Modified Biopolymer Sorbents for the Uptake of Naphthenic Acid Fraction Components (NAFCs) from Aqueous Solutions

The uncontrolled release of contaminants into the aquatic environment relates to industrial activities such as the extraction of bitumen from oil sands. This is a key challenge facing global water security due to reports on the toxicity of naphthenic acid fraction components (NAFCs). The overall objective of my thesis research concerns the development of biopolymer sorbent materials for the removal of NAFCs from aqueous solution. The research is further divided into various sub-themes as follows: i) synthesis of cellulose and chitosan based biopolymers through cross-linking reaction, ii) modification of cellulose via surface functionalization, iii) development of composite materials based on cellulose and chitosan and iv) sorption of NAFCs using cellulose and chitosan biopolymers. In the first theme (chapters 2 - 4), chitosan (CH) and cellulose (C) were cross-linked with glutaraldehyde (GL) and epichlorohydrin (EP), respectively, using variable cross-linker ratios. The effects of sonication on the cross-linking of cellulose with EP/aqueous ammonia was also studied. The polymer structure and physicochemical properties were characterized via Fourier transform infrared (FTIR) and CP-MAS 13C solids NMR spectroscopy, thermogravimetric analysis (TGA), elemental analysis (CHN), nitrogen adsorption, particle size analysis, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), swelling studies and a dye-based (p-nitrophenol; PNP, and phenolphthalein; phth) sorption method in aqueous solution. The characterization results provided complementary confirmation of the structural modification of the biopolymers as evidenced by the variable morphology and thermal stability of the cross-linked polymers. The adsorption capacity of the cross-linked chitosan and cellulose (CH-GL, C-EP and C-EP sonication/heating) biopolymers with NAFCs and phenolic dyes were greater when compared to the unmodified cellulose and chitosan. Uptake of adsorbates increased with greater cross-linker feed ratio except for C-EP polymers where the polymer with the medium feed ratio exhibited the highest sorption capacity. The CH-GL polymers displayed favorable uptake of model naphthenates as the hydrogen deficiency (z) decreased; whereas, the opposite trend was observed for the C-EP polymers. The molecular selectivity displayed by the CH-GL and C-EP polymers was due to steric hindrance as well as electrostatic repulsion between the negatively charged adsorbates and the negatively charged surface of the polymers. In the second theme (Chapter 5), cellulose was modified via cross-linking with EP and/or surface functionalization with glycidyl trimethyl ammonium chloride (GTAC) to form materials that behave like hydrogels. The successful modification of cellulose was confirmed via characterization by CHN analysis, TGA, and FTIR/13C solids NMR spectroscopy, where enhanced thermal stability and sorption capacity were observed for the hydrogels. Equilibrium sorption studies with naphthenates extracted from oil sands process affected water (OSPW) and a model naphthenic acid compound (2-naphthoxy acetic acid; S6) revealed that the cross-linked/surface functionalized hydrogel (C-EP-G) exhibited greater sorption capacity than the surface functionalized hydrogel. C-EP-G had similar binding affinity for the various OSPW naphthenate species in aqueous solution. Kinetic uptake of S6 at variable temperature, pH and adsorbent dosage showed that an increased temperature and adsorbent dosage favored the sorption process, while pH had negligible effects. Thermodynamic parameters obtained from the kinetic studies revealed that the sorption process was favored by enthalpy-driven electrostatic interactions. In the third theme (chapters 6 – 7), composite materials containing chitosan and cellulose biopolymers were prepared in order to evaluate the effects of cross-linking with glutaraldehyde and introduction of iron (III) species on the adsorption properties of the composites. The composite materials were characterized by various techniques: FTIR spectroscopy, CHN, 13C solid state NMR spectroscopy, powder X-ray diffraction (PXRD), TGA, SEM, equilibrium swelling, nitrogen adsorption and inductively coupled plasma-optical emission spectrophotometry (ICP-OES). The characterization techniques provided supportive evidence of composite formation between the biopolymers including cross-linking with glutaraldehyde. In chapter six, the chitosan-cellulose composite materials (CH-GL-C) displayed greater sorption capacity with phenolic dyes, OSPW and single component naphthenates relative to the CH-GL and C-EP polymers. The sorption results revealed that uptake of the adsorbates increased with cross-linker feed ratio, where the composite with the highest cross-linker ratio (CH-GL3-C) displayed greater uptake selectivity for naphthenates with lower double bond equivalence (DBE) values. Suspected charge screening at alkaline pH values may have attenuated the sorption capacity of the CH-GL-C composites. To overcome the related effects of charge screening observed in chapter 6, quaternary (CH-GL-CC-Fe) and ternary (CH-CC-Fe) composite materials with variable iron (Fe) contents were prepared from chitosan (CH) and carboxymethyl cellulose (CC) (see chapter 7). Equilibrium uptake studies with equimolar mixed and OSPW naphthenates in aqueous solution indicated that CH-GL-CC-Fe surpassed the sorption capacity of CH-CC-Fe and CH-GL3-C. According to high resolution electrospray ionization mass spectrometry (HR ESI-MS) analysis, the quaternary and ternary composite materials displayed little or no selectivity for OSPW naphthenates, irrespective of the double bond equivalence (DBE) or carbon number of the species, contrary to results obtained in chapter 6