Incorporation of halogenated substances into humic material

Abstract

Halogenated phenols and anilines are chemical analogues of aromatic compounds that naturally occur in soil and that, likewise, may be incorporated into humus. The incorporation may result from an oxidative coupling reaction mediated by enzymes or abiotic catalysts. Experiments with model humic substances such as syringic acid indicated that covalent bonds may be formed during oxidative coupling of the halogenated compounds to humus. In experiments with natural humic acids, the occurrence of covalent binding was confirmed by the use of ¹³C NMR for the detection of the bound ¹³C-2,4-dichlorophenol. It was found that the release of ¹⁴C-labeled chlorophenols from their complexes with humic acid by microbial activity was very limited (maximum 2.2% of initially bound radioactivity) and unlikely to cause any delayed pollution problem. In addition it was also determined that the covalent binding of chlorophenols to humic acid is accompanied by an extensive release of chloride ions (up to 41.4% of chlorine initially attached to the benzene ring). At the present time, it is not yet known if the chloride ions released in terrestrial systems could be of environmental concern.