Bacteria associated with Comamonadaceae are key arsenite oxidizer associated with Pteris vittata root

Pteris vittata (P. vittata), an arsenic (As) hyperaccumulator commonly used in the phytoremediation of As-contaminated soils, contains root-associated bacteria (RAB) including those that colonize the root rhizosphere and endosphere, which can adapt to As contamination and improve plant health. As(III)-oxidizing RAB can convert the more toxic arsenite (As(III)) to less toxic arsenate (As(V)) under As-rich conditions, which may promote plant survial. Previous studies have shown that microbial As(III) oxidation occurs in the rhizospheres and endospheres of P. vittata. However, knowledge of RAB of P. vittata responsible for As(III) oxidation remained limited. In this study, members of the Comamonadaceae family were identified as putative As(III) oxidizers, and the core microbiome associated with P. vittata roots using DNA-stable isotope probing (SIP), amplicon sequencing and metagenomic analysis. Metagenomic binning revealed that metagenome assembled genomes (MAGs) associated with Comamonadaceae contained several functional genes related to carbon fixation, arsenic resistance, plant growth promotion and bacterial colonization. As(III) oxidation and plant growth promotion may be key features of RAB in promoting P. vittata growth. These results extend the current knowledge of the diversity of As(III)-oxidizing RAB and provide new insights into improving the efficacy of arsenic phytoremediation.

Nitrate reduction coupling with As(III) oxidation in neutral As-contaminated paddy soil preserves nitrogen, reduces N2O emissions and alleviates As toxicity

In arsenic (As)-contaminated paddy soil, microbial-driven nitrate (NO3-) reduction coupled with arsenite (As(III)) oxidation can reduce As toxicity, but the whereabouts of NO3- remain unclear. In this study, the experiments were established using selective streptomycin (STP) and cyclohexylamine to inhibit bacterial and fungal functional responses, respectively, and metagenomic sequencing techniques were used to explain the biological mechanisms of NO3- reduction coupled with As(III) oxidation in neutral As-contaminated paddy soil. The results indicated that fungal denitrification resulted in stronger nitrous oxide (N2O) emissions (321.6 μg kg-1) than bacterial denitrification (175.9 μg kg-1) in neutral As-contaminated paddy soil, but NO3- reduction coupled with As(III) oxidation reduced the N2O emissions. Only adding STP led to ammonium (NH4+) generation (17.7 mg kg-1), and simultaneously more NH4+ appeared in NO3- reduction coupled with As(III) oxidation; this may be because it improved the electron transfer efficiency by 18.2 %. Achromobacter was involved in denitrification coupled with As(III) oxidation. Burkholderiales was responsible for NO3- reduction to NH4+ coupled with As(III) oxidation. This study provided a theoretical basis for NO3- reduction coupled with As(III) oxidation reducing N2O emissions, promoting the reduction of NO3- to NH4+, and reducing As toxicity in paddy soil.

Sulfurization alters phenol-formaldehyde resin microplastics redox property and their efficiency in mediating arsenite oxidation

Microplastics weathering by various types of oxidants in the oxic environment and their interaction with environmental contaminants have drawn numerous scientific attention. However, the environmental fate of microplastics under a reducing environment has been largely unresolved. Herein, the change of physicochemical and redox properties of microplastics during the weathering under a sulfate-reducing environment and the interaction with arsenite were addressed. The sulfurization of phenol-formaldehyde resin microplastics under a sulfate-reducing environment generated smooth and porous particles with the induction of organic S species. Multiple spectroscopic results demonstrated thioether and thiophene groups formed by the substitute removal of O-containing functional groups. Moreover, the sulfurization process induced the reduction of carbonyl groups and oxidation of phenolic hydroxyl groups and resulted in the formation of semiquinone radicals. The O-containing functional groups contributed to microplastics redox property and As(III) oxidation while S-containing functional groups showed no obvious effect. The sulfurized microplastics had lower efficiency in mediating arsenite oxidation than the unsulfurized counterparts due to the decreased electron donating capacity. Producing hydrogen peroxides by electron-donating phenol groups and semiquinone radicals and the direct semiquinone radicals oxidation could mediate arsenite oxidation. The findings of this study help us understand the fate of microplastics in redox fluctuation interfaces.

Solutions for an efficient arsenite oxidation and removal from groundwater containing ferrous iron

Manganese (Mn) oxides are extensively used to oxidize As(III) present in ground, drinking, and waste waters to the less toxic and more easily removable As(V). The common presence of multiple other cations in natural waters, and more especially of redox-sensitive ones such as Fe2+, may however significantly hamper As(III) oxidation and its subsequent removal. The present work investigates experimentally the influence of Mn(III) chelating agents on As(III) oxidation process in such environmentally relevant complex systems. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite in the presence of Fe(II) was investigated using batch experiments at circum-neutral pH. In the absence of PP, competitive oxidation of Fe(II) and As(III) leads to Mn oxide surface passivation by Fe(III) and Mn(II/III) (oxyhydr)oxides, thus inhibiting As(III) oxidation. Addition of PP to the system highly enhances As(III) oxidation by birnessite even in the presence of Fe(II). PP presence prevents passivation of Mn oxide surfaces keeping As and Fe species in solution while lower valence Mn species are released to solution. In addition, reactive oxygen species (ROS), tentatively identified as hydroxyl radicals (•OH), are generated under aerobic conditions through oxygen activation by Fe(II)-PP complexes, enhancing As(III) oxidation further. The positive influence of Mn(III) chelating agents on As(III) oxidation most likely not only depend on their affinity for Mn(III) but also on their ability to promote formation of these active radical species. Finally, removal of As(V) through sorption to Fe (oxyhydr)oxides is efficient even in the presence of significant concentrations of PP, and addition of such Mn(III) chelating agents thus appears as an efficient way to enhance the oxidizing activity of birnessite in large-scale treatment for arsenic detoxification of groundwaters.

Arsenic (As) oxidation by core endosphere microbiome mediates As speciation in Pteris vittata roots

Pteris vittata is an arsenic(As)-hyperaccumulator that may be employed in phytoremediation of As-contaminated soils. P. vittata-associated microbiome are adapted to elevated As and may be important for host survival under stresses. Although P. vittata root endophytes could be critical for As biotransformation in planta, their compositions and metabolisms remain elusive. The current study aims to characterize the root endophytic community composition and As-metabolizing potentials in P. vittata. High As(III) oxidase gene abundances and rapid As(III) oxidation activity indicated that As(III) oxidation was the dominant microbial As-biotransformation processes compared to As reduction and methylization in P. vittata roots. Members of Rhizobiales were the core microbiome and the dominant As(III) oxidizers in P. vittata roots. Acquasition of As-metabolising genes, including both As(III) oxidase and As(V) detoxification reductase genes, through horizontal gene transfer was identified in a Saccharimonadaceae genomic assembly, which was another abundant population residing in P. vittata roots. Acquisition of these genes might improve the fitness of Saccharimonadaceae population to elevated As concentrations in P. vittata. Diverse plant growth promoting traits were encoded by the core root microbiome populations Rhizobiales. We propose that microbial As(III) oxidation and plant growth promotion are critical traits for P. vittata survival in hostile As-contaiminated sites.

Coalescence of As(II) with •OH: The pivot for co-processing of As(III) and butyl xanthate

Although water metalloid pollution is widely studied, the effect of the combined pollution of organic matter and metalloids in mining water and, especially, the possible interaction mechanisms between metalloids and flotation reagents, are both poorly understood. Existence of mixed pollution of metalloids and organic compounds tends to cause more serious harm to natural organisms. In this study, a synergistic removal of arsenite (As(III)) and butyl xanthate (Bx) in an advanced oxidation system was reported using biochar-based catalyst loaded with nano-zero-valent iron from an inexpensive iron source (iron slag) to activate peroxodisulfate. The removal efficiencies were improved by 30 % in the co-existence of As(III) and Bx compared to those of the single pollutant. The theoretical calculations, especially frontier molecular orbital theory, revealed the generation of [AsO₂-OH]^•- by the combination of As(II) with •OH. This [AsO₂-OH]^•- participated in the oxidative degradation of Bx with high activity and combined with the sulfur falling off Bx after the reaction to form a novel Fe-As-S complex as indicated by X-ray absorption +fine structure analysis. Overall, this study reports the generation of low-valent arsenic active substances of [AsO₂-OH]^•- and their effect on the removal of organic pollution containing S atoms in advanced oxidation systems under typical mining water conditions with the coexistence of As(III) and expands the understanding and application of traditional free radicals.

Linking pyrogenic carbon redox property to arsenite oxidation: Impact of N-doping and pyrolysis temperature

Pyrogenic carbon-mediated arsenite (As(III)) oxidation shows great potential as a prerequisite for the efficient removal of arsenic in groundwater. Herein, the critical role of N-containing functional groups in low and high-temperature prepared pyrogenic carbons for mediating As(III) oxidation was systemically explored from an electrochemistry perspective. The pyrogenic carbon electron donating capacity and area-normalized specific capacitance were the key parameters explained the As(III) oxidation kinetics mediated by low electrical conductive 500 °C biomass-derived pyrogenic carbons (N contents of 0.36-7.72 wt%, R² = 0.87, p < 0.001) and high electrical conductive 800 °C pyrogenic carbons (N contents of 1.00-8.00 wt%, R² = 0.99, p < 0.001), respectively. The production of H₂O₂ from the reaction between electron donating phenol groups or semiquinone radicals and oxygen, and the direct electron transfer between semiquinone radicals and As(III) contributed to these pyrogenic carbons mediated As(III) oxidation. While the electron accepting quinone, pyridinic-N, and pyrrolic-N groups did not significantly contribute to the 500 °C pyrogenic carbons mediated As(III) oxidation, the direct electron conduction by these functional groups was responsible for the facilitated As(III) oxidation by the 800 °C pyrogenic carbons. Furthermore, the pyridinic-N and pyrrolic-N groups showed higher electron conduction efficiency than that of the quinone groups. The findings help to develop robust pyrogenic carbons for As(III) contaminated groundwater treatment.

Evaluation of pyrite/sodium hypochlorite for activating purification of arsenic from fractured-bedrock groundwater

In this work, the effectiveness of pyrite/sodium hypochlorite (FeS₂/NaClO) treatment to eliminate arsenic (As) from fractured-bedrock groundwater via oxidative adsorption was evaluated. The As concentration in the tested reactors decreased sharply during the initial 5 min, as the addition of NaClO effectively increased the As removal efficiency, attaining 98.6% removal within 60 min in the presence of 0.05 M NaClO. There was no coexisting anion effect (Cl^-, CO₃^-, HCO₃^-, NO₃^-, and F^-) on the As removal capacity of FeS₂/NaClO, except for the PO₄^3- which resulted in less removal of As. X-ray spectroscopy analysis of As(III)-sorbed FeS₂ surfaces revealed that a portion of As(III) was oxidized into As(V) during the adsorption process. Scanning electron microscopy-energy-dispersive spectrometer results of FeS₂ exhibited the distribution of adsorbed As on the newly formed iron (oxy) hydroxide surfaces, with an As element ratio of 1.27%. A continuous flow-bed column study further demonstrated the efficiency of FeS₂/NaClO treatment to lower the contamination level of As at the removal rates of 0.66-3.02 mg/L·day for 160 h. These results suggest that FeS₂/NaClO treatment can be considered an effective strategy for removing As in groundwater of bedrock aquifers.

Physiological, biochemical, and genomic elucidation of the Ensifer adhaerens M8 strain with simultaneous arsenic oxidation and chromium reduction

This study reports the simultaneous oxidation of As(III) and reduction of the Cr(VI) strain Ensifer adhaerens M8 screened from soils around abandoned gold tailings contaminated with highly complex metals (loids). Physiological, biochemical, and genomic techniques were used to explore the mechanism. The strain M8 could simultaneously oxidize 1 mM As(III) and reduce 45.3 % 0.1 mM Cr(VI) in 16 h, and the Cr(VI) reduction rate was increased by 5.8 % compared with the addition of Cr(VI) alone. Cellular debris was the main site of M8 arsenic oxidation. Chromium reduction was dominated by the reduction of extracellular hexavalent chromium (23.80-35.67 %). The genome of M8 included one chromosome and four plasmids, and a comparison of the genomes showed that M8 had two more plasmids than strains of the same genus, which may be related to strong environmental adaptations. M8 had 10 heavy metal resistance genes (HMRs), and plasmid D had a complete cluster of arsenic resistance-oxidation-transport genes (arsOHBCCR-aioSR-aioBA-cytCmoeA-phoBBU-PstBACS-phnCDEE). The genes involved in Cr(VI) detoxification include DNA repair (RecG, ruvABC, and UvrD), Cr(VI) transport (chrA, TonB, and CysAPTW) and Cr(VI) reduction. In summary, this study provides a molecular basis for As (III) and Cr (VI) remediation.

Effects of cobalt doping on the reactivity of hausmannite for As(III) oxidation and As(V) adsorption

Hausmannite is a common low valence Mn oxide mineral, with a distorted spinel structure, in surficial sediments. Although natural Mn oxides often contain various impurities of transitional metals (TMs), few studies have addressed the effect and related mechanism of TM doping on the reactivity of hausmannite with metal pollutants. Here, the reactivity of cobalt (Co) doped hausmannite with aqueous As(III) and As(V) was studied. Co doping decreased the point of zero charge of hausmannite and its adsorption capacity for As(V). Despite a reduction of the initial As(III) oxidation rate, Co-doped hausmannite could effectively oxidize As(III) to As(V), followed by the adsorption and fixation of a large amount of As(V) on the mineral surface. Arsenic K-edge EXAFS analysis of the samples after As(V) adsorption and As(III) oxidation revealed that only As(V) was adsorbed on the mineral surface, with an average As-Mn distance of 3.25-3.30 Å, indicating the formation of bidentate binuclear complexes. These results provide new insights into the interaction mechanism between TMs and low valence Mn oxides and their effect on the geochemical behaviors of metal pollutants.

Simultaneous redox transformation and removal of Cr(Ⅵ) and As(Ⅲ) by polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite: Insights into synergistic effects and mechanisms

Chromium(Ⅵ) and arsenic(Ⅲ) as typical anionic heavy metal pollutants normally coexist in the environment, greatly aggravating their environmental risks and elevating the difficulty of remediation. Here, a novel polyethyleneimine modified magnetic mesoporous polydopamine nanocomposite (Fe₃O₄ @mesoPDA/PEI) with abundant active functional groups was exploited as the synchronous adsorbent of Cr(Ⅵ) and As(Ⅲ). The results showed that Cr(Ⅵ) and As(Ⅲ) could mutually promote their conversions and adsorptions on Fe₃O₄ @mesoPDA/PEI. The adsorption mechanisms of Fe₃O₄ @mesoPDA/PEI were primarily redox chemistry and also involved electrostatic interactions and coordination. Cr(Ⅵ) was mainly reduced by reductive catechol, while As(Ⅲ) was oxidized to As(Ⅴ) by oxidative active substances (e.g., H₂O₂, •OH, and quinone). Meanwhile, active intermediate (semiquinone radicals) generated during the Cr(Ⅵ) reduction and As(Ⅲ) oxidation could constitute redox microcirculation with Cr(Ⅵ) and As(Ⅲ) to further accelerate redox reactions of Cr(Ⅵ) and As(Ⅲ) on Fe₃O₄ @mesoPDA/PEI, thereby exhibiting a synergistic effect. Moreover, newly immobilized Cr(Ⅲ) onto Fe₃O₄ @mesoPDA/PEI became extra active sites for As adsorption through cation bridges and then recovered by magnetic separation in favor of diminishing the environmental hazards of Cr and As. These findings also provide new inspirations for the roles of redox-active functional groups in the remediation of multiple redox-sensitive heavy metals including Cr(Ⅵ) and As(Ⅲ).

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.

Photochemical characterization of paddy water during rice cultivation: Formation of reactive intermediates for As(III) oxidation

Although the photochemical behavior of surface water and its effects on pollutant transformation have been studied extensively in recent years, the photochemistry of paddy water remains largely unknown. In this study, we examined the photochemical processes involving paddy water samples collected at four different cultivation stages of rice. Triplet dissolved organic matter (³DOM*), singlet oxygen (¹O₂), and hydroxyl radicals (^•OH) were found to be the dominant reactive intermediates (RIs), and their apparent quantum yields and steady-state concentrations were quantified. Compared with the typical surface water, quantum yields of ³DOM* and ^•OH were comparable, while quantum yields of ¹O₂ were about 2.4-6.7 times higher than those of surface water. Fluorescence emission-excitation matrix (EEM) spectra, Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), and statistical analysis revealed that DOM properties and nitrite concentration were the main factor influencing RIs generation. The results suggest that DOM with lower molecular weight and humification extent generated more RIs, and nitrite contributed to 23.9%-100% of ^•OH generation. EEM and FTICR-MS data showed that DOM with more saturated and less aromatic formulas could produce more ³DOM* under the irradiation, while the polyphenolic components of DOM inhibited the formation of RIs. Moreover, RIs significantly enhanced arsenite (As(III)) oxidation with oxidation rate increased by 1.8-4.1 times in paddy water, and ^•OH and ³DOM* were the main RIs responsible for As(III) oxidation. This study provides new insight into the pathways of arsenite abiotic transformation in paddy soil and water.

Complete arsenite removal from groundwater by UV activated potassium persulfate and iron oxide impregnated granular activated carbon

Removal of toxic arsenite [As(III)] from the contaminated surface and groundwater is essential for human health. However, direct arsenite removal is difficult compared to arsenate [As(V)]. Therefore, the peroxidation of arsenite to arsenate is vital for its effective removal from water. Herein, we investigated the removal efficiency of arsenic from groundwater by oxidizing it with UV activated potassium persulfate (KPS) and subsequently adsorbing it on iron oxide impregnated granular activated carbon (FeO/GAC). A batch experiment was carried out to determine the adsorption kinetics and thermodynamics. Further, the effects of the adsorbent mass (FeO/GAC), C/Fe molar ratio, pH, arsenic concentration, competing anions, and humic acid in arsenic adsorption was studied. The characterization of FeO/GAC adsorbent was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and zeta potential measurements. Using the UV activated KPS and FeO/GAC, a ∼100% removal amount was achieved for 10 ppm of the arsenic solution in 1 h. Also, the effect of pH showed the highest removal efficiency in the pH range of 6.0-7.0 and it decreased dramatically at higher and lower pH values. The groundwater collected from Cheongyang in South Korea was spiked with 10 ppm of the arsenic (III) and more than 82% removal of arsenic was achieved in 90 min even in the presence of natural contaminants. Therefore, the results suggest that the UV activated KPS with FeO/GAC provides an effective method for treating highly-arsenic-contaminated water sources and this may be a viable alternative method over the existing methods.

A powerful arsenite-oxidizing biofilm bioreactor derived from a single chemoautotrophic bacterial strain: Bioreactor construction, long-term operations and kinetic analysis

Microbial oxidation of As(III) by biofilm bioreactors followed by adsorption is a promising and environment friendly approach to remediate As(III) contaminated groundwater; however, poor activity, stability and expandability of the bioreactors hampered their industrious applications. To resolve this issue, we constructed a new biofilm bioreactor using a powerful chemoautotrophic As(III)-oxidizing bacterium Rhizobium sp. A219. This strain has strong ability to form biofilms and possesses very high As(III)-oxidizing activities in both planktonic and biofilm forms. Perlites were used as the biofilm carriers. Long-term operations suggest that the bioreactor has very high efficiency, stability and scalability under different geochemical conditions, and it is cheap and easy to construct and operate. During the operations, it is only required to supply air, nothing else. All the common contaminants in groundwater slightly affected the bioreactor As(III)-oxidizing activity. The common contaminants in groundwater can be largely removed through assimilation by the bacterial cells as nutrition. The bioreactor completely oxidize 1.0, 5.0, 10.0, 20.0 and 30.0 mg/L As(III) in 12, 18, 20, 25 and 30 min, respectively. Approximately 18, 18, 12, 12 and 21 min were needed to oxidize 1.1 mg/L As(III) at 20, 25, 30, 35 and 40 °C, respectively. The bioreactor works well under the pH values of 5-8, and the most optimal was 7.0. The data suggest that this bioreactor possesses the highest efficiency and stability, and thus has the great potential for industrial applications among all the described As(III)-oxidizing bioreactors derived from a single bacterium.

The biotransformation of arsenic by spent mushroom compost - An effective bioremediation agent

Spent mushroom compost (SMC) is a lignocellulose-rich waste material commonly used in the passive treatment of heavy metal-contaminated environments. In this study, we investigated the bioremediation potential of SMC against an inorganic form of arsenic, examining the individual abiotic and biotic transformations carried out by SMC. We demonstrated, that key SMC physiological groups of bacteria (denitrifying, cellulolytic, sulfate-reducing, and heterotrophic) are resistant to arsenites and arsenates, while the microbial community in SMC is also able to oxidize As(III) and reduce As(V) in respiratory metabolisms, although the SMC did not contain any As. We showed, that cooperation between arsenate and sulfate-reducing bacteria led to the precipitation of As_xS_y. We also found evidence of the significant role organic acids may play in arsenic complexation, and we demonstrated the occurrence of As-binding proteins in the SMC. Furthermore, we confirmed, that biofilm produced by the microbial community in SMC was able to trap As(V) ions. We postulated, that the above-mentioned transformations are responsible for the sorption efficiency of As(V) (up to 25%) and As(III) (up to 16%), as well as the excellent buffering properties of SMC observed in the sorption experiments.

Arsenic adsorbent derived from the ferromanganese slag

Arsenic-contaminated groundwater has a severe negative impact on the health of living beings. Groundwater majorly contains arsenite (As(III)) as well as arsenate (As(V)). Among these two, the arsenite species are more carcinogenic, mobile, and lethal. Hence, it is more difficult to remove by conventional water treatment methods. Ferromanganese slag, waste generated from steel industries, has been utilized in this study for the development of arsenic adsorbent. A chemical treatment method is applied to the ferromanganese slag to prepare efficient arsenic adsorbent, and it is easy to scale up. An adsorbent with the capacity for simultaneous oxidation of As(III) and adsorption of total arsenic species can be efficient for arsenic decontamination. X-ray photoelectron spectroscopy and X-ray absorption near edge spectra techniques prove the As(III) oxidation capability of the developed material is about 70 ± 5% based on initial As(III) concentration. The adsorbent not only oxidizes the As(III) species but also adsorbs both the arsenic species. The Langmuir isotherm model estimates the maximum adsorption capacities at the equilibrium concentration of 10 μg/L are 1.010 ± 0.004 mg/g and 1.614 ± 0.006 mg/g for As(III) and As(V), respectively. The rate of adsorption of As(III) was higher compared to the As(V), which was confirmed by the pseudo-second-order kinetic model. Therefore, the treated water quality meets the World Health Organization and Indian drinking water standards.

Active MnO2/biochar composite for efficient As(III) removal: Insight into the mechanisms of redox transformation and adsorption

In the present work, an active MnO2/rice husk biochar (BC) composite (MBC) was prepared to enhance As(III) removal for groundwater remediation. The MBC material obtained an improved porous structure (i.e., specific surface area, pore volume and mesoporosity) with MnO₂, providing abundant reaction or interaction sites for surface or interface-related processes such as redox transformation and adsorption of arsenic. As a result, a significant enhancement in arsenic removal can be achieved by using MBC. More specifically, MBC showed a high removal capacity for As(III), which was tenfold higher than that of BC. This improvement can be ascribed to the redox transformation of As(III) via MnO₂, resulting in the more effective removal of As(V) species. In addition, pH was an important factor that could influence the As(III) removal capacity. Under alkaline conditions, the As(III, V) removal capacity of MBC was clearly lower than those under acidic and neutral conditions due to the negative effects of electrostatic repulsion. Importantly, a powerful transformation capability of As(III) via MBC was presented; namely, only 5.9% As(III) remained in solution under neutral conditions. Both MnO₂ and the BC substrate contributed to the removal of arsenic by MBC. MnO₂ delivered Mn-OH functional groups to generate surface complexes with As(V) produced by As(III) oxidation, while the reduced Mn(II) and As(V) could precipitate on the MBC surface. The BC substrate also provided COOH and OH functional groups for As(III, V) removal by a surface complexation mechanism. Note that the application of MBC in the treatment of simulated groundwater demonstrated an efficient arsenic removal of 94.6% and a concentration of arsenic as low as the 10 µg L^-1 WHO guideline.

Highly enhanced oxidation of arsenite at the surface of birnessite in the presence of pyrophosphate and the underlying reaction mechanisms

Manganese(IV) oxides, and more especially birnessite, rank among the most efficient metal oxides for As(III) oxidation and subsequent sorption, and thus for arsenic immobilization. Efficiency is limited however by the precipitation of low valence Mn (hydr)oxides at the birnessite surface that leads to its passivation. The present work investigates experimentally the influence of chelating agents on this oxidative process. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite was investigated using batch experiments and different arsenic concentrations at circum-neutral pH. In the absence of PP, Mn(II/III) species are continuously generated during As(III) oxidation and adsorbed to the mineral surface. Field emission-scanning electron microscopy, synchrotron-based X-ray diffraction and Fourier transform infrared spectroscopy indicate that manganite is formed, passivating birnessite surface and thus hampering the oxidative process. In the presence of PP, generated Mn(II/III) species form soluble complexes, thus inhibiting surface passivation and promoting As(III) conversion to As(V) with PP. Enhancement of As(III) oxidation by Mn oxides strongly depends on the affinity of the chelating agent for Mn(III) and from the induced stability of Mn(III) complexes. Compared to PP, the positive influence of oxalate, for example, on the oxidative process is more limited. The present study thus provides new insights into the possible optimization of arsenic removal from water using Mn oxides, and on the possible environmental control of arsenic contamination by these ubiquitous nontoxic mineral species.

Genomic and physiological characterization of an antimony and arsenite-oxidizing bacterium Roseomonas rhizosphaerae

Antimony (Sb) and arsenic (As) are two toxic metalloids, which are listed as priority environmental pollutants by the European Union and the U.S. Environmental Protection Agency (EPA). Antimony taken up by plants enters the food chain and poses a threat to human health. Microbial oxidation of antimonite (Sb(III)) and arsenite (As(III)) to the less toxic antimonate (Sb(V)) and arsenate (As(V)), has great potential for the immobilization of Sb and As in the environment. A heterotrophic aerobic bacterium, Roseomonas rhizosphaerae YW11, oxidized both Sb(III) and As(III) in the modified R2A medium. In the same medium, strain YW11 preferred to oxidize Sb(III), whereas the As(III) oxidation rate was only 50%. Genomic analysis of YW11 confirmed the presence of several As-resistance gene islands. The aioAB genes encoding As(III) oxidase were also induced by Sb(III). The role of aioA in Sb(III) oxidation and resistance was confirmed by disrupting this gene in strain YW11, resulting in the loss of Sb(III) oxidation abilities. This study documents an enzymatic basis for microbial Sb(III) oxidation in strain YW11, which is a novel bacterial strain showing simultaneous oxidation of Sb(III) and As(III), and may be a potential candidate for bioremediation of heavy metal-contaminated environments.

Arsenite Oxidation by Dunaliella salina is Affected by External Phosphate Concentration

Arsenic (As) contamination in terrestrial and aquatic environments is a well-known global environmental problem. The biooxidation of arsenite [As(III)] and subsequent arsenate [As(V)] removal have increasingly been used for remediation of As-polluted groundwater. However, little is known about As(III) oxidation by microalgae, especially those from saltwater environments. In this study, we investigated As(III) toxicity and oxidation in the marine microalga Dunaliella salina in the presence of different phosphate concentrations. The results of the As(III) toxicity experiments showed that D. salina was tolerant to As(III) (5.4 ± 0.31 mg As L^-1 at 72 h of culture). The As(V) percentage in the P-enriched (11.2 mg L^-1) medium was 7.2-fold greater than in the P-deficient one after 24-h exposure, indicating As(III) oxidation by D. salina was more pronounced with increased phosphate levels. Treatment of As(III) with and without 2,4-dinitrophenol (DNP) on the algal cells showed that As(III) oxidation occurred mainly on the cell surface and in the cytoplasm of D. salina. The results of this study suggest that transformation of As(III) into As(V) may be an important pathway of detoxification in D. salina and that phosphate plays a key role in this oxidation process.

Minerals loaded with oxygen nanobubbles mitigate arsenic translocation from paddy soils to rice

Inhibiting reductive transformation of arsenic (As) in flooded paddy soils is fundamentally important for mitigating As transfer into the food chain. In this study, oxygen-nanobubble-loaded-zeolites (ZON) and -vermiculites (VON) were tested as a novel approach for supplying oxygen to paddy soils to inhibit As influx into rice. The dynamic physio- and bio-chemical variations in the rhizosphere and bulk soil were profiled in a rhizobox experiment. Upon adding ZON and VON, the redox potential and dissolved oxygen consistently increased throughout the cultivation period. The improved redox environment inhibited As(III) release into porewater and increased As(V) adsorbed on crystalline Fe (hydr)oxides, following the reduction of arsC and arrA gene abundances and enhancement of the aioA gene. Moreover, adding ZON and VON promoted root iron plaque formation, which increased As retention on iron plaque. Both ZON and VON treatments mitigated As translocation from soil to rice, meanwhile increasing root and shoot biomass. ZON was superior to VON in repressing As transfer and promoting rice growth due to its higher oxygen loading capacity. This study provides a novel and environment-friendly material to both mitigate the As translocation from paddy soil to rice and improve rice growth.

Simultaneous removal of chromate and arsenite by the immobilized Enterobacter bacterium in combination with chemical reagents

Simultaneous chromate [Cr(VI)] reduction and arsenite [As(III)] oxidation is a promising pretreatment process for Cr and As removal. Here, a facultative anaerobic bacterium, Enterobacter sp. Z1, presented capacities of simultaneous Cr(VI) reduction and As(III) oxidation during anoxic cultivation in a wild range of temperature (20-45 °C) and pH (Cerkez et al., 2015; Chen et al., 2015; China Environmental Prote, 1996; Fan et al., 2008, 2019) conditions. Strikingly, strain Z1 could simultaneously contribute up to 92.8% of the reduction of Cr(VI) and 45.8% of the oxidation of As(III) in wastewater. The cells of strain Z1 were embedded with sodium alginate to produce biobeads, and the biobeads exhibited stable ratio of Cr(VI) reduction (91.8%) and As(III) oxidation (29.6%) even in the 5 continuous cycles of wastewater treatment. Moreover, in a process pretreated with the Z1 biobeads followed a precipitation with Ca(OH)₂ and FeCl₃, the removal efficiencies in wastewater were 98.9% and 98.3% for total Cr and As, respectively, which were 44.1% and 9.8% higher than those of using Ca(OH)₂ and FeCl₃, only. The residual amounts of Cr and As met the national standard levels of wastewater discharge. Proteomics analysis showed that cysteine, sulfur and methionine metabolisms, As resistance and oxidoreductase (CysH, CysI, CysJ, NemA and HemF) were induced by Cr(VI) and As(III). Moreover, the addition of cysteine to the medium also significantly improved bacterial Cr(VI) reduction rate. Our results provide a novel microbial pretreatment approach for enhancing remediation of Cr(VI) and As(III) pollution in wastewater, and reveal the evident that cysteine, sulfur and methionine metabolisms, As resistance and oxidoreductases are associated with the redox conversion of Cr(VI) and As(III).

Microbes involved in arsenic mobilization and respiration: a review on isolation, identification, isolates and implications

Microorganisms play an important role in arsenic (As) cycling in the environment. Microbes mobilize As directly or indirectly, and natural/geochemical processes such as sulphate and iron reduction, oxidative sulphide mineral dissolution, arsenite (AsO₃^3-) oxidation and arsenate (AsO₄^3-) respiration further aid in As cycle in the environment. Arsenate serves as an electron donor for the microbes during anaerobic conditions in the sediment. The present work reviews the recent development in As contamination, various As-metabolizing microbes and their phylogenetic diversity, to understand the role of microbial communities in As respiration and mobilization. It also summarizes the contemporary understanding of the intricate biochemistry and molecular biology of natural As metabolisms. Some successful examples of engineered microbes by harnessing these natural mechanisms for effective remediation are also discussed. The study indicates that there is an exigent need to have a clear understanding of environmental aspects of As mobilization and subsequent oxidation-reduction by a suitable microbial consortium.

Simultaneous removal of As(III) and Cu(II) from real bottom ash leachates by manganese-oxidizing aerobic granular sludge: Performance and mechanisms

Manganese-oxidizing aerobic granular sludge (Mn-AGS) is a novel extension of AGS technology to treat arsenic (As) in organic wastewater. In this study, Mn-AGS was first applied to treat real wastewater (bottom ash leachates) containing high levels of As(III) and Cu(II) in a sequencing batch reactor (SBR) for 91 days. Influent and effluent As(III), As(V), Cu(II), as well as pH and chemical oxygen demand (COD) were monitored daily, and sludge was collected regularly for morphological observation, chemical characterization, and microbial analysis. The results indicated that As(III) and Cu(II) could be efficiently removed from wastewater (∼83% and ∼100%, respectively), but the performance was sensitive to pH variation, especially for As(III). The removed As and Cu were mostly bound to carbonates (60.2 ± 2.0% and 70.0 ± 0.6%, respectively) and Fe/Mn oxides (28.2 ± 1.6% and 14.6 ± 0.5%, respectively) in the final sludge. Influent As(III) was partially oxidized into As(V), and high fractions of As(V) were obtained in the Fe/Mn oxide-bound phase. Unexpectedly, microbial analysis revealed that community richness was only slightly changed when the influent was acidized (pH 4.0) but greatly reduced after the influent pH back to 6.0. It could be explained by that acid-fast bacteria rapidly grew after pH recovery and eliminated non-acid-fast bacteria. This work further supported the practical application of Mn-AGS to treat As(III)-containing organic wastewaters.

Interaction of graphene oxide with co-existing arsenite and arsenate: Adsorption, transformation and combined toxicity

The outstanding commercial application potential of graphene oxide (GO) will inevitably lead to its increasing release into the environment, and then affect the environmental behavior and toxicity of conventional pollutants. Interactions between arsenite [As (III)]/arsenate [As (V)] with GO and their combined toxicity to Chlorella pyrenoidosa were investigated. Under abiotic conditions, approximately 42% of the adsorbed As (III) was oxidized by GO with simulated sunlight illumination, which was induced by electron-hole pairs on the surface of GO. Co-exposure with GO greatly enhanced the toxicity of As (III, V) to alga. When adding 10 mg/L GO, the 72 h median effect concentration of As (III) and As (V) to C. pyrendoidosa decreased to 12.7 and 9.4 mg/L from 30.1 and 16.3 mg/L in the As alone treatment, respectively. One possible mechanism by which GO enhanced As toxicity could be that GO decreased the phosphate concentration in the algal medium, and then increased the accumulation of As (V) in algae. In addition, transmission electron microscope (TEM) images demonstrated that GO acted as a carrier for As (III) and As (V) transport into the algal cells. Also, GO induced severe oxidative stress, which could have subsequently compromised important detoxification pathways (e.g., As complexation with glutathione, As methylation, and intracellular As efflux) in the algal cells. Our findings highlight the significant impact of GO on the fate and toxicity of As in the aquatic environment.

Non-radical pathway dominated catalytic oxidation of As(III) with stoichiometric H2O2 over nanoceria

Heterogeneous catalysis is a promising approach to achieve efficient As(III) oxidation by H₂O₂ at circumneutral pH. However, radical-attack pathways dominated catalytic As(III) oxidation over most metal oxides is undesirably associated with low utilization of H₂O₂ induced by rapid self-quenching of radicals. In this study, we developed a non-radical catalytic route to improve H₂O₂ utilization efficiency for catalytic As(III) oxidation based on nanoceria. The performance and mechanism of As(III) oxidation by H₂O₂ was investigated by employing four types of nanoceria with different crystallite sizes (9.1-80.6 nm). At the H₂O₂/As(III) molar ratio of 2.0, nanoceria exhibited crystallite size-dependent catalytic activity for oxidation of As(III) over a wide pH range (5-9). Based on comprehensive study including scavenging, enzymatic, and pretreatment experiments and miscellaneous spectroscopy techniques, the catalytic As(III) oxidation over nanoceria was proved to be mainly mediated by the non-radical Ce-hydroperoxo surface complexes, whilst the surface hydroperoxyl radical served as a minor oxidant. In contrast, the roles of dissolved oxygen, homogeneous H₂O₂, and OH radical were all negligible. No obvious interconversion of Ce(III)/Ce(IV) was observed by XPS for the catalytic process. Such non-radical pathway enabled a stoichiometric reaction with high H₂O₂ utilization efficiency (90.5%-122%). Moreover, nanoceria could be recycled for five consecutive runs without desorption/regeneration treatment. This study sheds new light on the development of nanoceria-based catalytic processes for efficient oxidation of As(III).

Biological As(III) oxidation in biofilters by using native groundwater microorganisms

Arsenic (As) contamination in drinking water represents a worldwide threat to human health. During last decades, the exploitation of microbial As-transformations has been proposed for bioremediation applications. Among biological methods for As-contaminated water treatment, microbial As(III)-oxidation is one of the most promising approaches since it can be coupled to commonly used adsorption removal technologies, without requiring the addition of chemicals and producing toxic by-products. Despite the As(III) oxidation capability has been described in several bacterial pure or enrichment cultures, very little is known about the real potentialities of this process when mixed microbial communities, naturally occurring in As contaminated waters, are used. This study highlighted the contribution of native groundwater bacteria to As(III)-oxidation in biofilters, under conditions suitable for a household-scale treatment system. This work elucidated the influence of a variety of experimental conditions (i.e., various filling materials, flow rates, As(III) inflow concentration, As(III):As(V) ratio, filter volumes) on the microbially-mediated As(III)-oxidation process in terms of oxidation efficiency and rate. The highest oxidation efficiencies (up to 90% in 3 h) were found on coarse sand biofilters treating total initial As concentration of 100 μg L^-1. The detailed microbial characterization of the As(III) oxidizing biofilms revealed the occurrence of several OTUs affiliated with families known to oxidize As(III) (e.g., Burkholderiaceae, Comamonadaceae, Rhodobacteraceae, Xanthomonadaceae). Furthermore, As-related functional genes increased in biofilter systems in line with the observed oxidative performances.

Effects of arsenic on the biofilm formations of arsenite-oxidizing bacteria

Arsenite-oxidizing bacteria (AOB) play a key role in the biogeochemical cycle of arsenic in the environment, and are used for the bioremediation of As contaminated groundwater; however, it is not yet known about how arsenic affects biofilm formations of AOB, and how biofilm formations affect bacterial arsenite-oxidizing activities. To address these issues, we isolated seven novel AOB strains from the arsenic-contaminated soils. They can completely oxidize 1.0 mM As(III) in 22-60 h. Their arsenite oxidase sequences show 43-99% identities to those of other known AOB. Strains Cug1, Cug2, Cug3, Cug4, and Cug6 are able to form biofilms with thickness of 15-95 µm, whereas Cug8 and Cug9 cannot form biofilms. It is interesting to see that arsenite inhibited the biofilm formations of heterotrophic AOB strains, but promoted the biofilm formations of autotrophic strains in a concentration-dependent manner. The arsenite-oxidizing rates of Cug1 and Cug4 biofilms are 31.6% and 27.6% lower than those of their suspension cultures, whereas the biofilm activities of other strains are similar to those of their suspension cultures. The biofilm formation significantly promoted the bacterial resistance to arsenic. This work is the first report on the complex correlations among environmental arsenic, bacterial biofilm formations and bacterial arsenite-oxidizing activities. The data highlight the diverse lifestyle of different AOB under arsenic stress, and provide essential knowledge for the screening of efficient AOB strains used for constructions of bioreactors.

Enhanced oxidation of arsenite to arsenate using tunable K+ concentration in the OMS-2 tunnel

Cryptomelane-type octahedral molecular sieve manganese oxide (OMS-2) possesses high redox potential and has attracted much interest in its application for oxidation arsenite (As(III)) species of arsenic to arsenate (As(V)) to decrease arsenic toxicity and promote total arsenic removal. However, coexisting ions such as As(V) and phosphate are ubiquitous and readily bond to manganese oxide surface, consequently passivating surface active sites of manganese oxide and reducing As(III) oxidation. In this study, we present a novel strategy to significantly promote As(III) oxidation activity of OMS-2 by tuning K⁺ concentration in the tunnel. Batch experimental results reveal that increasing K⁺ concentration in the tunnel of OMS-2 not only considerably improved As(III) oxidation kinetics rate from 0.027 to 0.102 min^-1, but also reduced adverse effect of competitive ion on As(III) oxidation. The origin of K⁺ concentration effect on As(III) oxidation was investigated through As(V) and phosphate adsorption kinetics, detection of Mn²⁺ release in solution, surface charge characteristics, and density functional theory (DFT) calculations. Experimental results and theoretical calculations confirm that by increasing K⁺ concentration in the OMS-2 tunnel not only does it improve arsenic adsorption on K⁺ doped OMS-2, but also accelerates two electrons transfers from As(III) to each bonded Mn atom on OMS-2 surface, thus considerably improving As(III) oxidation kinetics rate, which is responsible for counteracting the adverse adsorption effects by coexisting ions.

Control of arsenic mobilization in paddy soils by manganese and iron oxides

Reductive mobilization of arsenic (As) in paddy soils under flooded conditions is an important reason for the relatively high accumulation of As in rice, posing a risk to food safety and human health. The extent of As mobilization varies widely among paddy soils, but the reasons are not well understood. In this study, we investigated As mobilization in six As-contaminated paddy soils (total As ranging from 73 to 122 mg kg^-1) in flooded incubation and pot experiments. Arsenic speciation in the solution and solid phases were determined. The magnitude of As mobilization into the porewater varied by > 100 times among the six soils. Porewater As concentration correlated closely with the concentration of oxalate-extractable As, suggesting that As associated with amorphous iron (oxyhydr)oxides represents the potentially mobilizable pool of As under flooded conditions. Soil containing a high level of manganese oxides showed the lowest As mobilization, likely because Mn oxides retard As mobilization by slowing down the drop of redox potential upon soil flooding and maintaining a higher arsenate to arsenite ratio in the solid and solution phases. Additions of a synthetic Mn oxide (hausmannite) to two paddy soils increased arsenite oxidation, decreased As mobilization into the porewater and decreased As concentrations in rice grain and straw. Consistent with previous studies using simplified model systems or pure mineral phases, the present study shows that Mn oxides and amorphous Fe (oxyhydr)oxides are important factors controlling reductive As mobilization in As-contaminated paddy soils. In addition, this study also suggests a potential mitigation strategy using exogenous Mn oxides to decrease As uptake by rice in paddy soils containing low levels of indigenous Mn oxides, although further work is needed to verify its efficacy and possible secondary effects under field conditions.

Characterization of Roseomonas and Nocardioides spp. for arsenic transformation

The metalloid arsenic predominantly exists in the arsenite [As(III)] and arsenate [As(V)]. These two forms are respectively oxidized and reduced by microbial redox processes. This study was designed to bioprospect arsenic tolerating bacteria from Lonar lake and to characterize their arsenic redoxing ability. Screening of sixty-nine bacterial species isolated from Lonar lake led to identification of three arsenic-oxidizing and seven arsenic-reducing species. Arsenite oxidizing isolate Roseomonas sp. L-159a being closely related to Roseomonas cervicalis ATCC 49957 oxidized 2mM As(III) in 60h. Gene expression of large and small subunits of arsenite oxidase respectively showed 15- and 17-fold higher expression. Another isolate Nocardioides sp. L-37a formed a clade with Nocardioides ghangwensis JC2055, exhibited normal growth with different carbon sources and pH ranges. It reduced 2mM As(V) in 36h and showed constitutive expression of arsenate reductase which increased over 4-fold upon As(V) exposure. Genetic markers related to arsenic transformation were identified and characterized from the two isolates. Moderate resistance against the arsenicals was exhibited by the two isolates in the range of 1-5mM for As(III) and 1-200mM for As(V). Altogether we provide multiple evidences to indicate that Roseomonas sp. and Nocardioides sp. exhibited arsenic transformation ability.

Simultaneous arsenite oxidation and nitrate reduction at the electrodes of bioelectrochemical systems

Arsenic and nitrate contaminations in the soil and groundwater have urged the scientific community to explore suitable technologies for treatment of both contaminants. This study reports, for the first time, a novel application of bioelectrochemical systems for coupling As detoxification at the anode and denitrification at the cathode. A similar As(III) oxidation efficiency was achieved when anode potential was controlled by a potentiostat or a direct current (DC) power supply. However, a slightly lower nitrate reduction rate was obtained in reactors using DC power supply during simultaneous operation of nitrate reduction and As(III) oxidation. Microbial community analysis by denaturing gradient gel electrophoresis indicated the presence of some autotrophic As(III)-oxidizing bacteria, including Achromobacter spp., Ensifer spp., and Sinorhizobium spp., that can flexibly switch their original metabolism of using oxygen as sole electron acceptor to a new metabolism mode of using solid-state anode as sole electron acceptor driving for As(III) oxidation under anaerobic conditions. Although further research is required for validating their applicability, bioelectrochemical systems represent a brilliant technology for remediation of groundwater contaminated with nitrate and/or arsenite.

Long-term performance of rapid oxidation of arsenite in simulated groundwater using a population of arsenite-oxidizing microorganisms in a bioreactor

A population of arsenite-oxidizing microorganisms enriched from the tailing of the Shimen realgar mine was used to generate biofilms on the surfaces of perlites. This bioreactor is able to completely oxidize 1100 μg/L As(III) dissolved in simulated groundwater into As(V) within 10 min; after 140 days of operation, approximately 20 min were required to completely oxidize the same concentration of As(III). Analysis for the 16S rRNA genes of the microbial community showed that Bacteroidetes and Proteobacteria are dominant in the reactor. Six different bacterial strains were randomly isolated from the reactor. Function and gene analysis indicated that all the isolates possess arsenite-oxidizing activity, and five of them are chemoautotrophic. Further analysis showed that a large diversity of AioAs and two types of RuBisCOs are present in the microbial community. This suggests that many chemoautotrophic arsenite-oxidizing microorganisms were responsible for quick oxidation of arsenite in the reactor. We also found that the reactor is easily regenerated and its number is readily expanded. To the best of our knowledge, the arsenite-oxidizing efficiency, which was expressed as the minimum time for complete oxidization of a certain concentration of As(III) under a single operation, of this bioreactor is the highest among the described bioreactors; it is also the most stable, economic and environment-friendly.

Arsenic redox transformation by Pseudomonas sp. HN-2 isolated from arsenic-contaminated soil in Hunan, China

A mesophilic, Gram-negative, arsenite[As(III)]-oxidizing and arsenate[As(V)]-reducing bacterial strain, Pseudomonas sp. HN-2, was isolated from an As-contaminated soil. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that the strain was closely related to Pseudomonas stutzeri. Under aerobic conditions, this strain oxidized 92.0% (61.4μmol/L) of arsenite to arsenate within 3hr of incubation. Reduction of As(V) to As(III) occurred in anoxic conditions. Pseudomonas sp. HN-2 is among the first soil bacteria shown to be capable of both aerobic As(III) oxidation and anoxic As(V) reduction. The strain, as an efficient As(III) oxidizer and As(V) reducer in Pseudomonas, has the potential to impact arsenic mobility in both anoxic and aerobic environments, and has potential application in As remediation processes.

Evidence for the generation of reactive oxygen species from hydroquinone and benzoquinone: Roles in arsenite oxidation

Natural organic matter (NOM) significantly affects the fate, bioavailability, and toxicity of arsenic in the environment. In the present study, we investigated the oxidation of As(III) in the presence of hydroquinone (HQ) and benzoquinone (BQ), which were selected as model quinone moieties for NOM. It was found that As(III) was oxidized to As(V) in the presence of HQ or BQ at neutral conditions, and the oxidation efficiency of As(III) increased from 33% to 92% in HQ solutions and from 0 to 80% in BQ solutions with pH increasing from 6.5 to 8.5. The oxidation mechanism was further explored with electron spin resonance (ESR) technique. The results showed that semiquinone radicals (SQ(-)) were generated from the comproportionation reaction between BQ and HQ, which mediated the formation of superoxide anion (O2(-)), hydrogen peroxide (H2O2) and hydroxyl radical (OH). Both the SQ(-), H2O2 and OH contributed to the oxidation of As(III). The increase of pH favored the formation of SQ(-), and thus promoted the generation of reactive oxygen species (ROS) as well as As(III) oxidation. Increasing concentrations of HQ and BQ from 0.1 to 1.0 mM enhanced As(III) oxidation from 65% to 94% and from 10% to 53%, respectively. The findings of this study facilitate our understanding of the fate and transformation of As(III) in organic-rich aquatic environments and highlight quinone moieties as the potential oxidants for As(III) in the remediation of arsenic contaminated sites.

Arsenite-oxidizing bacteria exhibiting plant growth promoting traits isolated from the rhizosphere of Oryza sativa L.: Implications for mitigation of arsenic contamination in paddies

Arsenite-oxidizing bacteria exhibiting plant growth promoting (PGP) traits can have the advantages of reducing As-uptake by rice and promoting plant growth in As-stressed soil. A gram-positive bacterium Bacillus flexus ASO-6 resistant to high levels of As (32 and 280 mM for arsenite and arsenate, respectively) and exhibiting elevated rates of As(III) oxidation (Vmax=1.34 μM min(-1) 10(-7) cell) was isolated from rhizosphere of rice. The presence of aoxB gene and exhibition of As(III)-oxidase enzyme activity of this strain was observed. The ability of the strain to produce siderophore, IAA, ACC-deaminase and to solubilize phosphate was verified. The rice seed treated with the strain exhibited significantly improved seed germination and seedling vigor compared with the un-inoculated seeds. The bacterial inoculation significantly increased root biomass, straw yield, grain yield, chlorophyll and carotenoid in the rice plant. Moreover, As uptake from root to shoot and As accumulation in straw and grain decreased significantly as a result of the bacterial inoculation. Noteworthy, the inoculation effect is more prominent in non-flooded soil than it is in flooded soil. Owing to its wide action spectrum, this As(III)-oxidizing PGPB could serve as a potential bio-inoculant for mitigation of As in paddies and sustainable rice production in As-contaminated areas.

Fate of low arsenic concentrations during full-scale aeration and rapid filtration

In the Netherlands, groundwater treatment commonly consists of aeration, with subsequent sand filtration without using chemical oxidants like chlorine. With arsenic (As) concentrations well below the actual guidelines of 10 μg As/L, groundwater treatment plants have been exclusively designed for the removal of iron (Fe), manganese and ammonium. The aim of this study was to investigate the As removal capacity at three of these groundwater treatment plants (10-26 μg As/L) in order to identify operational parameters that can contribute to lowering the filtrate As concentration to <1 μg/L. For this purpose a sampling campaign and experiments with supernatant water and hydrous ferric oxide (HFO) flocs were executed to identify the key mechanisms controlling As removal. Results showed that after aeration, As largely remained mobile in the supernatant water; even during extended residence times only 20-48% removal was achieved (with 1.4-4.2 mg/L precipitated Fe(II)). Speciation showed that the mobile As was in the reduced As(III) form, whereas, As(V) was readily adsorbed to the formed HFO flocs. In the filter bed, the remaining As(III) completely oxidized within 2 min of residence time and As removal efficiencies increased to 48-90%. Filter grain coating analysis showed the presence of manganese at all three treatment plants. It is hypothesized that these manganese oxides are responsible for the accelerated As(III) oxidation in the filter bed, leading to an increased removal capacity. In addition, pH adjustment from 7.8 to 7.0 has been found to improve the capacity for As(V) uptake by the HFO flocs in the filter bed. The overall conclusion is, that during groundwater treatment, the filter bed is crucial for rapid As(III) removal, indicating the importance to control the oxidation sequence of Fe and As for improved As removal efficiencies.

Microbial community composition of a household sand filter used for arsenic, iron, and manganese removal from groundwater in Vietnam

Household sand filters are used in rural areas of Vietnam to remove As, Fe, and Mn from groundwater for drinking water purposes. Currently, it is unknown what role microbial processes play in mineral oxide formation and As removal during water filtration. We performed most probable number counts to quantify the abundance of physiological groups of microorganisms capable of catalyzing Fe- and Mn-redox transformation processes in a household sand filter. We found up to 10(4) cells g(-1) dry sand of nitrate-reducing Fe(II)-oxidizing bacteria and Fe(III)-reducing bacteria, and no microaerophilic Fe(II)-oxidizing bacteria, but up to 10(6) cells g(-1) dry sand Mn-oxidizing bacteria. 16S rRNA gene amplicon sequencing confirmed MPN counts insofar as only low abundances of known taxa capable of performing Fe- and Mn-redox transformations were detected. Instead the microbial community on the sand filter was dominated by nitrifying microorganisms, e.g. Nitrospira, Nitrosomonadales, and an archaeal OTU affiliated to Candidatus Nitrososphaera. Quantitative PCR for Nitrospira and ammonia monooxygenase genes agreed with DNA sequencing results underlining the numerical importance of nitrifiers in the sand filter. Based on our analysis of the microbial community composition and previous studies on the solid phase chemistry of sand filters we conclude that abiotic Fe(II) oxidation processes prevail over biotic Fe(II) oxidation on the filter. Yet, Mn-oxidizing bacteria play an important role for Mn(II) oxidation and Mn(III/IV) oxide precipitation in a distinct layer of the sand filter. The formation of Mn(III/IV) oxides contributes to abiotic As(III) oxidation and immobilization of As(V) by sorption to Fe(III) (oxyhydr)oxides.

Construction of the recombinant broad-host-range plasmids providing their bacterial hosts arsenic resistance and arsenite oxidation ability

The plasmid pSinA of Sinorhizobium sp. M14 was used as a source of functional phenotypic modules, encoding proteins involved in arsenite oxidation and arsenic resistance, to obtain recombinant broad-host-range plasmids providing their bacterial hosts arsenic resistance and arsenite oxidative ability. An arsenite oxidation module was cloned into pBBR1MCS-2 vector yielding plasmid vector pAIO1, while an arsenic resistance module was cloned into pCM62 vector yielding plasmid pARS1. Both plasmid constructs were introduced (separately and together) into the cells of phylogenetically distant (representing Alpha-, Beta-, and Gammaproteobacteria) and physiologically diversified (unable to oxidize arsenite and susceptible/resistant to arsenite and arsenate) bacteria. Functional analysis of the modified strains showed that: (i) the plasmid pARS1 can be used for the construction of strains with an increased resistance to arsenite [up to 20mM of As(III), (ii) the presence of the plasmid pAIO1 in bacteria previously unable to oxidize As(III) to As(V), contributes to the acquisition of arsenite oxidation abilities by these cells, (iii) the highest arsenite utilization rate are observed in the culture of strains harbouring both the plasmids pAIO1 and pARS1, (iv) the strains harbouring the plasmid pAIO1 were able to grow on arsenic-contaminated mine waters (∼ 3.0 mg As L(-1)) without any supplementation.

Anaerobic arsenite oxidation with an electrode serving as the sole electron acceptor: a novel approach to the bioremediation of arsenic-polluted groundwater

Arsenic contamination of soil and groundwater is a serious problem worldwide. Here we show that anaerobic oxidation of As(III) to As(V), a form which is more extensively and stably adsorbed onto metal-oxides, can be achieved by using a polarized (+497 mV vs. SHE) graphite anode serving as terminal electron acceptor in the microbial metabolism. The characterization of the microbial populations at the electrode, by using in situ detection methods, revealed the predominance of gammaproteobacteria. In principle, the proposed bioelectrochemical oxidation process would make it possible to provide As(III)-oxidizing microorganisms with a virtually unlimited, low-cost and low-maintenance electron acceptor as well as with a physical support for microbial attachment.

Screening of plant growth-promoting traits in arsenic-resistant bacteria isolated from agricultural soil and their potential implication for arsenic bioremediation

Twelve arsenic (As)-resistant bacteria (minimum inhibitory concentration ranging from 10 to 30mM and 150 to 320mM for As(III) and As(V), respectively) were isolated from the agricultural soil of the Chianan Plain in southwestern Taiwan using enrichment techniques. Eight isolates capable of oxidizing As(III) (rate of oxidation from 0.029 to 0.059μMh(-1) 10(-9) cell) and exhibiting As(III)-oxidase enzyme activity belong to Pseudomonas, Acinetobacter, Klebsiella and Comamonas genera, whereas four isolates that did not show As(III)-oxidizing activity belong to Geobacillus, Bacillus, Paenibacillus, and Enterobacter genera. Assessment of the parameters of plant growth promotion revealed that Pseudomonas sp. ASR1, ASR2 and ASR3, Geobacillus sp. ASR4, Bacillus sp. ASR5, Paenibacillus sp. ASR6, Enterobacter sp. ASR10 and Comamonas sp. ASR11, and ASR12 possessed some or all of the studied plant growth-promoting traits, including phosphate-solubilization, siderophore, IAA-like molecules and ACC deaminase production. In addition, the ability of As-resistant isolates to grow over wide ranges of pH and temperatures signify their potential application for sustainable bioremediation of As in the environment.

Microbiology of inorganic arsenic: From metabolism to bioremediation

Arsenic (As) contamination of drinking water and soils poses a threat to a large number of people worldwide, especially in Southeast Asia. The predominant forms of As in soils and aquifers are inorganic arsenate [As(V)] and arsenite [As(III)], with the latter being more mobile and toxic. Thus, redox transformations of As are of great importance to predict its fate in the environment, as well as to achieve remediation of As-contaminated water and soils. Although As has been recognized as a toxic element, a wide variety of microorganisms, mainly bacteria, can use it as an electron donor for autotrophic growth or as an electron acceptor for anaerobic respiration. In addition, As detoxification systems in which As is oxidized to the less toxic form or reduced for subsequent excretion are distributed widely in microorganisms. This review describes current development of physiology, biochemistry, and genomics of arsenic-transforming bacteria. Potential application of such bacteria to removal of As from soils and water is also highlighted.

MM-1 isolated from a soil and identification of arsenite oxidase gene

A Gram-negative, arsenite-oxidizing bacterial strain, MM-1 tolerant to 20mM arsenite and 200 mM arsenate was isolated from a heavy metal contaminated soil which contained only 8.8 mg kg(-1) of arsenic. Based on 16S rRNA analysis, the strain was closely related to the genus Variovorax. This strain completely oxidized 500 μM of arsenite to arsenate within 3h of incubation in minimal salts medium. Kinetic studies of arsenite oxidation by the cells showed one of the lowest Km (17 μM) and highest Vmax (1.23 × 10(-7) μM min(-1) cell(-1)) values reported to date for whole cell suspension. PCR analysis using degenerate primers confirmed the presence of arsenite oxidase gene and its amino acid sequence was 70-91% identical to the large subunit of most reported arsenite oxidases. The significant arsenite oxidation capacity shown by the strain opens the way to its potential application in arsenic remediation process.