The Bioreduction of Selenite under Anaerobic and Alkaline Conditions Analogous to Those Expected for a Deep Geological Repository System

The environmental conditions for the planned geological disposal of radioactive waste —including hyper-alkaline pH, radiation or anoxia—are expected to be extremely harsh for microbial activity. However, it is thought that microbial communities will develop in these repositories, and this would have implications for geodisposal integrity and the control of radionuclide migration through the surrounding environment. Nuclear waste contains radioactive isotopes of selenium (Se) such as (79)Se, which has been identified as one of the main radionuclides in a geodisposal system. Here, we use the bacterial species Stenotrophomonas bentonitica, isolated from bentonites serving as an artificial barrier reference material in repositories, to study the reduction of selenite (Se(IV)) under simulated geodisposal conditions. This bacterium is able to reduce toxic Se(IV) anaerobically from a neutral to alkaline initial pH (up to pH 10), thereby producing elemental selenium (Se(0)) nanospheres and nanowires. A transformation process from amorphous Se (a-Se) nanospheres to trigonal Se (t-Se) nanowires, through the formation of monoclinic Se (m-Se) aggregates as an intermediate step, is proposed. The lesser solubility of Se(0) and t-Se makes S. bentonitica a potential candidate to positively influence the security of a geodisposal system, most probably with lower efficiency rates than those obtained aerobically.