Arsenic speciation in synthetic gypsum (CaSO4·2H2O): A synchrotron XAS, single-crystal EPR, and pulsed ENDOR study

Abstract

Gypsum (CaSO4·2H2O) is a major by-product of mining and milling processes of borate, phosphate and uranium deposits worldwide and, therefore, potentially plays an important role in the stability and bioavailability of heavy metalloids, including As, in tailings and surrounding areas. Gypsum containing 1900 and 185ppm As, synthesized with Na2HAsO4·7H2O and NaAsO2 in the starting materials, respectively, have been investigated by synchrotron X-ray absorption spectroscopy (XAS), single-crystal electron paramagnetic resonance spectroscopy (EPR), and pulsed electron nuclear double resonance spectroscopy (ENDOR). Quantitative analyses of As K edge XANES and EXAFS spectra show that arsenic occurs in both +3 and +5 oxidation states and the As3+/As5+ value varies from 0.35 to 0.79. Single-crystal EPR spectra of gamma-ray-irradiated gypsum reveal two types of arsenic-associated oxyradicals: [AsO3]2− and an [AsO2]2−. The [AsO3]2− center is characterized by principal 75As hyperfine coupling constants of A1=1952.0(2) MHz, A2=1492.6(2) MHz and A3=1488.7(2) MHz, with the unique A axis along the S–O1 bond direction, and contains complex 1H superhyperfine structures that have been determined by pulsed ENDOR. These results suggest that the [AsO3]2− center formed from electron trapping on the central As5+ ion of a substitutional (AsO4)3− group after removal of an O1 atom. The [AsO2]2− center is characterized by its unique A(75As) axis approximately perpendicular to the O1–S–O2 plane and the A2 axis along the S–O2 bond direction, consistent with electron trapping on the central As3+ ion of a substitutional (AsO3)3− group after removal of an O2 atom. These results confirm lattice-bound As5+ and As3+ in gypsum and point to potential application of this mineral for immobilization and removal of arsenic pollution.