Formation Of Inner-sphere Surface Complexes Research Articles

Comparison of arsenate and arsenite removal behaviours and mechanisms from water by Fe[sbnd]La binary composite (hydr)oxides

The present study compares the performances and mechanisms of As(V) adsorption with As(III) removal by a series of FeLa binary composite (hydr)oxides. The As(V) and As(III) adsorption properties were studied by batch adsorption experiments, and their adsorption mechanisms were explored by SEM-EDS, FTIR, XRD and XPS. The FeLa binary composite (hydr)oxides exhibited excellent As(V) removal performances, and their adsorption properties of As(V) were obviously enhanced with increasing La contents. However, their As(III) removal performances were extremely poor, and the increase of Fe contents could not significantly affect As(III) adsorption performances. The adsorption processes of As(V) were rapid, but the As(III) adsorption were slower. The maximum adsorption capacities of As(V) on Fe/La 3:1, Fe/La 1:1, and Fe/La 1:3 were 166.69, 271.65 and 360.81 mg/g, respectively, and those of As(III) were 30.71, 26.44 and 28.64 mg/g, respectively. Solution pH and presence of PO43−, and citrate can clearly influence the removal efficiencies of As(V) and As(III). The predominant mechanisms of As(V) were the formation of LaAsO4 through ligand exchange reactions under all experimental conditions and coprecipitation of La(III) and As(V) under acidic and neutral conditions, while the formation of inner-sphere surface complexes through ligand exchange reactions between Fe–OH groups and As(III) contributed to the As(III) removal. This work emphasizes the different adsorption properties between As(V) and As(III) by FeLa composite (hydr)oxides, and indicates great application potential in As(V) water treatment and the need for pretreatment for As(III) removal, such as pre-oxidation of As(III) to As(V).

Novel Application of MnO2–H2O2 System for Highly Efficient Arsenic Adsorption and Oxidation

A novel manganese dioxide–hydrogen peroxide (MnO2–H2O2) system was developed for effective Arsenic (As) removal. Under the specified conditions of no external mechanical stirring, a trace H2O2 concentration of 0.015 wt%, and a MnO2 concentration of 25 mg/L, high removal efficiency (88%) of As (100 µg/L) was achieved by the MnO2–H2O2 system within 30 min, which differs from conventional adsorption processes that require external mechanical stirring and conventional arsenite (As (III)) oxidation–adsorption processes that require high quantities of oxidants (such as ozone) and specially synthesized adsorbents/catalysts. The high removal efficiency of As (III) by the MnO2–H2O2 system was attributed to the turbulent conditions precipitated by the extensively generated oxygen (O2) from the catalytic decomposition of H2O2, the efficient adsorption of As on the surface of MnO2, and the effective generation of reactive radicals including hydroxyl and superoxide radicals (•OH and •O2−). Moreover, the MnO2 adsorbents before and after As removal were characterized systematically, and the generated radicals were verified using electron spin resonance (ESR). The results showed that the formation of inner-sphere surface complexes by the surface hydroxyl groups of MnO2 particles and As was responsible for the effective As adsorption process, and the oxidation of As (III) to arsenate (As (V)) was achieved via the generated radicals. The influences of representative environmental factors on As removal performance and the application of the MnO2–H2O2 system in river water and ground water were further studied and tested. In conclusion, the MnO2–H2O2 system offers several advantages, including low cost, ease of operation, and strong environmental adaptability, making it highly promising for practical water treatment applications.

Arsenic removal performance and mechanism from water on iron hydroxide nanopetalines

Human health has been seriously endangered by arsenic pollution in drinking water. In this paper, iron hydroxide nanopetalines were synthesized through a precipitation method using KBH4 and their performance and mechanism of As(V) and As(III) removal were investigated. The prepared material was characterized by SEM–EDX, XRD, BET, zeta potential and FTIR analyses. Batch experiments indicated that the iron hydroxide nanopetalines exhibited more excellent performance for As(V) and As(III) removal than ferrihydrite. The adsorption processes were very fast in the first stage, followed a relatively slower adsorption rate and reached equilibria after 24 h, and the reaction could be fitted best by the pseudo-second order model, followed by the Elovich model. The adsorption isotherm data followed to the Freundlich model, and the maximal adsorption capacities of As(V) and As(III) calculated by the Langmuir model were 217.76 and 91.74 mg/g at pH 4.0, respectively, whereas these values were 187.84 and 147.06 mg/g at pH 8.0, respectively. Thermodynamic studies indicated that the adsorption process was endothermic and spontaneous. The removal efficiencies of As(V) and As(III) were significantly affected by the solution pH and presence of PO43– and citrate. The reusability experiments showed that more than 67% of the removal efficiency of As(V) could be easily recovered after four cycles. The SEM and XRD analyses indicated that the surface morphology and crystal structure before and after arsenic removal were stable. Based on the analyses of FTIR, XRD and XPS, the predominant adsorption mechanism was the formation of inner-sphere surface complexes by the surface hydroxyl exchange reactions of Fe–OH groups with arsenic species. This research provides a new strategy for the development of arsenic immobilization materials and the results confirm that iron hydroxide nanopetalines could be considered as a promising material for removing arsenic from As-contaminated water for their highly efficient performance and stability.

Efficient Sorption of Arsenic on Nanostructured Fe-Cu Binary Oxides: Influence of Structure and Crystallinity.

To study the structure-performance relationship, a series of nanostructured Fe-Cu binary oxides (FCBOs) were prepared by varying synthesis conditions. The obtained binary oxides were well characterized using X-ray diffraction (XRD), transmission electron microscope (TEM), Brunner-Emmet-Teller (BET), magnetic and Zeta potential measurement techniques. Both As(V) and As(III) sorption on the FCBOs were evaluated by batch tests. Results show that the surface structure and crystallinity of FCBOs are greatly dependent on preparation conditions. The crystallinity of FCBOs gradually increases as the synthesis pH value increasing from 9.0 to 13.0, from amorphous phase to well-crystalline one. Simultaneously, the morphology change of FCBOs from irregular agglomerate to relatively uniform polyhedron has been observed. The sorption of arsenic is greatly influenced by the crystallinity and structure of FCBOs, decreasing with increasing degree of crystallinity. The amorphous FCBO has higher surface hydroxyl density than well-crystalline one, which might be the reason of higher sorption performance. As(V) is sorbed by the FCBOs via formation of inner-sphere surface complexes and As(III) is sorbed through formation of both inner- and outer-sphere surface complexes. This investigation provides new insights into structure-performance relationship of the FCBO system, which are beneficial to develop new and efficient sorbents.

Facile synthesis of novel tremella-like Mn0@Mn2O3 and its exceptional performance on removal of phosphate

Phosphorus is an essential nutrient for biological growth, but its excessive presence in water can lead to eutrophication. Reducing phosphorus concentrations is an important way to prevent eutrophication of surface water bodies. In this study, a novel zero-valent manganese (ZVM) Mn0@Mn2O3 sorbent was synthesized via a facile one-step reduction method at ambient temperature. The morphology and surface properties of the Mn0@Mn2O3 were analyzed by Scanning electron microscopy (SEM), X-ray diffraction (XRD), Zeta potential analyzer, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The obtained amorphous sample consists of many small tremella-like particles. The as-prepared sorbent shows exceptional phosphate removal performance and has a maximum adsorption capacity of 113.7 mg-P/g at pH 7.0, superior to many reported adsorbents. The removal of phosphate is obviously pH-dependent and is favorable under near-neutral and weak-alkaline conditions (pH 6.5–8.5). The kinetics data fit well to the pseudo-second-order model. The coexisting SO42-, CO32- and Cl- have no great influence on phosphate sorption, whereas, the SiO32- shows negative effect at high concentration. The sorbent is effective for phosphate removal from real municipal wastewater. The results of ionic strength effect experiment, SEM, XRD, FTIR and XPS analyses reveal that the phosphate was mainly removed through surface precipitation and formation of inner-sphere surface complexes. The Mn0@Mn2O3 is a promising sorbent for phosphate removal from surface water and wastewater, owning to its exceptional performance, high selectivity, environmental friendliness and easy preparation.

The importance of specifically adsorbed ions for electrokinetic phenomena: Bridging the gap between experiments and MD simulations

Molecular Dynamics (MD) simulations are uniquely suitable for providing molecular-level insights into the Electric Double Layer (EDL) that forms when a charged surface is in contact with an aqueous solution. However, simulations are only as accurate in predicting EDL properties as permitted by the atomic interaction models. Experimental ζ-potential values and surface charges could provide a potentially suitable reference to validate and tune the interaction models, if not for the fact that they themselves are a product of imperfect models used to interpret the raw measurement data. Here, we present an approach to tune an interaction model by comparing Electro-Osmotic Flow (EOF) MD simulations against experimental Streaming Current (SC) measurements while minimizing potential modeling errors arising from both approaches. The point that is least susceptible to interpretation and modeling errors is argued to be at the concentration for which zero flow velocity is observed in EOF simulations and a net zero electric current is measured in SC experiments. At this concentration, the ζ-potential is also zero. We were able to match the experimental concentration at which ζ = 0 in MD simulations for a CaCl2 solution at pH 7.5 in contact with fused silica by tuning the ion-surface Lennard-Jones cross interactions. These interactions were found to greatly affect the ion distribution within the EDL and particularly the formation of inner-sphere surface-complexes, which, in turn, affects the electrokinetic flow. With the ion distribution determined explicitly, a series of properties can be calculated unambiguously, such as the capacitance needed for surface complexation models.

Zirconium-modified natural clays for phosphate removal: Effect of clay minerals

Excessive amount of phosphate entering water bodies may cause eutrophication and have detrimental effects on ecosystems. Clay-based materials have been drawing attractive attention in mitigating phosphate release to aquatic environment. In this study, we prepared a series of zirconium (Zr)-modified clays to investigate the effect of clay structure and expansion property on phosphate adsorption. Kaolinite, montmorillonite, and vermiculite were selected as three representative natural clays for Zr modification, and the resulting Zr-modified clays were characterized using various techniques that included powder X-ray diffraction, scanning electron microscopy, and zeta potential measurement. Different Zr-modified clays exhibited substantially different phosphate adsorption behaviors, which may be related to the distinct structural and expansion properties of each clay substrate. Particularly, Zr-modified montmorillonite had fastest phosphate adsorption kinetics and highest phosphate adsorption capacity among all Zr-modified clays, which may be attributed to the good expansion property of montmorillonite that favored the uniform intercalation of Zr species, making the adsorption sites easily accessible by phosphate. Furthermore, all Zr-modified clays showed robust performance for phosphate adsorption under various water chemistry conditions. Combined aqueous sorption and solid characterization analyses suggested that formation of inner-sphere surface complexes may be the primary mechanism for phosphate adsorption by Zr-modified clays.

A nappies management by-product for the treatment of uranium-contaminated waters

The direct disposal of municipal solid waste such as nappies to the environment may create serious pollution problems. Based on the circular economy and waste management concepts, the conversion of nappies and/or their ingredients (such as super absorbent polymer (SAP)) to high added value products is of great importance. In this work, a modified SAP (MSAP) was examined as an adsorbent for treatment of contaminated waters and uranium recovery. Batch experiments and spectroscopic techniques were used to examine the effect of various parameters (pH, contact time, temperature, initial concentration, and ionic strength), and the mechanism of adsorption U(VI) and desorption process. The U(VI) concentration was determined by alpha spectroscopy after addition of 232U standard tracer solution to account for possible interferences during electrodeposition and alpha particle counting. The maximum adsorption monolayer capacity was found to be 217.4 mg/g at pH 4.0 and at 298 K. The adsorption of U(VI) on MSAP seems to occur mainly via the formation of inner-sphere surface complexes between U(VI) and the carboxylic surface moieties of MSAP. The MSAP could satisfactorily be regenerated with 0.1 M Na2CO3 (>90%) and it also shows a promising applicability to real wastewaters contaminated with U(VI).

Lanthanum hydroxide: a highly efficient and selective adsorbent for arsenate removal from aqueous solution.

In the present work, a lanthanum hydroxide adsorbent was prepared by a simple precipitation process, and its arsenic removal performances and adsorption mechanisms were investigated by batch experiments and various techniques including field emission scanning electron microscopy with energy-dispersive X-ray spectrophotometry (FESEM-EDX), Brunauer-Emmett-Teller (BET) analysis, powder X-ray diffraction (p-XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The influence of pH on arsenic removal showed that the lanthanum hydroxide adsorbent can effectively remove As(V) from solution, whereas the As(III) removal was very low, indicating that the lanthanum hydroxide adsorbent can selectively remove As(V) but not As(III). The isotherm study showed that the maximum adsorption capacities of As(V) at pH 5.0 and 9.0 were 299.4 and 192.3 mg/g, respectively, much higher than those of the widely used ferrihydrite. Significant interference on As(V) removal was caused by the presence of phosphate and natural organic acids (NOAs), such as citric acid. Powder XRD, FTIR, and XPS analysis showed that the lanthanum hydroxide was almost transformed into lanthanum arsenate after As(V) adsorption at pH 4.0, while a portion of lanthanum hydroxide remained after As(V) adsorption at pH 6.0 and 9.0. Furthermore, ligand exchange between the hydroxyl groups of the adsorbent and As(V) and the formation of inner-sphere surface complexes could play a central role in arsenic removal which needs further investigation.

Insights into the adsorption mechanism of Al30 polyoxocations-modified graphene oxide nanosheets for efficient removal of phosphate, chromate and selenate oxyanions: A comparative study

Keggin-type aluminum polyoxocation species, Al30, with interesting features resulting from a variety of surface oxygen functional groups, has high adsorption ability of different species so it can be served either as nanohybrids or singly in water purification. Insight into surface complex structures of various oxyanions with different activities on Al30 is fundamental for both mechanism and application in wastewater treatment. Reported here is an adsorption behavior investigation of three environmentally problematic oxyanions (phosphate, chromate, and selenate) with different activities on Al30. For this purpose, firstly, Al30 species were consolidated on graphene oxide to use as the sorbent material. Performance of the proposed sorbent evaluated by kinetics and isotherm studies was representative of good agreement of adsorption data with Langmuir isotherm model and pseudo-second-order kinetics for all oxyanions. The underlying molecular binding mechanisms of these oxyanions onto Al30 were revealed by FT-IR spectroscopy, desorption studies, TGA, as well as studying adsorption trends in different pH values. The obtained results boded predominant formation of inner-sphere surface complexes of phosphate and chromate and common contribution of inner- and outer-sphere complexes for selenate, increasing the mobility of selenate. Competitive adsorption studies exhibited a more adsorption affinity and selectivity of Al30 towards phosphate than chromate and selenate, while this was contrary to the result of the single-ion adsorption case, where less adsorption capacity was obtained for phosphate than the other ones.

Recyclable high-affinity arsenate sorbents based on porous Fe2O3/La2O2CO3 composites derived from Fe-La-C frameworks

Efficient removal of aqueous arsenic is still a great challenge in particular for producing clean drinking water and for waste water treatment. To this end, a novel magnetic porous Fe-La composite, comprising La2O2CO3 and Fe2O3, was fabricated via a self-sacrificing template method based on bimetallic (Fe, La)-MOFs calcined at 550 °C. Batch adsorption results demonstrated that Fe-LaXY (X and Y represent the feeding mole ratio of Fe and La precursors) exhibited a maximum adsorption capacity of As(V) (241, 251 and 410 mg g−1 for Fe-La51, Fe-La21 and Fe-La11, respectively), and they also showed satisfactory adsorption kinetics, which can be ascribed to a strong coordination between La and arsenate species. Coexisting sorbates including carbonate, silicate, sulfate, and humic acid had a slight influence on adsorption performance of arsenate, while phosphate isostructural to arsenate gave severe interference. More importantly, the Fe-La11 could be easily separated from water due to its magnetism (7.0 emu g−1), and it also exhibited excellent recyclability (above 80% of removal efficiency at the fifth cycle) as well as stability in terms of release of Fe and La ions (< 0.5 mg L−1 at pH 4.0–10.0). Based on results from X-ray photoelectron spectroscopy (XPS), Powder X-ray diffraction (PXRD) and Raman spectroscopy, it is demonstrated that ligand exchange of surface hydroxyl groups and the formation of inner-sphere surface complexes are main responsible for the As(V) removal mechanism. Combining the magnetic properties, the absorption properties and the recyclability make Fe-LaXY derived from bimetallic MOFs promising sorbents for arsenate removal.

U(VI) adsorption by biochar fiber–MnO2 composites

The removal of U(VI) by biochar fibers from aqueous solutions has been investigated prior and after MnO2 surface-deposition. The removal efficiency has been studied as a function of pH, U(VI) concentration, ionic strength, temperature and contact time. The fibers morphology and surface complexes were analyzed by SEM–EDX and FTIR, respectively. Evaluation of the experimental data indicates that the composite presents extraordinary adsorption capacity (qmax = 3.8 mmol g−1, 904 mg g−1), which is attributed to the formation of inner-sphere surface complexes, and that the adsorption reaction is a relatively fast, endothermic and entropy-driven process.

A facile one-pot hydrothermal synthesis of hydroxyapatite/biochar nanocomposites: Adsorption behavior and mechanisms for the removal of copper(II) from aqueous media

In this study, hydroxyapatite/biochar nanocomposites (HAP/BC-NCs) were synthesized through a simple one-pot hydrothermal process and utilized as an adsorbent for the removal of copper(II) from aqueous media. Characterization results revealed that rod-shaped HAP nanoparticles were successfully incorporated on the surfaces of synthesized HAP/BC-NCs. A set of systematically designed batch experiments were carried out to determine the influences of adsorbent dosage, solution pH, ionic strength, and temperature on the adsorption behavior of the HAP/BC-NCs. Overall findings from batch experiments and extended X-ray absorption fine structure analysis demonstrated that the potential mechanisms responsible for the removal of Cu(II) from aqueous media are cation exchange between Cu2+ in solution and Ca2+ in the HAP on the surfaces of the as-synthesized nanocomposites and the formation of inner-sphere surface complexes on the surfaces of the HAP/BC-NCs. Kinetic studies revealed that the adsorption process follows the pseudo-second-order model and that the overall adsorption rate is controlled by film diffusion as the dominant mechanism and intraparticle diffusion as a secondary mechanism. Adsorption isotherms were accurately represented by a Langmuir isotherm model and the maximum adsorption capacity was determined to be 99.01 mg/g at 298 K, which represents a higher efficiency for Cu(II) adsorption compared to previously reported composite materials. Thermodynamic analysis indicated that the process is thermodynamically spontaneous and endothermic process. Overall, the findings presented in this paper suggest that HAP/BC-NCs have promising applicability for the removal of heavy metals from aqueous media as an alternative, low-cost, and eco-friendly adsorbent for environmental remediation.

Fabrication of porous carbon and application of Eu(III) removal from aqueous solutions

An efficient and easy-made material for elimination of radioactive pollution from environment is highly desirable while remains challenging. Herein, porous carbon material (PC) was fabricated by hydrothermal carbonization method for radionuclide adsorption. The characterization revealed that PC had a large amount of micropores with an average size of 100–200 nm. The adsorption mechanism of Eu(III) on PC were explored by batch and spectroscopic techniques. Batch experiments showed that Eu(III) adsorption onto PC was independent of ionic strength, indicating the formation of inner-sphere surface complexes. The maximum adsorption capacity of PC calculated from Langmuir model were 28.6 mg·g−1 for Eu(III) at pH 6.0. FT-IR and XPS analysis revealed that the interaction of Eu(III) onto PC was mainly ascribed to the surface complexation and/or electrostatic interaction of Eu(III) and hydroxyl groups. This work indicated that PC can be regarded as a promising adsorbent for rapid and efficient sequestration of Eu(III) from aqueous solution.

Enhanced removal of arsenite and arsenate by a multifunctional Fe-Ti-Mn composite oxide: Photooxidation, oxidation and adsorption

In order to attain a high-efficiency and low-cost adsorbent for both arsenate (As(V)) and arsenite (As(III)) removal from As-contaminated water, a novel nanostructured Fe-Ti-Mn composite oxide (FTMO) was fabricated through a one-step simultaneous oxidation and co-precipitation method. Batch control experiments and series of spectroscopy detection technologies were carried out to investigate the surface change of the FTMO adsorbent and the respective role of Fe, Ti and Mn content in the arsenic adsorption process. The results showed that the FTMO adsorbent had a high adsorption capacity for both As(V) and As(III) (especially for the latter one) via the formation of inner-sphere complexes at the water/oxide interface under both darkness and light conditions. The material could effectively oxidize As(III) to As(V) and light illumination could further apparently enhance the As(III) oxidation, thus achieving high adsorption efficiency of As(III). Combined with the characterizations from FTIR, ESR and XPS, it was assumed that the predominant As(III) removal mechanism could be attributed to the coupling of various processes including photooxidation, oxidation and adsorption. The Ti and Mn contents were dominant for the As(III) oxidation, while the Fe content mainly played an important role for the adsorption of newly formed As(V). However, the involvement of surface hydroxyl groups and the formation of inner-sphere surface complexes were primarily responsible for the As(V) adsorption mechanism. Moreover, the successful removal of arsenic from real water matrices made the FTMO a potentially attractive adsorbent for both As(V) and As(III) removal.

Combining batch technique with theoretical calculation studies to analyze the highly efficient enrichment of U(VI) and Eu(III) on magnetic MnFe2O4 nanocubes

The progress of efficient and low-cost materials for environmental remediation is highly desirable but remains challenging. Herein, we fabricated magnetic bimetal oxide MnFe2O4 nanocubes (NCs) by co-precipitation phase inversion method. Characterization of the as-prepared nanomaterials revealed that MnFe2O4 NCs had an average size of 50–70 nm and possessed unique magnetic properties for adsorbent separation. The adsorption behavior and interaction mechanism of U(VI)/Eu(III) on MnFe2O4 were explored by batch experiment and spectroscopy analysis combined with density functional theory (DFT) calculations. Batch experimental results showed that U(VI)/Eu(III) sorption onto MnFe2O4 were independent of ionic strength, indicating the formation of inner-sphere surface complexes. The maximum sorption capacities of MnFe2O4 calculated from Langmuir isotherm model at 298.15 K were 119.90 mg·g−1 for U(VI) at pH 5.0 and 473.93 mg·g−1 for Eu(III) at pH 7.0, respectively. Furthermore, the excellent regeneration and reusability of MnFe2O4 could support long-term application in wastewater and sewage treatment. The analysis of FT-IR and XPS in combination with DFT calculations revealed that the interaction of U(VI)/Eu(III) onto MnFe2O4 was mainly ascribed to the strong ionic bonds (MO) and hydroxyl (OH) groups via electrostatic interaction and surface complexation. The resulting MnFe2O4 further exhibited high effective retention and recovery for U(VI)/Eu(III) from synthetic water systems and real seawater. This work highlighted that MnFe2O4 NCs were exceptionally capable in rapidly and efficiently sequestering U(VI)/Eu(III) from natural water and sewage, which would be a great prospect and further widely used in other lanthanides/actinides remediation.

Interaction of U(VI) with ternary layered double hydroxides by combined batch experiments and spectroscopy study

Uranium has attracted sustained attention due to its high mobility and biotoxicity. Among various techniques, sorption is considered as the most effective technique to treat the U(VI)-containing wastewater. In serving for low-cost adsorbents, layered double hydroxides (LDHs) have been proved to be promising candidates. However, the interaction mechanism of U(VI) with LDHs is still unclear, although plenty of LDHs have been used in this field. Herein, we developed two novel methods to fabricate ternary LDHs (MgFeAl LDHs and NiFeAl LDHs) and applied the ternary LDHs as adsorbents to investigate their sorption performance towards U(VI). The results showed that the sorption was endothermic and followed the pseudo-second-order kinetics through the formation of inner-sphere surface complexes, and the sorption isotherms were simulated by the Langmuir model well. Furthermore, systematic spectroscopy characterization indicated that the sorption occurred on the surface sites of Mg-OH and Ni2+–OH, while the CO32− in the gallery space also played an important role. This work highlighted the synthesis of high-efficient materials and their application in wastewater treatment.

Mechanisms of Uranyl Sequestration by Hydrotalcite.

Since the advent of large-scale U mining, processing, and enrichment for energy or weapons production, efficient capture and disposal of U, transuranics, and daughter radionuclides has constituted an omnipresent challenge. In this study, we investigated uranyl (UO22+) sequestration by hydrotalcite (HTC) as a coprecipitation or surface adsorption reaction scenario. The master variables of the study were pH (7.0 and 9.5) and CO2 content during the reactions (CO2-rich, CO2r vs CO2-depleted, CO2p). In addition, we compared the outcomes of U–HTC coprecipitation reactions between pristine salt precursors and barren U mine wastewater (lixiviant). Extended X-ray absorption fine structure spectra revealed that uranyl adsorbs on the HTC surface as inner-sphere complexes in CO2r and CO2p systems with U–Mg/Al interatomic distances of ∼3.20 and ∼3.35 Å indicative of single-edge (1E) and double-edge (2E) sharing complexes, respectively. Partial coordination of uranyl by carbonate ligands in CO2r systems does not appear to hinder surface complexation, suggesting ligand-exchange mechanisms to be operative for the formation of inner-sphere surface complexes. Uranyl symmetry is maintained when coprecipitated with Al and Mg from synthetic or barren lixiviant solutions, precluding incorporation into the HTC lattice. Uranyl ions, however, are surrounded by up to 3–5 Mg/Al atoms in coprecipitated samples interfering with HTC crystal growth. Future research should explore the potential of Fe(II) or Mn(II) to reduce U(VI) to U(V), which is conducive for U incorporation into octahedral crystal lattice positions of the hydroxide sheet.

Salt effects on Re(VII) and Se(IV) diffusion in bentonite

The through-diffusion method was employed to investigate the effect of the salt concentration on the diffusion behaviour of Re(VII) and Se(IV) in compacted bentonite. The accessible porosity, εacc, of Re(VII) increased from 0.25 to 0.38 with an increase of the ionic strength from 0.1 to 1.0mol/L. The dependency of the accessible porosity εacc on the salt concentration could be described approximately by a first-order decay exponential function. The interlayer porosity of GMZ bentonite was 21% of the total porosity at a bulk dry density of 1600kg/m3 at 12°C. The De values of Re(VII) and Se(IV) increased from 2.6×10−11 to 6.1×10−11m2/s and from 1.6×10−11 to 4.7×10−11m2/s in GMZ bentonite with an increase of the ionic strength from 0.1 to 1.0mol/L, respectively. In the case of Se(IV), the adsorption showed a different ionic strength dependency in GMZ bentonite and montmorillonite. The ionic strength has no significant effect on the adsorption (Kd=1.8×10−4m3/kg) on GMZ bentonite, indicating the formation of inner-sphere surface complexes. In the case of synthetic montmorillonite, the Kd values increased with increasing ionic strength. This can be explained by the fact that the electric potential of surface sites becomes less negative with increasing ionic strength resulting in an increase in the adsorption of Se(IV) at a pH above the point of zero salt effect (PZSE). The De values of both Re(VII) and Se(IV) increased with increasing salt concentration. Both anions are totally excluded from the interlayer space and partially from the diffuse double layer. Ionic strength alters the effective diffusion coefficient of anions by controlling the thickness of the diffuse double layer. Anion diffusion data from the literature (effective and apparent diffusion coefficients) were satisfactorily evaluated by Archie's relation.

Highly efficient removal of phosphate using a novel Fe-La binary (hydro) oxide as adsorbent: behavior and mechanism

Phosphate has been proved to be one of the main elements causing eutrophication of water bodies. However, the elevated phosphate concentrations in the water bodies are often due to the excess discharge of wastewater containing phosphate. Therefore, treatment of elevated phosphate-containing wastewater is of vital importance to alleviate the situation. In this study, a novel Fe-La binary (hydro) oxide adsorbent was synthesized via a facile coprecipitation method for effective phosphate removal from wastewater. The adsorbent grains were formed of primary nanoparticles. Batch experiments were carried out to investigate adsorption kinetics and equilibrium. The adsorption was fast and well fitted by the pseudo-second-order equation, indicating that the adsorption process might be chemical sorption. The adsorption isotherms could be well described by Freundlich model and the maximal adsorption capacity reached 80.7 mg/g at pH 6.0 +/- 0.1 and 67.4 mg/g at pH 9.0 +/- 0.1, respectively, much higher than many reported adsorbents. The phosphate adsorption was pH dependent and weakly acidic condition was favorable for the adsorption. The phosphatead sorption might be mainly achieved via the replacement of surface hydroxyl groups at the solid surface and the formation of inner-sphere surface complexes. Moreover, the spent adsorbent could be effectively regenerated using dilute NaOH solution and used repeatedly. All these results suggest that the adsorbent maybe a promising alternative for phosphate removal.