Arsenite Oxidation Research Articles

··A novel laminated Fe3O4/CaO2 composite for ultratrace arsenite oxidation and adsorption in aqueous solutions

In this study, for the first time, cheap CaO2 was introduced to combine with Fe3O4 nanoparticles by coprecipitation method to form multifunctional Fe3O4/CaO2 composites for arsenite (As (Ⅲ)) oxidation and adsorption. A novel laminated Fe3O4/CaO2 nanocomposite was obtained by varying the molar ratio of Fe3O4/Ca from 5:1 to 1:5. The laminated Fe3O4/CaO2 showed good oxidation and adsorption properties for As (Ⅲ), especially for ultratrace As (Ⅲ) (< 40 ppb) treatment. As (III) concentration can be effectively controlled below the international requirement of 10 ppb, which may be due to the oxidation function of Fe3O4/CaO2 since As (III) can be quickly oxidized into As (Ⅴ) within 15 min. The adsorption capacities of Fe3O4/CaO2 (the molar ratio of Fe3O4/Ca is 1:5) was 56.35 mg g−1 when pH is 6 and the amount of Fe3O4/CaO2 is 2 mg mL-1. Excitingly, the inhibition effects of phosphate for As (Ⅲ) adsorption by Fe3O4/CaO2 is much lower than that of most of the other reported materials. These results suggest that Fe3O4/CaO2 composite can be employed as a promising adsorbent to treat As (III), even in ultratrace concentration.

Diverse respiratory capacity among Thermus strains from US Great Basin hot springs.

Thermus species are thermophilic heterotrophs, with most capable of using a variety of organic and inorganic electron donors for respiration. Here, a combined cultivation-independent and -dependent approach was used to explore the diversity of Thermus in Great Boiling Spring (GBS) and Little Hot Creek (LHC) in the US Great Basin. A cultivation-independent 16S rRNA gene survey of ten LHC sites showed that Thermus made up 0-3.5% of sequences and were predominately Thermus thermophilus. 189 Thermus isolates from GBS and LHC were affiliated with T. aquaticus (73.0%), T. oshimai (25.4%), T. sediminis (1.1%), and T. thermophilus (0.5%), with T. aquaticus and T. oshimai forming biogeographic clusters. 22 strains were selected for characterization, including chemolithotrophic oxidation of thiosulfate and arsenite, and reduction of ferric iron, polysulfide, and nitrate, revealing phenotypic diversity and broad respiratory capability within each species. PCR demonstrated the wide distribution of aerobic arsenite oxidase genes. A GBS sediment metaproteome contained sulfite oxidase and Fe3+ ABC transporter permease peptides, suggesting sulfur and iron transformations in situ. This study expands our knowledge of the physiological diversity of Thermus, suggesting widespread chemolithotrophic and anaerobic respiration phenotypes, and providing a foundation for better understanding the ecology of this genus in thermal ecosystems.

Spontaneous oxidation of arsenite on platinized TiO2 through activating molecular oxygen under ambient aqueous condition

The pre-oxidation of As(III) is essential for effective removal of arsenic in contaminated water. Although diverse arsenic oxidation methods have been developed, most require the input of chemical oxidants and external energy (e.g., electricity, photon energy). This study investigated the spontaneous catalytic oxidation of As(III) over Pt nanoparticles loaded on TiO2 under ambient aqueous conditions (i.e., air-saturated and room temperature). The proposed catalytic process requires dissolved O2 only as an oxidant, instead of chemical oxidants and energy input, where electrons spontaneously transfer from As(III) to O2 on the Pt–TiO2 catalyst. The oxidation rate of As(III) was highly dependent on (i) the content of Pt, (ii) the oxidation state of Pt, (iii) the presence of an electron acceptor (e.g., O2 and Fe(III)), (iv) the Pt support (e.g., TiO2, Al2O3, SiO2, etc.), and (v) the hetero-junction between Pt and TiO2. In this catalytic system, zero-valent Pt was confirmed as an active species for As(III) oxidation, and TiO2 acted as not only a support for the dispersion of Pt nanoparticles but also as an activator of in situ-generated H2O2 that is utilized in As(III) oxidation. Although Pt is expensive, the operation of Pt–TiO2 at room temperature without needing additional energy and chemical inputs, along with its high durability, can compensate for the high material cost. Therefore, the method presented here represents a viable treatment option for the pre-treatment of As(III)-contaminated water.

Response to the letter to the editor

Arsenite is a highly toxic compound present in many water sources around the world. The removal of arsenite from water requires its oxidation to arsenate which is much more amenable to treatment using well attested technologies. Prior research has shown that the oxidation of arsenite by hydroxyl radicals is significantly accelerated in the presence of carbonate ions but the intrinsic mechanisms of the acceleration have not yet been established. The main goal of the present work was to examine the oxidation of arsenite in the framework of the density functional theory, to establish a detailed microscopic level mechanism of interactions between arsenite and hydroxyl radicals and to elucidate the nature of the catalytic effect of carbonate ions. Results of this study demonstrate that the [As(OH)2CO3]- complex is the thermodynamically most stable species formed in the system H3AsO3–CO32–/HCO3−–H2O. Interactions of the hydroxyl radical with the [As(OH)2CO3]- complex yield the pre-reaction complex [As(OH)3CO3]-∗ in the reaction of subsequent oxidation of arsenite. The structures of the reactants, products and transition states, as well as pre- and post-reaction complexes corresponding to several possible mechanisms of the first stage of As(III) oxidation to As(IV) intermediate using hydroxyl radicals in the absence and in the presence of [As(OH)2CO3]-, were determined in this study. The data demonstrate that the arsenite-carbonate complexes [As(OH)2CO3]- are characterized by a significantly lower activation energy of the first oxidation stage under the action of a hydroxyl radical (2.8 kcal/mol) compared to that for the free arsenite H3AsO3 (13.6 kcal/mol).

Ample Arsenite Bio-Oxidation Activity in Bangladesh Drinking Water Wells: A Bonanza for Bioremediation?

Millions of people worldwide are at risk of arsenic poisoning from their drinking water. In Bangladesh the problem extends to rural drinking water wells, where non-biological solutions are not feasible. In serial enrichment cultures of water from various Bangladesh drinking water wells, we found transfer-persistent arsenite oxidation activity under four conditions (aerobic/anaerobic; heterotrophic/autotrophic). This suggests that biological decontamination may help ameliorate the problem. The enriched microbial communities were phylogenetically at least as diverse as the unenriched communities: they contained a bonanza of 16S rRNA gene sequences. These related to Hydrogenophaga, Acinetobacter, Dechloromonas, Comamonas, and Rhizobium/Agrobacterium species. In addition, the enriched microbiomes contained genes highly similar to the arsenite oxidase (aioA) gene of chemolithoautotrophic (e.g., Paracoccus sp. SY) and heterotrophic arsenite-oxidizing strains. The enriched cultures also contained aioA phylotypes not detected in the previous survey of uncultivated samples from the same wells. Anaerobic enrichments disclosed a wider diversity of arsenite oxidizing aioA phylotypes than did aerobic enrichments. The cultivatable chemolithoautotrophic and heterotrophic arsenite oxidizers are of great interest for future in or ex-situ arsenic bioremediation technologies for the detoxification of drinking water by oxidizing arsenite to arsenate that should then precipitates with iron oxides. The microbial activities required for such a technology seem present, amplifiable, diverse and hence robust.

The intrinsic mechanism of catalytic oxidation of arsenite by hydroxyl-radicals in the H3AsO3–CO32–/HCO3−–H2O system: A quantum-chemical examination

Arsenite is a highly toxic compound present in many water sources around the world. The removal of arsenite from water requires its oxidation to arsenate which is much more amenable to treatment using well attested technologies. Prior research has shown that the oxidation of arsenite by hydroxyl radicals is significantly accelerated in the presence of carbonate ions but the intrinsic mechanisms of the acceleration have not yet been established. The main goal of the present work was to examine the oxidation of arsenite in the framework of the density functional theory, to establish a detailed microscopic level mechanism of interactions between arsenite and hydroxyl radicals and to elucidate the nature of the catalytic effect of carbonate ions. Results of this study demonstrate that the [As(OH)2CO3]- complex is the thermodynamically most stable species formed in the system H3AsO3–CO32–/HCO3−–H2O. Interactions of the hydroxyl radical with the [As(OH)2CO3]- complex yield the pre-reaction complex [As(OH)3CO3]-∗ in the reaction of subsequent oxidation of arsenite. The structures of the reactants, products and transition states, as well as pre- and post-reaction complexes corresponding to several possible mechanisms of the first stage of As(III) oxidation to As(IV) intermediate using hydroxyl radicals in the absence and in the presence of [As(OH)2CO3]-, were determined in this study. The data demonstrate that the arsenite-carbonate complexes [As(OH)2CO3]- are characterized by a significantly lower activation energy of the first oxidation stage under the action of a hydroxyl radical (2.8 kcal/mol) compared to that for the free arsenite H3AsO3 (13.6 kcal/mol).

ArxA From Azoarcus sp. CIB, an Anaerobic Arsenite Oxidase From an Obligate Heterotrophic and Mesophilic Bacterium.

Arsenic is a toxic element widely distributed in nature, but numerous bacteria are able to resist its toxicity mainly through the ars genes encoding an arsenate reductase and an arsenite efflux pump. Some “arsenotrophic” bacteria are also able to use arsenite as energy supplier during autotrophic growth by coupling anaerobic arsenite oxidation via the arx gene products to nitrate respiration or photosynthesis. Here, we have demonstrated that Azoarcus sp. CIB, a facultative anaerobic β-proteobacterium, is able to resist arsenic oxyanions both under aerobic and anaerobic conditions. Genome mining, gene expression, and mutagenesis studies revealed the presence of a genomic island that harbors the ars and arx clusters involved in arsenic resistance in strain CIB. Orthologous ars clusters are widely distributed in the genomes of sequenced Azoarcus strains. Interestingly, genetic and metabolic approaches showed that the arx cluster of the CIB strain encodes an anaerobic arsenite oxidase also involved in the use of arsenite as energy source. Hence, Azoarcus sp. CIB represents the prototype of an obligate heterotrophic bacterium able to use arsenite as an extra-energy source for anaerobic cell growth. The arsenic island of strain CIB supports the notion that metabolic and energetic skills can be gained by genetic mobile elements.

Electrocatalytical oxidation of arsenite by reduced graphene oxide via in-situ electrocatalytic generation of H2O2

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

Arsenic-resistance mechanisms in bacterium Leclercia adecarboxylata strain As3-1: Biochemical and genomic analyses.

Microbial arsenic transformation is important in As biogeochemical cycles in the environment. In this study, a new As-resistant bacterial strain Leclercia adecarboxylata As3-1 was isolated and its associated mechanisms in As resistance and detoxification were evaluated based on genome sequencing and gene annotations. After subjecting strain As3-1 to medium containing arsenate (AsV), AsV reduction occurred and an AsV-enhanced bacterial growth was observed. Strain As3-1 lacked arsenite (AsIII) oxidation ability and displayed lower AsIII resistance than AsV, probably due to its higher AsIII accumulation. Polymerase chain reaction and phylogenetic analysis showed that strain As3-1 harbored a typical AsV reductase gene (arsC) on the plasmids. Genome sequencing and gene annotations identified four operons phoUpstBACS, arsHRBC, arsCRDABC and ttrRSBCA, with 8 additional genes outside the operons that might have involved in As resistance and detoxification in strain As3-1. These included 5 arsC genes explaining why strain As3-1 tolerated high AsV concentrations. Besides ArsC, TtrB, TtrC and TtrA proteins could also be involved in AsV reduction and consequent energy acquisition for bacterial growth. Our data provided a new example of diverse As-regulating systems and AsV-enhanced growth without ArrA in bacteria. The information helps to understand the role of As in selecting microbial systems that can transform and utilize As.

Transcriptomic Analysis of Two Thioalkalivibrio Species Under Arsenite Stress Revealed a Potential Candidate Gene for an Alternative Arsenite Oxidation Pathway.

The genus Thioalkalivibrio includes haloalkaliphilic chemolithoautotrophic sulfur-oxidizing bacteria isolated from various soda lakes worldwide. Some of these lakes possess in addition to their extreme haloalkaline environment also other harsh conditions, to which Thioalkalivibrio needs to adapt. An example is arsenic in soda lakes in eastern California, which is found there in concentrations up to 3000 μM. Arsenic is a widespread element that can be an environmental issue, as it is highly toxic to most organisms. However, resistance mechanisms in the form of detoxification are widespread and some prokaryotes can even use arsenic as an energy source. We first screened the genomes of 76 Thioalkalivibrio strains for the presence of known arsenic oxidoreductases and found 15 putative ArxA (arsenite oxidase) and two putative ArrA (arsenate reductase). Subsequently, we studied the resistance to arsenite in detail in Thioalkalivibrio jannaschii ALM2T, and Thioalkalivibrio thiocyanoxidans ARh2T by comparative genomics and by growing them at different arsenite concentrations followed by arsenic species and transcriptomic analysis. Tv. jannaschii ALM2T, which has been isolated from Mono Lake, an arsenic-rich soda lake, could resist up to 5 mM arsenite, whereas Tv. thiocyanoxidans ARh2T, which was isolated from a Kenyan soda lake, could only grow up to 0.1 mM arsenite. Interestingly, both species oxidized arsenite to arsenate under aerobic conditions, although Tv. thiocyanoxidans ARh2T does not contain any known arsenite oxidases, and in Tv. jannaschii ALM2T, only arxB2 was clearly upregulated. However, we found the expression of a SoeABC-like gene, which we assume might have been involved in arsenite oxidation. Other arsenite stress responses for both strains were the upregulation of the vitamin B12 synthesis pathway, which can be linked to antioxidant activity, and the up- and downregulation of different DsrE/F-like genes whose roles are still unclear. Moreover, Tv. jannaschii ALM2T induced the ars gene operon and the Pst system, and Tv. thiocanoxidans ARh2T upregulated the sox and apr genes as well as different heat shock proteins. Our findings for Thioalkalivibrio confirm previously observed adaptations to arsenic, but also provide new insights into the arsenic stress response and the connection between the arsenic and the sulfur cycle.

Exploring Biodiversity and Arsenic Metabolism of Microbiota Inhabiting Arsenic-Rich Groundwaters in Northern Italy.

Arsenic contamination of groundwater aquifers is an issue of global concern. Among the affected sites, in several Italian groundwater aquifers arsenic levels above the WHO limits for drinking water are present, with consequent issues of public concern. In this study, for the first time, the role of microbial communities in metalloid cycling in groundwater samples from Northern Italy lying on Pleistocene sediments deriving from Alps mountains has been investigated combining environmental genomics and cultivation approaches. 16S rRNA gene libraries revealed a high number of yet uncultured species, which in some of the study sites accounted for more of the 50% of the total community. Sequences related to arsenic-resistant bacteria (arsenate-reducing and arsenite-oxidizing) were abundant in most of the sites, while arsenate-respiring bacteria were negligible. In some of the sites, sulfur-oxidizing bacteria of the genus Sulfuricurvum accounted for more than 50% of the microbial community, whereas iron-cycling bacteria were less represented. In some aquifers, arsenotrophy, growth coupled to autotrophic arsenite oxidation, was suggested by detection of arsenite monooxygenase (aioA) and 1,5-ribulose bisphosphate carboxylase (RuBisCO) cbbL genes of microorganisms belonging to Rhizobiales and Burkholderiales. Enrichment cultures established from sampled groundwaters in laboratory conditions with 1.5 mmol L-1 of arsenite as sole electron donor were able to oxidize up to 100% of arsenite, suggesting that this metabolism is active in groundwaters. The presence of heterotrophic arsenic resistant bacteria was confirmed by enrichment cultures in most of the sites. The overall results provided a first overview of the microorganisms inhabiting arsenic-contaminated aquifers in Northern Italy and suggested the importance of sulfur-cycling bacteria in the biogeochemistry of arsenic in these ecosystems. The presence of active arsenite-oxidizing bacteria indicates that biological oxidation of arsenite, in combination with arsenate-adsorbing materials, could be employed for metalloid removal.

Different Role of Bisulfite/Sulfite in UVC-S(IV)-O2 System for Arsenite Oxidation in Water.

It is of interest to use UV-sulfite based processes to degrade pollutants in wastewater treatment process. In this work, arsenic (As(III)) has been selected as a target pollutant to verify the efficacy of such a hypothesized process. The results showed that As(III) was quickly oxidized by a UV-sulfite system at neutral or alkaline pH and especially at pH 9.5, which can be mainly attributed to the generated oxysulfur radicals. In laser flash photolysis (LFP) experiments (λex = 266 nm), the signals of SO3•− and eaq− generated by photolysis of sulfite at 266 nm were discerned. Quantum yields for photoionization of HSO3− (0.01) and SO32− (0.06) were also measured. It has been established that eaq− does not react with SO32−, but reacts with HSO3− with a rate constant 8 × 107 M−1s−1.

Simultaneous 3-/4-Hydroxybenzoates Biodegradation and Arsenite Oxidation by Hydrogenophaga sp. H7.

Aromatic compounds and arsenic (As) often coexist in the environment. As(III)-oxidizing bacteria can oxidize the more toxic As(III) into the less toxic As(V), and As(V) is easily removed. Microorganisms with the ability to degrade aromatic compounds and oxidize arsenite [As(III)] may have strong potential to remediate co-contaminated water. In this study, a Gram-negative bacterium Hydrogenophaga sp. H7 was shown to simultaneously degrade 3-hydroxybenzoate (3-HBA) or 4-HBA (3-/4-HBA) and oxidize arsenite [As(III)] to arsenate [As(V)] during culture. Notably, the addition of As(III) enhanced the degradation rates of 3-/4-HBA, while the addition of 3-/4-HBA resulted in a slight delay in As(III) oxidation. Use of a 1% bacterial culture in combination with FeCl3 could completely degrade 250 mg/L 3-HBA or 4-HBA and remove 400 μM As(III) from simulated lake water within 28 h. Genomic analysis revealed the presence of As(III) oxidation/resistance genes and two putative 3-/4-HBA degradation pathways (the protocatechuate 4,5-dioxygenase degradation pathway and the catechol 2,3-dioxygenase degradation pathway). Comparative proteomics suggested that strain H7 degraded 4-HBA via the protocatechuate 4,5-dioxygenase degradation pathway in the absence of As(III); however, 4-HBA could be degraded via the catechol 2,3-dioxygenase degradation pathway in the presence of As(III). In the presence of As(III), more NADH was produced by the catechol 2,3-dioxygenase degradation pathway and/or by As(III) oxidation, which explained the enhancement of bacterial 4-HBA degradation in the presence of As(III). In addition, the key gene dmpB, which encodes catechol 2,3-dioxygenase in the catechol 2,3-dioxygenase degradation pathway, was knocked out, which resulted in the disappearance of As(III)-enhanced bacterial 4-HBA degradation from the dmpB mutant strain, which further confirmed that As(III) enhancement of 4-HBA degradation was due to the utilization of the catechol 2,3-dioxygenase pathway. These discoveries indicate that Hydrogenophaga sp. H7 has promise for the application to the removal of aromatic compounds and As co-contamination and reveal the relationship between As oxidation and aromatic compound degradation.

Arsenite Oxidation by a Newly Isolated Betaproteobacterium Possessing arx Genes and Diversity of the arx Gene Cluster in Bacterial Genomes.

Microbes play essential roles in arsenic transformation in the environment. Microbial arsenite oxidation is catalyzed by either of two distantly related arsenite oxidases, referred to as AIO and ARX. The arx genes encoding ARX and its regulatory proteins were originally defined in the genomes of gammaproteobacteria isolated from an alkaline soda lake. The arx gene cluster has been identified in a limited number of bacteria, predominantly in gammaproteobacteria isolated from lakes characterized by high pH and high salinity. In the present study, a novel arsenite-oxidizing betaproteobacterium, strain M52, was isolated from a hot spring microbial mat. The strain oxidized arsenite under both microaerophilic and nitrate-reducing conditions at nearly neutral pH. Genome analysis revealed that the strain possesses the arx gene cluster in its genome and lacks genes encoding AIO. Inspection of the bacterial genomes available in the GenBank database revealed that the presence of this gene cluster is restricted to genomes of Proteobacteria, mainly in the classes Gammaproteobacteria and Betaproteobacteria. In these genomes, the structure of the gene cluster was generally well-conserved, but genes for regulatory proteins were lacking in genomes of strains belonging to a specific lineage. Phylogenetic analysis suggested that ARX encoded in the genomes can be divided into three groups, and strain M52 belongs to a group specific for organisms living in low-salt environments. The ArxA protein encoded in the genome of strain M52 was characterized by the presence of a long insertion, which was specifically observed in the same group of ARX. In clone library analyses with a newly designed primer pair, a diverse ArxA sequence with a long insertion was detected in samples of lake water and hot spring microbial mat, characterized by low salinity and a nearly neutral pH. Among the isolated bacterial strains whose arsenite oxidation has been demonstrated, strain M52 is the first betaproteobacterium that possesses the arx genes, the first strain encoding ARX of the group specific for low-salt environments, and the first organism possessing the gene encoding ArxA with a long insertion.

Transcriptomics analysis defines global cellular response of Agrobacterium tumefaciens 5A to arsenite exposure regulated through the histidine kinases PhoR and AioS.

In environments where arsenic and microbes coexist, microbes are the principal drivers of arsenic speciation, which directly affects bioavailability, toxicity and bioaccumulation. Speciation reactions influence arsenic behaviour in environmental systems, directly affecting human and agricultural exposures. Arsenite oxidation decreases arsenic toxicity and mobility in the environment, and therefore understanding its regulation and overall influence on cellular metabolism is of significant interest. The arsenite oxidase (AioBA) is regulated by a three-component signal transduction system AioXSR, which is in turn regulated by the phosphate stress response, with PhoR acting as the master regulator. Using RNA-sequencing, we characterized the global effects of arsenite on gene expression in Agrobacterium tumefaciens 5A. To further elucidate regulatory controls, mutant strains for histidine kinases PhoR and AioS were employed, and illustrate that in addition to arsenic metabolism, a host of other functional responses are regulated in parallel. Impacted functions include arsenic and phosphate metabolism, carbohydrate metabolism, solute transport systems and iron metabolism, in addition to others. These findings contribute significantly to the current understanding of the metabolic impact and genetic circuitry involved during arsenite exposure in bacteria. This informs how arsenic contamination will impact microbial activities involving several biogeochemical cycles in nature.

Potential of biological arsenite oxidation in sand filtration units at arsenic\u2013iron removal plants (AIRPs) in Bangladesh

This study is aimed to investigate biological oxidation of arsenic [As(III)] in sand component of arsenic–iron removal plant (AIRP). Water and sand samples were collected from the municipal and household AIRP units (MAIRP and HAIRPs) at Manikganj district, Bangladesh. These samples were analyzed using arsenic speciation cartridge with inductively coupled plasma-mass spectrometry for arsenic and iron detection and phylogenetic analysis based on aoxB gene sequencing. It was observed that arsenic concentration in the raw water, before and after sand filtration, ranged from 13.0–81.0 to 4.0–21.0 μg/L, respectively, which majorly contained reduced arsenic [As(III)] in the inlet and oxidized arsenic [As(V)] in the outlet. The results of our batch experiments showed that indeed sand unit of MAIRP oxidized As(III) with 36 g/h estimated biological oxidation potential which was considered enough to oxidize all the As(III) loading (6.3 g/h) in the influent. Additionally, bacterial arsenite oxidase gene (aoxB) was detected in the sand sample and has 80% sequence similarity with Polymorphum gilvum, an alphaproteobacteria on the phylogenetic tree. This study, therefore, revealed that AIRP sand units have enough potential of biological activity to ensure overall arsenic removal through As(III) oxidation. However, future research is necessary to unfold the basic mechanistic approach explored in this plant for further modification if need arises.

Role of reactive oxygen species in As(III) oxidation by carbonate structural Fe(II): A surface-mediated pathway

Arsenite oxidation is commonly found during the removal of arsenite by iron minerals. The influencing factors and mechanisms of As(III) oxidation in reactive carbonate structural Fe(II) (CSF) suspensions were investigated in this study. Results showed that oxidation efficiency of As(III) was susceptible to CSF dosage, initial pH and dissolved oxygen (DO). It is interesting to note that the presence of iodide could conspicuously facilitate the oxidation efficiency of As(III) by accelerating Fe(III)/Fe(II) cycle. Scavenging experiments demonstrated that H2O2 and ·O2– were the crucial ROSs in the pH range of 5.0–9.0. Besides, the other predominant ROSs accounting for As(III) oxidation were Fe(IV) and surface-bound hydroxyl radical (·OH) in acid and neutral solutions while Fe(IV) became the main oxidant in alkaline solutions. Production of ROSs was attributed to the oxygenation of Fe2+ adsorbed on Fe(III) oxyhydroxides and complexed Fe(II) under neutral condition. No ROSs could be examined in the liquid phase, a surface-mediated mechanism was thus proposed: ROSs were generated on the surface of CSF while As(III) was adsorbed on the surface of CSF and then oxidized to As(V) by surface-bound ROSs. This study may shed light on molecular oxygen activation mechanism by carbonate structural Fe(II) during arsenite oxidation.

Development of a rate law for arsenite oxidation by manganese oxides

Abstract Arsenite (As(III)), may be rapidly oxidized to arsenate (As(V)) by manganese oxides over a wide range of pH. Interestingly, pseudo-first order rate constants for Mn(IV) oxides (MnO2) reported in the literature differ by orders of magnitude, mainly due to the fact that experimental conditions have not been normalized and surface passivation may hinder the reaction rate. As a result, a formal kinetic rate law for the oxidation of As(III) by MnO2 does not exist. In this study, a comprehensive rate law that describes the abiotic oxidation of As(III) by vernadite (δ-MnO2) was developed by systematically varying pH and the concentrations of δ-MnO2 and As(III). Experiments were performed with excess δ-MnO2 compared to As(III) in buffered solutions with an ionic strength typical of freshwater environments to reflect the geochemical conditions of most environments. The half-life of the pseudo-first order reaction is less than three minutes in all experiments. Results show the initial reaction rate is proportional to the concentrations of δ-MnO2 and As(III) and independent of pH in the pH range 4–9. However, below pH 6 and above pH 8 the pseudo-first order rate constant is slightly lower due to the protonation of the surface δ-MnO2 and the deprotonation of As(III). Overall, the reaction follows a first order rate law with respect to As(III) and effective surface sites of δ-MnO2 and zeroth order with respect to pH. The overall second order rate constant, k, was calculated to be 0.36 ± 0.11 L h−1 m−2. This rate constant is comparable to the vast majority of rate constants derived from previously published data when using the effective surface sites of MnO2 as reactive species only, indicating the most influential factors of As(III) oxidation by MnO2 are constrained in the developed rate law. Based on the present experiments, the oxidation of As(III) by MnO2 is proposed to proceed as one two-electron transfer reaction between As(III) and Mn(IV) to eventually form As(V) and Mn2+ species. Two steps of one-electron transfer, however, may be possible though it is likely that both Mn(IV) and Mn(III) species are ultimately reduced to Mn2+. Comproportionation of Mn2+ and Mn(IV) to Mn(III) does not seem to be important on the time scale of these experiments.

Immobilization of arsenic as scorodite by a thermoacidophilic mixed culture via As(III)-catalyzed oxidation with activated carbon.

In this study we describe the immobilization of arsenic as scorodite (FeAsO4.2H2O) by a thermophilic iron-oxidizing mixed culture from an acidic sulfate medium containing 500 mg L-1 of Fe(II), 500 mg L-1 As(III) and granular activated carbon (GAC) as the main arsenite oxidant. This study shows that crystalline scorodite can only be precipitated in the presence of the ferrous iron-oxidizing mixed culture (pH 1.3 and 70 °C). The efficiency of arsenite oxidation was over 99% with a maximum specific oxidation rate of 280 mgAs(III) gGAC-1 day-1. Ferrous iron and arsenite were also oxidized in the absence of the mixed culture, however, no scorodite precipitated under these conditions; consequently, scorodite precipitation was biologically induced. The precipitated scorodite particles had a size between 0.5 and 10 μm with an average of 5 μm, resulting in low settling rates. Ion activity product calculations and observations by Scanning Electron Microscopy (SEM) indicated that microbial cells served as surface for heterogeneous nucleation. The potential of the thermophilic mixed culture for the scorodite formation explored in this study provides the basis of a new approach for the treatment of As(III) polluted streams.

Mechanisms of arsenite oxidation and arsenate adsorption by a poorly crystalline manganese oxide in the presence of low molecular weight organic acids

Manganese oxides are considered as one of the effective oxides capable of oxidizing arsenite and reduce the toxicity of arsenic. Since low molecular weight organic acids (LMWOAs) commonly found in nature can act as reducing and chelating agents for manganese oxides, it is particularly important to investigate how these organic acids with different numbers of carboxyl groups like citrate and EDTA affect oxidation and adsorption of arsenic by manganese oxides. In this study, low As(V) adsorption on manganese oxide is slightly enhanced by citrate and EDTA, which results from the increase in active sites via reduction of manganese oxide by LMWOAs. However, citrate and EDTA have different effects on the oxidation of As(III). MnIII/II citrate autocatalytic cycle as a manganese-based redox system decreases As(III) oxidation rate, but EDTA does not yield autocatalysis, which slightly increases the oxidation rate of As(III). Reduction of manganese oxide by EDTA and chelation between Mn(II) and EDTA lead to exposure of more active sites. Our research highlights the different effects of low molecular weight organic acids on the reactions between arsenic and manganese oxide.