The sorption of Pu(VI) onto TiO2 was studied as a function of pH (2-10) and Pu concentration (10(-8)-10(-4) M) under an N-2 atmosphere, in 0.016 and 0.1 M NaClO4. A batch-wise method was used, in which pH was measured in separate experimental containers after removal of a sample to determine the amount of Pu that had been sorbed. As Pu is radioactive, it was used as a tracer and measured by liquid scintillation counting. No ionic strength dependence was discerned, which was taken as an indication of inner sphere complex formation. In the interval of pH 2-7 the system could be described by the formation of two positively charged surface complexes using a 1-pK Stern model. Sorption of the plutonyl ion (PuO22+) and the first hydrolysis species (PuO2(OH)(+)) was estimated using FITEQL to logK(1) = 6.9 and logK(2) = 1.4, respectively. (C) 2003 Elsevier Inc. All rights reserved.