TiO₂ Crystal Facet-dependent Antimony Adsorption And Photocatalytic Oxidation

Antimony （Sb） and arsenic （As） are both metalloids, having similar chemistry and toxicity, and belong to Group VA of the periodic table. In recent years, Sb has been widely used in industrial production-manufacturing. Along with the growing demand, the increased antimony contamination in water has created a global concern. The significance to human health and environment has explained the progressively attention on antimony speciation, behavior and removal.In this study, TiO₂ that exposed almost 100 % high-energy {001} facets with BET surface area of 205 m2/g was used to remove antimony. Adsorption experiments demonstrated that the maximum adsorption capacity for Sb（Ⅲ） and Sb（V） on TiO₂ was 200 mg/g and 156 mg/g, respectively. The PZC of TiO₂ shifted leftward upon Sb adsorption, indicating the formation of inner-sphere complexes. Coadsorption experiment indicated the existence of arsenic has no effect on Sb adsorption. DFT calculations proved that the adsorption configuration of Sb on TiO₂ can be both monodentate and bidentate （-4.99 eV）; whereas only bidentate adsorption configuration is favorable for Sb（V） with adsorption energy of -4.71 eV. EXAFS analysis indicated that antimony formed bidentate structures on TiO₂. This research demonstrated that TiO₂ with high-active {001} facets is a promising adsorbent for antimony removal.TiO₂ crystal facets are garnering increasing attention due their unique surface property. However, no specific relationship had been derived between the facet exposed on TiO₂ and the surface adsorption capacity as well as photocatalytic performance. This study systematically explored the facet effects on antimony （Sb） adsorption and photocatalytic oxidation using high-index {201} and low-index {101}, {001}, and {100}TiO₂. The results suggest that high-index {201} TiO₂ exhibits the best Sb（Ⅲ） adsorption and photocatalytic activity compared to the low-index TiO₂. Both the Sb（Ⅲ） adsorption density and the amount of · OH and O2·- generated in solution were correlated to the magnitude of surface energy on TiO₂ facets. Photocatalytically generated · OH and O₂^·- were responsible for Sb（Ⅲ） photooxidation as evidenced by radical-trapping experiments.The great contribution of’ OH was observed only on {201}, not on low-index TiO₂. This phenomenon was found to be attributable to the high surface energy on {201}, which enables the generation of a large amount of photogeneration ’OH to compensate for the fast rate of ’OH dissipation. Therefore, the predominant participation of ·OH in Sb（Ⅲ）photooxidation was only possible on high-index {201} TiO₂, which resulted in an enhanced photocatalytic rate. On the other hand, O2·- dominated the Sb（Ⅲ）photocatalytic oxidation on low-index TiO₂.