Lignosulfonates from sulfite spent liquor
Lignosulfonates stem from sulfite pulping processes, which are typically characterized by the pH of the processes and the bases used. The typical pH of the sulfite pulping process ranges from 1 to 5; neutral sulfite semichemical pulping (NSSC) processes are performed at pH 5–7. The conditions of pulping processes impart different properties to lignosulfonates. The most commonly used bases are sodium and calcium, although magnesium or ammonium may also be employed.In the sulfite pulping process, the two main reactions that occur to solubilize lignin are sulfonation and hydrolysis.
Scheme 1 outlines these two reactions in typical sulfite pulping processes under acidic conditions. During this process, the loss of a hydroxyl group or cleavage of the α-ether linkage occurs to form a quinone methide intermediate with either phenolic or nonphenolic substrates via a benzylic cation (hydrolysis). Next, sulfite ions present in solution add to this intermediate structure at the α position to form benzyl sulfonic acid units (sulfonation), which contributes to increasing the solubility of the lignosulfonates.
Alternatively, a condensation reaction may occur between the benzylic carbon of one molecule and the meta-carbon (6th) position of the aromatic ring of another molecule, due to the presence of the benzylic cation; this may prevent the sulfonation reaction because it occurs at the α position.
Scheme 2 outlines the sulfonation reaction occurring during neutral sulfite pulping to produce lignosulfonates.The difference between the mechanisms shown in Figures 1 and 2 is that, under neutral conditions (Scheme 2), hydrolysis occurs with only phenolic-type intermediates, whereas sulfonation occurs due to the electron-withdrawing effect of the first sulfonic group addition.
This sulfonic acid group on the α position facilitates the addition of another sulfonic acid group at the β position because of electron-withdrawing capabilities, resulting in the depolymerization reaction of the β-aryl ether bond.
It should be noted that the methoxy group shown on the aromatic ring structures in Figures 1 and 2 may be different, depending on the subunit of lignin. It is known that there are three main basic subunits of lignin: the p-hydroxyphenylpropene (H) unit (no substituted carbon adjacent to the para-oxy group), the guaiacyl (G) unit (one substituted carbon adjacent to the oxy group), or the syringyl (S) unit (substitution of both carbon atoms adjacent to the oxy group). For simplicity, the G-unit is shown in Figures 1 and 2.
After sulfite pulping, lignosulfonates are removed from pulp through filtration and remain in the spent liquor of the pulping process. In addition to lignosulfonates, the spent liquor of the sulfite pulping process contains hemicelluloses and residual pulping chemicals, wherein lignosulfonates account for 50–80 wt % of the total solids in the mixture, hemicelluloses constitute up to 30 wt % and inorganics represent roughly 10 wt %.
Due to the wide range of conditions under which sulfite pulping can be conducted and, as a result lignosulfonates produced, lignosulfonate structures may vary considerably. However, a full lignosulfonate model structure has also been proposed by Matsushita and may be found elsewhere.
Additional information on lignin and lignosulfonate structures and modification/biological engineering pathways for the purpose of biorefinery valorization has been covered extensively in recent years.
