Photolysis of polycyclic aromatic hydrocarbons (PAHs) in dilute aqueous solutions detected by fluorescence

Photodegradation kinetics of anthracene, pyrene, benz[a]anthracene and dibenz[a,h]anthracene were studied in aqueous solutions at pH 5.6 and 7.6 with purging gases of nitrogen, oxygen or synthetic air. Photolysis of PAHs was performed in an immersion well reactor at 254nm and was followed as decreasing fluorescence intensities. Photodegradation of PAHs was fast in all cases with anthracene and benz[a]anthracene being the fastest decomposed. The photochemical reactions followed a first order equation, and the apparent rate constants, k(r), varied from 10(-3) to 10(-4) s(-1). Oxygen concentration had minor effect upon the photodegradation rates of PAHs. At pH 5.6 the values of the rate constants were higher than those at pH 7.6, anthracene was an only exception of this. Based on photodegradation kinetics and fluorescence lifetime measurements, a simple scheme for a mechanism is proposed containing non-emissing excited intermediate species in equilibrium with the excited singlet state. (C) 2000 Elsevier Science S.A. All rights reserved.