Effect of pH to the surface precipitation mechanisms of arsenate and cadmium on TiO2.

Much attention has focused on the mutual effect of oxyanions and cations on surfaces, however, there is still a lack of knowledge on the molecular-level surface precipitation mechanisms for As(V) and Cd(II) on surfaces under different pH conditions in acid metallurgical industrial wastewater. The results of in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy show that different As(V) and Cd(II) surface precipitation mechanisms take effect at pH 5.0 and 7.0. Under acidic conditions, As(V) was preferentially adsorbed on TiO2 rather than Cd(II). Thus, a Cd(II)-As(V)-TiO2 ternary surface complex was formed with the adsorbed As(V) as the bridging molecule, and the subsequent layer-by-layer surface precipitate was generated. Under neutral conditions, the Cd(II)-As(V) surface precipitates were directly formed by using the aqueous Cd(II)-As(V) complex as a "seed", which took approximately 10 times longer than the acidic conditions to reach the adsorption equilibrium. Our findings provide spectroscopic evidence and elucidate the different simultaneous removal mechanisms of As(V) and Cd(II) on TiO2 under acidic and neutral conditions, which will further our understanding and application of the immobilization of multiple pollutants in industrial wastewaters.