Sequestration of Selenite and Selenate in Gypsum (CaSO4·2H2O): Insights from the Single-Crystal Electron Paramagnetic Resonance Spectroscopy and Synchrotron X-ray Absorption Spectroscopy Study

Abstract

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 Na₂SeO₄ solution at pH 7.5 and 0.02 M Na₂SeO₃ 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 Se⁴⁺- and Se⁶⁺-doped gypsum, respectively. The single-crystal EPR spectra of Se⁴⁺- and Se⁶⁺-doped gypsum after gamma-ray irradiation reveal five selenium-centered oxyradicals: SeO₂^-(I), SeO₂^-(II), SeO₂^-(III), SeO₃^-, and HSeO₄^2-. The former three radicals provide unequivocal evidence for the substitution of their paramagnetic precursor SeO₃^2- for SO₄^2- in the gypsum structure, while the latter two confirm the replacement of SeO₄^2- for SO₄^2-. 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.