Kinetic analysis of the photodegradation of polycyclic aromatic hydrocarbons in aqueous solution

The mechanisms by which polycyclic aromatic hydrocarbons (PAHs) photodegrade in dilute (less than or equal to25 nM) aqueous solutions under varying O-2 concentrations were examined. Employing previously measured photodegradation quantum yields (acquired in the absence and presence of electron donors to the PAH cation radicals, P+), excited singlet state (P-1*) lifetimes, P-1*/O-2 quenching rate constants and other parameters from the literature, a kinetic model was developed to analyze the fraction of PAH photodegradation that proceeds via the P-1* or excited triplet state (P-3*), as well as to determine the relative contribution of electron transfer to O-2 versus the direct reaction with O-2 from each state. The analysis supports the following conclusions: 1) P-1* is more reactive with O-2 than P-3*; 2) reaction via P-1* proceeds predominantly through electron transfer to produce the P+ intermediate, whereas reaction via P-3* proceeds primarily through direct reaction with O-2 within the collision complex, consistent with the thermodynamic driving forces for electron transfer from these states; 3) although P-3* is involved significantly in the degradation of many PAHs, such as anthracene, 9-methylanthracene, acenaphthene, and perylene, it is not involved in the degradation of others such as pyrene and benzo[a]pyrene. Under aerobic conditions, photodegradation is likely to be controlled largely by P-1* lifetime and reactivity.