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.