THE EFFECT OF METAL-COMPLEXES IN THE ELECTROCHEMICALLY MEDIATED OXYGEN BLEACHING OF WOOD PULP

The electrochemical behavior of transition-metal complexes is correlat ed with their effect in promoting electrochemically mediated oxygen bl eaching of unbleached kraft softwood pulp. Ions of Fe, Mn, Co, Cr, Cu, and V with various ligands were examined in oxygen bleaching experime nts at pH 9.4, 11.2, and 14. Only ferricyanide, iron/2,2'-dipyridyl, a nd ferrate exhibited significant synergistic effects with oxygen. Chem icals with reduction potential lower than that of the ferrocyanide cou ple at the pH of reaction, or those without reversible redox behavior, showed no synergistic effect. A mechanism is proposed in which the re action of lignin with oxygen is initiated by oxidation of lignin with the higher valent species of the metal redox couple The metal redox co uple may also catalyze peroxide decomposition via Fenton chemistry to generate the hydroxyl radical and thus promote degradation of cellulos e.