Abstract
As determined by transmission electron microscopy (TEM), the reduction of selenate and selenite by Desulfovibrio desulfuricans, a sulfate-reducing bacterium, produces spherical (Se, S) sub-micro particles outside the cell. The particles are crystalline or amorphous, depending on medium composition. Amorphous-like Se-rich spherical particles may also occur inside the bacterial cells. The bacteria are more active in the reduction of selenite than selenate. The Desulfovibrio desulfuricans bacterium is able to extract S in the (S, Se) solid solution particles and transform S-rich particles into Se-rich and Se crystals. Photoautotrophs, such as Chromatium spp., are able to oxidize sulfide (S2-). When the bacteria grow in sulfide- and selenide-bearing environments, they produce amorphous-like (S, Se) globules inside the cells. TEM results show that compositional zonation in the (S, Se) globules occur in Chromatium spp. collected from a top sediment layer of a Se-contaminated pond. S2- may be from the products of sulfate-reducing bacteria. Both the sulfate-reducing bacteria and photosynthetic Chromatium metabolize S preferentially over Se. It is proposed that the S-rich zones are formed during photosynthesis (day) period, and the Se-rich zones are formed during respiration active (night) period. The results indicate that both Desulfovibrio desulfuricans and Chromatium spp. are able to immobilize the oxidized selenium (selenate and/or selenite) in the forms of elemental selenium and (Se, S) solid solutions. The bacteria reduce S in the (Se, S) particles and further enrich Se in the crystalline particles. The reduced S combines with Fe2+ to form amorphous FeS.
Highlights
Selenium (79Se) is a fission product of nuclear fuel with a half-life time of 65,000 years
transmission electron microscopy (TEM) results show that compositional zonation in the (S, Se) globules occur in Chromatium spp. collected from a top sediment layer of a Se-contaminated pond
The results indicate that both Desulfovibrio desulfuricans and Chromatium spp. are able to immobilize the oxidized selenium in the forms of elemental selenium and (Se, S) solid solutions
Summary
Selenium (79Se) is a fission product of nuclear fuel with a half-life time of 65,000 years. Oxidative alteration of spent nuclear fuel will result in the formation of selenate ( SeO42 ) and selenite ( SeO32 ) oxy-anions together with uranyl oxy-cation in solution. Most reports about selenate or selenite reduction by bacteria only focus on solution changes, and do not on elemental selenium as the reaction product [2,5]. The elemental Se produced by Desulfovibrio desulfuricans was reported by Tomei et al [4], and their results indicate that reduction of selenate and selenite can occur under conditions where metabolism has no influence on electron acceptor activity by the culture [4]. The specimens for transmission electron microscopy study were prepared by depositing a drop of culture that contained both bacteria and reduced selenium on holey carbon-coated TEM Cu grids. Point-to-point resolution of the HRTEM is 0.19 nm, and accelerating voltage is 200 keV
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