Electron Paramagnetic Resonance and Synchrotron X-rayAbsorption Spectroscopy for Highly Sensitive Characterization ofCalcium Arsenates

Calcium arsenates such as pharmacolite (CaHAsO4middot2H2O), haidingerite (CaHAsO4middotH2O), and weilite (CaHAsO4) areimportant sinks for arsenic in mine tailings as well as other naturaland contaminated sites and are useful for reducing the mobility andbioavailability of this toxic metalloid in the environment. However,calcium arsenates usually occur in trace amounts dominated byother phases, making their detection, identification, and quantifi-cation challenging. In this contribution, pharmacolite, haidingerite,and weilite are shown to exhibit subtle but distinct postedgedifferences in As K-edge X-ray absorption near-edge structure(XANES) spectra and feature characteristic [AsO3]2-, [AsO4]2-,and [AsO4]4-radicals, all derived from the diamagnetic [HAsO4]2-precursor during gamma-ray irradiation, in electron paramagneticresonance (EPR) spectra. In particular, the75As (nuclear spinI= 3/2 and natural isotope abundance = 100%) hyperfine couplingconstants of the [AsO3]2-radicals in pharmacolite and haidingerite as well as other minerals (e.g., calcite and gypsum) are clearlydistinct, allowing the unambiguous identification of calcium arsenates by the EPR technique readily at similar to 0.1 wt %. Similarly, linearcombinationfittings of As K-edge XANES spectra demonstrate that pharmacolite and haidingerite at similar to 0.1 wt % each in gypsum-richmixtures can be detected and quantified as well. Therefore, a combination of the EPR and XANES techniques is a powerful approachfor the highly sensitive characterization of calcium arsenates in the quest for the safe management and remediation of arseniccontamination. This work demonstrates the highly sensitive characterization of calcium arsenates by integrated electronparamagnetic resonance and synchrotron X-ray absorption spectroscopy.