The degradation of the four most common hexachlorocyclohexane (HCH) isomers in sewage sludge was studied using chiral high-resolution gas chromatography/mass spectrometry. Pure isomers and the technical HCH mixture were incubated with sludge from a communal sewage sludge treatment 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-HCH > delta-HCH > beta-HCH with half-lives between 20 and 178 h for gamma- and beta-HCH, respectively. The rates correlate with the number of axial CI's in an isomer. Degradation in active sewage sludge was predominantly biotic (80-95%), as compared to the slower degradation in sterilized sludge. However, degradation in sterilized sludge was still significantly faster than hydrolysis in water. This enhanced chemical degradation must be due to additional compounds present in sludge and may possibly involve surface-catalyzed reactions. Suspected initial metabolites of HCHs such as tetra- and pentachlorocyclohexenes (TCCHs, PCCHs) and -hexanes were not detectable, presumably because these compounds degrade even faster than they are formed. Despite the faster degradation of(+)-alpha-HCH under anaerobic conditions and its faster degradation in surface waters (North Sea, Baltic Sea, Canadian freshwater lakes) under aerobic conditions, (+)-alpha-HCH is more accumulating in most aquatic biota. General conclusions on the fate of chiral compounds in the environment and on consequences for the monitoring of such compounds using enantio- and nonenantioselective analyses are given.
