Competitive adsorption of arsenic and fluoride on {201} TiO2

Elevated arsenic (As) and fluoride (F) in natural water present an urgent environmental concern. The demand for their effective removal underscores the fundamental understanding of their solid-liquid interface chemistry. Herein, the efficiency of {2 0 1} TiO2 in As(III/V) and F was explored using macroscopic and molecular-level techniques. Their adsorption isotherms followed the Langmuir equation, and the maximum adsorption capacity was 50.5, 29.3, and 5.0 mg/g for As(III), As(V) and F, respectively. Their adsorption kinetics of As(III), As(V) and F conformed to the pseudo-second-order model. The XPS and in situ ATR-FTIR results identified that the active adsorption sites on {2 0 1} TiO2 included surface hydroxyl groups, but not oxygen vacancies. As(III/V) and F form bidentate binuclear and monodentate mononuclear structures, respectively, regardless of exposed facets. Integrated with the molecular-level mechanism, the charge distribution multisite complexation model well predicted the pH edge behaviors in mono- and co-component systems. The shift of pH(PZC) of {2 0 1} TiO2 in competitive adsorption systems signified the formation of inner-sphere complexes. The results of this study shed new lights on the adsorption of coexisting ions using high-index faceted TiO2.