This thesis reports on the synthesis and characterisation of various types of oxidised multiwalled carbon nanotubes (O-MWCNTs) modified polymeric membranes. These OMWCNT modified polymeric membranes were then assessed in terms of their remediation
potential, in particular for the removal of estrogenic hormones, dissolved proteins and salts
from brackish water sources. The fabricated O-MWCNT-based polyethersulfone (PES)
membranes were applied as (i) adsorptive membranes, (ii) molecular-sieving membranes
and (iii) as membrane substrates for thin-film composite nanofiltration (NF) membrane
preparation.
The research work commences with the preparation of MWCNTs via a facile catalytic
chemical vapour deposition method and their chemical oxidation with strong acids in order
to introduce hydrophilic carboxylic (–COOH) and hydroxyl (–OH) surface functional group
moieties on the MWCNT outer walls. Intrinsically, MWCNTs are chemically inert and tend
to form agglomerated nanoclusters (due to van der Waals interaction forces), which induce
further difficulties in their homogenous dispersion in polar solvents (such as N-methyl-2-
pyrrolidone and dimethylacetamide) employed to dissolve the polymers in the study. The
introduction of these oxygen-containing moieties was therefore necessary to aid the
dispersion of MWCNTs in organic solvents and for their enhanced interaction with PES and
sulfonated polysulfone (SPSf).
The PES/O-MWCNT ultrafiltration (UF) membranes were produced via a non-solvent
induced phase separation (NIPS) method and employed in the adsorptive removal of natural
hormone estrone (E1). The PES/O-MWCNT UF membranes thus prepared were characterised using SEM, AFM, zeta potential measurements and MWCO experiments. It
was found that the adsorption of E1 initially increased with an increase in O-MWCNT
content followed by a constant decline on further increments. Moreover, the inclusion of OMWCNTs (0.5 wt.%) in the PES membrane matrix resulted in an increase in the maximum
adsorption capacity for E1 compared to pristine PES membrane, i.e., 31.25 mg/g adsorption
capacity was achieved for PES/O-MWCNT compared to 23.81 mg/g for bare PES UF
membrane. Based on the correlation coefficients, the Freundlich isotherm provided a better
fit for the adsorption data and the adsorption kinetics followed the pseudo-second order
kinetic model. Interestingly, after five regeneration cycles, the PES/O-MWCNT membranes
were found to maintain similar adsorption efficiencies. The PES/O-MWCNT membranes
thus prepared, present a viable approach for the removal of natural hormones and other
endocrine disruptors present in water systems compared to the use of common adsorbents
such as activated carbon, which end up generating large amounts of chemical sludge that
require disposal in the environment.
The third part of the study focused on the controlled formation of macrovoid-free
polyethersulfone/sulfonated polysulfone (PES/SPSf) UF membranes with high water
permeabilities, mechanical strength and antifouling properties, in the presence of
O-MWCNTs. To date, the majority of polymeric nanocomposite membranes modified with
O-MWCNTs as nanofillers, generally have finger-like structures and macrovoids in the
membrane sublayer. While the presence of finger-like structures is favoured for the reduction
in mass flow resistance, their presence induces mechanically weak points in the membrane
and reduces the nanocomposite membranes’ mechanical strength properties and long-term
performance stability. As such macrovoid-free PES/SPSf/O-MWCNT membranes were
fabricated via the NIPS techniques, using H2O and polyethylene glycol (PEG 20 kDa) as non-solvent additives. The SEM cross-sectional images showed that a fully sponge-like
morphology of the PES/SPSf membrane can be achieved in the presence of different
loadings of O-MWCNTs. This was attributable to the formation of stronger hydrogen bonds
between the SPSf polymer and non-solvent additives i.e., H2O, PEG 20kDa and OMWCNTs. The combination of the macrovoid-free morphology and homogenous
distribution of high mechanical strength O-MWCNTs in the membrane matrix provided
excellent mechanical strength enhancements for PES/SPSf/O-MWCNT membranes.
Additionally, pure water flux initially increased from 598 L/m2
.h to 713 L/m2
.h followed by
a decline to 578 L/m2
.h upon further increments in O-MWCNT contents, due to
agglomeration of O-MWCNTs at higher loadings. The fabricated PES/SPSf/O-MWCNT
membranes also displayed superior antifouling properties (FRR > 90%) and antibacterial
properties (99% bacterial killing ratio) against E. coli bacteria. The fabricated support fabricfree PES/SPSf/O-MWCNT UF membranes with macrovoid-free sublayer morphologies
displayed attractive features for use as UF membranes in the pre-treatment stages of water
treatment and as support substrates for the preparation of TFC membranes.
In general, sponge-like and macrovoid-free membrane structures are regarded as unfit for
use as support membranes for TFC membrane preparation since they increase the
membrane’s resistance to water flow, thereby reducing the overall TFC membrane
permeability. This assumption has largely been based on sponge-like and macrovoid-free
membranes structures achieved through the use of extremely high polymer concentrations,
particularly using polysulfone (PSf) polymer. Hence, the sponge-like structures formed are
very dense and less porous. Nevertheless, the macrovoid-free PES/SPSf/O-MWCNT
membranes produced in this study, consisted of open cellular network microstructures within
the membrane sublayer, which could be visualised at higher SEM magnifications.
This part of the work therefore investigated the role of hydrophilic, macrovoid-free
PES/SPSf and PES/SPSf/O-MWCNT as support membranes on the performance of TFC NF
membranes. The TFC NF membranes were prepared via an efficient interfacial
polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC). The
deposition of the polyamide thin-film layer was confirmed by ATR-FTIR, SEM, AFM,
contact angle and zeta potential measurements. Membrane performance results showed that
TFC NF membranes fabricated on PES/SPSf/O-MWCNT support membranes displayed a
35% improvement in pure water flux with comparable salt rejections from those prepared
on bare PES/SPSf support membranes. Salt rejection followed the order of Na2SO4 > MgSO4
> NaCl, which is typical for negatively charged NF membranes. It was established that the
presence of hydrophilic O-MWCNTs in the support membrane allowed for the formation of
a thin polyamide layer on the top surface of the support membrane, which gave rise to
enhanced water permeability of the TFC NF membrane and the possibility of polyamide
rejection layer within the support membrane pore channels. To further improve the
performance of the TFC NF membranes, in particular, the monovalent/bivalent salt
selectivity, a mixture of PIP and 2,4-diaminobenzene sulfonic acid (2,4-DABSA) at different
weight ratios was prepared in the aqueous solution and reacted with TMC in the organic
phase solution. It was found that the addition of low monomer weight ratio of 2,4-DABSA
in the amine mixture, lead to the generation of a sulfonated TFC NF membrane with superior
membrane performance in terms of pure water permeability (30.2 L/m2
.h),
monovalent/bivalent salt selectivity (𝛼NaCl/Na2SO4 = 25.0) at low operating pressures
(3 bar) and salt concentrations in the range of brackish waters. This was attributable to the
combined presence of sulfonic acid groups on the membrane surface and the formation of
the thin polyamide layer. Moreover, sulfonated TFC NF membranes exhibited good
antifouling properties against bovine serum albumin (BSA), with FRR of 96.4% after three cycles of fouling and cleaning, with a fairly stable membrane performance over a 10-day
period of pure water flux and Na2SO4 rejection testing. Indeed, the use of a macrovoid-free
PES/SPSf/O-MWCNT support membrane did not only provide the mechanical strength for
the deposition of TFC NF membrane, but also their open, cellular network microstructure,
combined with high hydrophilicity and large surface pore sizes were beneficial in the
reduction of polyamide layer thickness, and subsequently in the enhancement of TFC NF
membrane performance.
The study provided insightful information on lesser known aspects of O-MWCNT
incorporated polymeric membranes, with regards to membrane structural configurations in
relation to the membrane structure-performance relationships. It has been deduced that (i)
the right combination of membrane surface characteristics and adsorbate solution chemistry
is necessary for an open UF membrane to display reasonable removal efficiencies for low
molecular-weight solutes, (ii) the combination of macrovoid-free membrane morphology
with good dispersion of O-MWCNTs in the polymer matrix is necessary to realise significant
enhancements in the mechanical properties of sulfonated membrane and (iii) formation of a
thin sulfonated polyamide layer on top of the hydrophilic PES/SPSf/O-MWCNT support
membrane is necessary to achieve high salt selectivity, and allow for the sulfonated TFC NF
membrane to be operated at low pressures. / College of Engineering, Science and Technology
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:unisa/oai:uir.unisa.ac.za:10500/26700 |
| Date | 07 1900 |
| Creators | Gumbi, Nozipho Nonsikelelo |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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