Antimony (Sb) contamination poses an emerging environmental risk, whereas its removal remains a contemporary challenge due to the lack of knowledge in its surface chemistry and efficient adsorbent. In this study, self-assembly {001} TiO2 was examined for its effectiveness in Sb removal, and the molecular level surface chemistry was studied with X-ray absorption spectroscopy and density functional theory calculations. The kinetics results show that Sb adsorption followed the pseudo-second order reaction, and the Langmuir adsorption capacity was 200 mg/g for Sb(III) and 156 mg/g for Sb(V). The PZC of TiO2, which was 6.6 prior to the adsorption experiment, shifted to 4.8 and <0 after adsorption of Sb(III) and Sb(V), respectively, indicating the formation of negatively charged inner-sphere complexes. EXAFS results suggest that Sb(III/V) adsorption exhibited a bidentate binuclear surface complex. The orbital hybridizing of complexes was studied by XANES, molecular orbital theory (MO), and density of states (DOS) calculations. The change in orbital energy derived from orbital hybridizing of adsorbed Sb on surfaces is the driving force underlining the Sb surface chemistry. New bonds between Sb and TiO2 surface were formed with matched orbital energies. Integrating the molecular and electronic structures into surface complexation modeling reveals the nature of macroscopic Sb adsorption behaviors.

•Fe3O4@TiO2-GO (FTG) composites with different TiO2 shell thickness were synthesized.•The photodegradation mechanism of ENR under visible light irradiation was studied.•The effect of TiO2 shell thickness on photo-degradation performance was investigated.Multifunctional Fe3O4@TiO2-GO (FTG) magnetic composites have extensive application potential. However, the effect of TiO2 shell thickness as well as its underlying mechanism remains murky and motivates our study. Herein, FTG composites with the optimized TiO2 shell thickness were synthesized. The characterization results show that the FTG photocatalysts had the advantages of easy magnetic separation, extended light absorption range (>600 nm), and efficient charge separation properties. The photocatalytic activity of FTG was performed for photo-degradation of enrofloxacin (ENR) under visible light. The FTG composites with TiO2 layer thickness of 17 nm exhibited the highest photocatalytic activity, and significant recycle efficiency. The photocatalytic mechanism, as characterized using electrochemistry, ESR, and in-situ QXAFS analysis, indicates that this optimal TiO2 shell thickness delicately balances the generation and effective transport of electrons, and therefore is optimal for the production of hydroxyl radicals. This study provides new insights into the fabrication of TiO2-based materials and their photocatalytic application.

•Proposed subcellular Raman approach was applied in actual human tissues diagnosis.•Subcellular Raman analysis significantly enhanced the sensitivity and specificity of colon cancer diagnosis.•The approach has good stability and excellent diagnostic performance in other tissues diagnosis such as rectum.Subcellular Raman analysis is a promising clinic tool for cancer diagnosis, but constrained by the difficulty of deciphering subcellular spectra in actual human tissues. We report a label-free subcellular Raman analysis for use in cancer diagnosis that integrates subcellular signature spectra by subtracting cytoplasm from nucleus spectra (Nuc.-Cyt.) with a partial least squares-discriminant analysis (PLS-DA) model. Raman mapping with the classical least-squares (CLS) model allowed direct visualization of the distribution of the cytoplasm and nucleus. The PLS-DA model was employed to evaluate the diagnostic performance of five types of spectral datasets, including non-selective, nucleus, cytoplasm, ratio of nucleus to cytoplasm (Nuc./Cyt.), and nucleus minus cytoplasm (Nuc.-Cyt.), resulting in diagnostic sensitivity of 88.3%, 84.0%, 98.4%, 84.5%, and 98.9%, respectively. Discriminating between normal and cancerous cells of actual human tissues through subcellular Raman markers is feasible, especially when using the nucleus-cytoplasm difference spectra. The subcellular Raman approach had good stability, and had excellent diagnostic performance for rectal as well as colon tissues. The insights gained from this study shed new light on the general applicability of subcellular Raman analysis in clinical trials.

•Simultaneous As(III) and F removal is achieved by using granular TiO2-La composite.•LaCO3OH showing orientated growth on TiO2 {1 0 0} facet with matched lattice fringes.•Molecular-level coadsorption mechanism is investigated by EXAFS and DFT calculation.•The importance of crystal facet in material synthesis and adsorption is highlighted.Naturally co-occurring arsenic (As) and fluoride (F) in groundwater have caused increasing public concerns, and their simultaneous removal is still a challenge due to the lack of efficient adsorptive materials and their competition for adsorption sites. This study fabricated a novel TiO2-La granular composite adsorbent with LaCO3OH showing orientated growth on TiO2 {1 0 0} facet. The material exhibited high As(III) (114 mg/g) and F (78.4 mg/g) adsorption capacities and a wide application pH, ranging 3–9, achieving a high percentage (>90%) of As(III) and F adsorption. Coadsorption experiment results demonstrated that high F concentrations inhibit As(III) adsorption, whereas the coexisting As(III) has no significant effect on F removal. The molecular-level mechanisms with EXAFS and DFT studies demonstrated that As(III) adsorption is favorable only on Ti sites as evidenced by an As-Ti distance of 3.36 Å when pH < 7, while La adsorption sites can also be occupied with an As-La distance at 3.42 Å when pH > 10. The F adsorption is pH-dependent and mainly occurred on La sites. The granular TiO2-La with high As(III) and F adsorption capacities can be used to simultaneously remove As(III) and F. The insights gained from this study shed a new light on the interaction mechanism of As(III) and F with the TiO2-La composite.Download high-res image (201KB)Download full-size image

•A core-satellite structured Fe3O4-Au@TiO2 substrate was prepared.•The substrate can be used to monitor Cr(VI) with evident sensitivity lasting 30 days.•The Cr(VI) photo-reduction can be in situ monitored by SERS.•This platform provides an alternative method for quantitative determination of Cr(VI).Exposure to hexavalent chromium [Cr(VI)] poses a serious health threat. Thus, accurate field analysis and efficient Cr(VI) reduction are of great importance. The motivation of this research was to synthesize a multifunctional satellite Fe3O4-Au@TiO2 nano-structure for surface enhanced Raman scattering (SERS) detection and photo-reduction of Cr(VI). Our experimental results and three-dimensional finite difference time domain (FDTD) simulation suggest that coating Fe3O4@Au with a TiO2 layer 2–6 nm thick can protect SERS substrates from direct contact and reaction with Cr(VI), as well as enrich Cr(VI) in close proximity to Au NPs to facilitate its SERS detection. The SERS response exhibited a concentration dependence up to 5 μM with a detection limit of 0.05 μM. The SERS substrate exhibited an excellent selectivity for Cr(VI) over other coexisting ions and resulted in a satisfactory quantitation in complicated environmental matrices. In addition, the Cr(VI) reduction photo-catalyzed using this multifunctional material followed the first-order kinetics and the reduction process was monitored in situ with the SERS effect, and yielded a decrease of almost 80%. This novel platform provides an alternative method for the quantitative determination and remediation of Cr(VI).

Nanofabrication of multifunctional surface-enhanced Raman scattering (SERS) substrates is strongly desirable but currently remains a challenge. The motivation of this study was to design such a substrate, a versatile core–satellite Fe3O4@SiO2–Au (FA) hetero-nanostructure, and demonstrate its use for charge-selective detection of food dye molecules as an exemplary application. Our experimental results and three-dimensional finite difference time domain (FDTD) simulation suggest that tuning the Au nanoparticle (NP) gap to sub-10 nm, which could be readily accomplished, substantially enhanced the Raman signals. Further layer-by-layer deposition of a charged polyelectrolyte on this magnetic SERS substrate induced active adsorption and selective detection of food dye molecules of opposite charge on the substrates. Molecular dynamics (MD) simulations suggest that the selective SERS enhancement could be attributed to the high affinity and close contact (within a 20 Å range) between the substrate and molecules. Density function theory (DFT) calculations confirm the charge transfer from food dye molecules to Au NPs via the polyelectrolytes. This multifunctional SERS platform provides easy separation and selective detection of charged molecules from complex chemical mixtures.Keywords: charge transfer; food dye; selective detection; SERS; tunable gap

Bacterial adhesion to mineral surfaces is an important but underappreciated process. To decipher the molecular level process and mechanism, the adhesion of Shewanella oneidensis MR-1 cells to goethite was investigated using flow-cell attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy coupled with two-dimensional correlation spectroscopy (2D-COS) analysis. The FTIR results indicate that bacterial phosphate-moieties play an important role in the formation of mono- and bidentate inner-sphere complexes, whereas carboxylic groups on cell surface only have a minor contribution to its adhesion. The 2D-COS analysis in short-term (0–120 min) and long-term (2–18 h) stages reveal that the adhesion process was in the following sequence: change in H-bonds of proteins on cell surfaces > formation of monodentate inner-sphere surface complexes > formation of outer-sphere surface complexes > transformation of protein secondary structure on cell surfaces > formation of additional bridging bidentate surface complexes. In addition, the adhesion of MR-1 cells on goethite was pH dependent due to pH impacts on the cell structure and the interface charge. The in situ ATR-FTIR integrated with 2D-COS analysis highlights its great potential in exploring complex surface reactions with microbes involved. These results improve our understanding of microbe–mineral interactions at the molecular level and have significant implications for a series of biogeochemical processes.

Antibiotics pollution has become a critical environmental issue worldwide due to its high ecological risk. In this study, rapid degradation of enrofloxacin (ENR) was observed on goethite in the presence of Shewanella oneidensis MR-1 during the transition from anaerobic to aerobic conditions. The abiotic reactions also demonstrated that over 70% with initial concentration of 10 mg L–1 ENR was aerobically removed within 5 min by goethite with adsorbed Fe(II), without especial irradiation and strong oxidants. The results of spin trap electron spin resonance (ESR) experiments provide evidence that Fe(II)/Fe(III) complexes facilitate the generation of •OH. The electrophilic attack by •OH opens the quinolone ring of ENR and initiates further transformation reactions. Five transformation products were identified using high performance liquid chromatography-quadrupole time-of-flight mass spectrometry and the ENR degradation process was proposed accordingly. The identification of ENR transformation products also revealed that both the surface adsorption and the electron density distribution in the molecule determined the reactive site and transformation pathway. This study highlights an important, but often underappreciated, natural process for in situ degradation of antibiotics. With the easy migration of the goethite-MR-1 complex to the anaerobic/aerobic interface, the environmental fates of ENR and other antibiotics need to be seriously reconsidered.

Anatase TiO2 nanomaterials have been widely used in arsenic (As) remediation, although reports on their adsorption and photocatalytic capacity have been controversial. The motivation for our study is to explore the As adsorption and photooxidation processes on different TiO2 facets at the molecular level. Our results from multiple complementary characterization techniques suggest that anatase {001} facets have stronger Lewis acid sites than those on {101} facets, resulting in a higher As adsorption affinity. Density functional theory (DFT) calculations confirmed that the As surface complex is more energetically favorable on {001} than on the {101} facets. In addition, the strong interaction of the {001} facets with molecular O2 facilitates the transfer of photo-excited electrons to the adsorbed O2 to generate a superoxide radical (O2˙−), which is the primary As(III) oxidant as evidenced by our radical-trapping experiments. Meanwhile, the oxygen vacancies on the {001} facets could expedite the interfacial electron transfer and electron–hole separation, which promote the generation of O2˙− and, ultimately, the catalytic efficiency. The insights gained from this study provide a firm basis for the proposition that As adsorption and photoactivity can be mediated by tailoring the exposed TiO2 facets, which is of essence in the design and application of TiO2-based environmental technologies.

•SERS detection of the 16 EPA priority PAHs was conducted.•The satellite-core structure lead to high SERS enhancement by confined hotspots.•The approach does not require expensive instrumentation or large sample volumes.•The effective protocol is useful for the identification of hydrophobic molecules.Several methods and materials have been explored for the sensitive and practicable detection of polycyclic aromatic hydrocarbons (PAHs). However, it is still a challenge to develop simple and cost-effective sensing techniques for PAHs. Herein we report the synthesis and construction of Fe3O4@Au SERS substrate. This magnetic substrate was composed by Fe3O4 microspheres and Au NPs. The size, morphology, and surface composition of Fe3O4@Au were characterized by multiple complimentary techniques including scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction. The spatial distributions of electro-magnetic field enhancement around Fe3O4@Au was calculated using finite difference time domain (FDTD) simulations. As a result of its remarkable sensitivity, the Fe3O4@Au-based SERS assay has been applied to detect the 16 EPA priority PAHs. The LODs achieved by our method (100–5 nM, 16.6–1.01 μg L−1) make it promising for the rapid screening of highly contaminated cases. As a proof-of-concept study, the substrate was applied in SERS sensing of PAHs in river matrix. The 16 PAHs could be differentiated based upon their characteristic SERS peaks. Most importantly, the detection was successfully conducted using a portable Raman spectrometer, which could be used for on-site monitoring of PAHs.

Rare earth-modified adsorbents (REMAs) have been widely used to remove oxyanion pollutants from water, including arsenic (As). However, the molecular-level structural information and reactions at the liquid/solid interface are still murky, which limits the design of applicable REMAs. Herein, a lanthanum-impregnated activated alumina (LAA) was synthesized as a representative REMA, and its As uptake mechanisms were explored using multiple complementary characterization techniques. Our adsorption experiments showed that LAA exhibited 2–3 times higher As adsorption capacity than AA. In contrast to the bidentate configuration formed on most metal oxide surfaces, our EXAFS and DFT results suggest that As(III) and As(V) form monodentate surface complexes on LAA through As-O-La coordinative bonding. In situ flow cell ATR-FTIR observed a strong dependence of As-O peak positions on pH, which could be interpreted as the change in the fractions of As(V) surface complexes with zero- to double-protonation on LAA, AA, and LaOOH. As(V) on LAA existed as singly and doubly protonated surface species, and the pKa of transition from double to single protonation (∼5.8) was lower than that for its soluble counterpart (6.97). The surface reaction and structural configuration were incorporated in a CD-MUSIC model to satisfactorily predict macroscopic As adsorption behaviors. The insights gained from the molecular-level reactions shed light on the design and application of REMAs in environmental remediation for As and its structural analogues.Keywords: adsorption; arsenic; coordination modes; lanthanum; surface chemistry

Arsenic- and iron-reducing bacteria play an important role in regulating As redox transformation and mobility. The motivation of this study was to compare the contributions of different As- and Fe-reducing bacteria to As biotransformation. In this work, three bacteria strains with different functional genes were employed including Pantoea sp. IMH with the arsC gene, Alkaliphilus oremlandii OhILAs possessing the arrA gene, and Shewanella oneidensis MR-1, an iron reducer. The incubation results showed that Pantoea sp. IMH aerobically reduced 100% of As(V) released from waste residues, though total As release was not enhanced. Similarly, strain OhILAs anaerobically reduced dissolved As(V) but could not enhance As release. In contrast, strain MR-1 substantially enhanced As mobilization because of iron reduction, but without changing the As speciation. The formation of the secondary iron mineral pyrite in the MR-1 incubation experiments, as evidenced by the X-ray absorption near-edge spectroscopy (XANES) analysis, contributed little to the uptake of the freed As. Our results suggest that the arsC gene carriers mainly control the As speciation in the aqueous phase in aerobic environments, whereas in anaerobic conditions, the As speciation should be regulated by arrA gene carriers, and As mobility is greatly enhanced by iron reduction.

A pressing challenge in arsenic (As) adsorptive filtration is to decipher how the As atomic surface structure obtained in the laboratory can be used to accurately predict the field filtration cycle. The motivation of this study was therefore to integrate molecular level As adsorption mechanisms and capacities to predict effluent As from granular TiO2 columns in the field as well as its health impacts. Approximately 2,955 bed volumes of groundwater with an average of 542 μg/L As were filtered before the effluent As concentration exceeded 10 μg/L, corresponding to an adsorption capacity of 1.53 mg As/g TiO2. After regeneration, the TiO2 column could treat 2,563 bed volumes of groundwater, resulting in an As load of 1.36 mg/g TiO2. Column filtration and EXAFS results showed that among coexisting ions present in groundwater, only Ca2+, Si(OH)4, and HCO3– would interfere with As adsorption. The compound effects of coexisting ions and molecular level structural information were incorporated in the PHREEQC program to satisfactorily predict the As breakthrough curves. The total urinary As concentration from four volunteers of local residences, ranging from 972 to 2,080 μg/L before groundwater treatment, decreased to the range 31.7–73.3 μg/L at the end of the experimental cycle (15–33 days).

Insights from molecular-level mechanisms of arsenite [As(III)] and cadmium (Cd) co-adsorption on TiO2 can further our understanding of their synergistic removal in industrial wastewaters. The motivation for our study is to explore the interfacial interactions of neutrally charged As(III) and cationic Cd2+ on nanocrystalline TiO2 using multiple complementary techniques. The results of adsorption edge, ζ potential, and surface complexation modeling suggest that coexistence of As(III) and Cd2+ enhanced their synergistic adsorption on TiO2 and, consequently, resulted in the formation of a ternary surface complex. This ternary surface complex, in turn, inhibited the metal release into the aqueous phase and, therefore, facilitated the immobilization of the heavy metals. Our in situ flow-cell attentuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy evidence showed that, regardless of the order of contact, As(III) was preferentially adsorbed on TiO2 rather than Cd. In agreement with our spectroscopic analysis, quantum chemistry calculations also illustrated that the Cd–As(III)–TiO2 ternary surface complex should be formed with the adsorbed As(III) as the bridging molecule. At high As(III) concentrations, the formation of the Cd–As(III)–TiO2 complex is responsible for the Cd removal. The simultaneous removal mechanisms will further our understanding of the removal of multiple pollutants in industrial wastewaters.

High concentrations of arsenic (As) in groundwater pose a great threat to human health. The motivation of this study was to provide a practical solution for As-safe water in As geogenic areas using granular TiO2 (GTiO2). The kinetics results indicated that the As (III/V) adsorption on GTiO2 conformed to the Weber-Morris (WM) intraparticle diffusion model. The Langmuir isotherm results suggested that the adsorption capacities for As (III) and As (V) were 106.4 and 38.3 mg/g, respectively. Ion effect study showed that cationic Ca and Mg substantially enhanced As (V) adsorption, whereas no significant impact was observed on As (III). Silicate substantially decreased As (V) adsorption by 57 % and As (III) by 50 %. HCO3− remarkably inhibited As (V) adsorption by 52 %, whereas it slightly reduced As (III) adsorption by 8 %. Field column results demonstrated that ∼700 μg/L As was removed at an empty bed contact time (EBCT) of 1.08 min for 968 bed volumes before effluent As concentration exceeded 10 μg/L, corresponding to 0.96 mg As/g GTiO2. Two household filters loaded with 110 g GTiO2 in the on-off operational mode can provide 6-L/day As-safe drinking water up to 288 and 600 days from the groundwater containing ∼700 μg/L As and ∼217 μg/L As, respectively. Integration of batch experiments and column tests with systematic variation of EBCTs was successfully achieved using PHREEQC incorporating a charge distribution multisite complexation (CD-MUSIC) model and one-dimensional reactive transport block.

Surface-enhanced Raman scattering (SERS) analysis of environmental hydrophobic pollutants without chemical functionalization of a bare nanoparticle (NP) substrate presents a challenge. The motivation for our study is to develop a highly reproducible and robust portable SERS sensor for detection and identification of polycyclic aromatic hydrocarbons (PAHs) using bare Au NPs. Our hypothesis is that the coffee ring effect could separate PAHs from the bulk solution and concentrate them on the closely packed Au NP ring, consequently enhancing their Raman scattering. This premise was confirmed with the commonly used citrate-reduced Au NPs in 20 nm, having no structural uniqueness. Because of the coffee ring effect, however, closely packed but not aggregated Au NP arrays were formed and, consequently, facilitated the separation and concentration of hydrophobic PAHs. As a result, a prominent SERS enhancement can be obtained on the ring because of the electromagnetic mechanism. A mixture of six PAHs with different numbers of benzene rings, namely, naphthalene, anthracene, pyrene, benzo[a]pyrene, benzo[g,h,i]perylene, and indeno[1,2,3-cd]pyrene, could be readily identified in river water. This portable SERS sensor based on the coffee ring effect provides a robust and versatile approach in PAH detection without the need for stringent structural requirements for Au NPs.Keywords: Au nanoparticles; coffee ring effect; PAHs; SERS;

Rapid and sensitive SERS identification and quantification of arsenic species in multiple matrices have been realized using a Fe3O4@Ag magnetic substrate. The molecular structure of arsenic on Fe3O4@Ag characterized using EXAFS spectroscopy and DFT confirms the existence of a chemical effect on SERS enhancement.

Insights into the bonding of As(V) at the metal oxide/aqueous interface can further our understanding of its fate and transport in the environment. The motivation of this work is to explore the interfacial configuration of As(V) on single crystal rutile (110) using grazing-incidence X-ray absorption fine structure spectroscopy (GI-XAFS) and planewave density functional calculations with on-site repulsion (DFT+U). In contrast to the commonly considered corner-sharing bidentate binuclear structure, tetrahedral As(V) binds as an edge/corner-sharing tridentate binuclear complex on rutile (110), as evidenced by observation of three As–Ti distances at 2.83, 3.36, and 4.05 Å. In agreement with the GI-XAFS analysis, our DFT+U calculations for this configuration resulted in the lowest adsorption energy among five possible alternatives. In addition, the electron density difference further demonstrated the transfer of charge between surface Ti atoms and O atoms in AsO4. This charge transfer consequently induced the formation of a chemical bond, which is also confirmed by the partial density of states analysis. Our results may shed new light on coupling the GI-XAFS and DFT approaches to explore molecular-scale adsorption mechanisms on single crystal surfaces.

•The mechanisms of PFOS adsorption on clay minerals are examined at molecular level.•XANEFS analysis provides direct evidence for chemical adsorption.•FTIR spectra in situ monitors the interaction process of PFOS with clay minerals.•PFOS could insert into the interlayers of montmorillonite.•The HA coating decreased PFOS adsorption on kaolinite and montmorillonite.Adsorption of perfluorooctane sulfonate (PFOS) on clay minerals is pivotal in the fate and transport of PFOS in the environment. Understanding the PFOS adsorption mechanisms, however, presents challenges, especially in the presence of natural organic substances. In this study, the adsorption mechanism of PFOS on kaolinite and montmorillonite, with and without humic acids (HA), were investigated using batch experiments, ATR-FTIR spectroscopy, zeta potential, NEXAFS, and XRD. The results showed that PFOS had a higher adsorption capacity on kaolinite than on montmorillonite, both with a rapid formation of an outer-sphere complex followed by a slow ligand exchange reaction with the elimination of adsorbed H2O molecules. In addition to nonspecific hydrophobic and electrostatic interactions, specific chemisorption also exists between sulfonate group of PFOS and hydroxyl group on surfaces of clay minerals. The HA coating on clay materials inhibited PFOS adsorption due to its occupation of adsorption sites and increased electrostatic repulsion. The insights gained from this study improve our understanding on the mechanism of multilateral forces on PFOS adsorption at the molecular level.

•Phthalate adsorption at the goethite/aqueous interface was studied using ATR-FTIR.•Phthalate form one outer-sphere and two bidentate inner-sphere surface complexes.•Outer-sphere complex is suppressed with increasing ionic strength.•Adsorption follow pseudo-second-order kinetics and reach equilibrium within 60 min.Insights into the molecular-level behaviors of phthalate at the goethite/aqueous interface can further our understanding of the fate and transport of natural organic matter analogs in the environment. The motivation of this work is to explore the interfacial configuration and dynamic adsorption process of phthalate on goethite at the molecular scale. The flow-cell ATR-FTIR measurement, curve-fitting analysis, and pseudo-second-order kinetic simulation were used to investigate the adsorption mechanisms. The results showed that phthalate formed one electrostatic outer-sphere complex and two inner-sphere configurations in mononuclear bidentate and binuclear bidentate structures. The contribution of outer-sphere complex to the overall phthalate adsorption was suppressed with increasing ionic strength and decreasing pH values. The ratio of adsorption capacity between these two inner-sphere configurations was slightly affected under different experimental conditions. Furthermore, the dynamic adsorption process of these three interfacial configurations followed the pseudo-second-order kinetics model, and reached equilibrium rapidly within 60 min.

Elevated fluoride (F) in groundwater presents an obvious environmental concern. Adsorption using activated alumina (AA) is currently the best available technology for F removal, despite its low efficiency, pH dependence, and aluminium (Al) release. The motivation for our study is to synthesise a new F adsorbent by impregnating commercially available granulated AA with lanthanum oxide (LAA), and to explore its F adsorption mechanism on the molecular scale. Five cycles of lanthanum impregnation on AA followed by calcination at 573 K increased the La content up to 19.1% and the F removal from 18.1% by pristine AA to 92.4% by LAA. The SEM, TEM, XRD, TGA, and EXAFS results demonstrated that the 5–20 nm thin flakes of LaOOH on LAA were in an amorphous form, with 7.6 oxygen atoms around each La. This LaOOH layer was uniformly distributed inside the micropores of the 1–3 mm AA granules. LAA exhibited four fold higher F adsorption capacity than AA in the pH range 3.9 to 9.6, with substantially reduced Al release. The ability to regenerate and reuse LAA makes it an attractive sustainable material. Multiple complementary spectroscopic analyses demonstrated that ligand exchange between F and surface hydroxyl groups is the mechanism for F adsorption on LAA. Our work improves the understanding of F interaction with metal oxides on the molecular scale and presents an alternative solution for elevated F water treatment.

Arsenic removal using nanomaterials has attracted increasing attention worldwide, whereas the potential release of As from spent nanomaterials to groundwater in reducing environments is presently underappreciated. This research investigated the fate of As(V) adsorbed on nano-TiO2 in the presence of sulfate reducing bacteria (SRB) Desulfovibrio vulgaris strains DP4 and ATCC 7757. The incubation results demonstrated that As(V) was desorbed from nano TiO2, and subsequently reduced to As(III) in aqueous solution. The release of adsorbed As(V) was two to three times higher in biotic samples than that in abiotic controls. Reduction of As(V) to As(III) in biotic samples was coupled with the conversion of sulfate to sulfide, while no As(III) was observed in abiotic controls. STXM results provided the direct evidence of appreciable As(III) and As(V) on TiO2. XANES analysis indicated that As(V) was the predominant species for three As loads of 150, 300, and 5700 mg/g, whereas 15–28% As precipitated as orpiment for a high As load of 5700 mg/g. In spite of orpiment formation, As mobilized in higher amounts in the SRB presence than in abiotic controls, highlighting the key role of SRB in the fate of As in the presence of nanomaterials.

Chronic exposure to arsenic (As) threatens human health. To systematically understand the health risks induced by As ingestion, we explored water and diet contributions to As exposure, and compared As in biomarkers and the arsenicosis in a geogenic As area in China. In this study, high percentages of water (77% of n = 131 total samples), vegetables (92%, n = 120), cereals (32%, n = 25), urine (70%, n = 99), nails (76%, n = 176), and hair (62%, n = 61) contained As higher than the acceptable levels. Dietary As contributed 92% of the average daily dose (ADD) when the water As concentration was less than 10 μg/L, for which 5 out of 30 examined participants were diagnosed with arsenicosis symptoms. The distinct positive correlation between ADD and As concentrations in urine, nails, and hair suggests different applicability for these biomarkers. Methylated As as the predominant urinary As species confirms that the ingested inorganic As is methylated and is excreted through urine. In situ microdistribution and speciation analysis indicates that As is mainly associated with sulfur in nails and hair. Nails, rather than hair and urine, could be used as a proper biomarker for arsenicosis. High ADD from the environment and low excretion could result in As toxicity to humans.

The southeast Tibetan Plateau (TP) of China is characterized by mountain-valley topography and is usually the main channel for the warm and humid airstream from South Asia caused by the Indian monsoon. In this study, it is hypothesized that some semivolatile organic pollutants such as organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs) can be transported from the densely populated and intensely agricultural Indian Subcontinent via the Indian monsoon and then cold-trapped by the mountains of the southeast TP. Samples of soils, lichens, conifer barks, and needles were collected from five transects to investigate the accumulation patterns of OCPs and PAHs in this region. The OCP concentrations were found to generally increase with increasing altitude in transects 1, 3, and 4, while such trends were insignificant in most cases for PAHs. Total organic carbon/lipid based normalization of concentrations does not strengthen the correlations with altitude in most cases. Chemical concentration ratios in soils of forest areas to clearing sites without forest cover (F/C) showed significantly positive correlation with log Koa and negative correlation with log Kaw of PAHs, suggesting that the role of forests as a filter and forest soil as a final sink are more pronounced for more lipophilic compounds. A lower α-/γ-HCH ratio and higher ratios of DDT/DDE and o,p′-DDT/p,p′-DDT compared with the technical products suggest the usage of lindane (γ-HCH), DDT, and dicofol in neighboring countries. The suitability of using different sample matrices (soil, lichen, conifer bark, and needles) as passive air samplers in remote regions is evaluated.

Insights into the adsorption mechanisms of salicylate on goethite, especially its competition with catechol, can further our understanding of the fate and transport of natural organic matter analogues in the environment. The adsorption process was investigated using multiple complementary techniques including batch adsorption experiments, flow-cell ATR-FTIR measurement, and DFT calculations. The macroscopic results show that increasing pH and ionic strength had an adverse effect on salicylate adsorption because of electrostatic interactions. Salicylate formed an inner-sphere complex in a mononuclear monodentate configuration with carboxylate bound to the iron atom on goethite surface within pH 5–9. The electrostatic outer-sphere complex could be observed under high salicylate concentrations and low ionic strength. In the competitive adsorption of salicylate and catechol, their individual interfacial complexes coexisted and competed with each other. The surface salicylate could be replaced by catechol during adsorption under neutral and basic pH conditions. Accordingly, the macroscopic adsorption capacity of salicylate was depressed in the binary system.

Insights from molecular-level mechanisms of propranolol adsorption can further our understanding of the fate and transport of beta blockers in the environment. The motivation of our study is to explore the dynamic adsorption process of propranolol at the TiO2/aqueous interface on the molecular scale. Multiple complementary techniques including macroscopic adsorption experiments, flow-cell ATR-FTIR measurement, XPS, and quantum chemical calculations were used to study the adsorption mechanisms. Our results show that propranolol adsorption on TiO2 increased from 0.3 to 2.3 μmol/g in the pH range 5 to 9. The ATR-FTIR and XPS analysis indicated that the hydroxyl and amino groups of propranolol strongly interacted with the TiO2 surface. The DFT calculations suggested the molecular structure of surface complexes with hydrogen bonding and the charge transfer from propranolol to TiO2 surface upon adsorption.

Coexisting arsenic (As) and fluoride (F) in groundwater poses severe health risks worldwide. Highly efficient simultaneous removal of As and F is therefore of great urgency and high priority. The purpose of this study was to fabricate a novel composite adsorbent and explore the mechanism for concurrent removal of As(V) and F at the molecular level. This bifunctional adsorbent with titanium and lanthanum oxides impregnated on granular activated carbon (TLAC) exhibits a pronounced As(V) and F adsorption capacity over commercially available iron- and aluminum-based adsorbents for synthetic and real contaminated groundwater samples. Synchrotron-based X-ray microfluorescence analysis demonstrates that La and Ti were homogeneously distributed on TLAC. Extended X-ray absorption fine structure spectroscopic results suggest that As(V) formed bidentate binuclear surface complex as evidenced by an averaged Ti–As bond distance of 3.34 Å in the presence of F. Adsorption tests and Fourier transform infrared spectroscopy analysis indicate that F was selectively adsorbed on lanthanum oxides. The surface configurations constrained with the spectroscopic results were formulated in the charge distribution multisite complexation model to describe the competitive adsorption behaviors of As(V) and F. The results of this study indicate that TLAC could be used as an effective adsorbent for simultaneous removal of As(V) and F.Keywords: arsenic; fluoride; molecular level mechanisms; simultaneous removal; synchrotron-based techniques;

Insights from molecular-level mechanisms of catechol adsorption on goethite can further our understanding of the fate and transport of hydroxyaromatic compounds in the environment. The motivation for our study is to explore the dynamic adsorption process of catechol at the goethite/aqueous interface on the molecular scale. Multiple complementary techniques including macroscopic adsorption experiments, flow-cell ATR-FTIR measurement, 2D IR correlation analysis, and quantum chemical calculations were used to study the adsorption mechanisms. Our results show that the adsorption of catechol was elevated at high pH but was not affected by ionic strength because of the formation of inner-sphere complexes. Catechol adsorbed on goethite in mononuclear monodentate and binuclear bidentate configurations in the pH range of 5 to 9. Partial mononuclear monodentate structures could be converted to binuclear bidentate complexes under basic conditions and with increasing surface coverage.

In situ detection and identification of PAHs, a group of well-known persistent organic pollutants, presents a great challenge to environmental researchers. This work developed a novel substrate based on thiol-functionalized Fe3O4@Ag core–shell magnetic nanoparticles for surface enhanced Raman scattering (SERS) sensing of PAHs. The surface morphology, structure, and magnetic properties of the substrate were characterized using multiple complementary techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, vibrating sample magnetometry analysis, and extended X-ray absorption fine structure spectroscopy. The high saturation magnetization at 48.35 emu g–1 enabled the complete and rapid separation of the substrate from the PAH solution. Benzene, naphthalene, anthracene, phenanthrene, fluorene, pyrene, perylene, and BaP were chosen as probe molecules. Qualitative and quantitative determination of PAHs was achieved using a portable Raman spectrometer. The SERS sensitivity was positively correlated with the hydrophobic nature of PAHs. The SERS response exhibited a linear dependence on the PAHs concentration between 1 to 50 mg/L, and the detection limit in the order 10–5 to 10–7 M was obtained. The SERS platform with magnetic substrate provides a new way for in situ PAH monitoring.

Removal and recovery of high levels of arsenic (As) in copper smelting wastewater present a great environmental challenge. A novel approach was investigated for the first time using TiO2 for As adsorptive removal from wastewater and subsequent spent adsorbent regeneration and As recovery using NaOH. EXAFS results demonstrate that As(III), as the only As species present in the raw water, does not form an aqueous complex with other metal ions. An average of 3890 ± 142 mg/L As(III) at pH 1.4 in the wastewater was reduced to 59 ± 79 μg/L in the effluent with final pH at 7 in the 21 successive treatment cycles using regenerated TiO2. Coexisting heavy metals including Cd, Cu, and Pb with concentrations at 369 mg/L, 24 mg/L, and 5 mg/L, respectively, were reduced to less than 0.02 mg/L. As(III) adsorption followed pseudosecond-order rate kinetics, and the adsorption behavior was described with the charge distribution multisite surface complexation model. Approximately 60% As(III) in the waste solution after the TiO2 regeneration process was recovered by thermo vaporization and subsequent precipitation of sodium arsenite, as suggested by the EDX and XPS analysis. This “zero” sludge process sheds new light on successful As remediation technology for acidic metallurgical industry wastewater.

•Four media were evaluated for their ability to remove As(V) and Cd from wastewater.•EXAFS results exhibited similar uptake mechanism of As(V) and Cd on four adsorbents.•pH edge and dosage results can be well described using CD-MUSIC model.•Granular TiO2 with excellent adsorption-regeneration performance is the best media.With the wide application of adsorption technology for the remediation of heavy metal contaminated metallurgical wastewater, selecting appropriate adsorbent media is a prerequisite for advancement. In this study, four commonly used adsorbents including titanium dioxide (TiO2), granular ferric oxide (GFO), activated alumina (AA), and nanoscale zero-valent iron (nZVI) were systematically evaluated for their abilities to simultaneously remove arsenate [As(V)] and cadmium (Cd) from wastewater. In kinetics and dosage experiments, TiO2 and GFO adsorbed more As(V) and Cd than AA and nZVI, which suggests that these two are ideal candidates for the remediation of metal-contaminated wastewater. Furthermore, TiO2 and GFO exhibited a wider pH application range than AA and nZVI. The pH edge and dosage experimental results can be well described using the CD-MUSIC model under the EXAFS constraint. Finally, regeneration experiments demonstrated that only TiO2 exhibited high adsorption efficiency after regeneration, and its high chemical stability was further evidenced by in-situ XRD characterization. Our systematic comparisons of the four different media for As(V) and Cd adsorption demonstrate that TiO2 is an ideal adsorbent, which paves the way for further wastewater treatment.Download high-res image (206KB)Download full-size image

Concerns over exposure to mercury have motivated the exploration of cost-effective, rapid, and reliable method for monitoring Hg2 + in the environment. Recently, surface-enhanced Raman scattering (SERS) has become a promising alternative method for Hg2 + analysis. SERS is a spectroscopic technique which combines modern laser spectroscopy with the optical properties of nano-sized noble metal structures, resulting in substantially increased Raman signals. When Hg2 + is in a close contact with metallic nanostructures, the SERS effect provides unique structural information together with ultrasensitive detection limits. This review introduces the principles and contemporary approaches of SERS-based Hg2 + detection. In addition, the perspective and challenges are briefly discussed.Download high-res image (239KB)Download full-size image

Insightful knowledge of geochemical processes controlling fluoride (F) mobility is fundamental to understand the occurrence of elevated F in groundwater. Principal component analysis (PCA) was applied to explore the dominating factors in the F geochemistry in Shanxi and Inner Mongolia, two severely F-affected areas in China. Field sampling results of 111 tubewells showed that 26.1% of drinking water wells, with F concentrations in the range 0.3–5.6 mg/L, exceeded WHO standards of 1.5 mg/L. PCA with 16 geochemical parameters demonstrated that F occurrence in Shanxi could be the result of mineral weathering and water–rock interactions in the aquifer. Groundwater F concentrations increased with TDS in Shanxi, but not in Inner Mongolia. In agreement with our PCA, the occurrence of F in Inner Mongolia may be attributed to multiple processes including agriculture and mining activities, and water–rock interaction processes in the aquifer. Calcium is the scavenger of fluoride in Shanxi and Inner Mongolia. The results of this study further our understanding of the similarities and differences in the F occurrence and mobility at various locations.Highlights► F geochemistry in Shanxi and Inner Mongolia was compared. ► Principal component analysis was used to find the dominating factors. ► F occurrences in these two places were different. ► Calcium and HCO3 play major role in F occurrence.

Insightful knowledge of geochemical processes controlling As mobility is fundamental to understanding the occurrence of elevated As in groundwater. A comparative study of As geochemistry was conducted in the Datong Basin (Shanxi) and Hetao Basin (Inner Mongolia), two strongly As-enriched areas in China. The results show that As concentrations ranged from <1–1160 μg L−1 (n = 37) in the Datong Basin and <1–804 μg L−1 (n = 62) in the Hetao Basin. The groundwater is of the Na-HCO3 type in the Datong Basin and Na-Cl-HCO3 type in the Hetao Basin. Silicate mineral weathering and cation exchange processes dominated the groundwater geochemistry in the two study areas. Principal component analysis of 99 groundwater samples using 12 geochemical parameters indicated positive correlations between concentrations of As and Fe/Mn in the Datong Basin, but no correlation of As and Fe/Mn in the Hetao Basin. Phosphate correlated well with As in the Datong Basin and Hetao Basin, suggesting phosphate competition might be another process affecting As concentrations in groundwater. High concentrations of As, Fe, and Mn occurred in the pe range −2 to −4. The results of this study further understanding of the similarities and differences of As occurrence and mobility at various locations in China.Highlights► Comparative studies of As geochemistry in the Datong Basin and Hetao Basin, China. ► Positive correlations between concentrations of As and Fe/Mn in the Datong Basin. ► No correlation of As and Fe/Mn in the Hetao Basin. ► High concentrations of As, Fe, and Mn occurred in the pe range −2 to −4.

•As(V), either adsorbed or dissolved, was reduced in the presence of SRB.•Reduction was faster for adsorbed As(V) than for dissolved As(V).•SRB promoted As(V) desorption from TiO2 compared with abiotic sulfide.•As(V) desorption due to competition with phosphate surface complexationReduction of surface-bound arsenate [As(V)] and subsequent release into the aqueous phase contribute to elevated As in groundwater. However, this natural process is not fully understood, especially in the presence of sulfate-reducing bacteria (SRB). Gaining mechanistic insights into solid-As(V)-SRB interactions motivated our molecular level study on the fate of nano-TiO2 bound As(V) in the presence of Desulfovibrio vulgaris DP4, a strain of SRB, using incubation and in situ ATR-FTIR experiments. The incubation results clearly revealed the reduction of As(V), either adsorbed on nano-TiO2 or dissolved, in the presence of SRB. In contrast, this As(V) reduction was not observed in abiotic control experiments where sulfide was used as the reductant. Moreover, the reduction was faster for surface-bound As(V) than for dissolved As(V), as evidenced by the appearance of As(III) at 45 h and 75 h, respectively. ATR-FTIR results provided direct evidence that the surface-bound As(V) was reduced to As(III) on TiO2 surfaces in the presence of SRB. In addition, the As(V) desorption from nano-TiO2 was promoted by SRB relative to abiotic sulfide, due to the competition between As(V) and bacterial phosphate groups for TiO2 surface sites. This competition was corroborated by the ATR-FTIR analysis, which showed inner-sphere surface complex formation by bacterial phosphate groups on TiO2 surfaces. The results from this study highlight the importance of indirect bacteria-mediated As(V) reduction and release in geochemical systems.Download high-res image (112KB)Download full-size image

Anatase TiO2 crystal facets are garnering increasing attention due to their unique surface property. However, no specific linear relationship had been derived between the facet exposed on TiO2 and the surface adsorption capacity as well as photocatalytic performance. This study systematically explored the facet effects on antimony (Sb) adsorption and photocatalytic oxidation using high-index {201} and low-index {101}, {001}, and {100} TiO2. The results suggest that high-index {201} TiO2 exhibits the best Sb(III) adsorption and photocatalytic activity compared to the low-index TiO2. Both the Sb(III) adsorption density and the amount of OH and O2− generated in solution were correlated to the magnitude of surface energy on TiO2 facets. Photocatalytically generated OH and O2− were responsible for Sb(III) photooxidation as evidenced by radical-trapping experiments. The great contribution of OH was observed only on {201}, not on low-index TiO2. This phenomenon was found to be attributable to the high surface energy on {201}, which enables the generation of a large amount of photogeneration OH to compensate for the fast rate of OH dissipation. Therefore, the predominant participation of OH in Sb(III) photooxidation was only possible on high-index {201} TiO2, which resulted in an enhanced photocatalytic rate. On the other hand, O2− dominated the Sb(III) photocatalytic oxidation on low-index TiO2. The intrinsic facet-dependent adsorption and photocatalytic mechanism obtained from this study would be useful for developing TiO2-based environmental technologies.

Elevated arsenic (As) in groundwater poses a great threat to human health. Coagulation using mono- and poly-Fe salts is becoming one of the most cost-effective processes for groundwater As removal. However, a limitation comes from insufficient understanding of the As removal mechanism from groundwater matrices in the coagulation process, which is critical for groundwater treatment and residual solid disposal. Here, we overcame this hurdle by utilizing microscopic techniques to explore molecular As surface complexes on the freshly formed Fe flocs and compared ferric(III) sulfate (FS) and polyferric sulfate (PFS) performance, and finally provided a practical solution in As-geogenic areas. FS and PFS exhibited a similar As removal efficiency in coagulation and coagulation/filtration in a two-bucket system using 5 mg/L Ca(ClO)2. By using the two-bucket system combining coagulation and sand filtration, 500 L of As-safe water (< 10 μg/L) was achieved during five treatment cycles by washing the sand layer after each cycle. Fe k-edge X-ray absorption near-edge structure (XANES) and As k-edge extended X-ray absorption fine structure (EXAFS) analysis of the solid residue indicated that As formed a bidentate binuclear complex on ferrihydrite, with no observation of scorodite or poorly-crystalline ferric arsenate. Such a stable surface complex is beneficial for As immobilization in the solid residue, as confirmed by the achievement of much lower leachate As (0.9 μg/L–0.487 mg/L) than the US EPA regulatory limit (5 mg/L). Finally, PFS is superior to FS because of its lower dose, much lower solid residue, and lower cost for As-safe drinking water.Download full-size image

Elevated fluoride (F) in groundwater presents an obvious environmental concern. Adsorption using activated alumina (AA) is currently the best available technology for F removal, despite its low efficiency, pH dependence, and aluminium (Al) release. The motivation for our study is to synthesise a new F adsorbent by impregnating commercially available granulated AA with lanthanum oxide (LAA), and to explore its F adsorption mechanism on the molecular scale. Five cycles of lanthanum impregnation on AA followed by calcination at 573 K increased the La content up to 19.1% and the F removal from 18.1% by pristine AA to 92.4% by LAA. The SEM, TEM, XRD, TGA, and EXAFS results demonstrated that the 5–20 nm thin flakes of LaOOH on LAA were in an amorphous form, with 7.6 oxygen atoms around each La. This LaOOH layer was uniformly distributed inside the micropores of the 1–3 mm AA granules. LAA exhibited four fold higher F adsorption capacity than AA in the pH range 3.9 to 9.6, with substantially reduced Al release. The ability to regenerate and reuse LAA makes it an attractive sustainable material. Multiple complementary spectroscopic analyses demonstrated that ligand exchange between F and surface hydroxyl groups is the mechanism for F adsorption on LAA. Our work improves the understanding of F interaction with metal oxides on the molecular scale and presents an alternative solution for elevated F water treatment.

Rapid and sensitive SERS identification and quantification of arsenic species in multiple matrices have been realized using a Fe3O4@Ag magnetic substrate. The molecular structure of arsenic on Fe3O4@Ag characterized using EXAFS spectroscopy and DFT confirms the existence of a chemical effect on SERS enhancement.

Anatase TiO2 nanomaterials have been widely used in arsenic (As) remediation, although reports on their adsorption and photocatalytic capacity have been controversial. The motivation for our study is to explore the As adsorption and photooxidation processes on different TiO2 facets at the molecular level. Our results from multiple complementary characterization techniques suggest that anatase {001} facets have stronger Lewis acid sites than those on {101} facets, resulting in a higher As adsorption affinity. Density functional theory (DFT) calculations confirmed that the As surface complex is more energetically favorable on {001} than on the {101} facets. In addition, the strong interaction of the {001} facets with molecular O2 facilitates the transfer of photo-excited electrons to the adsorbed O2 to generate a superoxide radical (O2˙−), which is the primary As(III) oxidant as evidenced by our radical-trapping experiments. Meanwhile, the oxygen vacancies on the {001} facets could expedite the interfacial electron transfer and electron–hole separation, which promote the generation of O2˙− and, ultimately, the catalytic efficiency. The insights gained from this study provide a firm basis for the proposition that As adsorption and photoactivity can be mediated by tailoring the exposed TiO2 facets, which is of essence in the design and application of TiO2-based environmental technologies.