Solar photocatalysis for the abatement of emerging micro-contaminants in wastewater: Synthesis, characterization and testing of various TiO2 samples

The photocatalytic degradation of a mixture of three compounds spiked in secondary treated wastewater by means of simulated solar radiation over titania suspensions was investigated. Bisphenol-A (BPA) and 17 alpha-ethynylestradiol (EE2) were chosen as representatives of emerging micro-contaminants, while phenol was chosen as a reference contaminant. Ten titania samples were synthesized and employed to evaluate the effect of doping with nitrogen, phosphorous, calcium, silver, sodium and potassium, as well as platinum dispersion on photocatalytic activity. The catalysts were characterized by X-ray diffraction, diffuse reflectance UV-vis spectroscopy, nitrogen physisorption and selective chemisorption of CO or hydrogen. A 0.5% Pt/TiO2 catalyst (38 m(2)/g surface area, 72:28 anatase:rutile, 20 and 2 nm crystallite size for TiO2 and Pt, respectively) was highly active for the degradation of the contaminants, whose reactivity increased in the order: phenol < BPA <= EE2: a commercially available Aeroxide P25 TiO2 exhibited comparable activity. The effect of various operating conditions, such as 0.5% Pt/TiO2 concentration (125-1000 mg/L), initial contaminant concentration (100-300 mu g/L each), photon flux (17.4 x 10(-8)-58 x 10(-8) einstein/(Ls) provided by a 150 W Xenon lamp) and the water matrix (wastewater and ultrapure water), on degradation was then assessed. Reaction rates increased linearly with catalyst concentration and photon flux, confirming the photo-induced nature of the activation of the catalytic process: likewise, a linear dependence of rate on initial concentration occurred denoting first order kinetics. Degradation in wastewater was slower than in pure water by an order of magnitude, implying the scavenging behavior of effluent's constituents against hydroxyl radicals. The implications for tertiary wastewater treatment (e.g. mineralization, disinfection and removal of estrogenicity) are also discussed. (c) 2012 Elsevier B.V. All rights reserved.