Ion adsorption at metal (hydr)oxide surfaces is dependent on the surface structure and the coordination geometry of adsorption complexes. At the microscopic scale, surface structure and ion sorption have been examined extensively using various X-ray techniques. Additionally, theoretical simulations also provide molecular-scale details of mineralsolution interfaces. The binding configuration of adsorbed ions may change as a function of solution conditions; for example, surface complexes may protonate or deprotonate. Such changes are best investigated at the macroscopic scale as function of pH. Therefore, to develop an unambiguous understanding of the reactivity of mineral-solution interfaces it is necessary to scale from the molecular to the macroscopic scale. Surface Complexation Models (SCM), specifically the MUSIC model, provides a powerful framework in which microscopic information can be utilized to rationalize bulk experimental adsorption data. This contribution will concentrate on the adsorption of Y3+ on rutile. The sorption of Y3+ was studied as a function of pH and loading, in NaCl media at 25 and 50°C. The potentiometric titration data were rationalized successfully using the MUSIC model in combination with the charge distribution (CD) model. The CD values for the surface species were evaluated in relation to available structural information from X-ray techniques and MD simulations. Moreover, changes in surface speciation as a function of pH and surface loading, were also evaluated. X-ray studies have shown that on the predominant (110) rutile surface Y3+ adsorbs mainly as an inner-sphere tetradentate complex, comprising two bridged and two terminal rutile surface oxygens. In addition to this tetradentate complex, SCM revealed a bidentate inner-sphere complex (one bridged and one terminal group) with increased surface loading Furthermore, both inner-sphere Y3+ complexes hydrolyze with increasing pH.
|Original language||English (US)|
|Title of host publication||Geochimica et Cosmochimica Acta|
|Publisher||PERGAMON-ELSEVIER SCIENCE LTD|
|State||Published - 2009|