Reactivities of Fe(II) on Calcite: Selenium Reduction

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.