Arsenic Oxide Research Articles

Novel Dielectric Barrier Discharge Trap for Arsenic Introduced by Electrothermal Vaporization: Possible Mechanism and Its Application.

In this work, a novel integrated dielectric barrier discharge (IDBD) reactor coupled to an electrothermal vaporizer (ETV) was established for arsenic determination. It is for the first time gas-phase enrichment (GPE) was fulfilled based on the hyphenation of ETV and DBD. The mechanisms of evolution of arsenic atomic and molecular species during vaporization, transportation, trapping, and release processes were investigated via X-ray photoelectron spectroscopy (XPS) and other approaches. Tentative mechanisms were deduced as follows: the newly designed DBD atomizer (DBDA) tube upstream to the air inlet fulfills the atomization of arsenic nanoparticles in vaporized aerosol, leading to free arsenic atoms that are indispensable for forming arsenic oxides; the DBD trap (DBDT) tube traps arsenic oxides under an O2-domininating atmosphere and then releases arsenic atoms under H2-dominating atmospheres. In essence, this process is a physical-chemical process rather than an electrostatic particle deposition. Such a trap and release sequence separates matrix interference and enhances analytical sensitivity. Under the optimized conditions, the method detection limit (LOD) was 0.04 mg/kg and the relative standard deviations (RSDs) were within 6% for As standard solution and real seafood samples, indicating adequate analytical sensitivity and precision. The mean spiked recoveries for laver, kelp, and Undaria pinnatifida samples were 95-110%, and the results of the certified reference materials (CRMs) were consistent with certified values. This ETV-DBD preconcentration scheme is easy and green and has low cost for As analysis in seafood samples. DBD was proved a novel ETV transportation enhancement and preconcentration technique for arsenic, revealing its potential in rapid arsenic analysis based on direct solid sampling ETV instrumentation.

Nanomaterials as adsorbents for As(III) and As(V) removal from water: A review

Freshwater demand will rise in the next couple of decades, with an increase in worldwide population growth and industrial development. The development activities, on one side, have increased the freshwater demand. However, the ground water has been degraded. Among the various organic and inorganic contaminants, arsenic is one of the most toxic elements. Arsenic contamination in ground waters is a major issue worldwide, especially in South and Southeast Asia. Various methods have been applied to provide a remedy to arsenic contamination, including adsorption, ion exchange, oxidation, coagulation-precipitation and filtration, and membrane filtration. Out of these methods, adsorption of As(III)/As(V) using nanomaterials and biopolymers has been used on a wide scale. The present review focuses on recently used nanomaterials and biopolymer composites for As(III)/As(V) sorptive removal. As(III)/As(V) adsorption mechanisms have been explored for various sorbents. The impacts of environmental factors such as pH and co-existing ions on As(III)/As(V) removal, have been discussed. Comparison of various nanosorbents and biopolymer composites for As(III)/As(V) adsorption and regeneration of exhausted materials has been included. Overall, this review will be useful to understand the sorption mechanisms involved in As(III)/As(V) removal by nanomaterials and biopolymer composites and their comparative sorption performances.

Oxidation mechanism of the arsenopyrite surface by oxygen with and without water: Experimental and theoretical analysis

Arsenopyrite is a common mineral that is abundant in nonferrous metal tailings. Study of the atomic configuration, chemical state, and morphology of the oxidation products of the arsenopyrite surface is important for enhancing the resource utilization and environmental safety of cementitious backfill. In this paper, the chemical states of the surface atoms of arsenopyrite oxidized with and without water were analyzed by X-ray photoelectron spectroscopy (XPS). The (001) surface oxidation process of arsenopyrite by oxygen with and without water was characterized using density functional theory (DFT). Results of the XPS analysis showed that water molecules participated in the oxidation reaction and formed a large number of OH structures on the surface of arsenopyrite; the relative content of Sn2−, As(III), and Fe(II) increased significantly. The DFT calculations revealed that oxygen molecules are more easily adsorbed on the As2-Fe2 site by chemical adsorption (Fe2 is the second ortho Fe atom of As2), and water molecules are adsorbed on the Fe2 site by physical adsorption. The oxidation process of the (001) surface by oxygen can be divided into four stages: adsorption, dissociation, bridging oxygen formation, and desorption of oxidation products; the As2 atom with three coordination sites on the surface of (001) is oxidized first. In the presence of water, the water molecule dissociates into OH and H, resulting in the hydroxylation of As and Fe atoms on the (001) surface. Calculation of the energy difference and reaction energy barrier between different reaction steps showed that the participation of water molecules reduces the reaction energy barrier and increases the reaction energy. The oxidation products are derived from the arsenic oxidation of (AsO2) and (HAsO2) types without and with water, and (HAsO2) is easily desorbed from the surface. Water molecules accelerate the surface oxidation of arsenopyrite and cause the surface to form hydroxylated oxidation products.

Metal arsenic mediated enhancement of type-2 immunity in brains with altered locomotive activities in mice with autism-like behavioral characteristics.

Exposure to metal arsenic (As) has been proposed as a risk factor for autism spectrum disorders (ASDs), which are neurodevelopmental disorders with worldwide increasing in its incidence. In the present study, BTBR T + tf/J (BTBR) mice with ASD-like behavioral characteristics and control highly social FVB mice were orally exposed to 0.1mM arsenic(III)oxide for 4weeks, and were compared to investigate neuroimmunological or behavioral abnormalities. IgG1:IgG2a ratios in brain tissues from BTBR mice exposed to As (BTBR-As) were significantly higher than those of BTBR-control mice (BTBR-C), but this change did not occur in FVB mice exposed to As. Levels of IL-4, IFN-γ, IL-1β, IL-17, and TNF-α in brain tissue were lowered in BTBR-As relative to BTBR-C, but this tendency was not observed with FVB mice. BTBR-As mice demonstrated decrease in relative travel distance and time spent in the center vs. the periphery of open field arena compared to BTBR-C. Sociability evaluation using three-way chamber test did not clearly demonstrate As-mediated alteration in social interaction in BTBR mice. These findings suggest the potential for As-driven predominant TH2-like reactivity profile in the brain microenvironment of BTBR mice and for As-mediated locomotive impairment probably associated with ASD.

Middle-low-temperature oxidation and adsorption of arsenic from flue gas by Fe–Ce-based composite catalyst

Fe–Ce-based composite catalysts were prepared and used for As2O3 catalytic oxidation and adsorption. They were characterized by XRD, BET, H2-TPR, Raman, SEM and XPS. The results suggests Fe, La and Zr can be partially doped into CeO2 lattice to form solid solutions. Compared with pure Fe2O3, the composite catalysts have stronger low-temperature reducibility, especially La3+ doping is beneficial to the formation of more low-temperature active sites. Raman and XPS measurements disclose the presence of oxygen vacancy and surface adsorbed oxygen in composite catalysts and these are more prominent in FeCeLaO. Ce3+ ratio increases to 18.30% after reaction, which confirms part of Ce4+ can participate in As2O3 oxidation and be reduced to Ce3+. The oxidation and adsorption capacity for As2O3 were investigated at different temperatures and O2 concentrations. The results show FeCeLaO exhibits excellent activity at middle-low-temperatures of 200–400 °C, the oxidation efficiency of As2O3 can reach 100%, the total adsorbed arsenic at 400 °C reaches 583.7 μg/g, which is 1.8 times of pure Fe2O3 at 600 °C. As2O3 oxidation mechanism over FeCeLaO with/without O2 was proposed through the Mars-Maessen theory with the aid of surface-active oxygen. The abundant oxygen vacancy defects and active chemisorbed oxygen play important roles and guarantee an efficient As2O3 oxidation, which is also the essential reason why the composite catalysts can effectively oxidize and adsorb As2O3 at middle-low-temperature of 200–400 °C, while pure Fe2O3 can only be at high temperature of 600–700 °C.

Arsenic photocatalytic oxidation over TiO2-loaded SBA-15

Versatile materials for oxidizing As(III) into less hazardous As(V) species has triggered increasing attention. In this work, 7% anatase TiO2 was coated on pre-formed SBA-15 by means of impregnation to produce Ti-SBA-15. The materials were characterized by XRD, Raman, FTIR, N2 adsorption, DRS and TEM techniques. Photo-oxidation tests at different pHs were carried out. In the dark, the As(III) adsorption equilibrium on Ti-SBA-15 was reached, showing a moderated adsorption capacity at pHs 4, 7, and 9 (7%, 11%, and 30%, respectively), being more evident at alkaline conditions. This fact was attributed to the presence of surface hydroxyl groups on the deposited TiO2, which promotes the formation of surface monodentate As(III) complexes on Ti-SBA-15. Moreover, Ti-SBA-15 showed to be an efficient photocatalyst in all pHs, being more than 98% of As(III) photo-oxidized into As(V). Results demonstrated that the oxidation process is induced by reactive oxygen species, including •OH and 1O2, generated onto the TiO2 nanoparticles loaded on SBA-15 during the irradiation. SBA-15 instead showed a negligible impact on adsorption and photo-oxidation, indicating that these processes are the effect of the presence of TiO2 deposited on it. However, the use of SBA-15 as support promotes an adequate dispersion of TiO2, favoring these processes. Thus, a mechanism for the adsorption and photo-oxidation of As(III) on Ti-SBA-15 was proposed. Finally, despite the fact that TiO2 covers only a few percent of the overall Ti-SBA-15 surface it showed an unprecedented photocatalytic behavior, demonstrating to be a prominent photocatalyst for the removal of arsenic from water.

Water-induced variety of arsenic(III) oxide intercalates with alkali chlorides

Water-induced variety of arsenic(III) oxide intercalates with alkali chlorides

Gamma irradiation, thermal conductivity, and phase change tests of the cement‐hyperbranched poly amino‐ester‐block‐poly cabrolactone‐polyurathane plaster‐lead oxide and arsenic oxide composite for development of radiation shielding material

Gamma irradiation, thermal conductivity, and phase change tests of the <scp>cement‐hyperbranched</scp> poly <scp>amino‐ester‐block‐poly cabrolactone‐polyurathane plaster‐lead</scp> oxide and arsenic oxide composite for development of radiation shielding material

Biofilm formation of Ancylobacter sp. TS-1 on different granular materials and its ability for chemolithoautotrophic As(III)-oxidation at high concentrations

The oxidation of arsenic (As) is a key step in its removal from water, and biological oxidation may provide a cost-effective and sustainable method. The biofilm-formation ability of Ancylobacter sp. TS-1, a novel chemolithoautotrophic As oxidizer, was studied for four materials: polypropylene, graphite, sand, and zeolite. After seven days under batch mixotrophic conditions, with high concentrations of As(III) (225 mg·L−1), biofilm formation was detected on all materials except for polypropylene. The results demonstrate As(III)-oxidation of TS-1 biofilms and suggest that the number of active cells was similar for graphite, sand, and zeolite. However, the biofilm biomass follows the specific surface area of each material: 7.0, 2.4, and 0.4 mg VSS·cm−3 for zeolite, sand, and graphite, respectively. Therefore, the observed biofilm-biomass differences were probably associated with different amounts of EPS and inert biomass. Lastly, As(III)-oxidation kinetics were assessed for the biofilms formed on graphite and zeolite under chemolithoautotrophic conditions. The normalized oxidation rate for biofilms formed on these materials was 3.6 and 1.0 mg·L−1·h−1·cm−3, resulting among the highest reported values for As(III)-oxidizing biofilms operated at high-As(III) concentrations. Our findings suggest that biofilm reactors based on Ancylobacter sp. TS-1 are highly promising for their utilization in As(III)-oxidation pre-treatment of high-As(III) polluted waters.

A novel layer-layer crossed structure of bentonite/g-C3N4 for enhanced photocatalytic oxidation of arsenic(III) in a wide pH range

In this work, a novel 2-dimentional (2D)-2D bentonite/g-C3N4 composite (B/g-C3N4), with a layer-layer cross structure and large surface area, was synthesized by a self-assembly process, with the aim for the photocatalytic oxidation of arsenic (As(III)) in wastewater. The structure and properties of the mineral-based composite were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), and photocurrent measurements. The photocatalytic activity of 2D-2D B/g-C3N4 for As(III) removal was further investigated under visible light. The results reveal that the As(III) oxidation rate by 2D-2D B/g-C3N4 composite was three times higher than that by pure g-C3N4, owing to the enlarged specific surface area and the suppressed electron–hole recombination by the incorporated bentonite. Moreover, the composite exhibited a high photocatalytic oxidation efficiency in a wide pH adaptability from 3 to 8.5. Through electron spin resonance (ESR) technique and radical quenching experiments, the photocatalytic mechanism of As(III) oxidation was proposed, where superoxide radicals played a critical role in the photo-oxidation of As(III) in both acid and basic conditions. This work paved a new way to design the mineral-based photocatalyst for treatment of pollutants in a wide pH range.

Evidences on As(III) and As(V) interaction with iron(III) oxides: Hematite and goethite

Arsenic, which is ubiquitous in nature, was found associated with iron oxides in soils and sediments. Our interest was to utilize the same mechanism for the sorptive removal of arsenic from groundwater. The iron(III) oxides: hematite, goethite, were synthesized, characterized and sorption studies of arsenic [As(III) and As(V)] were carried out in batch mode. For studying the evidence of the interaction between arsenic and iron oxide during the process of sorption, a new electrochemical method was developed. Differential pulse voltammetry (DPV) study indicated that the sorbed arsenic species is redox active on the surface of the sorbent. X-ray photoelectron spectroscopy (XPS) measurement was performed for confirmation of the changes occurring to the oxidation states of iron as well as arsenic after the sorption. XPS studies confirmed that the behavior of arsenic species on hematite/goethite was similar and occurs via a partial redox reaction. During sorption of As(III), a partial oxidation occurs resulting in As(V) species, simultaneously the Fe(III) present in the iron oxide gets reduced to Fe(II). However, during the sorption of As(V), there occurs a Fe(II) oxidation followed by As(V) reduction. Based on the results, a mechanistic scheme for sorption of arsenic on iron(III) oxides as sorbents was proposed.

Soil enrichment with actinomycete mitigates the toxicity of arsenic oxide nanoparticles on wheat and maize growth and metabolism.

The use of plant growth-promoting bacteria (PGPB) to enhance plant growth and protection against heavy metal toxicity has been extensively studied. However, its potentiality to reduce arsenate toxicity, a threat to plant growth and metabolism, has been hardly investigated. Moreover, the toxic effect of arsenic oxide nanoparticles (As-NPs) on plants and possible mechanisms for its alleviation has not yet been explored. In this study, the impact of the bioactive actinomycete Streptomyces spp. on the growth, physiology and stress-related metabolites, such as sugars and proline, on As-NPs-stressed wheat and maize plants was investigated. Soil amendment with arsenic oxide nanoparticles (As-NPs) induced the uptake and accumulation of As in the plants of both species, resulting in reduced growth and photosynthesis, but less marked in maize than in wheat plants. Under As-NPs-free conditions, Streptomyces spp. treatment markedly improved growth and photosynthesis in wheat only. The application of Streptomyces spp. reduced As accumulation, recovered the As-NPs-induced growth, photosynthesis inhibition, and oxidative damage in plants of both species. Wheat plants specifically accumulated soluble sugars, while both species accumulated proline. Under As-NPs stress, the ornithine pathway of proline biosynthesis was more important in maize than in wheat plants, while the glutamine pathway was dominant in wheat ones. The addition of Streptomyces spp. further induced the accumulation of proline and starch in both plant species. Overall, despite a different response to Streptomyces spp. under nontoxic conditions, the amendment of as-contaminated soil with Streptomyces spp. induced similar metabolic responses in the two tested species, which trigger stress recovery.

Chemical solidification/stabilization of arsenic sulfide and oxide mixed wastes using elemental sulfur: Efficiencies, mechanisms and long-term stabilization enhancement by dicyclopentadiene

Large amounts of hazardous arsenic sulfide (As2S3) wastes are generated in many industries. These wastes, which are extremely unstable and can partially transform into highly soluble arsenic oxide (As2O3) and then transform into As2S3 and As2O3 mixed wastes (ASOW), are difficult to be solidified/stabilized using common binders. This study proposed a thermally initiated copolymerization method employing elemental sulfur (S8) to chemically solidify/stabilize ASOW. Under thermal conditions (140–200 °C), the elemental sulfur rings break and polymerize into diradical polymeric sulfur chains (•S-(S)m-S•). The ASOW is solidified/stabilized not only by transforming As2S3 into poly(As2S3-r-S) copolymers through copolymerization of •S-(S)m-S• with As2S3 but also by transforming As2O3 into As2S3 in the presence of poly(As2S3-r-S) copolymers. However, the sulfur chain in poly(As2S3-r-S) copolymers gradually crystallizes into S8 after long-term aging, resulting in the depolymerization of copolymers. Dicyclopentadiene (DCP) greatly improves the long-term stability of the solidified body through maintaining the sulfur chain form by forming highly stable poly(As2S3-r-S-r-DCP) copolymers. The solidified body showed high compressive strength (25.7 MPa) and low leaching concentration of arsenic (<1.2 mg L−1) even after 732 days of aging. This study provides a theoretical foundation for the S8-based chemical solidification/stabilization of ASOW as well as other sulfide-containing wastes.

Exploration of Cs–O co-adsorption on GaAs(100) β2(2×4) surface at different sites from first-principles calculation

The micro-mechanism of Cs–O co-adsorption on GaAs(100) β2(2 × 4) surface is clarified through first-principles calculation. In Cs–O adsorption models, several different adsorption sites including D, D′, T2, T2′, T3, T3′, T4, T4′, H and H′ are respectively considered to research the influence of adsorption sites on photoemission. Related crucial properties including adsorption energy, work function, charge transfer and dipole moment are analyzed. The calculation results show that the uppermost As-dimer D site has the maximum work function reduction, but its adsorption energy is not the lowest, while the hollow site H is considered to be the most undesirable site because of its minimum work function reduction and higher adsorption energy. Although T3 site is the most stable adsorption site, its weaker dipole moment resulting in smaller work function reduction is not favorable for photoemission. Moreover, it is inferred from Mulliken charge distribution that the Cs atom can transfer electrons to Ga atoms in the upper bilayer, especially for the sites D, D′, H and H’. Meanwhile, Cs–O adsorption at the lowest hollow H site promotes the formation of gallium oxide while the arsenic oxide tends to form for adsorption at the sites D, D′ and H’ nearby the As-dimers.

Elevated CO2 differently suppresses the arsenic oxide nanoparticles-induced stress in C3 (Hordeum vulgare) and C4 (Zea maize) plants via altered homeostasis in metabolites specifically proline and anthocyanin metabolism

Nano-sized arsenic oxide nanoparticles (As2O3-NP) limit crop growth and productivity. As2O3-NP represent a strong environmental hazard. The predicted rise in future atmospheric CO2 could boost plant growth both under optimal and heavy metal stress conditions. So far, the phytotoxicity of As2O3-NP and their interaction with eCO2 were not investigated at physiological and metabolic levels in crop species groups such as C3 and C4. We investigated how eCO2 level (620 ppm) alleviated soil As2O3-NP toxicity induced growth and mitigated oxidative damages through analysing photosynthetic parameters, primary (sugars and amino acids) and secondary (phenolics, flavonoids and anthocyanins) metabolism in C3 (barley) and C4 (maize) plants. Compared to maize, barley accumulated higher As2O3-NP level, which inhibited growth and induced oxidative damage particularly in barley (increased H2O2 and lipid peroxidation). Interestingly, eCO2 differently mitigated As2O3-NP toxicity on photosynthesis, which consequently improved sugar metabolism. Moreover, high carbon availability in eCO2 treated plants directed to produce osmo-protectant (soluble sugars and proline) and antioxidants (anthocyanins and tocopherols). In the line with increased proline and anthocyanins, their metabolism was also improved. Notable differences occurred between the two plant species. The ornithine pathway was preferred in maize while in barley proline accumulation was mainly through glutamate pathway. Moreover, under As2O3-NP stress, barley preferentially accumulated anthocyanins while maize accumulated total phenolics and flavonoids. This work contributes to improving our understanding of the differences in growth, physiological and biochemical responses of major crops of two functional photosynthetic groups (C3 and C4 plants) under ambient and elevated CO2 grown under As2O3-NP stress.

Insight into the homogenous and heterogeneous transformation behavior of arsenic on commercial V2O5-WO3-TiO2 and novel γ-Fe2O3 catalysts during selective catalytic reduction of NOx

Effect of arsenic on the activity of V2O5-WO3-TiO2 and γ-Fe2O3 catalysts for selective catalytic reduction of NOx with ammonia were studied in this work, and the more important objective was to reveal the heterogeneous and homogeneous reactions about arsenic on V2O5-WO3-TiO2 and γ-Fe2O3 catalysts during arsenic poisoning process. It was discovered that oxygen enhanced the deactivation of V2O5-WO3-TiO2 caused by arsenic. However, the increased oxygen exhibited a contrast impact on arsenic poisoning process of γ-Fe2O3 catalyst. On the other hand, temperature suppressed deactivation effect of arsenic both on V2O5-WO3-TiO2 and γ-Fe2O3. Further concluded from the results of arsenic content in poisoned catalysts, it was found that promotion effect of oxygen on the oxidation of trivalent arsenic in gas side intensified the combination between vanadium and arsenic while inhibited reactions between iron and arsenic. Besides, the aggravation effect of temperature rising on the decomposition of gaseous pentavalent arsenic weakened the combination between gaseous arsenic and vanadium on V2O5-WO3-TiO2 catalyst, while facilitated the combination between gaseous arsenic and non-active iron sites on γ-Fe2O3. Moreover, after trivalent arsenic combined on the surface of catalyst, adsorbed oxygen and pentavalent vanadium competed for providing vacant electron sites to proceed the oxidation of combined trivalent arsenic.

Study on the Formation Process of Oxide Ion and the Influence of Carbon Matrix in Inductively Coupled Plasma Mass Spectrometry Using 18O-Labeled Arsenous Acid

Abstract To obtain further insight into the mechanism of carbon-induced signal enhancement of arsenic (As) at m/z = 75 in inductively coupled plasma mass spectrometry (ICP-MS), the formation process of arsenic oxide ion (AsO+) and the influence of carbon matrix on the process were investigated. The formation process of AsO+ was investigated using arsenous acid isotopically labeled with a highly enriched stable oxygen isotope 18O (H3As18O3). H3As18O3 sample solutions with or without carbon matrix [i.e., 5% (v/v) isopropanol] were analyzed via ICP-MS, and axial intensity profiles of 75As18O+ and 75As16O+ in the ICP were obtained by measuring the signals at axial sampling positions from 3 to 28 mm away from the load coil. Results suggested that AsO+ was formed via two different processes in the ICP: process 1, which involved the decomposition of As-containing molecules (i.e., H3AsO3) to AsO+, and process 2, which involved the recombination between As+ and oxygen originating from liquids introduced into the ICP (e.g., solvents) or gases (e.g., entrained atmospheric gases). In addition, results suggested that carbon matrix has the potential to enhance process 1.

Monodisperse superparamagnetic nanoparticles separation adsorbents for high-yield removal of arsenic and/or mercury metals in aqueous media

The use of magnetic nanoparticles (MNPs) as adsorbent/separation agents for the removal of heavy metal-based compounds from aqueous environments has received much attention in the last few years. However, in most cases, the designed particles present large dimensions (at least several tens of nm) in order to provide them with high magnetic energy and the capability to be collected with a magnet, thus decreasing their surface-to-volume ratio and displaying a non-superparamagnetic behavior which leads to their irreversible aggregation and avoid their use in repetitive adsorption cycles.Here, we report the preparation of 8.50 nm iron oxide-based MNPs with high specific surface areas where large adsorption of heavy metals can efficiently take place. Through an exhaustive control over the synthetic procedures, we obtained MNPs with high magnetizations despite their reduced dimensions (ca. 80 emu·g−1) and superparamagnetic behavior, enabling their efficient separation with a single magnet whilst avoiding irreversible aggregation. After incubation of MNPs in heavy metal aqueous solutions for 24 h, extremely high adsorption values up to ca. 400 mg of arsenic oxide and 500 mg of mercury oxide per gram of MNPs were measured. The pH of the pollutant solutions has a great influence over the bioremediation capability of the MNPs, being the largest adsorption extents at pH 10.0 and 5.5 for arsenic and mercury oxides, respectively. Moreover, the efficiency of desorption protocols and the subsequent reusability of the MNPs for at least four bioremediation cycles was also evaluated. The adsorption data were modeled following a Langmuir-type behavior, while kinetics experiments demonstrated that pollutant ions are adsorbed onto MNPs surfaces through chemisorption processes.

In silico analysis of phylogeny, structure, and function of arsenite oxidase from unculturable microbiome of arsenic contaminated soil

BackgroundArsenite oxidase (EC 1.20.2.1) is a metalloenzyme that catalyzes the oxidation of arsenite into lesser toxic arsenate. In this study, 78 amino acid sequences of arsenite oxidase from unculturable bacteria available in metagenomic data of arsenic-contaminated soil have been characterized by using standard bioinformatics tools to investigate its phylogenetic relationships, three-dimensional structure and functional parameters. ResultsThe phylogenetic relationship of all arsenite oxidase from unculturable microorganisms was revealed their closeness to bacterial order Rhizobiales. The higher aliphatic content showed that these enzymes are thermostable and could be used for in situ bioremediation. A representative protein from each phylogenetic cluster was analysed for secondary structure arrangements which indicated the presence of α-helices (~63%), β-sheets (57–60%) and turns (13–15%). The validated 3D models suggested that these proteins are hetero-dimeric with two chains whereas alpha chain is the main catalytic subunit which binds with arsenic oxides. Three representative protein models were deposited in Protein Model Database. The query enzymes were predicted with two conserved motifs, one is Rieske 3Fe-4S and the other is molybdopterin protein. ConclusionsComputational analysis of protein interactome revealed the protein partners might be involved in the whole process of arsenic detoxification by Rhizobiales. The overall report is unique to the best of our knowledge, and the importance of this study is to understand the theoretical aspects of the structure and functions of arsenite oxidase in unculturable bacteria residing in arsenic-contaminated sites.

Arsenic removal from copper slag matrix by high temperature sulfide-reduction-volatilization

Arsenic contamination has been a major problem in copper slag utilization. Arsenic is easily incorporated into the silicate-based matrix, making the arsenic difficult to volatilize. In this study, pyrite was selected to depolymerize the matrix structure and volatilize the glassy arsenic by sulfide-reduction-volatilization reaction. The optimum technological parameters and mechanism of glassy arsenic volatilization by pyrite were further studied. The optimum operating parameters for glassy arsenic volatilization by pyrite were determined to be a temperature of 1200 °C, a holding time of 60 min, a heating rate of 5 °C/min, a basicity of 0.3, and a pyrite addition content of 15%. The arsenic volatilization ratio reached 80.9% under these experimental conditions. Besides, the mechanism of glassy arsenic volatilization was elucidated by XRD, XPS, FTIR, and SEM analyses. These results indicate that, with the increase in temperature, the pyrite decomposes to generate a variety of sulfur-based reducing substances (FeS, FeS1-x, S2(g)). Through “oxygen capture reaction”, these sulfur-based reducing substances depolymerize the bridging oxygen structure from the glass former ([AsO4], [FeO4], and [SiO4]) by the conversion of (Q2 +Q3)→(Q0 +Q1) and result in the precipitation of glass former ([AsO4], [FeO4] and [SiO4]) combining with the nearby cation. In this process, the glassy arsenic is released by the glass network and participates in reductive volatilization reaction with sulfur-based reducing substances, converting the glassy arsenic with high thermal stability to volatile arsenic oxide and arsenic sulfide. These findings provide a theoretical support for the in situ volatilization of arsenic in copper smelting and centralized control of arsenic contamination.