Humic acids as a distinct reactive phase in solution

The aliphatic, chlorinated hydrocarbons mirex and lindane are transformed in Aldrich and Fluka humic acid solutions in photochemical reactions mediated by the humic acids. Mechanistic studies suggest that these reactions are initiated by humic-generated hydrated electrons. Both mirex and lindane react with hydrated electrons produced in solutions of the electron photoejector N,N-dimethylaniline (DMA). However, while the apparent rate constant for the reaction of mirex (k{dollar}sb{lcub}rm M{rcub}spprime){dollar} is {dollar}sim{dollar}1.44 times that of lindane in DMA solution, it is {dollar}sim{dollar}30 times that of lindane in humic acid solution. This change in apparent rate constant reflects the different affinities mirex and lindane have for humic acids: the binding constant of mirex with humic acids is {dollar}{lcub}>{rcub}10sp6{dollar} while lindane has a binding constant {dollar}{lcub}<{rcub}10sp4.{dollar} Only bound mirex appears to react in humic acid solution and k{dollar}sb{lcub}rm M{rcub}spprime{dollar} varies with humic acid concentration in direct proportion to the fraction of mirex that is bound. The rate law describes the reaction of mirex as occurring wholly within humic acid molecules or molecular aggregates. The reaction is limited to within humic acid molecules because hydrated electron apparently is produced and consumed entirely within the aqueous humic acid matrix. The effect of scavenger concentrations on k{dollar}sb{lcub}rm M{rcub}spprime{dollar} is the same as has been found for reactions in micelles and colloids and corroborates the view that the reaction of mirex in humic acid solution is intra-humic. The action spectrum for the reaction of mirex in humic acid solution extrapolated to natural surface waters suggests that mirex should have a half-life in these waters of 6-8 days. These results indicate not only that humic solutions acids can comprise a distinct reaction phase in solution but also that intra-humic reactions can be significant in transforming environmental pollutants.