Lignin uptake by cellulose fibres from aqueous solutions

The dependence of lignin sorption by bleached kraft pulp fibres from aqueous Solution on pH and ionic strength was studied by direct measurements of sorbed amounts (kappa number), polyelectrolyte titration, spectrophotometry, confocal laser scanning microscopy and atomic force microscopy. It was found that dissolved lignin is imbibed into fibres from true lignin solutions and precipitated front coagulated lignin dispersions. In the former case the sorbed amount of lignin increased linearly with lignin concentration. Adding calcium and aluminium to the solutions induces precipitation of lignin floes onto the fibres as long as the negative charge on lignin is neutralised, Maximum uptake occurs when the flocs are more or less neutral and then decreases rapidly. In alkaline Solutions, calcium ions were more efficient than aluminium while in neutral solutions aluminium had a much greater effect. Most of the sorbed lignin bonds to fibres loosely and can be easily removed by washing with water. Only lignin-aluminium complexes formed at pH 7 were strongly attached to the fibres. Only in this case was sheet strength significantly affected by lignin sorption. In handsheets trapped and dried lignin is mostly located as lavers between the fibres. Retained lignin impeded paper wetting. and aluminium-lignin rendered sheet hydrophobic. The attachment of lignin to fibres is mainly governed by imbibition into the fibres with solution and floes retention in fibre mat due to filtration effect. Both the sorption studies and measurements of interaction forces by AFM indicate that the net van der Waals' interactions between lignin and fibres in aqueous dispersion are apparently very week and do not overcome the electrostatic repulsion between lignin and fibres, even when negative charge on lignin is very small.