Mechanisms for uranyl reduction by ferrous iron in solution and at the oxide mineral-water interface

The reduction of U(VI)aq to the insoluble U(IV) oxidation state is a relevant process for retarding U transport in the subsurface. However, experimental results have been inconclusive of whether U(VI)aq is reduced by Fe(II)aq in anoxic, homogeneous environments. Experimental and computational approaches were used here to understand mechanisms for uranyl removal from solution, focusing on whether Fe(II)aq reduces U(VI)aq via homogeneous or heterogeneous pathways. In an initially homogeneous system with 1 mM Fe(II)aq and 0.16 mM U(VI)aq, U(VI)aq concentrations dropped to 10-5 M in the first hour of reaction due to (meta)schoepite precipitation. Similarly, at a lower U(VI)aq concentration (0.02 mM), U(VI) precipitation occurred but more slowly. XRD and XPS analyses of the solids confirmed partially reduced (meta)schoepite phases, where 25-30% of U was reduced. Thus U(VI)aq was removed from solution by precipitation first, enabling partial reduction of the solid U phase via heterogeneous pathways. Ab initio methods and Marcus Theory were used to calculate the electron-transfer (ET) rate of U(VI)aq reduction to U(V)aq by Fe(II)aq. When U(VI)aq and Fe(II)aq were modelled as outer-sphere (OS) complexes, the reaction occurred as a proton-coupled ET reaction. Modelling ET to occur before proton-transfer (PT), the redox products were thermodynamically unfavorable (+102 kJ/mol) and ET was the rate-limiting step (10-12 s-1). If ET and PT occurred concurrently, the redox products were energetically favorable (-19 to -35 kJ/mol) though the reaction was still kinetically inhibited; the rate is effectively 0 s-1. In contrast, ET for an inner-sphere (IS) complex was thermodynamically favorable (-16 kJ/mol) and significantly faster (104 to 108 s-1). Significant thermodynamic and kinetic barriers were associated with the OS-complex becoming an IS-complex, such as dehydration of the first solvation shell (+96 kJ/mol) and hydrolysis of Fe(II), preventing IS-complex formation. These results substantiate studies where the reduction of U(VI)aq by Fe(II)aq occurs via heterogeneous pathways. Further studies on this system involving the impact of Al- and Fe-oxide minerals are underway to explore heterogeneous redox pathways and the effects of semiconducting and insulating minerals on redox rates.