Surface Complexation Studies in An (Th, U(IV), Pu(VI)) Systems

Abstract: Two-phase systems containing an electrolyte solution and a suspended solid were studied. The common denominator for the three systems examined in this work is that they contain actinides (elements of atomic numbers between 89 and 103) in either the solid phase or as a sorbate. Experiments were done on simple oxides. The surface charge of uranium oxide (UO2) and thorium oxide (ThO2) was studied as a function of pH at different ionic strengths to gain information that can be used in evaluating forthcoming sorption experiment data from these solid phases. The third solid phase studied is the model oxide TiO2, which is used because of its stability. As TiO2 is commonly used, there is ample data in the literature for comparison. The sorption of plutonium(VI) onto TiO2 was studied as a function of both pH and plutonium concentration at different ionic strengths. A 1 pK Stern surface complexation model was then used and fitted to experimental data using the FITEQL code to see how well surface complexation reactions can describe the measured sorption. The pH point of zero charge was estimated for UO2 and ThO2. It was also found that UO2 is more effective at accumulating charge than other oxides. The sorption of Pu(VI) onto TiO2 could be described well by using two surface complexation reactions, where the plutonyl ion was presumed to sorb as both unhydrolyzed (PuO22+) and the form of its first hydrolysis complex (PuO2(OH)+).

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