Solution medium effects on the photochemical degradation of pyrene in water

Poly-aromatic hydrocarbons (PAHs), carcinogenic compounds ubiquitous in the environment, are readily degraded photochemically. Understanding the kinetics and mechanisms of their photochemical degradation is important in assessing their overall toxicity. Here we have measured the photo-oxidation rate of pyrene in aqueous solution as a function of environmentally relevant solution media. The degradation rate of pyrene is 0.268 min−1 in air-equilibrated water at pH 8. Degradation appears to require water and is dependant on oxygen concentrations, ionic strength and the concentration of humic acid in solution. Rates decrease as oxygen concentrations decrease, reaching a rate of 0.0424 min−1 in argon-purged solutions. Degradation increases as ionic strength increases and decreases as humic acid concentration increases. Rates will vary by as much as an order of magnitude over the range of environmental ionic strengths and humic acid concentrations. Mechanistic studies showed the formation of the 1-hydroxypyrene intermediate, consistent with a previously proposed mechanism in the literature that proceeds via electron transfer from excited singlet state pyrene to oxygen.