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

我们的研究小组一直在开发一种考虑表面非均质性的吸附模型，从而建立了放射性核素在矿物表面吸附的模型。该模型唯一不能处理的现象是涉及氧化还原反应的吸附反应。由于表面氧化还原反应的动力学取决于吸附结构、电子转移途径等，因此了解这些反应机理是建模的必要条件。本研究的目的是揭示锕系元素在固/水界面氧化还原反应的途径和机理。首先，我们开发了一种激光光谱系统，使我们能够跟踪U(VI)-Fe（II）氧化还原对的时间演变。结果表明：α-Al(OH)_3是铝的氢氧化物，在三水铝石表面，Fe（II）将U（IV）还原为U（IV）。同样的激光光谱系统还用于研究共存配体分子对氧化还原反应的影响，发现自然界中发现的一些螯合配体，如柠檬酸，在体溶液和矿物表面上都促进了Fe（II）对U（VI）的还原。我们进一步从电子转移途径（内球和外球电子转移）的角度研究了配体效应，并应用密度泛函理论来确定吸附结构和阐明吸附物质的还原。

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.