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Whether reductive dehalogenation proceeds via a one-or a two-electron mechanism has been suggested to affect product distributions, hence potentially influencing the success of engineered treatment systems. In this work, we explore vicinal dibromide stereoisomers as "probes" of the concertedness of electron transfer in reduction by aqueous suspensions of iron and zinc metal. Dibromides consisted of 2,3-dibromopentane (diBP) stereoisomers and (()-1,2-dibromo-1,2-diphenylethane. All dibromides reacted with metals to give the same E:Z ratio of olefins observed during dehalogenation by iodide (a two-electron reductant). Reduction by Cr(II) (a one-electron reductant) yielded distinctly different proportions of E and Z olefins. Although this might be construed as evidence that metals function as two-electron reductants, high stereospecificity was also obtained for reduction of diBPs by Fe(II) adsorbed to goethite, a presumed one-electron reductant; this can be explained by two single-electron transfers in rapid succession, facilitated by the locally elevated concentration of reducing equivalents at the oxide-water interface. The results suggest that reduction of alkyl halides by metals is not likely to produce free radicals that persist long enough to undergo radical-radical coupling or hydrogen-atom abstraction from minor dissolved constituents. Apparent free-radical coupling products are more likely to result from (possibly surface-bound) organometallic intermediates.

Chlorpyrifos-methyl is widely used in the control of insects on certain stored grain, including wheat, barley, oats, rice, and sorghum. The reactions of chlorpyrifos-methyl with hydrogensulfide/bisulfide (H2S/HS-), polysulfides (Sn(2-)), thiophenolate (PhS-), and thiosulfate (S2O3(2-)) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. The resulting data indicate that chlorpyrifos-methyl undergoes a S(N)2 reaction with the reduced sulfur species. The transformation products indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of a methoxy group to form the desmethyl chlorpyrifos-methyl. The formation of trichloropyridinol, a minor degradation product, could be attributed entirelyto hydrolysis. The reaction of chlorpyrifos-methyl with thiophenolate leads to the formation of the corresponding methylated sulfur compound. The resulting pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate constant of 2.2 (+/- 0.1) x 10(-3) M(-1) s(-1). The determined second-order rate constants show that the reaction of chlorpyrifos-methyl with HS- is of the same order of magnitude as the reaction of chlorpyrifos-methyl with S2O3(2-) with a second-order rate constant of 1.0 (+/- 0.1) x 10(-3) M(-1) s(-1). The second-order rate constant for chlorpyrifos-methyl with polysulfides (3.1 (+/- 0.3) x 10(-2) M(-1) s(-1)) is of the same order of magnitude as the one with thiophenolate (2.1 (+/- 0.2) x 10(-2) M(-1) s(-1)). The second-order rate constant for the reaction of polysulfides is approximately 1 order of magnitude greater than that for the reaction with HS-. When the determined second-order rate constants are multiplied by the concentration of HS-, polysulfides and thiosulfate reported in salt marshes and porewaters, predicted half-lives show that the inorganic reduced sulfur species present at environmentally relevant concentrations may represent an important sink for phosphorothionate triesters in coastal marine environments.