Potentially low-cost nanocellulose based membranes for efficient removal of contaminants from water.
Mbakop, Sandrine
Mbakop, Sandrine
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Abstract
Industrial and urbanisation growth worldwide, have been associated with the increasing demand for freshwater and the release of large quantities of pollutants laden wastewaters into
the environment. This represents a major threat to the environment and the public health. Environmental awareness and existing regulatory measures have induced the growing interest
on greener, cheaper and more effective technologies for the treatment of industrial wastewaters before their discharge into the environment. This project investigated the performance of four novel nanocellulose-based composite membranes in removing dyes and pollutant ions from aqueous solutions. The non-solvent phase inversion precipitation technique was utilized to fabricate the membranes and standard characterisation techniques such as scanning electron microscopy (SEM), equilibrium water content (EWC), surface porosity, water contact angle
(WCA), pure water flux (PWF), membrane hydraulic resistance (MHR), flux recovery ratio (FRR) and Fourier transform infra-red (FTIR) analyses to name a few, were used to evaluate their inherent physicochemical properties. A polyethersulphone (PES) ultrafiltration (UF) membrane reinforced with synthesised cellulose nanocrystal-zinc oxide (CNC/ZnO) nanocomposite was fabricated. The prepared membrane recorded an average PWF of 72 L m- 2 h-1, an EWC of 74.5 % and a WCA value of 55.9o. The adsorptive performance of the composite membrane in removing methylene blue (MB) dye from aqueous solutions was investigated through batch equilibrium, kinetics and filtration experiments. Optimum adsorption at room temperature was achieved at initial solution pH of 7. The equilibrium experimental data fitted well to the Langmuir model with a maximum predicted adsorption capacity of 21.19 mg/g while the kinetics data followed the pseudo second-order model. Another UF membrane made of PES reinforced with pristine CNC nanofiller was also prepared with a coagulation bath made of 50% (V/V) pure water and isopropyl alcohol (IPA). The composite membrane exhibited an average PWF of 51.4 L m-2 h-1, a WCA measurement of 65o and an EWC of 74.5 %. The membrane was used for the adsorptive removal of lead ions (Pb2+) from aqueous solutions. An optimum pH of 7 was observed and the maximum adsorption capacity was predicted to be 21.37 mg/g. The experimental data fitted well to the Langmuir model and the kinetics data followed the pseudo second-order model. The second phase of this study explored the surface charge reversal of the CNC before embedding within the membrane polymeric matrix. Form this phase of the study, two membranes were fabricated. One membrane with pure water as non-solvent coagulation bath and the other membrane with a coagulation bath made of 50% (V/V) pure water and IPA. The former was used for the adsorptive removal of congo red dye from aqueous solutions. A PWF about 64 L m-2 h-1, an EWC of 82.11 % and a WCA value of 54.1o were observed. Optimum removal efficiency was recorded at initial solution pH of 7. The Langmuir isotherm was a better fit to the experimental data obtained with a predicted maximum adsorption capacity of 41.49 mg/g. The kinetic data followed the pseudo second-order model. The latter on the other hand, was explored as a potential membrane adsorbent for the removal of fluoride ions (F-) from aqueous solutions. The composite membrane recorded an optimum removal efficiency at pH 6. An average PWF of 45.5 L m-2 h-1 was observed as well as a WCA value of 59.9o and EWC of 76.8 %. The batch adsorption data collected confirmed the feasibility and spontaneous nature of the process. The monolayer adsorption mechanism was illustrated through the successful application of the Langmuir isotherm model. A maximum adsorption capacity of 26.81 mg/g was predicted. Predominant chemisorption interaction between the adsorbate and the functional groups at the surface of the membrane adsorbent was observed through the successful application of the pseudo second-order model to the kinetics data. In view of the above and considering the availability of sawdust as a biomass waste material; its conversion into CNC for the development of potentially low-cost membrane adsorbents could represent a promising alternative for the efficient treatment of dyes and dissolved pollutants laden effluents. However, in the water treatment field, the water matrix to be treated substantially influences the choice of the sorbent material.
Description
Submitted in partial fulfilment of the requirements for the degree Doctorate of Engineering: Chemical in the Department of Chemical, Metallurgical and Materials Engineering within the Faculty Of Engineering and the Built Environment at the Tshwane University of Technology.
Date
2023-11-01
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Tshwane University of Technology
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Keywords
Membrane, Nanocellulose, Adsorption, Wastewaters, Dyes, Pollutants, Water treatment