Sulfonation of Hydroxymethylated Lignin and Its Application

To valorize kraft lignin, hydroxymethylation and sulfonation were applied on this under-utilized, but vastly available, material. The hydroxymethylation pretreatment was conducted as means to improve the reactivity of lignin. The sulfonation of hydroxymethylated kraft lignin was investigated by sulfuric acid and sodium sulfite treatments under various conditions. The modified lignin samples were characterized for their charge density, solubility, elemental components, and molecular weights. The results showed that the hydroxymethylation facilitated the sulfonation by sodium sulfite, yielding a product (SSH-lignin) with the charge density of -1.2 meq/g and water solubility of 10 g/L under the conditions of Ns2SO3/lignin molar ratio of 0.49 mol/mol, 95°C and 3 h. On the other hand, hydroxymethylation hindered the sulfonation of lignin by sulfuric acid, yielding a product (SAH-lignin) with the charge density of -0.46 meq/g and solubility of 0.9 g/L under the conditions of H2SO4/lignin molar ratio of 14.8 mol/mol, 80 °C and 1 h. The SSH-lignin had a high adsorption on kaolinite (17 mg/g) at a lignin concentration of 40 g/L. The sulfonated lignins were also found to be effective coagulant for the cationic dye ethyl violet.