Uptake of uranium and trace elements in pyrite (FeS2) suspensions

Pyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5. 5. Iron(II) uptake can explain the non-stoichiometric [S](aq)/[Fe](aq) ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm-2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1. 25 * 10-6 mol g-1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4. 5 * 10-8 mol g-1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s). Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyper-stoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U-(VI). (authors)