A spectrophotometric study of neodymium(III) complexation in chloride solutions

The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250degreesC and a pressure of 50 bars. The simple ion, Nd3+, is dominant at 25degreesC, whereas NdCl2+ and NdCl2+ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions: Nd3+ + Cl- = NdCl2+ beta(1), Nd3+ + 2 . Cl- = NdCl2+ beta(2.) The values of P, were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of beta(2) obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200degreesC. Theoretical estimates of beta(1) and beta(2) by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl2+ and NdCl2+ at temperatures above 150degreesC than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for beta(1) and beta(2) similar to those obtained experimentally. Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of similar to2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth's crust. Copyright (C) 2002 Elsevier Science Ltd.