The experimental determination of REE partition coefficients in the water-calcite system

To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~ 6 and 25 °C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates. Partition coefficients (D^REE), determined from analysis of the aqueous solution compositions, range from 10^2.8 at the lowest aqueous La concentration to 10^1.4 at the highest aqueous concentration of Yb. D^REE was found to correlate with aqueous REE concentrations, as well as with the calcite saturation state. Experimental results, together with previous spectroscopic evidence, suggest the incorporation of REE³⁺ into the Ca²⁺ site of calcite while maintaining charge balance by the concurrent incorporation of Na⁺. The value of the equilibrium constants for the corresponding exchange reaction was determined for La, Eu and Yb by combining our results with previously published experimental data. Subsequently, these constants were used together with a lattice strain model to estimate corresponding values for the other REE. Application of this exchange model suggests that D^REE vary significantly with the composition of natural waters including seawater. Such variations are likely the origin for the large differences of D^REE values reported in the literature based on experimental and field evidence.