Reduction of substituted nitrobenzenes in aqueous solutions containing natural organic matter

Natural organic matter (NOM) from a variety of sources has been shown to mediate the reduction of substituted nitrobenzenes in aqueous solution containing hydrogen sulfide. Pseudo-first- order rate constants were proportional to NOM concentrations and increased with increasing pH and decreasing reduction potential (Eh) of the solution. At fixed pH and Eh, the carbon-normalized rate constants (kNOM) of a given compound varied less than 1 order of magnitude among NOMs derived from various natural waters. The effect of substituents on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log kNOM = aEh1'(ArNO₂) + b, where Eh1'(ArNO₂) is the one-electron reduction potential of the nitroaromatic compound. Such LFERs were applied successfully to predict the kNOM values of previously untested compounds. The results of this study suggest that hydroquinone moieties within the NOM may play a pivotal role in the mediation of electron-transfer reactions involving organic pollutants.