| A series of p-substituted nitrobenzenes have been shown to undergo abiotic reduction to the corresponding anilines in several anaerobic sediment samples. Pseudo-first-order disappearance rate constants are reported for 1,4-dinitrobenzene, 4-nitroacetophenone, 4-nitrobenzonitrile, 4-chloronitrobenzene, 4-bromonitrobenzene, nitrobenzene, 4-nitroanisole and 4 nitrotoluene. These compounds were selected to evaluate electronic effects on the kinetics of nitroaromatic reduction, and to provide insight into the reaction mechanism and the nature of the sediment-associated reducing agent(s). The disappearance rate constants were correlated with the Hammett sigma constants. The slopes of the linear free energy plots for samples from different sources—river sediment, pond sediment, and aquifer material—were similar. Temperature and sterility studies support the abiotic nature of these reactions. Nitrobenzene was reduced to yield aniline as a stable anaerobic product. Phenylhydroxylamine and nitrosobenzene were reactive intermediates. Aniline was oxidized rapidly when the sediment-water mixture was exposed to air. The disappearance rate constant for nitrobenzene correlated best with the organic carbon content of the solid phase. Results of studies in heterogeneous systems suggest that the reducing agent(s) are associated with the solid phase.; A quantum mechanical approach, described in this paper, has been developed that will use the calculated half-wavepotential to estimate the homogeneous and heterogeneous reaction rate constants within a homologous series. This approach will also allow kinetic estimations for compounds that are not well handled by the Hammett equation. The free energy of solvation is then used to calculate the Henry's Law constants for nitroaromatics and halocarbons. The results of this study are encouraging. The kinetic rate constants matched the literature valued within one order of magnitude. |
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