Toxicity changes during the UV treatment of pentachlorophenol in dilute aqueous solution

Pentachlorophenol (PCP) was photolysed using a 1 kW photochemical reactor. The degradation of PCP (0.15 mM) was carried out using both direct UV photolysis and photolysis in the presence of H2O2 (3.0, 6.7, 18.0 and 37.2 mM). The decay of PCP and the formation of photoproducts were followed by high performance liquid chromatography, ion chromatography and UV absorbance detection. The first-order rate constants k1 for the decay of PCP were estimated under various conditions: for direct UV photolysis, k1 = 0.115 min−1; for photolysis in the presence of various concentrations of H2O2, the rate constant increased to a plateau (k1≈0.7 min−1) at H2O2 concentrations greater than 6 mM. The correlation between photodegradation and toxicity was studied using a bacterial toxicity test and a 96 h Fathead Minnow toxicity test. In both cases, the toxicity decreased as the concentration of PCP or the total organic chlorine concentration fell. These results indicate that the UV treatment of PCP either does not generate significant levels of acutely toxic intermediates, or any toxic intermediates are rapidly degraded under the specific conditions used in these studies.