Modelling americium sorption onto colloids: effect of redox potential

Abstract

An analytical expression that evaluates the effect of the pH and the redox potential (Eh) was developed for studying the sorption of actinides onto colloids. It includes surface complexation with one type of surface site and ion exchange processes, and its formulation yields to a distribution coefficient (Kd) as function of the pH (hydrolysis) and Eh (redox sensitive species). The formulation considers also the values of the stability and hydrolysis constants for all species in solution and sorbed at the surface, and makes use of semi-empirical correlations between hydrolysis and surface complexation constants, for each colloid surface. The presence of complexing ligands in solution (such as carbonates) was also taken into account. The model was applied to the sorption of americium onto Al2O3, FeOOH and SiO2 colloids, in the presence and in the absence of carbonates in solution. From model testing it was shown that Am(III) sorption does not undergo redox change under natural conditions, as was expected. When carbonates are present in solution the calculated values of the distribution coefficient were lower than those calculated in the absence of carbonates, and no redox effect was detected. The distribution coefficient (Kd) values obtained with the developed model are in agreement with values reported for the sorption of americium onto colloids. It is known that in the water stability region Am(III) and their hydroxides are the only stable species; however, for the model application, all the possible redox couples of americium were considered. Moreover, this model is applicable to study the sorption of other redox sensitive elements.