The weight-average molecular weight (Mw) reported for softwood lignosulfonates are in the range of 10-60 kDa with high polidispersity [23, 60, 61]. These lignins present, in general, higher molecular weight and have fewer hydroxylphenolic (OHph) groups than kraft lignin [20, 23] (as depicted in Table 12.2) which is in accordance with low rate of cleavage of ether linkages of lignin reported for sulfite pulping. The condensation reactions in sulfite pulping and kraft pulping follow the same pattern, leading mainly to Ca-aryl linkages (diphenylmethane structures) [18]. The increase of condensed lignin in spent liquor results from these reactions, and also from the dissolution of condensed lignin moieties present in wood lignin. The frequency of sulfonation is about 0.4-0.5/ppu [62].

Published studies concern mostly softwood lignosulfonates. However, more recently, hardwood lignosulfonates have been the subject of increasing research and interest [55, 61, 63-65] and substantial differences on characteristics, partic­ularly concerning Mw, have been found. Recent results concerning Eucalyptus globulus lignosulfonates from Caima, Industria de Celulose S. A., in Portugal showed an high content of partially sulfonated fragments with rather low molec­ular weight (around 1 kDa) formed via cleavage of b-O-4 bonds [63]. The Mw of Eucalyptus globulus lignosulfonates (1-2.4 kDa) is even lower than the already low value reported for lignosulfonates from other hardwoods (5.7-12 kDa) as compared to typical Mw of softwood lignosulfonates [60, 61]. Some of lignins characteristics are summarized in Table 12.2.