Lignin reprecipitation on ethanol pulp fibres determined with AFM and XPS

AFM and XPS were used to further investigate the precipitation of dissolved lignin onto fibres during the pulping and washing steps of ethanol (organosolve) pulping of wheat straw pulp. Results have previously shown that during ethanol pulping lignin particles precipitate back onto the surface of fibre in both unwashed and washed pulp, while increasing pulp washing reduces lignin precipitation. Current results indicate that the majority of lignin precipitates are globular, ranging from a few nanometers to about 2.0 micrometers. AFM images showed that the fibre surface is effectively covered with lignin, as no microfibrils could be seen. XPS data revealed that under the conditions studied, the unwashed pulp fibre surface is approximately 92.5% covered by lignin, whereas the washed pulp fibre surface is approximately 65% covered by lignin. Both AFM and XPS reveal that more lignin is present on the fibre surface than has been previously observed in SEM studies.