Dispersion and flocculation affinity of anionically charged kraft lignin

Abstract

Softwood kraft lignin is a biodegradable, renewable, and environmentally friendly natural polymer. Although kraft lignin is widely produced in Canada, it is predominantly underutilized as fuel because it is only soluble in alkaline pH that limits its application as a value-added product. The main objective of this MSc study was to enhance the water solubility of softwood kraft lignin to be used as dispersants and flocculants. In one set of experiments, softwood kraft lignin was modified through a sulfonation reaction with sodium sulfite to produce water soluble sulfonated lignin with an anionic charge density. Alternatively, the kraft lignin was oxidized via hydrogen peroxide or nitric acid to improve its water solubility and charge density. The results confirmed that the modifications generated water soluble products with anionic charge densities. The modified lignin samples were also characterized via FTIR, TGA, DSC, GPC and elemental analyses. Furthermore, the performance of the modified lignin samples was studied and compared with a commercial lignosulfonate as a flocculant and dispersant. The results confirmed that the adsorption of modified lignin samples was higher than that of commercial lignosulfonate on alumina, bentonite, and kaolin. The oxidized lignin with nitric acid exhibited a greater adsorption capacity than other modified lignin samples on all types of particles studied. Generally, the modified lignin reduced the zeta potential of suspensions and the change was more pronounced for the oxidized lignin via nitric acid treatment. The dispersion studies confirmed that the modified lignin with nitric acid was more effective than other samples, but its flocculation performance was not as good. Furthermore, the efficiency of the lignin samples in removing basic blue and ethyl violet dyes from solutions were studied, and the results confirmed that the lignin samples modified with nitric acid had a higher efficiency than other samples. The higher charge density of the lignin sample modified with nitric acid (2.6 meq/g) was claimed to be the main factor for its better performance; whereas the lower charge density of commercial lignosulfonate (1.4 meq/g) was its main drawback for its application as a flocculant.