Actinyl Adsorption and Reduction on Pyrite Surfaces: Insights from DFT Calculations

Abstract

Interactions of actinides with pyrite surfaces are highly important in catalyzing their reductive immobilization, thereby controlling the movement of these species in the environment. Here, surface adsorption and subsequent reduction of aqueous actinyl­(VI) on pyrite surfaces were explored using density functional hybrid theory (DFT-B3LYP) combined with a first hydration sphere of water molecules and a dielectric continuum for solvation effects. Adsorption of cationic (AnO2(H2O)5)2+(An = U, Np, Pu) and neutral AnO2(OH)2(H2O)3 actinyl onto a small pyrite cluster (Fe4S8) and the effect of coadsorption on the energetics and electron transfer are evaluated by adding either hydroquinone, H2Q (reduced), or quinone, Q (oxidized). The pyrite surface instantaneously transfers an electron to the adsorbed cationic actinyl. Unpaired electron atomic spin densities confirm the electron transfer from the pyrite surface to An atoms. For the neutral actinyl adsorption, electron transfer is confirmed for neptunyl and plutonyl but not for uranyl. Several factors control the overall adsorption energetics and kinetics, such as the nature of the coadsorbate (H2Q/Q), pyrite surface, actinyl, and charge or protonation state (cationic or neutral). The surface-mediated reduction of adsorbed actinyl occurs by receiving electrons either directly from the sulfide or from the coadsorbed H2Q through the sulfide. In the direct reduction case, an H+ ion is added to the surface-bound cationic actinyl, and the mineral surface acts as an electron donor. In contrast, in the proton-coupled electron transfer (PCET) reduction, the surface mediates the electrons through the surface by synergistically aligning relevant orbitals in line. This results in the less soluble and stable An­(IV). Our results indicate that the pyrite surface promotes a faster PCET reaction for the actinyl reduction under circumneutral (pH 4–7) conditions