Arsenite Oxidation Research Articles

Microbial arsenite oxidation with oxygen, nitrate, or an electrode as the sole electron acceptor.

The purpose of this study was to identify bacteria that can perform As(III) oxidation for environmental bioremediation. Two bacterial strains, named JHS3 and JHW3, which can autotrophically oxidize As(III)-As(V) with oxygen as an electron acceptor, were isolated from soil and water samples collected in the vicinity of an arsenic-contaminated site. According to 16S ribosomal RNA sequence analysis, both strains belong to the ɤ-Proteobacteria class and share 99% sequence identity with previously described strains. JHS3 appears to be a new strain of the Acinetobacter genus, whereas JHW3 is likely to be a novel strain of the Klebsiella genus. Both strains possess the aioA gene encoding an arsenite oxidase and are capable of chemolithoautotrophic growth in the presence of As(III) up to 10mM as a primary electron donor. Cell growth and As(III) oxidation rate of both strains were significantly enhanced during cultivation under heterotrophic conditions. Under anaerobic conditions, only strain JHW3 oxidized As(III) using nitrate or a solid-state electrode of a bioelectrochemical system as a terminal electron acceptor. Kinetic studies of As(III) oxidation under aerobic condition demonstrated a higher V max and K m from strain JHW3 than strain JHS3. This study indicated the potential application of strain JHW3 for remediation of subsurface environments contaminated with arsenic.

Characterization of Thiomonas delicata arsenite oxidase expressed in Escherichia coli

Microbial arsenite oxidation is an essential biogeochemical process whereby more toxic arsenite is oxidized to the less toxic arsenate. Thiomonas strains represent an important arsenite oxidizer found ubiquitous in acid mine drainage. In the present study, the arsenite oxidase gene (aioBA) was cloned from Thiomonas delicata DSM 16361, expressed heterologously in E. coli and purified to homogeneity. The purified recombinant Aio consisted of two subunits with the respective molecular weights of 91 and 21 kDa according to SDS-PAGE. Aio catalysis was optimum at pH 5.5 and 50–55 °C. Aio exhibited stability under acidic conditions (pH 2.5–6). The Vmax and Km values of the enzyme were found to be 4 µmol min−1 mg−1 and 14.2 µM, respectively. SDS and Triton X-100 were found to inhibit the enzyme activity. The homology model of Aio showed correlation with the acidophilic adaptation of the enzyme. This is the first characterization studies of Aio from a species belonging to the Thiomonas genus. The arsenite oxidase was found to be among the acid-tolerant Aio reported to date and has the potential to be used for biosensor and bioremediation applications in acidic environments.

Preparation, characterization of Fe, Ce Co-doped flower-like TiO2 photocatalysts and their properties in photocatalytic oxidation of arsenite

Preoxidation process is usually needed in the treatment of arsenic containing water because arsenite (i.e., As(III)) is less easily to be removed by adsorption. Nano-scale titanium dioxide (TiO2) is an efficient photocatalyst for arsenite oxidation. In this study, a series of photocatalysts of un-doped, single-doped and co-doped flower-like TiO2 were successfully prepared by template method using Fe(NO3)3 · 9H2O, ammonium ceric nitrate and tetrabutyl titanate (Ti(OC4H9)4) as precursors and glucan as template. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and N2 adsorption-desorption measurement were employed to characterize the morphology, crystal structure and surface structure of the samples. The photoabsorbance of the obtained catalysts was measured by UV–Vis absorption spectroscopy, and the photocatalytic activities of the prepared samples under UV light were estimated by measuring the oxidation rate of As(III) in an aqueous solution. The characterizations indicated that the prepared photocatalysts had flowerlike structures, consisted of anatase phase and possessed high surface area of ca. 160 m2/g. It was shown that the Fe, Ce co-doped flower-like TiO2 could be used as an effective catalyst in photocatalytic oxidation reactions. The synergistic effect of Fe and Ce co-doping played an important role in improving the photocatalytic activity. In addition, the possibility of cyclic usage of the co-doped flower-like TiO2 was also confirmed, the photocatalytic activity of flower-like TiO2 remained 88% of that of the fresh sample after being used four times.

In silico approach for bioremediation of arsenic by structure prediction and docking studies of arsenite oxidase from Pseudomonas stutzeri TS44

Pseudomonas stutzeri TS44 is a moderately halotolerant, arsenite-oxidizing bacterium, containing genes for arsenite oxidation, arsenic resistance, and ectoine/hydroxyectoine biosynthesis. This paper reports in silico studies to understand bioremediation to eliminate toxic metal arsenic in water, air and soil by arsenite oxidase (AO), the bacterial enzyme from P. stutzeri TS44 that can be used for a low cost and eco-friendly removal of arsenite from the environment. To understand the activity of AO in elimination of arsenite, sequence analysis was carried out and homologs, orthologs, domains, family, and conserved residues were identified followed by model generation using various homology modeling tools. The generated models were validated for the best quality protein structure and the best model was used for further optimization using energy minimization approach. Molecular docking studies were performed to study the binding interaction of AO with arsenite. The study predicts and validates the 3D structure of P. stutzeri TS44 arsenite oxidase and reports four active site residues (His197, Glu205, Arg421, and His425) from a close structural homolog of AO from A. faecalis (PDB ID: 1G8K_A). The molecular docking studies suggested the formation of a stable complex and in silico site-directed mutagenesis revealed the importance of Arg421, which resulted in a decrease in stability of the complex when mutated. The study implicates P. stutzeri TS44 arsenite oxidase as a non virulent protein for low cost and eco-friendly bioremediation of arsenite.

Processes governing arsenic retardation on Pleistocene sediments: Adsorption experiments and model‐based analysis

AbstractIn many countries of south/south‐east Asia, reliance on Pleistocene aquifers for the supply of low‐arsenic groundwater has created the risk of inducing migration of high‐arsenic groundwater from adjacent Holocene aquifers. Adsorption of arsenic onto mineral surfaces of Pleistocene sediments is an effective attenuation mechanism. However, little is known about the sorption under anoxic conditions, in particular the behavior of arsenite. We report the results of anoxic batch experiments investigating arsenite (1–25 µmol/L) adsorption onto Pleistocene sediments under a range of field‐relevant conditions. The sorption of arsenite was nonlinear and decreased with increasing phosphate concentrations (3–60 µmol/L) while pH (range 6–8) had no effect on total arsenic sorption. To simulate the sorption experiments, we developed surface complexation models of varying complexity. The simulated concentrations of arsenite, arsenate, and phosphate were in good agreement for the isotherm and phosphate experiments while secondary geochemical processes affected the pH experiments. For the latter, the model‐based analysis suggests that the formation of solution complexes between organic buffers and Mn(II) ions promoted the oxidation of arsenite involving naturally occurring Mn‐oxides. Upscaling the batch experiment model to a reactive transport model for Pleistocene aquifers demonstrates strong arsenic retardation and could have useful implications in the management of arsenic‐free Pleistocene aquifers.

Modeling fate and transport of arsenic in a chlorinated distribution system

Experimental and modeling studies were conducted to understand the fate and transport properties of arsenic in drinking water distribution systems. Pilot scale experiments were performed in a distribution system simulator by injecting arsenic and measuring both adsorption onto iron pipe material and the oxidation of arsenite by hypochlorite in tap water to form arsenate. A mathematical model describing these processes was developed and simulated using EPANET-MSX, a hydraulic and multi-species water quality software for pipe networks. Model parameters were derived from the pilot-scale experiments. The model was applied to both the distribution system simulator and EPANET example network #3, a real-world model of a drinking water system serving approximately 78,000 customers. The model can be applied to systems-level studies of arsenic fate and transport in drinking water resulting from natural occurrences, accidental spills, or intentional introduction into water.

Flexible biological arsenite oxidation utilizing NOx and O2 as alternative electron acceptors

The feasibility of flexible microbial arsenite (AsIII) oxidation coupled with the reduction of different electron acceptors was investigated. The results indicated the acclimated microorganisms could oxidize AsIII with oxygen, nitrate and nitrite as the alternative electron acceptors. A series of batch tests were conducted to measure the kinetic parameters of AsIII oxidation and to evaluate the effects of environmental conditions including pH and temperature on the activity of biological AsIII oxidation dependent on different electron acceptors. Kinetic results showed that oxygen-dependent AsIII oxidation had the highest oxidation rate (0.59 mg As g−1 VSS min−1), followed by nitrate- (0.40 mg As g−1 VSS min−1) and nitrite-dependent AsIII oxidation (0.32 mg As g−1 VSS min−1). The kinetic data of aerobic AsIII oxidation were fitted well with the Monod kinetic model, while the Haldane substrate inhibition model was better applicable to describe the inhibition of anoxic AsIII oxidation. Both aerobic and anoxic AsIII oxidation performed the optimal activity at the near neutral pH. Besides, the optimal temperature for oxygen-, nitrate- and nitrite-dependent AsIII oxidation was 30 ± 1 °C, 40 ± 1 °C and 20 ± 1 °C, respectively.

Enhanced Detoxification of Arsenic Under Carbon Starvation: A New Insight into Microbial Arsenic Physiology.

Nutrient availability in nature influenced the microbial ecology and behavior present in existing environment. In this study, we have focused on isolation of arsenic-oxidizing cultures from arsenic devoid environment and studied effect of carbon starvation on rate of arsenite oxidation. In spite of the absence of arsenic, a total of 40 heterotrophic, aerobic, arsenic-transforming bacterial strains representing 18 different genera were identified. Nineteen bacterial species were isolated from tannery effluent and twenty-one from tannery soil. A strong co-relation between the carbon starvation and arsenic oxidation potential of the isolates obtained from the said niche was observed. Interestingly, low carbon content enhanced the arsenic oxidation ability of the strains across different genera in Proteobacteria obtained. This represents the impact of physiological response of carbon metabolism under metal stress conditions. Enhanced arsenic-oxidizing ability of the strains was validated by the presence of aio gene and RT-PCR, where 0.5- to 26-fold up-regulation of arsenite oxidase gene in different genera was observed. The cultures isolated from tannery environment in this study show predominantly arsenic oxidation ability. This detoxification of arsenic in lack of carbon content can aid in effective in situ arsenic bioremediation.

Arsenite oxidation regulator AioR regulates bacterial chemotaxis towards arsenite in Agrobacterium tumefaciens GW4

Some arsenite [As(III)]-oxidizing bacteria exhibit positive chemotaxis towards As(III), however, the related As(III) chemoreceptor and regulatory mechanism remain unknown. The As(III)-oxidizing bacterium Agrobacterium tumefaciens GW4 displays positive chemotaxis towards 0.5–2 mM As(III). Genomic analyses revealed a putative chemoreceptor-encoding gene, mcp, located in the arsenic gene island and having a predicted promoter binding site for the As(III) oxidation regulator AioR. Expression of mcp and other chemotaxis related genes (cheA, cheY2 and fliG) was inducible by As(III), but not in the aioR mutant. Using capillary assays and intrinsic tryptophan fluorescence spectra analysis, Mcp was confirmed to be responsible for chemotaxis towards As(III) and to bind As(III) (but not As(V) nor phosphate) as part of the sensing mechanism. A bacterial one-hybrid system technique and electrophoretic mobility shift assays showed that AioR interacts with the mcp regulatory region in vivo and in vitro, and the precise AioR binding site was confirmed using DNase I foot-printing. Taken together, these results indicate that this Mcp is responsible for the chemotactic response towards As(III) and is regulated by AioR. Additionally, disrupting the mcp gene affected bacterial As(III) oxidation and growth, inferring that Mcp may exert some sort of functional connection between As(III) oxidation and As(III) chemotaxis.

Functional genes and thermophilic microorganisms responsible for arsenite oxidation from the shallow sediment of an untraversed hot spring outlet.

Hot Springs have unique geochemical features. Microorganisms-mediated arsenite oxidation is one of the major biogeochemical processes occurred in some hot springs. This study aimed to understand the diversities of genes and microorganisms involved in arsenite oxidation from the outlet of an untraversed hot spring located at an altitude of 4226 m. Microcosm assay indicated that the microbial community from the hot springwas able to efficiently oxidize As(III) using glucose, lactic acid, yeast extract or sodium bicarbonate as the sole carbon source. The microbial community contained 7 phyla of microorganisms, of which Proteobacteria and Firmicutes are largely dominant; this composition is unique and differs significantly from those of other described hot springs. Twenty one novel arsenite oxidase genes were identified from the samples, which are affiliated with the arsenite oxidase families of α-Proteobacteria, β-Proteobacteria or Archaea; this highlights the high diversity of the arsenite-oxidizing microorganisms from the hot spring. A cultivable arsenite-oxidizer Chelatococcu sp. GHS311 was also isolated from the sample using enrichment technique. It can completely convert 75.0 mg/L As(III) into As(V) in 18 days at 45 °C. The arsenite oxidase of GHS311 shares the maximal sequence identity (84.7%) to that of Hydrogenophaga sp. CL3, a non-thermotolerant bacterium. At the temperature lower than 30 °C or higher than 65 °C, the growth of this strain was completely inhibited. These data help us to better understand the diversity and functional features of the thermophilic arsenite-oxidizing microorganisms from hot springs.

Diversity of bacterial communities inhabiting soil and groundwater of arsenic contaminated areas in West Bengal, India

Soil and water contaminated with arsenic (As) through natural or anthropogenic inputs are commonly considered as native source of tolerant bacterial strains. The present study was successful in characterizing 12 hyper-tolerant bacteria, satisfying maximum tolerable concentration (MTC) for arsenate (As5+) ≥ 300 mM and arsenite (As3+) ≥ 30 mM, isolated from As affected North 24 Parganas and South 24 Parganas districts of West Bengal, India. Most of the bacteria showing higher level of tolerance to As5+ and As3+ were found as gram-positive and bacilli in shape. Positive responses to different biochemical tests indicated that some of these bacteria could be potent sources of various biotechnologically important enzymes. Some of the hyper-tolerant bacteria could reduce As5+ to As3+ while all others could oxidise As3+ to As5+. Phylogenetic analysis revealed that those hyper-tolerant bacterial strains were distributed among three phyla such as Actinobacteria, Firmicutes, and γ-Proteobacteria. The Firmicutes were well represented in this study with more than half of the hyper-tolerant strains corresponding to members of this group. Moreover, majority of the isolates except SR10 belonging to this phylum were affiliated to different species of the genus Bacillus and showed different tolerance capability to As3+ and As5+. We present the first report of the genus Paenibacillus as being involved in arsenite oxidation with hyper-tolerance property to As. Four isolates named as SDe5, SDe12, SDe13, and SDe15 belonging to genera Bacillus and Rhodococcus exhibited highest tolerance to As and therefore represented as good candidates for bioremediation processes of native polluted soil and ground water.

Arsenic removal from arsenic-contaminated water by biological arsenite oxidation and chemical ferrous iron oxidation using a down-flow hanging sponge reactor

The down-flow hanging sponge (DHS) reactor was used for continuous As removal treatment of As-contaminated water. The treatment scheme was: (1) As(III) in contaminated water is oxidized by arsenite-oxidizing bacteria fixed in the sponges in the reactor; (2) Fe(II) naturally existing in the water is oxidized by dissolved oxygen; (3) Fe(III) is precipitated as iron hydroxide and As(V) is co-precipitated with the iron hydroxide; and finally (4) the co-precipitates are fixed in the sponges. This system could remove As from As-contaminated water on a small scale and at low cost. The results showed that, after using the DHS reactor, As and Fe concentrations in the treated water were lower than water quality standards for drinking water when Fe(II) concentration in the influent was lower than 10 mg/L and the Fe/As ratio was higher than 6.67–8.42, with dependence on the Fe concentration. Additionally, even if Fe concentration is higher than 10 mg/L, the treatment system is still applicable if the pH of the influent is higher than 7 or the retention time is longer than 2 h.

Photocatalytic Oxidation of Arsenite Using Goethite and UV LED

Arsenic (As) has been considered as the most toxic one among various hazardous materials and As contamination can be caused naturally and anthropogenically. Major forms of arsenic in groundwater are arsenite [(As(III)] and/or arsenate [(As(V)], depending on redox condition: arsenite and arsenate are predominant in reduced and oxidized environments, respectively. Because arsenite is much more toxic and mobile than arsenate, there have been a number of studies on the reduction of its toxicity through oxidation of As(III) to As(V). This study was initiated to develop photocatalytic oxidation process for treatment of groundwater contaminated with arsenite. The performance of two types of light sources (UV lamp and UV LED) was compared and the feasibility of goethite as a photocatalyst was evaluated. The highest removal efficiency of the process was achieved at a goethite dose of 0.05 g/L. Based on the comparison of oxidation efficiencies of arsenite between two light sources, the apparent performance of UV LED was inferior to that of UV lamp. However, when the results were appraised on the basis of their emitting UV irradiation, the higher performance was achieved by UV LED than by UV lamp. This study demonstrates that environmentally friendly process of goethite-catalytic photo-oxidation without any addition of foreign catalyst is feasible for the reduction of arsenite in groundwater containing naturally-occurring goethite. In addition, this study confirms that UV LED can be used in the photo-oxidation of arsenite as an alternative light source of UV lamp to remedy the drawbacks of UV lamp, such as long stabilization time, high electrical power consumption, short lifespan, and high heat output requiring large cooling facilities. Key words: Arsenite [As(III)], Arsenate [As(V)], Photocatalytic Oxidation, UV Lamp, UV LED, Goethite

An Oxidoreductase AioE is Responsible for Bacterial Arsenite Oxidation and Resistance

Previously, we found that arsenite (AsIII) oxidation could improve the generation of ATP/NADH to support the growth of Agrobacterium tumefaciens GW4. In this study, we found that aioE is induced by AsIII and located in the arsenic island near the AsIII oxidase genes aioBA and co-transcripted with the arsenic resistant genes arsR1-arsC1-arsC2-acr3-1. AioE belongs to TrkA family corresponding the electron transport function with the generation of NADH and H+. An aioE in-frame deletion strain showed a null AsIII oxidation and a reduced AsIII resistance, while a cytC mutant only reduced AsIII oxidation efficiency. With AsIII, aioE was directly related to the increase of NADH, while cytC was essential for ATP generation. In addition, cyclic voltammetry analysis showed that the redox potential (ORP) of AioBA and AioE were +0.297 mV vs. NHE and +0.255 mV vs. NHE, respectively. The ORP gradient is AioBA > AioE > CytC (+0.217 ~ +0.251 mV vs. NHE), which infers that electron may transfer from AioBA to CytC via AioE. The results indicate that AioE may act as a novel AsIII oxidation electron transporter associated with NADH generation. Since AsIII oxidation contributes AsIII detoxification, the essential of AioE for AsIII resistance is also reasonable.

Efficient purification of arsenic-contaminated water using amyloid-carbon hybrid membranes.

We show the purification of arsenic-contaminated water using amyloid fibril-based membranes, which adsorb both the arsenate (+5) and arsenite (+3) oxidation forms at efficiencies of ∼99%. Binding isotherms indicate that amyloid fibrils possess multiple binding residues capable of strongly adsorbing arsenic ions via metal-ligand interactions, delaying the saturation of the membrane. We also show that these membranes can be reused for several cycles without any efficiency drop, and validate our technology in purifying real contaminated ground water by removing arsenic with an efficiency as high as 99.6%. These results make this technology promising for inexpensive, efficient and low-energy removal of arsenic from contaminated water.

A microscopic and spectroscopic study of rapid antimonite sequestration by a poorly crystalline phyllomanganate: differences from passivated arsenite oxidation

Surface passivation during the adsorption of Mn(ii) and the formation of Mn(iii) was the predominant cause for the decrease in the As(iii) oxidation rate, whereas it may not have been the limiting factor during Sb(iii) oxidation.

Rhizospheric iron and arsenic bacteria affected by water regime: Implications for metalloid uptake by rice

Rice is characterized by high levels of arsenic accumulation, even if cultivated in non-contaminated soils. Given the limits for arsenic concentration in rice grain recently established by the European Community, it is essential to understand the mechanisms and find solutions to this issue. Arsenic bioavailability is strictly related to water management of the rice paddy as well as to iron- and arsenic-cycling bacterial populations inhabiting the rice rhizosphere.To evaluate the effect of different agronomic conditions on the root-soil microbiota involved in arsenic mobilization, rice plants were grown in macrocosms containing non contaminated field soil under either continuous flooding, aerobic rice regime or continuous flooding with a 14 day-period of drainage before flowering. Specific groups of iron- and arsenic-cycling bacteria were assessed by real time quantitative PCR and fluorescence in situ hybridization.Continuous flooding led to the release of arsenate and iron in soil solution and produced rice grains with arsenite and organic arsenic above the recently established limits, contrary to the other agronomic conditions.Iron-reducing bacteria affiliated to the family Geobacteraceae significantly increased under continuous flooding in rhizosphere soil, in concomitance to arsenate dissolution from iron minerals. The 14 day-period of drainage before flowering allowed the recycling of iron, with the increase of Gallionella-like iron-oxidizing bacteria. This phenomenon likely influenced the decrease of arsenic translocation in rice grains.Regardless of the water regime, genes for arsenite oxidation (aioA) were the most abundant arsenic-processing genes, explaining the presence of arsenate in soil solution. The presence of arsenite and organic arsenic in rice grains produced under continuous flooding might be related to the retrieval of genes for arsenate reduction (arsC) and for arsenite methylation (arsM) in the proximity of the roots.These outcomes indicate a potential active role of rhizospheric iron- and arsenic-cycling bacteria in determining arsenic accumulation in rice grains from plants cultivated under continuous flooding, even in soil with a low arsenic content.

Energy-Efficient Oxidation and Removal of Arsenite from Groundwater Using Air-Cathode Iron Electrocoagulation

Arsenic contamination of groundwater has affected many countries, especially in Southeast Asia. Although aerated electrocoagulation (EC) provides an effective way to remove arsenite, this process is energy-intensive because of aeration and relatively poor cathode performance. To overcome these disadvantages, a novel EC system was proposed using an air cathode to generate H2O2 in situ for improved energy efficiency of As(III) removal. With the air cathode, the H2O2 production rate was 3.7 ± 0.1 mg L–1 h–1, which indirectly promoted As(III) oxidation by interaction with Fe(II). At a current density of 4 A m–2, the average cell voltage during air-cathode electrocoagulation (ACEC) was 1.0 V, compared to 1.9 V in the EC and aerated EC systems. Energy consumption in the ACEC system was 17.0 ± 0.7 Wh log–1 m–3, much lower than those in the EC (67.8 ± 0.9 Wh log–1 m–3) and aerated EC (65.1 ± 0.8 Wh log–1 m–3) systems, which can be attributed to effective As(III) oxidation, no need for aeration, and improved catho...

PH dependent kinetic insights of electrocatalytic arsenite oxidation reactions at Pt surface

Kinetics of electrocatalytic oxidation of arsenite ions has been investigated at a Pt disk electrode using cyclic voltammetry, convolution potential sweep voltammetry and electrochemical impedance spectroscopy. A complimentary environment pertaining to oxidation reactions of arsenite ions is attained in the acidic medium compared to a neutral or basic medium. It is suggested that in the neutral and basic media, direct electron transfer from the solution to electrode instigate the oxidation process without any pre adsorption. Meanwhile, in the acidic medium, prior to oxidation, arsenite ions are adsorbed on the Pt surface and a stepwise reaction mechanism is involved. Using impedance analysis, it is suggested that different forms of surface oxides at various pH values control the oxidation kinetics.

A rapid monitoring method for inorganic arsenic in rice flour using reversed phase-high performance liquid chromatography-inductively coupled plasma mass spectrometry

A new rapid monitoring method by means of high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) following the heat-assisted extraction was developed for measurement of total inorganic arsenic species in rice flour. As(III) and As(V) eluted at the same retention time and completely separated from organoarsenic species by an isocratic elution program on a reversed phase column. Therefore, neither ambiguous oxidation of arsenite to arsenate nor the integration of two peaks were necessary to determine directly the target analyte inorganic arsenic. Rapid injection allowed measuring 3 replicates within 6min and this combined with a quantitative extraction of all arsenic species from rice flour by a 15min HNO3-H2O2 extraction makes this the fastest laboratory based method for inorganic arsenic in rice flour.