Laboratory Level Trials 2211 Delignification

As a major enzyme type secreted by white-rot fungi, laccases have been extensively studied for delignifying pulp. Although the conventional chlorine- or oxygen-based chemical oxidants are very effective, they can cause serious problems in byproducts disposal or cellulose fiber strength loss. Laccases might be applied to oxidatively degrade lignin, so that the use of chemical oxidants could be reduced [39].

Various laccases are able to degrade both natural and synthetic lignin [7, 40-45]. Most of the studies in the field have been carried out on monomeric or oligomeric model compounds (for recent reviews, see [7, 29, 44, 47, 48]). Experimental data suggest that in general, laccase first oxidizes accessible phenolic moieties in lignin to generate phenoxy radicals. From these radicals, quinone formation, cross-linking, or other reactions may take place. In the presence of a redox mediator, the non-phenolic moieties in lignin (particularly the benzylic Ca site) may also be oxidized, leading to other reactions such as peroxygenation, demethoxylation, Ca-CP cleavage, CP-O homolysis, or aromatic ring opening [39, 49-51].

Laccases alone tend to polymerize lignin, by generating cross-linking, O-centered phenoxy radical (Ph-O^). With a mediator, however, laccase catalysis tends to lead to lignin depolymerization, likely due to the action of bond-cleaving C radicals [29, 52, 53]. Delignification of pulp by laccase may be demonstrated by kappa number reduction or improvement for the downstream chemical bleaching.

Mediated laccase systems, particularly those based on N-hydroxy mediators, have made significant strides for delignification application. Satisfactory pulp bleaching could be achieved (for recent reviews, see [7, 47, 48, 52]). However, major hurdles still exist for future commercialization of the technology. Currently, the mediator has to be loaded at ~0.1-1% w/w based on pulp, a level too expensive for the industry [46, 54]. The high loading may also pose disposal problems for spent mediators, whose transformation during pulp bleaching needs further study for potential environmental impact.

It seems that some white-rot fungi employ highly efficient, natural laccase mediators in wood degradation. It is highly desirable to discover novel, natural, or artificial mediators having high reactivity as well as high turnover stability, so that the mediator dosage for laccase-based pulp delignification may be reduced by several orders of magnitude to make this technology economically viable [44, 55, 55a].

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