Effect of hexamethylenetetramine on the visible-light photocatalytic activity of C-N codoped TiO2 for bisphenol A degradation: evaluation of photocatalytic mechanism and solution toxicity

Our previous work showed that highly visible-light photoactive C-N codoped TiO2 (CN-TiO2) was synthesized by a solvothermal method. The CN-TiO2 prepared using hexamethylenetetramine (HMT) as the dopant source exhibited excellent photoactivity on bisphenol A (BPA) degradation under visible light irradiation. The influences of pH and inorganic anions on BPA degradation were evaluated. Three main studies were elucidated in this work. (1) Effect of HMT dosage on the physicochemical properties and photoactivity of CN-TiO2 was investigated. After 2 h of irradiation with white LED, 95% of BPA was removed by Ti-0.375HMT, which exhibited large surface area (157.7 m(2) g(-1)) and low band gap (2.46 eV). (2) Mineralization and evolution of solution toxicity upon BPA degradation under white LED, simulated solar light and UV irradiation were systematically studied. After 4 h of reaction, BPA was almost completely mineralized under simulated solar light and UV irradiation by the CN-TiO2, and generally the BPA solution became almost non-toxic irrespective of the light source used. (3) Photocatalytic mechanism of BPA degradation by the CN-TiO2 under visible light irradiation was proposed. The carbon and nitrogen codoping could create the intragap localized states located above the valence band of TiO2, and carbonate species formed on the TiO2 surface could serve as photosensitizer. As a result, O-2(center dot-), O-1(2) and h(+) could be generated on the surface of CN-TiO2 under visible light irradiation: they together contributed to the BPA degradation. Particularly, O-2(center dot-) was the predominant oxidative species to oxidize BPA. Trace OH center dot was produced but its role in BPA degradation was insignificant. (C) 2011 Elsevier B.V. All rights reserved.