Study of thorium association and surface precipitation on colloids

Thorium is known to sorb strongly on environmental particles, however, its sorption coefficient need to be formally quantified to justify its use as a tracer or to study his behaviour on environmental particles. A surface complexation model was developed to simulate metal sorption on colloids and precipitation by polymerization at their surface. Correlations between hydrolysis and surface complexation are systematically used to estimate all surface complexation constants on the basis of the corresponding hydrolysis constant. The model is based on the metal adsorption on free sites as well as on already contaminated sites. The distribution coefficient (Kd) is calculated for the Th(IV) sorption and polymerization onto Al2O3, TiO2, FeOOH, and SiO2 colloids. The calculations were performed for the effect of various polymer chain lengths, pH, the radius of the colloids, and the concentration of both the metal and colloid. It was verified that the highest sorption occurs at approximately neutral pH values. It is shown that at low metal concentrations, polymerization does not significantly affect the distribution coefficient, but as the metal concentrations reaches the concentration of the surface sites, the distribution coefficients could increase before tapering off at saturation for limited polymer sizes. As the size of the colloid particles decreases the distribution coefficients increase as expected for a given mass concentration. When comparing the effect of colloid material, TiO2 displays the highest Kd values followed by Al2O3, FeOOH, and finally SiO2. Since the polymerization is an effect of the metal and not of the type of colloid, once the polymerization begins affecting the Kd, the values for all four types of the colloids would become similar. However, thorium saturation in the solution apparently occurs prior to polymerization. The Kd values gained from the literature are comparable with those calculated using the presented model.