INTERPRETING STRONTIUM ADSORPTION AT THE MINERAL-WATER INTERFACE OF NANO- AND MICRO-CRYSTALLINE TITANIUM-DIOXIDE PHASES

Our understanding of the complex processes that take place at the interface between water and micro-crystalline minerals has been advanced by integrated experimental and computational studies. Surface Complexation Models (SCM) have been used to provide a framework to explicitly incorporate surface structural information with experimental studies, and to provide rigorous thermodynamic models of adsorption data.

The adsorption of Sr²⁺ on micro-crystalline rutile (α-TiO₂) was investigated by potentiometric titrations, X-ray measurements and theoretical calculations. All data were integrated into a SCM, specifically the MUSIC model in combination with the charge distribution (CD) model. Such an all-inclusive approach has been extended to examine particle-size effects on the adsorption of Sr²⁺ on nano-crystalline anatase phases.

The sorption of Sr²⁺ was studied as a function of nanoparticle size (3–40 nm diameter), pH and loading in NaCl media at 25ºC. The adsorption of Sr²⁺, as indicated by the development of proton charge curves, was similar for all anatase nano-particles when normalized with respect to their pH_znpc values. As anticipated, the adsorption charging curves changed as a function of surface loading, suggesting a change in the Sr²⁺ surface speciation. A CD-MUSIC model was used to rationalize all anatase experimental data. Furthermore, all fits were constrained by available DFT-MD simulation results for the interaction of Na⁺, Sr²⁺ and Cl^- ions with the anatase (100) surface. Strontium and sodium ions adsorb as inner-sphere species, whereas Cl^- electrolyte ions form outer-sphere complexes. The predominant Sr²⁺ surface complexes are bidentate species.