Arsenic remediation by formation of arsenic sulfide minerals in a continuous anaerobic bioreactor

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.

Reviewing arsenic biomineralization: An upcoming strategy for mining wastewater treatment

Human activities are the main cause of arsenic contamination in the environment and water resources, being the mining industry an important source of arsenic contamination because this element is released into the environment in solid, liquid, and gaseous wastes. Currently, several physical and chemical processes could be used for the removal of arsenic in water, but these alternatives depend on the concentration of arsenic. At low concentrations (nanograms or micrograms per liter) arsenic can be removed by membrane technologies. When arsenic is at high concentrations (milligrams or grams per liter), treatment options are reduced to inefficient processes of high economic cost and poor chemical stability of the precipitate, returning consequently arsenic into the environment. Biomineralization is a biological process where microorganisms induce the formation of minerals. This bioprocess has gained interest in recent years for the removal of contaminants from liquid effluents. This review details the harmful effects of arsenic on the health and exposes the relevance of arsenic contamination related to mining activity, whose effluents contain high concentration of arsenic. It also describes and analyzes advances in arsenic treatment strategies through biomineralization using microorganisms, such as sulfate-reducing bacteria, iron- and manganese-oxidizing microorganisms, and ureolytic microorganisms, detailing aspects of effectiveness, applicability, chemical stability of biominerals and future perspectives in their industrial application. To our knowledge, there are no previous reports compiling, analyzing, and explaining in detail the biomineralization of arsenic as a single element. The importance of this review is to deliver in a summarized and systematized way the main aspects and perspectives on the application of microorganisms to remove toxic elements, such as arsenic, from effluents.

Arsenic behavior in soil-plant system under the manure application with the combination of antibiotic and roxarsone

Limited attention has been given to the interaction between antibiotics and arsenic in the soil-plant system. In this investigation, Medicago sativa seedlings were grown in soil treated with cow manure containing oxytetracycline (OTC) or sulfadiazine (SD), as well as arsenic (introduced through roxarsone, referred to as ROX treatment). The study revealed a notable increase in As(III) and dimethylarsinic acid (DMA(V)) levels in rhizosphere soils and plant root tissues as arsenic contamination intensified in the presence of antibiotics, while concentrations of As(V) and monomethylarsonic acid (MMA(V)) decreased. Conversely, elevated antibiotic presence resulted in higher levels of As(V) but reduced DMA concentrations in both rhizosphere soils and plant root tissues in the presence of arsenic. The arsenic biotransformation gene aioA was inhibited by arsenic contamination when antibiotics were present, and suppressed by antibiotic contamination in the presence of arsenic, especially in SD treatments, resulting in reduced expression levels at higher SD concentrations. Conversely, the arsM gene exhibited consistent upregulation under all conditions. However, its expression was found to increase with higher concentrations of ROX in the presence of antibiotics, decrease with increasing SD concentrations, and initially rise before declining with higher levels of OTC in the presence of arsenic. Bacterial genera within the Proteobacteria phylum, such as Geobacter, Lusitaniella, Mesorhizobium, and Methylovirgula, showed significant co-occurrence with both aioA and arsM genes. Correlation analysis demonstrated associations between the four arsenic species and the two arsenic biotransformation genes, emphasizing pH as a critical factor influencing the transformation and uptake of different arsenic species in the soil-plant system. The combined stress of antibiotics and arsenic has the potential to modify arsenic behavior and associated risks in soil-plant systems, highlighting the necessity of considering this interaction in future research endeavors.

First Insight into the Mobilization and Sequestration of Arsenic in a Karstic Soil during Redox Changes

Karst terrains provide drinking water for about 25% of the people on our planet, particularly in the southwest of China. Pollutants such as arsenic (As) in the soil can infiltrate groundwater through sinkholes and bedrock fractures in karst terrains. Despite this, the underlying mechanisms responsible for As release from karst soils under redox changes remain largely unexplored. Here, we used multiple synchrotron-based spectroscopic analyses to explore As mobilization and sequestration in As-polluted karstic soil under biogeochemical conditions that mimic field-validated redox conditions. We observed that As in the soil exists primarily as As(V), which is mainly associated with Fe(oxyhydr)oxides. The concentration of the dissolved As was high (294 μM) and As(III) was dominant (∼95%) at low Eh (≤-100 mV), indicating the high risk of As leaching under reducing conditions. This As mobilization was attributed to the fact that the dissolution of ferrihydrite and calcite promoted the release and reduction of associated As(V). The concentration of the dissolved As was low (17.0 μM) and As(V) was dominant (∼68%) at high Eh (≥+100 mV), which might be due to the oxidation and/or sequestration of As(III) by the recrystallized ferric phase. Our results showed that the combined effects of the reductive release of As(V) from both ferric and nonferric phases, along with the recrystallization of the ferric phase, govern the redox-induced mobilization and potential leaching of As in soils within karst environments.

Impact of iron and sulfur cycling on the bioavailability of cadmium and arsenic in co-contaminated paddy soil

The biogeochemical cycling of iron (Fe) or sulfur (S) in paddy soil influences the cadmium (Cd) and arsenic (As) migration. However, the influence of coupled reduction effects and reaction precedence of Fe and S on the bioavailability of Cd and As is still not fully understood. This study aimed to reveal the influence of Fe and S reduction on soil Cd and As mobility under various pe + pH conditions and to elucidate the related mechanism in subtropical China. According to the findings, higher adsorption from Fe reduction caused high-crystalline goethite (pe + pH > 2.80) to become amorphous ferrihydrite, which in turn caused water-soluble Cd (62.0%) to first decrease. Cd was further decreased by 72.7% as a result of the transformation of SO42- to HS-/S2- via sulfate reduction and the formation of CdS and FeS. As release (an increase of 8.1 times) was consequently caused by the initial reduction and dissolution of iron oxide (pe + pH > 2.80). FeS had a lesser impact on the immobilization of As than sulfate-mediated As (V) reduction in the latter stages of the reduction process (pe + pH < 2.80). pe + pH values between 3 and 3.5 should be maintained to minimize the bioavailability of As and Cd in moderate to mildly polluted soil without adding iron oxides and sulfate amendments. The practical remediation of severely co-contaminated paddy soil can be effectively achieved by using Fe and S additions at different pe + pH conditions. This technique shows promise in reducing the bioavailability of Cd and As.

Arsenic in Drinking Water and Diabetes

Arsenic is ubiquitous in soil and water environments and is consistently at the top of the Agency for Toxic Substances Disease Registry (ATSDR) substance priority list. It has been shown to induce toxicity even at low levels of exposure. One of the major routes of exposure to arsenic is through drinking water. This review presents current information related to the distribution of arsenic in the environment, the resultant impacts on human health, especially related to diabetes, which is one of the most prevalent chronic diseases, regulation of arsenic in drinking water, and approaches for treatment of arsenic in drinking water for both public utilities and private wells. Taken together, this information points out the existing challenges to understanding both the complex health impacts of arsenic and to implementing the treatment strategies needed to effectively reduce arsenic exposure at different scales.

As and S speciation in a submarine sulfide mine tailings deposit and its environmental significance: The study case of Portmán Bay (SE Spain)

The dumping of an estimated amount of 57 million tons of hazardous sulfide mine waste from 1957 to 1990 into Portmán's Bay (SE Spain) caused one of the most severe cases of persistent anthropogenic impact in Europe's costal and marine environments. The resulting mine tailings deposit completely infilled Portmán's Bay and extended seawards on the continental shelf, bearing high levels of metals and As. The present work, where Synchrotron XAS, XRF core scanner and other data are combined, reveals the simultaneous presence of arsenopyrite (FeAsS), scorodite (FeAsO₄·2H₂O), orpiment (As₂S₃) and realgar (AsS) in the submarine extension of the mine tailings deposit. In addition to arsenopyrite weathering and scorodite formation, the, the presence of realgar and orpiment is discussed, considering both potential sourcing from the exploited ores and in situ precipitation from a combination of inorganic and biologically mediated geochemical processes. Whereas the formation of scorodite relates to the oxidation of arsenopyrite, we hypothesize that the presence of orpiment and realgar is associated to scorodite dissolution and subsequent precipitation of these two minerals within the mine tailings deposit under moderately reducing conditions. The occurrence of organic debris and reduced organic sulfur compounds evidences the activity of sulfate-reducing bacteria (SRB) and provides a plausible explanation to the reactions leading to the formation of authigenic realgar and orpiment. The precipitation of these two minerals in the mine tailings, according to our hypothesis, has important consequences for As mobility since this process would reduce the release of As into the surrounding environment. Our work provides for the first time valuable hints on As speciation in a massive submarine sulfide mine tailings deposit, which is highly relevant for similar situations worldwide.

Novel microbial consortia facilitate metalliferous immobilization in non-ferrous metal(loid)s contaminated smelter soil: Efficiency and mechanisms

Exposure to toxic metals from nonferrous metal(loid) smelter soils can pose serious threats to the surrounding ecosystems, crop production, and human health. Bioremediation using microorganisms is a promising strategy for treating metal(loid)-contaminated soils. Here, a native microbial consortium with sulfate-reducing function (SRB1) enriched from smelter soils can tolerate exposures to mixtures of heavy metal(loid)s (e.g., As and Pb) or various organic flotation reagents (e.g., ethylthionocarbamate). The addition of Fe²⁺ greatly increased As³⁺ immobilization compared to treatment without Fe²⁺, with the immobilization efficiencies of 81.0% and 58.9%, respectively. Scanning electronic microscopy-energy dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy confirmed that the As³⁺ immobilizing activity was related to the formation of arsenic sulfides (AsS, As₄S₄, and As₂S₃) and sorption/co-precipitation of pyrite (FeS₂). High-throughput 16S rRNA gene sequencing of SRB1 suggests that members of Clostridium, Desulfosporosinus, and Desulfovibrio genera play an important role in maintaining and stabilizing As³⁺ immobilization activity. Metal(loid)s immobilizing activity of SRB1 was not observed at high and toxic total exposure concentrations (220-1181 mg As/kg or 63-222 mg Pb/kg). However, at lower concentrations, SRB1 treatment decreased bioavailable fractions of As (9.0%) and Pb (28.6%) compared to without treatment. Results indicate that enriched native SRB1 consortia exhibited metal(loid) transformation capacities under non-toxic concentrations of metal(loid)s for future bioremediation strategies to decrease mixed metal(loid)s exposure from smelter polluted soils.

Arsenic and cadmium bioavailability to rice (Oryza sativa L.) plant in paddy soil: Influence of sulfate application

Arsenic (As) and cadmium (Cd) accumulate easily in rice grains that pose a non-negligible threat to human health worldwide. Sulfur fertilizer has been shown to affect the mobilization of As and Cd in paddy soil, but the effect of co-contamination by As and Cd has not been explored. This study selected three soils co-contaminated with As and Cd from Shangyu (SY), Tongling (TL) and Ma'anshan (MA). Incubation experiments and pot experiments were carried out to explore the effect of sulfate supply (100 mg kg^-1) on the bioavailability of As and Cd in soil and the rice growth. The results showed that the exogenous sulfate decreased As concentrations in porewater of SY and TL by 51.1% and 29.2% through forming arsenic-sulfide minerals. The exchangeable Cd in soil also declined by 25.6% and 18.6% and transformed into Fe and Mn oxides-bound Cd. The relative abundance of Desulfotomaculum, Desulfurispora and dsr gene increased remarkably indicated that sulfate addition stimulated the activity of sulfate-reducing bacteria. In MA soil, sulfate addition immobilized Cd but had little effect on As solubility, which was speculated to be related to the high sulfate background of the soil. Further pot experiments showed that sulfate application significantly increased rice tillers, biomass, chlorophyll content in shoots, and decreased electrolyte leakage in root. Finally, sulfate significantly reduced As and Cd in SY rice shoots by 60.2% and 40.8%, respectively, while As decreased by 39.6% in TL rice shoots and Cd decreased by 23.0% in MA rice shoots. These results indicate that the application of sulfate can reduce the bioavailability of As and Cd in the soil-rice system and promote rice growth, and it is possible to reduce the accumulation of As and Cd in rice plants simultaneously.

Impact of organic matter on As sulfidation in wetlands: An in situ experiment

Arsenic incorporation into newly formed As sulfides has recently been identified as an important As sequestration pathway in both laboratory experiments and natural As-wetlands. Here, we used an in situ experimental technique with double nylon experimental bags (10-μm mesh) to study the effect of low-cost organic materials (sawdust, wood cubes and hemp shives) on As sulfidation in three naturally As-enriched wetland soils under water-saturated (~1 m depth) and neutral pH conditions. After 15 months of in situ incubation, all of the organic materials and their corresponding inner bags were covered by yellow-black mineral accumulations, dominantly composed of crystalline As₄S₄ polymorphs (realgar and bonazziite) and reactive Fe(II) sulfides (probably mackinawite); while the major fraction of As (~80%) was sequestered as AsS minerals. The amount of As accumulation in the experimental bags varied significantly (0.03-4.24 g As kg^-1) and corresponded with different levels of As (0.23-9.4 mg As L^-1) in the groundwater. Our findings suggest an authigenic formation of AsS minerals in strongly reducing conditions of experimental bags by a combination of reduced exchange of solutes through the pores of the bag and comparatively fast microbial production of dissolved sulfide. Arsenic sulfide formation, as an effective treatment mechanism for natural and human-constructed wetlands, appears to be favored for As(III)-rich waters with a low Fe(II)/As(III) molar ratio. These conditions prevent the consumption of dissolved As and sulfide by their preferential incorporation into natural organic matter, and newly-formed Fe(II) sulfides, respectively.

Removal of Arsenate From Groundwater by Cathode of Bioelectrochemical System Through Microbial Electrosorption, Reduction, and Sulfuration

Arsenate [As(V)] is a toxic metalloid and has been observed at high concentrations in groundwater globally. In this study, a bioelectrochemical system (BES) was used to efficiently remove As(V) from groundwater, and the mechanisms involved were systematically investigated. Our results showed that As(V) can be efficiently removed in the BES cathode chamber. When a constant cell current of 30 mA (I_cell, volume current density = 66.7 A/m³) was applied, 90 ± 3% of total As was removed at neutral pH (7.20–7.50). However, when I_cell was absent, the total As in the effluent, mainly As(V), had increased approximately 2–3 times of the As(V) in influent. In the abiotic control reactor, under the same condition, no significant total As or As(V) removal was observed. These results suggest that As(V) removal was mainly ascribed to microbial electrosorption of As(V) in sludge. Moreover, part of As(V) was bioelectrochemically reduced to As(III), and sulfate was also reduced to sulfides [S(–II)] in sludge. The XANES results revealed that the produced As(III) reacted with S(–II) to form As₂S₃, and the residual As(III) was microbially electroadsorbed in sludge. This BES-based technology requires no organic or chemical additive and has a high As(V) removal efficiency, making it an environment-friendly technique for the remediation of As-contaminated groundwater.

Spatial variation in dissolved phosphorus and interactions with arsenic in response to changing redox conditions in floodplain aquifers of the Hetao Basin, Inner Mongolia

Increasing numbers of studies have reported groundwater with naturally high phosphorous (P) and arsenic (As) concentrations, which can potentially threaten the environment and human health. However, the cycling of P and its interactions with As in groundwater under changing redox conditions remain largely unknown. In this study, 83 groundwater samples and 14 sediment samples were collected from the Hetao Basin, Inner Mongolia, for systematic hydrogeochemical investigation and complementary geochemical evaluation. The results showed that P cycling in floodplain aquifers was tightly constrained by redox conditions. Under oxic/suboxic conditions, mineralization of organic matter and weathering of P-bearing minerals were the two dominant processes that mobilized considerable amounts of P in groundwater. When redox conditions became reducing, Fe(III)-oxide reduction dominated, resulting in enrichment of both P and As in groundwater. In Fe(III)-reducing conditions, secondary Ca/Fe(II)-minerals might serve as an important sink for P. When redox conditions became SO42--reducing, preferential adsorption and incorporation of P over As on Fe(II)-sulfides might constrain the As immobilization pathway, resulting in immediate retardation of P and hysteretic immobilization of As. This P-immobilization pathway in natural aquifers has not been described before. This study provides novel insights into P cycling and As enrichment in groundwater systems. Understanding the roles of Fe(II)- and S(-II)-minerals in the immobilization of and interaction between P and As in response to SO42- reduction may help to inspire effective in-situ remediation of contaminated groundwater, in which P and As coexist and remain mobile for decades or longer.

Nanoscale Sulfur Improves Plant Growth and Reduces Arsenic Toxicity and Accumulation in Rice (Oryza sativa L.)

Rice is known to accumulate arsenic (As) in its grains, posing serious health concerns for billions of people globally. We studied the effect of nanoscale sulfur (NS) on rice seedlings and mature plants under As stress. NS application caused a 40% increase in seedling biomass and a 26% increase in seed yield of mature plants compared to untreated control plants. AsIII exposure caused severe toxicity to rice; however, coexposure of plants to AsIII and NS alleviated As toxicity, and growth was significantly improved. Rice seedlings treated with AsIII + NS produced 159 and 248% more shoot and root biomass, respectively, compared to plants exposed to AsIII alone. Further, AsIII + NS-treated seedlings accumulated 32 and 11% less As in root and shoot tissues, respectively, than the AsIII-alone treatment. Mature plants treated with AsIII + NS produced 76, 110, and 108% more dry shoot biomass, seed number, and seed yield, respectively, and accumulated 69, 38, 18, and 54% less total As in the root, shoot, flag leaves, and grains, respectively, compared to AsIII-alone-treated plants. A similar trend was observed in seedlings treated with AsV and NS. The ability of sulfur (S) to alleviate As toxicity and accumulation is clearly size dependent as NS could effectively reduce bioavailability and accumulation of As in rice via modulating the gene expression activity of As transport, S assimilatory, and glutathione synthesis pathways to facilitate AsIII detoxification. These results have significant environmental implications as NS application in agriculture has the potential to decrease As in the food chain and simultaneously enable crops to grow and produce higher yields on marginal and contaminated lands.

Acetotrophic sulfate-reducing consortia develop active biofilms on zeolite and glass beads in batch cultures at initial pH 3

Sulfate-reducing microbial communities remain a suitable option for the remediation of acid mine drainage using several types of carrier materials and appropriate reactor configurations. However, acetate prevails as a product derived from the incomplete oxidation of most organic substrates by sulfate reducers, limiting the efficiency of the whole process. An established sulfate-reducing consortium, able to degrade acetate at initial acidic pH (3.0), was used to develop biofilms over granular activated carbon (GAC), glass beads, and zeolite as carrier materials. In batch assays using glycerol, biofilms successfully formed on zeolite, glass beads, and GAC with sulfide production rates of 0.32, 0.26, and 0.14 mmol H₂S/L·d, respectively, but only with glass beads and zeolite, acetate was degraded completely. The planktonic and biofilm communities were determined by the 16S rRNA gene analysis to evaluate the microbial selectivity of the carrier materials. In total, 46 OTUs (family level) composed the microbial communities. Ruminococcaceae and Clostridiaceae families were present in zeolite and glass beads, whereas Peptococcaceae was mostly enriched on zeolite and Desulfovibrionaceae on glass beads. The most abundant sulfate reducer in the biofilm of zeolite was Desulfotomaculum sp., while Desulfatirhabdium sp. abounded in the planktonic community. With glass beads, Desulfovibrio sp. dominated the biofilm and the planktonic communities. Our results indicate that both materials (glass beads and zeolite) selected different key sulfate-reducing microorganisms able to oxidize glycerol completely at initial acidic pH, which is relevant for a future application of the consortium in continuous bioreactors to treat acidic streams. KEY POINTS: • Complete consumption of glycerol and acetate at acidic pH by sulfate reduction. • Glass beads and zeolite are suitable materials to form sulfate-reducing biofilms. • Acetotrophic sulfate-reducing bacteria attached to zeolite preferably.

Enhancing Roxarsone Degradation and In Situ Arsenic Immobilization Using a Sulfate-Mediated Bioelectrochemical System

Roxarsone (ROX) is widely used in animal farms, thereby producing organoarsenic-bearing manure/wastewater. ROX cannot be completely degraded and nor can its arsenical metabolites be effectively immobilized during anaerobic digestion, potentially causing arsenic contamination upon discharge to the environment. Herein, we designed and tested a sulfate-mediated bioelectrochemical system (BES) to enhance ROX degradation and in situ immobilization of the released inorganic arsenic. Using our BES (0.5 V voltage and 350 μM sulfate), ROX and its metabolite, 4-hydroxy-3-amino-phenylarsonic acid (HAPA), were completely degraded within 13-22 days. In contrast, the degradation efficiency of ROX and HAPA was <85% during 32-day anaerobic digestion. In a sulfate-mediated BES, 75.0-83.2% of the total arsenic was immobilized in the sludge, significantly more compared to the anaerobic digestion (34.1-57.3%). Our results demonstrate that the combination of sulfate amendment and voltage application exerted a synergetic effect on enhancing HAPA degradation and sulfide-driven arsenic precipitation. This finding was further verified using real swine wastewater. A double-cell BES experiment indicated that As(V) and sulfate were transported from the anode to the cathode chamber and coprecipitated as crystalline alacranite in the cathode chamber. These findings suggest that the sulfate-mediated BES is a promising technique for enhanced arsenic decontamination of organoarsenic-bearing manure/wastewater.

Arsenic and the gastrointestinal tract microbiome

Arsenic is a toxin, ranking first on the Agency for Toxic Substances and Disease Registry and the Environmental Protection Agency Priority List of Hazardous Substances. Chronic exposure increases the risk of a broad range of human illnesses, most notably cancer; however, there is significant variability in arsenic-induced disease among exposed individuals. Human genetics is a known component, but it alone cannot account for the large inter-individual variability in the presentation of arsenicosis symptoms. Each part of the gastrointestinal tract (GIT) may be considered as a unique environment with characteristic pH, oxygen concentration, and microbiome. Given the well-established arsenic redox transformation activities of microorganisms, it is reasonable to imagine how the GIT microbiome composition variability among individuals could play a significant role in determining the fate, mobility and toxicity of arsenic, whether inhaled or ingested. This is a relatively new field of research that would benefit from early dialogue aimed at summarizing what is known and identifying reasonable research targets and concepts. Herein, we strive to initiate this dialogue by reviewing known aspects of microbe-arsenic interactions and placing it in the context of potential for influencing host exposure and health risks. We finish by considering future experimental approaches that might be of value.

Treatment of As-rich mine effluents and produced residues stability: Current knowledge and research priorities for gold mining

Refractory ores, in which gold is often embedded within As-bearing and acid-generating sulfide minerals, are becoming the main gold source worldwide. These ores require an oxidizing pre-treatment, prior to cyanidation, to efficiently breakdown the sulfides and enhance gold liberation. As a result, large volumes of As-rich effluents (> 500 mg/L) are produced through the pre-oxidation of refractory gold ores and/or the exposure of As-bearing tailings upon exposure to air and water. Limited information is available on performant treatment of these effluents, especially of pre-oxidation effluents characterized by a complex chemistry, extremely acidic or alkaline pH and high concentrations of arsenic. The treatment of As-rich effluents is mainly based on precipitation (using Al or Fe salts and/or Ca-based compounds) and (electro)-chemical or biological oxidation processes. A performant treatment process must maximize As removal from contaminated mine water and allow for the production of residues that are geochemically stable over the long term. An extensive literature review showed that Fe(III)-As(V) precipitates, especially bioscorodite and (nano)scorodite, appear to be the most appropriate forms to immobilize As due to their low solubility and high stability, especially when encapsulated within an inert material such as hydroxyl gels. Research is still required to assess the long-term stability of these As-bearing residues under mine-site conditions for the sustainable exploitation of refractory gold deposits.

Realgar (As4S4) bioprecipitation in microcosm fed by a natural groundwater and organic matter

Sequestration of arsenic to biogenic sulfide minerals is known from As-contaminated anoxic environments. Despite numerous successful laboratory experiments, the process remains difficult to predict in moderate arsenic conditions. We performed microcosm experiments using naturally contaminated groundwater (containing ca. 6 mg/L As) and natural organic matter (NOM) particles both collected from wetland soil. Macroscopic realgar precipitates, occasionally accompanied by bonazziite, a FeS phase, elementary S, calcite, and whewellite, appeared after 4 to 18 months. Realgar only precipitated in microcosms moderately poisoned by azide or antibiotics and those in which oxidation of hydrogen sulfide to sulfur took place. The biomineralization process was not affected by the presence of additional carbon sources or the diversity, community structure, and functional composition of the microbial community. Hydrogen sulfide concentration was greater in the realgar-free microcosms, suggesting that arsenic thiolation prevented precipitation of realgar. We compared our data to available microbial community data from soils with different rates of realgar precipitation, and found that the communities from realgar-encrusted NOM particles usually showed limited sulfate reduction and the presence of fermentative metabolisms, whereas communities from realgar-free NOM particles were strongly dominated by sulfate reducers. We argue that the limited sulfate supply and intensive fermentation amplify reducing conditions, which make arsenic sulfide precipitation plausible in high-sulfate, low-arsenic groundwaters.

Arsenite removal without thioarsenite formation in a sulfidogenic system driven by sulfur reducing bacteria under acidic conditions

Sulfidogenic process using sulfate-reducing bacteria (SRB) has been used to remove arsenite from the arsenic-contaminated waters through the precipitation of arsenite with sulfide. However, excessive sulfide production and significant pH increase induced by sulfate reduction result in the formation of the mobile thioarsenite by-products and the inefficiency and instability of arsenite removal, especially when the arsenite level fluctuates. In this study, we proposed a novel sulfidogenic process driven by sulfur reducing bacteria (S⁰RB) for the arsenite removal under acidic conditions. In a long term experiment, efficient sulfide production (0.42 ± 0.2 kg S/m³-d) was achieved without changing the acidic condition (pH around 4.3) in a sulfur reduction bio-reactor. With the acidic sulfide-containing effluents from the bio-reactor, over 99% of arsenite (10 mg As/L) in the arsenic-contaminated water was precipitated without the formation of soluble thioarsenite by-products, even in the presence of excessive sulfide. Maintaining the acidic condition (pH around 4.3) of the sulfide-containing effluent was essential to ensure the efficient arsenite precipitation and minimize the formation of thioarsenite by-products when the arsenite to sulfide molar ratios ranged from 0.1 to 0.46. An acid-tolerant S⁰RB, Desulfurella, was found to be responsible for the efficient dissimilatory sulfur reduction under acidic conditions without changing the solution pH significantly. The microbial sulfur reduction may proceed through the direct electron transfer between the S⁰RB and sulfur particles, and also through the indirect electron transport mediated by electron carriers. The findings of this study demonstrate that the proposed sulfidogenic process driven by S⁰RB working under acidic conditions can be a promising alternative to the SRB-based process for arsenite removal from the arsenic-contaminated waters.

Improved Method for Estimating Reaction Rates During Push-Pull Tests

The breakthrough curve obtained from a single-well push-pull test can be adjusted to account for dilution of the injection fluid in the aquifer fluid. The dilution-adjusted breakthrough curve can be analyzed to estimate the reaction rate of a solute. The conventional dilution-adjusted method assumes that the ratios of the concentrations of the nonreactive and reactive solutes in the injection fluid vs. the aquifer fluid are equal. If this assumption is invalid, the conventional method will generate inaccurate breakthrough curves and may lead to erroneous conclusions regarding the reactivity of a solute. In this study, a new method that generates a dilution-adjusted breakthrough curve was theoretically developed to account for any possible combination of nonreactive and reactive solute concentrations in the injection and aquifer fluids. The newly developed method was applied to a field-based data set and was shown to generate more accurate dilution-adjusted breakthrough curves. The improved dilution-adjusted method presented here is simple, makes no assumptions regarding the concentrations of the nonreactive and reactive solutes in the injection and aquifer fluids, and easily allows for estimating reaction rates during push-pull tests.

Lipidomic profiling of subchronic As4S4exposure identifies inflammatory mediators as sensitive biomarkers in rats

Arsenic sulfide compounds provide nearly all of the world's supply of arsenic.

A Case Study of Heavy Metal Pollution in Water of Bone River by Artisanal Small-Scale Gold Mine Activities in Eastern Part of Gorontalo, Indonesia

Bone River, which is located in the eastern part of Gorontalo, Indonesia, has an issue of contamination by heavy metals due to artisanal small-scale gold mine (ASGM) activities. This river is used as a source of water by inhabitants living along the river, due to the lack of another clean water supply. The purpose of this study is to investigate the concentration of As from the mining site alongside Bone River, and to reveal the As pollution source. Water and sediment samples were collected from ASGM and along Bone River to the delta. The concentrations of As, Hg, and Pb in water samples were identified by inductively coupled plasma mass spectroscopy (ICP–MS), while concentrations in sediment samples were determined by particle-induced X-ray emission (PIXE). Results showed that the concentrations of As, Hg, and Pb in water ranged from 66 to 82,500 µg/L, 17 to 2080 µg/L, and 11 to 1670 µg/L, respectively. These levels exceeded, by 1000 to 10,000 times, the safe limits of drinking water defined by the World Health Organization, which indicated that Bone River water is not safe for drinking or cooking purposes.

Engineering bacteria for biogenic synthesis of chalcogenide nanomaterials

Microbes naturally build nanoscale structures, including structures assembled from inorganic materials. Here, we combine the natural capabilities of microbes with engineered genetic control circuits to demonstrate the ability to control biological synthesis of chalcogenide nanomaterials in a heterologous host. We transferred reductase genes from both Shewanella sp. ANA-3 and Salmonella enterica serovar Typhimurium into a heterologous host (Escherichia coli) and examined the mechanisms that regulate the properties of biogenic nanomaterials. Expression of arsenate reductase genes and thiosulfate reductase genes in E. coli resulted in the synthesis of arsenic sulfide nanomaterials. In addition to processing the starting materials via redox enzymes, cellular components also nucleated the formation of arsenic sulfide nanomaterials. The shape of the nanomaterial was influenced by the bacterial culture, with the synthetic E. coli strain producing nanospheres and conditioned media or cultures of wild-type Shewanella sp. producing nanofibres. The diameter of these nanofibres also depended on the biological context of synthesis. These results demonstrate the potential for biogenic synthesis of nanomaterials with controlled properties by combining the natural capabilities of wild microbes with the tools from synthetic biology.

Iron Oxides Minimize Arsenic Mobility in Soil Material Saturated with Saline Wastewater

The soil material in constructed wetlands is effective in retaining Se from flue-gas desulfurization (FGD) wastewater (WW), but reducing conditions can enhance native-soil As mobility. A laboratory-based soil column experiment was performed to assess the effectiveness of ferrihydrite (Fh) in minimizing the mobility of native-soil As in water-saturated soil material. A saline FGD WW mixture (i.e., influent) was delivered to columns of untreated and Fh-treated soil for 60 d. One untreated column and one Fh-treated column were then subjected to drying, followed by an additional 30 d of influent delivery. Although the influent was low in As (∼1 μg L) and the soil As level was normal, the total dissolved As concentration of effluent from the untreated columns increased with time, from ∼1 μg L to a maximum of ∼27 μg L. In contrast, effluent from the Fh-treated columns remained low in As (i.e., <5 μg L). The strong correlation between total dissolved Fe and As in the effluent suggested that reductive dissolution of native-soil Fe minerals was responsible for releasing As into solution. Results from X-ray absorption spectroscopy showed newly precipitated Fe minerals in the Fh-treated soil, and the remaining As was mainly As(V) species in both the untreated and Fh-treated soils. Thus, native-soil As mobilized under saturated conditions can be sequestered by adding poorly crystalline Fe oxides to soil prior to saturation. Furthermore, results obtained by drying and rewetting the columns showed that saturated conditions must be maintained to minimize the remobilization of sequestered As and retained Se.

Phylogenetic Structure and Metabolic Properties of Microbial Communities in Arsenic-Rich Waters of Geothermal Origin

Arsenic (As) is a toxic element released in aquatic environments by geogenic processes or anthropic activities. To counteract its toxicity, several microorganisms have developed mechanisms to tolerate and utilize it for respiratory metabolism. However, still little is known about identity and physiological properties of microorganisms exposed to natural high levels of As and the role they play in As transformation and mobilization processes. This work aims to explore the phylogenetic composition and functional properties of aquatic microbial communities in As-rich freshwater environments of geothermal origin and to elucidate the key microbial functional groups that directly or indirectly may influence As-transformations across a natural range of geogenic arsenic contamination. Distinct bacterial communities in terms of composition and metabolisms were found. Members of Proteobacteria, affiliated to Alpha- and Betaproteobacteria were mainly retrieved in groundwaters and surface waters, whereas Gammaproteobacteria were the main component in thermal waters. Most of the OTUs from thermal waters were only distantly related to 16S rRNA gene sequences of known taxa, indicating the occurrence of bacterial biodiversity so far unexplored. Nitrate and sulfate reduction and heterotrophic As(III)-oxidization were found as main metabolic traits of the microbial cultivable fraction in such environments. No growth of autotrophic As(III)-oxidizers, autotrophic and heterotrophic As(V)-reducers, Fe-reducers and oxidizers, Mn-reducers and sulfide oxidizers was observed. The ars genes, involved in As(V) detoxifying reduction, were found in all samples whereas aioA [As(III) oxidase] and arrA genes [As(V) respiratory reductase] were not found. Overall, we found that As detoxification processes prevailed over As metabolic processes, concomitantly with the intriguing occurrence of novel thermophiles able to tolerate high levels of As.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

Complete removal of arsenic and zinc from a heavily contaminated acid mine drainage via an indigenous SRB consortium

Acid mine drainages (AMD) are major sources of pollution to the environment. Passive bio-remediation technologies involving sulfate-reducing bacteria (SRB) are promising for treating arsenic contaminated waters. However, mechanisms of biogenic As-sulfide formation need to be better understood to decontaminate AMDs in acidic conditions. Here, we show that a high-As AMD effluent can be decontaminated by an indigenous SRB consortium. AMD water from the Carnoulès mine (Gard, France) was incubated with the consortium under anoxic conditions and As, Zn and Fe concentrations, pH and microbial activity were monitored during 94days. Precipitated solids were analyzed using electron microscopy (SEM/TEM-EDXS), and Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy at the As K-edge. Total removal of arsenic and zinc from solution (1.06 and 0.23mmol/L, respectively) was observed in two of the triplicates. While Zn precipitated as ZnS nanoparticles, As precipitated as amorphous orpiment (am-As^III₂S₃) (33-73%), and realgar (As^IIS) (0-34%), the latter phase exhibiting a particular nanowire morphology. A minor fraction of As is also found as thiol-bound As^III (14-23%). We propose that the formation of the As^IIS nanowires results from As^III₂S₃ reduction by biogenic H₂S, enhancing the efficiency of As removal. The present description of As immobilization may help to set the basis for bioremediation strategies using SRB.

Adaptation of a methanogenic consortium to arsenite inhibition

Arsenic (As) is a ubiquitous metalloid known for its adverse effects to human health. Microorganisms are also impacted by As toxicity, including methanogenic archaea, which can affect the performance of process in which biological activity is required (i.e. stabilization of activated sludge in wastewater treatment plants). The novel ability of a mixed methanogenic granular sludge consortium to adapt to the inhibitory effect of arsenic (As) was investigated by exposing the culture to approximately 0.92 mM of AsIII for 160 d in an arsenate (AsV) reducing bioreactor using ethanol as the electron donor. The results of shaken batch bioassays indicated that the original, unexposed sludge was severely inhibited by arsenite (AsIII) as evidenced by the low 50% inhibition concentrations (IC50) determined, i.e., 19 and 90 μM for acetoclastic- and hydrogenotrophic methanogenesis, respectively. The tolerance of the acetoclastic and hydrogenotrophic methanogens in the sludge to AsIII increased 47-fold (IC50 = 910 μM) and 12-fold (IC50= 1100 μM), respectively, upon long-term exposure to As. In conclusion, the methanogenic community in the granular sludge demonstrated a considerable ability to adapt to the severe inhibitory effects of As after a prolonged exposure period.