Cationic Cellulose Nanocrystals for the Flocculation of Mature Fine Oil Sands Tailings

Abstract

As the volume of oil sands tailings continues to increase each year with a concurrent need for better flocculants to replace the inorganic electrolytes and synthetic polymers, natural polymers are being considered as good substitutes. Cellulose nanocrystals (CNCs) possess interesting properties such as high specific area, biodegradability, surface functionalization capabilities and their large scale availability. This thesis explores the development of CNC-based flocculants for bentonite clay removal. Cationic and thermo-responsive CNC-based copolymers were prepared via the free radical polymerization technique. The thermo-responsive characterization of the CNC-gpoly( oligo(ethylene glycol) methyl ether methacrylate) (CNC-POEGMA) indicated that the LCST could be tuned by adjusting the poly(2-methacryloyloxyethyl) trimethyl ammonium chloride (PDMC) content. The cloud point measurements revealed that the LCST of CNC-POEGMA decreased in the presence of salt following a typical salting-out effect. Additionally, CNCPOEGMA- PDMC displayed two salt-responsiveness behaviors depending on the DMC content. CNC-POEGMA-PDMC with low DMC ratio displayed salting-in effect at low salt concentrations due to the extension of adsorbed PDMC caused by the screening of negative charges on CNCs by sodium cations. On the other hand, higher salt concentrations led to salting-out effect due to the strong ion-pair formation that removed the hydration shell around PDMC along with the polarization of water around POEGMA. The flocculation results of CNC-POEGMA-PDMC revealed that the thermo-responsive property could facilitate the formation of compact flocs at low dosage. The use of CNC-PDMCs cationic copolymers synthesized via the Activators ReGenerated by Electron Transfer polymerization (ARGET-ATRP) as flocculants for bentonite removal was also investigated. CNC-PDMC with different chain length demonstrated pH and salt-responsiveness. The hydration and conformation of the CNC-PDMC were more sensitive in the presence of salt due to the higher hydrophobicity of the ion-pair formed between the chloride ions and the quaternary ammonium groups. The flocculation results indicated that CNC-PDMCs had better performance compared to inorganic electrolytes at low dosage. The flocculation performance of the CNC-PDMCs and inorganic electrolytes after 5 min settling followed the Hofmeister series, and the flocculation behavior after 24 h settling agreed with DLVO theory. Three cationic-biopolymers were synthesized: CNC-g-PDMC, PDADMAC-coated-CNC and PDADMAC-coated-CNF. The CNC-g-PDMC prepared via the ARGET-ATRP possessed a branched-cationic structure while the other two polymers prepared via physical adsorption had a flat-cationic structure. The effect of cationic charge distribution of three cationic-biopolymer based flocculants on bentonite removal was evaluated. The flocculation results indicated that the cationic charge distribution affected the performance of the flocculants. The extended-cationic brushes of CNC-PDMC possessed better approachability toward bentonite particles compared to the coatedcationic layer of PDADMAC-CNC and PDADMAC-CNF. These findings are in agreement with the Singh’s model on the dependence of flocculation efficiency on the conformation of the flocculants. The microscopic images showed that the flocs formed with CNC-PDMC were more compact compared to flocs formed with the PDADMAC-CNC and PDADMAC-CNF. The effect of brush sequence, charge density and salt concentration on the thermo-responsive behavior of cationic and thermos-responsive CNC-based copolymers was studied. The LCST of CNC-PMEO2MA was found to be 24.5 °C, and after grafting with two different ratios of PDMC brushes on PMEO2MA brushes, the LCST could be tuned by adjusting the PDMC content. The CNC-PMEO2MA-PDMC with a relatively low PDMC content exhibited a broad phase transition and the LCST increased to 28.5 °C. However, the CNC-PMEO2MA-PDMC with higher PDMC content, resulted in a sharp transition behavior similar to the CNC-PMEO2MA and the LCST was 25 °C. The observed trend was due to steric hindrance of the polymer brushes. The higher the grafting density yielded brushes with lower hydration and the stronger hydrophobic interactions between the polymethacrylate backbone of the chains being densely grafted on the CNC substrate. For the copolymers with the opposite sequence, CNC-PDMC-PMEO2MA, the LCST was very close to CNC-PMEO2MA indicating that the presence of PDMC brushes on the inner layer had no effect on its thermo-responsive property. The addition of salt tuned the LCST of the CNCPMEO2MA leading to a typical salting-out effect as the chloride ions polarized the water molecules around the polar groups of PMEO2MA. On the other hand, the CNC-PMEO2MAPDMC and CNC-PDMC-PMEO2MA displayed similar salt-responsiveness behavior; displaying a salting-out effect at low salt concentrations and salting-in effect at high salt concentrations.