Arsenic transformation and redistribution in groundwater induced by the complex geochemical cycling of iron and sulfur

Microbial reductive mobilization of As(V) in solid phase coupled with the oxidation of sulfur compounds: An overlooked biogeochemical reaction affecting the formation of arsenic-contaminated groundwater

Dissimilatory As(V)-respiring prokaryotes (DARPs) are recognized as having a crucial role in the formation of arsenic-contaminated groundwater. DARPs use small-molecule organic acids as electron donor to directly reduce As(V) in solid phase to more mobile As(III). Therefore, DARPs are considered to be heterotrophic bacteria. However, these cannot explain why high concentrations of As(III) are produced in environments lacking soluble organic carbon. We thus propose that reduced sulfur compounds may also be utilized by DARPs and affect the DARPs-mediated arsenic mobilization. This study sought to confirm this hypothesis. Metagenomic investigations on the DARP population derived from As-contaminated soil indicated that approximately 84 % of DARP MAGs possess the enzymes potentially catalyzing the oxidation of S2-, S0, SO32-, or S2O32-. Functional analysis of DARP population and a cultivable strain suggested that DARPs, in addition to small-molecule organic carbon, can effectively use sulfur compounds as electron donor to reduce As(V) to mobile As(III). Arsenic release experiments using DARP population and a cultivable DARP strain showed that DARPs indeed utilized sulfur compounds as the sole electron donors under autotrophic and anaerobic conditions to directly reduce adsorbed As(V) in the soils to mobile As(III). These findings provide new insights into the microbial mechanism responsible for the variation of As(III) concentrations in contaminated groundwater.

Influence of calcium carbonate on ferrihydrite bio-transformation and associated arsenic mobilization/redistribution

The sulfate-reducing bacteria (SRB)-induced ferrihydrite transformation is an important cause for arsenic (As) contamination in the aquifer near mining area. Calcium carbonate (CaCO3) is widespread and has the potential of regulating As fate directly or indirectly. However, the influence of CaCO3 on ferrihydrite transformation and the associated As mobilization/redistribution in SRB-containing environments remains unclear. Therefore, in this research, batch experiments coupled with a series geochemical, spectroscopic, and microscopic technologies were conducted collectively to address the research gap above. The results suggested that under low CaCO3 loading conditions, the reductive transformation of ferrihydrite to Fe-S minerals resulted in a significant release of adsorbed As into the solution and As(V) reduction to more mobile and toxic As(III). Inside the cell of SRB, there existed S and As zone (no Fe), where As(V) might be reduced by S2- without interference of Fe(III). Although the high CaCO3 loadings exerted little effect on Fe(III) reduction, they promoted ferrihydrite transformation into dufrenite and vivianite, which sequestered As effectively via structural incorporation process, retarding As mobilization. Besides, the coprecipitation of As with dufrenite and vivianite enhanced the stability of solid phase As. Overall, the high CaCO3 loadings altered the mineralogical transformation of ferrihydrite and the associated As biogeochemistry significantly in SRB-containing environments. Regulating ferrihydrite transformation and the associated As fate via CaCO3 addition is an effective approach for mitigation of As mobilization in the aquifer near mining area.

Thioarsenate sorbs to natural organic matter through ferric iron-bridged ternary complexation to a lower extent than arsenite

Understanding processes regulating thioarsenate (HxAsSnO4-n3-x; n = 1 - 3; x = 1 - 3) mobility is essential to predicting the fate of arsenic (As) in aquatic environments under anoxic conditions. Under such conditions, natural organic matter (NOM) is known to effectively sorb arsenite and arsenate due to metal cation-bridged ternary complexation with the NOM. However, the extent and mechanism of thioarsenate sorption onto NOM via similar complexation has not been investigated. By equilibrating monothioarsenate (representative of thioarsenate) with a peat (model NOM) with different Fe(III) loadings, this study shows that NOM can sorb monothioarsenate considerably via Fe(III)-bridging. Iron and As K-edge XAS analysis of the monothioarsenate-treated Fe-loaded peats revealed that monothioarsenate forms bidentate mononuclear edge-shared (1E) (RAs···Fe: 2.89 ± 0.02 Å) and bidentate binuclear corner-shared (2C) (RAs···Fe: 3.32 Å) complexes with organically bound Fe(O,OH)6 octahedra, in addition to direct covalent bonds with oxygen-containing functional groups (e.g., -COOH and -OH) (RAs···C: 2.74 ± 0.02 Å), upon equilibration with the Fe(III)-loaded peat. However, the extent of monothioarsenate sorption was considerably less than that of its precursor As species, arsenite, due to higher electrostatic repulsion between the negatively charged monothioarsenate and peat. This study implies that thioarsenate formation under anoxic conditions would increase As mobility by decreasing its sorption onto the NOM.

Effect of sulfidogenesis on ferrihydrite geochemistry and associated arsenic fate

The behavior of arsenic (As) in groundwater is closely related to the sulfidation of ferrihydrite. In the ternary ferrihydrite-As-sulfide system, ferrihydrite can either initially adsorb As before sulfide reduction or first encounter sulfide and then interact with the aqueous As, altering As fate. However, their relative contributions to the mineralogical transformation of ferrihydrite and subsequently associated As mobilization/redistribution remain poorly understood. Therefore, batch experiments combined with chemical, microscopic, and spectroscopic analyses were conducted to clarify the geochemistry of ferrihydrite and its influence on As behavior. Results indicated that in the pre-sorption groups, the secondary minerals were predominantly presented in amorphous phase due to the retardative effect of As. At low sulfide concentrations (S/Fe = 0.04), the content of residual ferrihydrite was large, which favored As immobilization. At high sulfide concentrations (S/Fe = 0.8), however, As was initially released into the solution and subsequently re-immobilized by secondary minerals. The adsorption capacity of the secondary minerals for As decreased with the increase in amorphous mackinawite formation. In the pre-sulfidation groups, rapid ferrihydrite reduction promoted the formation of crystalline minerals, significantly reducing their adsorption capacity. At low sulfide concentrations, the released As was partially adsorbed on the surface of crystalline goethite and lepidocrocite. At high sulfide concentrations, magnetite formed and favored As immobilization through its incorporation into magnetite particles. These results provide important insights into the geochemistry of Fe, S, and As in groundwater systems.

Biogenic Sulfide-Mediated Iron Reduction at Low pH

Acid mine drainage (AMD) pollutes natural waters, but some impacted systems show natural attenuation. We sought to identify the biogeochemical mechanisms responsible for the natural attenuation of AMD. We hypothesized that biogenic sulfide-mediated iron reduction is one mechanism and tested this in an experimental model system. We found sulfate reduction occurred under acidic conditions and identified a suite of sulfate-reducing bacteria (SRB) belonging to the groups Desulfotomaculum, Desulfobacter, Desulfovibrio, and Desulfobulbus. Iron reduction was not detected in microcosms when iron-reducing bacteria or SRB were selectively inhibited. SRB also did not reduce iron enzymatically. Rather, the biogenic sulfide produced by SRB was found to be responsible for the reduction of iron at low pH. Addition of organic substrates and nutrients stimulated iron reduction and increased the pH. X-ray diffraction and an electron microprobe analysis revealed that the polycrystalline, black precipitate from SRB bioactive samples exhibited a greater diversity of iron chalcogenide minerals with reduced iron oxidation states, and minerals incorporating multiple metals compared to abiotic controls. The implication of this study is that iron reduction mediated by biogenic sulfide may be more significant than previously thought in acidic environments. This study not only describes an additional mechanism by which SRB attenuate AMD, which has practical implications for AMD-impacted sites, but also provides a link between the biogeochemical cycling of iron and sulfur.

Characteristics and Mechanism of Hematite Dissolution and Release on Arsenic Migration in Heterogeneous Materials

The migration of arsenic in groundwater is influenced by the heterogeneity of the medium, and the presence of iron minerals adds complexity and uncertainty to this effect. In this study, a stratified heterogeneous sand column with an embedded hematite lens at the coarse-to-medium sand interface was designed. We introduced an arsenic-laden solution and controlled groundwater flow to investigate the spatiotemporal characteristics of arsenic migration and the impact of hematite dissolution. The results showed that the medium structure significantly influenced the arsenic migration and distribution within the lens-containing sand column. The clay layers directed the lateral migration of arsenic, and the arsenic concentrations in deeper layers were up to seven times greater than those on the surface. The extraction experiments of solid-phase arsenic revealed that the main adsorption modes on quartz sand surfaces were the specific adsorption (F2) and adsorption on weakly crystalline iron-aluminum oxides (F3), correlating to the specific and colloidal adsorption modes, respectively. Monitoring the total iron ions (Fe(aq)) revealed rapid increases within the first 14 days, reaching a maximum on day 15, and then gradually declining; these results indicate that hematite did not continuously dissolve. This study can aid in the prevention and control of arsenic contamination in groundwater.

Coupled redox cycling of arsenic and sulfur regulates thioarsenate enrichment in groundwater

High‑arsenic groundwater is influenced by a combination of processes: reductive dissolution of iron minerals and formation of secondary minerals, metal complexation and redox reactions of organic matter (OM), and formation of more migratory thioarsenate, which together can lead to significant increases in arsenic concentration in groundwater. This study was conducted in a typical sulfur- and arsenic-rich groundwater site within the Datong Basin to explore the conditions of thioarsenate formation and its influence on arsenic enrichment in groundwater using HPLC-ICPMS, hydrogeochemical modeling, and fluorescence spectroscopy. The shallow aquifer exhibited a highly reducing environment, marked by elevated sulfide levels, low concentrations of Fe(II), and the highest proportion of thioarsenate. In the middle aquifer, an optimal ∑S/∑As led to the presence of significant quantities of thioarsenate. In contrast, the deep aquifer exhibited low sulfide and high Fe(II) concentration, with arsenic primarily originating from dissolved iron minerals. Redox fluctuations in the sediment driven by sulfur‑iron minerals generated reduced sulfur, thereby facilitating thioarsenate formation. OM played a crucial role as an electron donor for microbial activities, promoting iron and sulfate reduction processes and creating conditions conducive to thioarsenate formation in reduced and high‑sulfur environments. Understanding the process of thioarsenate formation and the influencing factors is of paramount importance for comprehending the migration and redistribution of arsenic in groundwater systems.

Adsorption of Bichromate and Arsenate Anions by a Sorbent Based on Bentonite Clay Modified with Polyhydroxocations of Iron and Aluminum by the "Co-Precipitation" Method

The physicochemical properties of natural bentonite and its sorbents were studied. It has been established the modification of natural bentonites using polyhydroxoxides of iron (III) (mod.1_Fe_5-c) and aluminum (III) (mod.1_Al_5-c) by the "co-precipitation" method led to changes in their chemical composition, structure, and sorption properties. It was shown that modified sorbents based on natural bentonite are finely porous (nanostructured) objects with a predominance of pores of 1.5-8.0 nm in size. The modification of bentonite with iron (III) and aluminum compounds by the "co-precipitation" method also leads to an increase in the sorption capacity of the obtained sorbents with respect to bichromate and arsenate anions. A kinetic analysis showed that, at the initial stage, the sorption process was controlled by an external diffusion factor, that is, the diffusion of the sorbent from the solution to the liquid film on the surface of the sorbent. The sorption process then began to proceed in a mixed diffusion mode when it limited both the external diffusion factor and the intra-diffusion factor (diffusion of the sorbent to the active centers through the system of pores and capillaries). To clarify the contribution of the chemical stage to the rate of adsorption of bichromate and arsenate anions by the sorbents under study, kinetic curves were processed using equations of chemical kinetics (pseudo-first-order, pseudo-second-order, and Elovich models). It was found that the adsorption of the studied anions by the modified sorbents based on natural bentonite was best described by a pseudo-second-order kinetic model. The high value of the correlation coefficient for the Elovich model (R2 > 0.9) allows us to conclude that there are structural disorders in the porous system of the studied sorbents, and their surfaces can be considered heterogeneous. Considering that heterogeneous processes occur on the surface of the sorbent, it is natural that all surface properties (structure, chemical composition of the surface layer, etc.) play an important role in anion adsorption.

Inhibiting effects of humic acid on iron flocculation hindered As removal by electro-flocculation on air cathode iron anode

Activated carbon air cathode combined with iron anode oxidation-flocculation synergistic Arsenic (As) removal was a new groundwater purification technology with low energy consumption and high efficiency for groundwater with high As concentration. The presence of organic matter such as humic acid (HA) had ambiguous effects on formation of organic colloids in the system. The effects of the particle size distribution characteristics of these colloids on the formation characteristics of flocs and the efficiency of As purification was not clear. In this work, we used five different pore size alumina filter membranes to separate mixed phase solutions and studied the corresponding changes in iron and arsenic concentrations in the presence and absence of humic acid conditions. In the presence of HA, the arsenic concentration of < 0.05 µm particle size components was 1.01, 1.28, 3.07, 7.69, 2.85 and 1.24 times of that in the absence of HA. At the same time, the arsenic content in 0.05-0.1 µm and 0.1-0.45 µm particle size components was also higher than that in the system without HA, which revealed that the presence of HA hindered the flocculation behavior of As distribution to higher particle sizes in the early stage of the reaction. The presence of HA affected the flocculation rate of iron flocs from small to large particle size fractions and it had limited effect on the behavior of large-size flocs in adsorption of As. These results provide a theoretical basis for targeted, rapid, and low consumption synergistic removal of arsenic and organic compounds in high arsenic groundwater.

Unravelling the removal mechanisms of trivalent arsenic by sulfidated nanoscale zero-valent iron: The crucial role of reactive oxygen species and the multiple effects of citric acid

The remediation of arsenic-contaminated groundwater by sulfidated nanoscale zero-valent iron (S-nZVI) has raised considerable attention. However, the role of trivalent arsenic (As(III)) oxidation by S-nZVI in oxic conditions (S-nZVI/O2) remains controversial, and the comprehensive effect of citric acid (CA) prevalent in groundwater on As(III) removal by S-nZVI remains unclear. Herein, the mechanisms of reactive oxygen species (ROS) generation and multiple effects of CA on As(III) removal by S-nZVI/O2 were systematically explored. Results indicated that the removal efficiency of As(III) by S-nZVI/O2 (97.81 %) was prominently higher than that by S-nZVI (66.71 %), resulting from the significant production of ROS (mainly H2O2 and OH) under oxic conditions, which played a crucial role in promoting the As(III) oxidation. Additionally, CA had multiple effects on As(III) removal by S-nZVI/O2 system: (i) CA impeded the diffusion of As(III) towards S-nZVI and increased the secondary risk of immobilized As(III) re-releasing into the environment due to the Fe dissolution from S-nZVI; (ii) CA could significantly enhance the yields of OH from 25.29 to 133.00 μM via accelerating the redox cycle of Fe(II)/Fe(III) and increasing the oriented conversion rate of H2O2 to OH; (iii) CA could also enrich the types of ROS (such as O2- and 1O2) in favor of further As(III) oxidation. This study contributed novel findings regarding the control of As(III) contaminated groundwater using S-nZVI technologies.