Laccase/mediator bleaching of kraft pulps: Identification of mediator active species and their role in oxidation of residual lignin subunits

The bleaching of kraft pulps by laccase and mediators was first reported in 1992. Since the initial report with the mediator ABTS, there has been intense research activity to discover a cost-effective laccase/mediator combination, and several nitrogen aromatics are now known to be at least as effective as APTS. Using cyclic voltammetry, we have identified the redox features of APTS and HBT mediator intermediates required for oxidation of the various lignin subunits found in kraft pulp. With APTS, the radical cation was found to oxidize phenolic model compounds and some components of soluble kraft lignin. Nonphenolic lignin models were only oxidized by the dication (redox potential around 900 mV). HBT was found to oxidize nonphenolic beta-O-4 and beta-1 structures, initially to C-alpha carbonyls. The resulting structures are susceptible to alkaline degradation during oxidative extraction of pulp. Thus a picture emerges of laccase/mediator action during delignification: initial attack of phenolic subunits, followed by slower oxidation of non-phenolic units to give structures susceptible to alkaline degradation.