Enzymatic polymerization of sodium lignosulfonates: effect of catalysts, initial molecular weight, and mediators

The aim of this study was to investigate the effect of different parameters on the enzymatic polymerization of sodium lignosulfonates (SLS) by laccase, compared with the chemical treatment by manganese III. Different initial molecular weights of SLS (commercial SLS (17 800 Da), F-1 (4300 Da), F-2 (2500 Da), and F-3 (2300 Da)) were tested. Size exclusion chromatography (SEC-UV), Fourier transform infrared (FT-IR) and phenolic group determination showed that SLS molecular weight increases depending on the laccase origin, the enzyme, and the substrate concentrations and the initial molecular weight of the SLS fractions. The highest molecular weight (M-w) was obtained by fungal laccases, specifically when using laccase from Trametes versicolor, while no reactivity was observed by plant laccase (laccase from Rhus vernicifera). The largest increase of M-w (108 600 Da) is reached when using SLS (17 800 Da) at 50 g/L and 30 U/mL of laccase from Trametes versicolor. The laccase polymerization of SLS can be improved by the use of a mediator. In this study, 5 mediators were studied for F-1 polymerization by laccase from Trametes versicolor: acetosyringone (ASG), violuric acid (VLA), 1-hydroxy-benzotriazole (HBT), acetovanillone (ACV) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). Results of F-1 polymerization with mediators showed that only ASG and VLA lead to a higher molecular weight (7500 Da) compared with reactions carried without a mediator (6600 Da).