| Summary: | Widespread arsenic contamination with adverse effects to human health is a global problem. Most previous studies on arsenic contamination in natural environments and those associated with mining and agricultural activities focused largely on arsenic-rich minerals such as arsenates, arsenites, sulfarsenides, and sulfides. Rock-forming minerals generally contain only minor or trace amounts of arsenic but, owing to their sheer abundances, are potentially important (and sometimes dominant) sources of this metalloid and can play significant roles in the attenuation and sequestration of arsenic in various environments. However, there remains a significant gap in my knowledge about the uptake and speciation of arsenic in rock-forming minerals. This thesis research is intended to bridge this gap by investigating the uptake and speciation of arsenic in selected rock-forming sulfate and phosphate minerals (i.e., gypsum, struvite and newberyite).
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 1,900 and 185 ppm As, synthesized with Na2HAsO4•7H2O and NaAsO2 in the starting materials, respectively, has 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 X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (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. EPR spectra show that another sulfate boussingaultite is also sequestering both As5+ and As3+ at its S site. Synthesis experiments at pH from 2 to 14 also show that arsenic uptake in gypsum is pH dependent.
Struvite and newberyite, common biominerals and increasingly important green fertilizers recovered from wastewater treatment plants, are capable of sequestering a wide range of heavy metals and metalloids, including arsenic. Inductively coupled plasma mass spectrometric (ICPMS) analyses show that struvite formed under ambient conditions contains up to 547±15 ppm As and that the uptake of As is controlled by pH. Synchrotron As K-edge XANES spectra measured at 20 K show that As5+ is the predominant oxidation state in struvite, irrespective of Na2HAsO4•7H2O or NaAsO2 as the source for As. Modeling of As K-edge EXAFS data suggest that local structural distortion associated with the substitution of As5+ for P5+ in struvite reaches up to 3.75 Å. Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated struvite disclose five [AsO3]2- radicals and one [AsO4]2- radical. These arsenic-centered oxyradicals are all readily attributed to form from diamagnetic [AsO4]3- precursors during irradiation, providing further support for exclusive incorporation and local structural expansion beyond the first shell of As5+ at the P site in struvite.
Arsenic doped newberyite (MgHPO4•3H2O) obtained from the gel diffusion method has investigated by synchrotron XAS at the As K-edge (11,867 eV) at 8 K and single-crystal EPR spectroscopy at room temperature. XANES data show that As5+ is dominant and EXAFS analysis reveals a local environment typical of the arsenate species as well. Single-crystal EPR spectra of gamma-ray-irradiated newberyite contain two arsenic-associated oxyradicals: [AsO3]2− and [AsO2]2− derived from As5+ and As3+, respectively, at the P site in the newberyite structure. Elevated concentrations of arsenic have also been observed in natural newberyite from guano deposits and reflect the accumulation of this metalloid in the food chain. Therefore, struvite and newberyite can both sequester significant amounts of arsenic, and their direct use as fertilizers (irrespective of origins from guano deposits or wastewaters) is a potential source of arsenic contamination. On the other hand, the capacities of struvite and newberyite for accommodating significant amounts of arsenic in crystal lattices coupled with their simple chemistry and crystallization under ambient conditions make them attractive materials for immobilization and removal of arsenic contamination in aqueous environments.
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