BASE HYDROLYSIS OF ALPHA-HEXACHLOROCYCLOHEXANES AND GAMMA-HEXACHLOROCYCLOHEXANES

Hexachlorocyclohexanes (HCHs) are the most abundant organochlorine pesticides in the world's oceans and large lakes, and knowing their chemical reactivity is important for determining environmental fate. Hydrolysis of alpha- and gamma-HCHs was carried out in buffered distilled water poisoned with sodium azide in dark, tightly sealed bottles to avoid biodegradation, photolysis, and volatilization losses. Experiments were run at (a) constant temperature (45-degrees-C) and PH 7-9 and (b) constant pH (9) and 5-45-degrees-C over times of 4-248 d, depending on reaction conditions. At constant pH, breakdown of the HCHs followed pseudo-first-order kinetics. Second-order base rate constants (k(b), M-1 min-1) were calculated from pseudo-first-order rate constants (k', min-1), the measured pH, and the ion product of water as a function of temperature. At 20-degrees-C, values of k(b) were 1.57 (alpha-HCH) and 1.10 (gamma-HCH). From the variation in k(b) with temperature, activation energies of 78.3 and 84.6 kJ/mol were determined for alpha-HCH and gamma-HCH. At pH 7, reaction with H2O contributed to the breakdown of the HCHs; estimates of the neutral rate constant (k(n), min-1) were 1.1 X 10(-6) (alpha-HCH) and 2.0 x 10(-6) (gamma-HCH). At pH 8 and 5-degrees-C, hydrolytic half-lives of alpha-HCH and gamma-HCH were 26 and 42 yr. The relative contribution of hydrolysis to removing HCHs from cold, deep water in oceans and lakes requires that rates of other processes such as sedimentation and microbial attack be better established.