Lignin produced in the kraft pulping process is insoluble in water at neutral pH, which limits its application in industry. In this paper, kraft lignin (KL) was copolymerized with acrylic acid (AA) in an aqueous solution to produce a water soluble lignin-based copolymer. The copolymerization was carried out using K 2 S 2 O 8 -Na 2 S 2 O 3 as the initiator under alkaline aqueous conditions, and the influence of the reaction parameters, i.e. initiator dosage, reaction time and temperature, mole ratio of acrylic acid to lignin and reaction concentration, on resultant lignin copolymers were investigated. The mechanism of copolymerization of kraft lignin with acrylic acid was also discussed in this work. The resultant lignin copolymer was characterized by Fourier Transform Infrared (FTIR) spectrophotometry and Nuclear Magnetic Resonance (NMR). The successful copolymerization of AA and KL was confirmed by the new absorption peak of carboxyl anions in the FTIR spectrum and new peaks in the 1 H-NMR spectrum. At optimal conditions, the charge density and molecular weight of lignin copolymer reached 1.86 meq g −1 and 46 421 g mol −1 , respectively, and the solubility of lignin after reaction was increased from 1.80 g L −1 to 100 g L −1 at neutral pH. † Electronic supplementary information (ESI) available. See

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.