Formation of aryl-chlorinated aromatic acids and precursors for chloroform in chlorination of humic acid

Abstract

The formation of chloroform when humic substances are chlorinated is well known. Other chlorinated products that may be formed are chloral, di- and trichloroacetic acid, chlorinated C-4 diacids, and α-chlorinated aliphatic acids. Several of these compounds are formed in molar yields comparable to chloroform. The mechanism for the formation of these products is still largely unknown, due to the complex structure of humic material. Humic materials are geopolymers formed from lignin, carbohydrates, proteins, and fatty acids by microbial degradation and enzymatic or autooxidative coupling reactions. Although hummic materials are not well defined organic compounds, several structures have been proposed which contain moieties that may be converted into chloroform by chlorine in aqueous medium. For example, 1, 3-dihydroxybenzenes, 1, 3-diketo compounds, natural acids such as citric acid, and compounds with activated C-H bonds such as indoles or methylketones can form chloroform in high yield on chlorination in aqueaous medium. For humic substances 1 3-dihydroxybenzenes appear to be likely chloroform precursor candidates as suggested by the fact that 3, 5- dihydroxybenzoic acid is formed in the degradation of humic material with CuSO₄-NaOH at 175 to 180° C. However, the products of KOH fusion may be of no diagnostic value for the structure of humic substances. In recent KMnO₄ degradation studies of humic and fulvic acids no 1, 3- dihydroxybenzene structures were detected, probably because of complete oxidation of these structures. Although the possibility of l, 3-dihydroxybenzene structures as the precursor fragments for chloroform remains to be proven, Rook proposed a mechanism based on the chemistry of the reaction between chlorine and resorcinol. The identification of reaction intermediates is necessary to achieve a better understanding of this mechanism and to assist in the identification of the structural fragments in humic material that are converted into chloroform and chlorinated acids. In this study we describe (1) the identification and structural assignment of such intermediates in the reaction between terrestrial humic acid and chlorine in aqueous medium at pH 7.2, and (2) the attempts to demonstrate the presence of 1, 3-dihydroxybenzene structures in humic acids by means of Curiepoint pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS) and nuclear magnetic resonance (NMR) before and after the chlorination reaction.