“…In the latter case, the shuttling of electrons between reducing agents and adsorbed UO 2 2+ is facilitated by the highly conductive surface of magnetite (Becker et al, 2001;Missana et al, 2003;Rosso and Becker, 2003;Renock and Becker, 2010;Gorski et al, 2012;Latta et al, 2012;Singer et al, 2012a;Singer et al, 2012b). The two-electron reduction of UO 2 2+ to U 4+ is more energetically favored than the one-electron reduction of UO 2 2+ to UO 2 + based on standard reduction potentials (i.e., E 0 U(VI)/U(IV) = 0.070 V, E 0 U(VI)/U(V) = −0.135 V, all potentials in this manuscript are with respect to the Ag/AgCl, saturated KCl,0.197 V vs. NHE) (Morris, 2002;Konings et al, 2006). However, direct reduction from UO 2 2+ to U 4+ is kinetically hindered due to the significant change between the structures of the reactant and the product, where the UO 2 2+ exists as the stable linear uranyl molecule with the short U=O bond (Burns, 1999).…”