Sequestration of Selenite and Selenate in Gypsum (CaSO·2HO): Insights from the Single-Crystal Electron Paramagnetic Resonance Spectroscopy and Synchrotron X-ray Absorption Spectroscopy Study.

Gypsum is the most common sulfate mineral on Earth's surface and is the dominant solid byproduct in a wide variety of mining and industrial processes, thus representing a major source for heavy metal(loid) contamination, including selenium. Gypsum crystals grown from the gel diffusion technique in 0.02 M NaSeO solution at pH 7.5 and 0.02 M NaSeO solutions at pH 7.5 and 9.0 contain 828, 5198, and 5955 ppm Se, respectively. Synchrotron Se K-edge X-ray absorption spectroscopic analyses show that selenite and selenate are the dominant species in Se- and Se-doped gypsum, respectively. The single-crystal EPR spectra of Se- and Se-doped gypsum after gamma-ray irradiation reveal five selenium-centered oxyradicals: SeO(I), SeO(II), SeO(III), SeO, and HSeO. The former three radicals provide unequivocal evidence for the substitution of their paramagnetic precursor SeO for SO in the gypsum structure, while the latter two confirm the replacement of SeO for SO. These results demonstrate that gypsum has a significant capacity for sequestrating both selenite and selenate in the structure but has a marked preference for the former, thus confirming important controls on the mobility and bioavailability of selenium oxyanions and pointing to optimal applications of gypsum for remediating selenium contamination under neutral to alkaline conditions.