A laboratory study was performed to investigate the effects of desorption from soil aggregates on the biomineralization kinetics of a-hexachlorocyclohexane (a-HCH). Desorption and biodegradation of a-HCH in mixed soil suspensions at 20 "C were shown to be controlled by intraparticle mass-transfer processes. Two models were applied to the desorption and biodegradation kinetic data, a first-order model (FOM) and a sorption-retarded radial diffusion model (RDM). Only the RDM could explain the effect of aggregate size on HCH desorption and bioconversion rate. It also yielded the best fit to the desorption kinetic data. Estimated effective diffusivities were in the order of 5 X lo-'' m2/s. Biodegradation kinetics of a-HCH could only be described by the RDM by assuming that microorganisms could penetrate the inner parts of the aggregates.

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Aerobic conditions proved to be best for the microbial conversion of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry. The dry soil contained 400 mg of alpha-HCH per kg. This xenobiotic compound was mineralized within about 18 days at an initial rate of 23 mg/kg of soil per day by the mixed native microbial population of the soil. The only intermediate that was detected during breakdown was pentachlorocyclohexene, which was detected at very small concentrations. Alpha-HCH was also bioconverted under methanogenic conditions. However, a rather long acclimation period (about 30 days) was necessary before degradation started, at a rate of 13 mg/kg of soil per day. Mass balance calculations showed that about 85% of the initial alpha-HCH that was present was converted to monochlorobenzene, 3,5-dichlorophenol, and a trichlorophenol isomer, possibly 2,4,5-trichlorophenol. Under both denitrifying and sulfate-reducing conditions, no significant bioconversion of alpha-HCH was observed. The beta isomer of HCH was recalcitrant at all of the four redox conditions studied. We propose that the specific spatial chloride arrangement of the beta isomer is responsible for its stability. The results reported here with complex soil slurry systems showed that alpha-HCH is, in contrast to the existing data in the literature, best degraded biologically in the presence of oxygen.

The factors identified to be important for the aerobic biodegradation of alpha-hexachlorocyclohexane (alpha-HCH) in a soil slurry are temperature, auxiliary carbon source, substrate concentration, and soil inhomogeneities. Temperatures in the range of 20 to 30°C were determined to be most favorable for biodegradation of alpha-HCH. No alpha-HCH biodegradation was detected at temperatures below 4°C and above 40°C. The addition of auxiliary organic carbon compounds showed repressive effects on alpha-HCH biomineralization. Increased oxygen partial pressures reduced the repressive effects of added auxiliary organic carbon compounds. A linear relationship between alpha-HCH concentration and its conversion rate was found in a Lineweaver-Burk plot. Inhomogeneities such as clumping of alpha-HCH significantly affected its biodegradation. Inhomogeneity as an influence on biodegradation has not drawn sufficient attention in the past, even though it certainly has affected both laboratory studies and the application of biotechnological methods to clean up contaminated sites. On the basis of metabolites detected during degradation experiments, the initial steps of aerobic alpha-HCH bioconversion in a soil slurry are proposed.

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