Oxidative coupling of Kraft lignin mediated with hypervalent iodine reagent (III) for enhanced removal of dye in water

One of the most attractive routes to valorize lignin is to prepare lignin-based materials which can be used as high-value products, such as dye absorbents. Various processes have been investigated to improve the properties of lignin. In this study, a hypervalent iodine reagent (III) was employed to achieve oxidative coupling of Kraft lignin (KL). The structural changes were investigated by FTIR, quantitative P-31 NMR, 2D-HSQC, and GPC. Oxidative coupling between lignin aromatic C(sp(2))-H bonds and aromatic C(sp(2))-H functionalization with aliphatic hydroxyl group occurred via inter- or intramolecular modes, with resultant decreases of the aromatic protons and hydroxyl groups. In addition, selective modification of aliphatic OH groups in beta-O-4' linkages occurred leading to the formation of aromatic carbonyl groups. Increases in molecular weights with the appearance of conjugated C=O groups were also observed. The oxidative crosslinked lignin (KL-2.0) exhibited better adsorptive features for brilliant green (251.2 mg/g) than that of unmodified KL (59.4 mg/g). Their adsorption kinetics were investigated by pseudo-first-order, pseudo-second-order and mixed-order models. Furthermore, the Langmuir and Freundlich models were employed to study the adsorption isotherms.