The coprecipitation of Pu and other radionuclides with CaCO 3

Abstract

The record of fallout plutonium concentrations in annual bands of corals is strikingly similar to the record of atmospheric deposition of 90Sr. This similarity implies that corals may incorporate Pu from seawater with a constant partition coefficient (constant discrimination). To investigate physicochemical aspects of Pu incorporation, the following have been coprecipitated with CaCO 3 (calcite and aragonite): oxidized and reduced Pu; americium, thorium and uranium as analogs to Pu oxidation states (III, IV, VI), respectively; and 210Pb as a particle-reactive nuclide which may be incorporated by corals with constant discrimination. Americium, thorium and lead adsorb onto both calcite and aragonite, with more than 99% of the recovered activity found associated with the solids. Uranium exhibits a behavior consistent with lattice substitution. Partition coefficients for U in aragonite range from 1.8 to 9.8 and vary inversely with pH and/or rate of precipitation. The partition coefficient for U in calcite is less than 0.2 and may be as low as 0.046. Reduced Pu sorbs with 3 to 4% remaining in solution. Oxidized Pu may both sorb and coprecipitate. The coral record for Pb and U results primarily from biological, rather than physicochemical, effects; it is likely that the Pu coral record also reflects biological discrimination.