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

Abstract:

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.