Solid solution partitioning of Sr 2+, Ba 2+, and Cd 2+ to calcite

Abstract

Although solid solutions play important roles in controlling the concentrations of minor metal ions in natural waters, uncertainties regarding their compositions, thermodynamics, and kinetics usually prevent them from being considered. A range of precipitation rates was used here to study the nonequilibrium and equilibrium partitioning behaviors of Sr 2+, Ba 2+, and Cd 2+ to calcite (CaCO 3(s)). The distribution coefficient of a divalent metal ion Me 2+ for partitioning from an aqueous solution into calcite is given by D Me = ( X MeCO 3(s) /[Me 2+])/( X CaCO 3(s) /[Ca 2+]). The X values are solid-phase mole fractions; the bracketed values are the aqueous molal concentrations. In agreement with prior work, at intermediate to high precipitation rates R (nmol/mg-min), D Sr, D Ba, and D Cd were found to depend strongly on R. At low R, the values of D Sr, D Ba, and D Cd became constant with R. At 25 °C, the equilibrium values for D Sr, D Ba, and D Cd for dilute solid solutions were estimated to be 0.021 ± 0.003, 0.012 ± 0.005, and 1240 ± 300, respectively. Calculations using these values were made to illustrate the likely importance of partitioning of these ions to calcite in groundwater systems. Due to its large equilibrium D Me value, movement of Cd 2+ will be strongly retarded in aquifers containing calcite; Sr 2+ and Ba 2+ will not be retarded nearly as much.