The enhancement roles of layered double hydroxide on the reductive immobilization of selenate by nanoscale zero valent iron: Macroscopic and microscopic approaches

Zero-valent iron (ZVI) facilitated in-situ selenium (Se) immobilization and its recovery by magnetic separation: Mechanisms and implications for microbial ecology

Selenium (Se(VI)) is environmentally toxic. One of the most popular reducing agents for Se(VI) remediation is zero-valent iron (ZVI). However, most ZVI studies were carried out in water matrices, and the recovery of reduced Se has not been investigated. A water-sediment system constructed using natural sediment was employed here to study in-situ Se remediation and recovery. A combined effect of ZVI and unacclimated microorganisms from natural sediment was found in Se(VI) removal in the water phase with a removal efficiency of 92.7 ± 1.1% within 7 d when 10 mg L-1 Se(VI) was present. Soluble Se(VI) was removed from the water and precipitated to the sediment phase (74.8 ± 0.1%), which was enhanced by the addition of ZVI (83.3 ± 0.3%). The recovery proportion of the immobilized Se was 34.2 ± 0.1% and 92.5 ± 0.2% through wet and dry magnetic separation with 1 g L-1 ZVI added, respectively. The 16 s rRNA sequencing revealed the variations in the microbial communities in response to ZVI and Se, which the magnetic separation could potentially mitigate in the long term. This study provides a novel technique to achieve in-situ Se remediation and recovery by combining ZVI reduction and magnetic separation.

Overlooked encounter process that affects physical behaviors of stabilized nanoscale zero-valent iron during in situ groundwater remediation

Dynamic encountering between groundwater matrices and nanoscale zero-valent iron (NZVI) injected for in situ subsurface remediation affects NZVI's mobility and has not been well recognized. Polyacrylic acid (PAA)-stabilized NZVI (NZVI-PAA) and Mg(OH)2-coated NZVI (NZVI@Mg(OH)2) were investigated as representative NZVIs stabilized by enhanced electrostatic repulsion and reduced magnetic attraction, respectively. Encounters with divalent cations and humic acid (HA) induced the drastic aggregation and sedimentation (presedimentation) of NZVI-PAA owing to Lewis acid-base interactions and heteroaggregation. In addition, encountered groundwater electrolytes could not effectively provide electrostatic repulsion for NZVI-PAA, resulting in breakthrough ripening dynamics. The presedimentation and ripening behaviors of NZVI-PAA were eliminated and unheeded after mixing the NZVI slurry with groundwater by sonication. In comparison, the encountering process barely impacted NZVI@Mg(OH)2, for which settling was hindered. Although the particle-collector attraction promoted NZVI@Mg(OH)2 adsorption on pristine and hybrid-coated sands, the Langmuirian blocking dynamics of the NZVI@Mg(OH)2 breakthrough demonstrated its high mobility after adsorption sites of sand surface were exhausted. Extended Derjaguin-Landau-Verwey-Overbeek analysis and transport modeling provided insights into overlooked effects of encountering on physical behaviors of different stabilized NZVIs, which should be considered during practical applications under diverse subsurface conditions.

Bioremediation system consisted with Leclercia adecarboxylata and nZVI@Carbon/Phosphate for lead immobilization: The passivation mechanisms of chemical reaction and biological metabolism in soil

Bioremediation is one of the most promising strategies for heavy metal immobilization. A new remediation system was demonstrated in this research, which combined phosphate solubilizing bacteria (PSB) with nZVI@Carbon/Phosphate (nZVI@C/P) composite to remediate lead contaminated soil. Experimental results indicated that the new system (nZVI@C/P + PSB) could effectively convert the labile Pb into the stable fraction after 30 days of incubation, which increased the maximum residual fraction percentage of Pb by 70.58%. The characterization results showed that lead may exist in the forms of Pb₅(PO₄)₃Cl, PbSO₄ and 3PbCO₃·2Pb(OH)₂·H₂O in the soil treated with nZVI@C/P + PSB. Meanwhile, soil enzyme activities and Leclercia abundance were enhanced in the treated soil compared with CK during the incubation time. In addition, the specialized functions (e.g. ABC transporters, siderophore metabolism, sulfur metabolism and phosphorus metabolism) in PSB and nZVI@C/P + PSB group were also enhanced. These phenomena proved that the key soil metabolic functions may be maintained and enhanced through the synergistic effect of incubated PSB and nZVI@C/P. The study demonstrated that this new bioremediation system provided feasible way to improve the efficacy for lead contaminated soil remediation.

Highly efficient removal of Se(IV) using reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO): selenium removal mechanism

Se(IV) removal using nanoscale zero-valent iron (nZVI) has been extensively studied. Still, the synergistic removal of Se(IV) by reduced graphene oxide-supported nanoscale zero-valent iron (nZVI/rGO) has not been reported. In this study, nZVI/rGO was successfully synthesized for Se(IV) removal from wastewater. The effects of different environmental conditions (load ratio, dosage, initial pH) on Se(IV) removal by nZVI/rGO were investigated. When the load ratio is 10%, the dosage is 0.3 g/L, the initial pH is 3, and the removal rate is 99%. The adsorption isotherm and kinetics accorded with the Langmuir isotherm and first-order kinetics models (R² > 0.99). The fitted maximum adsorption capacity reached up to 173.53 mg/g. NZVI/rGo and Se(IV) is a spontaneous endothermic reaction (△G < 0, △H > 0) and is characterized by EDS, XRD, and XPS before and after the reaction, to further clarify the reaction mechanism. The XPS narrow spectrum analysis suggested that Se(IV) was reduced to elemental selenium (Se(0)), while the intermediate Fe(II) was oxidized to form hydroxide precipitation. Therefore, nZVI/rGO was favored for Se-contaminated wastewater remediation.

Selenium Oxoanions Removal from Wastewater by MoS42− Intercalated FeMgAl LDH: Catalytic Roles of Fe and Mechanism Insights

FeMgAl−MoS4 LDH was successfully synthesized by a one-pot hydrothermal process followed by ion-exchange methods, and this novel adsorbent was first conducted for aqueous selenite and selenate elimination. The Fe as a component for metal cation layers of LDHs could modulate the layer charge density, leading to more functional groups inserted into layers, and more importantly, this heterogeneous Fe can catalyze the surface reactions between Se(IV) or Se(VI) with S(-II) for oxoanions sequestration. The mechanisms are ion exchange between functional groups with HSeO3− and SeO32− for Se(IV) or SeO42− for Se(VI), followed by reduction by S(-II) from MoS42− groups. The existence of Fe in LDH cation layers, obviously enhanced the reactions (almost two times more for Se(IV) and three times more for Se(VI), respectively), resulting in satisfying adsorption capacities of 483.9 mg/g and 167.2 mg/g for Se(IV) and Se(VI), respectively. Mechanisms were further revealed by elementary analysis, XRD, FT−IR, SEM−EDX, and XPS, as well as the quantitative study. For sorption kinetics, the calculated values of capacities from the pseudo-second-order model are much closer to the experimental values. For sorption isotherms, Langmuir is better than the Freundlich isotherms model for closer capacities (505 mg/g for selenite and 172 mg/g for selenate). All these results demonstrated that the presence of heterogeneous Fe could catalyze the reduction of Se (IV/VI) for the aqueous system, and maybe other high oxidative states hazardous ions. So FeMgAl−MoS4 is a kind of novel adsorbent that offers a promising multi-functional and highly efficient solution for water selenium purification.

Impacts of shell structure on nitrate-reduction activity and air stability of nanoscale zero-valent iron

Nanoscale zero-valent iron (nZVI) has been intensively studied for pollution control because of its high reductive activity and environmental benignity, but the poor reaction selectivity and the aging problem have limited its practical decontamination application. Here, we shed light on the impacts of nZVI shell structure on its reactivity and air stability by systematically comparing two nZVI materials with distinct iron oxide shells. The nZVI with highly crystalline and weakly hydrophilic shell exhibited ninefold higher intrinsic activity for nitrate reduction and significantly improved air stability than that with amorphous, hydrophilic iron hydroxide oxide shell. The compact-structured crystalline shell of nZVI facilitated more efficient interfacial electronic transfer for nitrate reduction and suppressed side reaction of hydrogen evolution. The protective hematite shell endowed the nZVI with significantly improved anti-aging ability, and the reducing force remained 92.6% after exposed to air for 10 days due to decreased oxygen diffusion. This work provides a better understanding of the pollutant degradation behavior of nZVI and may guide an improved synthesis and environmental application of nZVI.

A Bibliometric Analysis of Research on Selenium in Drinking Water during the 1990-2021 Period: Treatment Options for Selenium Removal

A bibliometric analysis based on the Scopus database was carried out to summarize the global research related to selenium in drinking water from 1990 to 2021 and identify the quantitative characteristics of the research in this period. The results from the analysis revealed that the number of accumulated publications followed a quadratic growth, which confirmed the relevance this research topic is gaining during the last years. High research efforts have been invested to define safe selenium content in drinking water, since the insufficient or excessive intake of selenium and the corresponding effects on human health are only separated by a narrow margin. Some important research features of the four main technologies most frequently used to remove selenium from drinking water (coagulation, flocculation and precipitation followed by filtration; adsorption and ion exchange; membrane-based processes and biological treatments) were compiled in this work. Although the search of technological options to remove selenium from drinking water is less intensive than the search of solutions to reduce and eliminate the presence of other pollutants, adsorption was the alternative that has received the most attention according to the research trends during the studied period, followed by membrane technologies, while biological methods require further research efforts to promote their implementation.

Phosphate functionalized iron based nanomaterials coupled with phosphate solubilizing bacteria as an efficient remediation system to enhance lead passivation in soil

Bioremediation technology has attracted increasing interest due to it efficient, economical and eco-friendly to apply to heavy metal contaminated soil. This study presents a new biological remediation system with phosphate functionalized iron-based nanomaterials and phosphate solubilizing bacterium strain Leclercia adecarboxylata. Different phosphate content functionalized iron-based nanomaterials were prepared, and nZVI@C/P1 (nP: nFe: nC=1:10:200) with high passivation efficiency was selected to combine with PSB for the remediation experiments. The change in lead fraction and microbial community under five conditions (CK, PSB, nZVI@C, nZVI@C/P1, nZVI@C/P1 + PSB) during 10 days incubation were investigate. The results indicated that nZVI@C/P1 + PSB increased the residual fraction of lead by 93.94% compared with the control group. Meanwhile, inoculation of Leclercia adecarboxylata became the dominant microflora in the soil microbial community during the remediation time, improving the utilization rate of phosphate in nZVI@C/P1 and enhancing the passivation efficiency of lead. Experimental findings demonstrated that combining nZVI@C/P1 with PSB could be considered as an efficient strategy for the lead contaminated soil remediation.

Environmental impact of amino acids on the release of selenate immobilized in hydrotalcite: Integrated interpretation of experimental and density-functional theory study

The environmental impact of amino acids on the release of SeO₄^2- immobilized into hydrotalcite (Mg₂Al-LDH) which belongs to the layered double hydroxides (LDHs) family was investigated by experimental study and the observed layer structure of hydrotalcite was verified through density-functional theory (DFT) calculations. Glycine, l-cysteine, and l-aspartic acid, which have smaller molecular sizes, can release SeO₄^2- largely due to intercalation, unstabilization of Mg₂Al-LDH and simple dissolution, while l-tryptophan and l-phenylalanine caused limited SeO₄^2- release due to their larger sizes and aromaticity. XRD patterns for the solid residues after intercalation of amino acids revealed that the layer distance of Mg₂Al-LDH was partially expanded. The main peaks and shoulder features corresponding to d₀₀₃ diffraction were well explained by DFT simulations using glycine as a model: the layer spacing of the main peak is responsible for the remaining SeO₄^2- and singly stacked glycine molecule and the layer spacing of the shoulder peak was well explained by doubly stacked glycine molecules. Hydrogen bonds between amino acids and hydroxyl ions in the metallic layers of Mg₂Al-LDH were responsible for the stable configuration of the intercalated Mg₂Al-LDH. This study indicates potential limitations to the stability of low-level radioactive wastes of ⁷⁹Se in repositories which are affected by smaller molecules of amino acids released through degradation of organic matters in the pedosphere.

Simultaneous Removal of Selenite and Selenate by Nanosized Zerovalent Iron in Anoxic Systems: The Overlooked Role of Selenite

The application of nanosized zerovalent iron (nZVI) for reductive immobilization of selenite (Se(IV)) or selenate (Se(VI)) alone has been extensively investigated. However, as the predominant species, Se(IV) and Se(VI) usually coexist in the environment. Thus, it is essential to remove both species simultaneously in the solution by nZVI. Negligible Se(VI) removal (∼7%) by nZVI was observed in the absence of Se(IV). In contrast, the Se(VI) was completely removed in the presence of Se(IV), and the removal rate and electron selectivity of Se(VI) increased from 0.12 ± 0.01 to 0.29 ± 0.02 h^-1 and from 1% to 4.5%, respectively, as the Se(IV) concentration increased from 0.05 to 0.20 mM. Se(IV) was rapidly removed by nZVI, and Se(VI) exerted minor influence on Se(IV) removal. Se(IV) promoted the generation of corrosion products that were mainly composed of magnetite (26%) and lepidocrocite (67%) based on the Fe K-edge XANES spectra and k³-weighted EXAFS analysis. Fe(II) released during the Se(IV) reduction was not the main reductant for Se(VI) but accelerated the transformation of F(0) to magnetite and lepidocrocite. The formation of lepidocrocite contributed to the enrichment of Se(VI) on the nZVI surface, and magnetite promoted electron transfer from Fe(0) to Se(VI). This study demonstrated that Se(IV) acted as an oxidant to activate nZVI, thus improving the reactivity of nZVI toward Se(VI), which displays a potential application of nZVI in the remediation of Se(IV)- and Se(VI)-containing water.

Adsorption and mechanistic study of the invasive plant-derived biochar functionalized with CaAl-LDH for Eu(III) in water

Herein, we developed the invasive plant-derived biochar (IPB) functionalized with CaAl-LDH at five mass ratios using a physical mixture method, assessed their adsorption perform for Eu(III), and explored the relative mechanisms. Results show that the IPB successfully loaded CaAl-LDH in five composites and their Eu(III) sorption affinities were strongly affected by solution pH, contact time, temperature, and the mass ratio of LDH and IPB. All the sorpiton process for Eu(III) occurred on the heterogeneous surface of five composites and the boundary layer diffusion limited the chemical sorption rate. Interestingly, the CaAl-LDH/IPB composite with high ratio of IPB had higher sorption capacity than the one with high ratio of LDH due to larger porosity of the former. Three mechanisms containing ion exchange between Al and Eu ions, surface complexation with carboxyl- and oxygen-containing functional groups, and precipitation were involved in the Eu(III) sorption, but the dominant sorption mechanism for each CaAl-LDH/IPB composite differed with different mass ratio of CaAl-LDH and IPB. In composite with more IPB (e.g., CaAl-LDH/IPB-13), both ion exchange and surface complexes dominated the sorption process and the intensity of Eu³⁺ was identified with the one of Eu₂O_3. Whereas in composites with high LDH, ion exchange dominated the sorption and the intensity of Eu³⁺ was obviously higher than the one of Eu₂O₃. This research will provide a new perspective for the application of the LDH/biochar materials.

Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to groundwater. SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the maximum adsorption capacity of 93.7 mg g^-1 at pH 3.0. Coexisting anions and dissolved oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0-8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO₂ and V₂O₃. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.

Enhanced removal of tetrachloroethylene from aqueous solutions by biodegradation coupled with nZVI modified by layered double hydroxide

Chlorinated volatile organic compounds, such as tetrachloroethylene (PCE), are the most commonly detected toxic contaminants in groundwater. In this study, the performance of PCE removal by a microbial consortium combined with nZVI modified by layered double hydroxide (nZVI-LDH) was evaluated. The enriched PCE-degrading consortium consisted of 44.49% Clostridium and other potential PCE degraders, and 0.5-2.5 mg/L PCE was completely biodegraded within 4 days. The characterization of nZVI-LDH indicated that LDH was coated on the surfaces of nZVI particles with an increased surface area. The PCE removal kinetics by nZVI-LDH was well described by a second-order model, and the removal rate constant of nZVI-LDH was 0.12 L h/mg, higher than that of native nZVI (0.02 L h/mg). Interestingly, the presence of Cu²⁺ improved the removal efficiency of PCE by nZVI-LDH, owing to its role as a catalyst or medium for charge transfer during reduction. Removal of PCE was enhanced by coupling the PCE-degrading consortium and nZVI-LDH. The initial removal of PCE was mainly dominated by the abiotic degradation and adsorption of nZVI-LDH, and biodegradation then played a major role in the exhaustion of nZVI-LDH. These results suggest that biodegradation coupled with nZVI-LDH has a great potential for applications in the remediation of chlorinated-solvent contaminated groundwater.

Enhanced passivation of lead with immobilized phosphate solubilizing bacteria beads loaded with biochar/ nanoscale zero valent iron composite

Phosphate solubilizing bacteria (PSBs) can effectively enhance the stability of lead via the formation of insoluble Pb-phosphate compounds. This research presents a bio-beads, which was implemented with the help of a self-designed porous spheres carrier, by immobilized PSBs strains Leclercia adecarboxylata (hereafter referred as L1-5). In addition, the passivation efficiency of lead via bio-beads under different conditions and its mechanism were also investigated in this study. The results indicated that phosphate solubilized by bio-beads could reach 30 mg/L in Ca₃(PO₄)₂ medium containing 1 mM Pb²⁺, and the highest removal rate of Pb²⁺ in beef peptone liquid medium could reach 93%, which is better than that of free bacteria. Furthermore, it was also concluded that the lead could be transformed into stable crystal texture, such as Pb₅(PO₄)₃Cl and Pb₅(PO₄)₃OH. Both hydrophobic and hydrophilic groups in the bio-beads could capture Pb²⁺, which indicated that electrostatic attraction and ion-exchange were also the mechanism of Pb²⁺ adsorption. All the experimental findings demonstrated that this bio-bead could be consider as an efficient way for the lead immobilization in contaminated soil in the future.

Performance of Halloysite-Mg/Al LDH Materials for Aqueous As(V) and Cr(VI) Removal

This research focused on the investigation of layered double hydroxide (LDH)/halloysite materials’ adsorption efficiency and mechanisms in reactions with aqueous As(V) and Cr(VI) in a broad pH range. The materials consisting of Mg/Al LDH and halloysite were synthesized using both direct precipitation and physical mixing methods. The XRD, FTIR, DTA, SEM and XPS methods were used to evaluate the quality of the obtained materials and get insight into removal mechanisms. The XRD, FTIR and DTA confirmed LDH formation and showed the dominating presence of intercalated carbonates in the LDH structure. The SEM of the materials revealed characteristic agglomerates of layered LDH particles deposited on halloysite tubular forms. The raw LDH phases showed high removal efficiency of both As(V) and Cr (VI) for initial pH in the range of 3–7. In the studied concentration range the materials containing 25 wt % of LDH exhibited a removal efficiency very similar to the raw LDH. In particular, the halloysite presence in the materials’ mass had a positive effect in the reactions with As(V), which was removed by chemisorption. At a low pH the LDH component underwent partial dissolution, which lowered the adsorption efficiency. Apart from the anion exchange mechanism at a low pH the Cr(VI) was removed via formation of MgCrO₄ with Mg (II) being released from the LDH structure. The XPS spectra for As(V) did not show changes in oxidation state in the reactions. In turn, a partial reduction of Cr(VI) to Cr(III) was observed, especially at a high pH. The use of materials composed of two different minerals is promising due to reduction of costs as well as prevention of adsorbent swelling. This opens the possibility of its use in dynamic adsorption flow through systems.

Kinetics and mechanism of selenate and selenite removal in solution by green rust-sulfate

This work investigated the removal of selenite and selenate from water by green rust (GR) sulfate. Selenite was immobilized by simple adsorption onto GR at pH 8, and by adsorption-reduction at pH 9. Selenate was immobilized by adsorption-reduction to selenite and zero valent selenium (Se0) at both pH 8 and 9. In the process, GR oxidized to a mixture of goethite (FeOOH) and magnetite (Fe3O4). The kinetics of selenite and selenate sorption at the GR-water interface was described through a pseudo-second-order model. X-ray absorption spectroscopy data enabled to elucidate the concentration profiles of Se and Fe species in the solid phase and allowed to distinguish two removal mechanisms, namely adsorption and reduction. Selenite and selenate were reduced by GR through homogeneous solid-phase reaction upon adsorption and by heterogeneous reaction at the solid-liquid interface. The selenite reduced through heterogeneous reduction with GR was adsorbed onto GR but not reduced further. The redox reaction between GR and selenite/selenate was kinetically described through an irreversible second-order bimolecular reaction model based on XAFS concentration profiles. Although the redox reaction became faster at pH 9, simple adsorption was always the fastest removal mechanism.

Novel synthesis of nanoscale zerovalent iron from coal fly ash and its application in oxidative degradation of methyl orange by Fenton reaction

We firstly developed a novel synthesis method of nanoscale zerovalent iron (NZVI) using Fe sources in coal fly ash (CFA) for the oxidative degradation of methyl orange by Fenton reaction. Hydrochloric acid (HCl) and methyl isobutyl ketone (MIBK) were used for Fe dissolution from CFA and selective Fe(III) chelation, respectively. Among varied HCl concentrations, 7 N HCl showed the best performance for the oxidation of aqueous Fe(II) to Fe(III) and efficient chelation of Fe(III) with MIBK. The NZVI-CFA was synthesized by adding NaBH₄ to a solution of Fe(III)-chelated MIBK, yielding NZVI transformation >95% from Fe(III) in HCl. Various surface analyses were performed to characterize the NZVI-CFA, which was almost identical to typical NZVI-Bare. HCl and MIBK could be reused several times, indicating potential reusability of chemicals used in the synthesis. Remarkable >96% decolorization of methyl orange was obtained by the NZVI-CFA-induced Fenton reaction at pH 3, with a ∼22% decrease in total organic carbon in 7 min. The heterogeneous Fenton reaction initiated by NZVI-CFA with H₂O₂ showed reactivity similar to that of the homogeneous Fenton reaction (i.e., aqueous Fe(II) with H₂O₂), indicating the importance of homogeneous reaction for the oxidative degradation of methyl orange.

High-quality Al@Fe-MOF prepared using Fe-MOF as a micro-reactor to improve adsorption performance for selenite

High-quality Al@Fe-MOF was prepared by in situ modification of Fe-MOF with Al³⁺ to improve the adsorption performance for selenite (Se(Ⅳ)). The structures and properties of Al@Fe-MOF were characterized by powder X-ray diffraction, high resolution transmission electron microscope, X-ray photoelectron spectroscopy (XPS), nitrogen isothermal adsorption-desorption measurement and zeta potential. The adsorption performance of Al@Fe-MOF for Se(Ⅳ) was studied by batch adsorption experiments. A large number of pores in Al@Fe-MOF were filled by AlOOH and some bayerite formed on the surfaces. Compared with those of Fe-MOF, the specific surface area (SSA) and microporosity of Al@Fe-MOF decreased to 1368 m²/g and 38.5%, respectively. Hydrolysis occurred at pH > 5.0 for Fe-MOF, but did not for Al@Fe-MOF at the pH range of 3.0-7.0. Compared with in Fe-MOF, the adsorption capacity and efficiency of SSA for Se(Ⅳ) were increased by 77% and 112%, and the average free energy of adsorption was increased to 11.62 kJ/mol in Al@Fe-MOF. Besides, the Se(Ⅳ) adsorption amount of Al@Fe-MOF was almost not influenced by the pH from 3.0 to 7.0. The high resolution XPS (HR-XPS) and pH analysis indicated that Al species in Al@Fe-MOF could significantly increase the density of adsorption sites to improve its adsorption capacity for Se(Ⅳ).

Stability and stabilizing efficiency of Mg-Fe layered double hydroxides and mixed oxides in aqueous solutions and soils with elevated As(V), Pb(II) and Zn(II) contents

Although the mechanisms of metal(loid) removal from aqueous solutions using LDHs (layered double hydroxides) and mixed oxides (thermally treated LDHs; CLDHs) have been studied, research dealing with their stability, stabilizing efficiency and remediation potential for contaminated soils remains scarce. We present a complex study investigating the stabilizing efficiency of Mg-Fe LDHs and CLDHs at different conditions, including aqueous solutions and real soils with highly elevated As(V), Pb(II) and Zn(II) concentrations. All studied materials showed excellent (ad)sorption efficiency for As(V), Pb(II) and Zn(II) in aqueous solutions. Additionally, the reconstruction ability of CLDHs at different conditions that could improve their adsorption properties was also evaluated, and the dependence on time, pH and the concentrations of metal(loid)s was shown. In general, CLDHs showed higher stability and stabilizing efficiency in aqueous and soil solutions; however, LDHs were more efficient in contaminated soils. Furthermore, solid state analyses coupled with geochemical modeling showed the formation of new phases corresponding to Mg‑carbonates/silicates on the surfaces of LDH/CLDH after their incubation in soils. Both LDHs and CLDHs significantly decreased the bioavailable/labile fraction of As(V) and Zn(II) in the studied soils. In general, our work shows Mg-Fe LDHs and CLDHs as prospective materials for water and soil remediation.

A phenomenological reaction kinetic model for Cu removal from aqueous solutions by zero-valent iron (ZVI)

Zero-valent iron (ZVI) being an inexpensive and eco-friendly catalyst has drawn great attention in removal of heavy metals from wastewaters. However, quantitative understandings of ZVI processes are significantly deficient. To compensate for the lack of quantitative analyses of removal of heavy metals by ZVI, a phenomenological reaction kinetic model was newly developed for removal of Cu chosen as a typical heavy metal from acidic aqueous solutions by ZVI. The novel kinetic model is based on the adsorption of Cu²⁺ and H⁺ onto ZVI surface and subsequent Cu²⁺ reduction on ZVI surface and Fe²⁺ elution from ZVI. Batch experiments were conducted to elucidate effects of pH and Cu loading on Cu removal by ZVI in acidic aqueous solutions and to validate the proposed phenomenological reaction kinetic model. The quick and complete removals of 1.57 mM Cu were established in the rage of pH 2-5. Although the maximum Cu removal rate was obtained at pH 4, effects of pH were insignificant. In the range of Cu loading from 0.393 to 4.72 mM, almost complete Cu removals were obtained at pH 4 within 35 min. The changes in concentrations of Cu²⁺, Fe²⁺, H⁺ and dissolved oxygen were strongly linked with each other. They could be successfully simulated by the proposed model with the average correlation coefficient of 0.979. The capability of the phenomenological reaction kinetic model for dynamic simulation of Cu removal by ZVI under acidic conditions was confirmed.

Macroscopic and microscopic investigation of uranium elimination by Ca–Mg–Al-layered double hydroxide supported nanoscale zero valent iron

Ca–Mg–Al-LDH/nZVI nanocomposites showed excellent U(vi) removal performance from aqueous solutions through the coordination of reduction and adsorption reactions.