Effect of humic constituents on the transformation of chlorinated phenols and anilines in the presence of oxidoreductive enzymes or birnessite

Chlorinated phenols and anilines are transformed and detoxified in soil thr ough oxidative coupling reactions mediated by enzymes or metal oxides. The reactions may be influenced by humic constituents, such as syringaldehyde o r catechol, that originate from lignin decomposition and are also subject t o oxidative coupling. In this study, the effect of humic constituents on xe nobiotic transformation was evaluated in vitro based on the determination o f unreacted chlorophenols and chloroanilines. In experiments with peroxidas e, laccase, and birnessite (delta-MnO2), the transformation of most chlorop henols was considerably enhanced by the addition of syringaldehyde. Less en hancement was observed using 4-hydroxybenzoic acid, and the addition of cat echol resulted in a reduction of most transformations. The opposite was obs erved in experiments with tyrosinase, in which case catechol caused conside rable enhancement of chlorophenol transformation. The varying effect of cat echol can be explained by different transformation mechanisms involving eit her o-quinone coupling (with tyrosinase) or free radical coupling (with per oxidase, laccase, or birnessite). Regardless of the agent used to mediate t he reactions, chloroanilines seemed to undergo nucleophilic addition to qui none oligomers, which resulted from coupling of the humic constituents. Cat echol, which readily forms quinones and quinone oligomers, was most efficie nt in enhancing these reactions.