinsic to the process. These inherent inefficiencies are attributed to (i) frictional losses required to overcome hydraulic resistance and drive water permeation and (ii) unutilized energy due to the discontinuation of water permeation when the osmotic pressure difference becomes equal to the applied hydraulic pressure. The highest extractable work in constant-pressure PRO with a seawater draw solution and river water feed solution is 0.75 kWh/m3 while the free energy of mixing is 0.81 kWh/m3—a thermodynamic extraction efficiency of 91.1%. Our analysis further reveals that the operational objective to achieve high power density in a practical PRO process is inconsistent with the goal of maximum energy extraction. This study demonstrates thermodynamic and energetic approaches for PRO and offers insights on actual energy accessible for utilization in PRO power generation through salinity gradients.

The performance of an electrochemical multiwalled carbon nanotube (EC-MWNT) filter toward virus removal and inactivation in the presence of natural organic matter was systematically evaluated over a wide range of solution chemistries. Viral removal and inactivation were markedly enhanced by applying DC voltage in the presence of alginate and Suwannee River natural organic matter (SRNOM). Application of 2 or 3 V resulted in complete (5.8 to 7.4 log) removal and significant inactivation of MS2 viral particles in the presence of 5 mg L–1 of SRNOM or 1 mg L–1 of alginate. The EC-MWNT filter consistently maintained high performance over a wide range of solution pH and ionic strengths. The underlying mechanisms of enhanced viral removal and inactivation were further elucidated through EC-MWNT filtration experiments using carboxyl latex nanoparticles. We conclude that enhanced virus removal is attributed to the increased viral particle transport due to the applied external electric field and the attractive electrostatic interactions between the viral particles and the anodic MWNTs. The adsorbed viral particles on the MWNT surface are then inactivated through direct surface oxidation. Minimal fouling of the EC-MWNT filter was observed, even after 4-h filter runs with solutions containing 10 mg L–1 of natural organic matter and 1 mM CaCl2. Our results suggest that the EC-MWNT filter has a potential for use as a high performance point-of-use device for the removal of viruses from natural and contaminated waters with minimal power requirements.

Carboxyls are inherent functional groups of thin-film composite polyamide nanofiltration (NF) membranes, which may play a role in membrane performance and fouling. Their surface presence is attributed to incomplete reaction of acyl chloride monomers during the membrane active layer synthesis by interfacial polymerization. In order to unravel the effect of carboxyl group density on organic fouling, NF membranes were fabricated by reacting piperazine (PIP) with either isophthaloyl chloride (IPC) or the more commonly used trimesoyl chloride (TMC). Fouling experiments were conducted with alginate as a model hydrophilic organic foulant in a solution, simulating the composition of municipal secondary effluent. Improved antifouling properties were observed for the IPC membrane, which exhibited lower flux decline (40%) and significantly greater fouling reversibility or cleaning efficiency (74%) than the TMC membrane (51% flux decline and 40% cleaning efficiency). Surface characterization revealed that there was a substantial difference in the density of surface carboxyl groups between the IPC and TMC membranes, while other surface properties were comparable. The role of carboxyl groups was elucidated by measurements of foulant-surface intermolecular forces by atomic force microscopy, which showed lower adhesion forces and rupture distances for the IPC membrane compared to TMC membranes in the presence of calcium ions in solution. Our results demonstrated that a decrease in surface carboxyl group density of polyamide membranes fabricated with IPC monomers can prevent calcium bridging with alginate and, thus, improve membrane antifouling properties.

This study investigates the fouling behavior and fouling resistance of superhydrophilic thin-film composite forward osmosis membranes functionalized with surface-tailored nanoparticles. Fouling experiments in both forward osmosis and reverse osmosis modes are performed with three model organic foulants: alginate, bovine serum albumin, and Suwannee river natural organic matter. A solution comprising monovalent and divalent salts is employed to simulate the solution chemistry of typical wastewater effluents. Reduced fouling is consistently observed for the superhydrophilic membranes compared to control thin-film composite polyamide membranes, in both reverse and forward osmosis modes. The fouling resistance and cleaning efficiency of the functionalized membranes is particularly outstanding in forward osmosis mode where the driving force for water flux is an osmotic pressure difference. To understand the mechanism of fouling, the intermolecular interactions between the foulants and the membrane surface are analyzed by direct force measurement using atomic force microscopy. Lower adhesion forces are observed for the superhydrophilic membranes compared to the control thin-film composite polyamide membranes. The magnitude and distribution of adhesion forces for the different membrane surfaces suggest that the antifouling properties of the superhydrophilic membranes originate from the barrier provided by the tightly bound hydration layer at their surface, as well as from the neutralization of the native carboxyl groups of thin-film composite polyamide membranes.

We propose an innovative approach to impart nanomaterial-specific properties to the surface of thin-film composite membranes. Specifically, biocidal properties were obtained by covalently binding single-walled carbon nanotubes (SWNTs) to the membrane surface. The SWNTs were first modified by purification and ozonolysis to increase their sidewall functionalities, maximize cytotoxic properties, and achieve dispersion in aqueous solution. A tailored reaction protocol was developed to exploit the inherent moieties of hand-cast polyamide membrane surfaces and create covalent amide bonds with the functionalized SWNTs. The reaction is entirely aqueous-based and entails activation of the carboxylate groups of both the membrane and the nanomaterials to maximize reaction with ethylenediamine. The presence of SWNTs was verified after sonication of the membranes, confirming the strength of the bond between the SWNTs and the membrane surface. Characterization of the SWNT-functionalized surfaces demonstrated the attainment of membranes with novel properties that continued to exhibit high performance in water separation processes. The presence of surface-bound antimicrobial SWNTs was confirmed by experiments using E. coli cells that demonstrated an enhanced bacterial cytotoxicity for the SWNT-coated membranes. The SWNT membranes were observed to achieve up to 60% inactivation of bacteria attached to the membrane within 1 h of contact time. Our results suggest the potential of covalently bonded SWNTs to delay the onset of membrane biofouling during operation.Keywords: amide bonds; biocidal membrane; biofouling; nanocomposite; polyamide membranes; single-walled carbon nanotubes; surface modification; SWCNT; SWNT; thin-film composite; water purification;

Osmotically driven membrane processes have the potential to treat impaired water sources, desalinate sea/brackish waters, and sustainably produce energy. The development of a membrane tailored for these processes is essential to advance the technology to the point that it is commercially viable. Here, a systematic investigation of the influence of thin-film composite membrane support layer structure on forward osmosis performance is conducted. The membranes consist of a selective polyamide active layer formed by interfacial polymerization on top of a polysulfone support layer fabricated by phase separation. By systematically varying the conditions used during the casting of the polysulfone layer, an array of support layers with differing structures was produced. The role that solvent quality, dope polymer concentration, fabric layer wetting, and casting blade gate height play in the support layer structure formation was investigated. Using a 1 M NaCl draw solution and a deionized water feed, water fluxes ranging from 4 to 25 L m−2 h−1 with consistently high salt rejection (>95.5%) were produced. The relationship between membrane structure and performance was analyzed. This study confirms the hypothesis that the optimal forward osmosis membrane consists of a mixed-structure support layer, where a thin sponge-like layer sits on top of highly porous macrovoids. Both the active layer transport properties and the support layer structural characteristics need to be optimized in order to fabricate a high performance forward osmosis membrane.Graphical abstractResearch highlights▶ The ideal FO TFC membrane has both high performance active and support layers. ▶ The ideal support layer maximizes porosity to minimize concentration polarization. ▶ A finger-like morphology with large macrovoids maximizes porosity. ▶ Macrovoids spanning the entire support layer and open at its bottom maximize porosity. ▶ The ideal support layer facilitates the formation of an active layer with high A and low B.

Many conventional practices in the production and use of water, energy, and food are unsustainable. Existing technologies and concepts can be improved with the integration of forward osmosis, a membrane-based technology that uses osmosis as its driving force. This Feature highlights five emerging applications of forward osmosis that elegantly bypass the difficult step of draw solution regeneration and make common processes more sustainable. These applications enhance the efficiency of the production and use of water, energy, and food; utilize wastes and abundant, low value resources; and better protect the environment.

Pressure retarded osmosis has the potential to utilize the free energy of mixing when fresh river water flows into the sea for clean and renewable power generation. Here, we present a systematic investigation of the performance limiting phenomena in pressure retarded osmosis—external concentration polarization, internal concentration polarization, and reverse draw salt flux—and offer insights on the design criteria of a high performance pressure retarded osmosis power generation system. Thin-film composite polyamide membranes were chemically modified to produce a range of membrane transport properties, and the water and salt permeabilities were characterized to determine the underlying permeability-selectivity trade-off relationship. We show that power density is constrained by the trade-off between permeability and selectivity of the membrane active layer. This behavior is attributed to the opposing influence of the beneficial effect of membrane water permeability and the detrimental impact of reverse salt flux coupled with internal concentration polarization. Our analysis reveals the intricate influence of active and support layer properties on power density and demonstrates that membrane performance is maximized by tailoring the water and salt permeabilities to the structural parameters. An analytical parameter that quantifies the relative influence of each performance limiting phenomena is employed to identify the dominant effect restricting productivity. External concentration polarization is shown to be the main factor limiting performance at high power densities. Enhancement of the hydrodynamic flow conditions in the membrane feed channel reduces external concentration polarization and thus, yields improved power density. However, doing so will also incur additional operating costs due to the accompanying hydraulic pressure loss. This study demonstrates that by thoughtful selection of the membrane properties and hydrodynamic conditions, the detrimental effects that limit productivity in a pressure retarded osmosis power generation process can be methodically minimized to achieve high performance.

Pressure retarded osmosis has the potential to produce renewable energy from natural salinity gradients. This work presents the fabrication of thin-film composite membranes customized for high performance in pressure retarded osmosis. We also present the development of a theoretical model to predict the water flux in pressure retarded osmosis, from which we can predict the power density that can be achieved by a membrane. The model is the first to incorporate external concentration polarization, a performance limiting phenomenon that becomes significant for high-performance membranes. The fabricated membranes consist of a selective polyamide layer formed by interfacial polymerization on top of a polysulfone support layer made by phase separation. The highly porous support layer (structural parameter S = 349 μm), which minimizes internal concentration polarization, allows the transport properties of the active layer to be customized to enhance PRO performance. It is shown that a hand-cast membrane that balances permeability and selectivity (A = 5.81 L m–2 h–1 bar–1, B = 0.88 L m–2 h–1) is projected to achieve the highest potential peak power density of 10.0 W/m2 for a river water feed solution and seawater draw solution. The outstanding performance of this membrane is attributed to the high water permeability of the active layer, coupled with a moderate salt permeability and the ability of the support layer to suppress the undesirable accumulation of leaked salt in the porous support. Membranes with greater selectivity (i.e., lower salt permeability, B = 0.16 L m–2 h–1) suffered from a lower water permeability (A = 1.74 L m–2 h–1 bar–1) and would yield a lower peak power density of 6.1 W/m2, while membranes with a higher permeability and lower selectivity (A = 7.55 L m–2 h–1 bar–1, B = 5.45 L m–2 h–1) performed poorly due to severe reverse salt permeation, resulting in a similar projected peak power density of 6.1 W/m2.

The transport properties (adsorption and aggregation behavior) of virus-like particles (VLPs) of two strains of norovirus (“Norwalk” GI.1 and “Houston” GII.4) were studied in a variety of solution chemistries. GI.1 and GII.4 VLPs were found to be stable against aggregation at pH 4.0−8.0. At pH 9.0, GI.1 VLPs rapidly disintegrated. The attachment efficiencies (α) of GI.1 and GII.4 VLPs to silica increased with increasing ionic strength in NaCl solutions at pH 8.0. The attachment efficiency of GI.1 VLPs decreased as pH was increased above the isoelectric point (pH 5.0), whereas at and below the isoelectric point, the attachment efficiency was erratic. Ca2+ and Mg2+ dramatically increased the attachment efficiencies of GI.1 and GII.4 VLPs, which may be due to specific interactions with the VLP capsids. Bicarbonate decreased attachment efficiencies for both GI.1 and GII.4 VLPs, whereas phosphate decreased the attachment efficiency of GI.1, while increasing GII.4 attachment efficiency. The observed differences in GI.1 and GII.4 VLP attachment efficiencies in response to solution chemistry may be attributed to differential responses of the unique arrangement of exposed amino acid residues on the capsid surface of each VLP strain.

Abstract

In recent years, numerous large-scale seawater desalination plants have been built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, we review the possible reductions in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technological solution to global water shortages.

Osmotically driven membrane processes are an emerging set of technologies that show promise in water and wastewater treatment, desalination, and power generation. The effective operation of these systems requires that the reverse flux of draw solute from the draw solution into the feed solution be minimized. A model was developed that describes the reverse permeation of draw solution across an asymmetric membrane in forward osmosis operation. Experiments were carried out to validate the model predictions with a highly soluble salt (NaCl) as a draw solution and a cellulose acetate membrane designed for forward osmosis. Using independently determined membrane transport coefficients, strong agreement between the model predictions and experimental results was observed. Further analysis shows that the reverse flux selectivity, the ratio of the forward water flux to the reverse solute flux, is a key parameter in the design of osmotically driven membrane processes. The model predictions and experiments demonstrate that this parameter is independent of the draw solution concentration and the structure of the membrane support layer. The value of the reverse flux selectivity is determined solely by the selectivity of the membrane active layer.

The impact of single-walled carbon nanotubes (SWNTs) on the different developmental stages of biofilms has been investigated using E. coli K12 as a model organism. Specifically, we investigated (i) the impact of SWNT concentration on cell growth and biofilm formation, (ii) toxic effects of SWNTs on mature biofilms, and (iii) formation of biofilm on SWNT-coated surfaces. The results show that at the initial stage of biofilm formation, SWNTs come into contact with bacterial cells prior to biofilm maturation and inhibit their growth. Furthermore, the results suggest that bacteria in mature biofilms are less sensitive to the presence of SWNTs than cells in other biofilm stages, similar to previous observations of biofilm resistance to antimicrobials. In mature biofilms, the soluble exopolymeric substances (EPS) secreted by the biofilm play an important role in mitigating the toxic effects of SWNTs. Upon exposure to SWNTs, biofilms without soluble EPS in the supernatant had a much more significant loss of biomass because of cell detachment from the biofilm than biofilms containing soluble EPS. To observe similar cell loss, biofilms with soluble EPS needed SWNT concentrations that were 10 times higher compared to biofilms without soluble EPS. Finally, SWNTs deposited onto surfaces affected significantly the subsequent biofilm development. Analysis of the total biomass and the area occupied by cells indicates that a SWNT-coated substratum has 10 times less biofilm colonization and biomass production than a control substratum without SWNTs.

We examined the resistance to bacterial adhesion of a novel polyacrylonitrile (PAN) ultrafiltration membrane incorporating the amphiphilic comb copolymer additive, polyacrylonitrile-graft-polyethylene oxide (PAN-g-PEO). The adhesion of bacteria (E. coli K12) and the reversibility of adhered bacteria were tested with the novel membrane, and the behavior was compared to a commercial PAN ultrafiltration membrane. Under static (no flow) bacterial adhesion tests, we observed no bacterial adhesion to the PAN/PAN-g-PEO membrane at all ionic strengths tested, even with the addition of calcium ions. In contrast, significant adhesion of bacterial cells was observed on the commercial PAN membrane, with increased cell adhesion at higher ionic strengths and in the presence of calcium ions. Under crossflow filtration conditions, initial bacterial deposition rate increased with ionic strength and with addition of calcium ions for both membranes, with generally lower bacterial deposition rate with the PAN/PAN-g-PEO membrane. However, deposited bacteria were readily removed (between 97 and 100%) from the surface of the PAN/PAN-g-PEO membrane upon increasing the crossflow and eliminating the permeate flow (i.e., no applied transmembrane pressure), suggesting reversible adhesion of bacteria. In contrast, bacterial adhesion on the commercial PAN membrane was irreversible, with approximately 50% removal of adhered bacteria at moderate ionic strengths (10 and 30 mM) and less than 25% removal at high ionic strength (100 mM). The resistance to bacterial adhesion of the PAN/PAN-g-PEO membrane was further analyzed via measurement of interaction forces with atomic force microscopy (AFM). No adhesion forces were detected between a carboxylated colloid probe and the PAN/PAN-g-PEO membrane, while the probe exhibited strong adhesion to the commercial PAN membrane, consistent with the bacterial adhesion tests. The exceptional resistance of the PAN/PAN-g-PEO membrane to bacterial adhesion is attributable to steric repulsion imparted by the dense brush layer of polyethylene oxide (PEO) chains.

Recent studies show that osmotically driven membrane processes may be a viable technology for desalination, water and wastewater treatment, and power generation. However, the absence of a membrane designed for such processes is a significant obstacle hindering further advancements of this technology. This work presents the development of a high performance thin-film composite membrane for forward osmosis applications. The membrane consists of a selective polyamide active layer formed by interfacial polymerization on top of a polysulfone support layer fabricated by phase separation onto a thin (40 μm) polyester nonwoven fabric. By careful selection of the polysulfone casting solution (i.e., polymer concentration and solvent composition) and tailoring the casting process, we produced a support layer with a mix of finger-like and sponge-like morphologies that give significantly enhanced membrane performance. The structure and performance of the new thin-film composite forward osmosis membrane are compared with those of commercial membranes. Using a 1.5 M NaCl draw solution and a pure water feed, the fabricated membranes produced water fluxes exceeding 18 L m2−h−1, while consistently maintaining observed salt rejection greater than 97%. The high water flux of the fabricated thin-film composite forward osmosis membranes was directly related to the thickness, porosity, tortuosity, and pore structure of the polysulfone support layer. Furthermore, membrane performance did not degrade after prolonged exposure to an ammonium bicarbonate draw solution.

The effective removal of viruses by a multiwalled carbon nanotube (MWNT) filter is demonstrated over a range of solution chemistries. MS2 bacteriophage viral removal by the MWNT filter was between 1.5 and 3 log higher than that observed with a recently reported single-walled carbon nanotube (SWNT) filter when examined under similar loadings (0.3 mg/cm2) of carbon nanotubes (CNTs). The greater removal of viruses by the MWNT filter is attributed to a more uniform CNT-filter matrix that allows effective removal of viruses by physicochemical (depth) filtration. Viral removal by the MWNT filter was examined under a broad range of water compositions (ionic strength, monovalent and divalent salts, solution pH, natural organic matter, alginate, phosphate, and bicarbonate) and filter approach velocities (0.0016, 0.0044, and 0.0072 cm/s). Log viral removal increased as the fluid approach velocity decreased, exhibiting a dependence on approach velocity in agreement with colloid filtration theory for Brownian particles. Viral removal improved with increasing ionic strength (NaCl), from 5.06 log removal at 1 mM NaCl to greater than 6.56 log removal at 100 mM NaCl. Addition of calcium ions also enhanced viral removal, but the presence of magnesium ions resulted in a decrease in viral removal. Solution pH also played an important role in viral removal, with log removals of 8.13, 5.38, and 4.00 being documented at solution pH values of 3.0, 5.5, and 9.0, respectively. Dissolved natural organic matter (NOM) had a negligible effect on viral removal at low concentration (1 mg/L), but higher concentrations of NOM significantly reduced the viral removal by the MWNT filter, likely due to steric repulsion. Addition of alginate (model polysaccharide) also caused a marked decrease in viral removal by the MWNT filter. This highly scalable MWNT-filter technology at gravity-driven pressures presents new, cost-effective options for point-of-use filters for viral removal.

We describe the concept and demonstrate the efficacy of a novel SWNT−MWNT hybrid filter for the removal and inactivation of microbial pathogens from water. The filter is composed of a thin SWNT layer (0.05 mg cm−2) on top of a thicker MWNT layer (0.27 mg cm−2) supported by a microporous support membrane. The SWNT−MWNT filter exhibits high log removal of several model viruses (MS2, PRD1, and T4 bacteriophages) by depth filtration, which predominantly takes place in the thicker and more uniform MWNT layer. The filter removes all bacteria by a sieving mechanism, with the top SWNT layer providing high levels of inactivation of model bacteria (Escherichia coli K12 and Staphylococcus epidermidis), as well as microbes from river water and treated wastewater effluent. The dual-layer SWNT−MWNT filter lays the framework for new possibilities in point-of-use water filtration.

Concentrating sugar solutions is a common process used in the production of many food products for either dewatering a high value product or concentrating waste streams prior to disposal. Thermal and pressure-driven dewatering methods are widely used, but they are prohibitively energy intensive and hence, expensive. Osmotically driven membrane processes, like forward osmosis, may be a viable and sustainable alternative to these current technologies. Using NaCl as a surrogate draw solution, this investigation shows that forward osmosis processes can lead to sucrose concentration factors that far exceed current pressure-driven membrane technologies, such as reverse osmosis. For instance, a concentration factor of 5.7 was achieved by forward osmosis with a starting sucrose concentration of 0.29 M, compared to reported concentration factors of up to 2.5 with reverse osmosis. Water fluxes were found to be lower than those commonly obtained in reverse osmosis, which is a consequence of the significantly higher concentration factors in conjunction with internal concentration polarization. The latter is a common problem in forward osmosis processes that utilize current generation anisotropic polymeric membranes. Further advances in forward osmosis membrane technology would yield higher water fluxes and concentration factors.

The increased production and commercial use of nanomaterials combined with a lack of regulation to govern their disposal may result in their introduction to soils and ultimately into groundwater systems. In this study, we investigated the transport behavior of carboxyl-functionalized single-walled carbon nanotubes (SWNTs) in columns packed with a natural soil. In general, SWNT deposition (filtration) rate increased with increasing solution ionic strength, with divalent cations (Ca2+) being more effective in increasing SWNT retention than monovalent cations (K+). However, SWNT deposition rate over a very wide range of monovalent and divalent cation concentrations (0.03 to 100 mM) was relatively high and changed only slightly above 0.3 mM KCl or 0.1 mM CaCl2. In contrast, filtration of another type of engineered carbon-based nanomaterial, namely aqueous fullerene (C60) nanoparticles (radius of 51 nm), was more sensitive to solution ionic strength, displaying lower deposition rate and more effective transport in soil than SWNTs. These observations indicate that physical straining governs SWNT filtration and transport under all the solution chemistries investigated in the present study. It is proposed that SWNT shape and structure, particularly the very large aspect ratio and its highly bundled (aggregated) state in aqueous solutions, as well as the heterogeneity in soil particle size, porosity, and permeability, collectively contribute to straining in flow through soil media. Our results suggest that SWNTs of comparable properties to those used in the present study will not exhibit substantial transport and infiltration in soils because of effective retention by the soil matrix.

This study evaluates the cytotoxicity of four carbon-based nanomaterials (CBNs)—single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), aqueous phase C60 nanoparticles (aq-nC60), and colloidal graphite—in gram negative and gram positive bacteria. The potential impacts of CBNs on microorganisms in natural and engineered aquatic systems are also evaluated. SWNTs inactivate the highest percentage of cells in monocultures of Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus epidermis, as well as in the diverse microbial communities of river water and wastewater effluent. Bacterial cytotoxicity displays time dependence, with longer exposure times accentuating toxicity in monocultures with initial tolerance for SWNTs. In Bacillus subtilis, an additional 3.5 h of incubation produced a five fold increase in toxicity. Elevated concentration of NOM reduces the attachment of bacteria on SWNT aggregates by 50%, but does not mitigate toxicity toward attached cells. CBN toxicity in bacterial monocultures was a poor predictor of microbial inactivation in chemically and biologically complex environmental samples.

We developed a method for preparing live bacterial cell probes for atomic force microscopy (AFM) using a bioinspired polydopamine wet adhesive. Microscopic examinations with bacterial and yeast cells indicated that cells were successfully glued to the end of the AFM cantilevers and remained viable for the duration of the force measurements. Interaction forces measured with live single-cell microorganism probes differed markedly from those obtained with glutaraldehyde-fixed microorganism probes. Interaction forces between live cell probes and quartz surfaces involved both repulsive steric forces and multimodal weak adhesion forces, which were attributed to the soft exocellular polymeric layers and the heterogeneity of the cell membrane surfaces.

Excess of boron in water poses a problem due to adverse effects on crop production as well as human health and aquatic life. This study examined the influence of biofouling of NF and RO membrane on the performance of the membranes in removing boron from a synthetic wastewater effluent. Accelerated laboratory-scale biofouling experiments were carried out with commercial thin film composite NF and RO membranes under controlled conditions. Permeate flux decline, down to less than 25% of its initial value, and substantial decrease in boron rejection were attributed to extensive biofilm growth on the membranes. For the RO membrane, boron rejections declined by 45 and 34% of the initial values for influent boron concentrations of 5.5 and 1.1 mg B/L, respectively, whereas the corresponding declines in boron rejection for the NF membrane were 44 and 13% of the initial values. These adverse effects of biofilm growth on permeate water flux and boron rejection are attributed to both an increase in hydraulic resistance to permeate flow due to bacterial extracellular polymeric substances (EPS) and a biofilm-enhanced concentration polarization near the membrane surface.

The presence of norovirus (NoV) genogroup I (GI) and II (GII) was evaluated using real-time reverse transcription polymerase chain reaction (rRT-PCR) in the influent, two midtreatment locations, and final effluent of a three-pond serial waste stabilization pond system from December 2005 through June 2006. Additionally, influent and effluent samples were filtered through a cascade of three membrane filters with sequentially smaller pores to determine the size range of particles with which GI and GII were associated. NoV GI and GII removal occurs primarily in the third pond. Viruses were found on large settleable particles (retained on a 180 μm filter), on smaller suspended particles (retained on a 0.45 μm filter), on colloidal particles (retained on a positively charged 0.45 μm filter), and in the final filtrate. Both GI and GII in influent samples were found to be dominantly associated with particles smaller than 180 μm, thereby suggesting that particle settling is not the main virus removal mechanism in the waste stabilization pond system. On average, NoV detected in filtered effluent samples were associated with particles between 0.45 and 180 μm in diameter (47 and 67% of detected GI and GII, respectively). The presence of NoV GI and GII in the final filtrate of influent and effluent samples shows that positively charged membrane filters often used for viral concentration methods are not capable of trapping all viruses present in wastewater samples.

Rational modification of carbon nanotubes (CNTs) to isolate their specific physical and chemical properties will inform a mechanistic understanding of observed CNT toxicity in bacterial systems. The present study compares the toxicity of commercially obtained multiwalled carbon nanotubes (MWNTs) before and after physicochemical modification via common purification and functionalization routes, including dry oxidation, acid treatment, functionalization, and annealing. Experimental results support a correlation between bacterial cytotoxicity and physicochemical properties that enhance MWNT-cell contact opportunities. For example, we observe higher toxicity when the nanotubes are uncapped, debundled, short, and dispersed in solution. These conclusions demonstrate that physicochemical modifications of MWNTs alter their cytotoxicity in bacterial systems and underline the need for careful documentation of physical and chemical characteristics when reporting the toxicity of carbon-based nanomaterials.

The influence of bacterial motility on cell transport and deposition was investigated in a well-characterized radial stagnation point flow (RSPF) chamber. Deposition experiments were conducted with nonmotile (PAO1 ΔfliC ΔpilA) and motile (PAO1 ΔpilA) strains of Pseudomonas aeruginosa, and oppositely (positively) charged modified quartz surfaces. Deposition dynamics of the two bacterial strains were determined over a wide range of solution ionic strengths and at two flow velocities. The observed deposition dynamics were modeled using a modified expression of the random sequential adsorption (RSA) blocking function accounting for the impacts of hydrodynamic and electrostatic interactions on cell deposition. Results for the nonmotile bacteria indicated that the changes in blocking rate and surface coverage with ionic strength and flow rate were similar to those expected for nonbiological, “soft” particles, for which the coupling of hydrodynamic interactions and electrostatic repulsion governs the deposition dynamics. In contrast, deposition dynamics of the motile bacterial cells reduced blocking rates and enhanced maximum coverages, approaching the jamming limit predicted for “hard” ellipsoids of 0.583. We hypothesized that cell motility allows the upstream swimming of bacteria and subsequent cell deposition on regions which are otherwise inaccessible to nonmotile cell deposition due to the “shadow effect”.

Deposition of nanomaterials onto surfaces is a key process governing their transport, fate, and reactivity in aquatic systems. We evaluated the transport and deposition behavior of carboxyl functionalized single-walled carbon nanotubes (SWNTs) in a well-defined porous medium composed of clean quartz sand over a range of solution chemistries. Our results show that increasing solution ionic strength or addition of calcium ions result in increased SWNT deposition (filtration). This observation is consistent with conventional colloid deposition theories, thereby suggesting that physicochemical filtration plays an important role in SWNT transport. However, the relatively insignificant change of SWNT filtration at low ionic strengths (≤3.0 mM KCl) and the incomplete breakthrough of SWNTs in deionized water (C/C0 = 0.90) indicate that physical straining also plays a role in the capture of SWNTs within the packed sand column. It is proposed that SWNT shape and structure, particularly the very large aspect ratio and its highly bundled (aggregated) state in aqueous solutions, contribute considerably to straining in flow through porous media. We conclude that both physicochemical filtration and straining play a role at low (<3.0 mM) ionic strength, while physicochemical filtration is the dominant mechanism of SWNT filtration at higher ionic strengths. Our results further show that deposited SWNTs are mobilized (released) from the quartz sand upon introduction of low ionic strength solution following deposition experiments with monovalent salt (KCl). In contrast, SWNTs deposited in the presence of calcium ions were not released upon introduction of low ionic strength solution to the packed column, even when humic acid was present in solution during SWNT deposition.

The initial aggregation kinetics of multiwalled carbon nanotubes (MWNTs) were examined through time-resolved dynamic light scattering. Aggregation of MWNTs was evaluated by varying solution pH and the concentration of monovalent (NaCl) and divalent (CaCl2 and MgCl2) salts. Suwannee River humic acid (SRHA) was used to study the effect of background natural organic matter on MWNT aggregation kinetics. Increasing salt concentration and addition of divalent calcium and magnesium ions induced MWNT aggregation by suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. The critical coagulation concentration (CCC) values for MWNTs were estimated as 25 mM NaCl, 2.6 mM CaCl2, and 1.5 mM MgCl2. An increase in solution pH from acidic (pH 3) to basic (pH 11) conditions resulted in a substantial (over 2 orders of magnitude) decrease in MWNT aggregation kinetics, suggesting the presence of ionizable functional groups on the MWNT carbon scaffold. The presence of humic acid in solution markedly enhanced the colloidal stability of MWNTs, reducing the aggregation rate by nearly 2 orders of magnitude. The enhanced MWNT stability in the presence of humic acid is attributable to steric repulsion imparted by adsorbed humic acid macromolecules. Our results suggest that MWNTs are relatively stable at solution pH and electrolyte conditions typical of aquatic environments.

The unique and tunable properties of carbon-based nanomaterials enable new technologies for identifying and addressing environmental challenges. This review critically assesses the contributions of carbon-based nanomaterials to a broad range of environmental applications: sorbents, high-flux membranes, depth filters, antimicrobial agents, environmental sensors, renewable energy technologies, and pollution prevention strategies. In linking technological advance back to the physical, chemical, and electronic properties of carbonaceous nanomaterials, this article also outlines future opportunities for nanomaterial application in environmental systems.

Injection of nanoscale zero-valent iron (NZVI) is potentially a promising technology for remediation of contaminated groundwaters. However, the efficiency of this process is significantly hindered by the rapid aggregation of the iron nanoparticles. The aim of this study was to enhance the colloidal stability of the nanoparticles through the addition of the “green” polymer guar gum. We evaluated the properties of guar gum and its influence on the surface properties, particle size, aggregation, and sedimentation of iron nanoparticles. Commercial iron nanoparticles were dispersed in guar gum solutions, and their aggregation and sedimentation behaviors were compared to those of bare iron nanoparticles and commercial nanoparticles modified with a biodegradable polymer (polyaspartate). High performance size exclusion chromatography, charge titration, and viscosity assessment showed that guar gum is a high molecular weight polymer which is nearly neutrally charged, rendering it suitable for steric stabilization of the iron nanoparticles. Electrophoretic mobility measurements demonstrated the ability of guar gum to adsorb on the nanoparticles, forming a slightly negatively charged layer. Dynamic light scattering experiments were conducted to estimate the particle size of the different nanoparticle suspensions and to determine the aggregation behavior at different ionic strengths. Guar gum effectively reduced the hydrodynamic radius of the bare nanoparticles from 500 nm to less than 200 nm and prevented aggregation of the nanoparticles even at very high salt concentrations (0.5 M NaCl and 3 mM CaCl2). Sedimentation profiles of the different nanoparticle suspensions confirmed the improved stability of the iron nanoparticles in the presence of guar gum. The results strongly suggest that guar gum can be used to effectively deliver stabilized zero-valent iron nanoparticles for remediation of contaminated groundwater aquifers.

We provide the first evidence that the size (diameter) of carbon nanotubes (CNTs) is a key factor governing their antibacterial effects and that the likely main CNT-cytotoxicity mechanism is cell membrane damage by direct contact with CNTs. Experiments with well-characterized single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) demonstrate that SWNTs are much more toxic to bacteria than MWNTs. Gene expression data show that in the presence of both MWNTs and SWNTs, Escherichia coli expresses high levels of stress-related gene products, with the quantity and magnitude of expression being much higher in the presence of SWNTs.

A novel method of converting thermal energy into mechanical work is presented, using semi-permeable membranes to convert osmotic pressure into electrical power. This method, a closed cycle pressure-retarded osmosis (PRO) process known as an osmotic heat engine (OHE), uses a concentrated ammonia–carbon dioxide draw solution to create high osmotic pressures which generate water flux through a semi-permeable membrane against a hydraulic pressure gradient. The depressurization of the increased draw solution volume in a turbine produces electrical power. The process is maintained in steady state operation through the separation of the diluted draw solution into a re-concentrated draw solution and (nearly) deionized water working fluid, both for reuse in the engine. The use of deionized water working fluid has been shown to allow for high membrane water flux and efficient mass transport, as internal concentration polarization effects are eliminated. Modeling of the engine indicates that membrane power density may exceed 200 W/m2, given appropriate operating conditions. The thermal efficiency of the engine is predicted to approach a maximum of 16% of Carnot efficiency (maximum theoretical engine efficiency), with practical efficiencies most likely in the range of 5–10% of Carnot efficiency. The temperature of heat used for the engine may be very low (40 °C with a 20 °C ambient temperature), allowing for the production of potentially low cost, carbon neutral power from waste heat, low temperature geothermal reservoirs, or other non-combustion thermal energy sources. This combination of a highly concentrated NH3/CO2 draw solution and a deionized working fluid may allow for highly effective power generation from osmotic pressure gradients.

The early stage aggregation kinetics of fullerene C60 nanoparticles were investigated in the presence of Suwannee River humic acid and common monovalent and divalent electrolytes through time-resolved dynamic light scattering (DLS). In the absence of humic acid, the aggregation behavior of the fullerene nanoparticles in the presence of NaCl, MgCl2, and CaCl2 was found to be consistent with the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloidal stability. In the presence of humic acid and NaCl or MgCl2 electrolytes, the adsorbed humic acid on the fullerene nanoparticles led to steric repulsion, which effectively stabilized the nanoparticle suspension. This behavior manifested in a dramatic drop in the rate of aggregation, an increase in the critical coagulation concentration (CCC), and an attained value of less than unity for the inverse stability ratio (or attachment efficiency) at high MgCl2 concentrations. While the increase in the nanoparticle stability was similarly observed in the presence of humic acid at low CaCl2 concentrations, enhanced aggregation occurred at higher CaCl2 concentrations. Measurement of scattered light intensities over time indicated significant aggregation of the humic acid macromolecules in solutions of high CaCl2 concentrations. Transmission electron microscopy (TEM) imaging of the fullerene aggregate structures in the presence of humic acid revealed that bridging of the fullerene nanoparticles and aggregates by the humic acid aggregates is the likely mechanism for the enhanced aggregation at high CaCl2 concentrations.Fullerene nanoparticle aggregates with humic acid and divalent cations.

The role of membrane pore size and solution chemistry in ultrafiltration (UF) membrane fouling by colloidal natural organic matter (NOM) has been investigated. Fouling experiments were conducted with two laboratory-made cellulose acetate UF membranes, with estimated pore sizes of 2 and 10 nm, under identical hydrodynamic conditions. Flux decline with colloidal NOM was independent of pH and increased in the presence of calcium. The membrane pore size has been found to influence membrane flux decline. Permeate flux for the more permeable membrane (10 nm pore size) decreased faster than the flux of the tightest, less permeable membrane (2 nm pore size). The adhesion forces between the foulant (colloidal NOM) and the clean membrane, and between the foulant in bulk solution and foulant in the fouling layer, were determined by atomic force microscopy (AFM). Force measurements confirmed the fouling trends with calcium ions. For a given solution chemistry, foulant–foulant adhesion was identical for both membranes, indicating that the more pronounced flux decline observed for the largest pore-size membrane was not related to differences in intermolecular chemical interactions among NOM molecules. At the earlier stage of filtration, the dominant fouling mechanism was pore blocking for both membranes. For longer filtration times, there was a transition in the fouling mechanism from pore blocking to cake formation. This transition occurred first for the more permeable membrane, indicating that colloidal NOM penetrates more readily into the pores of this membrane and that the relative size of colloidal NOM and the membrane pore influences the rate of pore blocking. The results further suggest that the structure of the fouling layer is dependent on the operating pressure. At high pressure, the compactness and specific resistance of the colloidal NOM layer substantially increase, and thus, significantly affect the extent of flux decline.

The combined influence of natural organic matter (NOM) and colloidal particles on the fouling of thin-film composite nanofiltration (NF) membranes is systematically investigated. Combined fouling is compared to the individual fouling behaviors (i.e., colloid or NOM alone) with respect to fouling mechanisms and the effect of concentration factor (or recovery). Results demonstrate that (1) “cake-enhanced osmotic pressure” (CEOP) is a key fouling mechanism for individual colloidal fouling, (2) NOM–calcium complexation is the dominant factor governing individual NOM fouling, and (3) combined fouling is affected by both CEOP and NOM–calcium complexation. The extent of flux decline for combined fouling, however, is less than what inferred from additivity of the individual contributions of colloidal and NOM fouling to flux decline. This observation implies that the contributions of the fouling mechanisms appear to be relatively less significant for combined fouling compared to their separate influences on individual colloidal and NOM fouling. An increase in colloidal stability in presence of NOM and the competition between colloids and NOM for calcium are likely explanations for this behavior. It is further shown that NF membrane salt rejection increases noticeably in case of combined fouling compared to individual colloidal fouling due to the formation of an active rejecting layer by the accumulated NOM on the membrane surface. Results from combined fouling runs involving EDTA treatment confirm that both CEOP and NOM–calcium complexation take place simultaneously.

Reclamation and reuse of agricultural tile drainage water in the San Joaquin Valley, California, by membrane desalination has been given serious consideration for nearly 30 years. The agricultural drainage water has very high levels of calcium and sulfate ions, rendering it nearly saturated with gypsum in some locations in this region. The present paper focuses on the conditions of membrane surface gypsum scale formation during nanofiltration (NF) of agricultural drainage water. Experimental studies with drainage water samples and model solutions were conducted in laboratory glassware as well as with a small plate-and-frame cross-flow membrane re-circulation unit. NF membrane experiments consisted of two types: permeate disposal to measure the effect of increasing recovery on the onset of scaling, and feed water re-circulation at a fixed concentration factor to establish performance change over time. An expression relating NF product water recovery to feed water concentration factor enabled numerous membrane scale formation studies that have direct relevance to full-scale NF systems. Glassware experiments showed that magnesium ions decreased the amount of gypsum incipient nuclei because of MgSO40 complexation, which reduces the availability of sulfate ions for nucleus formation. In membrane experiments, model solutions showed that bicarbonate, magnesium ions, and humic acid retarded the onset of gypsum scaling by tying up calcium ions that would have otherwise been used for formation of gypsum incipient nuclei. Both calcium carbonate and gypsum scales may result from particulate deposition rather than membrane surface (wall) crystallization, with the likelihood of particulate fouling increasing with supersaturation.

Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure. Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties. Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. It is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable insights into the mechanisms governing colloidal fouling. At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the “valleys” of rough membranes, resulting in “valley clogging” which causes more severe flux decline than in smooth membranes.

The role of spatial distribution of porous medium patchwise chemical (charge) heterogeneity in colloid transport in packed bed columns is investigated. Colloid transport experiments with carboxyl latex particles flowing through columns packed with chemically heterogeneous sand grains were carried out. Patchwise chemical heterogeneity was introduced to the granular porous medium by modifying the surface chemistry of a fraction of the quartz sand grains via reaction with aminosilane. Colloid transport experiments at various degrees of patchwise charge heterogeneity and several spatial distributions of heterogeneity were conducted at different flow rates and background electrolyte concentrations. Colloid deposition rate coefficients were determined from analysis of particle breakthrough curves as a response to short-pulse colloid injections to the column inlet. Experimental colloid deposition rate coefficients compared well with theoretical predictions based on a colloid transport model that incorporates patchwise chemical heterogeneity. The results revealed the particle deposition rate and transport behavior to be independent of the spatial distribution of porous medium chemical heterogeneity. It is the mean value of chemical heterogeneity rather than its distribution that governs the colloid transport behavior in packed columns.

Re-engineering the support layers of membranes for forward and pressure retarded osmosis is critical for making these technologies commercially viable. Real-world applications of forward and pressure retarded osmosis, especially those involving natural and waste waters, will require membranes to withstand significant stresses. Therefore, structural changes to the support layer, which are necessary in minimizing internal concentration polarization, must not compromise its critical abilities to resist mechanical stress and provide a suitable surface for the interfacial polymerization of a robust and selective active layer. Electrospinning can provide nanofibers for support layers to potentially overcome the limitations of traditional membrane fabrication techniques in fulfilling these challenging design criteria. In this work, we present the fabrication and evaluation of thin-film composite membranes composed of electrospun polyethylene terephthalate nanofibers, a phase separation formed microporous polysulfone layer, and a polyamide selective layer formed by interfacial polymerization. These membranes have active and support layer transport properties that are suitable for engineered osmosis, with water permeability of 1.13 L m− 2 h− 1 bar− 1 (3.14 × 10− 7 m s− 1 bar− 1), salt permeability of 0.23 L m− 2 h− 1 (6.4 × 10− 8 m s− 1), and a structural parameter of 651 μm. Relevant and easily reproducible tests for membrane resistance to mechanical stress were performed. The use of electrospun fibers in the support layer enhanced membrane resistance to delamination at high cross-flow velocities because the 340 nm diameter electrospun fibers enmesh with the microporous polysulfone layer. A broader discussion of the most promising approaches for using electrospun materials to improve membranes for engineered osmosis is provided.Highlights► Thin-film composite membranes for forward and pressure retarded osmosis were fabricated. ► Electrospun mats served as a base onto which a support layer was formed by phase separation. ► Thin-film composite membranes had high salt rejection and high water flux in forward osmosis. ► Membrane resistance to shear stress and hydraulic pressure was evaluated. ► Nanofibers enmeshed with microporous layer to enhance resistance to delamination.

•Batch-like processes yield similar energy savings as staging with energy recovery.•Semi-batch RO and two-stage RO show similar promise for seawater RO.•A practical batch RO process shows promise for high recovery brackish water RO.•Process inefficiencies hinder batch-like processes more than staged processes.•Capital cost and process robustness should be considered in addition to energy use.Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the minimum practical energy of desalination substantially exceeds the thermodynamic minimum. Batch processes can theoretically approach the thermodynamic minimum, suggesting the possibility for further energy savings. In this study, we aim to quantify what energy reductions may be possible for batch-like processes when process inefficiencies such as frictional losses and concentration polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive analytical approximations and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this analysis, we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and two-stage RO would save 13% and 15% energy, respectively, over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.Download high-res image (261KB)Download full-size image

•A method for in situ functionalization Cu-NPs of thin-film composite RO membranes is presented.•In situ loading of Cu-NPs had minor impact on membrane water permeability and selectivity.•Functionalized TFC membranes exhibit strong antibacterial activity.•Cu-NPs can be cost-effective for in situ TFC membrane functionalization.Biofouling may lead to severe operational challenges that can significantly impair membrane desalination processes. In recent years, copper-based nanoparticles (Cu-NPs) have gained increased attention as a potentially viable anti-biofouling agent in membrane processes, due to their strong antibacterial activity and relatively low cost. This study presents a novel and facile method to attach biocidal Cu-NPs on the surface of a thin-film composite reverse osmosis membrane. Herein, we suggest a method for membrane surface functionalization with Cu-NPs that is performed without disassembling the membrane module, which highlights its practicality and potential application for reverse osmosis desalination plants. The loading of Cu-NPs on the membrane was confirmed both by scanning electron microscope imaging and X-ray photoelectron spectroscopy analysis, indicating that the deposited nanoparticles were composed of either metallic copper or copper-oxide. The impact of the in situ Cu-NP modification on membrane transport properties was found to be minor, with only a slight increase of the water and salt permeability. Furthermore, except for a slight increase in hydrophobicity, the modified membrane exhibited surface properties comparable to those of the pristine membrane. Finally, the in situ formed Cu-NPs imparted a strong antibacterial activity to the membrane surface, leading to 90% reduction in the number of attached live Escherichia coli bacteria on the modified membrane compared to the pristine reverse osmosis membrane. This study demonstrates that in situ grafting of Cu-NPs on reverse osmosis membranes is a potential alternative to reduce biofouling.

•New RO system designs can reduce the energy of desalination.•Minimum specific energies were formulated for single- and multiple-stage RO systems.•Staged RO operations are more energy efficient than single-stage RO.•Multi-stage direct pass RO theoretically consumes less energy than close-circuit RO.•Closed-circuit RO is a more practical/economical approach for energy reduction.Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodynamics of separation of saline solutions. Therefore, pushing the energy consumption towards the thermodynamic limit of separation would lead to significant long-term savings in energy cost. In this work, we quantitatively demonstrate the potential of energy reduction for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the minimum specific energy of desalination (i.e., the minimum energy required to generate a unit volume of permeate water) to the number of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our analysis suggests that although it is theoretically impossible to reach the thermodynamic minimum energy of separation with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.Download full-size image

•We present a critical review of the current state of forward osmosis (FO).•We analyze the energy efficiency of FO and emphasize relevant applications.•We discuss the key required membrane properties for FO and future implications.•We highlight fouling reversibility of FO and relevant benefits and applications.•We discuss applications where FO outperforms current technologies.Forward osmosis (FO) has been extensively investigated in the past decade. Despite significant advancements in our understanding of the FO process, questions and challenges remain regarding the energy efficiency and current state of the technology. Here, we critically review several key aspects of the FO process, focusing on energy efficiency, membrane properties, draw solutes, fouling reversibility, and effective applications of this emerging technology. We analyze the energy efficiency of the process, disprove the common misguided notion that FO is a low energy process, and highlight the potential use of low-cost energy sources. We address the key necessary membrane properties for FO, stressing the importance of the structural parameter, reverse solute flux selectivity, and the constraints imposed by the permeability–selectivity tradeoff. We then dispel the notion that draw solution regeneration can use negligible energy, highlighting the beneficial qualities of small inorganic and thermolytic salts as draw solutes. We further discuss the fouling propensity of FO, emphasizing the fouling reversibility of FO compared to reverse osmosis (RO) and the prospects of FO in treating high fouling potential feed waters. Lastly, we discuss applications where FO outperforms other desalination technologies and emphasize that the FO process is not intended to replace RO, but rather is to be used to process feed waters that cannot be treated by RO.

The energy requirements of ammonia–carbon dioxide forward osmosis (FO) desalination are predicted by the use of chemical process modeling software (HYSYS). The FO process is modeled using single or multiple distillation columns to separate draw solution solutes from the product water for solute recycling within the FO system. Thermal and electrical energy requirements of the process are calculated, as well as a combined term for equivalent electrical work. The results of the simulations are compared to the energy requirements of current desalination technologies. Energy savings of FO compared to current technologies, on an equivalent work basis, are projected to range from 72% to 85%. Forward osmosis desalination is in an early stage of its development, and several areas of future work promise opportunities to improve its energy utilization and cost.

•Comparing biofilm structures using dry, confined, and unconfined mounting.•Characterizing unconfined biofouling with custom-made chamber and CLSM.•Dry and confined mounting techniques cause significant biofilm deformation.•Unconfined mounting yields biofilms with greater 3-D complexity.•The importance of using unconfined microscopic mounting is highlighted.Confocal laser scanning microscopy (CLSM) is often used to evaluate biofilm development or biofouling mitigation in membrane systems. However, several methods of CLSM sample preparation exist. In this paper, we evaluate the effects of three preparation techniques — dry, confined (wet), and unconfined (immersed) mounting — on CLSM-derived biofilm architecture and dimensions. Although placing a wet or dry biofilm between a slide and a coverslip before viewing is relatively common, our results show that this confinement significantly alters the biofilm observed. Therefore, biofilms should be viewed in an unconfined and hydrated state that allows for full extension of the biofilm structure in a media-filled viewing well of fixed depth (~ 250 μm). Pseudomonas aeruginosa biofilms were grown on thin-film composite reverse osmosis membranes and glass coupons. Dry and confined mounting of 24 and 48 h biofilms resulted in biofilms with low 3-D complexity and thickness (14 and 18 μm, respectively). Measured biofilm thickness was significantly higher on samples prepared using unconfined mounting (55 μm). Additionally, the reduction in biofilm thickness and biovolume observed after treatment with biocidal compounds was significantly less on the dry and confined biofilms than the unconfined samples. Our results strongly suggest that biofilms on membranes be prepared for microscopy using unconfined mounting to accurately assess biofilm structure and dimensions. Unconfined mounting will allow for accurate CLSM assessment of membrane biofilm structure, dimensions, and biofouling mitigation measures in membrane systems.Download full-size image

Graphene-based materials are gaining heightened attention as novel materials for environmental applications. The unique physicochemical properties of graphene, notably its exceptionally high surface area, electron mobility, thermal conductivity, and mechanical strength, can lead to novel or improved technologies to address the pressing global environmental challenges. This critical review assesses the recent developments in the use of graphene-based materials as sorbent or photocatalytic materials for environmental decontamination, as building blocks for next generation water treatment and desalination membranes, and as electrode materials for contaminant monitoring or removal. The most promising areas of research are highlighted, with a discussion of the main challenges that we need to overcome in order to fully realize the exceptional properties of graphene in environmental applications.

A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine–pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.