Mechanisms of Solid-Water Interfacial Reactions Involving Redox Reactions Studied by Laser Spectroscopy and Quantum Chemical Calculation

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Our research group has been developing a sorption models that consider heterogeneity of surfaces, leading to modeling of the sorption of radionuclides on mineral surfaces. The only phenomenon that the model cannot deal with is sorption reactions involving redox reactions. Since the kinetics of surface redox reactions depend on sorption structures, the routes of electron transfer, etc., understanding such reaction mechanisms is mandatory for the modeling. The purpose of this research is to reveal the routes and the mechanisms of redox reaction of actinide elements at solid/water interaces.At first, we have developed a laser spectroscopy system that enabled us to follow the temporal evolution of the U(VI)-Fe(II) redox couple. It was turned out that U(IV) was reduced to U(IV) by Fe(II) on the surface of gibbsite, a hydroxide mineral of aluminum, α-Al(OH)_3. The same laser spectroscopy system was also used to investigate the effects of coexisting ligand molecules on the redox reaction and revealed that some chelating ligands found in nature such as citric acid facilitated the reduction of U(VI) by Fe(II) both in bulk solution and on mineral surfaces. We further looked into the ligand effects from the point of electron transfer routes (innersphere vs outersphere electron transfer) and applied the density functional theory for the determination of adsorption structures as well as elucidation of the reduction of adsorbed species.