ISOMER AND ENANTIOSELECTIVE DEGRADATION OF HEXACHLOROCYCLOHEXANE ISOMERS IN SEWAGE-SLUDGE UNDER ANAEROBIC CONDITIONS

The degradation of the four most common hexachlorocyclohexane (HCH) is omers in sewage sludge was studied using chiral high-resolution gas ch romatography/mass spectrometry. Pure isomers and the technical HCH mix ture were incubated with sludge from a communal sewage sludge treatmen t plant. High enantioselectivity was observed for alpha-HCH with the ( +)-enantiomer faster degraded than the (-)-enantiomer. The degradation rates of the different HCHs were in the order of gamma-HCH > alpha-HC H > delta-HCH > beta-HCH with half-lives between 20 and 178 h for gamm a- and beta-HCH, respectively. The rates correlate with the number of axial CI's in an isomer. Degradation in active sewage sludge was predo minantly biotic (80-95%), as compared to the slower degradation in ste rilized sludge. However, degradation in sterilized sludge was still si gnificantly faster than hydrolysis in water. This enhanced chemical de gradation must be due to additional compounds present in sludge and ma y possibly involve surface-catalyzed reactions. Suspected initial meta bolites of HCHs such as tetra- and pentachlorocyclohexenes (TCCHs, PCC Hs) and -hexanes were not detectable, presumably because these compoun ds degrade even faster than they are formed. Despite the faster degrad ation of(+)-alpha-HCH under anaerobic conditions and its faster degrad ation in surface waters (North Sea, Baltic Sea, Canadian freshwater la kes) under aerobic conditions, (+)-alpha-HCH is more accumulating in m ost aquatic biota. General conclusions on the fate of chiral compounds in the environment and on consequences for the monitoring of such com pounds using enantio- and nonenantioselective analyses are given.