Climate change, water stress, and rapid population growth have increased the need
to manage water resources through innovative sustainable technologies. Decentralized
systems such as membrane treatment trains have become increasingly important to provide
high volumes of potable water to millions of people. Pressure-driven membrane systems
have dominated separation processes due to their low cost, small footprint, ease of
operation, and high permeate quality. Conventionally, pressure-driven membranes are
classified into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse
osmosis (RO). MF and UF membranes operate under low pressure (< 7 bar, <~100 psi).
They can separate a variety of large particles such as bacteria, natural organic matter,
suspended solids, and colloids. In contrast, NF and RO membranes are more energy-intense
due to operating at high pressures (7 – 80 bar, ~100 – 1200 psi) and can remove small
molecules such as ions, pharmaceuticals, and heavy metals. Fouling is a primary challenge
with membranes that compromises the membrane performance, increases energy
consumption, and reduces the membrane lifetime. Many strategies are used to address
fouling, such as pre-treatment (pH adjustment, screening, coagulation), membrane
modification (chemical and morphological properties), and membrane cleaning (physical,
chemical). However, such strategies increase operational expenditures, produce waste
products that can impact the environment, and negatively impact membrane lifetimes.
Recently, electrically conductive membranes (ECMs) have been introduced to
address the challenges with traditional membranes. They contain conductive surfaces that
offer self-cleaning and antifouling properties across the surface in response to electrical potential externally applied to them. ECMs are advantageous as compared to traditional
membranes because (a) they are more effective in treating foulants as they target foulants
at the membrane/solvent interface, (b) they are more economical and environmentally
friendly as they reduce the need for chemical consumption, and (c) they can be responsive
to fouling conditions as their antifouling mechanisms can be easily manipulated by
changing the applied current type (positive, negative, direct current, alternating current) to
match the foulant.
ECMs have been formed from all categories of membranes (MF, UF, NF, MD, FO,
and RO) with a range of applications. Despite the remarkable progress in demonstrating
their excellent antifouling performance, there are many hurdles to overcome before they
can be commercialized. Two of these are (a) a fundamental understanding of their
underlying mechanisms, (b) surface materials that can withstand extreme chemical and
electrical conditions. In this work, we have comprehensively discussed antifouling
mechanisms with respect to surface polarization and elaborated on the impact of
electrically-induced mechanisms on four major fouling categories. i.e., biofouling, organic
fouling, mineral scaling, and oil wetting. In addition, we characterized surface properties
of a common electrically conductive composite membrane composed of carbon nanotubes
(CNTs) and polyvinyl alcohol (PVA). We then investigated the impact of cross-linkers in
CNT/PVA network on transmembrane flux, electrical conductivity, hydrophilicity, and
surface roughness. In addition, we proposed standard, practical, and straightforward
methodologies to detect and quantify the electrochemical, physical, and mechanical
stability of ECMs, using chronoamperometry and cyclic voltammetry, an evaluation of polymer leaching from membranes, and micro mechanical scratch testing, respectively. Our
methods can readily be extended to all membrane-based separation processes and different
membrane materials (carbonaceous materials, ceramics, metal-based, and polymers).
To demonstrate the antifouling properties of ECMs, we challenged ECMs with
mixed-bacterial cultures in a flow-through system. Although ECMs showed high rejection,
comparable flux, and excellent self-cleaning performance under application of electrical
potential, understanding the relationship between applied electrical currents and antifouling
mechanisms demands a well-controlled investigation. To this end, we quantified the impact
of electrochemically-induced acidic conditions, alkaline conditions, and H2O2
concentration on model bacteria, Escherichia Coli. We first quantified the electrochemical
potential of CNT-based ECMs in generating stressors such as protons, hydroxyl ions, and
H2O2, under a range of applied electrical currents (± 0-150 mA). Next, these individual
stressors with identical magnitude were imposed on E. Coli cells and biofilms in batch and
flow-through systems, respectively. This thesis guides researchers to understand the
underlying antifouling mechanisms associated with ECMs, how to match the mechanisms
to the application of ECMs, and offers benchmarks for making practical ECMs. / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/26707 |
| Date | January 2021 |
| Creators | Halali, Mohamad Amin |
| Contributors | de Lannoy, Charles-François, Chemical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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