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 through oxidative coupling reactions mediated by enzymes or metal oxides. The reactions may be influenced by humic constituents, such as syringaldehyde or catechol, that originate from lignin decomposition and are also subject to oxidative coupling. In this study, the effect of humic constituents on xenobiotic transformation was evaluated in vitro based on the determination of unreacted chlorophenols and chloroanilines. In experiments with peroxidase, laccase, and birnessite (delta-MnO2), the transformation of most chlorophenols was considerably enhanced by the addition of syringaldehyde. Less enhancement was observed using 4-hydroxybenzoic acid, and the addition of catechol resulted in a reduction of most transformations. The opposite was observed in experiments with tyrosinase, in which case catechol caused considerable enhancement of chlorophenol transformation. The varying effect of catechol can be explained by different transformation mechanisms involving either o-quinone coupling (with tyrosinase) or free radical coupling (with peroxidase, laccase, or birnessite). Regardless of the agent used to mediate the reactions, chloroanilines seemed to undergo nucleophilic addition to quinone oligomers, which resulted from coupling of the humic constituents. Catechol, which readily forms quinones and quinone oligomers, was most efficient in enhancing these reactions.