The comparison of Se(IV) and Se(VI) sequestration by nanoscale zero-valent iron in aqueous solutions: The roles of solution chemistry

Effective selenate removal using pH modulated synthesis of biogenic jarosite: Comparative insight with non-biogenic jarosite and biogenic schwertmannite

Selenium, a crucial trace element for many organisms, including prokaryotes and humans, is toxic at high concentrations, necessitating its removal from wastewater. This study investigates the use of jarosite, a naturally occurring iron sulfate mineral with excellent heavy metal attenuation properties, for selenate (Se(VI)) removal for the first time. Biogenic jarosite was synthesized through Fe(II) oxidation by Acidithiobacillus ferrooxidans at an initial pH ranging from 1.5 to 4.0 (J-1.5 to J-4.0). This resulted in the formation of morphologically diverse particles of biogenic jarosite owing to varying Fe(II) oxidation and precipitation rates. For comparative analysis, non-biogenic jarosite (J-90C) and biogenic schwertmannite (S-2.5) were also synthesized. At 0.2 mM initial Se(VI) concentration, J-2.5 demonstrated superior Se(VI) removal compared to J-3.5 and J-90C. At 2.0 mM Se(VI), J-2.5 still outperformed J-3.5 and J-90C although its overall removal efficiency decreased. Notably, at 0.2 mM concentration, Se(VI) removal by J-2.5 was 63 %, which is comparable to 77 % removal by S-2.5. Furthermore, sulfate release from J-2.5 was significantly lower than that from S-2.5 in both Se-free and Se-containing solutions. This study provides critical insights into the synthesis and application of biogenic jarosite as a replacement for metastable schwertmannite, emphasizing its potential as an excellent Se sink for wastewater treatment.

Effects of common dissolved anions on the efficiency of Fe0-based remediation systems

Quiescent batch experiments were conducted to evaluate the influences of Cl-, F-, HCO3-, HPO42-, and SO42- on the reactivity of metallic iron (Fe0) for water remediation using the methylene blue (MB) method. Strong discoloration of MB indicates high availability of solid iron corrosion products (FeCPs). Tap water was used as an operational reference. Experiments were carried out in graduated test tubes (22 mL) for up to 45 d, using 0.1 g of Fe0 and 0.5 g of sand. Operational parameters investigated were (i) equilibration time (0-45 d), (ii) 4 different types of Fe0, (iii) anion concentration (10 values), and (iv) use of MB and Orange II (O-II). The degree of dye discoloration, the pH, and the iron concentration were monitored in each system. Relative to the reference system, HCO3- enhanced the extent of MB discoloration, while Cl-, F-, HPO42-, and SO42- inhibited it. A different behavior was observed for O-II discoloration: in particular, HCO3- inhibited O-II discoloration. The increased MB discoloration in the HCO3- system was justified by considering the availability of FeCPs as contaminant scavengers, pH increase, and contact time. The addition of any other anion initially delays the availability of FeCPs. Conflicting results in the literature can be attributed to the use of inappropriate experimental conditions. The results indicate that the application of Fe0-based systems for water remediation is a highly site-specific issue which has to include the anion chemistry of the water.

A critical review on selenium removal capacity from water using emerging non-conventional biosorbents

Anthropogenic-driven selenium (Se) contamination of natural waters has emerged as severe health and environmental concern. Lowering Se levels to safe limits of 40 μg-L-1 (recommended by WHO) presents a critical challenge for the scientific community, necessitating reliable and effective methods for Se removal. The primary obectives of this review are to evaluate the efficiency of different biosorbents in removing Se, understand the mechanism of adsorption, and identify the factors influencing the biosorption process. A comprehensive literature review is conducted to analyze various studies that have explored the use of modified biochars, iron oxides, and other non-conventional biosorbents for selenium removal. The assessed biosorbents include biomass, microalgae-based, alginate compounds, peats, chitosan, and biochar/modified biochar-based adsorbents. Quantitative data from the selected studies analyzed Se adsorption capacities of biosorbents, were collected considering pH, temperature, and environmental conditions, while highlighting advantages and limitations. The role of iron impregnation in enhancing the biosorption efficiency is investigated, and the mechanisms of Se adsorption on these biosorbents at different pH levels are discussed. A critical literature assessment reveals a robust understanding of the current state of Se biosorption and the effectiveness of non-conventional biosorbents for Se removal, providing crucial information for further research and practical applications in water treatment processes. By understanding the strengths and limitations of various biosorbents, this review is expected to scale-up targeted research on Se removal, promoting the development of innovative and cost-effective adsorbents, efficient and sustainable approaches for Se removal from water.

Coupling of selenate reduction and pyrrhotite oxidation by indigenous microbial consortium in natural aquifer

Pyrrhotite is ubiquitously found in natural environment and involved in diverse (bio)processes. However, the pyrrhotite-driven bioreduction of toxic selenate [Se(VI)] remains largely unknown. This study demonstrates that Se(VI) is successfully bioreduced under anaerobic condition with the participation of pyrrhotite for the first time. Completely removal of Se(VI) was achieved at initial concentration of 10 mg/L Se(VI) and 0.56 mL/min flow rate in continuous column experiment with indigenous microbial consortium and pyrrhotite. Variation in hydrochemistry and hydrodynamics affected Se(VI) removal performance. Se(VI) was reduced to insoluble Se(0) while elements in pyrrhotite were oxidized to Fe(III) and SO₄^2-. Breakthrough study indicated that biotic activity contributed 81.4 ± 1.07% to Se(VI) transformation. Microbial community analysis suggested that chemoautotrophic genera (e.g., Thiobacillus) could realize pyrrhotite oxidation and Se(VI) reduction independently, while heterotrophic genera (e.g., Bacillus, Pseudomonas) contributed to Se(VI) detoxification by utilizing metabolic intermediates generated through Fe(II) and S(-II) oxidation, which were further verified by pure culture tests. Metagenomic and qPCR analyses indicated genes encoding enzymes for Se(VI) reduction (e.g., serA, napA and srdBAC), S oxidation (e.g., soxB) and Fe oxidation (e.g., mtrA) were upregulated. The elevated electron transporters (e.g., nicotinamide adenine dinucleotide, cytochrome c) promoted electron transfer from pyrrhotite to Se(VI). This study gains insights into Se biogeochemistry under the effect of Fe(II)-bearing minerals and provides a sustainable strategy for Se(VI) bioremediation in natural aquifer.

Supporting nanoscale zero-valent iron onto shrimp shell-derived N-doped biochar to boost its reactivity and electron utilization for selenite sequestration

Nanoscale zero-valent iron (nZVI) has been widely used in the reductive removal of contaminants from water, yet it still fights against the inherent passive cover and the raise of medium pH. In this study, nZVI was supported onto a nitrogen-doped biochar (NBC) that was prepared by pyrolyzing shrimp shell for efficiently sequestrating aqueous selenite (Se(IV)). The resultant composite (NBC-nZVI) revealed a higher reactivity and electron utilization efficiency (EUE) than the bare nZVI in Se(IV) sequestration because of the positive charge, the buffering effect and the good conductivity of NBC. The kinetic rate and EUE of NBC-nZVI were increased by 143.4% and 15.3% compared to the bare nZVI, respectively, at initial pH of 3.0. The high removal capacity of 605.4 mg g^-1 for NBC-nZVI was obtained at Se(IV) concentration of 1000 mg L^-1, initial pH of 3.0, NBC-nZVI dosage of 1.0 g L^-1 and contact time of 12 h. Moreover, NBC-nZVI exhibited a strong tolerance to solution pHs and coexisting compounds (e.g., humic acid) and could reduce the Se(IV) concentration from 5.0 mg L^-1 to below the limit of drinking water (50 μg L^-1) in real-world samples. This work exemplified a utilization of shrimp shell-derived NBC to simultaneously enhance the reactivity and EUE of nZVI for reductively removing contaminants.

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.

Removal of estrogen pollutants using biochar-pellet-supported nanoscale zero-valent iron

Nanoscale zero-valent iron-supported biochar pellets (nZVI)-(BP) were synthesized via liquid-phase reduction and applied to estrogen removal, including estrone (E1), 17β-estradiol (E2), and estriol (E3). The performance of nZVI-BP, with respect to its characterization, removal kinetics, and isotherms, was investigated. The results showed that the adsorption equilibrium was reached within 10 min of exposure. The adsorption capacity of estrogen decreased with increasing solute pH and nZVI-BP dosage. The adsorptivity increased with increasing initial estrogen concentration. The estrogen behavior followed a pseudo-second-order kinetic model. The adsorption data of different initial estrogen concentrations fitted to Freundlich adsorption isotherms. In addition, a preliminary discussion of the adsorption mechanism of nZVI-BP for estrogens was provided.

Selenium removal from water using adsorbents: A critical review

Selenium (Se) has become an increasingly serious water contamination concern worldwide. It is an essential micronutrient for humans and animals, however, can be extremely toxic if taken in excess. Sorption can be an effective treatment for Se removal from a wide range of water matrices. However, despite the synthesis and application of numerous adsorbents for remediation of aqueous Se, there has been no comprehensive review of the sorption capacities of various natural and synthesized sorbents. Herein, literature from 2010 to 2021 considering Se remediation using 112 adsorbents has been critically reviewed and presented in several comprehensive tables including: clay minerals and waste materials (presented in Table 1); zero-valent iron, iron oxides, and binary iron-based adsorbents (Table 2); other metals-based adsorbents (Table 3); carbon-based adsorbents (Table 4); and other adsorbents (Table 5). Each of these tables, and their relevant sections, summarizes preparation/modification methods, sorption capacities of various Se adsorbents, and proposed model/mechanisms of adsorption. Furthermore, future perspectives have been provided to assist in filling noted research gaps for the development of efficient Se adsorbents for real-world applications. This review will help in preliminary screening of various sorbent media to set up Se treatment technologies for a variety of end-users worldwide.

Microbial selenate detoxification linked to elemental sulfur oxidation: Independent and synergic pathways

Elevated selenium levels in the environment, with soluble selenate [Se(VI)] as the common chemical species, pose a severe threat to human health. Anaerobic Se(VI) bioreduction is a promising approach for selenium detoxification, and various organic/inorganic electron donors have proved effective in supporting this bioprocess. Nevertheless, autotrophic Se(VI) bioreduction driven by solid inorganic electron donors is still not fully understood. This work is the first to employ elemental sulfur [S(0)] as electron donor to support Se(VI) bioreduction. A batch trial with mixed culture demonstrated the feasibility of this bioprocess, with Se(VI) removal efficiency of 92.4 ± 0.7% at an initial Se(VI) concentration of 10 mg/L within 36 h. Continuous column tests showed that increased initial concentration, flow rate, and introduction of NO₃^--N depressed Se(VI) removal. Se(VI) was mainly bioreduced to solid elemental Se with trace selenite in the effluent, while S(0) was oxidized to SO₄^2-. Enrichment of Thiobacillus, Desulfurivibrio, and Sulfuricurvum combined with upregulation of genes serA, tatC, and soxB indicated Se(VI) bioreduction was coupled to S(0) oxidation. Thiobacillus performed S(0) oxidation and Se(VI) reduction independently. Intermediate metabolites as volatile fatty acids, hydrogen and methane from S(0) oxidation were utilized by heterotrophic Se(VI) reducers for Se(VI) detoxification, indicative of microbial synergy.

Hydrodynamics- and hydrochemistry-affected microbial selenate reduction in aquifer: Performance and mechanisms

Selenate [Se(VI)] with higher content in groundwater will be harmful for human beings. Hence, effective treatment for Se(VI) in aquifer should be conducted reasonably. Microbial reduction of Se(VI) to elemental selenium with weak movability and toxicity has attracted significant attention due to its high efficiency and no secondary contamination. However, hydrodynamic and hydrochemical influences with corresponding mechanisms during Se(VI) bioreduction are still not clear. In this study, influences of flow rate, initial Se(VI) and organic concentrations, coexisting nitrate were evaluated. Se(VI) removal efficiency and capacity reached 96.42 ± 6.82% and 41.28 ± 3.41 (g/m³·d) with flow rate of 0.56 mL/min, initial Se(VI) and chemical organic demand concentrations of 10 mg/L and 400 mg/L. Dechloromonas and Pseudomonas were presumably contributed to Se(VI) reduction, with upregulated serA and tatC genes. Solid Se⁰ was identified as the final product from Se(VI) reduction. These results will be beneficial for the further comprehending of Se(VI) remediation in aquifer.

Distribution, origin and speciation of soil selenium in the black soil region of Northeast China

Selenium (Se) is an essential trace element within human beings that hold with crucial biological functions. Investigating the complex origin of soil Se is of great importance to scientifically approach the land use of Se-rich land use, and the respective promotion of regional economic development. In this study, 160 soil samples from 10 profiles in farmland and woodland were collected in Hailun city, which is a typical black soil region in Northeast China, in order to characterize the distribution and speciation of Se in the black soil, and to identify the origin of soil Se. The total selenium content in the soil ranges from 0.045 to 0.444 μg g^-1, with an average selenium content in black soil (0.318 μg g^-1) of three times greater than that found in the yellow-brown soil (0.114 μg g^-1). The land-use type has a significant influence on the distribution of selenium in the black soil. Moreover, Se and heavy metals have a significant (positive or negative) correlation, in which TOC plays an important role. The black soil presents a consistent REE distribution pattern with underlying yellow-brown soil indicating black soil originates from yellow-brown soil. REE geostatistical analysis suggests that the soil Se partly originates from shale weathering and enriches in black soil. Moreover, elemental geochemical analysis and XRD results show that the paleoclimate change from humid and warm to dry and cold is favorable for organic matter accumulation, resulting in less leaching and enhanced adsorption of selenium into the black soil.

Efficient and selective removal of SeVI and AsV mixed contaminants from aqueous media by montmorillonite-nanoscale zero valent iron nanocomposite

Nanoscale zero-valent iron (NZVI) and NZVI supported onto montmorillonite (NZVI-Mt) were synthetized and used in this study to remove Se^VI and As^V from water in mono- and binary-adsorbate systems. The adsorption kinetics and isotherm data for Se^VI and As^V were adequately described by the pseudo-second-order (PSO) (r²>0.94) and Freundlich (r²>0.93) equations. Results from scanning electron microscopy showed that the dimension of the NZVI immobilized on the Mt was smaller than pure NZVI. Using 0.05 g of adsorbent and an initial 200 mg L^-1 As^V and Se^VI concentration, the maximum adsorption capacity (q_max) and partition coefficient (PC) for As^V on NZVI-Mt in monocomponent system were 54.75 mg g^-1 and 0.065 mg g^-1·μM^-1, which dropped respectively to 49.91 mg g^-1 and 0.055 mg g^-1·μM^-1 under competitive system. For Se^VI adsorption on NZVI-Mt in monocomponent system, q_max and PC were 28.63 mg g^-1 and 0.024 mg g^-1·μM^-1, respectively. Values of q_max and PC were higher for NZVI-Mt than NZVI and montmorillonite, indicating that the nanocomposite contained greater adsorption sites for removing both oxyanions, but with a marked preference for As^V. Future research should evaluate the effect of different operational variables on the removal efficiency of both oxyanions by NZVI-Mt.

Promotional effect of Mn(II)/K2FeO4 applying onto Se(IV) removal

Promotional effect of Mn(II)/K₂FeO₄ [Fe(VI)] applying onto Se(IV) removal was determined for the first time, with description of reaction mechanisms. Four different combinations of water treatment agents [K₂FeO₄ alone, K₂FeO₄ with Al(III) ions, K₂FeO₄ with Fe(III) ions, and K₂FeO₄ with Mn(II) ions] were used for Se removal in spiked deionized water, and K₂FeO₄ in combination with Mn(II) ions showed great removal efficiency. Over 90% of Se(IV) (200 μg/L) was removed within 2 min by using 1 mg/L of K₂FeO₄ and 9 mg/L of Mn(II) ions (pH 7.0, 23 °C). XPS analysis identified that in the reaction process, Se(0) formed on the settlement. It was speculated that Se(IV) was oxidized to Se(VI) by K₂FeO₄, and the Se(VI) species was reduced to insoluble Se(0) by γ-Fe₂O₃-Mn(II) nanocomplex. Insoluble Se(0) adsorbed on the surface of Fe-Mn particle and coprecipitated, thus removed from aqueous solution. As solution pH varied from 4.0 to 8.0, Se(IV) removal ratio ranged from 89% to 98% in the system. Co-existing ions such as Na⁺, Ca²⁺ and SO₄^2- had no intense effect on Se removal, while PO₄^3- and humic acid (HA) inhibited Se removal in Mn(II)/K₂FeO₄ system. Mn(II)/K₂FeO₄ was an effective and convenient way for Se(IV) removal from polluted water.

Influence of humic acid and its different molecular weight fractions on sedimentation of nanoscale zero-valent iron

The effects of humic acid (HA) and its different molecular weight (MW) fractions on the sedimentation of nanoscale zero-valent iron (NZVI) in the absence and presence of cations (i.e., Na⁺/Mg²⁺/Ca²⁺) were investigated. Ultrafiltration (UF) was used as the method of fractionation to obtain four different MW fractions (separated by ultrafiltration membranes of 10 kDa, 50 kDa, and 100 kDa). Differing sedimentation behavior was observed for NZVI with different MW fractions of HA. Generally, the degree of settling of NZVI particles in the presence of high MW fractions of HA was lower than that of low MW fractions of HA and that without HA. The results were mainly attributed to the steric stabilization provided by the high MW fractions of HA. The presence of Na⁺/Mg²⁺/Ca²⁺ alone had insignificant influence on the settling of NZVI, but both Mg²⁺ and Ca²⁺ exerted an obvious influence on the settling of NZVI in the co-presence of HA. The settling behavior of NZVI was further examined in the co-presence of different MW fractions of HA and Ca²⁺. The co-presence of low MW HA fractions and Ca²⁺ led to a lower settling of NZVI. This might be due to the formation of a layer of HA-Ca²⁺ complex on the particle surface, providing stronger steric stabilization. Nevertheless, in the co-presence of high MW HA fractions and Ca²⁺, the settling of NZVI was initially reduced but accelerated with time, which might be due to the gradual aggregation of NZVI with time resulted from the bridging effect of HA-Ca²⁺ complex.

Nanoscale zero valent iron supported on MgAl-LDH-decorated reduced graphene oxide: Enhanced performance in Cr(VI) removal, mechanism and regeneration

The scaled application of nanoscale zero-valent iron nanoparticles (nZVI or Fe° NPs) in environmental remediation is challenged by easy surface passivation and particle aggregation. To improve this situation and enhance their performance in Cr(VI) removal from water phase, we present one novel strategy to hybridize nZVI with layered double hydroxide (LDH) decorated reduced graphene oxide (rGO). The as-prepared ternary (Fe@LDH/rGO) composites possess better dispersibility, improved hydrophilicity and more positive surfaces that allows higher removal efficiency and capacity for Cr(VI) oxyanions. Composition proportion are optimized and influences of surroundings (solution pH, Cr(VI) concentration and temperature) are evaluated. Also, we demonstrate that Fe@LDH/rGO can be reused with suitable post-treatments, which combines alkaline solution desorption and NaBH₄ revivification possess. Cr desorption and Fe leaching ratio during regeneration should be critical indicators that determine the recovery efficiency. Synergistic effect within this ternary system not only contributes to its superiorities in stability, but also continuous iron corrosion via the formation of micro Fe-C batteries, where rGO acts as cathode and alternative electron conductor. The present work suggests great potentials of Fe@LDH/rGO composites in groundwater remediation.

Comparison of biochar- and activated carbon-supported zerovalent iron for the removal of Se(IV) and Se(VI): influence of pH, ionic strength, and natural organic matter

Biochar (BC) and activated carbon (AC) were both produced from corn straw. Biochar-supported zerovalent iron (BC-ZVI) and activated carbon-supported zerovalent iron (AC-ZVI) were synthesized and applied for Se(IV)/Se(VI) removal. The sorption capacity of BC-ZVI for Se(IV) and Se(VI) was reported at 62.52 and 35.39 mg g^-1, higher than that of AC-ZVI (56.02 and 33.24 mg g^-1), respectively, due to its higher iron content and more positive charges. The spectroscopic analyses showed that Se(IV)/Se(VI) were reduced to Se(0)/Se(-II) of less toxicity and solubility. The effects of various factors such as pH, ionic strength, co-existing cations and anions, and natural organic matter (NOM) were also investigated. Ionic strength showed no significant effect on Se(IV)/Se(VI) removal, but pH was critical. The presence of NO₃^- and SO₄^2- did not cause obvious inhibition to the removal, while PO₄^3- inhibited the sorption capacity of BC-ZVI and AC-ZVI for Se(IV)/Se(VI) significantly. Common cations (K⁺, Ca²⁺, and Mg²⁺) were found to slightly enhance the removal, while NOM significantly decreased the sorption capacity of BC-ZVI and AC-ZVI for Se(IV)/Se(VI). Besides, NOM showed stronger inhibition effect on AC-ZVI than that on BC-ZVI. These results indicated that BC-ZVI, compared with AC-ZVI, could be a promising sorbent to remove Se(IV)/Se(VI) due to its low cost and high efficiency.

Kinetics and mechanism of selenite reduction by zero valent iron under anaerobic condition activated and enhanced by dissolved Fe(II)

Batch test was conducted to investigate Se(IV) removal kinetics and mechanism by zero valent iron (ZVI) in presence of Fe(II) under anaerobic condition. Dissolved Fe(II) activated and enhanced Se(IV) reduction by ZVI, which also determined the removal efficiency, reduction rate, final corrosion products and their structures. Se(IV) was completely removed at initial Fe(II)/Se(IV) ≥ 1.0, and the specific rate constant significantly increased from 0.6 to 3.44 L h^-1 m^-2 with the augment of ratio from 1.0 to 1.4. At Fe(II)/Se(IV) < 1.0 (take 0.6 as an example), Raman, XPS, SEM-EDS and XRD results suggested that Se(IV) was reduced to amorphous Se(0) in forms of red suspended solids, amorphous FeSe and crystal maghemite (γ-Fe₂O₃) coated on ZVI surface. At Fe(II)/Se(IV) ≥ 1.0 (take 1.0 and 1.4 as examples), crystal FeSe and magnetite (Fe₃O₄) deposits formed on ZVI surface with a core-shell structure. Additionally, final pH increased due to Se(IV) reduction. This study suggested that traditional ZVI passivation problem could be overcome through the addition of excess dissolved Fe(II) under anaerobic condition, which also provided an alternative method to produce a reactive ammonia-free Fe₃O₄/ZVI/Fe(II) system.

Evaluation of electrokinetics coupled with a reactive barrier of activated carbon loaded with a nanoscale zero-valent iron for selenite removal from contaminated soils

The range between dietary deficient and toxic levels for selenium is quite narrow. In this study, the synergistic effects of electrokinetics (EK) and a permeable reactive barrier (PRB) on the reductive sequestration of Se(IV) oxyanions from spiked soils were investigated in detail. Activated charcoal (AC)-supported Fe(II) and nanoscale zero-valent iron (nZVI) were prepared as the PRB media for use in an electrolyzer. In aqueous equilibrium adsorption tests, the AC-supported nZVI medium had a higher adsorption capacity than that of the other adsorbents. The Se(IV) removal isotherms were well-fitted using the Langmuir model. The Se(IV) removal rates were accurately predicted by both pseudo-first- and pseudo-second-order kinetic models. For the coupled systems, a moderate increase in the number of PRBs and decrease in the PRB thickness in the electrolyzer enhanced the removal and catalytic recovery of Se(IV) from the spiked soil samples. A Se(VI) removal efficiency of approximately 95% and Se(VI) reduction efficiency of 90% were achieved in the optimized electrochemical system. The Se(IV) species were reduced to Se° and FeSe by the AC-supported nZVI regardless of the pH distribution. The experimental results provide guidance for the multichannel recovery of Se from abandoned ore tailings or solid wastes.

Toxicity of sulfide-modified nanoscale zero-valent iron to Escherichia coli in aqueous solutions

Sulfide-modified nanoscale zero-valent iron (S/nZVI) has been widely studied for groundwater remediation, but the potential environmental risks are poorly understood. This study examined the toxicity of S/nZVI to Escherichia coli in aqueous solutions. The sulfidation could reduce toxicity of nZVI, and S/nZVI exhibited a weaker toxicity at lower Fe/S molar ratio, resulting from the lower Fe⁰ content and higher sulfate and iron oxide. The toxicity of S/nZVI was significantly alleviated in the presence of N-Acetyl-L-cysteine (a scavenger for reactive oxygen species (ROS)), revealing that the ROS-induced oxidative stress was the principal mechanism. Moreover, Transmission Electron Microscopy images elucidated that the membranes of S/nZVI-treated cells were disrupted and S/nZVI existed on E. coli surface and in the cytoplasm. S/nZVI might have interacted with the amine, carboxyl, and ester groups on E. coli cell surface, as demonstrated by Fourier Transform Infrared Spectroscopy analysis. However, the presence of individual groundwater component (e.g., Ca²⁺, SO₄^2-, HCO₃^- and humic acid) could more or less alleviate the toxicity of S/nZVI. Furthermore, S/nZVI only exhibited slight toxic effect (<0.15-log after 1 h) in the presence of the mixed components. The same faint toxicity was observed for the aged S/nZVI, indicating that S/nZVI could lose its toxicity over time.

Integration of nanoscale zero-valent iron and functional anaerobic bacteria for groundwater remediation: A review

The technology of integrating nanoscale zero-valent iron (nZVI) and functional anaerobic bacteria has broad prospects for groundwater remediation. This review focuses on the interactions between nZVI and three kinds of functional anaerobic bacteria: organohalide-respiring bacteria (OHRB), sulfate reducing bacteria (SRB) and iron reducing bacteria (IRB), which are commonly used in the anaerobic bioremediation. The coupling effects of nZVI and the functional bacteria on the contaminant removal in the integrated system are summarized. Generally, nZVI could create a suitable living condition for the growth and activity of anaerobic bacteria. OHRB and SRB could synergistically degrade organic halides and remove heavy metals with nZVI, and IRB could reactive the passivated nZVI by reducing the iron (hydr)oxides on the surface of nZVI. Moreover, the roles of these anaerobic bacteria in contaminant removal coupling with nZVI and the degradation mechanisms are illustrated. In addition, this review also discusses the main factors influencing the removal efficiency of contaminants in the integrated treatment system, including nZVI species and dosage, inorganic ions, organic matters, pH, type of pollutants, temperature, and carbon/energy sources, etc. Among these factors, the nZVI species and dosage play a fundamental role due to the potential cytotoxicity of nZVI, which might exert a negative impact on the performance of this integrated system. Lastly, the future research needs are proposed to better understand this integrated technology and effectively apply it in groundwater remediation.

Performance of selenate removal by biochar embedded nano zero-valent iron and the biological toxicity to Escherichia coli

The application of nano zero-valent iron (nZVI) in water environment was limited by its easily aggregation and potential biological toxicity. In this study, biochar embedded nZVI (BC-nZVI) was prepared by carbon-thermal reduction method, and the SEM-EDX mapping results showed that nZVI was successfully embedded on biochar. Meanwhile, BC-nZVI with the optimal Fe/C of 2/1 showed a similar Se(vi) removal efficiency to pure nZVI. Effects of pH, BC-nZVI loading, and initial Se(vi) concentration were studied. Se(vi) removal rates (at 30 min) by BC-nZVI at pH 4.0 and 5.0 were 98.2% and 95.9%, respectively. But Se(vi) removal rate (at 30 min) was sharply decreased to 25.8% at pH 6.0. With the increase of BC-nZVI loading from 0.5 g L⁻¹ to 1 g L⁻¹, Se(vi) removal rate (at 30 min) significantly increased from 25.5% to 95.9%. And the continuous increase of BC-nZVI loading to 2 g L⁻¹ did not improve Se(vi) removal rate. Se(vi) less than 3 mg L⁻¹ was completely removed by BC-nZVI in 30 min, but Se(vi) more than 6 mg L⁻¹ only was removed about 25.9% at 30 min. Optimal parameters were pH 4.0, 2 g L⁻¹ BC-nZVI, and 1.5 mg L⁻¹ Se(vi). Variation of calculated amount, SOD activity, and protein content of Escherichia coli with nZVI and BC-nZVI indicated that nZVI and BC-nZVI both produced negative effects on the growth of E. coli. But the amount and SOD activity of E. coli with pure nZVI was lower than that with BC-nZVI. Moreover, E. coli with nZVI released more protein than that with BC-nZVI. So modified nZVI by biochar was less harmful to E. coli than nZVI.

The application of nano zero-valent iron (nZVI) in water environment was limited by its easily aggregation and potential biological toxicity.

Difunctional chitosan-stabilized Fe/Cu bimetallic nanoparticles for removal of hexavalent chromium wastewater

Bimetallic Fe/Cu nanoparticles were successfully stabilized by chitosan used for remediating hexavatlent chromium contaminated wasterwater. However, the over-loaded chitosan on the surface of Fe/Cu particles limited the Cr(VI) reduction due to the occupation of the surface reactive sites. Weighing the colloid stability and the reduction reactivity, the optimal dosage of chitosan is 2.0 wt% and the optimal Cu doping dosage is 3.0 wt%. SEM and TEM images showed that the chitosan-stabilized Fe/Cu bimetallic nanoparticles (CS-Fe/Cu nanoparticles) were uniformly dispersed, which had loose and porous surface. FTIR characterization showed that the binding sites of nZVI and chitosan. XRD demonstrated that the presence of copper and chitosan did not change the existence form of zero-valent iron. Most importantly, the contribution of chitosan and Cu in the removal mechanism was studied by the reduction experiments and the XPS analysis. On the one hand, chitosan could effectively combine with Cr(VI) due to chelation, on the other hand, Cu played an important role in the precipitation and coprecipitation phenomena. These findings indicate that CS-Fe/Cu has the potential to be a promising material for wastewater treatment.

A novel combined process for efficient removal of Se(VI) from sulfate-rich water: Sulfite/UV/Fe(III) coagulation

The efficient removal of Se(VI) from sulfate-rich water is challenging as most reported processes last for hours to days. In this study, a combined sulfite/UV/Fe(III) coagulation process was proposed for efficient Se(VI) removal from sulfate-rich water within a short time (∼1 h). In the presence of sulfate (1000 mg L^-1), over 99% of Se(VI) (initially at 10 mg L^-1) could be reduced by sulfite (5.0 mM) with a UV dose of 16 J cm^-2 (within 20 min) into Se(IV) as the sole observed product. An alkaline pH (>9) was required for the reduction process, which was naturally obtained with the addition of sulfite. Scavenging experiments with N₂O and NO₃^- both indicated that hydrated electrons (e_aq^-) were responsible for Se(VI) reduction by sulfite/UV. The presence of chloride, sulfate, phosphate, and carbonate (up to 10 mM) showed negligible influence on Se(VI) reduction, whereas nitrate and humic acid inhibited Se(VI) reduction to different extents depending on their concentrations. By Fe(III) coagulation, Se(IV) in the co-presence of sulfite and sulfate was efficiently removed at an OH^-/Fe molar ratio of 1.8-2.8. The removal of Se(IV) by Fe(III) coagulation responded insignificantly to chloride, nitrate, or sulfate (up to 10 mM), whereas it was adversely affected at high levels of carbonate (10 mM) and phosphate (1 mM). The combined sulfite/UV/Fe(III) coagulation process was validated for the efficient removal of Se(VI) from synthetic sulfate-rich solution, simulated wastewater, and authentic smelting wastewater (in 1.1 h). The introduced sulfite underwent minor consumption during UV irradiation and was almost (∼90%) removed after coagulation.

Effects of sulfate on simultaneous nitrate and selenate removal in a hydrogen-based membrane biofilm reactor for groundwater treatment: Performance and biofilm microbial ecology

Effects of sulfate on simultaneous nitrate and selenate removal in a hydrogen-based membrane biofilm reactor (MBfR) for groundwater treatment was identified with performance and biofilm microbial ecology. In whole operation, MBfR had almost 100% removal of nitration even with 50 mg mL^-1 sulfate. Moreover, selenate degradation increased from 95% to approximate 100% with sulfate addition, indicating that sulfate had no obvious effects on nitrate degradation, and even partly promoted selenate removal. Short-term sulfate effect experiment further showed that Gibbs free energy of reduction (majority) and abiotic sulfide oxidation (especially between sulfate and selenate) contributed to degradable performance with sulfate. Microbial ecology showed that high percentage of Hydrogenophaga (≥75%) was one of the contributors for the stable and efficient nitrate degradation. Chemoheterotrophy (ratio>0.3) and dark hydrogen oxidation (ratio>0.3) were the majority of functional profile for biofilm in MBfR, and sulfate led to profiles of sulfate respiration and respiration of sulfur compounds in biofilm. Additionally, no special bacteria for selenate degradation was identified in biofilm microbial ecology, and selenate degradation was relied on Hydrogenophaga (75% of ecology percentage with sulfate addition) and Desulfovibrionaceae (4% of ecology percentage with sulfate addition). But with overloading sulfate, Desulfovibrionaceae was prior to sulfate degradation for energy supply and thus inhibited selenate removal.

Enhanced removal of Se(VI) from water via pre-corrosion of zero-valent iron using H2O2/HCl: Effect of solution chemistry and mechanism investigation

Although the removal of Se(VI) from water by using zero-valent iron (ZVI) is a promising method, passivation of ZVI severely inhibits its performance. To overcome such issue, we proposed an efficient technique to enhance Se(VI) removal via pre-corrosion of ZVI with H₂O₂/HCl in a short time (15 min). The resultant pcZVI suspension was weakly acidic (pH 4.56) and contained abundant aqueous Fe²⁺. ⁵⁷Fe Mössbauer spectroscopy showed that pcZVI mainly consisted of Fe⁰ (66.2%), hydrated ferric oxide (26.3%), and Fe₃O₄ (7.5%). Efficient removal of Se(VI) from sulfate-rich solution was achieved by pcZVI compared with ZVI (in the absence and presence of H₂O₂) and acid-pretreated ZVI. Moreover, the efficient removal of Se(VI) by pcZVI sustained over a broad pH range (3-9) due to its strong buffering power. The presence of chloride, carbonate, nitrate, and common cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) posed negligible influence on the removal of Se(VI) by pcZVI, while the inhibitory effect induced by sulfate, silicate, and phosphate indicated the significance of Se(VI) adsorption as a prerequisite step for its removal. The consumption of aqueous Fe²⁺ was associated with Se(VI) removal, and X-ray absorption near edge structure revealed that the main pathway for Se(VI) removal by pcZVI was a stepwise reduction of Se(VI) to Se(IV) and then Se⁰ as the dominant final state (78.2%). Moreover, higher electron selectivity of pcZVI was attributed to the enhanced enrichment of Se oxyanions prior to their reduction.

Physicochemical transformation of Fe/Ni bimetallic nanoparticles during aging in simulated groundwater and the consequent effect on contaminant removal

To assess the fate and long-term reactivity of bimetallic nanoparticles used in groundwater remediation, it is important to trace the physicochemical transformation of nanoparticles during aging in water. This study investigated the short-term (within 5 d) and long-term (up to 90 d) aging process of Fe/Ni bimetallic nanoparticles (Fe/Ni BNPs) in simulated groundwater and the consequent effect on the particle reactivity. Results indicate that the morphological, compositional and structural transformation of Fe/Ni BNPs happened during the aging. In the 5-d short-term aging, Fe⁰ corrosion occurred rapidly and was transformed to ferrous ions which were adsorbed onto the surface of Fe/Ni BNPs, accompanied by the elevation of solution pH and the negative redox potential. In the long-term aging, scanning electron microscopy (SEM) images show that the particles transformed from spherical to rod-like and further to sheet-like and needle-like. X-ray diffraction (XRD) analysis reveals that the main aging product was magnetite (Fe₃O₄) and/or maghemite (γ-Fe₂O₃) after aging for 60-90 d. Energy dispersive spectrometer (EDS) analysis demonstrates that the mass ratio of Fe/Ni increased with aging, revealing that Ni were possibly gradually entrapped and covered by the iron oxides. Besides, the release of Ni into solution was also detected during the aging. The reactivity of the aged Fe/Ni BNPs was examined by studying its performance in tetracycline (TC) removal. The aged Fe/Ni BNPs within 2 d kept similar removal efficiency of TC as the fresh particles. However, the removal efficiency of TC by Fe/Ni BNPs aged for 5-15 d dropped by 20-50% due to aggregation and oxidation of particles, and the removal efficiency further decreased slowly with the prolongation of aging time up to 90 d. This reveals that Fe/Ni BNPs were vulnerable to passivation in water environments.