Hydroxypropyl sulfonated kraft lignin as a coagulant for cationic dye

In this paper, kraft lignin was reacted with 3-chloro-2-hydroxypropansulfonic acid sodium (CHPS) to produce hydroxypropyl sulfonated lignin (SL). The effects of reaction conditions, e.g., the molar ratio of CHPS to lignin, the concentration of NaOH and the reaction temperature and time, on the charge density of resultant SL were investigated. The optimized conditions were found to be 80 °C, 4 h reaction time, 0.05 M NaOH concentration and 5/1 mol/mol of CHPS/lignin, which generated SL with the charge density and molecular weight of −3.33 meq/g and 45,536 g/mol, respectively. The solubility of KL was 9.6 g/L while that of SL was 100 g/L at the neutral pH. The reaction mechanism between kraft lignin and CHPS was fundamentally discussed. The SL was also characterized by elemental analysis and proton nuclear magnetic resonance (1H NMR). The thermal stability and surface activity of the SL were also evaluated and compared with those of commercially produced lignosulfonate (LSS). The efficiency of SL as a coagulant for dye (basic blue 41 and ethyl violet) removal from simulated wastewater was compared with that of LSS. The zeta potential and particle size analyses were conducted to determine the interaction mechanism of dye and lignin derivatives, and the kinetic studies confirmed their fast complexation.