Dissolved Versus Colloidal U and Th Under Acid Mine Drainage Conditions in Río Tinto Area (Spain)

Results of ultrafiltration experiments performed in acid waters from the Río Tinto acid mine drainage system in order to evaluate the role of colloids under acid mine drainage conditions indicate that for U and Th, a constant permeation model is applicable and does produce results that are compatible with the predicted behavior of these elements under acidic conditions. Results show that around 8,7% of total U is in the colloidal fraction. Results considering the 234, 235 and 238 U isotopes show similar percentages of retention in the colloidal fraction. Th is dissolved in the acid waters of Río Tinto, which agrees with the predicted dissolved character of Th under acid and high sulfate concentration conditions