Visible light-induced degradation of sulfa drugs on pure TiO2 through ligand-to-metal charge transfer

The degradation of sulfisoxazole (SSX) on pure TiO2 under visible light irradiation was investigated. 50 pM SSX was completely degraded in a suspension of pure TiO2 (Hombikat UV100, 15 mg/30 mL) at pHi = 4.0 after 30min of visible light irradiation (lambda > 420 nm and light intensity = 0.26 x 10(-3) einstein min(-1) L-1). Although neither SSX nor pure TiO2 alone absorbs visible light, the complexation of SSX on the TiO2 surface induces visible light absorption and electron transfer from SSX to the TiO2 conduction band (CB) (i.e., ligand-to-metal charge transfer (LMCT)) in the SSX-TiO2 complex. The visible light absorption and formation of an SSX-TiO2 complex were confirmed by measuring the UV-visible absorption spectra, attenuated total reflection Fourier transform infrared spectra, and elemental mapping images of the SSX-TiO2 complex powder. The reduction of Cr(VI) to Cr(III) proceeded in a suspension of pure TiO2 with SSX under visible light irradiation. 93% reduction was achieved after 1 h of visible light irradiation. This result corroborates the LMCT mechanism of the SSX-TiO2 complex. The visible light-induced degradation of SSX on pure TiO2 is ascribed to both electron transfer from SSX to the TiO2 CB in the SSX-TiO2 complex and the reaction of SSX with reactive oxygen species that are generated from oxygen reduction. The pseudo-first-order degradation rate constant (k) for SSX was higher at lower pH (e.g., 0.103 min(-1) at pHj = 4.0 and 0.011 min(-1) at pH; = 6.0), higher TiO2 loading (e.g., 0.103 min(-1) at [TiO2} = 0.5 g/L and 0.048 min(-1) at [TiO2] = 0.1 g/L), and higher TiO2 surface area (e.g., 0.103 min(-1) at 316 m(2)/g and 0.025 min(-1) at 85 m(2)/g), which enhanced the formation of the SSX-TiO2 complex by providing better conditions for the adsorption of SSX on the TiO2 surface. Not only SSX (k = 0.103 min(-1)) but also other sulfa drugs such as sulfathiazole (k = 0.010 min(-1)), sulfamoxole (k = 0.009 min(-1)), sulfamethizole (k = 0.007 min(-1)), and sulfamethoxazole (k = 0.006 min(-1)) were degraded on pure TiO2 under visible light irradiation. In addition, the degradation efficiency was not reduced by repeated cycles of SSX degradation. Therefore, the pure TiO2/visible light system is proposed as a practical method for the treatment of sulfa drug-contaminated water.