Topochemistry of delignification in Douglas-fir wood with soda, soda-anthraquinone and karft pulping as determined by SEM-EDXA

Abstract

Delignification studies on 0.5 μm sections of Douglas-fir earlywood tracheids pulped by soda, soda-anthraquinone (soda/AQ) and kraft pulping processes were performed by determining bromine concentrations in various morphological regions with SEM-EDXA technique. Soda/AQ pulping was much more selective in removing lignin from the middle lamella regions than either soda or kraft pulping. However, up to 50% delignification, more lignin was removed from the secondary wall by soda or kraft, compared to soda/AQ pulping. The kinetics of lignin removal in the various morphological regions were established. Addition of AQ and sodium sulfide resulted in an earlier transition from a slow initial to a rapid bulk delignification, particularly in the middle lamella, and in an enhanced bulk delignification in the secondary wall. Anthraquinone was also found to promote residual delignification in the secondary wall, where sodium sulfide was not effective. The opposite was observed for the bulk delignification in the middle lamella, where only sodium sulfide addition improved the rate significantly. The great differences observed in the bulk delignification rates between middle lamella and secondary wall in soda pulping as well as their response to additives suggest structural differences between middle lamella and secondary wall lignins.