Copolymerization of pretreated kraft lignin and acrylic acid to produce flocculants for suspension and solution systems

Abstract

The presence of dyes in the wastewater of textile industry poses a number of environmental concerns due to their toxic nature and the removal of dyes from the wastewater is a major challenge nowadays. Synthetic polymers are commonly used in the flocculation of various colloidal systems. However, these synthetic polymers cause environmental concerns due to their nonbiodegradability and toxicity to human and animals. Their high costs also limit their use in many applications. These disadvantages may be overcome by using polymers derived from natural sources. Lignin is the most abundant renewable resource after cellulose, and is an inexpensive raw material that can be used to produce high-value products, such as flocculants, dispersants, phenols etc. Native lignin does not possess suitable properties to be used as is, however, its attributes can be changed by means of chemical modifications. In this thesis, kraft lignin was initially modified via sulfation, sulfomethylation or phenolation methods and subsequently copolymerized with acrylic acid. The resulting copolymers were characterised using a variety of methods including NMR, FTIR, TGA, molecular weight analysis, and elemental analysis, all of which demonstrated remarkable changes in the chemical and physical structure of lignin after modification. The flocculant property of the produced copolymers was investigated in alumina suspensions via studying a) the adsorption behaviour of copolymers on alumina particles b) changes in the zeta potential of alumina particles, and c) the ability of the copolymers to alter the relative turbidity of alumina colloids. The results demonstrated that the copolymers could indeed act as effective flocculants, with their performance being partly dependent on their charge density, molecular weight and their ability to decrease the zeta potential of the alumina particles. Indeed, the copolymers with the highest charge density and molecular weight adsorbed more dye. This work demonstrated the potential of using widely available, inexpensive, and renewable kraft lignin as a flocculant after modification. It also provided information on the flocculation of ethyl violet dyes from synthetic solutions. The data also provides further information about the mechanism and best ways to chemically modify lignin, as well as how these products function as flocculants.