This study investigates uranium and technetium sorption onto aluminum and iron hydroxides during titration of acidic groundwater. The contaminated groundwater exhibits oxic conditions with high concentrations of NO3-, SO42-, U, Tc, and various metal cations. More than 90% of U and Tc was removed from the aqueous phase as Al and Fe precipitated above pH 5.5, but was partially resolublized at higher pH values. An equilibrium hydrolysis and precipitation reaction model adequately described variations in aqueous concentrations of metal cations. An anion exchange reaction model was incorporated to simulate sulfate, U and Tc sorption onto variably charged (pH-dependent) Al and Fe hydroxides. Modeling results indicate that competitive sorption/desorption on mixed mineral phases needs to be considered to adequately predict U and Tc mobility. The model could be useful for future studies of the speciation of U,Tc and co-existing ions during pre- and post-groundwater treatment practices. (C) 2010 Elsevier B.V. All rights reserved.