Chemical characterization of residual lignins from eucalypt paper pulps

Residual lignins were isolated from eucalypt kraft pulps by enzymatic hydrolysis of cellulose followed by purification using a combination of proteinase hydrolysis and solvent extraction. The residual lignins isolated from brown (unbleached) and totally chlorine free (TCF) bleached pulps were analyzed by Fourier-transform infrared (FT-IR) spectroscopy and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and compared with eucalypt milled-wood lignin and kraft lignin. The lignin in eucalypt wood and pulps showed a strong predominance of syringyl (S) over guaiacyl (G) units. The strongest lignin modification was observed in the kraft lignin recovered from the alkaline pulping liquor. The chemical composition of the residual lignin from pulp was closer to milled-wood lignin than to the kraft lignin, indicating that most of the lignin modified during pulping was released to the cooking liquor. During TCF bleaching of pulp, which included oxygen delignification and peroxide stages, modifications of lignin were produced resulting in increased amount of non-conjugated carbonyl groups. We concluded that residual lignin in eucalypt kraft pulp reflects native lignin moieties entrapped into the cellulosic network thus partially avoiding their attack by chemical reagents, although different modifications are produced during the bleaching stages.