Complexation of Arsenite, Arsenate, and Monothioarsenate with Oxygen-Containing Functional Groups of Natural Organic Matter: An XAS Study

A critical review on arsenic and antimony adsorption and transformation on mineral facets

Arsenic (As) and antimony (Sb), with analogy structure, belong to VA group in the periodic table and pose a great public concern due to their potential carcinogenicity. The speciation distribution, migration and transformation, enrichment and retention, as well as bioavailability and toxicity of As and Sb are influenced by several environmental processes on mineral surfaces, including adsorption/desorption, coordination/precipitation, and oxidation/reduction. These interfacial reactions are influenced by the crystal facet of minerals with different atomic and electronic structures. This review starts with facets and examines As and Sb adsorption and transformation on mineral facets such hematite, titanium dioxide, and manganese dioxide. The main focus lies on three pressing issues that limit the understanding of the environmental fate of As and Sb: the facet-dependent intricacies of adsorption and transformation, the mechanisms underlying facet-dependent phenomena, and the impact of co-existing chemicals. We first discussed As and Sb adsorption behaviors, structures, and bonding chemistry on diverse mineral facets. Subsequently, the reactivity of various mineral facets was examined, with particular emphasis placed on their significance in the context of environmental catalysis for the oxidation of As(III) and Sb(III). Finally, the impact of co-existing cation, anion, or organic substances on the processes of adsorption and transport of As and Sb was reviewed. This comprehensive review enhances our understanding of the facet-dependent phenomena governing adsorption, transformation, and fate of contaminants. It underscores the critical role of mineral facets in dictating environmental reactions and paves the way for future research in this intriguing field.

Release and Redistribution of Arsenic Associated with Ferrihydrite Driven by Aerobic Humification of Exogenous Soil Organic Matter

Humification of exogenous soil organic matter (ESOM) remodels the organic compositions and microbial communities of soil, thus exerting potential impacts on the biogeochemical transformation of iron (hydr)oxides and associated trace metals. Here, we conducted a 70-day incubation experiment to investigate how aerobic straw humification influenced the repartitioning of arsenic (As) associated with ferrihydrite in paddy soil. Results showed that the humification was characterized by rapid OM degradation (1-14 days) and subsequent slow maturation (14-70 days). During the degradation stage, considerable As (13.1 mg·L-1) was released into the aqueous phase, which was reimmobilized to the solid phase in the maturation stage. Meanwhile, the low-crystalline structural As/Fe was converted to a more stable species, with a subtle crystalline phase transformation. The generated highly unsaturated and phenolic compounds and enriched Enterobacter and Sphingomonas induced ferrihydrite (∼3.1%) and As(V) reduction, leading to As release during the degradation stage. In the maturation stage, carboxylic-rich alicyclic molecules facilitated the aqueous As reimmobilization. Throughout the humification process, organo-mineral complexes formed between OM and ferrihydrite via C-O-Fe bond contributed to the solid-phase As/Fe stabilization. Collectively, this work highlighted the ESOM humification-driven iron (hydr)oxide transformation and associated As redistribution, advancing our understanding of the coupled biogeochemical behaviors of C, Fe, and As in soil.

Arsenic source-sink dynamics under phosphorus competition in sediments from river-lake connected systems

Multiple factors in the river-lake connected system can affect the transport and transformation of arsenic (As) in sediments. In this study, As source and sink characteristics under phosphorus (P) competition in sediments from river (R), lake (L) and lake-centre (LC) of river-lake connected system were analyzed, and the As release regionality and continuity were also discussed. The study showed that the ORP of the sediments at each site was negative with a mean value of -151 mV. The sediments in the R region were weakly acidic and those in the LC region were weakly alkaline. The R region had relatively high EAsC0 concentrations, while the LC region had the largest EPC0 value of 0.17 mg L-1. As in the sediments competed very strongly with P for sources and sinks, with the sediments in the R region as a source of strong As and weak P, in the L region as a source of weak As and weak P, and in the LC region as an As sink and P source. Regions of high risk of As release from sediments were the R and L regions. Sediments in the L and LC regions had a high value of DAsS (the degree of As saturation), 8.9-13.1% and 8-13%, respectively, and the P release risk and saturation were inversely proportional to this result. The contribution analysis showed that surface sediments provided the largest contribution of As release, and the release contribution of As(V) was greater than that of As(III). In addition, the microbial community analysis identified Proteobacteria as the most abundant species in the sediments, with a relative abundance ranging from 42.4% to 53.2%. This bacterium, known for its As-metabolizing capabilities, was found to be positively correlated with the risk of As release. The analysis of As content in sediments revealed a gradient of R region < L region < LC region, confirming that rivers are significant contributors of As contamination to the lake.

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.

Microwave-assisted supramolecular double crosslinked chitosan-based molecularly imprinted polymer for synergistic recognition and selective recovery of Cd(II) and As(V) from water: Performance and mechanistic insights

A novel model of the sustainable double crosslinked molecularly imprinted polymer (D-Crosslinked MIP) represented as a supramolecular imprinted polymer was synthesized via the bulk polymerization method. The primary crosslinking was fabricated using biomacromolecule chitosan as a functional monomer and glutaraldehyde as a crosslinker. The primary crosslinked was subjected to dynamic interactions in a secondary crosslinking by binding Al2O3-NPs and TiO2-NPs, forming the supramolecular D-Crosslinked-MIP. The dually crosslinked formed from combining three distinct crosslinkers in one system for the interaction with As(V) and Cd(II). A microwave was employed to evaluate the performance of the designed material in selectivity and extraction of metal ions from water. The FT-IR, XRD, TG/DTA, SEM-EDX, TEM, and XPS were used to verify the characteristics of (D-nano-Al2O3@Crosslinked Chitosan@D-nano-TiO2). The type of solvents, selectivity, interferences, microwave-contact time, pH, temperatures, concentrations, and regeneration were investigated. By using the D-Crosslinked-MIP, at 15 s, Cd(II) revealed a recovery capacity of 99.03 %, Qmax 862.07 mg/g, while As(V) demonstrated a recovery capacity of 99.06 %, Qmax 850.75 mg/g. The D-Crosslinked-MIP exhibited BETs of 69.01 m2/g with a pore volume of 0.2340 cm3/g owing to polymeric crosslinking by metal oxide NPs. The kinetics, isotherm models, and mechanisms of dually crosslinking and extraction of toxic metals were discussed.

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.

Low-molecular-weight organic acids inhibit the methane-dependent arsenate reduction process in paddy soils

Anaerobic methane oxidation (AOM) can drive soil arsenate reduction, a process known as methane-dependent arsenate reduction (M-AsR), which is a critical driver of arsenic (As) release in soil. Low molecular weight organic acids (LMWOAs), an important component of rice root exudates, have an unclear influence and mechanism on the M-AsR process. To narrow this knowledge gap, three typical LMWOAs-citric acid, oxalic acid, and acetic acid-were selected and added to As-contaminated paddy soils, followed by the injection of 13CH4 and incubation under anaerobic conditions. The results showed that LMWOAs inhibited the M-AsR process and reduced the As(III) concentration in soil porewater by 35.1-65.7 % after 14 days of incubation. Among the LMWOAs, acetic acid exhibited the strongest inhibition, followed by oxalic and citric acid. Moreover, LMWOAs significantly altered the concentrations of ferrous iron and dissolved organic carbon in the soil porewater, consequently impacting the release of As in the soil. The results of qPCR and sequencing analysis indicated that LMWOAs inhibited the M-AsR process by simultaneously suppressing microbes associated with ANME-2d and arrA. Our findings provide a theoretical basis for modulating the M-AsR process and enhance our understanding of the biogeochemical cycling of As in paddy soils under rhizosphere conditions.

A kinetics-coupled multi-surface complexation model deciphering arsenic adsorption and mobility across soil types

The diversity of soil adsorbents for arsenic (As) and the often-overlooked influence of manganese (Mn) on As(III) oxidation impose challenges in predicting As adsorption in soils. This study uses Mössbauer spectroscopy, X-ray diffraction of oriented clay, and batch experiments to develop a kinetic coupled multi-surface complexation model that characterizes As adsorbents in natural soils and quantifies their contributions to As adsorption. The model integrates dynamic adsorption behaviors and Mn-oxide interactions with unified thermodynamic and kinetic parameters. The results indicate that As adsorption is governed by five primary adsorbents: poorly crystalline Fe oxides, well crystalline Fe oxides, Fe-rich clay, Fe-depletion clay, and organic carbon (OC). Fe oxides dominate As adsorption at low As concentrations. However, at higher As concentrations, soils from carbonate strata, with higher content of Fe-rich clay, exhibit stronger As adsorption capabilities than soils from Quaternary sediment strata. The enrichment in Fe-rich clay can enhance the resistance of adsorbed As to reduction processes affecting Fe oxides. Additionally, extensive redox cycles in paddy fields increase OC levels, enhancing their As adsorption compared to upland fields. This model framework provides novel insights into the intricate dynamics of As within soils and a versatile tool for predicting As adsorption across diverse soils.

Transformation of dissolved organic matter during groundwater arsenite removal using air cathode iron electrocoagulation

Dissolve organic matters (DOM) usually showed negative effect on the removal of inorganic arsenic (As) in groundwater by electrochemical approaches, yet which parts of sub-component within DOM played the role was lack of evidence. Herein, we investigated the effects of land-source humic-like acid (HA) on groundwater As(III) removal using air cathode iron electrocoagulation, based on the parallel factor analysis of three-dimensional excitation-emission matrix and statistical methods. Our results showed that the land-source HA contained five kinds of components and all components presented significantly negative correlations with the removal of both As(III) and As(V). However, the high aromatic fulvic-like acid and low aromatic humic-like acid components of land-source HA presented the opposite correlations with the concentration of As(III) during the reaction. The high aromaticity fulvic-like components of land-source HA (Sigma-Aldrich HA, SAHA) produced during the reaction facilitated the oxidation of As(III) due to its high electron transfer capacities and good solubility in wide pH range, but the low aromaticity humic-like ones worked against the oxidation of As(III). Our findings offered the novel insights for the flexible activities of DOM in electron Fenton system.

Arsenite adsorption and oxidation affected by soil humin: The significant role of persistent free radicals and reactive oxygen species

Humin (HM), as the main component of soil organic matter, carries various reactive groups and plays a crucial regulatory role in the transformation of arsenic (As). However, current research on the redox pathway of As and its interactions with HM is relatively limited. This study aimed to explore the impact of different HM samples on the redox characteristics of As. The results showed that HM can not only adsorb arsenite [As(III)] but also oxidize As(III) into arsenate [As(V)]. However, once As(III) is adsorbed on the HM, it cannot undergo further oxidation. HMNM (extracted from peat soil) exhibited the highest adsorption capacity of As(III), with a maximum amount of 1.95 mg/kg. The functional groups of HM involved in As complexation were primarily phenolic hydroxyl and carboxyl groups. The adsorption capacity of HM samples for As(III) was consistent with their carboxyl group contents. The oxygen-containing functional groups and environmentally persistent free radicals (EPFRs) on HM can directly oxidize As(Ⅲ) through electron transfer, or indirectly induce the production of reactive oxygen species (ROS), such as hydroxyl radicals, to further oxidize As(Ⅲ). This study provides new insight into the transport and transformation process of As mediated by soil HM, and establishes a theoretical basis for As remediation.

Arsenic(III) sorption on organo-ferrihydrite coprecipitates: Insights from maize and rape straw-derived DOM

The mobility of arsenic (As) specie in agricultural soils is significantly impacted by the interaction between ferrihydrite (Fh) and dissolved organic material (DOM) from returning crop straw. However, additional research is necessary to provide molecular evidence for the interaction of toxic and mobile As (As(III)) specie and crop straw-based organo- Fh coprecipitates (OFCs). This study investigated the As(III) sorption behaviours of OFCs synthesized with maize or rape derived-DOM under various environmental conditions and the primary molecular sorption mechanisms using As K-edge X-ray absorption near edge structure (XANES) spectroscopy. According to our findings, pure Fh adsorbed more As(III) relative to the other two OFCs, and the presence of natural organic matter in the OFCs induced more As(III) adsorption at pH 5.0. Findings from this study indicated a maximum As(III) sorption on Ma (53.71 mg g⁻1) and Ra OFC (52.46 mg g⁻1) at pH 5.0, with a sharp decrease as the pH increased from 5.0 to 8.0. Additionally, As K-edge XANES spectroscopy indicated that ∼30% of adsorbed As(III) on the OFCs undergoes transformation to As(V) at pH 7-8. Functional groups from the DOM, such as O-H, COOH, and CO, contributed to As(III) desorption and its oxidation to As(V), whereas ionic strength analysis revealed inner complexation as the dominant As(III) sorption mechanism on the OFCs. Overall, the results indicate that the interaction of natural organic matter (NOM) with As(III) at higher pH promotes As(III) mobility, which is crucial when evaluating As migration and bioavailability in alkaline agricultural soils.

Degradation of algae promotes the release of arsenic from sediments under high-sulfate conditions

Sulfate concentrations in eutrophic waters continue to increase; however, the transformations of arsenic (As) in sediments under these conditions are unclear. In this study, we constructed a series of microcosms to investigate the effect of algal degradation on As transformations in sediments with high sulfate concentrations. The results showed that both the elevated sulfate levels and algal degradation enhanced the release of As from sediments to the overlying water, and degradation of algal in the presence of elevated sulfate levels could further contribute to As release. Sulfate competed with arsenate for adsorption in the sediments, leading to As desorption, while algal degradation created a strongly anaerobic environment, leading to the loss of the redox layer in the surface sediments. With high sulfate, algal degradation enhanced sulfate reduction, and sulfur caused the formation of thioarsenates, which may cause re-dissolution of the arsenides, enhancing As mobility by changing the As speciation. The results of sedimentary As speciation analysis indicated that elevated sulfur levels and algal degradation led to a shift of As from Fe2O3/oxyhydroxide-bound state to specifically adsorbed state at the sediment water interface. This study indicated that algal degradation increases the risk of As pollution in sulfate-enriched eutrophic waters.

Seasonal Formation of Low-Sorbing Methylthiolated Arsenates Induces Arsenic Mobilization in a Minerotrophic Peatland

Peatlands are known sinks for arsenic (As). In the present study, seasonal As mobilization was observed in an acidic, minerotrophic peatland (called Lehstenbach) in late summer, accompanied by a peak in dissolved sulfide (S(-II)). Arsenic speciation revealed the lowest seasonal porewater concentrations of arsenite and arsenate, likely due to As(III)-S-bridging to natural organic matter. Arsenic mobilization was driven by the formation of arsenite-S(-II) colloids and formation of methylthiolated arsenates (up to 59% of the sum of As species) and to a minor extent also of inorganic thioarsenates (6%-30%) and oxymethylated arsenates (5%-24%). Sorption experiments using a purified model peat, the Lehstenbach peat, natural (to mimic winter conditions) and reacted with S(-II) (to mimic late summer conditions) at acidic and neutral pH confirmed low sorption of methylthiolated arsenates. At acidic pH and in the presence of S(-II), oxymethylated arsenates were completely thiolated. This methylthiolation decreased As sorption up to 10 and 20 times compared with oxymethylated arsenates and arsenite, respectively. At neutral pH, thiolation of monomethylated arsenates was incomplete, and As could be partially retained as oxymethylated arsenates. Dimethylated arsenate was still fully thiolated and highly mobile. Misidentification of methylthiolated arsenates as oxymethylated arsenates might explain previous contradictory reports of methylation decreasing or increasing As mobility.

Arsenic accumulation pattern in water-soil-rice systems: A study of tolerance mechanisms and associated health risks

Regardless of the daunting challenge of arsenic (As) contamination in Pakistan, literature on tolerance and responsible factors in paddy fields remain elusive. In this regard, we aimed to explore physiochemical factors responsible for As availability in water-soil-rice systems. The study highlighted rice defense mechanisms to mitigate As toxicity on growth and yield. In the present study, basmati rice samples were collected along with irrigation and soil samples from control (<10 μg/L), low (11-25 μg/L), medium (26-100 μg/L), and high (>100 μg/L) contaminated regions. Oxidative stress markers (MDA and H2O2) and antioxidant enzymatic assays (SOD, CAT, POD, APX) were measured by spectrophotometer. The Durov diagram was constructed by using Grapher software to identify prevalent water types in irrigation wells. Total As was measured in water, soil, and rice tissues by hydride generation-atomic absorption spectroscopy (HG-AAS). The Durov diagram showed that the majority of irrigation water was Ca-Mg-Cl type. Furthermore, the FTIR analysis identified different organic compounds, i.e., OH, CC, CI, and CBr, particularly in soil from high regions. The results indicated higher accumulation and translocation of As in the water-soil-rice system from a high region compared to control and other regions. Phenotypic traits, i.e., grain yield, biological yield, chlorophyll, and root parameters were significantly impacted under high As-contaminated region. A concentration-dependent increase was indicated in oxidative stress and antioxidant activities except for APX. Risk assessment indicated a higher hazard quotient (1.09) and carcinogenic risk (5.0 × 10-03) due to grain consumption in high As-contaminated regions. The present study emphasized the need for strict regulations and policies to mitigate As calamity at the local level and protect human health.

Effect of landfill leachate on arsenic migration and transformation in shallow groundwater systems

Arsenic contamination of groundwater has affected human health and environmental safety worldwide. Hundreds of millions of people in more than 100 countries around the world are directly or indirectly troubled by arsenic-contaminated groundwater. In addition, arsenic contamination of groundwater caused by leakage of leachate from municipal solid waste landfills has occurred in some countries and regions, which has attracted widespread attention. Understanding how domestic waste landfill leachate affects the arsenic's migration and transformation in shallow groundwater is crucial for accurate assessment of the distribution and ecological hazards of arsenic in groundwater. Based on literature review, this study systematically summarized and discussed the basic characteristics of landfill leachate, the mechanism of arsenic pollution in groundwater, and the effect of landfill leachate on the migration and transformation of arsenic in groundwater. Combined with relevant research findings and practical experience, countermeasures and suggestions to limit the impact of landfill leachate on the migration and transformation of arsenic in groundwater are put forward.

Synergistic mechanism of iron manganese supported biochar for arsenic remediation and enzyme activity in contaminated soil

This study prepared and characterized bamboo-derived biochar loaded with different ratios of iron and manganese; evaluated its remediation performance in arsenic-contaminated soil by studying the changes in various environmental factors, arsenic speciation, and arsenic leaching amount in the soil after adding different materials; proposed the optimal ratio and mechanism of iron-manganese removal of arsenic; and explained the multivariate relationship between enzyme activity and soil environmental factors based on biological information. Treatment with Fe-Mn-modified biochar increased the organic matter, cation exchange capacity, and N, P, K, and other nutrient contents. During the remediation process, O-containing functional groups such as Mn-O/As and Fe-O/As were formed on the surface of the biochar, promoting the transformation of As from the mobile fraction to the residual fraction and reducing the phytotoxicity of As, and the remediation ability for As was superior to that of Fe-modified biochar. Mn is indispensable in the FeMn-BC synergistic remediation of As, as it can increase the adsorption sites and the number of functional groups for trace metals on the surface of biochar. In addition to electrostatic attraction, the synergistic mechanism of ferromanganese-modified biochar for arsenic mainly involves redox and complexation. Mn oxidizes As(Ⅲ) to more inert As(V). In this reaction process, Mn(Ⅳ) is reduced to Mn(Ⅲ) and Mn(II), promoting the formation of Fe(Ⅲ) and the conversion of As into Fe-As complexes, while As is fixed due to the formation of ternary surface complexes. Moreover, the effect of adding Fe-Mn-modified biochar on soil enzyme activity was correlated with changes in soil environmental factors; catalase was correlated with soil pH; neutral phosphatase was correlated with soil organic matter; urease was correlated with ammonia nitrogen, and sucrase activity was not significant. This study highlights the potential value of FM1:3-BC as a remediation agent in arsenic-contaminated neutral soils.

Mechanistic Insight into the Abiotic Interactions of Selenate and Selenite with Natural Organic Matter

Natural organic matter (NOM) decreases the selenium (Se) mobility in soil and sediment. Biotic dissimilatory reduction of selenate and selenite and assimilation of the reduced Se species into biomolecules are thought to be primarily responsible for this decreased Se mobility. However, the possibility of Se immobilization due to the abiotic interaction of Se species with NOM is still poorly understood. Equilibrating selenate and selenite with a model NOM (Pahokee peat soil), followed by X-ray absorption spectroscopic analysis, this study shows that the NOM can abiotically reduce highly mobile selenate into relatively less mobile selenite. NOM can sorb Se species, especially selenite, considerably. Preloading of the NOM with Fe(III) increases the sorption of selenite and selenate by several orders of magnitude. Modeling of the Se and Fe K-edge EXAFS data revealed that Se species are sorbed to NOM due to indirect complexation with the organically complexed Fe(O,OH)6 octahedra through the corner- (2C) and edge-sharing (1E) and direct complexation with the oxygen-containing functional groups of the NOM. This study concludes that the abiotic reduction and complexation of the Se species with NOM can be the additional or alternative route of Se immobilization in the NOM-rich soil and sediment.

Modeling the Interaction and Uptake of Cd-As(V) Mixture to Wheat Roots Affected by Humic Acids, in Terms of root cell Membrane Surface Potential (ψ0)

The joint toxicological effects of Cd²⁺ and As(V) mixture on wheat root as affected by environmental factors, such as pH, coexisting cations, and humic acids etc., were investigated using hydroponic experiments. The interaction and toxicological mechanisms of co-existing Cd²⁺ and As(V) at the interface of solution and roots in presence of humic acid were further explored by incorporating root cell membrane surface potential ψ₀ into a mechanistic model of combined biotic ligand model (BLM)-based Gouy-Chapman-Stern (GCS) model and NICA-DONNAN model. Besides, molecular dynamics (MD) simulations of lipid bilayer equilibrated with solution containing Cd²⁺ and H₂AsO₄^- further revealed the molecular distribution of heavy metal(loid) ions under different membrane surface potentials. H₂AsO₄^- and Cd²⁺ can be adsorbed on the surface of the membrane alone or as complexes, which consolidate the limitation of the macroscopic physical models.

The spatial distribution, health risk, and cytotoxicity of metal(loid)s in contaminated field soils: The role of Cd in human gastric cells damage

The spatial distribution and pollution level of heavy metal(loid)s in soil (0-6 m) from a typical industrial region in Jiangmen City, Southeast China was investigated. Their bioaccessibility, health risk, and human gastric cytotoxicity in topsoil were also evaluated using an in vitro digestion/human cell model. The average concentrations of Cd (87.52 mg/kg), Co (106.9 mg/kg), and Ni (1007 mg/kg) exceeded the risk screening values. The distribution profiles of metal(loid)s showed a downward migration trend to reach a depth of 2 m. The highest contamination was found in topsoil (0-0.5 m), with the concentrations of As, Cd, Co, and Ni being 46.98, 348.28, 317.44, and 2395.60 mg/kg, respectively, while Cd showed the highest bioaccessibility in the gastric phase (72.80 %), followed by Co (21.08 %), Ni (18.27 %), and As (5.26 %) and unacceptable carcinogenic risk. Moreover, the gastric digesta of topsoil suppressed the cell viability and triggered cell apoptosis, evidenced by disruption of mitochondrial transmembrane potential and increase of Cytochrome c (Cyt c) and Caspases 3/9 mRNA expression. Bioaccessible Cd in topsoil was responsible for those adverse effects. Our data suggest the importance to reduce Cd in the soil to decrease its adverse impacts on the human stomach.

Remobilization of legacy arsenic from sediment in a large subarctic waterbody impacted by gold mining

Arsenic contamination from mining poses an environmental challenge due to the mobility of this redox-sensitive element. This study evaluated arsenic mobility in sediments of Yellowknife Bay (Canada), a large subarctic water body impacted by gold mining during the 20th century. Short-term measurements of arsenic flux from sediment, arsenic profiling of the water column and sediment porewater, and mass balance modelling were conducted to assess the importance of sediment as an arsenic source. Sediment arsenic fluxes were highly variable throughout Yellowknife Bay and ranged from - 65-1520 µg m^-2 day^-1. Elevated fluxes measured near the mine site were among the highest published for well-oxygenated lakes. Redox boundaries were typically 2-3 cm below the sediment surface as indicated by porewater profiles of iron, manganese, and arsenic, with arsenic maxima of 65-3220 µg L^-1 predominately as arsenite. Sediment arsenic flux was positively related to its solid-phase concentration. Modelling indicated sediment was a principal source of arsenic to the water column. Adsorption and precipitation processes in the oxidizing environment of near-surface sediments did not effectively attenuate arsenic remobilized from contaminated sediments. Internal recycling of legacy arsenic between sediment and surface water will impede a return to background conditions in Yellowknife Bay for decades.

Biochar Promotes Arsenopyrite Weathering in Simulated Alkaline Soils: Electrochemical Mechanism and Environmental Implications

Oxidation dissolution of arsenopyrite (FeAsS) is one of the important sources of arsenic contamination in soil and groundwater. Biochar, a commonly used soil amendment and environmental remediation agent, is widespread in ecosystems, where it participates in and influences the redox-active geochemical processes of sulfide minerals associated with arsenic and iron. This study investigated the critical role of biochar on the oxidation process of arsenopyrite in simulated alkaline soil solutions by a combination of electrochemical techniques, immersion tests, and solid characterizations. Polarization curves indicated that the elevated temperature (5-45 °C) and biochar concentration (0-1.2 g·L^-1) accelerated arsenopyrite oxidation. This is further confirmed by electrochemical impedance spectroscopy, which showed that biochar substantially reduced the charge transfer resistance in the double layer, resulting in smaller activation energy (E_a = 37.38-29.56 kJ·mol^-1) and activation enthalpy (ΔH* = 34.91-27.09 kJ·mol^-1). These observations are likely attributed to the abundance of aromatic and quinoid groups in biochar, which could reduce Fe(III) and As(V) as well as adsorb or complex with Fe(III). This hinders the formation of passivation films consisting of iron arsenate and iron (oxyhydr)oxide. Further observation found that the presence of biochar exacerbates acidic drainage and arsenic contamination in areas containing arsenopyrite. This study highlighted the possible negative impact of biochar on soil and water, suggesting that the different physicochemical properties of biochar produced from different feedstock and under different pyrolysis conditions should be taken into account before large-scale applications to prevent potential risks to ecology and agriculture.

Polyethylene and polypropylene microplastics reduce chemisorption of cadmium in paddy soil and increase its bioaccessibility and bioavailability

Microplastics (MPs) usually coexist with heavy metals (HMs) in soil. MPs can influence HMs mobility and bioavailability, but the underlying mechanisms remain largely unexplored. Here, polyethylene and polypropylene MPs were selected to investigate their effects and mechanisms of sorption-desorption, bioaccessibility and bioavailability of cadmium (Cd) in paddy soil. Batch experiments indicated that MPs significantly reduced the Cd sorption in soil (p < 0.05). Accordingly, soil with the MPs had lower boundary diffusion constant of Cd (C₁= 0.847∼₁.020) and the Freundlich sorption constant (K_F = 0.444-0.616) than that without the MPs (C₁ = 0.894∼₁.035, K_F = 0.500-0.655). X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses suggested that the MPs reduced Cd chemisorption, by covering the soil active sites and thus blocking complexation of Cd with active oxygen sites and interrupting the formation of CdCO₃ and Cd₃P₂ precipitates. Such effects of MPs enhanced about 1.2-1.5 times of Cd bioaccessibility and bioavailability in soil. Almost the same effects but different mechanisms of polyethylene and polypropylene MPs on Cd sorption in the soil indicated the complexity and pervasiveness of their effects. The findings provide new insights into impacts of MPs on the fate and risk of HMs in agricultural soil.

Mechanisms and health implications of toxicity increment from arsenate-containing iron minerals through in vitro gastrointestinal digestion

Inadvertent oral ingestion is an important exposure pathway of arsenic (As) containing soil and dust. Previous researches evidenced health risk of bioaccessible As from soil and dust, but it is unclear about As mobilization mechanisms in health implications from As exposure. In this study, we investigated As release behaviors and the solid-liquid interface reactions toward As(V)-containing iron minerals in simulated gastrointestinal bio-fluids. The maximum As release amount was 0.57 mg/L from As-containing goethite and 0.82 mg/L from As-containing hematite at 9 h, and the As bioaccessibility was 10.8% and 21.6%, respectively. The higher exposure risk from hematite-sorbed As in gastrointestinal fluid was found even though goethite initially contained more arsenate than hematite. Mechanism analysis revealed that As release was mainly coupled with acid dissolution and reductive dissolution of iron minerals. Proteases enhanced As mobilization and thus increased As bioaccessibility. The As(V) released and simultaneously transformed to high toxic As(III) by gastric pepsin, while As(V) reduction in intestine was triggered by pancreatin and freshly formed Fe(II) in gastric digests. CaCl2 reduced As bioaccessibility, indicating that calcium-rich food or drugs may be effective dietary strategies to reduce As toxicity. The results deepened our understanding of the As release mechanisms associated with iron minerals in the simulated gastrointestinal tract and supplied a dietary strategy to alleviate the health risk of incidental As intake.

As(III) removal by a recyclable granular adsorbent through dopping Fe-Mn binary oxides into graphene oxide chitosan

Graphene oxide chitosan composite (GOCS) is recognized as an environmentally friendly composite adsorbent because of its stability and abundant functional groups to adsorb heavy metals, and Fe-Mn binary oxides (FMBO) have attracted increasing interest due to their high removal capacity of As(III). However, GOCS is often inefficient for heavy metal adsorption and FMBO suffers poor regeneration for As(III) removal. In this study, we have proposed a method of dopping FMBO into GOCS to obtain a recyclable granular adsorbent (Fe/MnGOCS) for achieving As(III) removal from aqueous solutions. Characterization of BET, SEM-EDS, XRD, FTIR, and XPS are carried out to confirm the formation of Fe/MnGOCS and As(III) removal mechanism. Batch experiments are conducted to investigate the effects of operational factors (pH, dosage, coexisting ions, etc.), as well as kinetic, isothermal, and thermodynamic processes. Results show that the removal efficiency (R_e) of As(III) by Fe/MnGOCS is about 96 %, which is much higher than those of FeGOCS (66 %), MnGOCS (42 %), and GOCS (8 %), and it increases slightly with the increasing molar ratio of Mn and Fe. This is because amorphous Fe (hydro)oxides (mainly in the form of ferrihydrite) complexation with As(III) is the major mechanism to remove As(III) from aqueous solutions, and it is accompanied by As(III) oxidation mediated by Mn oxides and the complexation of As(III) with oxygen-containing functional groups of GOCS. Charge interaction plays a weaker role in As(III) adsorption, therefore R_e is persistently high over a wide range of pH values of 3-10. But the coexisting PO₄^3- can greatly decrease R_e by 24.11 %. As(III) adsorption on Fe/MnGOCS is endothermic and its kinetic process is controlled by pseudo-second-order with a determination coefficient of 0.95. Fitted by the Langmuir isotherm, the maximum adsorption capacity is 108.89 mg/g at 25 °C. After four times regeneration, there is only a slight decrease of <10 % for the R_e value. Column adsorption experiments show that Fe/MnGOCS can effectively reduce As(III) concentration from 10 mg/L to <10 μg/L. This study provides new insights into binary polymer composite modified by binary metal oxides to efficiently remove heavy metals from aquatic environments.

Field and laboratory investigations on factors affecting the diel variation of arsenic in Huangshui Creek from Shimen Realgar Mine, China: implications for arsenic transport in an alkali stream

The release of arsenic and related species from mining activities has been investigated widely at both seasonal and diel scales, contributing to the understanding of arsenic cycles, its ultimate fate, and enabling accurate estimates of arsenic flux in specific areas. To enrich the research in this area, a case study was undertaken in Huangshui Creek, Hunan province, China. Here, arsenic is present in the sediment at the Creek entrance to a reservoir and in the widely developed alkali realgar(α-As₄S₄)-calcite(CaCO₃)-dolomite[CaMg(CO₃)₂] strata (pH 7-11). Water from different levels in the Huangshui Creek, the Creek/reservoir entrance, and the downstream reservoir together with corresponding sediments were collected and analyzed. The local algae were separated and cultured. A diel variation of arsenic (688 ug/L in AM 3:50-1152 ug/L in PM 19:50) was observed in the Creek. The largest difference in arsenic concentration between the upper and lower water body was at the mixed creek/reservoir site (364 ug/L). Laboratory experiments showed that arsenic release from Creek sediment and pristine realgar was 1.3-2.7 times and 2.0-2.3 times at 25 and 37 °C, respectively, than low-temperature samples (8 °C) over 24 h. However, temperature variation is not the only factor controlling arsenic release from Huangshui Creek. Batch experiments show that both sediment and pristine realgar can release arsenic(III). In addition, the presence of bicarbonate promotes arsenic(V) release by 15.2-24.3 times for the sediment and by 1.7-3.4 times for pristine realgar compared to the control, though it restrains arsenic(III) release. High levels of algae have a complex effect on arsenic release; it increases arsenic(V) release by accelerating dissolution of realgar but decreases arsenic(III) release through adsorption. The field observations-variation of bicarbonate (67 mg/L in day and 201 mg/L in night) and chlorophyll-a (0.06-0.87)-support that both dissolved bicarbonate and algae affect arsenic concentration. These factors establish a circadian rhythm in the Creek, which coupled with arsenic release, ultimately affect the fate of arsenic.

Arsenic enriched groundwater discharge to a tropical ocean: Understanding controls and processes

The role of submarine groundwater discharge (SGD) in transporting terrestrial-sourced arsenic (As) to the global oceans is not well documented. In the present study, executed on a coast adjoining the extensive groundwater As-contaminated Ganges river delta, we hypothesize that As-enriched groundwater discharges to the adjoining Bay of Bengal (BoB) through SGD flow paths. We conducted high-resolution, field-based investigations and thermodynamic modeling to understand the SGD-sourced As discharge and geochemical cycling of As and other redox-sensitive solutes along the discharge path under varying redox conditions and water sediment interactions. The As distribution and other solutes were measured in a series of multi-depth observation wells and sediment cores, extending from the high tide line (HTL) to 100 m toward the sea, for pre- and post-monsoon seasons. Results reveal the presence of a plume carrying up to 30 μg/L dissolved load of As toward the sea. Arsenic is associated with a plume of Fe and exhibits similar shore-perpendicular variability. Arsenic distribution and transport is controlled by the Fe-Mn redox cycle and influenced by terrestrial groundwater discharge. Field-observations and geochemical modeling demonstrate that Fe-hydroxide precipitates in the subterranean estuary and acts as an interim sink for As , which is eventually mobilized on alteration of geochemical conditions with the season. Fluctuating plume size can be attributed to seasonal variation in fresh groundwater input to the site. Estimates indicate up to 55mg/m²/d As is released to BoB from the site. Based on physicochemical observations this study demonstrates the yet to be studied SGD derived As cycles and the role of SGD dynamics in controlling the fate of redox-sensitive contaminants and their discharge into global oceans.

Influence of DOM and microbes on Fe biogeochemistry at a riverbank filtration site

Riverbank filtration (RBF) constitutes an important part of the water cycle, which involves active natural filtration leading to pollution of river water being intercepted and retained. The RBF has the function of water purification, but retention of exogenous pollutants in the RBF system complicates biogeochemical processes due to the presence of primary active components. In this study, we verified the essential role of microbial mediation during the interactions between primary Fe minerals in the RBF system and dissolved organic matter (DOM) in river water based on lab-scale experiments. The results demonstrated that DOM from infiltration of river water increased the amount of iron (Fe) released from the sediment in RBF, leading to an increase in Fe concentration in groundwater by higher than one order of magnitude. In particular, the existence of Fe bacteria even made this effect more thorough and more complex. Abiotic reduction was shown to play a more significant role in increasing Fe release than microbe-mediated reduction. Increasing the amount of Fe released could change the distribution of Fe minerals at the sediment surface, thereby affecting the structure of the microbial community in the RBF system and decreasing the DOM concentration in the groundwater. Moreover, As and Mn were found to behave in a similar manner as Fe due to their close biochemical properties when interacting with primary minerals in sediment. This study not only provides mechanistic insight into the higher Fe concentrations encountered in the groundwater of nearby rivers but also has important practical implications for developing nature-based technologies for water pollution control and environmental remediation.

Geochemical partitioning and spatial distribution of heavy metals in soils contaminated by lead smelting

Heavy metals (HMs) pollution is a universal and complex problem at lead smelting sites. Further understanding on the distribution, coexistence relationship and occurrence form of multi-metals in soils should be taken prior to restoration on the contaminated sites. In this study, 222 soil samples in a typical abandoned lead smelting site were investigated to understand the spatial distribution and geochemical partitioning of HMs. The results showed that soil quality was seriously threatened by As, Pb and Cd, which expressed high spatial heterogeneity. Integration of sequential extraction, X-ray photoelectron spectroscopy and mineral liberation analysers were employed to qualify the geochemical partitioning of HMs. The data showed that Pb and As were mainly partitioned in the reducible phase and residue phase, where the maximum of As were 18% and 79%, and the maximum of Pb were 31% and 64%, respectively, whilst Cd was mainly partitioned with residue phase (about 25%) and weakly acid soluble phase (about 18%). Paulmooreite was the major important mineral host for Pb and As, whereas Cd predominantly existed in willemite. These minerals containing HMs could usually with Fe reside in the octahedral layer of clay minerals such as montmorillonite, and may also reside in the interlayer. Quartz, montmorillonite and goethite were closely associated with HMs minerals in contaminated soils, which limited vertical migration of HMs and potential risks to groundwater. The results enhanced the understanding of spatial distribution and occurrence behavior of HMs, whilst providing potential benefits to heavy metal stabilization and risks control at abandoned non-ferrous metal smelting sites.

Structural and mechanistic study of antimonite complexation with organic ligands at the goethite-water interface

Antimony is a re-emerging contaminant, and its complexation with natural organic matter is rising to ever-increasing levels due to global climate change, which has far-reaching impacts on its environmental fate and mobility. A molecular-level understanding of the interactions between Sb(III) and organic ligands at the solid-liquid interface is of paramount importance in deciphering the effect of these organic ligands. Herein, we identified and characterized Sb(III)-organic ligand complexes in solution and at the goethite-water interface using complementary techniques. The FT-ICR MS, XANES, and DFT calculations show that organic ligands bind Sb(III) through nucleophilic functional groups, such as -COO^-, -OH and -HS. The formation of surface ternary Sb(III)-bridging complexes retarded the Sb(III) surface precipitation starting from 3.8 mg-Sb/L to a much higher level at 8.3-13.5 mg-Sb/L. The strong bond between Sb(III) and organic ligands is the key factor to inhibit Sb(III) adsorption, surface precipitation and oxidation under sunlight irradiation. Our results showed the chemical basis for the multifaceted functions of organic ligands in stabilizing trace metalloids such as Sb(III) in the environment.

Tungsten distribution and vertical migration in soils near a typical abandoned tungsten smelter

As an emerging contaminant, tungsten's distribution and speciation in soils are far from understood. In this study, two soil profiles near a typical abandoned tungsten smelter in Hunan Province, China were collected and investigated, to ascertain the binding and association of tungsten with different soil components and subsequently to understand its mobility. The data showed that past tungsten smelting activities resulted in elevated concentrations of both tungsten and arsenic in the soil profiles, both of which ranged from dozens of to a few hundred mg/kg. Nano-scale secondary ion mass spectrometry (NanoSIMS) was employed to quantify the distribution and association of tungsten with various other elements. Combined with sequential extraction and mineralogical analysis, the data from NanoSIMS showed that aluminosilicates including kaolinite and illite were the most important mineral hosts for tungsten, whereas arsenic was predominantly bound to iron (oxyhydr)oxides. Additional data from ¹³C nuclear magnetic resonance and X-ray photoelectron spectroscopy revealed that soil organic matter retained tungsten in deep soils (>70 cm) by binding tungsten through carboxyls on aromatic rings. Compared to arsenic, tungsten migrated deeper in the soil profiles, suggesting its higher mobility and potential risk to groundwater quality.

Suppression mechanism of model humic constituents on laccase-enabled 17β-estradiol oxidation and oligomerization

Humic constituents (HCs) are ubiquitous in the aquatic ecosystems, and contain various functional groups that seriously impact the conversion of 17β-estradiol (17β-E2) by fungal laccase. The purpose of this study was to explore the influencing mechanism of HCs on Trametes versicolor laccase-enabled 17β-E2 oxidation and oligomerization. Herein, T. versicolor-secreted laccase could rapidly convert 99.2% of 17β-E2 (rate constant = 3.7 × 10^-2 min^-1, half-life = 18.7 min) into multifarious oligomers at 25 °C and pH 5.0, by phenolic radical-caused C-C and/or C-O self-linking routes, whereas HCs with O-phenolic hydroxyl groups (O-p-OH, i.e., catechol, pyrogallol, gallic acid, and caffeic acid) dramatically suppressed 17β-E2 oligomerization. Compared with HC-free, 17β-E2 rate constants weakened 6.3-15.8 fold in the presence of HCs containing O-p-OH. It is largely because the O-p-OH was preferentially oxidized by T. versicolor laccase to create the electrophilic O-quinone monomers/oligomers. These unstable reactive O-quinone intermediates strongly reversed 17β-E2 phenolic radicals to their monomeric molecules via two proton-transfer versus two electron-transfer channels, thus intercepting 17β-E2 oxidation and oligomerization. These findings highlight new insights into the effect of HCs containing O-p-OH on T. versicolor laccase-started 17β-E2 conversion, which is beneficial to re-understanding the fate and geochemical behavior of 17β-E2 in water.

Titanium nanoparticles fate in small-sized watersheds under different land-uses

Surface waters from three catchments having contrasting land-uses (forested, agricultural, and urban) were sampled monthly and analysed for nanoparticulate titanium dioxide (NPs-TiO₂) by single particle ICPMS and electron microscopy. We report one-year of data for NPs-TiO₂ having average number and mass concentrations of 9.1 × 10⁸ NPs-TiO₂ particles L^-1 and 11 µg NPs-TiO₂ L^-1 respectively. An increase in concentration during warmer months is observed in the forested and agricultural catchments. Both concentrations of NPs-TiO₂ are within the range of recently reported values using similar analytical approaches. The positive correlations for NPs-TiO₂ mass concentration or particle number with the concentration of some trace elements and DOC in the forested and agricultural catchments suggest the detected NPs-TiO₂ in these two systems are mostly from geogenic origin. Additionally, microscopy imaging confirmed the presence of NPs in the three catchments. Furthermore, the land-area normalized annual flux of NPs-TiO₂ (1.65 kg TiO₂ year^-1 km^-2) was highest for the agricultural catchment, suggesting that agricultural practices have a different impact on the NPs-TiO₂ dynamics and exports than other land-uses (urban or forestry). A similar trend is also found by the reanalysis of recent literature data.

Mediation of arsenic mobility by organic matter in mining-impacted sediment from sub-Arctic lakes: implications for environmental monitoring in a warming climate

Arsenic (As) is commonly sequestered at the sediment-water interface (SWI) in mining-impacted lakes through adsorption and/or co-precipitation with authigenic iron (Fe)-(oxy)hydroxides or sulfides. The results of this study demonstrate that the accumulation of organic matter (OM) in near-surface sediments also influences the mobility and fate of As in sub-Arctic lakes. Sediment gravity cores, sediment grab samples, and porewaters were collected from three lakes downstream of the former Tundra gold mine, Northwest Territories, Canada. Analysis of sediment using combined micro-X-ray fluorescence/diffraction, K-edge X-ray Absorption Near-Edge Structure (XANES), and organic petrography shows that As is associated with both aquatic (benthic and planktonic alginate) and terrestrially derived OM (e.g., cutinite, funginite). Most As is hosted by fine-grained Fe-(oxy)hydroxides or sulfide minerals (e.g., goethite, orpiment, lepidocrocite, and mackinawite); however, grain-scale synchrotron-based analysis shows that As is also associated with amorphous OM. Mixed As oxidation states in porewater (median = 62% As (V), 18% As (III); n = 20) and sediment (median = 80% As (-I) and (III), 20% As (V); n = 9) indicate the presence of variable redox conditions in the near-surface sediment and suggest that post-depositional remobilization of As has occurred. Detailed characterization of As-bearing OM at and below the SWI suggests that OM plays an important role in stabilizing redox-sensitive authigenic minerals and associated As. Based on these findings, it is expected that increased concentrations of labile OM will drive post-depositional surface enrichment of As in mining-impacted lakes and may increase or decrease As flux from sediments to overlying surface waters.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12665-022-10213-2.

Review on the interactions of arsenic, iron (oxy)(hydr)oxides, and dissolved organic matter in soils, sediments, and groundwater in a ternary system

High concentrations of arsenic (As) in groundwater threaten the environment and public health. Geogenically, groundwater As contamination predominantly occurs via its mobilization from underground As-rich sediments. In an aquatic ecosystem, As is typically driven by several underlying processes, such as redox transitions, microbially driven reduction of iron (Fe) oxide minerals, and release of associated As. Notably, dissolved As mobilized from soils and sediments exhibits high affinity for dissolved organic matter (DOM). Thus, high DOM concentrations can increase As mobility. Therefore, it is crucial to understand the complex interactions and biogeochemical cycling of As, DOM, and Fe oxides. This review collates knowledge regarding the fate of As in multicomponent As-DOM-Fe systems, including ternary complexes involving both Fe and DOM. Additionally, the release mechanisms of As from sediments into groundwater in the presence of both Fe and DOM have been discussed. The mechanisms of As mobilization/sorption at the solid-water interface can be affected by negatively charged DOM competing for sorption sites with As on Fe (oxy)(hydr)oxides and may be further modified by other anionic ubiquitous species such as phosphate, silicic acid, or sulfur. This review emphasizes the need for a comprehensive understanding of the impact of DOM, Fe oxides, and related biogeochemical processes on As mobilization to aquifers. The review identifies important knowledge gaps that may aid in developing applicable practices for preventing the spread of As contamination in aquatic resources and traditional soil management practices.

Environmental geochemistry of thioantimony: formation, structure and transformation as compared with thioarsenic

Antimony (Sb), a redox-sensitive toxic element, has received global attention due to the increased awareness of its rich geochemistry. The past two decades have witnessed the explosive development in geochemistry of oxyanionic Sb(OH)₃ and Sb(OH)₆^-. Emerging thioantimony species (Sb-S) have recently been detected, which actually dominate the Sb mobility in sulfate-reducing environments. However, the instability and complexity of Sb-S present the most pressing challenges. To overcome these barriers, it is urgent to summarize the existing research on the environmental geochemistry of Sb-S. Since Sb-S is an analogous species to thioarsenic (As-S), a comparison between Sb-S and As-S will provide insightful information. Therefore, this review presents a way of comparing environmental geochemistry between Sb-S and As-S. Here, we summarize the formation and transformation of Sb-S and As-S, their chemical structures and analytical methods. Then, the challenges and perspectives are discussed. Finally, the important scientific questions that need to be addressed are also proposed.

Ferrihydrite-organo composites are a suitable analog for predicting Cd(II)-As(V) coexistence behaviors at the soil solid-liquid interfaces

Organomineral assemblages are building units of soil micro-aggregates and exert their essential roles in immobilizing toxic elements. Currently, our knowledge of the adsorption and partitioning behaviors of coexisting Cd-As onto organomineral composites is limited. Herein, we carefully studied Cd-As cosorption onto ferrihydrite organomineral composites made with either living or non-living organics, i.e., bacteria (Delftia sp.) or humic acid (HA), using batch adsorption and various spectroscopies. Batch results show that As(V) only enhances Cd(II) sorption on pure Fh at pH < 6 but cannot promote Cd(II) sorption to Fh-organo composites. However, Cd(II) noticeably promotes As(V) sorption at pH>~5-6. Synchrotron micro X-ray fluorescence indicates that Cd(II) adsorbs predominately to the bacterial fraction (Cd versus P, r = 0.924), whereas As(V) binds mainly to the Fh fraction (As versus Fe, r = 0.844) of the Fh-bacteria composite. On Fh-HA composite, however, Cd(II) and As(V) are both primarily sorbed by the Fh fraction (Cd/As versus P, r > 0.8), based on the scanning transmission electron microscopy-energy disperse spectroscopy analyses. Elemental distribution characterization also manifests the co-localization of Cd(II) and As(V) within the organomineral composite, particular in Fh-HA composite (Cd versus As, r = 0.8), which is further identified as the Fh-As-Cd ternary complex based on the observations (higher frequencies at ~753-761 cm^-1) of attenuated total reflection Fourier-transform infrared spectroscopy. Moreover, this ternary interaction is more pronounced in Fh-HA than in Fh-bacteria. In summary, our results suggest that Cd-As coadsorption behaviors on Fh-organo composites are different from those on pure minerals, and the presence of bacteria/HA can significantly affect metal (loid)s speciation, distribution, and ternary interaction. Therefore organomineral composites are a more suitable analog than pure mineral phases to predict the mobility and fate of Cd-As in natural environments.

Effects of different dissolved organic matter on microbial communities and arsenic mobilization in aquifers

Dissolved organic matter (DOM) play key roles in the biotransformation of arsenic in groundwater systems. However, the effects of different types of DOM on arsenic biogeochemistry remain poorly understood. In this study, four typical DOM compounds (acetate, lactate, AQS and humic acid) were amended to high As aquifer sediments to investigate their effects on arsenic/iron biotransformation and microbial community response. Results demonstrated that different DOM drove different microbial community shifts and then enhanced microbially-mediated arsenic release and iron reduction. With labile DOM (acetate and lactate) amendment, the abundance of putative dissimilatory iron and sulfate reducers Desulfomicrobium and Clostridium sensu stricto increased within the first week, and subsequently the anaerobic fermentative bacterial genus Acetobacterium and arsenate/sulfate-reducing bacterial genus Fusibacter became predominant. In contrast, recalcitrant DOM (AQS and humic acid) mainly stimulated the abundances of sulfur compounds respiratory genus Desulfomicrobium and fermentative bacterial genus Alkalibacter in the whole incubation. Accompanied with the microbial community structure and function shifts, dissolved organic carbon concentration and oxidation-reduction potential changed and the arsenic/iron reduction increased, which resulted in the enhanced arsenic mobilization. Collectively, the present study linked DOM type to microbial community structure and explored the potential roles of different DOM on arsenic biotransformation in aquifers.

New insights on nanostructure of ordered mesoporous FeMn bimetal oxides (OMFMs) by a novel inverse micelle method and their superior arsenic sequestration performance: Effect of calcination temperature and role of Fe/Mn oxides

A series of ordered mesoporous FeMn bimetal oxides (OMFMs) were fabricated by using a novel inverse micelle method, and the texture, nanostructure and interface chemistry properties of OMFMs were closely correlated to the calcination temperature. Due to the amorphous regular inner-connected nanostructure and bimetallic synergistic effect, the obtained OMFMs exhibited superior arsenic sequestration performance than pure mesoporous Fe oxides (PMF) and Mn oxides (PMM). The optimum ratio of Fe/Mn and calcination temperature for arsenic removal was 3/1 and 350 °C (OMFM-3), and the maximum As(III) and As(V) adsorption capacities of OMFM-3 were 174.59 and 134.58 mg/g, respectively. Solution pH value negligibly affected the uptake of arsenic (ranged from 3.0 to 7.0), while SiO₃^2-/PO₄^3- ions and humic acid (HA) displayed significant inhibitory effect on arsenic removal by OMFM-3. According to the mechanism of arsenic removal, which simultaneously analyzed the arsenic redox transformation in aqueous phase and on solid phase interface, it was concluded that manganese oxides in OMFM-3 mainly played the role as a remarkable As(III) oxidant in water, whereas iron oxides dominantly acted as an excellent arsenic species adsorbent. Finally, the prominent arsenic sequestration behavior and performance in surface water suggested that OMFM-3 could be a promising and hopeful candidate for arsenic-contaminated (especially As(III)) surface water and groundwater remediation and treatment.

Fractionation of carbon isotopes of dissolved organic matter adsorbed to goethite in the presence of arsenic to study the origin of DOM in groundwater

Natural dissolved organic matter (DOM) in groundwater plays a crucial role in mobilizing arsenic (As). The complex contribution of DOM sources makes it hard to predict how the variation of environmental conditions would affect the distribution of As concentrations. Identifying the carbon isotope fractionation of DOM is the key to quantify DOM sources based on stable carbon isotopes. To understand the magnitude and variability in the carbon isotopic fractionation of DOM in competitive adsorption with As(V), this study investigated the δ¹³C values of fulvic acid (FA) and DOM during adsorption to goethite in the presence of As(V), at a specific pH and temperature. The carbon isotopic enrichment factor (ε) of FA in the adsorption to goethite was 0.65 ± 2.11‰ at pH 4.1, 25 °C, suggesting that FA molecules containing ¹³C were more easily adsorbed to goethite. An increasing temperature increased ε_FA from 0.32 ± 1.17‰ to 0.82 ± 5.39‰ at 15-35 °C. For dissolved sediment organic matter (DSOM) cases, molecules containing ¹³C were more easily adsorbed to goethite. However, enrichment factors were not detected due to a reduction in DSOM adsorption and the diversity of natural humic substances or groups. The findings provide basic data for accurately ascertaining DOM sources through carbon isotopes, which is significant for predicting As fluctuation in aquifers affected by monsoon climate and/or human activities.

Potential of high pH and reduced sulfur for arsenic mobilization - Insights from a Finnish peatland treating mining waste water

Peatlands, used for purification of mining waste waters, have shown efficient solid-phase sequestration of contaminants such as arsenic (As). However, contaminant re-mobilization may occur related to management changes or chemical alteration of original peatland conditions. For a treatment peatland in Finnish Lapland, we here confirm efficient As retention in near-surface peat layers close to the mining waste water inflow, likely due to binding to Fe^III-phases. Seven years into operation of the treatment peatland, there appears to be further retention potential, as large areas downstream still had solid-phase As concentrations at background levels. However, via depth-resolved pore water analysis we observed a hotspot 170 m from the inflow at 10-50 cm depth, where As pore water concentrations exceeded input concentrations by a factor of 20, indicating substantial As re-mobilization. At the same spot, a peak of reduced sulfur (S) species was found. Arsenic species detected were arsenite and up to 26% methylated oxyarsenates, 15% methylated and 7.9% inorganic thioarsenates. We postulate that As mobilization is a result of short-term re-equilibration to a changed inflow chemistry after installation of a process water treatment plant and a long-term consequence of changing pore water pH from acidic to near-neutral, releasing reduced S and As. We infer that the co-occurrence of reduced S and As leads to formation of methylated and/or thiolated As species with known low sorption affinity, thereby further enhancing As mobility. Laboratory incubation studies with two peat cores confirmed a high S-induced As mobilization potential, especially when As-Fe-rich, oxic surface layers were incubated anoxically at near-neutral pH. Highest risk of As re-mobilization from this treatment peatland is expected in a scenario in which mining waste water inflow has stopped but the peatland remains flooded, and near-surface layers transition from oxic to anoxic conditions.

Removal of As(III) from Water Using the Adsorptive and Photocatalytic Properties of Humic Acid-Coated Magnetite Nanoparticles

The oxidation of highly toxic arsenite (As(III)) was studied using humic acid-coated magnetite nanoparticles (HA-MNP) as a photosensitizer. Detailed characterization of the HA-MNP was carried out before and after the photoinduced treatment of As(III) species. Upon irradiation of HA-MNP with 350 nm light, a portion of the As(III) species was oxidized to arsenate (As(V)) and was nearly quantitatively removed from the aqueous solution. The separation of As(III) from the aqueous solution is primarily driven by the strong adsorption of As(III) onto the HA-MNP. As(III) removals of 40–90% were achieved within 60 min depending on the amount of HA-MNP. The generation of reactive oxygen species (•OH and ¹O₂) and the triplet excited state of HA-MNP (³HA-MNP*) was monitored and quantified during HA-MNP photolysis. The results indicate ³HA-MNP* and/or singlet oxygen (¹O₂) depending on the reaction conditions are responsible for converting As(III) to less toxic As(V). The formation of ³HA-MNP* was quantified using the electron transfer probe 2,4,6-trimethylphenol (TMP). The formation rate of ³HA-MNP* was 8.0 ± 0.6 × 10⁻⁹ M s⁻¹ at the TMP concentration of 50 µM and HA-MNP concentration of 1.0 g L⁻¹. The easy preparation, capacity for triplet excited state and singlet oxygen production, and magnetic separation suggest HA-MNP has potential to be a photosensitizer for the remediation of arsenic (As) and other pollutants susceptible to advanced oxidation.

Arsenic Fate in Peat Controlled by the pH-Dependent Role of Reduced Sulfur

Reduced sulfur (S) has a contrasting role in the fate of arsenic (As) in peatlands. Sulfur bridges provide efficient binding of As to organic carbon (C), but the formation of aqueous As-S species, so-called thioarsenates, leads to a low to no sorption tendency to organic C functional groups. Here, we studied how pH changes the role of reduced S in desorption and retention of presorbed As in model peat. Control desorption experiments without S addition revealed that As was mobilized, predominantly as arsenite, in all treatments with relative mobilization increasing with pH (4.5 < 7.0 < 8.5). Addition of sulfide or polysulfide caused substantial As retention at acidic conditions but significantly enhanced As desorption compared to controls at neutral to alkaline pH. Thioarsenates dominated As speciation at pH 7.0 and 8.5 (maximum, 79%) and remained in solution without (re)sorption to peat. Predominance of arsenite in control experiments and no evidence of surface-bound thioarsenates at pH 7.0 suggest mobilization to proceed via arsenite desorption, reaction with dissolved or surface-bound reduced S, and formation of thioarsenates. Our results suggest that natural or management-related increases in pH or increases in reduced S in near-neutral pH environments can turn organic matter from an As sink into a source.

Influence of late-Holocene climate change on the solid-phase speciation and long-term stability of arsenic in sub-Arctic lake sediments

Sediment cores were collected from two lakes in the Courageous Lake Greenstone Belt (CLGB), central Northwest Territories, Canada, to examine the influence of late-Holocene warming on the transport and fate of arsenic (As) in sub-Arctic lakes. In both lakes, allochthonous As-bearing minerals (i.e. arsenopyrite and scorodite) were identified in sediment deposited during times of both regional warming and cooling, suggesting that weathering of bedrock and derived surficial materials provides a continual source of As to lakes of the CLGB. However, maximum porewater As (84 μg·L^-1 and 15 μg·L^-1) and reactive organic matter (OM; aquatic and terrestrial-derived) concentrations in each lake are coincident with known periods of regional climate warming. It is inferred that increased biological production in surface waters and influx of terrigenous OM led to the release of sedimentary As to porewater through reductive dissolution of As-bearing Fe-(oxy)hydroxides and scorodite during episodes of regional warming. Elevated sedimentary As concentrations (median: 36 mg·kg^-1; range: 29 to 49 mg·kg^-1) are observed in sediment coeval with the Holocene Thermal Maximum (ca. 5430 ± 110 to 4070 ± 130 cal. years BP); at these depths, authigenic As-bearing framboidal pyrite is the primary host of As in sediment and the influence of organic matter on the precipitation of As-bearing framboidal pyrite is apparent petrographically. These findings suggest that increased biological productivity and weathering of terrestrial OM associated with climate warming influences redox cycles in the near-surface sediment and enhances the mobility of As in northern lakes. Knowledge generated from this study is relevant for predicting future climate change-driven variations in metal(loid) cycling in aquatic systems and can be used to interpret trends in long-term environmental monitoring data at historical, modern, and future metal mines in northern environments.

Anthropogenic influences on dissolved organic matter transport in high arsenic groundwater: Insights from stable carbon isotope analysis and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

In East and Southeast Asia, the health of over 100 million people is threatened by the consumption of groundwater containing high concentrations of arsenic (>10 μg L^-1), which is released from sediments through reductive dissolution of arsenic-bearing iron/manganese oxides. Dissolved organic matter (DOM) is known to play a crucial role in the process of arsenic mobilization in shallow aquifers, and its availability and reactivity are key factors controlling the variation of arsenic concentrations in groundwater. However, it is unclear how human activities influence the transport of DOM and how the transportation affects the DOM molecular properties in high arsenic groundwater. This study provides insights on the sources and molecular compositions of DOM in groundwater from the Jianghan Plain, central China, a newly discovered area with seasonal fluctuations in arsenic concentrations in shallow groundwater. Monitoring of water levels and stable carbon isotope compositions in groundwater from different depths and canal water over a year indicated that terrestrial DOM was the dominant source, accounting for 54.2%-85.5% of groundwater DOM. Electrospray ionization combined with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that canal water infiltration transferred aliphatic, tannin-like and leached aromatic DOM from sediments into groundwater. Therefore, groundwater recharge through irrigation using canal water not only inputs terrestrial DOM, but also accelerates the release of sedimentary DOM. Furthermore, carboxylic-rich alicyclic molecule (CRAM)-like DOM that is derived from biomolecules has the highest proportion (60.1%-65.5%) among the identified DOM structures. And, it might be reused in biochemical processes during arsenic mobilization, suggesting a third source of groundwater DOM in addition to canal water and sediments. The findings in this study advance the understanding on transport processes and molecular properties of DOM in high arsenic groundwater under extensive anthropogenic influences.