In Situ EXAFS Study of Sr Adsorption on TiO₂(110) under High Ionic Strength Wastewater Conditions

In order to provide important details concerning the adsorption reactions of Sr, batch reactions and a set of both ex situ and in situ Grazing Incidence X-ray Absorption Fine Structure (GIXAFS) adsorption experiments were completed on powdered TiO₂ and on rutile(110), both reacted with either SrCl₂ or SrCO₃ solutions. TiO₂ sorption capacity for strontium (Sr) ranges from 550 ppm (SrCl₂ solutions, second order kinetics) to 1400 ppm (SrCO₃ solutions, first order kinetics), respectively, and is rapid. Sr adsorption decreased as a function of chloride concentration but significantly increased as carbonate concentrations increased. In the presence of carbonate, the ability of TiO₂ to remove Sr from the solution increases by a factor of ~4 due to rapid epitaxial surface precipitation of an SrCO₃ thin film, which registers itself on the rutile(110) surface as a strontianite-like phase (d-spacing 2.8 Å). Extended X-ray Absorption Fine Structure (EXAFS) results suggest the initial attachment is via tetradental inner-sphere Sr adsorption. Moreover, adsorbates from concentrated SrCl₂ solutions contain carbonate and hydroxyl species, which results in both inner- and outer-sphere adsorbates and explains the reduced Sr adsorption in these systems. These results not only provide new insights into Sr kinetics and adsorption on TiO₂ but also provide valuable information concerning potential improvements in effluent water treatment models and are pertinent in developing treatment methods for rutile-coated structural materials within nuclear power plants.