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