Effect of soil microorganisms on arsenite oxidation in paddy soils under oxic conditions

The contribution of abiotic and biotic processes to the oxidation of arsenite [As(III)] when anaerobic paddy soils were shifted to oxic conditions was investigated. Soil slurries were first incubated under reducing conditions to allow indigenous arsenic (As) to be dissolved into the liquid phase as As(III), and were then switched to oxic incubation conditions with shaking. One day after switching to oxic incubation, As(III) almost disappeared from the liquid phase without any increase of arsenate [As(V)], suggesting that dissolved As(III) was co-precipitated or adsorbed on the soil solid phase. X-ray adsorption near-edge structure (XANES) analysis revealed that the predominant species of As in the solid phase before the oxic incubation was As(III), ranging from 74 to 85% of total As. After 1 d of the oxic incubation, the proportion of As(III) decreased to 46–47%. This oxidation step was an abiotic process and 28–38% of As(III) was oxidized per day on average. However, the abiotic oxidation ceased within 24 h probably due to passivation of reactive sites on the mineral surface. Afterward, a second slow oxidation step (2.5–2.8% per day on average) became predominant. Interestingly, this step was microbiologically mediated, since it did not occur in sterilized soil slurries. Determination of putative arsenite oxidase gene (aioA) sequences suggested that arsenite-oxidizing bacteria are actually present in our soil slurries. Our results suggest that microbial As(III) oxidation accounts for more than 30% of total As(III) oxidation, and thus it is an important process especially after abiotic oxidation ceases.