| Pharmaceutical compounds are emerging contaminants in aquatic environments because they are bioactive, polar and persistent which may cause adverse effects in aquatic wildlife and humans. Degradation of these compounds by conventional activated sludge treatment ranged from complete to very poor, however, recent studies on new treatment technologies showed that photocatalytic oxidation is one of the most promising treatment technologies to remove these compounds from water. This study examined photocatalytic oxidation of four pharmaceuticals, using titanium dioxide (TiO2). Adsorption of naproxen, ibuprofen, clofibic acid and diclofenac on TiO2 under ambient conditions decreased for all compounds with increasing pH. Diclofenac, naproxen, ibuprofen and clofibric acid were 100%, 65%, 50% and 22% adsorbed at pH 3, respectively but adsorption decreased to 20%-5% at pH > 5. Adsorption of all compounds on TiO2 at pH 3 likely resulted from electrostatic attraction and hydrogen bonding. Diclofenac showed the highest adsorption probably because of the combined effects of carboxyl group and amine lone pair electrons to form bidentate complexes.; Photocatalytic oxidation of naproxen, ibuprofen, clofibric acid and diclofenac were studied in UV illuminated (lambda > 300nm) TiO2 suspensions at pH 3, 5 and 7 and 25°C with emphasis on kinetic rates, intermediate reaction products and oxidation pathways. The faster rates occurred at pH 3 except for diclofenac and coincided with the larger extent of adsorption on TiO2. Diclofenac showed the opposite behavior: oxidation rates increased as pH increased although adsorption decreased. The first order rate constants at pH 5 showed the order NP > IBN > CFA. > DC in the range from 0.098 to 0.026 min-1. The kinetics also followed Langmuir-Hinshelwood kinetics and showed a similar order. Two pH dependent oxidation pathways were identified. Electron transfer from a carboxyl group to a valence band hole dominated at pH 3 yielding a ketone and alcohol from ibuprofen and naproxen oxidation and chlorophenol from clofibric acid oxidation. The hydroxyl radical ring oxidation pathway was dominant at pH > 5 giving the Cl- ion from clofibric acid and diclofenac oxidation. Diclofenac oxidized only through hydroxyl radical substitution on aromatic rings with a carboxyl group to form phenolic intermediates and Cl- ions. |
