The surface speciation of specifically adsorbed ions at metal (hydr)oxide surfaces is dependent on mineral surface structure and solution chemistry. Theoretical simulations and X-ray techniques provide molecular-scale detail of adsorption complexes and mineral surfaces. Whereas, the effects of solution chemistry on ion adsorption, particularly pH, are best investigated using macroscopic experimental techniques. Surface complexation models (SCM) provide a framework for integrating molecular-scale detail with macroscopic experimental results. Such integrated approaches have been applied successfully to better understand the specific adsorption of ions onto various micro-crystalline metal (hydr)oxide phases. For example, potentiometric titration data, X-ray measurements and theoretical calculations for the adsorption of Sr2+ on micro-crystalline rutile (-TiO2) have been integrated successfully using the MUSIC model in combination with the charge distribution (CD) model. The surface reactivey of nano-crystalline materials have not been studied as extenisively, thus fewer compariable integrated studies have been performed. This contribution will present adsorption results of Sr2+ on nano-crystalline anatase (TiO2). The sorption of Sr2+ was studied as a function of nanoparticle size (3–40 nm diameter), pH and loading in NaCl media at 25°C. Moreover, the experimental solution conditions were selected carefully so as to evaluate Sr2+ sorption on nano-particles relative to compariable macroscopic phases. The development of proton charge curves with the adsorption of Sr2+ were similar for all nano-samples, when normalized to their respective pHznpc values. Furthermore, the charging curves for Sr2+ sorption on the nano-sized anatase were similar to compariable adsorption curves for micro-crystalline rutile. Adsorption charging curves change as funtion of surface loading, suggesting a change in surface speciation. All experimental results were rationalized using a CD-MUSIC model combination. Moreover, the SCMs for Sr2+ sorption on rutile were used to help constrain all nanocrystalline anantase modeling efforts.
|Original language||English (US)|
|Title of host publication||Geochimica et Cosmochimica Acta|
|Publisher||PERGAMON-ELSEVIER SCIENCE LTD|
|State||Published - 2010|