Operational characteristics of membranes.
Membrane processes which combine the physical separation through filtration and pollutant degradation or antibacterial properties achieved by photocatalysis in a single unit are gaining popularity as wastewater treatment alternatives. There has been considerable progress in the development of photocatalytic membranes through incorporation of metal-oxide photocatalysts to enhance the performance of the membranes. An optimum amount of the photocatalyst should be incorporated into the membrane in order to realise reasonable photocatalytic activity with minimal consequences on water flux. Besides TiO2 loading, membrane performance is also affected by light intensity and irradiation time. This chapter highlights some of the recent progresses in photocatalytic membrane fabrication, reactor configuration and membrane application in disinfection and pollutant removal from wastewater.
- titanium dioxide
- photocatalytic membrane
- membrane fouling
- water treatment
The past few decades have seen membrane-based technologies gaining global popularity largely due to the high separation efficiencies, relatively low costs, small footprint and ease of operation. A membrane is a physical porous barrier between two phases that allows substances to be selectively transported through it. The efficiency and effectiveness of a membrane as a separation barrier depends on a number of factors, including the type of membrane, the pore structure and sizes, polarity, roughness, hydrophilicity as well as its mechanical properties .
In the water sector, membranes have found applications as physical barriers to specific size ranges of water contaminants. They act as filters or selective sieves, removing contaminants that are larger than the membrane pore size and allowing smaller contaminants and water molecules to pass through . However, the use of membranes is without drawbacks. Membrane separation efficiency is compromised mainly due to two effects: concentration polarisation and membrane fouling. Concentration polarisation is a result of increase in concentration of the rejected suspended or dissolved solids near the membrane surface. Membrane fouling is a result of irreversible deposition of suspended or dissolved solids on the external membrane surface or membrane pores compromising the overall performance of the membrane. Once fouled, complex and often expensive cleaning procedures that affect the continuous operation of the membrane filtration process, such as forward and reverse flushing, backwashing, air scouring and back permeation, need to be performed. Although membrane systems may incur higher capital or operational costs than conventional processes (such as evaporation, use of activated carbon, deep-bed filtration, coagulation, sedimentation or chemical treatment), they are generally able to achieve superior quality water, while imposing a smaller footprint at the plant level. Fouling remediation is currently receiving much research attention in an effort to mitigate these challenges through modification of traditional membranes to improve their antifouling properties. Recently, a whole new field of study focused on membrane modification has emerged. One such innovation is the modification of membrane characteristics with nanomaterials to fine-tune performance for specific pollutant types or improve resistance to fouling.
2. Classification of membranes
Membrane technology has developed to a point where there are a variety of membranes tailored for specific contaminants and even different membrane configurations and technologies designed for specific industries such as milk production, beer production, desalination, solvent separation and material regeneration. Depending on their pore sizes, which determine their selectivity, membranes can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes  (Table 1). The so-called nanoporous membranes consist of a film in which molecules are transported through solution-diffusion mechanism. Transport through such membranes is driven by pressure, concentration or potential gradient across the membrane. Nanoporous membranes are typically used for processes such as reverse osmosis (RO) and, more recently, forward osmosis (FO). Nanofiltration (NF) membranes have very small pores (0.1–1 nm). Microporous membranes are used for other pressure-driven processes such as microfiltration (MF) and ultrafiltration (UF). MF membranes can separate particles between 0.1 and 10 μm, whereas UF membranes can remove particles of size range 1–100 nm [1,4].
|Microfiltration (MF)||0.1 to −10 μm||Suspended solids,
|Ultrafiltration (UF)||0.001–0.1 μm||Colloids, proteins,
|1–5 bar (cross-flow)
0.2–0.3 bar (dead-end
|Nanofiltration (NF)||0.0001–0.001 μm||Viruses, natural
(including water hardness)
|Reverse osmosis (RO)||<0.0001 μm||Almost all
3. Photocatalytic membranes
Materials in which one or more phases with nanoscale dimensions are embedded in another matrix phase are called nanocomposites. The motive behind fabricating such materials is to create synergies between the various phase constituents to result in smart materials capable of meeting or even exceeding the intended design expectations . Membranes have been functionalised through incorporation of engineered nanoparticles deposited on their surfaces or embedded into their matrices to result in tailored properties such as capability to bind specific contaminants or catalyse degradation reactions. The resulting mixed matrix membranes (MMMs) play a multifunctional role in water treatment. Variables such as the nanomaterial loading, the matrix material, degree of particle dispersion, size, shape and orientation of the nanoscale phase as well as interactions between the phases have a bearing on the overall properties of the hybrid material. In photocatalytic membranes, nanoscale inorganic photocatalysts are embedded in a membrane matrix to enhance the properties of the resultant polymer.
The first attempts at combining photocatalysis with membrane technology were aimed at achieving the separation and reuse of the photocatalyst nanoparticles from the reactor slurry. However, the approach faced the usual problems of membrane fouling, photocatalyst deactivation and nanoparticle agglomeration. This stirred interest in the development of stand-alone hybrid photocatalysis/filtration water treatment processes exhibiting both photocatalytic and separation efficiency. The advantages of such innovations include the absence of any requirement for post- or pre-treatment stage, an attractive feature for upscaling the technology in membrane wastewater treatment applications .
Among various photocatalysts, TiO2 has been proven to be an attractive and promising semiconductor catalyst in heterogeneous photocatalysis and in advanced oxidation processes owing to its stability, low cost, availability, non-toxicity, unique photocatalytic efficiency and its promise for applications in water and wastewater treatment . Several methods have recently been developed and optimised for the fabrication of TiO2-based photocatalytic membranes. Among them are dip-coating or spin-coating of porous supports using TiO2 precursor sols, filtration of TiO2 nanofibers through glass filters followed by hot pressing or liquid phase pressurisation, hydrothermal growth of free-standing TiO2 nanowire membranes, anodisation of titanium films sputtered on to stainless steel substrates, embedding TiO2 nanoparticles into the polymeric membranes matrix, electrospinning TiO2 fibres or flat membranes by recasting, development of TiO2 layers with rapid atmospheric plasma spray coating and the fabrication of free-standing and flow-through TiO2 nanotube membranes among others .
Photocatalytic membranes can be broadly classified into four categories, based on the location of the nanoparticles on the membrane  (Figure 1):
Thin-film nanocomposites (TFC)
Thin film nanocomposite with nanocomposite substrate
3.1. TiO2––polymer membranes
Polymer membranes have gained popularity for use in wastewater treatment and water purification. A number of polymers have successfully been used as supports for photocatalysts, and these include polymers such as polyamide, polyvinylidene fluoride (PVDF), polysulfone (PSf), polyethersulfone (PES), sulfonated polyethersulfone (SPES), polyurethane (PU), polyethylene terephthalate (PET), polyester, polyacrylonitrile (PAN) and polytetrafluoroethylene (PTFE) [4,9].
The photocatalyst nanoparticles can either be deposited onto the membrane surface or dispersed in the polymer dope solution prior to membrane casting. Several other methods have been proposed and tried for the fabrication of polymeric and ceramic photocatalytic membranes (Figure 2). The overall objective is to incorporate the photocatalyst onto the membrane to impart some photocatalytic activity to the membrane.
3.1.1. Surface-located TiO2 polymer membranes
22.214.171.124. Dip-coating or self-assembly
Surface-located TiO2 polymer membranes are prepared through dip-coating or self-assembly , chemical or photochemical processes based on chemical grafting and photopolymerisation  and physical deposition of a TiO2 layer via ultrafiltration of a TiO2 suspension through a polymer membrane . In the dip-coating or self-assembly method, a layer of TiO2 is coated on the surface of a membrane through dipping the membrane into a sol–gel TiO2 slurry solution, followed by drying and pressing with a compressed gas. The TiO2 particles tend to self-assemble on the membrane due to the anchoring effect of the functional groups (e.g. carbonyl, sulfone) located on the membrane surface or through hydrogen bonding of the membrane surface groups and TiO2. Kim et al. prepared a hybrid thin-film composite (TFC) membrane by self-assembly of TiO2 nanoparticles through coordination and H-bonding interaction with the COOH functional group of aromatic polyamide thin-film layer. The TFC membrane with self-assembled TiO2 nanoparticles showed photocatalytic activity towards the destruction of microorganisms. Such membranes can be used to reduce membrane biofouling . In another study, reverse osmosis (RO) membranes composed of aromatic polyamide thin films underneath titanium dioxide (TiO2) nanosized particles were fabricated by a self-assembly process. Sol−gel colloidal TiO2 were self-assembled on a TFC aromatic polyamide membrane containing COOH groups along its surface. The TiO2 hybrid membrane was examined for its photocatalytic efficiency for the destruction of
126.96.36.199. Chemical grafting and photopolymerisation
In this method, appropriate monomers containing the photocatalyst to be immobilised and an initiator are photo-irradiated on a support. In a study by Razmjou et al., commercial TiO2 nanoparticles were functionalised by aminopropyltriethoxysilane (APTES) to initiate coupling with PES in the dope solution, prior to casting and precipitation, to form hollow fibre membranes. Initial pure water flux was significantly enhanced while there was a small improvement in the fouling performance in comparison with the control. The modified hollow fibre membrane showed increased glass transition temperature, membrane porosity and pore size, stiffness and hydrophilicity, whereas tensile strength and elongation at break decreased .
In a chemical grafting and photopolymerisation study, TiO2 was added to a suitable acrylic acid monomer and sonicated for about 20 min to facilitate the interaction between TiO2 and acrylic acid. Ethylene glycol was then added as a cross-linker, and potassium persulfate (1 wt.%) as an initiator. The PVDF support membrane was then dipped in the polymerisation solution for 2 min and placed on a glass plate. The grafted membranes were irradiated with UV light (160 W) for 15 min. The covalent attachment of TiO2 to PAA network in the membrane composite resulted in improved fouling resistance due to the hydrophilic functional groups of PAA and the photocatalytic effect of TiO2 that reduced the hydrophobic adsorption between the whey protein and the modified membrane surface .
188.8.131.52. Physical deposition
In this method, the photocatalyst is physically deposited on the membrane surface through pressure-driven filtration of the photocatalyst suspension slurry. A photocatalyst cake builds on the surface of the membrane, the thickness of which can be controlled by varying the filtration time and operational pressure . Bai et al. synthesised TiO2 nanothorn spheres and assembled them on a piece of cellulose acetate polymer membrane through compression on a dead-end filtration set-up. The resulting TiO2 nanothorn membrane was reported to exhibit a hierarchical porous structure, which gave a high water flux and a multifunctional TiO2 surface for mitigation of membrane fouling .
3.1.2. TiO2-entrapped polymer membranes
184.108.40.206. Phase inversion
Phase inversion is by far the most common and popular method for synthesising nanoparticle-entrapped membranes. In this method, the preformed photocatalyst is added to the polymer dope solution to form a homogenous solution. The solution is then cast on a glass plate and immersed in a coagulation bath to effect gelation. In the dry method, the solvent is allowed to evaporate until gelation is complete after casting . Phase inversion membranes may be fabricated in different forms, such as hollow fibres or flat sheets, which can be incorporated modules designed to produce optimal hydrodynamic conditions for separation (Figure 3). TiO2-embedded membranes have been reported to show improved membrane properties such as porosity, wettability, permeability and antifouling propensity. Application of photocatalytic membranes requires irradiation of the membrane to activate the photocatalyst. However, sulfone group containing membranes (PES and PSf) are generally sensitive to UV radiation, and irradiation may lead to destruction of some membrane bonds. The least affected membranes are PTFE and PVDF .
Although embedding nanoparticles onto membranes eliminate the need for post-treatment removal of the nanoparticles, one major limitation of immobilising nanoparticles onto membrane supports is the reduction in the available surface area of the nanoparticles due to coverage by the polymer phase material.
220.127.116.11. Biaxial stretching
Biaxial stretching is a physical method where membranes are stretched to modify their properties in an effort to improve overall membrane performance. This modification induces significant and permanent membrane pore deformation, which may result in improved porosity . In a typical study, catalytic membranes were fabricated via biaxial stretching of a polytetrafluoroethylene (PTFE) extrusion containing 2 wt.% of anatase TiO2. The catalyst particles were uniformly distributed throughout the membrane. The photocatalytic membrane could degrade organic solutes in ultra-pure water .
18.104.22.168. In-situ generated photocatalyts
The formation of mixed matrix membranes through in-situ generation of the desired nanomaterials is a relatively new innovation in membrane technology. The main advantages of in-situ generation of nanoparticles include control over nanoparticle–membrane matrix compatibility, nanomaterial loading capacity, water flux as well as selectivity .
Luisa Di Vona et al. successfully synthesised a sulfonated poly(ether ether ketone) (SPEEK) nanocomposite membrane via an in situ mixed sol–gel process in which titanium butoxide/2,4-pentanedione in DMAc was added to a SPEEK solution and the solvents allowed to evaporate at 120°C. The resulting composite membranes were flexible and transparent, and showed improved mechanical and thermal properties as well as hydrolytic stability . In another study, a mixed matrix membrane was fabricated through in situ polymerisation of titanium isopropoxide in polyvinyl acetate, PVAc. The titanium alkoxide can cross-link with PVAc through a transesterification reaction. The strong chemical interaction prevents phase separation and improves TiO2 dispersion within the PVAc matrix. One major drawback is that the Ti-based clusters contain some residual isopropyl groups and the TiO2 phase may show limited crystallinity .
3.2. TiO2-based ceramic membranes
Ceramic membranes are currently receiving significant attention as candidates for water decontamination because of their various properties, including sufficiently high mechanical strength, thermal stability, high flux, lower life cycle cost and resistance to corrosive environments. Depending on their pore diameters, they can be classified as MF, UF, NF, RO, pervaporation (PV), gas separation (GS) or ceramic membrane reactors (CMR). Oxides such as Al2O3, TiO2, ZrO2, SiO2 and combinations thereof are some of the common ceramic membrane fabrication materials .
Ceramic membranes are fabricated through methods such as dip-coating, where a photocatalyst is deposited on a porous ceramic support (e.g. SiO2, Al2O3, ZrO2, Al2O3/SiO2/ZrO2, SiN/SiC, Al2O3/SiC), and hydrothermal synthesis followed by filtration and calcination, where a suspension of hydrothermally synthesised TiO2 is filtered through a glass fibre filter, dried and calcined at high temperature. Another method is grafting of TiO2 onto ceramic membranes, where TiO2 nanotubes are grafted onto channels of ceramic membranes such as alumina (Al2O3) . Other methods such as chemical vapour deposition (CVD) and atmospheric plasma spraying (APS) have also been used to fabricate ceramic membranes [27,28].
3.3. Polymeric versus ceramic photocatalytic membranes
Ceramic membranes generally have many advantages compared to polymeric membranes from an application point of view, mainly because of their superior thermal properties and resistance to corrosion by harsh chemicals. With polymeric membranes, there are some risks associated with membrane degradation under extreme chemical conditions. Ceramic membranes are also less prone to membrane fouling because of their hydrophilic surfaces. Ceramic membranes generally have relatively uniform pore structures and thus significantly higher flux and lower membrane resistance compared to polymeric membranes. Unlike polymeric membranes which are soft and rather difficult to analyse by most physicochemical characterisation techniques, the composition of ceramic membranes can be well-characterised. Ceramic membranes have a lower membrane resistance and therefore require a lower pressure to produce the same volume of water as the polymeric ones. They also have a longer lifespan. However, the capital costs are huge .
Anchoring photocatalytic nanoparticles on commercially available ceramic membranes requires appropriate anchor sites on the membrane in order to form bonds between the nanoparticles and the ceramic support. This is currently a big hindrance to ceramic membrane modification attempts.
4. Photocatalytic membrane reactor configurations
A number of engineering designs and configurations have been proposed for possible scaling up of the photocatalytic membrane technology. The photocatalytic membrane reactors can be classified into four different configurations based on the stage at which the membrane will be used in the water treatment train (Figure 4):
Slurry photocatalytic reactor followed by a membrane filtration unit,
Ceramic or polymeric membrane submerged in a slurry photocatalytic reactor,
Membrane placed inside a photocatalyst-coated (TiO2) photoreactor,
Photocatalytic membranes (pure TiO2 or TiO2-mixed matrix membranes).
While a number of these configurations have been put to test, the photocatalytic membrane is currently receiving immense research focus because of its potential advantages over the other configurations. Some of its advantages include the duality of physical separation during membrane filtration, and degradation of organics and bacteria, all achieved in a single unit. Generally, photocatalytic membranes tend to outperform conventional membranes in terms of reducing membrane fouling and improving permeate quality [6,18].
Despite the potential advantages using these different reactor configurations, the research on a combined use of photocatalysis and membranes is still not sufficiently developed.
5. Application of TiO2 photocatalytic membranes
Photocatalytic membranes have evolved as promising innovations for providing sustainable quality water to end users. The use of multimembrane systems or coupling membrane separation with other biological, chemical or physical treatments is envisaged to have a huge positive impact on the provision of quality water at low cost and lower energy consumption. TiO2-based catalytic membranes have found applications in a wide range of wastewater and water treatment applications.
Membrane biofouling is the deposition of microorganisms and microbial products on the surface of a membrane. Incorporation of TiO2 into polymeric or ceramic membranes has been reported to mitigate membrane biofouling. In a study on self-assembly of TiO2 nanoparticles on hybrid thin-film composite (TFC) membrane, dramatic photoactivity for bactericidal degradation of
5.2. Removal of pollutants
By and large, removal of pollutants in water is the widely used application of photocatalytic membranes. A number of studies have recently emerged on batch and reactor scale application of photocatalytic membranes for fouling mitigation through photodegradation of foulants. A PVDF/TiO2 mixed matrix UF membrane was reported to show enhanced performance for the removal of methylene blue (MB) due to the extra adsorption sites provided by the embedded TiO2 nanoparticles . In a similar study, Hua et al. fabricated a PVDF membrane with self-assembled TiO2 nanoparticles, and the permeability and antifouling ability of self-assembled membranes were significantly improved . A multifunctional polyurethane (PU)/TiO2/fly ash composite membrane fabricated through electrospinning showed enhanced adsorption of heavy metals (Pb, Hg), enhanced MB dye removal, superior water flux and antibacterial properties attributed to the photocatalytic properties of TiO2 as well as the adsorptive property of fly ash . Bae and Tak observed a positive effect of the presence of dip-coated and entrapped TiO2 on membrane antifouling in a membrane bioreactor (MBR) . Cao et al. observed remarkable improvement in the antifouling property of PVDF membrane with different sizes of nanosized TiO2 particles, with smaller particles having greater effect than the larger ones . Kim et al. self-assembled TiO2 nanoparticles on carboxylic acid groups on the surface of a thin-film composite (TFC) polymer membrane. The membrane was observed to mitigate biofouling through photodegradation effect under UV radiation .
A ceramic TiO2/Al2O3 composite membrane was prepared using a sol–gel technique and applied in a filtration/photodegradation membrane reactor under UV radiation. UV irradiation enhanced the wettability of the ceramic membrane, and a stable water flux was rapidly archived. 25% of Acid Orange dye was degraded within several milliseconds. The membrane could easily be cleaned by irradiating the membrane surface under static conditions . Syafei et al. observed greater flux decline with TiO2-coated alumina/titania/zirconia (ATZ) ultrafiltration ceramic membrane disk under UV irradiation than with uncoated membrane in the treatment of natural organic matter (NOM). It was postulated that UV irradiation alters the molecular weight distribution of humic materials to below1 kDa, which facilitates their attachment to TiO2 nanoparticles . In a similar approach, TiO2 nanoparticles were coated on Al2O3 nanoparticles through solid-state sintering method. The resulting membrane was reported to show enhanced photocatalytic properties and reduced membrane fouling in oil emulsion wastewater treatment . The effect of UV irradiation on membrane performance was evaluated on TiO2-entrapped and self-assembled PES membranes. The TiO2-coated membranes showed a uniform distribution of the TiO2 nanoparticles on the membrane surface. The non-irradiated membranes showed a flux decline while the irradiated membranes exhibited higher fluxes and superior antifouling properties. It was then concluded that dip-coating TiO2 on PES membrane surface is a superior technique to minimise membrane fouling than entrapping the TiO2 . Similar observations were reported by other researchers on TiO2/PES, TiO2/poly(styrene-alt-maleic anhydride)/poly(vinylidene fluoride) (TiO2/SMA/PVDF) and TiO2/PES/polyimide (PI) composite membranes, using polyethylene glycol-5000  and Bovine serum albumin (BSA) [34,43]. In a study by Lin et al., it was concluded that the effectiveness of photodegradation of 1,2-dichlorobenzene was higher with TiO2 immobilised on low-density polyethylene (LDPE) support than on quartz support using four UV lamps of 15 W each .
Inasmuch as progress has been realised in the application of photocatalytic membranes for water decontamination, further research is needed to investigate the combined effects of water chemistry, nature of nanoparticles, loading capacity and the photocatalyst incorporation conditions on membrane performance.
6. Conclusions and perspectives
As engineering technology innovation continues to grow, new opportunities and prospects to solve the ever-increasing environmental challenges continue to emerge. Innovative techniques for the integration of photocatalyst in both polymeric and ceramic membranes need to be continuously developed in order to open new insights into the development of functional membranes capable of mitigating fouling with minimal consequences on water flux. Polymer and ceramic membranes have both been used as TiO2 supports, though the stability of polymeric membranes under UV irradiation still requires further investigation. Commercialisation of photocatalytic membranes will depend on the design of the membrane reactors as well as optimisation of the radiant flux on the membrane surface during the short time frame that the polluted water filters through the membrane. The use of solar light as an energy source instead of UV light could reduce the cost of the filtration process. This requires band-tuning of the TiO2 through doping with non-metals or metals to shift the absorption edge to the visible region. Currently, the membrane fouling mechanisms are not fully understood. Another challenge is possible leaching of the entrapped photocatalyst particles into the permeate solution during the filtration process. Given the present knowledge gaps regarding possible health and ecological hazards of nanoparticles’ aquatic environment, caution should be exercised to avoid discharge of the particles into the water stream.
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