Americium interaction with calcite and aragonite surfaces in seawater

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Observations of the distribution of 241Am in the marine environment indicate that Am has a high affinity for solid surfaces. The adsorption of Am onto calcite and aragonite surfaces from seawater and related solutions has been studied, in order to establish the interaction of Am with a major component of many marine sediments. Results indicate that Am is rapidly and strongly adsorbed. This occurs even when both dissolved Am concentrations and solid to solution ratios are low. The minimum value for K D determined is 2 × 10 5. Measurements of reaction kinetics established that Am is adsorbed from seawater at 40 times the rate per unit surface area on synthetic aragonite that it is on synthetic calcite. Approximately 15% of the difference is attributable to epitaxial influences, with the remainder being due to enhanced site competition by Mg on calcite relative to aragonite. The adsorption rate is first order with respect to Am concentration, but follows approximately the square root of the solid surface area to solution volume ratio. Adsorption rate of Am on biogenic aragonite and Mg-calcites are, within a given particle size range, close to equal. It is not possible to normalize these adsorption rates to surface area due to the differing microporous structure of biogenic carbonates. The Am adsorption rates on a shallow water calcium carbonate-rich sediment gave results which were predicted from, its mineralogie mixture of components.