Insolubilization of Technetium by Indigenous Bacteria in Water Covering Paddy Soils

Microbially mediated insoluble technetium (Tc) formation was studied using surface water covering paddy fields. Surface water samples were collected from three kinds of flooded paddy soils: andosol, gley soil, and gray lowland soil. Insoluble Tc (> 0.2 μm in size) was formed in all surface water samples and the amounts ranged from 48.7 ± 2.2% of total Tc (gley soil) to 88.5 ± 3.1% (gray lowland soil). However, elimination of microbial cells from the surface water resulted in an extremely low amount of insoluble Tc being formed. This result suggested there were microbial contributions to insoluble Tc formation. In the investigated samples, fungi and bacteria were dominant species among microorganisms. The inhibition of fungal growth resulted in little change in the insoluble Tc amount. In contrast, the inhibition of bacterial growth led to obvious decreases of insoluble Tc amount. Sulfate-reducing bacteria are known to electrochemically reduce and remove Tc from solution. In the surface water samples, the insoluble Tc formation by bacteria other than sulfate-reducing bacteria was also demonstrated, because inhibition of sulfate-reducing bacteria did not significantly influence the formation of insoluble Tc. It was concluded from these results that the indigenous bacterial community in surface water covering paddy fields would affect Tc behavior in this agricultural environment through insoluble Tc formation.