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Crystallography is used for studying the synthetic gypsum coprecipitated with arsenic at different concentrations and pH values and neutron and X-ray diffraction are used for analyzing the solid phases. The charge redistribution within the structure ion substitutions of either the protonated or the unprotonated arsenate species has shown an increase in the covalency in the interaction between [Ca.sup.2+] and As[O.sub.4.sup.3-], whereas the interaction between [Ca.sup.2+] and HAs[O.sub.4.sup.2-] has remained predominantly ionic.

Recent studies demonstrated that synthetic calcite may host considerable amounts of arsenic (As). In this paper, the concentration of As in natural calcite was determined using two novel, specifically designed, sequential extraction procedures. In addition, the oxidation state of As and its distribution between calcite and coexisting Fe-oxyhydroxides was unravelled by μXRF elemental mapping and As K-edge μXAS spectroscopy. Our results conclusively demonstrate that arsenic can be found in natural calcite up to 2 orders of magnitude over the normal crustal As abundances. Because of the large diffusion of calcite in the environment, this phase may exert an important control on As geochemistry, mobility, and bioavailability. [ABSTRACT FROM AUTHOR]

The reductive immobilization of Se(IV) by micrometer-sized (100-200 μm) calcite containing sorbed or coprecipitated Fe(II) was investigated at pH 7 under anoxic conditions (O2 < 1 ppmv) using X-ray absorption near-edge structure (XANES) spectroscopy. The Se(IV) sorption on calcite increased in the presence of sorbed Fe(II) compared to that of Fe-free pure calcite. XANES spectra of Se K-edge shows that nearly half of the total sorbed Se(IV) is reduced to Se(0) by Fe(II) sorbed on calcite within 24 h. The extent of reduction decreases with increasing equilibration time of calcite with Fe(II) solution before Se(IV) addition. The combined results of field emission scanning electron microscopy and X-ray diffraction have shown that needle-shaped red monoclinic elemental Se with diameters of 30-50 nm and lengths of up to 100 nm is precipitated on the calcite surface. Fe(II) coprecipitated calcite does not contribute to Se(IV) reduction within 72 h. Therefore, the reduction capacity of Fe(II) linked to calcite critically depends on its location (either on the surface or in the bulk solid), and less extensively on the pre-equilibration time of calcite with Fe(II) solution. Such understanding is important to predict the transport, transformation, and attenuation of Se in subsurface and in nuclear waste repositories. [ABSTRACT FROM AUTHOR]