[Groundwater Arsenic and Silicate Adsorption on TiO2 and the Regeneration of TiO2].

Titanium dioxide (TiO2) was widely used to remove arsenic (As) from groundwater due to its excellent properties. Previous studies show that the coexisting silicate ions (Si) could occupy the available surface sites of TiO2 and further inhibit As adsorption and TiO2 regeneration. To investigate the effect of Si adsorption on the As molecular surface structure, an extended X-ray absorption fine structure (EXAFS) analysis was conducted in this work. The results indicated that the presence of Si exhibited no impact on the As adsorption configuration on TiO2. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy results demonstrated that the polymerization of Si that formed on the TiO2 surface compete with As adsorption sites, increasing the difficulty for TiO2 regeneration. To effectively regenerate TiO2, the removal efficiency of Si polymers on TiO2 via sodium fluoride (NaF) was studied. The results showed that NaF could remove Si monomer and polymer from TiO2, and the regenerated TiO2 could be reused with a stable adsorption performance. In situ ATR-FTIR spectroscopy suggested that NaF desorbed the Si monomer and polymer effectively. When spent TiO2 was regenerated with NaOH and NaF in three treatment cycles, As and Si desorption rates were 86.8%-100.3% and 67.9%-82.0%, respectively. The present study provides a new insight into regenerating absorbents with coadsorbed As and Si in groundwater.