Mechanisms of Long-Term U Transport under Oxidizing Conditions

摘要

Understandings of U transport at the Earth's surface are quite important especially for the disposal of radioactive waste and the remediation of contaminated ground water. We examined long-term U transport in and around the Koongarra U deposit, Australia, which has been subjected to weathering for the past 2 million years. The formation of saleeite [Mg(UO_2)_2(PO_4)_2·10H_2O] occurred at the rim of apatite [Ca_5(PO_4)_3(F,OH)], where the ground waters had about 200 - 400 ppb U (～ 10~(-7) mol~(-1)) and were undersaturated with respect to saleeite. The laboratory experiments using apatite and U-containing solutions and Rutherford backscattering spectrometry of the apatite surface have suggested that a leached layer of about 100 nm thick is formed on dissolving apatite, saleeite is precipitated only in the leached layer by local saturation, and saleeite precipitated is added to the interface between the leached layer and solution. At lower U concentrations (～ 10~(-8) mol·l~(-1)), U occurred as nanocrystals (20-100 nm in size) of uranyl phosphates such as metatorbernite [Cu(UO_2)_2(PO_4)·8H_2O], scattered between and attached firmly to nanocrystals (2-50 nm in size) of goethite and hematite. A possible mechanism is that U, P, and Mg or Cu adsorbed onto ferrihydrite form nanocrystals of uranyl phosphates during crystallization of goethite and hematite from ferrihydrite. The above two mechanisms significantly lower the U concentration in the ground water that flows into the Koongarra creek, 200 m downstream from the ore deposit, where the U concentration is ～ 10~(-10) mol·l~(-1). We conclude that the two mechanisms of uranyl-phosphate mineralization control the long-term U transport at Koongarra.