Abiotic formation of elemental selenium and role of iron oxide surfaces.

The possible abiotic reduction of selenite to form elemental Se was studied under controlled conditions in the presence of ferrous iron. The reduction of selenite and formation of Se (0) was found to be a surface-mediated reaction by iron oxides. Without the presence of the reactive surface of freshly precipitated iron oxides, the reduction reaction could not be detected, even under a well controlled low oxygen environment (30 ppmv). Our results clearly illustrate the crucial role of iron oxide in this redox process. In lake sediments, the complex sediment matrix seems to strongly adsorb SeO(2-) and make it less available for reduction into Se(0). When lake sediments were submitted to UV irradiation to eliminate bacteria, the percentage of iron oxides increased and it was found that higher levels of iron oxides, generated by the UV treatment, correlated with the higher formation of Se(0). Moreover, the results of our field studies on two distinctively different lake sediments also showed a strong influence of iron oxides on the formation of elemental Se, which agrees well with our laboratory simulations.