Anthracene Photodegradation Kinetics in Salty Aqueous and Organic Solutions

The reactivity of organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs) in environmental condensed phases depends strongly on matrix composition. Pollutants such as PAHs are often associated with atmospheric aerosols, so an understanding of the effects of changing aerosol composition is required to accurately predict their environmental chemical fate. We have measured the photodegradation kinetics of the PAH anthracene in aqueous solutions containing sodium or potassium chloride (NaCl or KCl) at chloride concentrations greater than those observed in seawater (0.56 M) but relevant to environmental matrices such as atmospheric aerosols, industrial brines, and saline wastewater. At concentrations greater than 0.56 M but below the saturation limit (4.6 M for KCl, 6.1 M for NaCl), the rate constant did not depend on Cl^- concentration, but rather remained relatively constant at a value approximately a factor of 2 greater than the rate constant measured in deionized water. We attribute this behavior to competition between a heavy atom effect that suppresses photodegradation by promoting intersystem crossing from anthracene's excited triplet state to its singlet ground state, and the salting out effect, which we demonstrate increases photodegradation by promoting anthracene self-association. At KCl and NaCl concentrations exceeding the saturation limit, the photodegradation rate constant increased with increasing salt concentration. This is hypothesized to be due to anthracene-NaCl(s) interactions. This is supported by observed monotonic increases in anthracene's photodegradation rate constant in octanol containing up to 1.5 M solid NaCl. The results suggest that the fate of anthracene, and potentially other aromatic pollutants, in high-salt environments may not be accurately predicted by reaction mechanisms and kinetics from existing literature, which is based almost exclusively on measurements performed in low-salt (seawater concentrations and lower) conditions.