Bacteria play crucial roles in the biogeochemical cycle of arsenic (As) and selenium (Se) as these elements are metabolized via detoxification, energy generation (anaerobic respiration) and biosynthesis (e.g. selenocysteine) strategies. To date, arsenic and selenium biomineralization in bacteria were studied separately. In this study, the anaerobic metabolism of As and Se in Shewanella sp. O23S was investigated separately and mixed, with an emphasis put on the biomineralization products of this process. Multiple analytical techniques including ICP-MS, TEM-EDS, XRD, Micro-Raman, spectrophotometry and surface charge (zeta potential) were employed. Shewanella sp. O23S is capable of reducing selenate (SeO₄²⁻) and selenite (SeO₃²⁻) to red Se(-S)⁰, and arsenate (AsO₄³⁻) to arsenite (AsO₃³⁻). The release of H₂S from cysteine led to the precipitation of AsS minerals: nanorod AsS and granular As₂S₃. When As and Se oxyanions were mixed, both As–S and Se(-S)⁰ biominerals were synthesized. All biominerals were extracellular, amorphous and presented a negative surface charge (−24 to −38 mV). Kinetic analysis indicated the following reduction yields: SeO₃²⁻ (90%), AsO₄³⁻ (60%), and SeO₄²⁻ (<10%). The mix of SeO₃²⁻ with AsO₄³⁻ led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO₄²⁻ incubated with AsO₄³⁻ boosted the Se removal (71%). The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process characteristic of various Shewanella species and strains. This is the first study documenting a complex interplay between As and Se oxyanions: selenite decreased arsenate reduction, whereas arsenate stimulated selenate reduction. Further investigation needs to clarify whether Shewanella sp. O23S employs multi-substrate respiratory enzymes or separate, high affinity enzymes for As and Se oxyanion respiration.

Arsenic (As) pollution remains a major threat to the quality of global soils and drinking water. The health effects of As pollution are often severe and have been largely reported across Asia and South America. This study investigated the possibility of using unmodified biochar derived from rice husk (RB) and aspen wood (WB) at 400 °C and 700 °C to enhance the precipitation of calcium/arsenic compounds for the removal of As(III) from solution. The approach was based on utilizing calcium to precipitate arsenic in solution and adding unmodified biochar to enhance the process. Using this approach, As(III) concentration in aqueous solution decreased by 58.1% when biochar was added, compared to 25.4% in the absence of biochar. Varying the pH from acidic to alkaline enabled an investigation into the pH dependent dynamics of the approach. Results indicated that significant precipitation was only possible at near neutral pH (i.e. pH = 6.5) where calcium arsenites (i.e. Ca(AsO₂)₂, and CaAsO₂OH•½H₂O) and arsenates (i.e. Ca₅(AsO₄)₃OH) were precipitated and deposited as aggregates in the pores of biochars. Arsenite was only slightly precipitated under acidic conditions (pH = 4.5) while no arsenite was precipitated under alkaline conditions (pH = 9.5). Arsenite desorption from wood biochar was lowest at pH 6.5 indicating that wood biochar was able to retain a large quantity of the precipitates formed at pH 6.5 compared to pH 4.5 and pH 9.5. Given that the removal of As(III) from solution is often challenging and that biochar modification invites additional cost, the study demonstrated that low cost unmodified biochar can be effective in enhancing the removal of As(III) from the environment through Ca–As precipitation.

Arsenic is a potent toxicant, and long-term exposure to inorganic arsenic causes lung damage. M2 macrophages play an important role in the pathogenesis of pulmonary fibrosis. However, the potential connections between arsenic and M2 macrophages in the development of pulmonary fibrosis are elusive. C57BL/6 mice were fed with drinking water containing 0, 10 and 20 ppm arsenite for 12 months. We have found that, in lung tissues of mice, arsenite, a biologically active form of arsenic, elevated H19, c-Myc, and Arg1; decreased let-7a; and caused pulmonary fibrosis. For THP-1 macrophages (THP-M) and bone-marrow-derived macrophages (BMDMs), 8 μM arsenite increased H19, c-Myc, and Arg1; decreased let-7a; and induced M2 polarization of macrophages, which caused secretion of the fibrogenic cytokine, TGF-β1. Down-regulation of H19 or up-regulation of let-7a reversed the arsenite-induced M2 polarization of macrophages. Arsenite-treated THP-M and BMDMs co-cultured with MRC-5 cells or primary lung fibroblasts (PLFs) elevated levels of p-SMAD2/3, SMAD4, α-SMA, and collagen I in lung fibroblasts and resulted in the activation of lung fibroblasts. Knockout of H19 or up-regulation of let-7a in macrophages reversed the effects. The results indicated that H19 functioned as an miRNA sponge for let-7a, which was involved in arsenite-induced M2 polarization of macrophages and induced the myofibroblast differentiation phenotype by regulation of c-Myc. In the sera of arseniasis patients, levels of hydroxyproline and H19 were higher, and levels of let-7a were lower than levels in the controls. These observations elucidate a possible mechanism for arsenic exposure-induced pulmonary fibrosis.

Salinity stress affects aquatic microalgal growth and their physiological responses have been studied extensively. However, arsenic (As) accumulation and biotransformation by freshwater phytoplankton under a salinity gradient have never been addressed. This study reports a distinctive pattern of As uptake, accumulation, and biotransformation by four axenic freshwater phytoplankton species, i.e., Scenedesmus acutus, Closterium aciculare, Staurastrum paradoxum, and Pediastrum duplex. Phytoplankton cells were incubated in sterilised C medium modified with varying salinity levels (0–5‰) in association with arsenate and phosphate concentrations. The biotransformation of arsenate (i.e., As(V)) to arsenite (As(III)) and to further methylated species decreased with increasing salinity in the culture medium whereas As accumulation increased. Among the four strains, only S. acutus and S. paradoxum converted As(V) to As(III), with no detected methylated species. In contrast, C. aciculare and P. duplex biotransformed As(V) to As(III) and further to methyl arsenic species, such as DMAA. S. acutus and S. paradoxum exhibited higher accumulation tendency than the other two species. S. paradoxum showed the lowest As reduction rate (i.e., As(V) to As(III)) compared to other species, although, without significant variations. The morphological changes were observed in phytoplankton cells in response to increased salinity stress. Moreover, As(V) concentrations in the culture medium significantly decreased by day 7–14. Thus, this study presents a conceptual model of the As biotransformation pattern by axenic freshwater phytoplankton.

Arsenite [As(III)] toxicity causes impeded growth, inadequate productivity of plants and toxicity through the food chain. Using various chemical residues for priming is one of the approaches in conferring arsenic tolerance in crops. We investigated the mechanism of abscisic acid (ABA)-induced As(III) tolerance in rice genotypes (cv. Swarna and Swarna Sub1) pretreated with 10 μM of ABA for 24 h and transferred into 0, 25 and 50 μM arsenic for 10 days. Plants showed a dose-dependent bioaccumulation of As(III), oxidative stress indicators like superoxide, hydrogen peroxide, thiobarbituric acid reactive substances and the activity of lipoxygenase. As(III) had disrupted cellular redox that reflecting growth indices like net assimilation rate, relative growth rate, specific leaf weight, leaf mass ratio, relative water content, proline, delta-1-pyrroline-5-carboxylate synthetase and electrolyte leakage. ABA priming was more protective in cv. Swarna Sub1 than Swarna for retrieval of total glutathione pool, non-protein thiols, cysteine, phytochelatin and glutathione reductase. Phosphate metabolisms were significantly curtailed irrespective of genotypes where ABA had moderated phosphate uptake and its metabolizing enzymes like acid phosphatase, alkaline phosphatase and H⁺/ATPase. Rice seedlings had regulated antioxidative potential with the varied polymorphic expression of those enzymes markedly with antioxidative enzymes. The results have given the possible cellular and physiological traits those may interact with ABA priming in the establishment of plant tolerance with As(III) over accumulation and, thereby, its amelioration for oxidative damages. Finally, cv. Swarna Sub1 was identified as a rice genotype as a candidate for breeding program for sustainability against As(III) stress with cellular and physiological traits serving better for selection pressure.

A highly efficient, eco-friendly and relatively low-cost catalyst is necessary to tackle bottlenecks in the treatment of industrial wastewater laden with heavy metals and antibiotic such as livestock farm and biogas liquids. This study investigated co-oxidative removal of arsenite (As(III)) and tetracycline (TC) by iron nanoparticles (Fe NP)-impregnated carbons based on heterogeneous Fenton-like reactions. The composites included Fe NP@biochar (BC), Fe NP@hydrochar (HC), and Fe NP@HC-derived pyrolysis char (HDPC). The functions of N and S atoms and the loading mass of the Fe NP in the Fe NP@BC in heterogeneous Fenton-like reactions were studied. To sustain its cost-effectiveness, the spent Fe NP@BC was regenerated using NaOH. Among the composites, the Fe NP@BC achieved an almost complete removal of As(III) and TC under optimized conditions (1.0 g/L of dose; 10 mM H₂O₂; pH 6; 4 h of reaction; As(III): 50 μM; TC: 50 μM). The co-oxidative removal of As(III) and TC by the Fe NP@ BC was controlled by the synergistic interactions between the Fe NPs and the active N and S sites of the BC for generating reactive oxygen species (ROS). After four consecutive regeneration cycles, about 61 and 95% of As(III) and TC removal were attained. This implies that the spent carbocatalyst still has reasonable catalytic activities for reuse. Overall, this suggests that adding technological values to unused biochar as a carbocatalyst like Fe NP@BC was promising for co-oxidative removal of As(III) and TC from contaminated water.

Previous studies have indicated that natural organic matter in the aquatic environment could affect arsenic bioaccumulation and biotransformation to aquatic organisms. However, the differences between the effects of arsenite and arsenate exposure have not been studied and compared in fish exposure models. In this study, adult zebrafish (Danio rerio) were exposed to 5 mg/L inorganic As solutions, in the presence of a range of humic acid (HA) concentrations (1, 2.5, 5, 10, 20 mg/L) in 96 h waterborne exposure. Results showed that in the presence of HA, total As bioaccumulation was significantly reduced in zebrafish following arsenite exposure, while this reduction was not observed during arsenate exposure. The reduction in total arsenic bioaccumulation for arsenite exposure can be explained by the fact that HA forming a surface coating on the cell surface, hindering transport and internalization. However, this reduction in total As was not observed due to differences in uptake pathways for arsenate exposure. Results also showed that Arsenobetaine (AsB) was the main biotransformation product in zebrafish following inorganic As exposure, accounting for 44.8%–64.7% of extracted arsenic species in all exposure groups. The addition of HA caused levels of MMA and As(III) to decrease, while the distribution of AsB significantly increased in arsenite exposure groups. The increase in AsB could be because the As(III)-HA complex was formed, affecting the methylation of As(III). In contrast, the addition of HA to arsenate exposure groups, did not affect the reduction of As(V) to As(III) and therefore, an increase in the distribution of AsB was not observed in arsenate exposure groups. This study provides useful information on the mechanisms of toxicity, for improved risk assessment of As in natural aquatic environments.

The present study proposes a maize sprouts hydroponic growth model to evaluate the As, Cd, Cr and Pb translocation from multinutrient fertilizer and to do speciation of As and Cr in this fertilizer and As in parts of plant in order to predict their phytoavailability. X-ray absorption near edge structure (XANES) was employed to speciate As and Cr directly on fertilizer solid sample. Arsenate (Asⱽ) and a solid solution of FeCrO₃ were the major species identified in the samples. The sprouts were hydroponically cultivated in water, fertilizer slurry and fertilizer extract media. Concentrations of As, Cd and Pb measured on leaves of maize sprouts ranged from 0.061 to 0.31 mg kg⁻¹, whereas Cr was not translocated to the aerial parts of sprouts. High performance liquid chromatographic with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) analysis was used to determine As speciation in maize sprouts, as well as in the fertilizer extracts and slurries. Arsenate was the only species identified in the initial fertilizer extract and this information is in agreement with the XANES results. However, the reduction of arsenate to arsenite was observed in extracts and slurries collected after sprout growth, probably due to the action of exudates secreted by plant roots. Arsenite was the predominant species identified in sprouts, the high phosphate concentration in the medium may have contributed to reduce arsenate phytoavailability.

Preoxidation of As(III) to As(V) is required for the efficient removal of total arsenic in the treatment of wastewater. In this work, the electro-Fenton oxidation of As(III) with a high efficiency was successfully achieved by using the system of the stainless steel net (SSN) coating with reduced graphene oxide (RGO@SSN) as the cathode and stainless steel net (SSN) as the sacrificial anode. The RGO@SSN was synthesized by electrophoretic deposition-annealing method. The carbon disorder and defects of RGO resulted from the remained oxygen-containing functional groups facilitated the electrocatalytically active sites for two-electron oxygen reduction reaction (ORR). A high concentration (up to 1000 μmol/L) of H₂O₂ was in-situ produced through two-electron oxygen reduction reaction of electro-catalysis, and then served as the electro-Fenton reagent for the oxidation of As(III). HO generated by H₂O₂ participating the electro-Fenton reaction or decomposed at the surface of RGO@SSN cathode at acid condition endowed the strong oxidizing ability for As(III). The electro-Fenton equipped with RGO@SSN cathode has a promising application in the oxidation and removal of organic or inorganic pollutants in wastewater.

The bioavailability of the metalloid arsenic (As) in paddy soil is controlled by microbial cycling of As and other elements such as iron (Fe) and sulfur (S), which are strongly influenced by water management in paddy fields. In this study, we evaluated how water management affects As bioavailability by growing rice plants in a geogenic As-contaminated soil. We determined As speciation in soil porewater and the diversity of the associated microbial community. Continuous flooding enhanced the release of Fe and As and increased arsenite (As(III)) and methylated As species concentrations in the rice grain compared with aerobic treatment. Total inorganic and organic As in the grain was 84% and 81% lower, respectively, in the aerobic treatment compared with the continuous flooding treatment. The amounts of Fe(III)-reducing bacteria (FeRB) increased in the flooded rhizosphere soil. The abundance of FeRB in the soil correlated with the dissolution of Fe and As. Among the As-transformation genes quantified, the aioA gene for As(III) oxidation and arsM gene for As(III) methylation were most abundant. The arsM copy number correlated positively with the levels of dsrB (dissimilatory (bi) sulfite reductase β-subunit), suggesting that dissimilatory sulfate-reducing bacteria (SRB) may play an important role in dimethylarsenate (DMAs(V)) production in soil. Our results show that decreased populations of rhizosphere FeRB and SRB contributed to a lower bioavailability of As, and decreased production of methylated arsenicals under oxic conditions.