Sulfonation of Phenolated Kraft Lignin to Produce Water Soluble Products

Kraft lignin is water insoluble and has limited end-use applications. To produce water soluble lignin-based products, the modification of softwood kraft lignin through phenolation followed by sulfonation of sulfuric acid or sodium sulfite treatment were investigated in this work. Fourier transform infrared (FTIR) spectrophotometer, nuclear magnetic resonance (NMR), and thermo-gravimetric analysis were also considered for characterizing the lignin-based products. The results showed that phenolation pretreatment was effective to generate sulfonated lignin (SAP-lignin) by sulfuric acid treatment with a high charge density (3.12 meq/g) and solubility, which is due to the addition of sulfonation sites on the phenolic ring. However, sodium sulfite treatment of phenolated lignin generated sulfonated lignin (SSP-lignin) with the charge density of 1.20 meq/g due to hindered sulfonation by occupation of reactive α-position. SAP-lignin was soluble across the tested pH range of 1–13, but SSP became insoluble at a pH lower than 3. Thermogravimetric analysis revealed that phenolation pretreatment reduced the thermal resistance of modified lignin when compared to kraft lignin, while SAP-lignin exhibited the highest thermal resistance due to condensation under sulfuric acid treatment. SAP- and SSP-lignin were successfully used as a coagulant for dye removal from simulated solutions as they could remove 72.1 and 90.4% of ethyl violet from a simulated dye solution, respectively.