Bacteria are candidates for the biotransformation of environmental arsenic (As), while As metabolism in bacteria is not yet fully understood. In this study, we sequenced the genome of an As-resistant bacterium strain Stenotrophomonas maltophilia SCSIOOM isolated from the fish gut. After arsenate (As(V)) exposure, S. maltophilia transformed As(V) to organoarsenicals, along with the significant change of the expression of 40 genes, including the upregulation of arsH, arsRBC and betIBA. The heterogeneous expression of arsH and arsRBC increased As resistance of E. coli AW3110 by increasing As efflux and transformation. E. coli AW3110 (pET-betIBA) could transform inorganic As into dimethylarsinate (DMA) and nontoxic arsenobetaine (AsB), which suggested that AsB could be synthesized through the synthetic pathway of its analog-glycine betaine. In addition, the existence of arsRBC, betIBA and arsH reduced the reactive oxygen species (ROS) induced by As exposure. In total, these results demonstrated that S. maltophilia adopted an As metabolism strategy by reducing As accumulation and synthesizing less toxic As species. We first reported the production and potential synthetic pathway of AsB in bacteria, which improved our knowledge of As toxicology in microorganisms.

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.

The main intent of the current research was to appraise if combined application of hydrogen sulfide (H₂S, 0.2 mM) and silicon (Si 2.0 mM) could improve tolerance of tomato plants to arsenic (As as sodium hydrogen arsenate heptahydrate, 0.2 mM) stress. Plant growth, chlorophylls (Chl), PSII maximum efficiency (Fv/Fm), H₂S concentration and L-cysteine desulfhydrase activity were found to be suppressed, but leaf and root As, leaf proline content, phytochelatins, malondialdehyde (MDA) and H₂O₂ as well as the activity of lipoxygenase (LOX) increased under As stress. H₂S and Si supplied together or alone enhanced the concentrations of key antioxidant biomolecules such as ascorbic acid, and reduced glutathione and the activities of key antioxidant system enzymes including catalase (CAT), superoxide dismutase (SOD), dehydroascorbate reductase (DHAR), glutathione reductase (GR), and glutathione S-transferase (GST). In comparison with individual application of H₂S or Si, the joint supplementation of both had better effect in improving growth and key biochemical processes, and reducing tissue As content, suggesting a putative collaborative role of both molecules in improving tolerance to As-toxicity in tomato plants.

Arsenic (As) is among the most dangerous metalloids and is harmful to human wellbeing. In this laboratory study, Al(III)-modified kapok fibres (Al-Kapok) were used to remove As(V) from water. The sorbent was characterised using Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX). Batch experiments were performed to observe the performance of Al-Kapok in the removal of As(V) and to examine the effects of pH, temperature, adsorbent dose, and coexisting ions on the adsorption process. The surface of the sorbent changed after aluminium modification, and the results of the batch experiments showed that the adsorption of As(V) occurred mainly via endothermic-spontaneous chemisorption at the solution and solid interface of Al-Kapok. The As(V) removal efficiency was approximately 76%–84%, and it was slightly affected at pH levels below 8.0. Further study showed that the maximum adsorption capacity of Al-Kapok for As(V) was 118 μg/g at 30 °C and pH 6, and notable adverse effects were caused by the presence of SO42−and PO43−. It was also found that the boundary layer and film diffusion contributed more to As(V) adsorption. After five adsorption/desorption cycles, regeneration recovered approximately 92% of the adsorption capacity of Al-Kapok used. Overall, Al-Kapok appears to be a suitable adsorbent material for the purification of As-contaminated water.

Dissolved organic matter (DOM) and dissolved ions are two integral parameters to affect the environmental fate of As in different ways. Numerous studies chose surrogate of DOM, humic substances (HSs), to investigate the As complexation behavior. However, microbial secretion (protein and polysaccharide) was also considered for a great proportion in surface aquatic system, and its effect was still not fully understood. The present research distinguished the As complexation behavior with different DOM components (HSs, protein, polysaccharide and synthetic organic matter) in natural and simulated water samples. The results indicated that different DOM components exhibited various binding capacities for As. HSs showed the strongest affinity for As, followed by long-chain compounds (polysaccharide and synthetic organic matter) and proteins. In water source, HSs were probably the primary parameter for As complexation. In eutrophic water system, however, polysaccharide maybe the main DOM component to bind As. Cationic bridge function was prone to occur in the presence of HSs, but not observed in the presence of protein. PO₄³⁻ competed for binding sites with As, consequently decreasing the As complexation with all the DOM components. The research implied that a comprehensive and meticulous analyses of DOM fractions and coexist ions are the prerequisite to understanding the behavior of As (or other pollutants) in different natural aquatic systems.

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.

High-arsenic wastewater derived from the metallurgical industry of nonferrous minerals is one of the most dangerous arsenic (As) sources that usually follow the emission of massive hazardous arsenic-bearing wastes. Considering the properties of red mud (RM), we propose an alternative and environmentally friendly method for the efficient remediation of high-arsenic wastewater using RM through formation of AlAsO₄@silicate precipitate, aiming at ''zero-emission of hazardous solid waste''. The results show nearly 100% of arsenic could be stepwisely removed from high-arsenic wastewater and reduce the arsenic concentration from 6100 mg/L to 40 μg/L using RM at room temperature. The highest arsenic removal capacity of RM reaches 101.5 mg/g at a RM-to-wastewater ratio of 40 g/L due to the superior arsenic adsorption and the co-precipitation of arsenate and Al³⁺ to form insoluble aluminum arsenate. The silicate shell of arsenic-loaded RM created at an alkaline condition acts as an arsenic stabilizer, resulting in a leached arsenic concentration of 1.2 mg/L in TCLP tests. RM acts as a highly effective arsenic remover and stabilizer for the disposal of high-arsenic wastewater. It shows great potential for the remediation of wastewater containing heavy metals with varying concentrations to produce clean water available for industrial purpose.

The bioaccessibility of arsenic and its speciation are two important factors in assessing human health risks exposure to contaminated soils. However, the effects of human gut microbiota on arsenic bioaccessibility and its speciation are not well characterized. In this study, an improved in vitro model was utilized to investigate the bioaccessibility of arsenic in the digestive tract and the role of human gut microbiota in the regulation of arsenic speciation. For all soils, arsenic bioaccessibility from the combined in vitro model showed that it was <40% in the gastric, small intestinal and colon phases. This finding demonstrated that the common bioaccessibility approach assuming 100% bioaccessibility would overestimate the human health risks posed by contaminated soils. Further to this, the study showed that arsenic bioaccessibility was 22% higher in the active colon phase than that in the sterile colon phase indicating that human colon microorganisms could induce arsenic release from the solid phase. Only inorganic arsenic was detected in the gastric and small intestinal phases, with arsenate [As(V)] being the dominant arsenic species (74%–87% of total arsenic). Arsenic speciation was significantly altered by the active colon microbiota, which resulted in the formation of methylated arsenic species, including monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] with low toxicity, and a highly toxic arsenic species monomethylarsonous acid [MMA(III)]. Additionally, a high level of monomethylmonothioarsonic acid [MMMTA(V)] (up to 17% of total arsenic in the extraction solution) with unknown toxicological properties was also detected in the active colon phase. The formation of various organic arsenic species demonstrated that human colon microorganisms could actively metabolize inorganic arsenic into methylated arsenicals and methylated thioarsenicals. Such transformation should be considered when assessing the human health risks associated with oral exposure to soil.

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.