A spectrophotometric study of samarium (III) speciation in chloride solutions at elevated temperatures

The speciation of samarium (III) in chloride-bearing solutions was investigated spectrophotometrically at temperatures of 100–250 °C and a pressure of 100 bars. The simple hydrated ion, Sm3+, is predominant at ambient temperature, but chloride complexes are the dominant species at elevated temperatures. Cumulative formation constants for samarium chloride species were calculated for the following reactions:Sm3++Cl-=SmCl2+β1Sm3++2Cl-=SmCl2+β2Within experimental error, the values for the first formation constant (β1), are identical to the values predicted by Haas et al. [Haas J. R., Shock E. L. and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta, 59, 4329–4350]. The values for the second formation constant (β2) at 200 and 250 °C are in fair agreement with those of Haas et al. (1995) and Gammons et al. [Gammons C. H., Wood S. A. and Li Y. (2002) Complexation of the rare earth elements with aqueous chloride at 200 °C and 300 °C and saturated water vapor pressure. Special Publication—The Geochemical Society, (Water–Rock Interactions, Ore Deposits, and Environmental Geochemistry), pp. 191–207]. Calculations of monazite solubility indicate that Sm is less mobile in chloride-bearing solutions than Nd, which may indicate that the HREE are less mobile than the LREE.