A titania thin film annular photocatalytic reactor for the degradation of polycyclic aromatic hydrocarbons in dilute water streams

An external lamp, annular photocatalytic reactor with titania immobilized on a quartz tube was used to dearade two polycyclic aromatic hydrocarbons (PAHs), viz. phenanthrene (PHE) and pyrene (PYR) from a dilute water stream. The thin film geometry was used to obtain both the mass transfer coefficients and intrinsic reaction rate constants for the two compounds on immobilized titania (Degussa P-25) particles. Beyond a feed velocity of 7 cm min(-1), the conversion was solely reaction rate controlled and was not subjected to mass transfer limitations from the aqueous phase to the immobilized titania film. The overall reaction rate constant was independent of the feed concentration as large as the saturation aqueous solubility of the two compounds. However, the conversion was dependant on the ultraviolet (UV) light illumination intensity at the reactor. The quantum efficiency ranged from 3.7 x 10(-5) to 2.7 x 10(-4) which was somewhat low because of the very low aqueous concentrations of the chemicals. The overall reaction rate constant was 1.6 times larger for pyrene than for phenanthrene. Seven reaction intermediates were identified for the conversion of phenanthrene, while for the degradation of pyrene two intermediates were identified. The presence of the phthalate ester as an intermediate product in the degradation of both PAHs indicates the presence of a quinone in both cases which degrades to the products CO2 and H2O, along with other stable intermediates. Mass balance in a batch reactor showed that only 28.6-40.1% of phenanthrene is mineralized to CO2 in 1-3 h of reaction although 35-67% of the parent compound has disappeared, confirming that a substantial fraction of the parent compound has been converted to stable intermediates that remain in the reactor. A plausible mechanism based on these observations is proposed. (C) 2003 Elsevier Science B.V. All rights reserved.