Microbial oxidation and solubilization of precipitated elemental selenium in soil

Oxidation of elemental selenium (Se-0) leads to increased solubilization and enhanced bioavailability. In this work, laboratory microcosm experiments were conducted to study oxidation of Se-0 in soil and liquid cultures. Major objectives were to examine the oxidation rates of four San Joaquin Valley, California soils, and to assess the contribution of biological vs, chemical processes. For these experiments, red, crystalline Se-0 was prepared by both chemical and biological synthesis, and its presence was confirmed by synchrotron-based x-ray absorption spectroscopy, The amount of Se-0 oxidized over 125 d was from 1 to 10% of Se-0 in soils spiked to 250 mg Se-0 kg(-1) and approximately half that in soils spiked to 100 mg S(e)0 kg(-1). First order rate constants fur oxidation of Se-0 were from 0.05 to 0.32 yr(-1) and 0.04 to 0.39 yr(-1) at 250 and 100 mg Se-0 kg(-1) soil, respectively. The amount of Se-0 oxidized was generally correlated with prior exposure of the soil to Se. Products included either selenite (SeO32-), or both SeO32- and selenate (SeO42-). Biotic processes were shown to be of major importance. Both heterotrophic and autotrophic oxidation were observed, and an inorganic C source (NaHCO3) was favored relative to glucose. This study demonstrates that Se-0 oxidation in soils is largely biotic in nature, occurs at relatively slow rates and yields both SeO32- and SeO42-.