Photochemical Degradation of Polynuclear Aromatic Hydrocarbons in Aquatic Systems.

Abstract

The present study examined the photochemical fate of polynuclear aromatic hydrocarbons (PAH) in aqueous systems. The rates of photolysis for naphthalene, anthracene and pyrene were calculated, the reaction mechanisms were investigated and some of the photoproducts were identified. The reaction media were primarily Lake Michigan water (LMW), Lake Michigan microlayer (LMM) and distilled water. The rates observed for naphthalene in distilled water were two times faster than those in LMW and four times faster than in LMM. Similar trends were observed for anthracene and pyrene. Three concentrations (1, 5, 20 ppm total organic carbon, TOC) of humic acid (HA) were added to lake water to simulate higher concentrations of organic carbon found in other natural waters. The association constants for each PAH - humic acid system were calculated and found to increase with increasing size of the PAH and decrease with increasing TOC concentration of the humic acids. The presence of humic acids decreased the photolytic rates for all PAH: The decrease in the rate constant per mg/L TOC is 0.000073 for naphthalene, 0.000037 for anthracene and 0.000110 for pyrene. Calcium carbonate in saturated solution were also found to decrease the photolysis rates of naphthalene and anthracene by 20% and by 50% for pyrene. Algal suspensions were found to enhance the photodegradation of naphthalene twice, but no apparent enhancment was shown by anthracene or pyrene. The photochemical reactions appear to proceed mainly via free radical formation evidenced by quenching of the reaction with tertiary butyl alcohol, an effective free radical quencher. The photoproducts are of oxidation reactions, mainly quinones, esters and hydroperoxides. The information obtained from the data suggests that laboratory experiments in distilled water provide only for direct photolysis and will overestimate photolytic rates for naphthalene, anthracene and pyrene in natural systems. The estimate is based on the overall effect of the constituents added into the system in simulated environmental conditions.