Experimental study of strontium adsorption on anatase nanoparticles as a function of size with a density functional theory and CD model interpretation

The effect of particle size on the adsorption of Sr²⁺ onto monodisperse nanometer diameter (4, 20, and 40 nm) anatase samples has been evaluated quantitatively with macroscopic experimental studies. The adsorption of Sr²⁺ onto the anatase particles was evaluated by potentiometric titrations in NaCl media, at two ionic strengths (0.03 and 0.3 m), and over a wide range of pH (3-11) and surface loadings, at a temperature of 25 °C. Adsorption of Sr²⁺ to the surface of the 20 and 40 nm diameter samples was similar, whereas the Sr²⁺ adsorption titration curves were shallower for the 4 nm diameter samples. At high pH, the smallest particles adsorbed slightly less Sr²⁺ than was adsorbed by the larger particles. At the molecular scale, density functional theory (DFT) calculations were used to evaluate the most stable Sr²⁺ surface species on the (101) anatase surface (the predominant crystal face). An inner-sphere Sr-tridentate surface species was found to be the most stable. The experimental data were described with a charge distribution (CD) and multisite complexation (MUSIC) model, with a Basic Stern layer description of the electric double layer. The resulting surface complexation model explicitly incorporated the molecular-scale information from the DFT simulation results. For 20 and 40 nm diameter anatase, the CD value for the Sr-tridentate species was calculated using a bond valence interpretation of the DFT-optimized geometry. The CD value for the 4 nm sample was smaller than that for the 20 and 40 nm samples, reflecting the shallower Sr²⁺ adsorption titration curves. The adsorption differences between the smallest and larger anatase particles can be rationalized by water being more highly structured near the 4 nm anatase sample and/or the Sr-tridentate surface species may require more well-developed surface terraces than are present on the 4 nm particles.