Bioremediation for seleno-oxyanions in tunnel-excavated muck using selenate-reducing bacterium Stutzerimonas stutzeri NT-I

Selenium is a rare metal found mainly in volcanic sediments; it is naturally distributed in Japan owing to its sulfide deposits. Selenium exists in soil as highly toxic and soluble seleno-oxyanions, namely, selenate and selenite. The Japanese government has established standards for soluble selenium concentrations in soil, achieved by water shielding or the addition of insolubilization agents. If the selenium (and/or other heavy metals) in tunnel-excavated muck exceeds the environmental standards, it should be treated similarly to contaminated soil. However, conventional treatments leave selenium residues in tunnel-excavated muck, preventing their reuse due to the risk of selenium re-elution. This study attempted to insolubilize soluble seleno-oxyanions in tunnel-excavated muck by adding the bacterium Stutzerimonas stutzeri NT-I, which reduces selenate to insoluble elemental selenium and then to volatile dimethyl diselenide. Selenate reduction tests were conducted by adding heavy metals, including As(III), As(V), Cd(II), Pb(II), and Fe(III), to examine the selenate reduction ability and growth of S. stutzeri NT-I in the presence of their ions. The results showed that the S. stutzeri NT-I reduced the selenate by 50 mg/L concentrations for all the tested contaminants. In naturally contaminated tunnel-excavated muck, heavy metal elements, nitrate, and nitrite contents were detected at levels of <50 mg/L, suggesting that they do not adversely affect the selenate reduction by S . stutzeri NT-I. The results indicate that selenium insolubilization using S . stutzeri NT-I can be applied to a wide range of tunnel-excavated muck.