Flocculation performance of lignin-acrylamide derivatives

Abstract

Wastewater produced in different industries contains fine suspended particles that raise a number of environmental concerns due to their potential toxicity and physicochemical properties and make their removal from industrial wastewater a challenging process. Synthetic polymers are commonly used as flocculants in colloidal systems due to their effectiveness at low dosages and their inertness to pH changes. However, in addition to their high costs, these synthetic polymers are toxic and nonbiodegradable. These disadvantages of synthetic flocculants can be overcome by using flocculants derived from natural sources. Lignin, after cellulose, is the most abundant biopolymer; it's an inexpensive raw material, and readily available as a by-product of pulping processes. Kraft lignin does not possess suitable properties to be used as a flocculant, however, lignin attributes can be changed by means of chemical modification. In this thesis, kraft lignin-acrylamide polymer was produced and then hydrolyzed to a product with properties that favor its use as a flocculant. The flocculant performance of hydrolyzed kraft lignin-acrylamide polymer was compared with analyzed kraft lignin-acrylamide and polyacrylamide polymers synthesized under the same conditions, in an alumina suspension. Alternatively, kraft lignin was modified via carboxymethylation and oxidation, and subsequently polymerized with acrylamide monomer to produce an effective flocculant for alumina suspension. The properties of the resulting polymers were characterized using different methods including TGA, DSC, elemental analysis, molecular weight analysis, and NMR. All of these methods illustrated remarkable changes in the chemical and physical properties of kraft lignin after modification. The flocculant properties of the produced polymers were investigated in alumina suspension by studying a) the adsorption behavior of polymer on alumina particles, b) the effect of modified lignin on the zeta potential of alumina suspension, c) the affinity of the modified polymers to alter the relative turbidity of alumina colloids. The flocculation of the lignin polymers in alumina suspension was also analyzed by an FBRM. The results obtained demonstrated that modified lignin could indeed act as an effective flocculant in an alumina suspension with its performance being mainly dependent on charge density, molecular weight, and affinity to decrease the zeta potential of the alumina suspension. Modified lignin with a higher charge density showed high adsorption on alumina particles and significantly reduced the relative turbidity of the suspension. This work demonstrates the potential of using renewable and inexpensive material, i.e., kraft lignin, in the production of environmentally friendly flocculants.