Characterisation of residual lignins in pulps delignified by laccase/N-hydroxyacetanilide

A pine kraft pulp and a two-stage oxygen-delignified pine kraft pulp were delignified with laccase in the presence of N-hydroxyacetanilide (NHA). Laccase/NHA was selective in its reactions with lignin: 30% delignification was obtained under the moderate reaction conditions employed, while the carbohydrates remained unattacked. Laccase/NHA did not react with neutral pulp carbohydrates or with uronic acids. Beside delignification, laccase/NHA caused chemical modification of fiber lignin, lowering the content of free phenolic hydroxyl groups and thus verifying their importance in delignification. Oxidation of the residual lignins was clearly observed as a decrease in methoxyl groups and a simultaneous increase in conjugated carbonyl groups, carboxyl and/or ester groups. Only 50-60% of the pulp residual lignin retained its aromatic structure. Pyrolysis-GC/MS gave important information, not only of the chemical structure of residual lignins, but also of the behavior of the mediator during delignification. Residuals of aniline and acetanilide, i.e. degradation products of NHA, were found among thepyrolysis products of residual lignins. However, only trace amounts of NHA-derived nitrogen-containing products were present in the lignins.