Developing sustainable and bio-based alternatives to petrochemical building blocks for material synthesis is an immediate need for reducing our carbon footprint and for advancing the bioeconomy. The current study aims at developing new uses for lignin via enzymatic treatment to modify its molecular characteristics and functionalities and enable subsequent upcycling. A laccase-ABTS ((2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) mediator system was first optimized, in terms of pH and enzyme loading, to treat three different types of isolated lignins, namely organosolv, alkali and kraft lignin. After optimization, scaled-up laccase treatments were carried out in a 1 L bioreactor, at 2% (w/v) solid loading and a pH of 9.0, for 24 h with constant aeration. Laccase-treated lignins remaining in the solid phase were recovered via centrifugation and characterized for physico-chemical changes. Kraft lignin remained mostly unchanged after laccase treatment potentially due to the impact of its thiol functionalities. In contrast, organosolv and alkali lignins showed 15 – 60% increase in molecular weight and significant changes in chemical signature. However, they exhibited a slightly lowered glass transition temperature likely influenced by the added hygroscopicity. The total hydroxyl groups increased by 35% and 62% in organosolv and alkali lignins, respectively, whereas the condensed inter-unit linkages like β-β’ increased by 3- to 10-fold in all three lignins. Despite these chemical changes, the thermo-tolerance (~ 370 °C) and molecular uniformity (dispersity index ~ 2) did not change for all three lignins. Thus, our results demonstrated that alkali and organosolv lignins are the suitable candidates for laccase mediated structural modification and the resulting enrichment in functional groups may enhance the compatibility and mechanical property of lignin-based engineered bio-composites.

Amending lignin functionalities by laccase enzyme treatment: Impact of lignin isolation method

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