3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The synthesis, characterization and performance evaluation of polyphelenediamine- and polypyrrole- clay composites for removal of oxo-anionic wastewater contaminants(2017) Mdlalose, Lindani MbalenhleContamination of water bodies by numerous pollutants is a worldwide problem that endangers the environment and health of human beings, animals and aquatic life. Hexavalent chromium (Cr(VI)) for example is used in different metal products and processes which makes it a common environmental contaminant. Because of its high mobility in aqueous phase, and improper storage or unsafe disposal practices, leakage of Cr(VI) into water streams and ground water is a common occurrence. While Cr(III) is an essential micronutrient, Cr(VI), however is highly toxic posing serious health risks. Additionally, phosphorus is a limiting nutrient for the growth of organisms in most ecosystems, but, excessive discharge of phosphate ions in water systems leads to profuse algal growth, and is detrimental to both the environment and the ecosystem. This research focused on the development of suitable functional adsorbents for the removal of Cr(VI) complexes and phosphate ions from wastewater. Poly(para-phenylene)- (PpPD) and polypyrrole-based composites were synthesized through chemical oxidation polymerization, and investigated for Cr(VI) remediation. Poly(phenylenediamine) isomers were synthesized through different chemical oxidation methods for the uptake of phosphate ions in wastewater. Transition metals modified bentonite clay adsorbents were developed to remove phosphate ions in aqueous solution.The adsorbents were characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), Brunauer-Emmett-Teller (BET), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Thermogravimetric analyzer (TGA) and X-ray photoelectron spectroscopy (XPS) instruments. Adsorption kinetics and isotherm models were investigated. In the first study of this work (paper I), PpPD and PpPD-clay composite were successfully prepared and applied for Cr(VI) removal and reduction in aqueous solution. Characterization by XRD demonstrated that PpPD molecules intercalated into clay galleries. Additionally, PpPD functional groups dominated in the composite even though the signal bands were smaller than the pristine polymer bands indicating that there was a formation of polymeric structure inside the organoclay interlayer spaces. Batch adsorption studies showed that pH, adsorbent dosage, contact time and Cr(VI) concentration affected the degree of adsorption. The Langmuir maximum adsorption capacity for Cr(VI) was 217.4 mg/g and 185.2 mg/g whereas for total Cr it was 193.3 mg/g and 148.8 mg/g for PpPD and PpPD-organoclay, respectively at an optimum pH of 2. Paper II focused on the chemistry of Cr(VI) adsorption by PpPD and adsorbent regeneration. The adsorption mechanism on the material surface was revealed by XPS and FT-IR. Cr(VI) was reduced to Cr(III) which complexed onto the adsorbent surface at the studied pH of 2 and 8. Desorption of the adsorbed Cr was conducted using NaOH (0.05 M) and HCl (0.1 M). The PpPD adsorbent performed optimally for eight cycles and still retained about 80% adsorption efficiency at the 10th cycle using an initial Cr(VI) concentration of 100 mg/L. Treatment with the regenerants showed irreversible oxidation reaction for the adsorbents while still removing Cr(VI) for several cycles. To investigate the toxicological impact on seed germination due to contact with used adsorbents, phytotoxicity test was investigated. Seed germination severely diminished to 35% and 14% (respective to control) in the presence of P-p-PD-MMT and P-p-PD. In paper III polypyrrole-clay composite was synthesized and also proved to be an effective adsorbent for Cr(VI) removal. A percentage Cr(VI) removal of 99% was obtained at pH 2 using adsorbent dosage of 0.15g for 100 mg/L Cr(VI) concentration for 3h in a batch mode. Due to its excellent adsorption properties, the composite was regenerated using different varied concentrations of eluents (NaOH, NH4OH, HCl, NH4Cl and HNO3). Desorption and regeneration using 0.01 M NaOH and 0.5 M HCl gave more regeneration cycles where the first 5 regeneration efficiencies were still greater than 80%. EDX determined the elemental components of the polypyrrole-clay composite before and after Cr(VI) adsorption. It demonstrated a significant decrease of Cl- ions after adsorption which is attributed to ion exchange mechanism between Cl- ions and Cr(VI) during the adsorption process. Investigation of the adsorption behaviour revealed a decrease in thermal stability of the composite after several adsorption cycles while treating the adsorbent with the regenerants as a result of material oxidation and deterioration due to Cr(VI) exposure in acidic medium and the impact of the regenerants. According to FT-IR analysis, polypyrrole-clay bands shifted to a higher wavenumber after Cr(VI) adsorption due to the change in skeletal vibrations as a result of Cr(VI) species adsorbed onto its surface Paper (IV) described synthesized adsorbents for phosphate removal. The study presented the development and performance of two sets of poly(phenyelenediamine) (PPD) isomers synthesized from ammonium persulphate ((NH4)2S2O8) and potassium dichromate (K2Cr2O7) as oxidants. The chemical structure of the adsorbents were determined using FT-IR, TGA and XRD. Amorphous morphology dominated in all the polymers with poly(m-phenylenediamine) PmPD being more amorphous and PpPD was the least. Batch adsorption studies showed improved adsorption capacity for K2Cr2O7 synthesized polymers. K2Cr2O7 oxidant played a major role in providing trivalent chromium metal which improved the phosphate uptake. This is attributed to the Lewis acid-base interaction where trivalent chromium acts as an acid and phosphate ions serve as a base. Batch adsorption results showed that solution pH, contact time and initial concentration influenced phosphate adsorption with the maximum adsorption capacities of 143 mg/g, 217 mg/g and 69.0 mg/l for PoPD, PmPD and and PpPD adsorbents, respectively. Adsorption reached equilibrium at about 300 min at an optimum pH of 2.0. The adsorption isotherms were described by Langmuir isotherm and the kinetic data were described better by pseudo-second order kinetic rate model implying adsorption onto homogeneous surfaces and the mechanism of adsorption was attributed to chemisorption. Desorption was conducted on the meta substituted PPD using NaOH (0.05 M) which displayed effective desorption capacity and exhibited commendable adsorption for re-use. The adsorbent also proved to be selective to phosphate ions at the background of much higher concentrations of sulphate and nitrate anions due to the presence of Coulombic and Lewis-acid-base interactions. In paper (V), remediation of phosphate ions was examined using modified bentonite clay as an adsorbent. Modification was achieved by incorporating Fe, Ni and Co metal salts using precipitation method. Adsorbents were characterized by FT-IR and XRD. The results showed significant amorphosity for metal modified bentonite compared to the parent bentonite. The adsorption capacity for all studied bentonite-based materials increased with increasing initial phosphate concentration and adsorption mechanisms were influenced by the solution pH. The maximum adsorption capacity of 6.57 mg/g, 20.88 mg/g, 29.07 mg/g and 46.95 mg/g were obtained for Bent, Fe-Bent, Ni-Bent and Co-Bent, respectively. The adsorption rate fitted pseudo-second order for all adsorbents. Langmuir isotherm model described the phosphates removal for all adsorbents at an optimum pH of 3.Item Evaluating the wear and corrosion resistance of plasma-sprayed tungsten carbide coatings on aluminium-6082 alloy(2017) MacGregor, Oluwadamilola SolomonThe benefits of Al alloys to industry are significant. For the truck loading application in this study, the use of Al can deliver a greater payload than most other metals, due to its low relative density. However, it has poor tribological properties. This study investigated ways to improve the wear resistance of AA6082, an Al alloy widely used in transport. This was done by studying WC-based coatings to improve the wear resistance of the AA6082 surface. To ensure a sufficient WC-substrate bond, Al powder was used as a binder for the coatings. Although these coatings improved the wear resistance of the AA6082, it was imperative to test and establish that their corrosion resistance was not inferior to that of the AA6082 itself. Differences in hardness and tensile properties of the substrate were evaluated for varying ageing durations. The plasma spray technique was used to coat the AA6082 substrate. The heat input from this coating process on an Al alloys with high thermal conductivity could have lowered the hardness and mechanical properties of the AA6082 substrate. Therefore, hardness tests were carried out on the cross-section of each coated substrate. The hardnesses of all coated AA6082 samples were not lowered by the heat input from the plasma spray process. The coatings were varied to contain 20%, 40%, 60%, 80% and 100% volume of WC admixed with Al-102 powder. The wear resistance of AA6082 was significantly improved as WC content increased. From the wear resistance results, both 60% and 80% WC showed the highest wear resistance. The 60% WC coating, which contained a lower quantity of WC than the 80% WC coating, was the preferred option for truck loading applications because of cost. The 60% WC coating had a lower hardness value than the 80% WC coating, giving the 60% WC coating a higher allowance for work-hardening in service as ductile Al-102 is present in higher quantity. Al and its alloys, with no coating, are known to have good corrosion resistance. It was therefore imperative to evaluate and compare the corrosion resistance of the coated samples with AA6082. The lowest corrosion rate of all coated samples exposed to the neutral and acidic media was the 20% WC coating. Generally, all the coated samples had very low corrosion rates in the neutral solution, which is more applicable to typical truck loading conditions. Therefore, the 60% WC coating was most preferred for wear and hardness tests and also had acceptable corrosion behaviour for the proposed truck loading application.Item Hard, wear resistant Fe-B-C composites produced using spark plasma sintering(2017) Rokebrand, Patrick PierceFe-B-C composites were produced, from boron carbide and iron powders, using spark plasma sintering. This provided information on the effects of rapid sintering on densification, composition and the microstructure of the materials produced. The composition range included a selection high Fe contents (69.3, 78 and 80.9 vol. % Fe-B4C) and high B4C concentrations (1, 3, 5 vol. % Fe-B4C). The properties of the materials were investigated to determine the potential for using relatively cheap Fe and B4C powders to produce hard, wear resistant materials. High Fe-B4C composites were sintered at 900, 1000 and 1100°C at 60 MPa. Densification increased with increasing temperature and at 1100° each composition achieved ≥ 97 % densification. The materials reacted during sintering with the main phases observed being Fe2B and Fe3(B,C) whilst additional phases formed were FeB, C and Fe23(B,C)6.Comparing the phases that were produced to Fe-B-C phase diagrams showed deviations from expected compositions, indicating the non-equilibrium nature of producing the composites using SPS. Although the composites were not at equilibrium, all the B4C reacted and could not be maintained, even with fast heating and cooling rates. The properties of the materials were dependent on both densification and the phases that were present after sintering. Materials containing higher amounts of the Fe2B phase showed higher hardness and fracture toughness results, up to 13.7 GPa and 3.5 MPa.m0.5 respectively for the 69.3 vol. % Fe-B4C. The materials were sensitive to grain and pore growth which negatively affected properties at 1100°C. The transverse rupture strength of 388.3 MPa for 80.9 vol. % Fe-B4C composite was the greatest, and showed evidence of both intergranular and transgranular fracture. The strength was affected by a fine dispersion of porosity at the grain boundaries, throughout the material, and free carbon in the structure was detrimental to the strength of the 69.3 % Fe-B4C. The wear rates were lower using Si3N4 wear balls compared to stainless steel balls, where 69.3 vol. % Fe-B4C showed the best wear rates, 8.9×10-6 mm3/Nm (stainless steel ball) and 1.77×10-6 mm3/Nm (Si3N4 ball), due to the higher Fe2B composition and free carbon acting as a lubricant during sliding. 1, 3 and 5 vol. % Fe-B4C composites were sintered to densities above 97 % of theoretical at 2000°C and 30 MPa. The formation of a transient FeB liquid phase assisted densification. 1 % Fe-B4C attained hardness and fracture toughness up to 33.1 GPa and 5.3 MPa.m0.5 with a strength of 370.5 MPa. Thermal mismatch between the FeB phase and B4C caused high residual stresses at the interface which led to cracking and pull-out of the FeB phase. Residual carbon at the grain boundary interface exacerbated the pull-out effect. Increasing Fe and the subsequent FeB phase had an embrittling effect. The materials suffered severe wear of up to 36.92×10-6 mm3/Nm as a result of the pull-out with the remaining porosity acting as a stress raiser. 20 vol. % of the Fe in each system was substituted with Ti to reduce the presence of residual carbon. Although in some case the properties of the respective compositions improved, residual carbon was still present in the composites.Item The effect of montmorillonite clay on the mechanical properties of kenaf reinforced polypropylene composite(2017) Govinden, SumilanAn investigation was carried out to determine the effect of the addition of clay on the mechanical properties of a Natural Fibre Composite consisting of a polypropylene matrix with kenaf fibre reinforcement. The kenaf fibres were treated using various chemical treatments to improve the strength of the composites manufactured. Four treatments using different 3-mercaptopropyltrimethoxy silane (MPS) concentrations were investigated to determine which treatment resulted in the best mechanical properties. [Abbreviated Abstract. Open document to view full version]Item Electrospun nano-mat strengthened aramid fibre hybrid composites : improved mechanical properties by continuous nanofibres(2016) Jinasena, Isuru Indrajith KosalaAramid fibre reinforced epoxy composites were hybridised by the addition of electrospun PAN (polyacrylonitrile) and ECNF (electrospun carbon nanofibre) doped PAN nanomats. One of the major concerns in polymer composites is the effect of the interlaminar properties on the overall mechanical properties of the composite. Electrospun carbon nanofibres were used as doping agents within PAN nanofibres, and coated in between aramid epoxy laminates to improve the interlaminar properties. PAN nanomats and ECNF doped PAN nanomats were created by the use electrospinning on the surface of aramid fibre sheets. Multiscale hybrid aramid reinforced composites were then fabricated. Mechanical characterization was carried out to determine the effect of PAN and CNF doped PAN nanofibre mats on aramid fibre reinforced epoxy. It was found that PAN reinforced nanomats had improved the mechanical properties and more specifically, when doped by ECNFs, the volume fraction of ECNFs played a vital role. An addition of 1% vol. CNF doped 0.1% vol. PAN reinforcement within a 30% vol. aramid fibre composite (control composite), improved the tensile strength and elastic modulus by 17.3% and 730% respectively. The 0.5% vol. PAN reinforced AFC (aramid fibre composite) specimens revealed a major increase in the flexural strength by 9.67% and 12.1%, when doped by both 0.5% vol. ECNFs and 1% vol. ECNFs respectively. The 0.5% vol. CNF doped reinforcement increased the impact energy by over 40%, for both the 0.1% vol. and 0.2 % vol. PAN reinforced aramid hybrid specimens. The 0.5% vol. CNF doped 0.5% vol. PAN had increased by 30% when compared to a non-doped sample. Morphological studies indicated interlaminar shearing between plies was affected by CNF agglomerations. This was discovered when determining the impact properties of the multiscale doped hybrid composites. Electrospun nanofibres however, assisted in improving the interlaminar regions within aramid epoxy by mechanical locking within the epoxy, and creating an adhesive bond using Van der Waals forces and electrostatic charges between nanofibre and macro fibre. Hybridising aramid epoxy with the use of nanofibres assisted in improving various mechanical properties. Impact degradation was one disadvantage of hybridising using CNF doped PAN nanofibre reinforcements.Item Characterisation of the structural properties of ECNF embedded pan nanomat reinforced glass fiber hybrid composites(2016-10-11) Bradley, PhilipIn this study, hybrid multiscale epoxy composites were developed from woven glass fabrics and PAN nanofibers embedded with short ECNFs (diameters of ~200nm) produced via electrospinning. Unlike VGCNFs or CNTs which are prepared through bottom-up methods, ECNFs were produced through a top-down approach; hence, ECNFs are much more cost-effective than VGCNFs or CNTs. Impact absorption energy, tensile strength, and flexural strength of the hybrid multiscale reinforced GFRP composites were investigated. The control sample was the conventional GFRP composite prepared from the neat epoxy resin. With the increase of ECNFs fiber volume fraction up to 1.0%, the impact absorption energy, tensile strength, and flexural strength increased. The incorporation of ECNFs embedded in the PAN nanofibers resulted in improvements on impact absorption energy, tensile strength, and flexural properties (strength and modulus) of the GFPC. Compared to the PAN reinforced GRPC, the incorporation of 1.0% ECNFs resulted in the improvements of impact absorption energy by roughly 9%, tensile strength by 37% and flexural strength by 29%, respectively. Interfacial debonding of matrix from the fiber was shown to be the dominant mechanism for shear failure of composites without ECNFs. PAN/ECNFs networks acted as microcrack arresters enhancing the composites toughness through the bridging mechanism in matrix rich zones. More energy absorption of the laminate specimens subjected to shear failure was attributed to the fracture and fiber pull out of more ECNFs from the epoxy matrix. This study suggests that, the developed hybrid multiscale ECNF/PAN epoxy composite could replace conventional GRPC as low-cost and high-performance structural composites with improved out of plane as well as in plane mechanical properties. The strengthening/ toughening strategy formulated in this study indicates the feasibility of using the nano-scale reinforcements to further improve the mechanical properties of currently structured high-performance composites in the coming years. In addition, the present study will significantly stimulate the long-term development of high-strength high-toughness bulk structural nanocomposites for broad applications.