Lignin-based flocculant and dispersant for wastewater treatment

Abstract

The wastewaters produced from different industries contain fine and charged suspended particles and other impurities. Today, the removal of these colloidal particles from the wastewater has become a serious challenge for industry. Flocculation of the fine particles using polymers followed by settling is a popular technique in industry. Synthetic flocculants have been used in wastewater treatment systems, which are not biodegradable and eco-friendly. Therefore, natural based flocculants have been attracting wide interest of researchers because they are of biodegradable and are environmentally friendly. In this study, kraft lignin derived from black liquor of kraft pulping process, was copolymerized with acrylamide (AM) and (2-methacryloyloxyethyl) trimethyl ammonium chloride (DMC) in an aqueous solution in the presence of as an initiator potassium persulfate (K2S2O8) to produce a water-soluble lignin-based copolymer. The influence of the reaction conditions on the charge density and solubility of resultant lignin copolymers were investigated. The resultant lignin copolymer was characterized by Fourier transform infrared (FTIR) spectrophotometry, nuclear magnetic resonance (HNMR), thermogravimetric analyzer (TGA), molecular weight and elemental analyses. The applications of the resultant copolymer as a flocculant in kaolin and bentonite suspensions were systematically assessed. The flocculation studies allowed for correlating the polymer characteristics, namely the charge density and molecular weight, with its adsorption affinity as well as the zeta potential and relative turbidity of kaolin and bentonite suspensions. This study showed that a highly charged cationic lignin adsorbed more than low charged ones, and an increase in the molecular weight of cationic lignin enhanced its adsorption. Thus, cationic flocculants with higher molecular weights and charge densities were more effective in reducing the turbidity of clay suspensions. One of the important findings of this work was that both polymer bridging and charge neutralization mechanisms facilitated the destabilizing of the colloidal particles to form flocs. An improved reflocculation ability of cationic polymers was observed as the molecular weight and charge density of the polymers increased. The flocculation studies also confirmed that the iii flocculation efficiency of these cationic lignin polymers depended on the adsorbed amount of polymer on kaolin and bentonite particles, but not on the unadsorbed amount present in the suspensions. In this study, the flocs size and structure of cationic lignin in kaolin suspension was determined by a focused beam reflectance measurement (FBRM) and the results were correlated with flocs properties obtained by small-angle laser light scattering technology (SALLS). The results showed that the flocs produced were larger and more porous as the polymer's charge density and molecular weight increased. Also, the flocs strength decreased as the flocs size increased. A strong correlation between the size of flocs and sedimentation behavior of kaolin suspension was established by a vertical scan analyzer. The results demonstrated that the maximum rate of settling increased with the increase in floc size. The effect of solution pH and salt concentration on the dispersant performance of anionic kraft lignin in kaolin suspensions was also studied. The adsorbed anionic kraft lignin on kaolin particles induced electrostatic repulsion between the particles at a more basic pH and thereby improved the dispersibility of suspensions. The results showed that the adsorption of lignin polymers decreased with pH increase, but increased with ionic strength increase. In this study, the mechanism of self-assembly of kraft lignin-based polymers in aqueous solutions was investigated using dynamic light scattering (DLS) and the results were correlated with conformation and viscoelastic properties of the adsorbed polymer layers on particles via Quartz crystal microbalance with dissipation (QCM-D) analysis. The results showed that a higher molecular weight lignin polymer was adsorbed in a greater quantity, and that more mass interacted as the molecular weight increased. The results in this work provided insights into the fundamental understanding of the flocculation, dispersion and self-assembly behavior of kraft lignin-based polymers in various systems. These results can help establish the criteria for selecting and developing kraft lignin based flocculants or dispersants for altered applications. The results of this thesis contributed to knowledge on the chemical modification and characterization of lignin products and to the fundamentals associated with the performance analysis of flocculation and dispersion systems.