Comprehensive lignin balance and new insights into softwood lignosulphonates from neutral sulphite pulping

Accurate material balance of lignosulphonates in sulphite pulping and biorefinery processes is essential for their efficient recovery and utilization. By measuring the absorbances of dissolved lignosulphonates from pulp and spent liquor as lignin phenylpropane units at 280 nm by UV spectroscopy, the challenges caused by varying sulphonic acid group content in the dissolved fractions can be mitigated, ensuring a more accurate balance. This method was validated by compiling comprehensive lignin balances for six softwood neutral sulphite pulps, with yield levels ranging from 84.0 % to 57.7 %. The total lignin balances included lignosulphonates from the spent liquors and those isolated from the pulps by alkaline extraction and enzymatic hydrolysis, representing the distribution of wood lignin across the isolated fractions based on the degree of delignification and lignin alteration. Analyses of dissolved fractions, accounting for 74–95 % of the wood lignin, provided insight into softwood lignin sulphonation and fragmentation during neutral sulphite pulping. The spent liquor lignosulphonates had sulphonic acid group contents of 1.02–1.29 mmol/g and the average molecular weights between 5 200 and 13 400 g/mol, consistent with values reported in literature for hardwood lignosulphonates from NSSC pulping. Lignosulphonates isolated by alkaline extraction exhibited characteristics similar to those of spent liquor lignosulphonates, whereas those isolated by enzymatic hydrolysis were less sulphonated, had a higher average molecular weight, and appeared to be closer to the native type of lignin.