Arsenite Oxidation Research Articles

Rapid arsenite oxidation by Paenarthrobacter nicotinovorans strain SSBW5: unravelling the role of GlpF, aioAB and aioE genes.

A novel arsenite resistant bacterial strain SSBW5 was isolated from the battery waste site of Corlim, Goa, India. This strain interestingly exhibited rapid arsenite oxidation with an accumulation of 5mM arsenate within 24h and a minimum inhibitory concentration (MIC) of 18mM. The strain SSBW5 was identified as Paenarthrobacter nicotinovorans using 16S rDNA sequence analysis. Fourier-transformed infrared (FTIR) spectroscopy of arsenite-exposed cells revealed the interaction of arsenite with several important functional groups present on the cell surface, possibly involved in the resistance mechanism. Interestingly, the whole genome sequence analysis also clearly elucidated the presence of genes, such as GlpF, aioAB and aioE encoding transporter, arsenite oxidase and oxidoreductase enzyme, respectively, conferring their role in arsenite resistance. Furthermore, this strain also revealed the presence of several other genes conferring resistance to various metals, drugs, antibiotics and disinfectants. Further suggesting the probable direct or indirect involvement of these genes in the detoxification of arsenite thereby increasing its tolerance limit. In addition, clumping of bacterial cells was observed through microscopic analysis which could also be a strategy to reduce arsenite toxicity thus indicating the existence of multiple resistance mechanisms in strain SSBW5. In the present communication, we are reporting for the first time the potential of P. nicotinovorans strain SSBW5 to be used in the bioremediation of arsenite via arsenite oxidation along with other toxic metals and metalloids.

Arsenite Oxidation and Arsenic Adsorption Strategy Using Developed Material from Laterite and Ferromanganese Slag: Electron Paramagnetic Resonance Spectroscopy Analysis

Arsenite Oxidation and Arsenic Adsorption Strategy Using Developed Material from Laterite and Ferromanganese Slag: Electron Paramagnetic Resonance Spectroscopy Analysis

Detailed genomic and biochemical characterization and plant growth promoting properties of an arsenic-tolerant isolate of Bacillus pacificus from contaminated groundwater of West Bengal, India

In this study, arsenic tolerating bacteria Bacillus pacificus (AKS1a) was isolated from arsenic contaminated groundwater of Purbasthali, Purba Bardhaman, West Bengal, India and its bioremediation potential was preliminary screened. This multimetal resistant strain was able to grow against more than 20 mM arsenate and 10 mM arsenite salts. The genome was more than 5.16 Mb in length, with an average of around 35.2% GC content, bearing 5403 protein coding genes. Arsenic resistant genes like arsC, arsB, arsR, etc. were also identified. Rapid Annotation using Subsystem Technology (RAST) identified 328 subsystems within the genome. Presence of six Genomic Islands (GIs) and five phage virus genomic parts indicated its ecological adaptations to overcome environmental stresses. The production of about 415 μg mL−1 indole acetic acid (IAA), 258.0 μg mL−1 gibberellic acid (GA), and 183 μg mL−1 proline by the bacterium, along with nitrogen fixation ability under in-vitro conditions, indicate its plant growth promoting potential. This was further confirmed through rice seedling growth enhancement under arsenic stress. Beside arsenite oxidation to arsenate, its arsenic adsorption property was confirmed through X-ray Fluorescence spectroscopy (XRF), Fourier Transform Infrared spectroscopy (FTIR), and Energy Dispersive X-ray spectroscopic (EDS) analysis. Genomic comparisons among 25 different strains of B. pacificus showed that there are tremendous genetic differences in respect to their accessory genome content. In future, this strain can be applied as biofertilizer or biostimulant for improving rice plant growth.

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.

Detoxification of arsenite by iodide in frozen solution

The oxidation of arsenite (As(III)) to arsenate (As(V)) has received significant attention because it helps mitigate the hazardous and adverse effects of As(III) and subsequently improves the effectiveness of arsenic removal. This study developed an efficient freezing technology for the oxidative transformation of As(III) based on iodide (I−). For a sample containing a very low concentration of 20 μM As(III) and 200 μM I− frozen at −20 °C, approximately 19 μM As(V) was formed after reaction for 0.5 h at pH 3. This rapid conversion has never been achieved in previous studies. However, As(V) was not generated in water at 25 °C. The acceleration of the oxidation of As(III) by I− in ice may be attributed to the freeze-concentration effect. During freezing, all components (i.e., As(III), I−, and protons) are highly concentrated in the ice grain boundary regions, resulting in thermodynamically and kinetically favorable conditions for the redox reaction between As(III) and I−. The efficiency of the oxidation of As(III) using I− increased at high I− concentrations and low pH values. The low freezing temperature (below −20 °C) hindered the oxidative transformation of As(III) by I−. The efficiency of the oxidation of As(III) significantly increased using a fixed initial concentration of I− by subjecting the system to six freezing–melting cycles. The outcomes of this study suggest the possibility of the self-detoxification of As(III) in the natural environment, indicating the potential for developing an eco-friendly method for the treatment of As(III)-contaminated areas in regions with a cold climate. It also demonstrates radical remediation to almost completely remove a very small amount of As(III) that was input in As(III)-contaminated wastewater detoxification, a benchmark that existing methods have been unable to achieve.

Towards an understanding of the factors controlling bacterial diversity and activity in semi-passive Fe: and As-oxidizing bioreactors treating arsenic-rich acid mine drainage.

Semi-passive bioreactors based on iron and arsenic oxidation and co-precipitation are promising for the treatment of As-rich acid mine drainages. However, their performance in the field remains variable and unpredictable. Two bioreactors filled with distinct biomass carriers (plastic or a mix of wood and pozzolana) were monitored during one year. We characterized the dynamic of the bacterial communities in these bioreactors, and explored the influence of environmental and operational drivers on their diversity and activity. Bacterial diversity was analyzed by 16S rRNA gene metabarcoding. The aioA genes and transcripts were quantified by qPCR and RT-qPCR. Bacterial communities were dominated by several iron-oxidizing genera. Shifts in the communities were attributed to operational and physiochemical parameters including the nature of the biomass carrier, the water pH, temperature, arsenic and iron concentrations. The bioreactor filled with wood and pozzolana showed a better resilience to disturbances, related to a higher bacterial alpha diversity. We evidenced for the first time aioA expression in a treatment system, associated with the presence of active Thiomonas spp. This confirmed the contribution of biological arsenite oxidation to arsenic removal. The resilience and the functional redundancy of the communities developed in the bioreactors conferred robustness and stability to the treatment systems.

Brevibacterium sp. strain CS2: A potential candidate for arsenic bioremediation from industrial wastewater

A multiple metal-resistant Brevibacterium sp. strain CS2, isolated from an industrial wastewater, resisted arsenate and arsenate upto 280 and 40 mM. The order of resistance against multiple metals was Arsenate > Arsenite > Selenium = Cobalt > Lead = Nickel > Cadmium = Chromium = Mercury. The bacterium was characterized as per morphological and biochemical characteristics at optimum conditions (37 ℃ and 7 pH). The appearance of brownish color precipitation was due to the interaction of silver nitrate confirming its oxidizing ability against arsenic. The strain showed arsenic processing ability at different temperatures, pH, and initial arsenic concentration which was 37% after 72 h and 48% after 96 h of incubation at optimum conditions with arsenite 250 mM/L (initial arsenic concentration). The maximum arsenic removal ability of strain CS2 was determined for 8 days, which was 32 and 46% in wastewater and distilled water, respectively. The heat-inactivated cells of the isolated strain showed a bioremediation efficiency (E) of 96% after 10 h. Genes cluster (9.6 kb) related to arsenite oxidation was found in Brevibacterium sp. strain CS2 after the genome analysis of isolated bacteria through illumine and nanopore sequencing technology. The arsenite oxidizing gene smaller subunit (aioB) on chromosomal DNA locus (Prokka_01508) was identified which plays a role in arsenite oxidation for energy metabolism. The presence of arsenic oxidizing genes and an efficient arsenic oxidizing potential of Brevibacterium sp. strain CS2 make it a potential candidate for green chemistry to eradicate arsenic from arsenic-contaminated wastewater.

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.

Sustainable Immobilization of Arsenic by Man-Made Aerenchymatous Tissues in Paddy Soil.

Arsenic (As) is a major environmental pollutant and poses a significant health risk to humans through rice consumption. Elevating the soil redox potential (Eh) has been shown to reduce As bioavailability and decrease As accumulation in rice grains. However, sustainable methods for managing the Eh of rice paddies are lacking. To address this issue, we propose a new approach that uses man-made aerenchymatous tissues (MAT) to increase soil Eh by mimicking O2 release from wet plant roots. Our study demonstrated that the MAT method sustainably increased the soil Eh levels from -119 to -80.7 mV (∼30%), over approximately 100 days and within a radius of around 5 cm from the surface of the MAT. Moreover, it resulted in a significant reduction (-28.5% to -63.3%) in dissolved organic carbon, Fe, Mn, and As concentrations. MAT-induced Fe(III) (oxyhydr)oxide minerals served as additional adsorption sites for dissolved As in soil porewater. Furthermore, MAT promoted the oxidation of arsenite to the less mobile arsenate by significantly enhancing the relative abundance of the aioA gene (130% increase in the 0-5 cm soil zone around MAT). The decrease in As bioavailability significantly reduced As accumulation in rice grains (-30.0%). This work offers a low-cost and sustainable method for mitigating As release in rice paddies by addressing the issue of soil Eh management.

Geochemical enrichment, speciation and mobilization of arsenic and antimony in black shales (southern China): Evidence from sequential fractionation and XANES spectroscopy

The high geogenic levels of trace metal(loid)s preserved within black shale can be released during natural weathering, causing global concern. Currently, the accumulation, speciation and potential mobilization of two highly toxic metal(loid)s, arsenic (As) and antimony (Sb), from black shale to soils remain unresolved, which hinders the evaluation of their potential risk to surrounding environments. Here, we demonstrate the geochemical enrichment, species transformation, and leaching behavior of As and Sb by applying weathering-induced alteration, sequential extraction and synchrotron-based X-ray absorption near edge spectroscopy (XANES) in the analysis of a range of black shale and weathered samples from two profiles collected from typical Cambrian sedimentary strata of the Pearl River Delta, southern China. The results showed significant accumulation of trace metal(loid)s, including As (43.8–458.0 mg/kg), Sb (7.2–34.5 mg/kg), Cu (8.8–451.0 mg/kg) and Ni (22.2–108.0 mg/kg), in the black shale and weathered products, and high enrichment factors for As (22.0–266.8) and Sb (1.2–6.4) were observed, consistent with the very high degrees of weathering (chemical index of alteration: 83–91). Arsenic in the shale was dominated by arsenate (46–85%), as identified using XANES, and this was resulted from extensive weathering, which facilitated the oxidation of arsenite and arsenopyrite species. Accordingly, significant decreases in sulfide-associated As and Sb were observed in highly weathered products. As a result, As and Sb were primarily in the immobilizable fractions associated with amorphous and well-crystallized Fe/Mn oxides, largely due to the increased formation of secondary Fe (oxyhydr)oxides, including goethite-like minerals, which mainly derived from the oxidative weathering of chlorite-like minerals. Nevertheless, high concentrations of As (<64 μg/L), Sb (14–70 μg/L), and Ni (3–92 μg/L) still leached from black shale samples exposed to synthetic acid rain in the 18-h batch leaching experiment. Our combined characterization results indicated that a small percentage of the high contents of geogenic As and Sb could potentially be released. However, soluble trace metal(loid)s in shale leachate could still endanger surrounding ecosystems, and continual monitoring during long-term natural weathering processes is merited for global black shale regions.

Potential Self-Attenuation of Arsenic by Indigenous Microorganisms in the Nakdong River.

The toxic element arsenic (As) has become the major focus of global research owing to its harmful effects on human health, resulting in the establishment of several guidelines to prevent As contamination. The widespread industrial use of As has led to its accumulation in the environment, increasing the necessity to develop effective remediation technologies. Among various treatments, such as chemical, physical, and biological treatments, used to remediate As-contaminated environments, biological methods are the most economical and eco-friendly. Microbial oxidation of arsenite (As(III)) to arsenate (As(V)) is a primary detoxification strategy for As remediation as it reduces As toxicity and alters its mobility in the environment. Here, we evaluated the self-detoxification potential of microcosms isolated from Nakdong River water by investigating the autotrophic and heterotrophic oxidation of As(III) to As(V). Experimental data revealed that As(III) was oxidized to As(V) during the autotrophic and heterotrophic growth of river water microcosms. However, the rate of oxidation was significantly higher under heterotrophic conditions because of the higher cell growth and density in an organic-matter-rich environment compared to that under autotrophic conditions without the addition of external organic matter. At an As(III) concentration > 5 mM, autotrophic As(III) oxidation remained incomplete, even after an extended incubation time. This inhibition can be attributed to the toxic effect of the high contaminant concentration on bacterial growth and the acidification of the growth medium with the oxidation of As(III) to As(V). Furthermore, we isolated representative pure cultures from both heterotrophic- and autotrophic-enriched cultures. The new isolates revealed new members of As(III)-oxidizing bacteria in the diversified bacterial community. This study highlights the natural process of As attenuation within river systems, showing that microcosms in river water can detoxify As under both organic-matter-rich and -deficient conditions. Additionally, we isolated the bacterial strains HTAs10 and ATAs5 from the microcosm which can be further investigated for potential use in As remediation systems. Our findings provide insights into the microbial ecology of As(III) oxidation in river ecosystems and provide a foundation for further investigations into the application of these bacteria for bioremediation.

Bayesian network highlights the contributing factors for efficient arsenic phytoextraction by Pteris vittata in a contaminated field

Phytoextraction is a low-cost and eco-friendly method for removing pollutants, such as arsenic (As), from contaminated soil. One of the most studied As hyperaccumulators for soil remediation include Pteris vittata. Although phytoextraction using plant-assisted microbes has been considered a promising soil remediation method, microbial harnessing has not been achieved due to the complex and difficult to understand interactions between microbes and plants. This problem can possibly be addressed with a multi-omics approach using a Bayesian network. However, limited studies have used Bayesian networks to analyze plant–microbe interactions. Therefore, to understand this complex interaction and to facilitate efficient As phytoextraction using microbial inoculants, we conducted field cultivation experiments at two sites with different total As contents (62 and 8.9 mg/kg). Metabolome and microbiome data were obtained from rhizosphere soil samples using nuclear magnetic resonance and high-throughput sequencing, respectively, and a Bayesian network was applied to the obtained multi-omics data. In a highly As-contaminated site, inoculation with Pseudomonas sp. strain m307, which is an arsenite-oxidizing microbe having multiple copies of the arsenite oxidase gene, increased As concentration in the shoots of P. vittata to 157.5 mg/kg under this treatment; this was 1.5-fold higher than that of the other treatments. Bayesian network demonstrated that strain m307 contributed to As accumulation in P. vittata. Furthermore, the network showed that microbes belonging to the MND1 order positively contributed to As accumulation in P. vittata. Based on the ecological characteristics of MND1, it was suggested that the rhizosphere of P. vittata inoculated with strain m307 was under low-nitrogen conditions. Strain m307 may have induced low-nitrogen conditions via arsenite oxidation accompanied by nitrate reduction, potentially resulting in microbial iron reduction or the prevention of microbial iron oxidation. These conditions may have enhanced the bioavailability of arsenate, leading to increased As accumulation in P. vittata.

Speciation of inorganic arsenic with mixed mode HPLC-ESI-MS and Arsenite Oxidation

It has been challenging to analyze inorganic arsenic (iAs) with anion exchange HPLC-Electrospray Ionization-Mass spectrometry (HPLC-ESI-MS), because arsenite (As(III)) is difficult to retain on column and the salts in mobile phase causes ionization suppression of iAs. To address these issues, a method has been developed involving the determination of arsenate (As(V)) with mixed mode HPLC-ESI-MS and the conversion of As(III) to As(V) for total iAs. As(V) was separated from other chemicals on Newcrom B, a bi-modal HPLC column involving anion exchange and reverse phase interaction. The elution employed a two-dimensional gradient, including a formic acid gradient to elute As(V) and a concurrent alcohol gradient to elute organic anions used in sample preparations. As(V) was detected by Selected Ion Recording (SIR) in negative mode at m/z = 141 with a QDa (single quad) detector. As(III) was quantitatively converted to As(V) by mCPBA oxidation and measured for total iAs. By replacing salt with formic acid in elution, the ionization efficiency for As(V) was greatly enhanced in ESI interface. The limit of detection (LOD) for As(V) and As(III) were 0.0263 μM (1.97 ppb) and 0.0398 μM (2.99 ppb), respectively. The linear range was 0.05–1 μM.The method has been used to characterize iAs speciation change in the solution and precipitation in a simulated iron-rich groundwater caused by air exposure.

The structure of the complex between the arsenite oxidase from Pseudorhizobium banfieldiae sp. strain NT-26 and its native electron acceptor cytochrome c552.

The arsenite oxidase (AioAB) from Pseudorhizobium banfieldiae sp. strain NT-26 catalyzes the oxidation of arsenite to arsenate and transfers electrons to its cognate electron acceptor cytochrome c552 (cytc552). This activity underpins the ability of this organism to respire using arsenite present in contaminated environments. The crystal structure of the AioAB/cytc552 electron transfer complex reveals two A2B2/(cytc552)2 assemblies per asymmetric unit. Three of the four cytc552 molecules in the asymmetric unit dock to AioAB in a cleft at the interface between the AioA and AioB subunits, with an edge-to-edge distance of 7.5 Å between the heme of cytc552 and the [2Fe-2S] Rieske cluster in the AioB subunit. The interface between the AioAB and cytc552 proteins features electrostatic and nonpolar interactions and is stabilized by two salt bridges. A modest number of hydrogen bonds, salt bridges and relatively small, buried surface areas between protein partners are typical features of transient electron transfer complexes. Interestingly, the fourth cytc552 molecule is positioned differently between two AioAB heterodimers, with distances between its heme and the AioAB redox active cofactors that are outside the acceptable range for fast electron transfer. This unique cytc552 molecule appears to be positioned to facilitate crystal packing rather than reflecting a functional complex.

Unexpected genetic and microbial diversity for arsenic cycling in deep sea cold seep sediments

Cold seeps, where cold hydrocarbon-rich fluid escapes from the seafloor, show strong enrichment of toxic metalloid arsenic (As). The toxicity and mobility of As can be greatly altered by microbial processes that play an important role in global As biogeochemical cycling. However, a global overview of genes and microbes involved in As transformation at seeps remains to be fully unveiled. Using 87 sediment metagenomes and 33 metatranscriptomes derived from 13 globally distributed cold seeps, we show that As detoxification genes (arsM, arsP, arsC1/arsC2, acr3) were prevalent at seeps and more phylogenetically diverse than previously expected. Asgardarchaeota and a variety of unidentified bacterial phyla (e.g. 4484-113, AABM5-125-24 and RBG-13-66-14) may also function as the key players in As transformation. The abundances of As cycling genes and the compositions of As-associated microbiome shifted across different sediment depths or types of cold seep. The energy-conserving arsenate reduction or arsenite oxidation could impact biogeochemical cycling of carbon and nitrogen, via supporting carbon fixation, hydrocarbon degradation and nitrogen fixation. Overall, this study provides a comprehensive overview of As cycling genes and microbes at As-enriched cold seeps, laying a solid foundation for further studies of As cycling in deep sea microbiome at the enzymatic and processual levels.

Anoxygenic phototrophic arsenite oxidation by a Rhodobacter strain.

Microbially mediated arsenic redox transformations are key for arsenic speciation and mobility in rice paddies. Whereas anaerobic anoxygenic photosynthesis coupled to arsenite (As(III)) oxidation has been widely examined in arsenic-replete ecosystems, it remains unknown whether this light-dependent process exists in paddy soils. Here, we isolated a phototrophic purple bacteria, Rhodobacter strain CZR27, from an arsenic-contaminated paddy soil and demonstrated its capacity to oxidize As(III) to arsenate (As(V)) using malate as a carbon source photosynthetically. Genome sequencing revealed an As(III)-oxidizing gene cluster (aioXSRBA) encoding an As(III) oxidase. Functional analyses showed that As(III) oxidation under anoxic phototrophic conditions correlated with transcription of the large subunit of the As(III) oxidase aioA gene. Furthermore, the non-As(III) oxidizer Rhodobacter capsulatus SB1003 heterologously expressing aioBA from strain CZR27 was able to oxidize As(III), indicating that aioBA was responsible for the observed As(III) oxidation in strain CZR27. Our study provides evidence for the presence of anaerobic photosynthesis-coupled As(III) oxidation in paddy soils, highlighting the importance of light-dependent, microbe-mediated arsenic redox changes in paddy arsenic biogeochemistry.

Enhanced simultaneous arsenite oxidation and sorption by Mn-modified biochar: Insight into the mechanisms under optimal modification condition

This study successfully prepared two types of Mn-modified biochar using chemical co-precipitation (MBC1) and impregnation pyrolysis methods (MBC2) for simultaneous oxidation and adsorption of arsenic species efficiently. Both Mn-modified biochars were able to remove significantly greater quantities of As(Ⅲ) from solution than the pristine biochar. Among them, MBC1 showed better oxidation and adsorption capacity, especially at a pyrolysis temperature of 600 °C and a ratio of 3:10 (MnOx: BC). The As(Ⅲ) adsorption on MBC1 conformed to the pseudo-second-order kinetics and Freundlich isothermal models. The maximum adsorption capacity of MBC1 for As(Ⅲ) at pH 7.0 calculated from the Langmuir adsorption isotherm was 32.06 mg·g−1. The related mechanisms were characterized using elemental analysis, SEM, BET, FTIR, XRD, XPS and HPLC-ICP-MS techniques. Results showed that Mn-oxides significantly improved the surface physicochemical properties of biochar, with significant increase of functional groups (e.g., hydroxyl and carboxyl), the specific surface area and pore structure, which not only promoted the adsorption of arsenic species but mitigated the passivation of biochar surface and Mn-oxides by Mn(Ⅱ). The oxidation of As(Ⅲ) via Mn(Ⅲ) and Mn(Ⅳ) in Mn-oxides on the modified biochar subsequently resulted in more efficient production of As(Ⅴ). The Mn(Ⅱ) and As(Ⅴ) could subsequently precipitate and be re-adsorbed on the biochar surface. The re-adsorbed Mn(Ⅱ) increased the positive charge on the biochar surface, and the strong electrostatic attraction can promote As(Ⅴ) adsorption at pH < pHPZC. This study further revealed the favorable conditions and underlying mechanisms for the enhanced simultaneous arsenite oxidation and sorption by Mn-modified biochar.

Arsenic pollution remediation mechanism and preliminary application of arsenic-oxidizing bacteria isolated from industrial wastewater

Microbial remediation is vital for improving heavy metal-polluted water. In this work, two bacterial strains, K1 (Acinetobacter gandensis) and K7 (Delftiatsuruhatensis), with high tolerance to and strong oxidation of arsenite [As(III)], were screened from industrial wastewater samples. These strains tolerated 6800 mg/L As(III) in a solid medium and 3000 mg/L (K1) and 2000 mg/L (K7) As(III) in a liquid medium; arsenic (As) pollution was repaired through oxidation and adsorption. The As(III) oxidation rates of K1 and K7 were the highest at 24 h (85.00 ± 0.86%) and 12 h (92.40 ± 0.78%), respectively, and the maximum gene expression levels of As oxidase in these strains were observed at 24 and 12 h. The As(III) adsorption efficiencies of K1 and K7 were 30.70 ± 0.93% and 43.40 ± 1.10% at 24 h, respectively. The strains exchanged and formed a complex with As(III) through the –OH, –CH3, and C]O groups, amide bonds, and carboxyl groups on the cell surfaces. When the two strains were co-immobilized with Chlorella, the adsorption efficiency of As(III) improved (76.46 ± 0.96%) within 180 min, thereby exhibiting good adsorption and removal effects of other heavy metals and pollutants. These results outlined an efficient and environmentally friendly method for the cleaner production of industrial wastewater.

PH dependent electro-oxidation of arsenite on gold surface: Relative kinetics and sensitivity

A detailed kinetic investigation of As(III) oxidation was performed on gold surface within pH between ∼3.0 and ∼9.0. It was found that the As(III) oxidation on the gold surface follows a purely adsorption-controlled process irrespective of pH. The evaluated adsorption equilibrium constant decreased from 3.21 × 105 to 1.61 × 105 mol L−1 for acidic to basic medium, which implies the strong affinity of the arsenic species in the acidic medium. Besides, the estimation of Gibbs free energy revealed that an acidic medium promotes arsenic oxidation on gold surface. In mechanistic aspect, the oxidation reaction adopts a stepwise pathway for acidic medium and a concerted pathway for neutral and basic medium. From the substantial kinetic evaluation, it is established that a conducive and compatible environment for the oxidation of arsenic was found in an acidic medium rather than a basic or neutral medium on gold surface. Besides, in sensitivity concern, neutral and highly acidic medium is quite favourable for the arsenite oxidation on gold surface.

Bicarbonate-mediated enhanced successive oxidation and adsorption of As(III) in heterogeneous PMS activation process: Mechanism and practicability

Successive oxidation and adsorption of arsenite (As(III)) for its immobilization are vital for purifying high-arsenic groundwater. Bicarbonate (HCO3ˉ), one of the most abundant anions in high-arsenic groundwater, usually inhibits As(III) immobilization in heterogeneous persulfate-based oxidation processes. In order to inhibit the negative effect of HCO3ˉ, we constructed a nanoscale MIL-100(Fe)-coupled peroxymonosulfate (NMIL-PMS) system. The results showed that when the initial concentration of HCO3ˉ increased from 0 to 1.2 mM, HCO3ˉ significantly accelerated the As(III) (C0 = 1000 µg/L) immobilization rate (k1, fitted by pseudo-first order model) from 0.018 min−1 to 0.409 min−1 and enhanced the As(III) immobilization capacity (calculated by Langmuir isotherm model) from 7962.7 µg/g to 12,082.6 µg/g. For the oxidation of As(III), HCO3ˉ shifted the dominant oxidation pathway from a 1O2-dominated one to the electron transfer-dominated non-radical pathway. As for the adsorption of evolved arsenate (As(V)), the process changed from chemisorption-driven to physical adsorption-driven, with hydrogen bonding and Van Der Waals interaction between NMIL and As(V) facilitated in the presence of HCO3ˉ. Furthermore, the NMIL-PMS-HCO3ˉ system exhibited excellent resistance towards complex water matrices as well as outstanding reusability and stability. This study could provide an avenue for the development of a very promising and economical technology for the purification of As(III)-contaminated groundwater.