| Polycyclic aromatic hydrocarbons (PAHs) are common carcinogenic and toxic pollutants in the aquatic environment. One significant transformation pathway in this media is photochemical degradation. Thus, the aqueous photodegradation of PAHs was studied using dilute PAH solutions and polychromatic radiation (>290 nm) in order to mimic natural conditions and better understand this transformation pathway in the environment. Photodegradation rate constants in the presence of natural water constituents, such as dissolved organic matter, were largely unchanged compared to rate constants in pure water. Photodegradation quantum yields varied from 10−5 to 10−2 in air-equilibrated solutions. While photoionization is insignificant in PAH photodegradation, photodegradation quantum yields were highly dependent on O2 concentration. In the presence of electron donors, it was observed that for most PAHs, only a small portion (<30%) of air-equilibrated photodegradation occurs through the PAH cation radical intermediate (P +). This intermediate is formed following electron transfer from the PAH to O2 after the absorption of light by the PAH.; Photodegradation not occurring through P+ proceeds by the direct reaction of O2 and electronically excited PAH (P*) within the collision complex. The dependence of photodegradation quantum yields on O2 concentration and significant quantum yields in N2 purged solutions for some PAHs implies degradation pathways through both the excited singlet (1P*) and triplet states (3P*). A model was developed which included PAH degradation through four pathways, direct reaction with O2 and electron transfer to O2 from both 1P* and 3P*. The relative contributions of reaction through 1P* and 3P* varied greatly among the PAHs; some compounds had large contributions from 3P*, while others had no contribution from 3P* in air-equilibrated solutions. |
