THE MECHANISM OF ARSENITE OXIDATION IN SOIL COLUMN EXPERIMENTS

Although inorganic arsenite oxidation is thermodynamically favored in contact with air, it is well known that the oxidation rate is extremely slow. While the half life of arenite in water open to the air has been reported by Eary et al. (1990) as being up to one year, much higher oxidation rates were observed in arsenic trioxide-spiked soil column experiments. The arsenate concentrations measured in column effluent solutions (up to 273 ppm) cannot be explained by inorganic arsenite oxidation. To determine the arsenite oxidation mechanism taking place in the soil columns, serum bottle experiments were conducted to compare the arsenite oxidation rate between the autoclaved series and the inoculated series of experiments. The results showed that arsenite in the inoculated series was completely oxidized to arsenate after 15 days, while no oxidation of arsenite was observed in the sterilized experiments.

Approximately thirty bacterial isolates were obtained from the column soil and inoculated serum bottle materials. The isolates were screened for arsenite-oxidizers by a modified microplate method using colorimetric silver nitrate-arsenite/arsenate precipitation reactions. The 16S rDNA of the isolates was amplified by polymerase chain reaction (PCR) and sequenced. The 16S rDNA sequences indicate that the isolates include members of the genera Burkholderia, Sphingomonas, and Bacillus, including several new species which will be further characterized.