Redox reactions and transport of selenium through fractured granite

Selenium (VI) migration through drill-core columns of granite was investigated as a function of water velocities under oxic and anoxic conditions. The measurement of Se(VI) recovery at the column outlet showed that a loss due to adsorption occurred. Comparison with the breakthrough curves of conservative tracers indicated that nonequilibrium of the sorption reaction did not contribute to the observed dependence of Se(VI) retention from the water velocity. A kinetically controlled redox transformation of Se(VI) to more strongly adsorbed Se species was postulated. Adsorption studies of selenate with pure minerals representative of granite weathering products indicated that iron sulfide inclusions can be important adsorbents for Se(VI). Synchrotron-based X-ray absorption spectroscopy measurements demonstrated that partial reduction of Se(VI) to Se(II) and Se(IV) occurred at the mineral surface.