The direct and sensitized photo-oxidation of hexachlorocyclopentadiene

Hexachlorocyclopentadiene (C5Cl6) is an intermediate in the production of cyclodiene pesticides and flame retardants. To test the treatability in a chemical production waste stream, hexachlorocyclopentadiene was tested as a substrate for direct photolysis and for sensitized photo-oxidation in this laboratory study. The initial hexachlorocyclopentadiene concentration was 100 mg/l in aqueous buffers. Direct photolysis experiments were at pH values 5, 7.4, 9.2, and 11. Riboflavin and methylene blue were used as sensitizers. Samples in batch reactors were exposed to Durotest Vitalite® lights in the laboratory and the hexachlorocyclopentadiene concentration decreased in all samples. Based on first order kinetic rates for the photodegradation of hexachlorocyclopentadiene, riboflavin at 5 mg/l was the best removal treatment, with an average photodegradative half-life of 47 min after light exposure in the laboratory compared to a 100 min half-life for direct photolysis after light exposure in the laboratory. This corresponds to quantum yields of 0.76 and 0.49, respectively. Four photoproduct peaks were identified by GC/MS analysis of methylene chloride extracts. The major photoproduct in the direct photolysis samples and in the riboflavin sensitized samples was hexachlorocyclopentenone (C5Cl6O). The major photoproduct in the methylene blue sensitized samples was a compound containing C4Cl4, either tetrachlorobutadiene (C4H2Cl4) or tetrachlorobutyne (C4Cl4).