Synthesis and incorporation of selenide in pyrite and mackinawite

According to performance assessment reports for high-level radioactive waste repositories, 79Se is a radionuclide that could represent a potential threat to the biosphere. Therefore knowledge of its geochemical behavior during its transport in aqueous solutions is fundamental to assess the safety of repository sites. Selenium is often associated with sulfides such as pyrite, a frequent minor constituent of host rocks and bentonite backfills considered for radioactive waste disposal. In this study, we investigated the uptake of Se by Fe sulfides synthesized in batch experiments under anoxic conditions. Results from X-ray diffraction and scanning electron microscopy analyses reveal the formation of pyrite and mackinawite. More than 98% of the Se added to solutions (initial concentration: 10-3-10-6 mol/l) was taken up by the Fe sulfide minerals. Focused ion beam analysis shows an inhomogeneous Se distribution with a higher accumulation in the center of the pyrite grains, probably due to the progressive depletion of Se from solution with regard to S. The results imply that pyrite and its most important precursor phase, mackinawite, are efficient in removing selenium from solution, which may contribute in reducing the mobility of 79Se released from radioactive waste.