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Kraft lignin is currently under-utilized since it is primarily used as fuel in pulping processes. However, it can be used as a raw material to prepare a functional polymer such as flocculant. A cationic lignin polymer was synthesized via graft polymerization of lignin and [2- (methacryloyloxy)­ethyl] trimethylammonium chloride (METAC) in an acidic environment (i.e., a heterogeneous system). The reaction, which was optimized under the conditions of pH 4.0, METAC/lignin ratio of 1.8 mol/mol, 3 h, 80 °C, 0.3 mol/L of lignin concentration, and 1.5 wt % of initiator dosage, generated a lignin-graft-PMETAC polymer having 2.93 mequiv/g charge density and 1.53 × 106 g/mol molecular weight. The lignin-graft-PMETAC polymer can be dissolved in water (at 10 g/L concentration) and a pH range of 0.5–13. The flocculation performance of the polymer was evaluated in a 0.25 wt % kaolin suspension using a particle dispersion analyzer, and the results demonstrated the superiority of the lignin-graft-PMETAC polymer to unmodified lignin and PMETAC. These results confirmed that lignin-graft-PMETAC polymer could replace PolyMETAC (i.e., PMETAC, a synthetic polymer) as a flocculant, which promotes the use of sustainable products in industry. This paper introduces a new process for inducing a cationic lignin-based flocculant.

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Lignin is an underutilized byproduct of pulping and cellulosic ethanol production plants. However, if utilized efficiently, it can facilitate the development of sustainable processes. In this work, oxidized kraft lignin (OKL) was prepared via treating kraft lignin (KL) with hydrogen peroxide, an environmentally friendly and industrially attractive oxidizing agent, under alkaline conditions. The oxidized kraft lignin with a carboxylate group content of 1.53 mequiv/g was obtained under the optimal oxidation conditions of 80 °C, 2 h treatment, at a 0.77 molar ratio of NaOH/H2O2, and 2.85 molar ratio of H2O2/lignin, which was then employed as an anionic dispersant for kaolin suspensions. The zeta potential, particle size, and specific surface area as well as the relative turbidity and flocculation index of the kaolin suspension were affected by the pH of the suspension. By increasing the dosage of OKL to 40 mg/L, the relative turbidity of the suspension was increased to 1.18 at pH 5 and the kaolin concentration of 4 g/L, which made its performance superior to that of commercially produced lignosulfonate.