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Microbial Oxidation and Solubilization of Precipitated Elemental Selenium in Soil

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside.

^* Corresponding author (william.frankenberger{at}vcr.edu).

ABSTRACT

Oxidation of elemental selenium (Se⁰) leads to increased solubilizalion and enhanced bioavailability. In this work, laboratory microcosm experiments were conducted to study oxidation of Se⁰ 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 Se⁰ was prepared by both chemical and biological synthesis, and its presence was confirmed by synchrotron-based x-ray absorption spectroscopy. The amount of Se⁰ oxidized over 125 d was from 1 to 10% of Se⁰ in soils spiked to 250 mg Se⁰ kg^–1 and approximately half that in soils spiked to 100 mg Se⁰ kg^–1. First order rate constants for oxidation of Se⁰ were from 0.05 to 0.32 yr^–1 and 0.04 to 0.39 yr^–1 at 250 and 100 mg Se⁰ kg^–1 soil, respectively. The amount of Se⁰ oxidized was generally correlated with prior exposure of the soil lo Se. Products included either selenite (SeO^2–₃), or both SeO^2–₃ and selenate (SeO^2–₄). 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 Se⁰ oxidation in soils is largely biotic in nature, occurs at relatively slow rates and yields both SeO^2–₃ and SeO^2–₄.

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