Wander Matthew C F, Kerisit Sebastien, Rosso Kevin M, Schoonen Martin A A
Department of Geosciences and Center for Environmental and Molecular Science, Stony Brook University, Stony Brook, NY 11794-2100, USA.
J Phys Chem A. 2006 Aug 10;110(31):9691-701. doi: 10.1021/jp062325t.
Uranium is a pollutant whose mobility is strongly dependent on its oxidation state. While U(VI) in the form of the uranyl cation is readily reduced by a range of natural reductants, by contrast complexation of uranyl by carbonate greatly reduces its reduction potential and imposes increased electron transfer (ET) distances. Very little is known about the elementary processes involved in uranium reduction from U(VI) to U(V) to U(IV) in general. In this study, we examine the theoretical kinetics of ET from ferrous iron to triscarbonato uranyl in aqueous solution. A combination of molecular dynamics (MD) simulations and density functional theory (DFT) electronic structure calculations is employed to compute the parameters that enter into Marcus' ET model, including the thermodynamic driving forces, reorganization energies, and electronic coupling matrix elements. MD simulations predict that two ferrous iron atoms will bind in an inner-sphere fashion to the three-membered carbonate ring of triscarbonato uranyl, forming the charge-neutral ternary Fe(2)UO(2)(CO(3))(3)(H(2)O)(8) complex. Through a sequential proton-coupled electron-transfer mechanism (PCET), the first ET step converting U(VI) to U(V) is predicted by DFT to occur with an electronic barrier that corresponds to a rate on the order of approximately 1 s(-1). The second ET step converting U(V) to U(IV) is predicted to be significantly endergonic. Therefore, U(V) is a stabilized end product in this ET system, in agreement with experiment.
铀是一种污染物，其迁移性在很大程度上取决于其氧化态。虽然铀酰阳离子形式的U(VI)很容易被一系列天然还原剂还原，但相比之下，碳酸根与铀酰的络合大大降低了其还原电位，并增加了电子转移(ET)距离。一般来说，对于从U(VI)还原为U(V)再还原为U(IV)所涉及的基本过程知之甚少。在本研究中，我们研究了水溶液中从亚铁离子到三碳酸铀酰的电子转移的理论动力学。采用分子动力学(MD)模拟和密度泛函理论(DFT)电子结构计算相结合的方法，计算了Marcus电子转移模型中的参数，包括热力学驱动力、重组能和电子耦合矩阵元。MD模拟预测，两个亚铁离子原子将以内球方式与三碳酸铀酰的三元碳酸环结合，形成电荷中性的三元Fe(2)UO(2)(CO(3))(3)(H(2)O)(8)络合物。通过顺序质子耦合电子转移机制(PCET)，DFT预测将U(VI)转化为U(V)的第一步电子转移发生时的电子势垒对应的速率约为1 s(-1)。将U(V)转化为U(IV)的第二步电子转移预计是显著吸热的。因此，U(V)是该电子转移系统中稳定的终产物，这与实验结果一致。
