Anthracene and Pyrene Photooxidation Kinetics in Saltwater Environments

Solutes can greatly affect pollutant photodegradation kinetics in atmospheric aqueous phases such as surface waters, atmospheric aerosols, and cloud and fog droplets. We have measured photooxidation rate constants of the polycyclic aromatic hydrocarbons (PAHs) pyrene and anthracene in aqueous solutions containing environmentally relevant concentrations of halide salts (NaCl, NaBr, and NaI) and in seawater. Chloride, bromide, and iodide did not affect pyrene photodegradation kinetics but increased anthracene photodegradation rate constants at low halide concentrations. The largest anthracene rate constant measured, in the presence of 0.2 M NaCl, was 3.4 times larger than that in deionized water. Smaller enhancements were observed in the presence of NaBr and NaI, with maximum rate constants observed at concentrations of 1 × 10–5 and 1 × 10–8 M, respectively. We determined that this enhancement was due to singlet oxygen (1O2) generation resulting from interactions between halides and electronically excited anthracene. Interactions between pyrene and halides also formed 1O2, but this did not affect pyrene’s observed photodegradation rate constant. At higher halide concentrations, anthracene photodegradation rate constants showed a weak negative dependence on halide concentration, likely due to quenching of anthracene’s excited triplet state. Our results suggest that the fates of some PAHs in saline environments could differ from those predicted by kinetics measured in deionized water in the absence of solutes.