Electrochemical oxidation of transition metal-based mediators for pulp delignification

In order to depolymerize and remove the lignin that remains in paper pulp following the pulping process, the pulp must be bleached. Environmental considerations and system closure requirements have created a need for replacement of the traditionally used chlorine bleaching technologies. Mediated laccase oxidation of residual lignin in pulp has gained in popularity during the last 10 years and could be considered as a good alternative bleaching method. The mechanisms leading to mediated lignin oxidation are still unclear. To clarify the role of the mediator during mediated kraft pulp delignification and to increase the efficiency of the system, we have studied the oxidation of K4Mo(CN)(8) (MoCN) mediator in an electrolysis cell prior to its reaction with pulp. The electrochemical oxidation of MoCN gave similar delignification levels to enzymatic oxidation under similar conditions, while the iron-based mediators gave better delignification than enzymatic oxidation. The electrodelignification setup allowed us to study the effect of pH and oxygen on the lignin degradation. We also report on the use and efficiency of high formal potential mediators that are suspected to be good lignin oxidants but which are not readily oxidized by the laccase. By studying the charge transferred to the solution to achieve mediator oxidation we were able to show that the mediator acts as catalyst, i.e., is being continuously regenerated throughout the process, to perform efficient pulp delignification. (C) 2001 The Electrochemical Society.