This indicates that bioavailability is the rate limiting step in the sandy soil slurry system. Meanwhile, in the clay slurry system, the phenanthrene concentration in the aqueous phase appeared to be zero, except for a very small amount that was detected at 12 h. This means that the rate of phenanthrene biodegradation is faster than that of desorption during the whole test period. Thus, mass transfer from the solid to the aqueous and micelle phases is the rate limiting step in the clay soil slurry system.