Comparative photocatalytic degradation of estrone in water by ZnO and TiO2 under artificial UVA and solar irradiation

Zinc oxide (ZnO) was studied as a photocatalyst under artificial ultraviolet (UV) and solar irradiation for degradation of estrone, a ubiquitous micropollutant widely detected in treated effluent from sewage treatment facilities. Despite its lower specific surface area and larger agglomerate size in water, ZnO enabled significantly more rapid estrone degradation in water under artificial UVA irradiation compared to the benchmark photocatalyst, Aeroxide (R) titanium dioxide P25 (P25 TiO2). Dissolution of ZnO photocatalyst, predominantly driven by its aqueous solubility, occurred during the photocatalytic degradation process which could be mitigated by pH adjustment. Solar irradiation was found to be a highly effective UV source for estrone photocatalytic degradation using either ZnO or P25 TiO2. Previous comparative studies on ZnO and TiO2 photocatalysts were reviewed, and the origin of the superior performance of ZnO under artificial UVA irradiation was investigated. Diffuse reflectance spectroscopy measurements showed that ZnO exhibited markedly higher UV absorbance than P25 TiO2 within the wavelength range of 320-385 nm, which supports the observation that ZnO showed evidently better performance than P25 TiO2 for estrone degradation under weak artificial UVA irradiation. Furthermore, a detailed mechanism was proposed on the role of intrinsic defects during ZnO optical excitation and charge transfer process, and its potential effect on ZnO surface redox reactions. The mechanism was supported by the observation that higher levels of hydrogen peroxide were generated in UV-irradiated ZnO suspension in the absence of added electron donors, suggesting that the photocatalytic activity of ZnO may be enhanced by an effective charge separation mechanism induced by its intrinsic defects. (C) 2012 Elsevier B.V. All rights reserved.