A spectrophotometric study of Nd(III), Sm(III) and Er(III) complexation in sulfate-bearing solutions at elevated temperatures

The speciation of Nd(III), Sm(III), and Er(III) in sulfate-bearing solutions has been determined spectrophotometrically at temperatures from 25 to 250 °C and a pressure of 100 bars. The data obtained earlier on the speciation of Nd in sulfate-bearing solutions (Migdisov et al., 2006) have been re-evaluated and corrected using a more appropriate activity model and are compared with the corresponding data for Sm(III) and Er(III) and new data for Nd(III). Based on this comparison, the dominant species in the solution are interpreted to be REESO4+ and REE(SO4)2-, with the latter complex increasing in importance at higher temperature. Equilibrium constants were calculated for the following reactions:REE3++SO42-=REESO4+β1;REE3++2·SO42-=REE(SO4)2-β2;REESO4++SO42-=REE(SO4)2-K2.The values of the first formation constant (β1) are in reasonably good agreement with those predicted by Haas et al., 1995 and Wood, 1990a; those for the second formation constant (β2, K2) also agree reasonably well with predictions of Wood, 1990a and Wood, 1990b at low temperatures, but predict higher stability of REE(SO4)2- at T > 150 °C.