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 model compounds for soil humic substances, were used as cosubstrates, applying phenoloxidases (laccase from Trametes villosa and peroxidase from horseradish) as oxidative biocatalysts. Without the addition of a cosubstrate, only 30% of 4-amino-2,6-dinitrotoluene (4ADNT) and no 2,4,6-trinitrotoluene (TNT) were transformed in the presence of the laccase. Adding various phenolic 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 monomer catechol and the enzyme, up to 100% of 4ADNT and up to 80% of TNT were transformed. Enzymatic transformation of 4ADNT in the presence of catechol reached a maximum at pH 6.8. TNT transformation, however, further increased at pH values above 6.8, even in the absence of the enzyme, due to chemical polymerization of catechol. We postulate a two-step reaction mechanism. The humic monomer is initially oxidized to a semi-quinone radical by a phenoloxidase. Subsequent oxidative coupling involves reactions with additional humic monomers or anilinic products derived from TNT, forming an anilinoquinone via nucleophilic addition or a benzoquinone-imine through condensation.