Structural changes in lignin during the deinking of old newsprint with laccase-violuric acid system

Old newsprint (ONP) could be deinked with laccase-violuric acid system (LVS). To study the mechanism of LUS deinking, lignins from the control and LVS-deinked pulp were isolated and characterized by chemical, spectroscopic and chromatographic methods. The molecular weight change of lignin was tracked by gel permeation chromatogram (GPC), and structural changes in lignin extracted from the control and LVS-deinked pulps were characterized by elemental analysis, functional groups measurement, FT-IR, H-1 NMR and H-1-C-13 correlation 2D NMR. The experimental results showed that the molecular weight of lignin from LUS-deinked pulp was lower than that of lignin from the control pulp. The contents of free phenolic hydroxyl groups and methoxyl groups of lignin from LUS-deinked pulp decreased, while the contents of carboxyl and carbonyl groups increased. Guaiacyl units in lignin were oxidized by LUS. Stilbene and 5-5' substructures in lignin were stable to LUS treatment, while beta-O-4 and beta-beta substructures were partially destroyed during the LUS deinking. According to structural changes in lignin, the mechanism of LUS deinking was deduced. (c) 2006 Elsevier Inc. All rights reserved.