BASE HYDROLYSIS OF ALPHA-HEXACHLOROCYCLOHEXANES AND GAMMA-HEXACHLOROCYCLOHEXANES

Hexachlorocyclohexanes (HCHs) are the most abundant organochlorine pes ticides in the world's oceans and large lakes, and knowing their chemi cal reactivity is important for determining environmental fate. Hydrol ysis of alpha- and gamma-HCHs was carried out in buffered distilled wa ter poisoned with sodium azide in dark, tightly sealed bottles to avoi d biodegradation, photolysis, and volatilization losses. Experiments w ere 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 p seudo-first-order kinetics. Second-order base rate constants (k(b), M- 1 min-1) were calculated from pseudo-first-order rate constants (k', m in-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, activa tion energies of 78.3 and 84.6 kJ/mol were determined for alpha-HCH an d gamma-HCH. At pH 7, reaction with H2O contributed to the breakdown o f 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-d egrees-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 proc esses such as sedimentation and microbial attack be better established .