BIOAVAILABILITY AND BIODEGRADATION RATE OF DDT BY BACILLUS SP B75 IN THE PRESENCE OF DISSOLVED HUMIC SUBSTANCES

The effects of dissolved humic substances, prepared from a humic volca nic ash soil, on the bioavailability and biodegradation rate of DDT (1 ,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) by Bacillus sp. B75 in t enfold diluted nutrient broth were examined. Dissolved humic acid (DHA ) decreased the biodegradation rate of DDT significantly while the eff ect of dissolved fulvic acid (DFA) was negligible. The initial DDT deg radation rates followed first order kinetics. The higher the DHA conce ntration, the greater the decrease in the degradation rate. The increa se in the apparent solubility of DDT by DHA and DFA suggested that DDT was bound to DHA and DFA. The addition of DHA significantly decreased the amount of DDT sorbed by the bacterial cells whereas in the case o f DFA addition the decrease was negligible. These data suggest that th e rate constants for DDT degradation are correlated with the amount of DDT sorbed to the Bacillus cells which represents the bioavailable DD T.