Oxidation of residual lignin with alkaline hydrogen peroxide. Part II: Elimination of chromophoric groups

Isolated residual lignin from art oxygen-bleached kraft pulp was subjected to oxidation with alkaline hydrogen peroxide under different time and tempe rature conditions in both the absence and presence of stabilizing agents an d transition metal ions. The resulting lignin samples were analyzed by UV/V IS and FTIR spectroscopy. The analyses demonstrate the occurrence at a temp erature as low as 50 degreesC: of a rapid but limited elimination of chromo phoric groups, resulting in the formation of carboxyl groups. The presence of DTPA and magnesium ions stabilizes the hydrogen peroxide towards decompo sition and the chromophore elimination is improved. At a higher oxidation t emperature, 80 degreesC, the effect of added stabilizer(s) is particularly pronounced. Addition of transition metal ions resulted in a rapid decomposi tion of hydrogen peroxide and the introduction of new chromophoric groups.