Sorption of NpO2+ and Co2+ onto TiO2

The sorption of cobalt and neptunyl ions onto titanium dioxide from aqueous solution was studied as a function of pH and ionic strength in a CO, free environment at moderate radionuclide concentrations (<3 x 10(-8) M Co (Co-6 0) and similar to 2.5 x 10(-5) M Np (Np-237). TWO different experimental me thods were employ ed; one with samples withdrawn from the same solution at specific pH, separated and measured, and one batch-wise where the pH of the individually prepared samples was measured after separation of the phases. The first and second acid dissociation constants, pK(a1) and pK(a2) of the titanium dioxide surface were determined from potentiometric titrations gi ving a pH(pzc), = 5.8. There is no difference in sorption between the diffe rent ionic strengths for neptunyl and cobalt ions indicating the formation of inner sphere complexes with the TiO2 surface. Neptunyl ion forms a neutr al complex with the titanium dioxide, pK(Np) = 1.5, cobalt ion forms a comp lex with a positive charge, pK(Co) = 3.4. The sorption of neptunyl and coba lt ions has been shown in this work to be quantitatively controlled by the hydrolysis if no other strong complexing agents are present.