Strengthened Removal of Tetracycline by a Bi/Ni Co-Doped SrTiO3/TiO2 Composite under Visible Light

A two-step hydrothermal method was used to first obtain a SrTiO3/TiO2 composite then to dope the composite with Bi, Ni and Bi/Ni. Morphology, crystalline structures, surface valances and optical features of SrTiO3/TiO2 and Bi-, Ni-, Bi/Ni-doped SrTiO3/TiO2 were assessed. XRD and XPS analysis showed that Bi and Ni were successfully doped and existed in Bi(3+) and Ni(2+) oxidation state. UV–vis analysis further revealed that the bandgap energies of TiO2 and SrTiO3/TiO2 were calculated to be 3.14 eV and 3.04 eV. By comparison, Bi, Ni and Bi/Ni doping resulted in the narrowing of bandgaps to 2.82 eV, 2.96 eV and 2.69 eV, respectively. The removal ability of SrTiO3/TiO2 and doped SrTiO3/TiO2 were investigated with tetracycline as the representative pollutant. After 40 min of exposure to visible light, Bi/Ni co-doped SrTiO3/TiO2 photocatalyst was able to remove 90% of the tetracycline with a mineralization rate of about 70%. In addition, first-order removal rate constant was 0.0074 min−1 for SrTiO3/TiO2 and increased to 0.0278 min−1 after co-doping. The strengthened removal by co-doped photocatalyst was attributed mainly to the enhanced absorption of visible light as co-doping resulted in the decreases of bandgap energies. At the same time, the co-doped material was robust against changes in pH. Removal of tetracycline was stable as pH changed from 5 to 9. Tetracycline removal was inhibited to a certain degree by the presence of nitrate, phosphate and high concentration of humic acid. Moreover, the co-doped material exhibited strong structural stability and reusability. In addition, a photocatalysis mechanism with photogenerated holes and ·O2− radicals as main oxidative species was proposed based on entrapping experiments and EPR results.