Experimental simulation of arsenite leaching on soils and mechanism of arsenite oxidation

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Arsenic trioxide was widely applied to soils in North America as an herbicide during the 1950-60s. These herbicide applications led to soil arsenic contamination at numerous sites. Decades after the herbicide application, the contaminated soil served as a secondary source for long-term leaching of arsenic into the groundwater system. To understand the history of arsenic contamination at these sites, column experiments were conducted to simulate the herbicide application and subsequent arsenic leaching processes. The experimental data showed that the effluent solution arsenic became dominantly As(V) after 180 pore volumes (equivalent to ~60 years of natural leaching, assuming a 50% recharge rate) of leaching, which represented an abnormally rapid arsenite oxidation rate (up to 60 mg/L/6.5 hours As(V)) (Yue and Donahoe, 2009). During peak arsenic release, the arsenite oxidation rate doubled (120 mg/mL/6.5 hours). Homogeneous As(III) oxidation cannot be responsible for the observed oxidation rate because the half life of As(III) in air can be up to one year (Eary and Schramke, 1990). Incubation experiments were designed, where sterilized and inoculated serum bottles with added aqueous As(III) and soil were compared for their aqueous total arsenic and As(V) concentrations. The aqueous As(V) in the innoculated series increased with time but remained below detection in the sterile series. This indicated that the As(III) oxidation observed in the column experiments was caused by the microbes in the soil. Bacterium strains A4 and A12 were isolated from the column soil and were tested to be the efficient arsenite oxidizers in the column experiments. Strain A12 shared 100% 16S rDNA sequence with Burkholderia. fungorum LMG 16225^T, while strain A4 shared 99.1%, 97.3% and 96.7% 16S rDNA sequences with strains Burkholderia. zhejiangensis CCTCC AB 2010354^T, B. glathei DSM 50014^T and B. sordidicola KCTC 12081^T, respectively. A polyphasic characterization, including phenotypic and biochemical characterization, 16S rDNA sequence analysis, DNA-DNA hybridization, and fatty acid analysis, was conducted on strain A4 to determine its taxonomic position. The results showed that strain A4 represented a novel species in the genus Burkhoderia, for which the name Burkholderia arsenicoxydans sp. nov. is proposed. The type strain is A4^T (=ATCC BAA-2404^T=CCTCC AB 2012027^T).

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Geochemistry, Geobiology, Environmental science