Molecular Insights into Ternary Surface Complexation of Arsenite and Cadmium on TiO_2

摘要

Insights from molecular-level mechanisms of arsenite [As(Ⅲ)] and cadmium (Cd) co-adsorption on TiO_2 can further our understanding of their synergistic removal in industrial wastewaters. The motivation for our study is to explore the interfacial interactions of neutrally charged As(Ⅲ) and cationic Cd~(2+) on nanocrystalline TiO_2 using multiple complementary techniques. The results of adsorption edge, ζ potential, and surface complexation modeling suggest that coexistence of As(Ⅲ) and Cd~(2+) enhanced their synergistic adsorption on TiO_2 and, consequently, resulted in the formation of a ternary surface complex. This ternary surface complex, in turn, inhibited the metal release into the aqueous phase and, therefore, facilitated the immobilization of the heavy metals. Our in situ flow-cell attentuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy evidence showed that, regardless of the order of contact, As(Ⅲ) was preferentially adsorbed on TiO_2 rather than Cd. In agreement with our spectroscopic analysis, quantum chemistry calculations also illustrated that the Cd-As(Ⅲ)-TiO_2 ternary surface complex should be formed with the adsorbed As(Ⅲ) as the bridging molecule. At high As(Ⅲ) concentrations, the formation of the Cd-As(Ⅲ)-TiO_2 complex is responsible for the Cd removal. The simultaneous removal mechanisms will further our understanding of the removal of multiple pollutants in industrial wastewaters.