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.

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.