Rare earth element partitioning between Fe oxyhydroxide precipitates and aqueous NaCl solutions doped with NaHCO3: Determinations of rare earth element complexation constants with carbonate ions

The distribution coefficients of rare earth elements (REEs; all lanthanides except Pm, Y and Sc) between Fe oxyhydroxide precipitates and 0.5 M NaCl solutions with NaHCO3 (0.0∼12.0 mM) at 25°C and 1 bar have been determined. This experimental system is a simple model for REE partitioning between deep-sea ferromanganese nodules and seawater. The distribution coefficients, Kd(REE: precipitate/solution), show systematic variations with increasing NaHCO3 concentrations. We have determined REE(III)-carbonate complexation constants from the experimental distribution coefficients as a function of NaHCO3 concentration and pH. The REE(III)-carbonate complexation constants show almost the same series variations as reported values obtained by the solvent-extraction method, although our results by the precipitation method are rather higher by 1.0∼1.5 in log unit than the literature data at zero ionic strength.