CHROMOPHORE CHANGES DURING OXYGEN DELIGNIFICATION OF A RADIATA PINE KRAFT PULP

Changes to the chromophores, or light-absorbing groups, during oxygen delignification were investigated using isolated spent liquor lignins. Radiata pine kraft and kraft-oxygen spent liquor lignins were isolate d and their absorption spectra compared both before and after chemical treatments. The changes in the absorption spectra of kraft lignin aft er oxygen delignification were associated with removal and formation o f chromophores. Removal of stilbenes accounted for a major portion of the change in the ultraviolet region on oxygen delignification. Eviden ce was also seen in the spectra for removal of phenolic guaiacyl units and formation of new phenolic cx-carbonyl units. Elimination and form ation of quinones and products derived from them may also occur during oxygen delignification and contribute to the changes in the absorptio n spectra. Much of the absorbance of the kraft and kraft-oxygen lignin s in the visible region of the spectrum was removed by reduction with borohydride or dithionite, suggesting the colour of both these lignins was due to carbonyl-containing chromophores, including, but not restr icted to quinones.