Experimental study of La(OH)3 and Gd(OH)3 solubilities (25 to 150°C), and La-acetate complexing (25 to 80°C)

The solubilities of pure synthetic La and Gd hydroxides were measured in aqueous solutions over a wide range of pH (5-9.5) and temperature (40-150 degrees C). The regression of these data is used to determine the first dissociation constant( K-S0*) of the minerals as a function of temperature. The logarithm of these constants is found to be linearly related to the reciprocal temperature, which allows their reliable extrapolation to 25 degrees C. In the range of temperature and pH investigated, only the REE3+ ion(where REE denotes La or Gd) is present in solution, indicating that the first hydrolysis constants proposed by Baes and Mesmer (1976), which are generally admitted, are overestimated. The complexation of La with acetate ion (AcO-) was studied by both solubility and potentiometric measurements. Potentiometric titrations are interpreted in terms of the formation of two complexes: LaAcO2+ and La(AcO)(2)(+), and are used to generate the corresponding stability constants in the temperature range 25-80 degrees C. These data allow accurate description of the La(OH), solubility increase, evidenced in acetate-bearing solutions. (C) 1998 Elsevier Science B.V. All rights reserved.