Abiotic reduction reactions of anthropogenic organic chemicals in anaerobic systems: A critical review

This review is predicated upon the need for a detailed process-level understanding of factors influencing the reduction of anthropogenic organic chemicals in natural aquatic systems. In particular, abiotic reductions of anthropogenic organic chemicals are reviewed. The most important reductive reaction is alkyl dehalogenation (replacement of chloride with hydrogen) which occurs in organisms, sediments, sewage sludge, and reduced iron porphyrin model systems. An abiotic mechanism involving a free radical intermediate has been proposed. The abstraction of vicinal dihalides (also termed dehalogenation) is another reduction that may have an abiotic component in natural systems. Reductive dehalogenation of aryl halides has recently been reported and further study of this reaction is needed. Several other degradation reactions of organohalides that occur in anaerobic environments are mentioned, the most important of which is dehydrohalogenation. The reduction of nitro groups to amines has also been thoroughly studied. The reactions can occur abiotically, and are affected by the redox conditions of the experimental system. However, a relationship between nitro-reduction rate and measured redox potential has not been clearly established. Reductive dealkylation of the N- and O-heteroatom of hydrocarbon pollutants has been observed but not investigated in detail. Azo compounds can be reduced to their hydrazo derivatives and a thorough study of this reaction indicates that it can be caused by extracellular electron transfer agents. Quinone-hydroquinone couples are important reactive groups in humic materials and similar structures in resazurin and indigo carmine make them useful as models for environmental redox conditions. The interconversion of sulfones, sulfoxides, and sulfides is a redox process and is implicated in the degradation of several pesticides though the reactions need more study. Two reductive heterocyclic cleavage reactions are also mentioned. Finally, several difficulties (both semantic and experimental) that recur in the studies reviewed are discussed. The subtle effects of various sterilization techniques on extracellular biochemicals and complex chemical reducing agents in sediment have stifled attempts to separate abiotic from biological degradation reactions. The characterization of redox conditions in a natural system is still problematic since measured redox potential is not adequate. Suggestions for future research toward a process-level understanding of abiotic chemical reductions are made.