Dehalogenation of xenobiotics as a consequence of binding to humic materials

Chlorinated phenols and anilines were transformed by oxidoreductive catalys ts with release of chloride ions in both the absence and the presence of hu mic substances (syringaldehyde, catechol, and humic acid). Dehalogenation o f these xenobiotics resulted from oxidative coupling reactions occurring at the chlorinated sites of the substrates. The effect of humic substances on dehalogenation depended on the mechanism of oxidative coupling. In a free- radical reaction mediated by peroxidase, laccase, or birnessite (delta-MnO2 ), syringaldehyde enhanced the dehalogenation of most of the chlorinated ph enols, but it did not enhance the dehalogenation of the chloroanilines. Wit h catechol, which does not form free radicals, dehalogenation was reduced o r remained the same for both the chlorophenols and the chloroanilines. Howe ver, in tyrosinase-mediated reactions controlled by nucleophilic addition, catechol enhanced the dehalogenation of most of the chlorophenols, whereas syringaldehyde had little effect. Humic acid in most cases enhanced the deh alogenation of the chlorophenols, but it had little effect on the dehalogen ation of the chloroanilines. On a molar basis, changes in dehalogenation ca used by humic substances were proportional to the respective changes in sub strate transformation. Only syringaldehyde was capable of releasing disprop ortionately high amounts of chloride ions from chlorophenols, apparently as a result of multiple cross-couplings to one molecule of the: substrate.