Precipitation kinetics of uranium by sedimentary organic matter under diagenetic and hydrothermal conditions

The reduction kinetics of uranyl cations to uraninite in aqueous solution b y two lignites with different maturities were experimentally determined und er diagenetic or hydrothermal conditions (180 degrees-200 degrees C). Both the uranyl reduction and lignite dehydrogenation reactions observed confirm previous mechanistic hypotheses (Nakashima et al., 1984, 1987; Nakashima, 1992a, b). Reduction is a first-order reaction with respect to uranyl catio n and organic substrate. For both of the reactions studied, reduction and d ehydrogenation, the activation energies measured in experiments using two d ifferent lignites were within experimental error. These values are 115 +/- 15 kJ/mol for uranyl reduction, and 59 +/- 13 kJ/mol for lignite dehydrogen ation. Estimates of the half-lives of U in aqueous solutions have been comp leted based on the kinetic parameters by assuming that the rate-determining step is uranyl reduction to uraninite. The results suggest a rough time-sc ale range of U deposition rates with large uncertainties on the pre-exponen tial factor. The half-life of U precipitation is estimated to be on the ord er of 3 h to 1 yr under modest thermal conditions (200 degrees-100 degrees C), 340 yr for radioactive waste repositories (50 degrees C), and 10(4) to 10(5) yr at the Earth's surface (25 degrees-4 degrees C).