Interaction between MnO2 and Oxalate: Formation of a Natural and Abiotic Lignin Oxidizing System

This paper reports for the first time the efficient moidification/degradation of in situ lignin by manganese complexes formed by the association of MnO2, accumulated as black deposits during wood decay, and oxalic acid, produced by fungi or resulting from oxidative degradation of cell wall components. The Mn oxidants formed are shown to modify both the chemical and physical properties of the woodcell wall. Microscopic analysis revealed the disruption of ray cell in oxidized wood and the formation of amorphous globular material not yet characterized. Thioacidolysis analysis of the oxidized lignin showed a reduction up to 25-30% in the recovery of ether-linked guaiacyl (G) and syringyl monomers and up to 50-80% of several diarylpropane and phenylcoumarane dimers composing the polymer.Preferential oxidation of G moieties in all structures examined was observed during catalysis. The MnO2/oxalate system also appears to selectively oxidize the lignin macromolecule, as no xylose or glucose loss was observed in all treated samples. The oxidation process catalyzed by MnO2 and oxalate may play an important role in the general pathway of lignin degradation and also in thetransformation of the lignin polymer into humus and/or its precursors.