Abstract
South African municipalities are faced with a crisis of receiving heavy metal and salt
contaminated industrial wastewater from large scale manufacturing industries. This study
fabricated a polyethersulfone (PES) membrane infused with acid-functionalised carbon
nanotubes (CNTs) and dip-coated with a polyvinyl alcohol (PVA) layer for the effective
treatment of industrial wastewater as its primary objective, providing an alternative solution to
the ineffective conventional methods used in the aforementioned industries. As described and
enshrined in this study, the primary objective was achieved in two parts; a) by investigating the
effect of carbon nanotube (CNT) loading on the physiochemical properties and performance of
a synthesised PES/CNT membrane, and b) by investigating the effect of a coated PVA layer of
the physiochemical properties and performance of a synthesised PES/CNT/PVA membrane.
In part A of this study, pristine CNTs were characterised using Transmission Electron
Microscopy (TEM) and X-ray Diffractometer (XRD) which confirmed the CNTs to be multiwalled
carbon nanotubes (MWCNTs). Particle size distribution (PSD) studies showed the
average particle size of the CNTs to be 216.90 nm. Fourier-Transform Infrared Spectroscopy
(FTIR) and XRD studies were conducted to confirm the successful functionalisation of the
CNTs with H2SO4:HNO3 and showed the successful attachment of hydrophilic functional
groups on the surface of the CNTs. The acid functionalised CNTs were blended into PES in
loadings of 2.5 wt%, 5 wt%, 7.5 wt% and 10 wt% to fabricate PC2.5, PC5, PC7.5, PC10
membranes respectively using non-solvent induced phase inversion method. Scanning Electron
Microscopy (SEM) morphological studies revealed the formation of porous and asymmetric
membrane structures with a thin dense skin layer and a porous finger-like bulk sub-layer
containing macro voids which increased with increasing CNT loading. Brunauer-Emmett-
Teller (BET) studies revealed that the average membrane pore sizes generally increased with
increasing CNT loading. The evaluation of tensile properties revealed that the mechanical
strength of the membranes decreased with the addition of CNTs. Thermogravimetric analysis
(TGA) showed that the thermal stability of the fabricated PC2.5, PC5, PC7.5, PC10 membranes
increased with the addition of CNTs, with a maximum increase of 8.70 % achieved at 10 wt%
CNT loading. Contact angle studies were conducted using the sessile drop method and it was
found that the degree of wettability of the membranes decreased with the addition of CNTs,
with a maximum decrease of 15.83 % achieved at 2.5 wt% loading indicating improvement in
hydrophilicity. All fabricated membranes were evaluated at 1 bar, 3 bar, 5 bar, 7 bar, 9 bar and
vii
11 bar with industrial wastewater using a dead-end stirred cell, it was found that the PC5
membrane achieved remarkable trade-off results with a flux of 43.65 L/m2h and rejection of
89.58 % Cu2+, 100 % Fe2+, 90.47 % Ni2+, 68.75 % Zn2+ and 99.99 % Cl- at a transmembrane
pressure of 3 bar.
In part B of this study, the PES/CNT membranes were dip-coated in a 1 wt% PVA solution to
significantly improve the membrane properties and performance, thus fabricating PC2.5P,
PC5P, PC7.5P, PC10P membranes. SEM morphology studies revealed the successful formation
of a coated PVA layer. TGA studies revealed that the PVA layer increased the membrane
thermal stability, with the highest increase of 19.57 % achieved by the PC10P membrane.
Contact angle measurements were conducted using the sessile drop method and it was found
that the degree of wettability of the membranes significantly decreased with the coating of the
PVA film, with a maximum decrease of 57.42 % achieved by the PC10P membrane, thus
indicating a significant increase in hydrophilicity. Performance evaluation studies for the
PES/CNT/PVA membranes were also conducted using industrial wastewater in a dead-end
stirred cell at transmembrane pressures of 1 bar, 3 bar, 5 bar, 7 bar, 9 bar and 11 bar.
Considering physiochemical properties and performance evaluation, the PC7.5P membrane was
found to be the optimum membrane at 5 bar with adequate trade off balance; rejecting 100 %
of Zn2+, 99.98 % of Cl-, 96.15 % of Fe2+, 89.06 % of Ni2+ and 88.56 % of Cu2+ with a flux of
7.14 L/m2h. The anti-fouling property of the optimum PC7.5P membrane as evaluated using the
cross-flow filtration rig at a pressure of 5 bar by measuring the flux recovery ratio (FRR), which
was found to be 70.77% at 180 min. SEM results showed that the PC7.5P membrane suffered
mechanical and structural fracturing at 240 min during the FRR evaluation as a result of the
relatively low tensile properties.
This study demonstrated that the addition of CNTs at various loadings alters the membrane
morphology and while the CNTs also occupy the internal membrane pores so as to increase the
surface area of adsorption, directly influencing the rejection capacity of the membrane. This
study also observed that increasing CNT loading increased the membrane surface pore sizes,
thermal stability and membrane hydrophilicity. Notably, the presence of hydrophilic groups on
the surface of the acid-functionalised CNTs and in the coated PVA layer significantly improved
the rejection capacity. The study effectively demonstrated that a PES/CNT/PVA membrane
can effectively remove Cu2+, Fe2+, Ni2+, Zn2+ and Cl- from industrial wastewater and endure
low fouling.