CHAPTER 1: The Role of Functional Nanomaterials for Wastewater Remediation Free

Environmental pollution caused by anthropogenic activities is increasing at an alarming rate. The phenomenon has caused severe health issues and reduced the availability of clean water. Numerous technologies such as adsorption, biological oxidation and chemical oxidation have been used for the removal of all types of organic pollutants. These technologies have several limitations including high energy requirement, incomplete pollutant removal and formation of hazardous waste products.¹  The limitations of conventional methods and materials used to eliminate unwanted and toxic contaminants for water remediation have prompted the use of more effective and sustainable approaches. The use of nanomaterials in addressing issues related to the environment is emerging due to their interesting features that satisfy the desired requirements. The constant fresh water demand has motivated the constant development and improvement of nanotechnology in various forms in wastewater treatment.^2,3  As defined by the National Nanotechnology Initiative (NNI), nanomaterial is a material with dimensions between 1 and 100 nm.⁴  The size effect of nanomaterials is the main contributor to achieving better performance. The size of nanomaterials is closely related to their exceptionally high surface area and surface reactivity. Many of them have shown other interesting properties including superior surface to volume ratio, photocatalytic properties, improved solubility, large surface charge and abundant reactions sites.^5–8  To date, nanoscience and nanotechnology developments have opened new opportunities for the advancement of water treatment.⁹ 

In the past two decades, exciting achievements have been made in the development of novel nanomaterials for different aspects of wastewater treatment, exploration of new materials, establishment of simple, green or economic synthesis routes, and fabrication of nanocomposite materials. The roles of a wide range of nanomaterials have been witnessed in their applications for wastewater remediation. In this chapter, we highlight the importance of nanostructured materials including hybrids of metallic and non-metallic nanomaterials for environmental remediation application. Current research on synthesis and characterization, as well as the roles of nanomaterials are reviewed. Lastly, the future directions of hybrid nanomaterials for environmental remediation are highlighted. The general roles of nanomaterials as absorbents, photocatalysts, membranes and chemical disinfectants for environmental remediation are shown in Figure 1.1.

Nanomaterials are categorized by their size, composition, shape and origin. It has been of great interest in the research field to explore new types of nanomaterials with different structural designs that can fit new applications. Moreover, the uniqueness of these materials promises the design of materials with tunable properties, with improved properties and performances that are comparable to those of long-established counterparts available in the market.¹⁰  The size of nanomaterials can be affected by several parameters, like the method used for synthesis, temperature, pressure, time, pH and concentration.^11,12  On the basis of their function, nanomaterials have been synthesized into various dimensions and shapes, including spheres, fibres, tubes, sheets and interconnected architectures.¹³  As shown in Figure 1.2, the zero-dimensional (0D) structure is characterized by spherical shape, fibers and tubes are the common shapes of one-dimensional (1D) structure, two-dimensional (2D) structure presents in the form of sheet-like structures and interconnected architectures are normally characterized as three-dimensional (3D) structures. The construction of nanostructure materials with multi-dimensions offers very interesting morphologies, properties and functions. The surface area, adhesion, adsorption, reflectance and carrier transportation properties have been harnessed for various applications, including wastewater remediation.

Due to their versatility, metallic nanomaterials have huge potential for different types of environmental remediation. There is an assorted class of nanomaterials that consists of one, two, or three metals and their oxides. The characteristics of metal based nanomaterials, including shape-controlled, stable and monodispersed, have been extensively investigated via physical and chemical approaches. These advantageous characteristics can be feasibly utilized for water treatments including adsorption, photodegradation, membrane separation and chemical disinfection.

TiO₂, silver (Ag) and zinc oxide (ZnO) with their 0D nanostructures are the most popularly investigated metal and metal-oxide in addressing energy and environmental issues.^14–18  They are notable for their significant antibacterial, antifungal and antiviral activity and are applied as water disinfectants, hence they have been used for self-cleaning surfaces, air and water purification systems and act as photocatalysts in water treatment.^13,19  In addition, these oxides and their hybrids can perform simultaneous actions in degrading organic moieties and also killing different organisms in a single wastewater treatment.²⁰ 

In the field of water purification, along with the development and improvement, the morphological structures of TiO₂ nanoparticles have been widely explored including spherical titania (TNP), titania nanotube (TNT) and titania nanosheets (TNS). TNP usually exhibits high specific surface area, pore volume and pore size.^6,7  As such, TNP provides a highly accessible active site that can be used for organic pollution adsorption. TNT is usually prepared from TNP via a hydrothermal method in a potassium hydroxide/sodium hydroxide (KOH/NaOH) solution.^6,7  Compared to TNP, TNT is expected to have more advantages, owing to its tubular structures it not only offers the high hydrophilicity properties of TiO₂, but also higher pore volume and a high interfacial charge carrier transfer rate.²¹  The 1D structure of TNT can reduce the transport dimensionality and recombination of photoinjected electrons.²²  As there is a much lower number of grain boundaries in 1D nanostructures compared to 0D nanostructures, as shown in Figure 1.3, as well as increasing the delocalization of carriers in rods, they can move freely throughout the length of TNT.²³  The presence of abundant adsorption sites of the large surface area resulting in improvement of the adsorption and degradation of the pollutants. In addition, the presence of abundant hydroxyl groups at the TNT surface not only promotes the surface hydrophilicity properties but can also be used as an active site for further modifications. For instance, the surface of TNT has been feasibly grafted with amino groups to further enhance its surface properties.^24,25  The introduction of functional groups can also enhance the compatibility of the nanomaterials when they are used for the preparation of a nanocomposite such as a polymer nanocomposite.

Much research has reported the advantages of carbon-based nanomaterials based on their interesting properties such as low cost, superior chemical and mechanical stability and highly integrated operation.^7,26  Due to the unique structural properties of most carbon-based nanomaterials, they normally exhibit high specific surface area, area functionalization and chemical stability. Various carbon-based nanomaterials such as graphene oxide (GO),²⁷  single-walled carbon nanotubes (SWCNT)²⁸  and multiwalled carbon nanotubes (MWCNT)⁷  have been explored as having high-potential for water purification.

Carbon nanotubes (CNT) have gained popularity as they can serve as robust pores in membranes for water treatment. CNT can be divided into two major types i.e. SWCNT and MWCNT.²⁹  These allotropes of carbon of cylindrical nanostructure offer frictionless transport of water molecules to enhance water permeability. Moreover, to balance between the permeation and rejection of ions through the CNT, it is possible to modify the membrane to provide an appropriate pore diameter.^30,31  Gupta et al. incorporated SWCNT into a polysulfone (PSf) membrane for heavy metal removal.³²  The result showed that the SWCNT–PSf membrane had a reduction in pore size and a smooth surface. Compared with the pristine PSf membrane, the SWCNT–PSf membrane improved the rejection of metal ions by up to 69, 68 and 70% for Cr⁶⁺, As³⁺ and Pb²⁺ ions, respectively. Thus, CNT can improve adsorption capability based on the effortless way in which they can be functionalized with various organic molecules. The increase in the number of active binding sites results from the formation of new functional groups upon chemical modification.³³  Among the chemical modifications, oxidation is a straightforward approach by introducing hydroxyl and carbonyl groups to the sidewalls of CNT. Lu et al. employed CNT that was oxidized by sodium hypochlorite (NaOCl) solution to improve its adsorption towards benzene and toluene.³⁴  SWCNT has a higher specific surface area and is able to provide a higher adsorption capacity for organic contaminants. Bina et al. studied the removal of benzene and toluene by using MWCNT and SWCNT.³⁵  From the results, SWCNT performed better with 9.98 and 9.96 mg g⁻¹ for benzene and toluene, respectively. The adsorption mechanisms were due to the interaction between the electron donor of the carboxylic groups of the CNT surface and the electron-acceptor aromatic rings of benzene and toluene.

GO that contains primarily epoxide and hydroxyl with a very low amount of carboxylic acid groups could be an attractive candidate for environmental remediation.³⁶  These functional groups are found at the edges and basal plane of the nanosheets. When they are used as a nanocomposite membrane, these functional groups can provide good compatibility with the polymer. GO has shown a potentially effective reinforcement with filler stiffness and strength when mixed with different polymers and other materials.³⁷  When GO is incorporated into the polymer matrix, the GO is able to enhance the mechanical properties as well as polymer host performance. Ionita et al. studied the mechanical strength of GO with various concentrations (0–2 wt%) incorporated into PSf membrane.³⁸  They found that the mechanical properties of tensile modulus and tensile strength increased as the GO concentration increased. At 2 wt% of GO, the mechanical properties reduced because of poor dispersion in the polymer matrix. Cano et al. covalently functionalized GO with polyvinyl alcohol f-(PVA) GO by esterification between the carboxylic group of GO and the hydroxyl group of PVA into a paper-like material and PVA-based composites.³⁹  They observed that there was an improvement in both Young's moduli and tensile strength with 40% f-(PVA) GO in PVA composite compared to the stand alone paper-like material due to the covalently bonded PVA. Gonalves et al. modified GO with poly(methyl methacrylate) (PMMA) through atom transfer radical polymerization (ATRP) as a reinforcement filler.³⁷  The effect of 1 wt% PMMA grafted graphene (GPMMA) was to yield a much more ductile and tougher material film than pure PMMA. This is because the GO-based membrane focused on adjusting membrane structure and improving its mechanical strength. Besides, GO has the potential to improve the resistance to chlorine in a membrane. Shao et al. modified a PA reverse-osmosis membrane with few-layered GO to enhance chlorine resistance.⁴⁰  From the result, after 16 hours of chlorine exposure, the salt rejection was 75% compared to only 63% with the unmodified membrane. This was attributed to the function of layered GO that can avoid the adsorption of chlorine radicals on a PA layer by forming an O–Cl bond.

Carbon-based nanomaterials like GO and CNT can be found in functionalized or non-functionalized forms.⁴¹  For functionalized GO, oxygen-containing functional groups are needed in order to form bonds with metals.⁴²  The oxygen-containing functional group and defects on the GO surface provide metal-capturing sites which show promise for the removal of metal ions in wastewater. Giwa et al. fabricated functionalized GO with maleic acid, hyperbranched polyethyleneimine (HPEI) and chitosan, respectively, for the treatment of electrokinetically remediated wastewater.⁴³  The high specific surface area provides abundant accessible sites for the functional nanoparticles. The purposes of the functionalization are to improve hydrophilicity and dispersion in the aqueous phase. The oxygen content can be controlled by tuning the oxidation conditions of graphite or the reduction of GO. The well dispersibility in aqueous solution makes the surface of each nanoparticle vulnerable to microbial and chemical contaminants.⁴⁴  Moreover, the dependence of the surface charge properties of the target contaminant is another factor. For example, the negative charges of functionalized CNT or GO could be the main driving force for the adsorption of positively charged compounds in wastewater via electrostatic interaction.⁴¹  Chen et al. studied the adsorbent GO–chitosan hydrogel for the removal of organic dyes and heavy metal ions from water.⁴⁵  From the study, high adsorption capacities of 70 and 90 mg g⁻¹ were observed for Cu(ii) and Pb(ii), respectively. The oxygen-containing groups allow for the binding of metal ions and positively charged organic molecules via electrostatic interaction and coordination.

In the field of hybrid coordination networks, significant progress has been made in developing a wide range of metal–organic frameworks (MOFs). MOF is classified as a 3D interconnected structure that has large pore volume ratios enabling efficient diffusion pathways for guest species into the framework.¹³  MOF has been applied for the purification and separation of contaminated water. Several strategies have been used to functionalize the MOF through pre- or post-synthesis modification to address some of their limitations including limited activity and structural stability.^46,47  The functionalization of organic ligands can be performed during the MOF synthesis or during the post-synthesis modification through coordinate bonds by using organic groups that can attach to a metal centre.⁴⁸  These approaches could fine-tune the pore dimensions and functional properties of the MOF to improve catalytic activity, leaching of the functional site and its stability.

The integration of MOF with zeolite has been developed to provide structural stability by incorporating inorganic zeolites into the nanocomposite material. Zeolites are aluminosilicate minerals with a large number of electrostatic holes on the surface structure that are occupied by cations and water molecules. The size can be within the range 10–500 nm. It has greater adsorption capacity, higher density of adsorption sites and higher surface area (400–1000 m² g⁻¹). The functional silanol groups of zeolite can be functionalized with a carboxylic acid to grow MOFs on its surface structure. Zhang et al. prepared an MOF with a high quality zeolite imidazole framework-8 (ZIF-8) to be coated on a polyacrylonitrile membrane by a self-assembly strategy.⁴⁹  This method has improved the production stability and the uniform dispersion of a ZIF-8/poly(sodium 4-styrene-sulfonate) (ZIF-8/PSS) hybrid in membranes as well as providing outstanding separation ability. Zhao et al. utilized the integration of a MOF [benzene-1,3,5-tricarboxylate; Cu3(BTC)2; “HKUST-1”] and magnetic ferrous ferric oxide (Fe3O4) nanoparticles for the removal of methylene blue (MB).⁵⁰  In this study, the magnetic MOF composite provides a large specific surface area (79.52 m² g⁻¹), excellent magnetic response (14.89 emug⁻¹) and meso-porous channel volume (0.09 cm³ g⁻¹) for excellent adsorption performance. The higher mean size of the magnetic MOF composite with 4.4 nm pores (mesopores) could allow solution to pass through and reach most of the potential adsorptive sites compared to the activated carbon with 1 nm pores (micropores).

Novel MOF-derived carbon materials are interesting materials due to their nonporous structure with high surface area and tunable chemical and physical properties.⁴⁸  Camilli et al. prepared a 3D porous CNT framework by a sulfur-addition strategy during an ambient-pressure chemical vapour deposition process.⁵¹  A sponge-like structure was obtained due to the enhancement of CNT's length and contorted morphology. Liu et al. fabricated 3D GO sponge from GO suspension via a centrifugal vacuum evaporation method for dye removal.⁵²  The organized and 3D interconnected GO sheets contributed to the high mechanical strength. Besides, due to the hydrophilicity and the exposed surface property of GO, the GO sponge is well dispersed and stable in aqueous phase. Thus, it can serve as an advanced adsorbent platform to capture aromatic organic dyes through strong π–π interaction and anion–cation interaction.

MIL-101 is an example that has been used in treating wastewater and in photocatalysis due to its high specific surface area, large pore volume and uniform pores. Du et al. synthesized MOF MIL-101 by using hydrogen fluoride chromium(iii) nitrate nonahydrate [HF-Cr(NO₃)3-1,4-dicarboxylic acid (Cr/H₂BDC)] and water/benzene-1,4-dicarboxylic acid [H₂O/H₂BDC] and studied its photocatalytic degradation.⁴⁷  They achieved a large surface area and high crystallinity of 1.25 and 350 m² g⁻¹, respectively. This catalyst also exhibits excellent catalytic recyclability and stability for the treatment of organic pollutants. MIL-101 is stable in polar and nonpolar solvents at high temperatures. The mechanism of the underlying photochemical degradation depends on the RBB dye on MIL-101 with strong oxidation reaction.

Nowadays, hybrid nanomaterials are almost everywhere because they have a wide range of applications such as in polymer nanocomposites, health and the environment. The integration of different kinds of materials could result in a new multifunctional hybrid and allow the combination of the advantages of each into one composite material.⁵³  These materials possess extraordinary physical and chemical properties derived from their size in nanoscale. Hybrid nanoparticles not only have the characteristics of both nanomaterials, but also have unique properties that surpass those for the original components. Compared to the nanoparticle alone, adhering nanoparticles to a scaffold can improve the stability of the material.⁵⁴  Ahmed et al. synthesized a ZnO/MWCNT hybrid by dispersing MWCNT into a Zn-based solution by sonication.⁵⁵  A high concentration of the hybrid nanomaterial (4 mg mL⁻¹) was observed to have better antibacterial properties compared to a low concentration (0.125 mg mL⁻¹). Moreover, the surface chemical properties were responsible for the increase in the selectivity and efficiency of the material towards the targeted contaminant molecules.⁵⁶  Suzaimi et al. grafted branched poly-ethyleneimine (bPEI) into husk porous silica (RSi) for nitrate adsorption.⁸¹  A maximum adsorption capacity of 94.49 mg g⁻¹ can be achieved with RSi–bPEI compared to only 47.46 mg g⁻¹ with non-hybrid RSi. The presence of bPEI could result in strong interaction between the positively charged adsorbent and negatively charged phosphate species.

Ramashkumar et al. synthesized a few layers of molybdenum disulfide (MoS₂) sheets using the facile and efficient surfactant-assisted liquid-phase exfoliation method.⁵⁷  Anionic surfactant was absorbed to the surface of MoS2 and improved the stability of the exfoliated MoS₂ sheets dispersion. By using the exfoliated MoS₂ sheets decorated with ZnO, the nanoparticles exhibited a reduced bandgap energy from 3.20 to 2.77 eV, which is beneficial for the degradation of MB dye. The hybrid nanomaterial also showed improved adsorption capacity and removal efficiency. Mamah et al. synthesized water-dispersible palygorskite with the aid of carboxylated chitin nanofibers (ChNFs) by using a facile ball milling method under the collision and shear force of ball mill.⁵  The rigid crystalline ChNFs with rich surface chemical groups aided hybridization between the ChNFs and palygorskite. Kanakaraju and Wong reported a carefully designed and synthesized TiO₂-biomass-loaded mixture for wastewater.⁵⁸  TiO₂ was modified with sago bark generated during the debarking step of a starch extraction process. The hybrid materials showed promising adsorption results. Gade et al. synthesized layer-type perovskite materials via ion-exchange and characterized them thoroughly by spectroscopic techniques to degrade organic dyes and industrial wastewater.⁵⁹  The ion-exchange of Na⁺ ions in layer-type perovskite NaLaTi₂O₆ (NLTO) with Ag⁺ or Cu²⁺ ions led to new perovskites, AgLaTi₂O₆ (ALTO) and Cu0.5LaTi₂O₆ (CLTO), respectively.

As far as hybrid material is concerned, the hybrid approach is still focused on material synthesis/modification and characterization. Thorough characterization is paramount to understanding the composite formation and making a fair correlation with the resultant characteristics.

The potential of nanomaterials has allowed the development and improvement of photocatalysis, absorption, membrane filtration and chemical disinfection for efficient wastewater treatment. The rapid development of nanotechnology has shown remarkable potential for environmental applications. In this section, the roles and application of various functional materials and their hybrids are discussed.

Photocatalysis is one of the approaches for the remediation of environmental contaminants. Nanomaterials have an important role in photolysis for water and wastewater treatment processes. Photocatalytic degradation is an advanced oxidation process (AOP) using ultraviolet (UV) light as an energy source.⁶⁰  Irradiation by UV light generates the highly reactive hydroxyl radical (˙OH) for the degradation of recalcitrant chemicals present in wastewater.⁶¹  Photocatalysis possesses several advantages over other AOPs including Fenton and photon-Fenton catalytic reactions and hydrogen peroxide: it is able to operate under ambient conditions, and uses inexpensive and non-toxic photocatalysts as well as atmospheric air as an oxidant.⁶² 

Figure 1.4 depicts the mechanism of the homogeneous photocatalytic degradation mechanism, which involves the migration of electrons (e⁻) and holes (h⁺) to the surface of the photocatalyst through UV light irradiation.⁶³  Both oxidation and reduction can take place on the surface of the photoexcited semiconductor photocatalysts. The role of the light energy (photons) is to generate e⁻–h⁺ pairs, which take place in the semiconductor particle because of the excitation of an electron from the valence band (VB) to the conduction band (CB). On excitation, the fate of separated e⁻ and h⁺ can follow several pathways. At the surface, the e⁻ and h⁺ could undergo subsequent oxidation and reduction reactions with any species to give the necessary products. Generally, in competition with charge transfer to adsorbed species is e⁻ and h⁺ recombination; a separate e⁻ and h⁺ recombination can occur in the volume of the semiconductor particles at the surface with the release of heat. Thus, it is important to develop an efficient redox catalyst in order to make product formation more competitive against these recombination processes. Photocatalysts used for wastewater treatment can be classified into solubilized/suspended photocatalyst particles and photocatalyst particles immobilized in/on a membrane.⁶⁴  The improved photocatalytic activity was attributed to a smaller band gap, enhanced adsorption and more charge separation of the e⁻ and h⁺.⁶⁵ 

Solubilized photocatalyst particles have been reported to exhibit high efficiency performance.⁶⁶  They have larger active surface area, which promises a good contact between the photocatalyst and the pollutants compared to the photocatalyst particles immobilized in/on a membrane. Besides that, to increase the photocatalysis performance, the solubilized photocatalyst particles are doped with another alien ions non-metal doping⁶⁷  and transitional metal doping.^68–71  Nishiyama et al. demonstrated improvement in photocatalytic degradation of 4-chlorophenol (4-CP) by doping Cu into TiO₂ nanoparticle.⁷⁰  Results showed the increase in photocatalytic degradation efficiency of 4-CP by the successful entry of Cu atoms into the TiO₂ lattice and created charge compensating anion vacancies in the TiO₂ lattice. Hydrochloric acid, carbon dioxide and water were generated during the process. Besides that, the doping with alkaline metals likes, potassium (K), lithium (Li) and sodium (Na) could lead to the creation of the negative charges on the surface of the photocatalysts. Bessekhouad et al. utilized alkaline metals and successfully created the negatives charges on the photocatalyst surface.⁷¹  The cationic molecules of malachite green oxalate (MG) contaminants can be adsorbed via electrostatic interactions. Lv et al. developed magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba)-doped TiO₂ with honeycomb-like inverse opal structure for photocatalysis.⁶⁹  Sr-doped TiO₂ that exhibited the largest specific area and the highest carrier separation rate and transfer efficiency contributed to the strongest light-absorption capability compared to the other alkali earth metals. The CB level of TiO₂ could be shifted to a more negative value by Sr-doped TiO₂ that generates ˙O₂⁻ and contributed to a higher photocatalytic H₂-production efficiency as shown in Figure 1.5.

Yu et al. reported a field study on the performance of modified ZnO with silver as photocatalysts in degrading a dye contaminant by photolysis.⁶⁸  They found that the degradation rate of dye by modified Ag–ZnO photocatalysts is faster than that by pure ZnO. The increased degradation rate was attributed to the reduction in band-gap energy and improved charge separation between the photogenerated e⁻ and h⁺. ZnO integrated with graphene was used under UV irradiation for photocatalytic reduction of Cr⁶⁺. As compared to pure ZnO material, the removal rate obtained for the composite was 40% higher. Kanakaraju and Wong investigated a novel TiO₂/modified sago bark (MSB) for sago wastewater treatment by deploying a response surface methodology with the target parameter of chemical oxygen demand (COD).⁵⁸  Under optimisation, the highest COD removal was 64.92%, obtained using a TiO₂/MSB mixture.

Rhatigan et al. prepared TiO₂ rutile surface-modified with alkaline earth oxide nanoclusters of magnesium oxide (MgO) for photocatalytic performance.⁶⁷  At low loading of MgO, the oxygen vacancies and reducible Ti³⁺ ions promoted the oxygen evolution reaction (OER) and identified interfacial sites. Zhang et al. fabricated photocatalytic nanocomposite of CdS–graphene for remediation of benzene. The effect of different weight addition ratios of graphene has been investigated.⁷²  Based on the results, the excess loading of graphene lowered the photoactivity of the nanocomposite. 5% graphene was suggested as the best loading to achieve optimal photocatalytic performance as shown Figure 1.6. It was able to achieve 100% reduction of Cr⁶⁺ in water over 20 min.

Gade et al. demonstrated layer-type perovskite materials for photodegradation of organic dyes under visible light irradiation.⁵⁹  In this study, it was observed that the type of perovskite material achieving the highest photocatalytic degradation efficiency of dyes was ALTO, followed by CLTO and NLTO. The improvement was due to their excellent stability, and the hydroxyl radicals significantly improved mineralization of the dyes. Du et al. prepared MOF MIL-101(Cr) for photocatalytic degradation of Remazol Black B (RBB) dye.⁴⁷  It was demonstrated that RBB colour was completely removed under UV irradiation while without UV irradiation the removal efficiency was 43%. The decolourization of RBB dye was dominated by the photocatalytic activity of MIL-101 rather than the adsorption, thanks to electron transfer from photoexcited organic ligands to the metallic cluster in MIL-101.

On the other hand, the development of a photocatalytic membrane, coupling membrane filtration and photocatalysis in a single unit, has shown a great potential for water purification.⁷³  It has gained increasing attention over the standalone solubilized photocatalyst particles configuration due toits ability to induce photocatalytic reactions on the membrane surface and pore of the membrane. Novel photocatalytic membrane reactors are continuously emerging due to the significant contribution of the photocatalytic activity and membrane filtration processes. The photocatalyst and membrane separation processes take place in the same unit in the development of photocatalytic membranes, allowing simultaneous separation and degradation of the pollutant. Subramaniam et al. demonstrated a novel photocatalytic polyvinylidene fluoride/TNT (PVDF/TNT) hybrid photocatalytic membrane to remove pigments present in AT-POME.⁶  They found that the removal of Reactive Black 5 was enhanced significantly under the synergistic effects of photocatalysis and membrane separation, with colour removal of 34.2% compared to the bare membrane. Similar approaches were reported by Martinez et al. who investigated the coupling of photocatalytic oxidation and separation performances of TiO₂ and hematite iron oxide supported over a santa barbara amorphous support (TiO₂ and Fe₂O₃/SBA) for the removal of pharmaceutical pollutants.⁷⁴  The results showed a permeate stream free of pharmaceutical that <0.5 mg L⁻¹ the removal percentage was between 80 and 100 after UV irradiation. They claimed that the membrane not only achieved effective separation of the contaminants but also good degradation by photo-oxidation.

Goa et al. designed a photocatalyst by combining a TiO₂ suspension onto a SWCNT nanocomposite film for oily wastewater treatment.²⁸  The resultant membrane achieved up to 30 000 L m⁻² h⁻¹ bar⁻¹ with separation efficiency of 99.99%. Moreover, it has excellent antifouling and self-cleaning performance for multiples cycles, aided by the photocatalytic properties of TiO₂ nanoparticles. However, the modified membranes have some limitations in slurry/submerged or continuous photocatalytic membrane reactors, involving photocatalyst regeneration, poor photocatalytic activity and excessive membrane resistance.⁶⁶ 

Immobilisation of the active photocatalytic layer on the membrane surface rather than bulk incorporation in photocatalytic membranes has been introduced to overcome the aforementioned limitations.^57,75  By introducing the photocatalyst on the surface, the maximum photocatalytic efficiency can be utilized as the membrane surface can be inherently tailored with desired multi-functionalities. Besides that, membrane modification has been targeted to achieve better adhesion. This is done by introducing onto the membrane surface key functional groups that can stably hold the TiO₂-based catalyst. Rameshkumar et al. reported that the membrane deposited the hybrid ZnO–MoS₂ photocatalyst on the PVDF in the synergistic mechanism of photocatalytic-assisted dye degradation.⁵⁷  ZnO nanoparticles on the exfoliated MoS₂ sheets offered a bandgap energy reduction of about 2.77 eV and achieved a significant result with 87.12% degradation of MB, with only 56.89% for the pristine ZnO. Thus, they showed a complete removal of MB dye, 99.95%, by using photocatalysis-enhanced membrane filtration. It is the exfoliated MoS₂ that promotes a bandgap energy reduction and activates the electron transfer rate of the ZnO semiconductor to suppress the faster recombination of its e⁻–h⁺ pairs, thus enhancing the membrane properties such as surface adsorption, hydrophilicity and filtration performance.

Hu et al. developed a single-walled CNT coated with polydopamine (PDA) to form an ultrathin bilayer membrane for oil/water separation.⁷⁶  The presence of the SWCNT layer changed the behaviour in responding to the pressure applied across the membrane. The authors claimed that the water-in-oil emulsion could be separated due to the higher pressures that allow the separation of emulsion drops. Besides that, the integration of CNT/TiO₂ enables create a self-cleaning under mild condition via a sol–gel reaction. Moreover, the self-cleaning and antifouling properties under UV-light resulted in successful degradation of organic materials that accumulate on the membrane surface.

Lv et al. described a photocatalytic layer of β-FeOOH nanorods on a PDA/PEI-nanofiltration membrane.⁷⁵  In long-term continuous filtration, the water flux of the membrane under visible light after 6 h of filtration could be recovered almost to its original value compared to the membrane that was not subjected to visible light irradiation. The latter experienced a decline in water flux of 35% due to severe accumulation of methyl blue on the membrane surface. The photocatalytic activities demonstrated antifouling and self-cleaning functions. The β-FeOOH-fabricated photocatalytic layer has high photo-Fenton reaction activity in the presence of H₂O₂ under visible light irradiation. Besides, the strong coordination complex between the PDA−PEI layer and the β-FeOOH layer exhibited excellent stability during cyclic photocatalytic filtration.

Adsorption is one of the best-suited wastewater treatments because of its high efficiency, low-cost and ease of operation. The limitation is that pollutants may only transfer to the adsorbent, which needs to be generated regularly.⁶¹  A well-designed adsorbent is the critical component of an adsorption system.⁷⁷  Nano adsorbents exhibit advantages including high adsorption capacity, cost effectiveness, environmental non-toxicity, ease of separation and robustness and reusability.⁷⁸  Usually, nanosized particles are more efficient than their larger-sized counterpart.⁷⁹  Generally, the mechanism of adsorption includes the diffusion to adsorbent surface, migrate into pores of adsorbent and monolayer build-up of adsorbate, as shown in Figure 1.7. The mechanism of adsorption relies on the surface features, electrochemical potential and the ion-exchange process.²⁹  For example, the charged contaminants tend to adsorb via electrostatic attraction on adsorbents with an oppositely charged surface. The adsorbents with mesoporous structures and larger surface area show high adsorption capacities and kinetics.⁸⁰  Metal and metal-oxide nanoparticles play an important role in the efficiency of contaminant adsorption. Features of nanoparticles that make them suitable for this purpose include having a large number of active sites, low cost, good mechanical properties and environmental friendliness.

Mamah et al. prepared a palygorskite/chitin (PAL-CHI) hybrid nanomaterial to increase the number of adsorbent active sites.⁵  The abundant amino and hydroxyl groups of chitin as well as the hydrophilicity, anti-fouling properties and biocompatibility of palygorskite made the nanomaterial suitable for adsorption of metals. Thus, the hybrid adsorbent showed good adsorption capacity (q_e) of 53.7 mg g⁻¹ and removal efficiency of 92.95%. Figure 1.8 shows the proposed mechanisms of metal ion adsorption. The possible adsorption of lead ions onto PAL-CHI was caused by electrostatic attraction, ion exchange and chemical adsorption. Electrostatic attraction initiated binding of the lead ions onto the surface of the PAL-CHI hybrid. Meanwhile, ion exchange was found to take place based on the interaction of lead ions and hydroxyls group of the PAL-CHI matrix. Another possible mechanism was chemical adsorption via surface complexation, which happens when lead, as the heavy metal, binds strongly and sorbs on the oppositely charged surface.

Silica materials with hydroxyl groups present on the surface have been further modified through methods such as via grafting of functional groups onto the pore wall. Suzaimi et al. fabricated functionalized RSi–bPEI for treating nitrate-rich wastewater.⁸¹  At the optimal conditions of RSi–bPEI at pH 4, 0.5 g L⁻¹ dose, 50 mg L⁻¹ concentration and 120 min adsorption time, the functionalized adsorbent has significantly improved adsorption capacity of 94.49 mg g⁻¹ about 40% higher. The high adsorption capacity was ascribed to the porous structure, specific surface area and abundant amines of bPEI. The adsorption of nitrate on RSi–bPEI involved electrostatic interactions, ion exchange and hydrogen bonding.

Anvari and Shadjou used a functionalized absorbent of dithiocarbamate-dendritic fibrous nanosilica (KCC-1-NH-CS₂) for the removal of malachite green (MG) from wastewater.^82 ,107  The adsorption capacity was 98% and the KCC-1-NH-CS₂ nanoadsorbent was found to have narrow size distribution, high surface area and effortlessly available pores. Moreover, the mechanism involved is due to the electrostatic interaction between the negatively charged NH-CS₂ and positively charged MG dye and hydrogen bonding between the hydroxyl groups of KCC-1 in nanopores and MG dye. Liu et al. used 3D GO sponge to remove both MB and methyl violet (MV) from textile waste.⁵²  About 99.1% of MB and 98.8% of MV can be removed by adsorption. This high capability of 3D GO was attributed to the large surface area of GO and strong π–π interactions on the surface.

Disinfectants are the chemical agents that are used for inactivating and destroying microorganisms on inert surfaces, especially in the wastewater purification. Disinfection commonly causes cell wall corrosion, interfering with cell permeability, the protoplasm or enzyme activity,⁸³  thus disturbing the activity of microorganisms so they can no longer multiply, causing them to die. However, disinfection also faces the issue of the harmful disinfection by-products (DBP) of conventional chemical disinfectants, including chlorine, chloramines and ozone, that can react with various wastewater materials to form DBP. The use of nanomaterials can get rid of this issue as they can act without creating DBP. The most common inorganic antimicrobial nanomaterials are Ag, TiO₂ and ZnO nanoparticles.

Dimaphilis et al. demonstrated the disinfection activity of ZnO nanoparticles for application to water treatment.¹⁰⁸  Several mechanisms were involved during the disinfection activity, including the release of reactive oxygen species (ROS), release of Zn²⁺ and direct contact with cell membrane. Both ROS and Zn²⁺ ions can penetrate the intracellular contents and cause fatal damage to the microorganisms. As the ZnO nanoparticles contact the bacterial cell walls, the cells tend to become damaged and disorganized. Similarly, Narayanan et al. investigated the antimicrobial activity of ZnO nanoparticles with different sizes against human pathogens.⁸⁴  Based on a study of different ZnO particle sizes, smaller ZnO particles (41.6 nm) had the best antibacterial activity compared to particle sizes of 167.61 and 76.42 nm. They claimed that the smaller the particle, the higher the specific surface area and the stronger the antibacterial activity.⁸⁵  da Silva et al. studied the size and surface of a ZnO nanoparticle suspension and powders, which were carefully monitored to evaluate ZnO antibacterial activity against Staphylococcus aureus and Escherichia coli.⁸⁶  To increase the contact surface area and improve the interaction with the cell, they used a sol–gel technique and (3-glycidyloxypropyl) trimethoxysilane (GPTMS) modifier to coat the different sizes of ZnO nanoparticles. Based on the results, 4.5 nm ZnO nanoparticles showed higher antibacterial activity against S. aureus and E. coli compared to 21.20 nm ZnO nanoparticles. The surface modifier and smaller size of ZnO nanoparticle allowed the dissolution of ZnO to Zn²⁺ ions through the production of ROS. The high interfacial area of smaller size ZnO nanoparticles can easily penetrate and interact with the bacterial membrane. Pasquet et al. observed higher antibacterial activity for 14.7 nm compared to 17.5 and 76.2 nm ZnO nanoparticles.⁸⁷  Meanwhile, Ahmed et al. studied the effect of different ZnO/MWCNTs hybrid concentrations (0.125, 0.5, 2, 4 mg mL⁻¹) for the removal of microbial pathogens.⁵⁵  The results revealed that the highest concentration of ZnO/MWCNTs hybrid has stronger antibacterial properties against S. aureus, Salmonella typhi and Pseudomonas aeruginosa with 99.3–100%, 95.1–100% and 97.1–99.95% effectiveness, respectively. The formation of the carbon bond between ZnO and MWCNT resulted in the death of the bacteria by penetrating the cell wall and membrane and inhibiting their breeding.

Besides ZnO nanoparticles, TiO₂ is also a common nanoparticle that has been widely used as an antibacterial agent for wastewater treatment. Abdulazeem et al. studied TiO₂ nanoparticles as antibacterial agents against some pathogenic bacteria.⁸⁸  With 35 nm TiO₂ nanoparticles, bacterial biofilm growth was reduced at minimum inhibitory concentration (MIC) concentration and minimum bactericidal concentration (MBC). A higher MBC of 500 µg mL⁻¹ TiO₂ caused a large reduction of biofilm growth for Proteus vulgaris and S. aureus. Meanwhile, when the MBC of TiO₂ was 125 µg mL⁻¹, it showed a large reduction of biofilm formation for Acinetobacter baumannii and Strptococcus pyogenes on a glass surface. This is attributed to the thinner peptidoglycan layer of both A. baumannii and S. pyogenes (gram negative bacteria) that offers an easy route for TiO₂ nanoparticles to attack sulfhydryl (thiol) (–SH) groups of the proteins on the cytoplasmic membrane. Thus, the nanoparticles penetrate the membrane, deforming the structure of cellular components and finally causing the death of the microbial cell.

Azizi-Lalabadi et al. developed a novel antimicrobial nanomaterial by supporting ZnO and TiO₂ nanoparticles in the channels of a porous matrix of 4A zeolite.¹⁸  4A zeolite was used as a carrier for ZnO and TiO₂ nanoparticles to decrease the possibility of leaching as much as possible and increase the antimicrobial activity. As shown in Figure 1.9A, the highest zone of inhibition was seen against E.coli: 6.86 ± 0.03, 9.13 ± 0.03 and 10.73 ± 0.04 mm for ZnO/4A z, TiO₂/4A z and ZnO and TiO₂/4A z, respectively. Meanwhile, S. aureus demonstrated the lowest inhibitory zone in all treatments. Indeed, the gram negative bacterium facilitated the mobility of metal-ion nanoparticles into the cell due to its thin peptidoglycan layer. Moreover, it helped in the interaction between nanoparticles and bacterial cell walls. The interaction disrupted biomolecules such as DNA and protein, and prevented processes such as DNA replication and protein synthesis, as shown in Figure 1.9B. The free metal-ion toxicity is generated from oxidative stress via the ROS using hydrogen peroxide (H₂O₂) and organic hydro peroxides (OHP) on the surface of nanoparticles and dissolution of metals from the surface of nanoparticles.

Kim et al. studied the antibacterial activity of Ag nanoparticles against both Staphylococcus as gram positive bacteria and E.coli as gram negative bacteria.¹⁷  Under various concentrations of Ag, incubation times, incubation temperatures and pH, the results demonstrated there was no fluctuation in response to changes in temperature and pH. Ag can cause protein leakage and the inactivation of lactate dehydrogenase (LDH), which converts lactate to pyruvate. Moreover, the high specific surface area of Ag nanoparticles make it easier to release Ag⁺ and can create oxidative stress in microbes associated with ROS release. Anaya-Esparza et al. studied TiO₂–ZnO–MgO mixed oxide nanoparticles against microbial growth and toxicity of Artemia salina.⁸⁹  Compared to the control TiO₂ nanoparticles, the result reported no viable E. coli cells after 15 min when exposed to the mixed oxide nanoparticles. The toxic effect of TiO₂ may be reduced due to the intake of nanoparticles by the organism reducing the capacity of TiO₂ to interact with water molecules to produce ROS. However, with the added nanoparticles of ZnO and MgO, antibacterial features can be significantly increased. Qi et al. evaluated the performance of copper-loaded chitosan nanoparticles in treating various microorganisms.⁹⁰  Chitosan is a natural non-toxic biopolymer and is applied as an antimicrobial agent. The MIC and MBC values were <0.25 and ∼1 µg mL⁻¹, respectively and exhibited higher antibacterial activity. This is due to the chitosan nanoparticles providing higher affinity with bacterial cells for a quantum-size effect and the interaction with negatively charged bacterial membranes and site-specific targeting in vivo. The larger surface area of the chitosan nanoparticles provides a larger surface area for adsorption of the bacterial cells.

Membrane separation processes have become an emerging technology in the field of water and wastewater treatment due to their efficiency in contaminant removal. The structure of the membrane is one of the main factors responsible for determining the separation characteristics and transport mechanisms across the membrane. This technology has been demonstrated to be more effective not only in removal efficiency, but also because of its smaller footprint, and ease of installation, operation and scaling up.⁹¹  However, the main limitation that is commonly faced by membrane technology is membrane fouling. Fouling is the main limitation on the formation of membrane thin film as it can influence the salt and organics rejection as well as water permeability performance. Fouling results in unwanted accumulation of material on solid surfaces to the detriment of function.⁹² 

Great efforts have been made to develop and enhance membrane filtration through the modification of membrane surface properties.⁹³  In general, membrane separation depends on absorption, sieving, and electrostatic phenomena. These mechanisms correspond with the solute and membrane hydrophilic/hydrophobic behaviour. Thus, the researchers have developed membranes with the incorporation of nanomaterial to produce synergic effects on the filtration process. The incorporation of functional nanomaterials offers a great diversity of membranes with different levels of rejection, mechanical strength and fouling liability. Various nanomaterials with different dimensions have been considered as modifiers to improve selectivity, strength, antifouling and permeability by enhancing the hydrophilicity of the polymeric membrane.⁹⁴ 

Nanocomposite membranes are generally broken down into two types: (1) mixed matrix nanocomposite membranes and (2) thin-film nanocomposite (TFN) membranes. The former refers to the nanoparticles that disperse into polymer casting solution before membrane casting and are known as nanoparticles blend membranes. The latter refers to nanoparticles that are allowed to embed within the thin film PA layer or self-assemble on the top-most membrane surface by dispersion and dip-coating or pressurized deposition on the prepared membrane, respectively. Gupta et al. designed an SWCNT-PSf nanocomposite-based membrane for the removal of heavy metals in wastewater.³²  The embedment of SWCNT into PSf nanocomposite was found to reduce the pore size of the membrane and resulted in a smoother surface. The modified nanocomposite membrane showed an improvement in its efficiency with a 3-fold higher rejection capability for metal ions. Qin et al. developed a TFN membrane with PES substrate incorporated with GO and a thin film selective layer of GO/PVA to address the intrinsic issue of forward osmosis (FO).⁹⁵  GO (0.2 wt%) which was sonicated with N,N-dimethylformamide (DMF) was mixed with 1 wt% of polyvinyl pyrrolidone (PVP) and 15 wt% of polyethersulfone (PES). The GO was entrapped inside the mixed matrix nanocomposite membrane by a phase-inversion method. As the result, the infused GO in substrate could reduce the internal concentration polarization (ICP) with a reduction of the structural parameter of 20%. It was the addition of GO nanomaterials that improved the hydrophilicity and increased the porosity and size of the substrate. Similarly, Salehi et al. modified PES with zeolite to form a TFN membrane to reduce the ICP in FO for desalination.⁹⁶  In their study, PES substrate was mixed with zeolite concentrations ranging from 0 to 0.6 wt% and upon the incorporation of 0.4% of zeolite the ICP was reduced. The low structural parameter of 0.4% TFN membrane was influenced by the controlled number of pores and the surface roughness of the substrate. Thus, resulting in an increase in the effective area for water transportation.

Amin et al. fabricated PSf substrates by using 15 wt% of PSf and different loading of organoclay to form TFN for desalination.⁹⁷  Compared to the pristine membrane, the modified membrane resulted in enhanced pure water and water permeability by 60.5 and 44.3%, respectively. The performance was improved due to the presence of organoclay in the membrane matrix which created extra void space for water transport. Aziz et al. synthesized a TFN membrane by incorporating 0.5 wt% positive carbon nitrate (pCN) in the PSf substrate.⁹⁸  From the result, the CN–pCN–TFN membrane exhibited improvement in terms of water permeability and salt rejection as well as better anti-fouling properties compared to pristine TFN membrane. The embedment of pCN within the substrate improved the interaction between the PA layer and the substrate by forming a denser layer of PA to achieve high salt rejection. Moreover, both hydrophilic pCN and CN also created an additional pathway for the transportation of water molecules. Furthermore, these studies indicated that the nanomaterials used in the polymeric membrane are capable of reorganizing the membrane macro void structure by increasing the hydraulic resistance as a result of void volume reduction.³  Also, preferential paths for water passage are created without compromising membrane selectivity.

Surface modification of membranes also aims to impart anti-fouling properties. Zhang et al. developed a MOF hybrid membrane by a self-assembly strategy for dye removal.⁴⁹  The fabricated ZIF-8/PSS membrane achieved high retention and flux for MB of 98.6% and 265 Lm⁻² h⁻¹ bar⁻¹. The performance was enhanced by the hydrophilicity of the membrane as PSS coordinates with ZIF-8 particles through bonding between the hydrophilic sulfonate group and zinc ion, and the ZIF-8 particles are modified to become more hydrophilic. Ahmad et al. coated oppositely charged titania nanosheets (TNS) on a PA–PSf membrane via a layer by layer approach for desalination.¹⁶  A 60% improvement in water permeability was achieved by two TNS bilayers, with 98.45% salt rejection. It is due to the hydrophilicity of TNS that the hydration layer was formed, hindering direct contact of salt ions with the surface of the TFN membrane.

Ng et al. developed a GO/PVA nanofiller-based layer over a microporous substrate for desalination.⁹⁹  Comparing PA on top of GO/PVA and the pristine membrane, the results demonstrated that GO/PVA with a top PA layer showed better separation efficiency, with ∼98% salt rejection and better antifouling properties. The adsorption of foulants on the membrane surface could be prevented with the formation of a hydrated layer. The increase in hydrophilicity of the GO/PVA coating layer tends to promote hydrogen bonding between the water molecules and the membrane surface. Yu et al. assembled 2D–2D reduced graphene oxide (rGO)–TiO₂ material on a seedling membrane.⁴⁶  The membrane exhibited >97.3% removal of organic dyes (Rhodamine B, Methyl orange, MB and Congo red) at ∼9.36 Lm⁻² h⁻¹ bar⁻¹. In Figure 1.10a and b, the higher the molecular size, the higher the rejection rate of the rGO–TiO₂ membrane, which indicates that the selectivity performance depends on the size-exclusion characteristic. The slit-like structure of TiO₂ nanosheet stacking served as an effective rejection route and provided an abundant number of reaction sites for dyes.

Engineering interfaces with various nanostructures can enhance their properties and is fascinating to study. In light of the limitations of traditional treatment technologies, nanotechnologies offer and present new approaches. They have been shown to perform well as an alternative to conventional treatment technologies. The application of nanoparticles in wastewater treatment, including adsorption, photocatalysis, disinfection and membrane separation, has been successfully utilized to achieve maximum efficiency. Metal and metal oxide-based, carbon-based, metal–organic framework nanoparticles have been described in order to provide a better understanding of their unique structures and address the special requirements needed for them to achieve excellent performance.

Nanomaterials, like all newly explored materials, face limitations that need to be addressed in the near future.^7,16  Based on the field study of water remediation, little progress has been made due to technological limitations in the use of nanomaterials. Therefore, improvements in nanomaterial properties are constantly being made to meet the high demand from researchers, industry and policy makers. Moreover, particular emphasis is placed on the construction of nano-structures with versatile dimensions and morphologies, and in particular their production routes, in order to improve their properties and functions as well as to ensure environmental sustainability.¹² 

In particular, there are still uncertainties about the fate of nanomaterials as well as regulatory challenges with regard to the environment.³⁶  Some of the discussed nanomaterials have been suggested to have high potential risks to human health and the environment.¹⁰⁰  Thus, to protect and control the public and the environment from nanotoxicity risks, an exigent requirement is required for effective exploitations of these nanomaterials. The properties of nanoparticles and the ecotoxicity effects of the release of nanoparticles into the environment need to be investigated further for drinking water purposes. During nanoparticle synthesis, the modification of the nanoparticle surfaces for a specific application could have a major impact on their toxicity and safety. Because of the nanoscale and large surface area of nanoparticles, such modifications could result in major toxic effects, which could be expressed in different ways once the nanoparticles are released into the environment. Thus, the in situ measurement of the concentration of nanomaterials in water has been suggested. The diffusive gradient in thin film can be used to measure the concentration of metal oxide nanomaterial in environmental media.¹⁰¹ 

On the other hand, green nanotechnology is always a major focus in order to achieve environmental sustainability. It is generally related to the formation of green nano-products and use of these products to achieve sustainable development. To achieve the aims of nanotechnology with respect to the environment, innovative designs with respect to greener materials and protocols are required. For example, the production of nanoparticles by using extracts from natural substances such as leaves of fruit trees is more sustainable and effective than using physico-chemical methods.¹⁰²  Many studies currently use chemical methods and generate huge amounts of hazardous and toxic by-products. These might harm human health due to composition ambiguity and lack of predictability.¹⁰³  Therefore, green synthesis of nanoparticles will promote eco-friendly, sustainable, less expensive and free of chemical contaminant production and applications.¹⁰⁴ 

Overall, researchers' efforts are the main driving force behind innovation in the field of nanotechnology, through evaluation of the processes used to produce nanomaterials for water and wastewater treatment, and these technologies should be perfected in the future. Undoubtedly, nanomaterials have played very significant roles in advancing the technologies used for wastewater treatment. However, it is important to point out that the development of nanomaterials should be accompanied by several concerns as discussed previously. In particular, an awareness of the related adverse toxicological and environmental impacts need to be placed at the centre of the research in order to avoid the possibility of these nanomaterials becoming the source of environmental contamination.¹⁰⁵  Effective education in the use of nanotechnology for sustainability could be an important step in creating learning experiences to promote the knowledge and skills needed to conduct change in a sustainable way.¹⁰⁶ 

The author would like to acknowledge the financial support provided by Universiti Teknologi Malaysia High Impact Research Grant 08G81 and Malaysia Research University Network Grant 4L862.