MECHANISTIC IMPLICATIONS OF THE INTRAHUMIC DECHLORINATION OF MIREX

The hydrophobic chlorocarbon mirex readily binds to aqueous humic acid s (HAs). In HA solutions irradiated at sunlight wavelengths (lambda gr eater than or equal to 290 nm), mirex molecules bound to HAs are trans formed to photomirex by dechlorination. The mechanism of this intrahum ic dechlorination was investigated both in HA solutions and in model s ystems simulating select photochemical capabilities of HAs. In HA solu tion, the reaction was unaffected by methanol and pentanol; was accele rated by hydroxide; and was inhibited by oxygen, 2-chloroethanol, nitr ate, and hydrogen ion, all four of which can scavenge hydrated electro n. Additional experiments probing for involvement of hydrated electron were consistent with it as the reactant. In irradiated N,N-dimethylan iline solution, a model system for generating hydrated electron, mirex was dechlorinated to form photomirex, the same product that is found in HA solution. Relative rate experiments in HA solution, while incons istent with the reaction with hydrated electron in homogeneous solutio n, were consistent with reaction with a purely intrahumic hydrated ele ctron. However, other humic-generated reductants cannot be eliminated as possible reactants. The potential confounding influence of hydropho bic partitioning to HAs on investigations using molecular probes is ev aluated in terms of current data and previous reports.