Sequestration of Selenite and Selenate in Gypsum (CaSO4•2H2O): Insights from Single-Crystal EPR and Synchrotron XAS 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 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. 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 HSeO42-. The former three radicals provide unequivocal evidence for the substitution of their paramagnetic precursor SeO32- for SO42- in the gypsum structure, while the latter two confirm the replacement of SeO42- for SO42-. These results demonstrate tha...