Zirconium oxide archored onto reduced graphene oxides (ZrO2@rGO) was fabricated via a hydrothermal method and used for Re(VII) removal from aqueous solutions. Scanning electron microscopy, Fourier transferred infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron spectroscopy were used to characterize the as-prepared ZrO2@rGO. The results indicated that ZrO2 was successfully decorated on rGO. The maximum sorption capacity of ZrO2@rGO toward Re(VII) was 43.55 mg/g. ZrO2@rGO exhibited enhanced sorption capacity for Re(VII) in comparison with bare ZrO2 or rGO. The sorption kinetics could be described by the pseudo-second-order equation. The sorption process of Re(VII) on ZrO2@rGO was endothermic and spontaneous. X-ray photoelectron spectroscopy indicated the formation of an ionic bond of Zr–O with Re(VII). According to the density functional theory calculations, ORe–Zr bonds on the surface of the monoclinic ZrO2 plane (m-ZrO2) (111) plane and tetragonal ZrO2 (t-ZrO2) (111) plane were formed when Re(VII) sorbs. The sorption energy of Re(VII) onto the t-ZrO2 (111) plane was 3.87 eV, being higher than that of Re(VII) onto m-ZrO2 (1.26 eV).Keywords: Interaction mechanism; Re(VII); Sorption; ZrO2@rGO;

•A novel diatomite sorbent was satisfactorily fabricated by hydrothermal method.•Interaction mechanism sorbent was investigated by batch, spectroscopic and modeling.•Novel diatomite could be a favorable sorbent for the efficient removal of U(VI).A novel magnesium silicate/diatomite sorbent (MSD) was fabricated using a facile hydrothermal method. The interaction mechanism of U(VI) on MSD was investigated by batch, spectroscopic and modeling techniques. The maximum adsorption capacity of MSD for U(VI) (31.54 mg/g) is significantly higher than that of diatomite (12.74 mg/g). XPS analysis indicated that the oxygen-containing groups (e.g., SiOH) of MSD were responsible for the high efficient removal of U(VI) from aqueous solutions. The fitted results of surface complexation modeling showed that the adsorption of U(VI) on MSD at pH < 4.0 and pH > 4.5 was attributed to cation exchange and inner-sphere surface complexation, respectively. These findings suggested that MSD could be a favorable sorbent for the efficient removal of U(VI) in the environmental cleanup.Schematic illustration of the synthesis process and mechanism.

•Attapulgite exhibits relatively high sorption capacity for U(VI) removal in the presence of HA.•Thermodynamic parameters indicate a spontaneous and endothermic sorption process.•Strong chemical interactions and/or surface complexation are the primary mechanisms.•pH and HA greatly influence the surface properties of attapulgite.Humic acid (HA) can markedly change the surface charge of clay minerals and influences the sorption of uranium(U) on clay minerals in aquatic environments. Furthermore, HA can potentially affect the mobility of U(VI) in the natural environment. The effect of HA on the surface charge, aggregation, sorption and bonding properties of attapulgite [(Mg,Al)4(Si)8(O,OH,H2O)26·nH2O] was investigated using fluorescence spectroscopy technique in combination with other analytical tools. The UV–vis absorbance spectra and fluorescence peaks obtained in the experiments are attributed to a large number of electron donating substitutes, such as carboxylic acid-like and phenolic acid-like fluorophores, in HA. Electrostatic attraction and ligand exchanges are the primary interaction mechanisms between attapulgite and HA. A significant modification of excitation-emission matrix (EEM) spectra and emission intensity of HA are observed after the interaction, indicating a different binding complex of HA on attapulgite at the molecular level. The sorption kinetics indicate that strong chemical interactions and/or surface complexation primarily contributed to U(VI) sorption. An analysis of U(VI) sorption isotherms and model fitting reveals that the sorption of U(VI) onto attapulgite is a spontaneous and endothermic process. The presence of HA is an important factor that greatly influences the immobilization and removal of U(VI) ions from aqueous solutions.Download high-res image (203KB)Download full-size image

•The aggregation of silica aerogels in the presence of coexistent electrolyte ions and HA were investigated.•The sorption mechanism of U(VI) sorption on silica aerogels is the inner-sphere complexation.•The influence of HA, silicate and phosphate on U(VI) sorption in the whole pH range were discussed.•The kinetic and thermodynamic data of U(VI) sorption on silica aerogels were analyzed.•U(VI) sorption is mainly due to the interaction of U(VI) with –OH on silica aerogel surfaces.The very large surface associated with aerogels made them attractive in the field of sorption. Herein, silica aerogels were characterized and used for the sorption of U(VI) in solution. The effects of specific surface area (SSA), pH and coexistent electrolyte ions on the surface site density, aggregation and sorption properties have been carefully investigated. The site distributions of silica aerogels were calculated with Visual MINTEQ. The sorption of U(VI) is affected significantly by the solution pH. The sorption increases with increasing SSA due to the increased amount of surface active sites of silica aerogels. Electrolyte ions (Na+, K+, NO3−, Cl−) were added to investigate the competition ability to U(VI) adsorbed on active sites. The influence of humic acid (HA), silicate and phosphate on U(VI) sorption in multiphase systems were also discussed. The kinetics data were used to investigate the sorption process and pseudo-second-order model was used to simulate the kinetic data. Spectroscopic analyses were also utilized to explain the role of OH groups in the U(VI) sorption.U(VI) sorption on silica aerogels in multi-component systemsDownload high-res image (131KB)Download full-size image

The synthesis of reclaimable adsorbents with satisfactory adsorption performance and easy separation properties is necessary for environment-related applications. In this study, novel amine-functionalized magnetic Fe3O4 (Fe3O4–NH2) nanoparticles coated with poly(m-phenylenediamine) (Fe3O4–NH2@PmPDs) were synthesized successfully via oxidation polymerization. The as-prepared Fe3O4–NH2@PmPDs with a well-defined core–shell structure were characterized, and their extraordinary Cr(VI) removal capability was investigated. Fe3O4–NH2@PmPDs exhibit high adsorption capacity (508 mg g−1) and fast adsorption rate towards Cr(VI). The abundant nitrogen-containing functional groups on the surface of Fe3O4–NH2@PmPDs greatly contribute to the adsorption/reduction of Cr(VI). Moreover, the intraparticle diffusion model can be used to provide a good explanation of every stage of the process. The calculated thermodynamic parameters suggest that the adsorption of Cr(VI) onto Fe3O4–NH2@PmPDs is endothermic and spontaneous. Fe3O4–NH2@PmPDs can be easily separated, and the regenerated adsorbents still maintain high adsorption capacity. The results imply that Fe3O4–NH2@PmPDs can be regarded as a suitable material for the treatment of Cr(VI) from contaminated water.

The surface and sorption properties of kaolinite were analyzed as a function of silicate. Batch experiments indicate that the U(VI) sorption is promoted by the addition of silicate at low pH while is depressed at high pH. The sorption was acceptably predicted by the formation of a ternary silicate surface complex under the experimental conditions. The pseudo-second order kinetic model fit the sorption kinetics better. The sorption isotherms are more in accordance with Langmuir model and the thermodynamic parameters indicate a spontaneous and endothermic sorption process.

•The dissolved Si leads to a rapid increase in the Ni sorption.•The coprecipitation of Si and Ni ions to Ni phyllosilicate occurs.•The nucleation of a (Ni, Al) phyllosilicate phase was confirmed by DRS and EXAFS analysis.•Kaolinite act as a nucleating surface for the formation of (Ni, Al) phyllosilicates in the presence of substantial Si.Unraveling the formation process of Ni precipitates at molecular scale is important for understanding the fate and mobility of Ni species in the real environment. Dissolved Si presents in the natural environment ubiquitously, which can alter Ni sorption as well as incorporation into neoformed precipitates. Batch experiments show that the dissolved Si leads to a rapid increase in the Ni sorption rate and interferes with the formation of Ni precipitates. The results of diffuse reflectance spectroscopy (DRS) and extended X-ray absorption fine structure (EXAFS) spectroscopy analyses suggest that the nucleation of a (Ni,Al) phyllosilicate phase involves a kaolinite-like local structure. Then, the substantial presence of Si affects the initial formation of Ni precipitate nucleation and the resulting crystal growth. Dioctahedral kaolinite may act as a nucleating surface for the heterogeneous formation of trioctahedral (Ni,Al) phyllosilicates under environmentally relevant conditions. This study provides experimental evidence on nucleation and epitaxial growth processes of Ni precipitate on kaolinite and provides insight on the relationship between substrates and precipitation, which is crucial for understanding the physicochemical behavior of Ni on mineral surfaces.

The current methods for chromium and natural organic matter decontamination from wastewater present limitations, such as high cost, poor reproducibility, and detrimental environmental effects as well as by secondary waste. Herein, we synthesized a core–shell structure of polyaniline/hydrogen-titanate nanobelt (PANI/H-TNB) composites through chemical oxidation in the presence of phytic acid, which played an important role in the formation and regeneration of PANI. The adsorption performance of PANI/H-TNB composites as an adsorbent of Cr(VI) and humic acid (HA) from aqueous solutions was tested. A batch technique was adopted to investigate the removal efficiency toward Cr(VI) and HA under various environmental conditions. The PANI/H-TNB composites exhibited excellent adsorption capacity toward Cr(VI) (156.94 mg g−1) and HA (339.46 mg g−1), outperforming that of PANI nanowires and many other materials. Large Kd values (>104 mL g−1) demonstrated the high affinity of the composites for both of Cr(VI) and HA. The analysis of Fourier transformed infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the removal of Cr(VI) was a combined effect of reduction Cr(VI) to Cr(III) and chemical sorption, while HA adsorption was mainly via surface complexation between the disassociated HA macromolecules and the positively charged PANI. The PANI/H-TNB composites presented satisfactory regeneration performance and reusability, which greatly reduced the wastewater disposal expenses. For the sake of industrial application, the PANI/H-TNB composites with high adsorption capacities can be applied as a suitable adsorbent for simultaneous removal of Cr(VI) and HA in wastewater cleanup.

This study establishes the relationship between the graphene oxide (GO) colloidal behavior and the co-adsorption of Cd(II) and phosphate (P(V)) on GO. Results reveal that the interactions among GO, Cd(II), and P(V) exhibit a significant dependence on solution chemistry and addition sequences and that these interactions subsequently affect the GO colloidal behavior. The GO aggregation is pH-dependent at pH < 4.0 and depends apparently on the binding ability of Cd(II) to GO at pH > 4.0. When the components were added simultaneously, the presence of P(V) enhances the GO binding capacity toward Cd(II), confirmed by theoretical calculation, resulting in the greater destabilizing influence of Cd(II) + P(V) on GO than Cd(II) at pH 3.0–9.5, while the formation of Cd3(PO4)2 precipitate leads to a lower destabilizing influence of Cd(II) + P(V) on GO than Cd(II) at pH > 9.5. Both pH and addition sequence affect the destabilizing ability of Cd(II) + P(V). These new insights are expected to provide valuable information not only for the application of GO as a potential adsorbent in multicomponent systems for heavy metal ion and oxyanion co-removal but also for the fate and risk assessment of GO after serving as heavy metal ion and oxyanion carrier.

•90Sr(II) sorption on Na-montmorillonite was strongly dependent on pH and ionic strength.•The DLM modeled the sorption of 90Sr(II) on Na-montmorillonite well with the aid of Visual Minteq 3.0.•HA enhanced 90Sr(II) sorption on Na-montmorillonite at low pH, whereas suppressed 90Sr(II) sorption at high pH.•The Freundlich model simulated the sorption isotherms of 90Sr(II) better than the Langmuir model.•The sorption process of 90Sr(II) on Na-montmorillonite was spontaneous and endothermic.Clay minerals have been studied extensively due to their strong sorption and complexation ability toward various environmental pollutants. In this study, the sorption of 90Sr(II) on Na-montmorillonite was studied as a function of various environmental conditions such as pH, ionic strength, humic acid (HA) and temperature. The results indicated that the sorption of 90Sr(II) on Na-montmorillonite was strongly dependent on pH and ionic strength. The experimental data of 90Sr(II) sorption was simulated by the diffuse-layer model (DLM) well with the aid of Visual Minteq 3.0. At low pH, the sorption of 90Sr(II) was dominated by outer-sphere surface complexation and ion exchange with Na+/H+ on Na-montmorillonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA enhanced 90Sr(II) sorption at pH < 7.0 but decreased 90Sr(II) sorption at pH > 7.0. Langmuir and Freundlich models were used to simulate the sorption isotherms of 90Sr(II) at three different temperatures of 303, 318 and 333 K. The thermodynamic parameters (ΔH, ΔS and ΔG) calculated from the temperature-dependent sorption isotherms indicated that the sorption of 90Sr(II) on Na-montmorillonite was an endothermic and spontaneous process. The thermodynamic parameters calculated from temperature-dependent sorption data were crucial to understand the interaction behavior of 90Sr(II) with Na-montmorillonite.

This paper presents a comparative study of Cu(II) decontamination by three different carbonaceous materials, i.e., graphene oxide, multiwalled carbon nanotubes, and activated carbon. The three carbonaceous materials were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, N2-BET surface area analysis, and potentiometric acid–base titrations in detail. Also, Cu(II) adsorption on the three types of carbonaceous materials as a function of pH and Cu(II) ion concentration were investigated. The constant capacitance model was used to determine the appropriate surface reactions of Cu(II) adsorption on carbonaceous materials with the aid of FITEQL 4.0 software. In addition, how the surface area and the total concentration of acidic functional groups influencing the adsorption capacities of the three carbonaceous materials for Cu(II) removal were elucidated. The results have an important role in predicting the adsorption capacity of surface modified carbonaceous materials.

The influence of humic acid (HA) on Ni(II) sorption to Ca-montmorillonite was examined by using a combination of batch sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The sorption of Ni(II) on HA–montmorillonite hybrids is strongly dependent on pH and temperature. At low pH, the sorption of Ni(II) is mainly dominated by Ni–HA–montmorillonite and outer-sphere surface complexation. The EXAFS results indicate that the first coordination shell of Ni(II) consists of ∼6 O atoms at the interatomic distances of ∼2.04 Å in an octahedral structure. At high pH, binary Ni–montmorillonite surface complexation is the dominant sorption mechanism. EXAFS analysis indicates the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5, while a Ni–Al layered double hydroxide (LDH) phase at the Ca-montmorillonite surface formed with pH 8.5. At pH 10.0, the dissolved HA–Ni(II) complexation inhibits the precipitation of Ni hydroxide, and Ni–Al LDH phase forms. The rise of temperature increases the sorption capacity of Ni(II), and promotes Ni–Al LDH phase formation and the growth of crystallites. The results are important to evaluate the physicochemical behavior of Ni(II) in the natural environment.

The interaction between radionuclides and solid/water interfaces is important to understand the physicochemical processes of radionuclides in the natural environment. Herein, the interaction of 60Co(II) with TiO2 in aqueous solution as a function of pH and ionic strength was studied by using batch technique combined with surface complexation model and density functional theory (DFT) calculations. The batch experimental results showed that the adsorption of 60Co(II) was dependent on pH and independent of ionic strength, indicating the formation of inner-sphere surface complexes on TiO2 surfaces. The results of surface complexation models and DFT calculations indicated that the surface species of 60Co(II) adsorbed on TiO2 followed the trend: B structure (i.e., 60Co(II) was linked to one bridge oxygen site) was the dominant surface species at low pH, and TT structure (i.e., 60Co(II) was linked to two terminal oxygen sites) became the important surface complex at neutral and alkaline pH values. These results demonstrated that a multi-technique approach could lead to definitive information on the structures of adsorbed 60Co(II) at the molecular level at the TiO2/water interfaces, as well as realistic models to rationalize and accurately evaluate the macroscopic manifestations of radionuclide adsorption phenomena.

The sorption speciation of Ni(II) on Ca-montmorillonite was evaluated using a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and modeling. The pH and temperature at the aqueous-montmorillonite interface affects both the extent of Ni(II) sorption as well as the local atomic structure of the adsorbed Ni(II) ions. At 0.001 mol L−1 Ca(NO3)2 and low pH, the study reveals that the majority of Ni(II) is adsorbed in the interlayers of Ca-montmorillonite coordinated by six water molecules in an octahedron as an outer-sphere complex. At higher pH, inner-sphere surface complexes are formed. The Ni–Si/Al distances (RNi–Al = 3.00 Å, RNi–Si1 = 3.10 Å and RNi–Si2 = 3.26 Å) determined by EXAFS confirm the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5 and 8.5. At pH 10.0, the Ni–Ni/Si distances (RNi–Ni = 3.07 Å and RNi–Si = 3.26 Å) indicates the formation of Ni-phyllosilicate precipitates. A rise in temperature promotes inner-sphere complexation, which in turn leads to an increase in Ni(II) sorption on Ca-montmorillonite. Sorption edges are fitted excellently by surface complexation model (SCM) with the aid of surface species determined from EXAFS spectroscopy.

This paper presents a comparative study of Cu(II) decontamination by three different carbonaceous materials, i.e., graphene oxide, multiwalled carbon nanotubes, and activated carbon. The three carbonaceous materials were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, N2-BET surface area analysis, and potentiometric acid–base titrations in detail. Also, Cu(II) adsorption on the three types of carbonaceous materials as a function of pH and Cu(II) ion concentration were investigated. The constant capacitance model was used to determine the appropriate surface reactions of Cu(II) adsorption on carbonaceous materials with the aid of FITEQL 4.0 software. In addition, how the surface area and the total concentration of acidic functional groups influencing the adsorption capacities of the three carbonaceous materials for Cu(II) removal were elucidated. The results have an important role in predicting the adsorption capacity of surface modified carbonaceous materials.

The sorption speciation of Ni(II) on Ca-montmorillonite was evaluated using a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and modeling. The pH and temperature at the aqueous-montmorillonite interface affects both the extent of Ni(II) sorption as well as the local atomic structure of the adsorbed Ni(II) ions. At 0.001 mol L−1 Ca(NO3)2 and low pH, the study reveals that the majority of Ni(II) is adsorbed in the interlayers of Ca-montmorillonite coordinated by six water molecules in an octahedron as an outer-sphere complex. At higher pH, inner-sphere surface complexes are formed. The Ni–Si/Al distances (RNi–Al = 3.00 Å, RNi–Si1 = 3.10 Å and RNi–Si2 = 3.26 Å) determined by EXAFS confirm the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5 and 8.5. At pH 10.0, the Ni–Ni/Si distances (RNi–Ni = 3.07 Å and RNi–Si = 3.26 Å) indicates the formation of Ni-phyllosilicate precipitates. A rise in temperature promotes inner-sphere complexation, which in turn leads to an increase in Ni(II) sorption on Ca-montmorillonite. Sorption edges are fitted excellently by surface complexation model (SCM) with the aid of surface species determined from EXAFS spectroscopy.

The influence of humic acid (HA) on Ni(II) sorption to Ca-montmorillonite was examined by using a combination of batch sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The sorption of Ni(II) on HA–montmorillonite hybrids is strongly dependent on pH and temperature. At low pH, the sorption of Ni(II) is mainly dominated by Ni–HA–montmorillonite and outer-sphere surface complexation. The EXAFS results indicate that the first coordination shell of Ni(II) consists of ∼6 O atoms at the interatomic distances of ∼2.04 Å in an octahedral structure. At high pH, binary Ni–montmorillonite surface complexation is the dominant sorption mechanism. EXAFS analysis indicates the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5, while a Ni–Al layered double hydroxide (LDH) phase at the Ca-montmorillonite surface formed with pH 8.5. At pH 10.0, the dissolved HA–Ni(II) complexation inhibits the precipitation of Ni hydroxide, and Ni–Al LDH phase forms. The rise of temperature increases the sorption capacity of Ni(II), and promotes Ni–Al LDH phase formation and the growth of crystallites. The results are important to evaluate the physicochemical behavior of Ni(II) in the natural environment.

Graphene oxide (GO), one of the most important graphene derivatives, has many oxygen-containing functional groups on its basal plane and on the edges in the form of epoxy, hydroxyl and carboxyl groups. It has attracted increasing interest in multidisciplinary research because of its unique structure and exceptional physicochemical properties. In particular, GO-based materials have great potential in environmental remediation and energy applications. Herein, we review the recent advances in GO-based materials for the sorption of radionuclides, mainly from the last decade. This review summarizes the preparation of GO-based materials and their application in the sorption of radionuclides (such as U(VI), Eu(III), Sr(II), etc.) from aqueous systems. The main sorption mechanisms are investigated using kinetic analysis, thermodynamic analysis, surface complexation models, spectroscopic techniques and theoretical calculations. It is evident that GO-based materials have good potential for the removal of radionuclides from aqueous systems. However, it is necessary to carry out more research focusing on the development of lower cost, higher efficiency and more environmentally friendly GO-based materials, either for scientific interest or practical applications.