Interaction of 2,4,6-trinitrotoluene (TNT) and 4-amino-2,6-dinitrotoluene with humic monomers in the presence of oxidative enzymes

Oxidative coupling of nitroaromatic compounds involving soil organic matter was examined as a means of soil remediation. Humic monomers, serving as mo del compounds for soil humic substances, were used as cosubstrates, applyin g phenoloxidases (laccase from Trametes villosa and peroxidase from horsera dish) as oxidative biocatalysts, Without the addition of a cosubstrate, onl y 30% of 4-amino-2,6-dinitrotoluene (4ADNT) and no 2,4,6-trinitrotoluene (T NT) were transformed in the presence of the laccase. Adding various phenoli c monomers produced differing effects on the enzyme-mediated transformation , which indicated that xenobiotics are preferentially bound to quinoid and phenolic moieties of soil hUMiC substances. In the presence of the humic mo nomer catechol and the enzyme, up to 100% of 4ADNT and up to 80% of TNT wer e transformed. Enzymatic transformation of 4ADNT in the presence of catecho l reached a maximum at pH 6.8. TNT trans format ion, however, further incre ased at pH values above 6.8, even in the absence of the enzyme, due to chem ical polymerization of catechol. We postulate a two-step reaction mechanism . The humic monomer is initially oxidized to a semi-quinone radical by a ph enoloxidase. Subsequent oxidative coupling involves reactions with addition al humic monomers or anilinic products derived from TNT, forming an anilino quinone via nucleophilic addition or a benzoquinone-imine through condensat ion.