Advances in the Applications of Nanomaterials for Wastewater Treatment

: Freshwater is in limited supply, and the growing population further contributes to its scarcity. The effective treatment of wastewater is essential now more than ever, because waterborne infections signiﬁcantly contribute to global deaths, and millions of people are deprived of safe drinking water. Current wastewater treatment technologies include preliminary, primary, secondary, and tertiary treatments, which are effective in removing several contaminants; however, contaminants in the nanoscale range are often difﬁcult to eliminate using these steps. Some of these include organic and inorganic pollutants, pharmaceuticals, pathogens and contaminants of emerging concern. The use of nanomaterials is a promising solution to this problem. Nanoparticles have unique properties allowing them to efﬁciently remove residual contaminants while being cost-effective and environmentally friendly. In this review, the need for novel developments in nanotechnology for wastewater treatment is discussed, as well as key nanomaterials and their corresponding target contaminants, which they are effective against. The nanomaterials of focus in this review are carbon nanotubes, graphene-based nanosheets, fullerenes, silver nanoparticles, copper nanoparticles and iron nanoparticles. Finally, the challenges and prospects of nanoparticle utilisation in the context of wastewater treatment are presented.


Introduction
The rapid growth of the world population and climate change, are two key factors that immensely affect the availability of freshwater. In total, 80% of diseases worldwide are directly or indirectly caused by a poor water quality supply, and 19% of global fatalities are attributable to infections caused by pathogenic microorganisms in water [1]. The developing world is mostly at risk; and this compounded by the fact, the cost of analysing the microbial and chemical content of water is high and relatively complex. In addition to having a negative impact on human life, contaminated water also negatively impacts wildlife and the environment [2,3]. Globally, the construction of water pipes connected to lodgings has increased in the last 20 years, but a large portion of the population still do not have access to clean drinking water, comprising approximately 780 million people [4]. In 2010, 690 million people had access to water which was only partially treated [5]. The conservation of water is a strategy used to reduce the amount of water usage, but other solutions are needed because conservation alone is insufficient due to the demands of the ever-growing population. Poor sanitation also puts pressure on the delivery of clean and safe water. Therefore, new innovative and cost-effective ways to treat wastewater and the corrosive and hazardous properties of some coagulants make them less desirable (e.g., ferric chloride). Coagulants also need to be recovered or reused. This recovery process may increase the cost of the treatment process [16]. Although coagulants are effective in further removing solids, they do not remove endocrine-disrupting compounds (EDCs), which have adverse effects on aquatic and human health. The application of nanotechnology can also target these harmful compounds [17]. Preliminary and primary treatments are very important steps in the WWT process, but they do not remove pathogens, dissolved solids and pharmaceuticals. Thus, further treatment is needed. Figure 1. (a) Wastewater treatment steps [18], showing (b) the removal efficacy of antibiotic resistance genes (ARG) (emerging contaminants) in this step [19].

Secondary Treatment
By utilising biological treatment, this step targets dissolved solids which cannot be removed by filtration or sedimentation. One example of biological treatment is the use of sequencing batch reactors (SBRs), which involves two or more tanks in series [20]. The primary wastewater is released into the first tank, after which oxygen is supplied (aeration) to the tank by blowers. Oxygen is required by the microorganisms to enable them metabolise and decompose the present organic material. The aeration stage may last for up to 105 min [21]. Subsequent to this phase is the settling step, during which the biomass settles in the tank. The clean water can be separated with the aid of a decant arm. Although this step removes a large portion of organic and inorganic pollutants, pollutants in the  [18], showing (b) the removal efficacy of antibiotic resistance genes (ARG) (emerging contaminants) in this step [19].

Secondary Treatment
By utilising biological treatment, this step targets dissolved solids which cannot be removed by filtration or sedimentation. One example of biological treatment is the use of sequencing batch reactors (SBRs), which involves two or more tanks in series [20]. The primary wastewater is released into the first tank, after which oxygen is supplied (aeration) to the tank by blowers. Oxygen is required by the microorganisms to enable them metabolise and decompose the present organic material. The aeration stage may last for up to 105 min [21]. Subsequent to this phase is the settling step, during which the biomass settles in the tank. The clean water can be separated with the aid of a decant arm. Although this step removes a large portion of organic and inorganic pollutants, pollutants in the nanoscale range remain. Secondary treatment are usually incapable of effectively eliminating contaminants of emerging concern (CEC), such as medicines and domestic cleaning products, which are harmful to the environment when released as effluents [22]. However, Pei et al. [19] highlighted that antibiotic resistance genes (ARGs), a class of emerging contaminants, may be more effectively removed during secondary treatment, as illustrated in Figure 1b.

Tertiary Treatment
The final stage is the tertiary treatment, and this differs depending on the WWTP. After the treatment steps mentioned above, there are still pollutants and residual toxins in the effluent which must be removed [23,24]. This step removes nutrients, pathogens and odours [25]. In this way, the quality of the discharged effluent is improved and meets stricter standards that are not obtainable through secondary treatment. Most tertiary treatment methods involve the application of physicochemical techniques, such as activated carbon adsorption, additional disinfection and even reverse osmosis [26][27][28]. It should be mentioned that the advanced oxidation technology, such as photocatalysis and photoelectrocatalysis, outlined as part of the tertiary treatment in Figure 1, are relevant nanotechnological approaches for water remediation due to the use of semiconductor catalysts in the form of nanomaterials.

Limitations of Current Treatment Steps
Despite the successes of the outlined methods, their large-scale applications are often plagued by a myriad of challenges. For example, chlorination, which is used to remove pathogens, leaves an undesired taste and smell in the water, and a further dechlorination step is often required to prevent this. An alternative to chlorine is ozonation, which is also effective in inactivating pathogens [29,30]. However, this method can be expensive due to the energy costs. Other methods of microorganism removal are ion exchange and ultraviolet (UV) photocatalysis, but these are not very convenient for the complete removal of pathogens. Nanofiltration (NF), reverse osmosis (RO), microfiltration (MF) and ultrafiltration (UF) are highly effective in removing micropollutants but may induce equipment fouling. There is a need for newer technologies that can adequately eliminate newly emerging contaminants. Nanotechnology has been deemed an effective technology in this regard [17].

The Use of Nanotechnology for Contaminant Removal
Nanoparticles can be used to improve the quality of an effluent which would otherwise still contain contaminants. The use of nanotechnology to treat wastewater in developing countries may be greatly beneficial, as major infrastructure is not needed, and it can be cost-effective and easy to operate. Nanotechnology for WWT has been utilised for laboratory-scale tests and has been very successful [9,31]. Nanoparticles can adsorb organic and inorganic pollutants from wastewater, which are otherwise difficult to remove. Another advantage of using nanoparticles is that during the manufacturing process, harmful byproducts are not released. Their unique properties, including their small size, large surface area, high reactivity, large surface-area-to-volume ratio and high porosity, provide them with these advantages. They also have unique optical properties, such as their transparency and the presence of iridescent films [32]. The large surface area is the most important, as it allows for more active adsorbing sites. Some nanoparticles can also be reused, which is a major sustainable attribute. Magnetic nanoparticles can be collected and separated from the wastewater after treatment and reused many times. These magnetic nanoparticles are capable of removing radionuclides and heavy metals [6].
In a study by Jiang et al. [33], adsorption and membrane filtration, as nanotechnologybased treatment methods, were examined. Adsorption was preferred over the membrane filtration method, as it was the simplest to use and could treat organic and inorganic pollutants. It is an attractive method for removing organic materials, as well as salts, bases, acids and toxic compounds. The efficiency of adsorption was determined by the pore structure of the adsorbent and the interaction between the adsorbent and the contaminant [34]. The authors also examined carbon-based nanoparticles, graphene-based nanoparticles and carbon nanotubes, all of which had a high performance with regard to the removal of heavy metals and organic pollutants. The hydrophobicity of carbon-based nanoparticles was also tested, and it was realised that the adsorption energy was increased, and the surface area was reduced as loose aggerates were formed. To alleviate this, functional groups or metal Environments 2022, 9, 141 6 of 27 oxide particles were added. The authors also remarked that carbon nanoparticles can be reused if the pH is lowered, without a decrease in the adsorption capacity. Membrane filtration (MF, UF and NF) was also examined. The difference between MF, UF and NF is the size of the pores of the membranes. However, the main disadvantage of these processes is the fouling of the membranes. This issue can be overcome via modification (with silica, aluminium, zeolite and titanium oxide). Silver can also be added to prevent biofilm growth and to kill bacteria and viruses [35].
A major challenge affecting many contaminants is the fact that they can be present in very low concentrations, which can be difficult to detect. The advantage of using nanomaterials is that they can concentrate pollutants to a level high enough to allow them to be detected and removed. An example of this is the use of Au-TiO 2 nanoparticles to concentrate low levels of insecticide in wastewater, which can then be removed [36]. New developments on effective sensors for detecting nanosized contaminants are needed, as the problem of the false detection of contaminants and pathogens is predominant in most treatment applications.

Types of Nanoparticles for Wastewater Treatment
It is worth mentioning that several parameters should be considered, such as the quality standards of the effluent that are to be met and the efficiency of the nanomaterial, its recyclability, environmental impact and cost, before applying nanotechnology for WWT [37]. Adequate knowledge of the properties, characteristics and functions of different nanoparticles is also important when deciding which type to use for contaminant removal. In this section, the characteristics of carbon nanotubes, graphene-based nanoparticles and silver, copper and iron nanoparticles and their applications to water treatment are discussed.

Carbon Nanotubes
Carbon nanotubes (CNTs) are comprised of graphene sheets rolled into a cylindrical tube shape, which is shown in Figure 2, referred to as a single-walled carbon nanotube (SWCNT). It is important to mention that this is not the procedure for fabricating CNTs but only indicates a general representation of CNTs. Multiple layers of graphene sheets are referred to as multi-walled carbon nanotubes (MWCNTs). The fabrication methods for CNTs and MWCNTs (including chemical vapour deposition, laser ablation, arc discharge, and electrophoretic deposition) are extensively covered in the literature [38,39]. structure of the adsorbent and the interaction between the adsorbent and the contaminant [34]. The authors also examined carbon-based nanoparticles, graphene-based nanoparticles and carbon nanotubes, all of which had a high performance with regard to the removal of heavy metals and organic pollutants. The hydrophobicity of carbon-based nanoparticles was also tested, and it was realised that the adsorption energy was increased, and the surface area was reduced as loose aggerates were formed. To alleviate this, functional groups or metal oxide particles were added. The authors also remarked that carbon nanoparticles can be reused if the pH is lowered, without a decrease in the adsorption capacity. Membrane filtration (MF, UF and NF) was also examined. The difference between MF, UF and NF is the size of the pores of the membranes. However, the main disadvantage of these processes is the fouling of the membranes. This issue can be overcome via modification (with silica, aluminium, zeolite and titanium oxide). Silver can also be added to prevent biofilm growth and to kill bacteria and viruses [35].
A major challenge affecting many contaminants is the fact that they can be present in very low concentrations, which can be difficult to detect. The advantage of using nanomaterials is that they can concentrate pollutants to a level high enough to allow them to be detected and removed. An example of this is the use of Au-TiO2 nanoparticles to concentrate low levels of insecticide in wastewater, which can then be removed [36]. New developments on effective sensors for detecting nanosized contaminants are needed, as the problem of the false detection of contaminants and pathogens is predominant in most treatment applications.

Types of Nanoparticles for Wastewater Treatment
It is worth mentioning that several parameters should be considered, such as the quality standards of the effluent that are to be met and the efficiency of the nanomaterial, its recyclability, environmental impact and cost, before applying nanotechnology for WWT [37]. Adequate knowledge of the properties, characteristics and functions of different nanoparticles is also important when deciding which type to use for contaminant removal. In this section, the characteristics of carbon nanotubes, graphene-based nanoparticles and silver, copper and iron nanoparticles and their applications to water treatment are discussed.

Carbon Nanotubes
Carbon nanotubes (CNTs) are comprised of graphene sheets rolled into a cylindrical tube shape, which is shown in Figure 2, referred to as a single-walled carbon nanotube (SWCNT). It is important to mention that this is not the procedure for fabricating CNTs but only indicates a general representation of CNTs. Multiple layers of graphene sheets are referred to as multi-walled carbon nanotubes (MWCNTs). The fabrication methods for CNTs and MWCNTs (including chemical vapour deposition, laser ablation, arc discharge, and electrophoretic deposition) are extensively covered in the literature [38,39].   Table 1 describes a comparison between SWCNTs and MWCNTs. While SWCNTs are usually present in stiff rope-like bundles (resulting from their small diameter/surface area and increased van der Waals forces), MWCNTs can be present in an agglomerated needle-like structures [41]. SWCNTs tend to be in the diameter range of 0.7-3 nm, whereas MWCNTS may possess a diameter of 10-200 nm [41,42]. Poor High Thermal conductivity (W/(m K)) 6000 2000 Specific gravity (g/cm 3 ) 0.8 1.8 Electrical conductivity (S/cm) 10 2 -10 6 10 3 -10 5 Electron mobility (cm 2 /(Vs))~10 5 10 4 -10 5 Thermal stability in air ( • C) >600 >600 CNT is one of the allotropes of carbon and was discovered approximately three decades ago. It is one of the lightest and toughest materials on earth and possesses hydrophobic properties [44]. To attract water functional groups that are hydrophilic, they are coated on the top of each nanotube. The water flows into the CNT but is pushed out very quickly as it is repelled by the tube walls. The pollutants are caught on the top of the CNT by the functional groups, leaving only clean water to flow out [40,45]. CNTs are chemically stable and have mesopores, distinguishing them from traditional adsorbents such as clay, zeolites, metal oxides, activated carbon and polymers. The unique thermal, chemical, electrical and mechanical properties of CNTs sets them apart, as they are able to remove many impurities from aqueous solutions [43]. CNTs are becoming very popular not only for wastewater purification but also for energy storage, space applications and electronics [34]. Organic material is typically adsorbed on the external and internal portions of open-ended CNTs. Inorganic pollutants can also be adsorbed on the external side through the addition of certain functional groups. Polycyclic aromatic hydrocarbons (PAHs), which are persistent organic pollutants, can be adsorbed in the interstitial channel of the CNT [45]. The ease of functionalization, large surface area, high aspect ratio and fast water transport are features that make CNTs an evolving nanomaterial. They can be used on their own as a filter, or they can be implemented in existing membranes to improve the performance.
Another feature of CNTs that renders their use highly attractive is their antifouling properties, disinfection capacity, permeability and strength, as enabled by their sp 2 chemical bonds. CNTs can be synthesized by photoablation, chemical vapour deposition or arc discharge and can be free-standing or mixed. Free-standing CNTs can be split into verticallyaligned CNTs, where water flows very quickly through their inner section, and those manufactured as bucky-paper members, where their arrangement is random, leading to a large 3D network with a high surface area. CNT membranes can supersede or possibly improve the performance of NF, RO, MF, UF and forward osmosis (FO). The hydrophobic hollow section indicates that little external energy is required to move the water molecules through the CNT [46], which greatly reduces the cost.
CNTs have been used to remove pollutants such as dyes, pharmaceuticals and herbicides from water. The oxygen groups on the CNT surface facilitate adsorption processes via chemisorption, physisorption and electrostatic interaction. Balarak et al. used SWCNTs to remove AB29 dye, which is an acid dye [47]. Their study revealed that the mechanism of the adsorption of AB29 dye by SWCNT occurred mainly via London dispersion force, π − π electrostatic interaction, hydrogen bonding and the hydrophobic effect. Approximately 99.4% of the dye was removed using SWCNT with the optimal conditions of 0.12 g/L, an initial concentration of 10 mg/L, a pH of 3 and a contact time of 75 min.
A major challenge of CNTs is their hydrophobic nature, which causes them to undergo agglomeration in water, and due to massive Van der Waals forces, the adsorptive surface area is severely reduced. To increase the adsorption capacity of CNTs, functionalization with polymers and amorphization, with many defects, have been exploited. For instance, a reported study on the removal of Methyl Orange and Rhodamine B using a CNT revealed that the short-term temperature treatment (200 • C for 30 min) of Ferrocene and ammonium chloride (analytically pure) could result in an appreciable hydrophilicity [48]. However, the adsorption capacity was still low with 21.39 mg/g and 25.5 mg/g in the case of Methyl orange and Rhodamine B, respectively.
The use of carboxylic-acid-modified CNTs as a linking skeleton for metal-organicframeworks has been reported for methylene blue dye removal. The functionalized CNT-MOF was wrapped in a gelatine and crosslinked to increase the chemical stability. The CNT provided a greater surface area for a greater adsorption, and the adsorption capacity of 106 mg/g of methylene blue was achieved under the conditions of a 289K and 100 mg/L initial dye concentration. The adsorption isotherm and kinetics suggested a chemisorption mechanism with an activated energy of 83.33 kJ/mol, and the adsorbent could be reused six times [49]. Another study on methylene blue removal using a polyethylene terephthalate nanofiber-MWCNT adsorbent revealed a physisorption mechanism with a very low maximum adsorption capacity of 7.047 mg/g [50]. Heavy metal removal has been achieved using polymer-metal-organic-framework-CNT composites. A recent study on the removal of arsenic-spiked water with two adsorbents, Zn-BDC@chitosan/CNT and Zn-BDC@chitosan/graphene oxide (GO), revealed that the graphene oxide-metal-organicframework-chitosan outperformed the CNT analogue because of the GO higher specific surface area and active sites [51]. This shows not only that MOF and polymers are necessary to increase interactions with the adsorbate, but also that the surface functional groups and specific surface area of the CNTs are vital for achieving a good adsorption capacity. Furthermore, CNT-grafted poly[(sodium methacrylate)-co-2-(methacryloyloxy)ethyl acetoacetate] has been used to remove high concentrations of lead (II) from water. The synthesized material possesses both acid and basic functional groups. The maximum adsorption reached 1178 mg/g, and an adsorbent dose of 2.5 g/L could reduce Pb 2+ from 1000 ppb to 2 ppb, which was significantly higher than the values described in most of the reported studies in the literature [52]. A similar study on polymer-grafted CNTs reported that the material could remove Pb 2+ with a relatively lower capacity [53,54].
The studies conducted revealed that chemisorption dominates in terms of the removal capacity of CNTs for most pollutants, and the pseudo-second-order kinetics can effectively describe the mechanism of adsorption. The pH value is the most critical parameter that influences the CNT adsorption capacity, especially for divalent metals. However, the low dispersion of CNTs and their high agglomeration and hydrophobic nature present serious challenges that ultimately result in a lower adsorption capacity compared to GO and active carbons. The grafting of CNTs on polymers and their crosslinking with mainly glutaraldehyde to increase their mechanical and chemical strength are noteworthy endeavours. The polymer grafting contributed to a greater capacity and reusability of most reported CNT-modified adsorbents. However, with the exception of natural polymers, synthesis is usually complex and time consuming. A natural method which seems to be especially appealing is the use of biological self-assembly microorganism-CNT composites. This is a form of natural crosslinking with microorganisms such as fungi, bacteria, even algae and can be achieved at the lower temperature of 30 • C. This idea has been used for porous carbon derived from starch, and it showed a higher surface area and higher adsorption capacity than porous carbon alone [55]. Table 2 shows some reported CNTs, along with their target pollutants, optimal conditions and adsorption capacities.  Note: * is in g/L, ** is in percentage (%).

Graphene-Based Nanoparticles
Graphene-based nanosheets (GBN), which are flexible and transparent, consist of three nanosheets which are all similar in structure, namely graphene, graphene oxide (GO) and reduced graphene oxide (rGO), as shown in Figure 3 [61]. GBN has become popular in the fields of optics, mechanics, electrics and, of course, environmental remediation. The final properties of GBNs depend on the route through which they are manufactured. Note: * is in g/L, ** is in percentage (%).

Graphene-Based Nanoparticles
Graphene-based nanosheets (GBN), which are flexible and transparent, consist of three nanosheets which are all similar in structure, namely graphene, graphene oxide (GO) and reduced graphene oxide (rGO), as shown in Figure 3 [61]. GBN has become popular in the fields of optics, mechanics, electrics and, of course, environmental remediation. The final properties of GBNs depend on the route through which they are manufactured. GO and rGO have surface-oxygen-containing groups (OCGs), chips on their surface, defined edges and structural wrinkles, which make them attractive for water decontamination. Studies have shown that these two materials significantly reduce the amount of organic waste and metallic ions in polluted aqueous solutions. GBNs have a higher adsorption capacity than CNTs, as well as resins and activated carbon, making them a popular choice for pollution control. Graphene can be manufactured using graphite as a raw GO and rGO have surface-oxygen-containing groups (OCGs), chips on their surface, defined edges and structural wrinkles, which make them attractive for water decontamination. Studies have shown that these two materials significantly reduce the amount of organic waste and metallic ions in polluted aqueous solutions. GBNs have a higher adsorption capacity than CNTs, as well as resins and activated carbon, making them a popular choice for pollution control. Graphene can be manufactured using graphite as a raw material. The graphene is peeled off the graphite flakes by a process called mechanical exfoliation until a monolayer of graphene is obtained. Mechanical exfoliation cannot be used for the mass production of graphene and, therefore, it has only been used in laboratory-scale research. Monolayer graphene is very popular because of its high thermal conductivity, high electrical conductivity, flexibility and high resistance. Liquid exfoliation is another method used to produce graphene, and this method can produce it on a large scale but yields a less pure form than chemical exfoliation [62]. GO can be manufactured by fuming different chemicals as oxidizing agents. Traditionally, HNO 3 and KClO 3 have been used, but these release NO x and ClO 2 , which are very toxic gases. Thus, they were replaced with H 3 PO 4 . rGO can be obtained by applying reducing agents to GO and is manufactured as an alternative to graphene. It is highly desirable for water purification due to its functional groups (e.g., C-OH, C-O-C, C=O) and its structure of wrinkles and cracks, which increase the surface area. Chemical reduction using borohydrides, hydrazine and acidic/alkaline reductions are the main production routes for GO. Thermal reduction and microbiological reduction can also be used [63].
The defined edges and defects of GBNs, which are produced during processing, are key attributes that make them useful for water decontamination, as they provide a large surface area (2630 m 2 /g) [64]. Due to the sp 2 aromatic structure of GBN, they are chemically stable in acid and alkaline conditions, making them suitable for use in WWTPs. GO and rGO are low in cost and have many adsorption sites that can adsorb PAHs, dyes, organic pollutants and antibiotics, all of which can be difficult to remove using the current wastewater treatment technologies. Graphene can also adsorb organic material, but the adsorption capacity depends on the pH, the temperature, the natural organic matter and the ionic strength. GO has been heavily relied on for the removal of different dyes, including methylene blue and methyl violet. It also has hydrophilic and hydrophobic properties, making it efficient in separating contaminants from wastewater [65,66]. The key attributes that make GO a promising method for the removal of several wastewater contaminants are its large theoretical specific surface, high negative charge density, surface hydrophobic interaction, hydrophilicity and ease of synthesis from readily available natural graphite using exfoliation or chemical oxidation methods [67,68].
Yang et al. [69] conducted a study on the adsorption capacity of polar and non-polar compounds by colloidal GO. They realised a strong adsorption affinity of GO for all the tested compounds. rGO adsorbed non-polar aromatic organic compounds, such as PAHs and nitroaromatic compounds, better than GO. This reinforces the importance of wastewater characterisation before the administration of the most effective nanomaterial for its treatment. GO, which has oxidative debris on the surface, can reduce the adsorption performance by blocking the adsorption sites. A sample of GO without oxidative debris was tested; it was more effective in absorbing 1-nitropyrene compared to raw GO by 75%. The conditions of the solution in which the GBNs are placed play a role in the adsorption. The adsorption of antibiotics (doxycycline, tetracycline and oxytetracycline) by GO was also tested and yielded adsorption capacities of 398.4, 212.3 and 313.5 mg/g, respectively. When the pH was decreased, the adsorption capacity increased, and for rGO, the adsorption capacity increased with the increase in pH [69]. Generally, GO-based materials present a higher sorption capacity, even for trace metals. This makes them appealing; however, the high cost of their synthesis and different forms of complex functionalization limits their large scale application. Table 3 presents the recent reported studies on the use of GO-based nanomaterials for wastewater treatment.

Fullerenes
Fullerenes are very important carbon-based materials that have been tested for the treatment of water and wastewater. These materials are obtained through the very slow condensation of vaporized carbon [81]. They were discovered when experiments were conducted to understand how long-chain molecules can be formed in the circumstellar and interstellar spaces in the presence of a laser beam [82]. This carbon structure is very similar to graphite but has rolled-up layers and can take the form of tubular, spherical, and ring-like geometric shapes. The difference between graphite and fullerenes is that while graphite has a hexagonal carbon atomic structure, the latter has pentagonal and hexagonal carbon rings [83]. Normally, the slow condensation of carbon vapor results in spherical fullerenes. However, the use of a catalyst during synthesis can yield tubular or ring-like structures. Fullerenes are usually represented as C 20+n , and the C 60 spherical family has been widely explored because of its unique sp2 hybridization and mechanical strength [84]. It can withstand high pressures of up to 3000 psi without deformation and has a bulk modulus of 668 GPa, which makes it harder than diamond [85]. Moreover, it has a high dielectric constant of 4, high affinity fir electrons, large surface-to-volume ratio and a high refractive index. It is also hydrophobic i.e., insoluble, or slightly soluble in water, although it can dissolve in benzene, toluene and carbon disulphide [86]. Additionally, it is the only carbon allotrope that can be dissolved at room temperature, making its synthesis straightforward.
To increase the solubility of nC 60 , it must be functionalized, as this is obtainable through the use of carbon nanotubes. The incorporation of carboxyl, hydroxyl, epoxy groups and heteroatoms increases the material's capacity to bind organic molecules in an aqueous medium via covalent or non-covalent bonding. These characteristics make C 60 a suitable material for environmental applications. It has been reported that under specific conditions, C 60 is not cytotoxic or harmful [87], and it has a neutral biological consequence [88,89]. Water-soluble C 60 and its derivates have been investigated for their antibacterial activities, and they were found to be toxic to Escherichia Coli and Bacillus subtilis, hindering their survival under low salt concentrations [90]. Their effectiveness against fungal spores has also been demonstrated [91], as shown in Figure 4E,F. Another study on the biodegradation of sewage sludge using C 60 via anaerobic digestion indicated no significant effect on the anaerobic community's structure or performance [92]. An in vitro analysis suggested that C 60 is not harmful to humans and animals; however, there is an observable, serious toxicity to animals in vivo [93]. This cytotoxicity normally arises from surface-modified C 60 . In general, C 60 fullerenes are not cytotoxic and can be deployed as adsorbents in water treatment, as fillers in a membrane and even in electrochemical treatment.
Glyphosate, a herbicide used for weed control, is a notable source of water pollution with little or no regulation in many countries. The theoretical elucidation of the adsorption of glyphosate on C 60 in a vacuum or water reveals that it occurs in at least three different forms, with adsorption energy minima of −0.575 (−0.431) eV, −0.480 (−0.372) eV and −0.451 (−0.402) eV, respectively [94]. However, the effect of the ionic state of glyphosate should be considered carefully, since it can reduce adsorption. Another study demonstrated that the solid-state mixing of ZnFe 2 O 4 and fullerene CNT yielded magnetic fullerene nanoparticles capable of removing heavy metals (Hg(II), Cd(II), Sn(II), and Pb(II)) from aqueous solution [95]. The addition of ZnFe 2 O 4 improved the adsorption capacity of the fullerene by about 25%, and the adsorbent could be reused after chemical treatment with either EDTA or HNO 3 .
Recent developments in the application of this nanomaterial have featured the utilisation of C 60 and TiO 2 for the degradation of a wide variety of pollutants, as well as the application of computational methods (such as density functional theory, DFT) to elucidate the molecular interactions between C 60 and TiO 2 . Qi et al. [96] demonstrated that the modified C 60 -aTiO 2 nanocomposite possessed an enhanced dye degradation activity against methylene blue. A photocatalytic mechanism was proposed using DFT, and it was realised that incorporating C 60 into the TiO 2 surface introduced an additional doping site, resulting in an improved performance. The application of water-soluble fullerol for the activation of TiO 2 under visible light was demonstrated to be a viable route for inducing the reduction of toxic Cr VI to less toxic Cr III in water [97]. Given the ease of synthesis of this class of nanomaterials and their enhanced visible light activity, the application of fullerene nanomaterials for water and wastewater treatment is expected to grow in the coming years.

Silver Nanoparticles
The overuse, incorrect use and incorrect disposal of antibiotics has led to the rapid growth of antibiotic resistance. WWTPs are said to be a reservoir for antibiotic-resistant bacteria, as they are not killed before their release into the environment. This increased resistance is a great concern for the future development of new antibiotic drugs [98]. For many decades, silver nanoparticles have been known to have excellent antimicrobial properties. While they are extremely toxic to bacteria such as E. coli, they are only slightly toxic to animal cells at a certain concentrations. Silver has been and is still used for drinking water disinfection. Silver nanoparticles can be manufactured by inert gas condensation (IGC) [99], where temperatures above 2000 °C are used. The temperature can be varied to change the size of the nanoparticles.
Another method used to manufacture silver nanoparticles is co-condensation, where the use of higher temperatures leads to larger particle sizes and a narrower distribution [100]. When grown using the ICG method, the average particle size is 75 nm, whereas an average particle size of 15 nm can be achieved when the co-condensation method is applied [101]. As the co-condensation method produces smaller particles (with an increased surface area), it is a viable synthesis route, as smaller particle sizes possess better adsorption properties. In addition to these methods, chemical reduction, microemulsion, UVinitiated photoreduction and micro-assisted synthesis can be applied for the synthesis of silver nanoparticles. The interested reader may refer to the extensive review by Iravani et al. [102], which discusses several synthesis methods. In the study by Baker et al. [101], the antimicrobial effect of silver nanoparticles was tested using E. coli. The nanoparticles were mixed with E. coli and spread on agar plates. Plates containing silver nanoparticles were also used, and the E. coli was spread on these plates. Figure 5 shows the colony-forming unit (CFU) on each of the plates using different methods. Increasing the concentration of

Silver Nanoparticles
The overuse, incorrect use and incorrect disposal of antibiotics has led to the rapid growth of antibiotic resistance. WWTPs are said to be a reservoir for antibiotic-resistant bacteria, as they are not killed before their release into the environment. This increased resistance is a great concern for the future development of new antibiotic drugs [98]. For many decades, silver nanoparticles have been known to have excellent antimicrobial properties. While they are extremely toxic to bacteria such as E. coli, they are only slightly toxic to animal cells at a certain concentrations. Silver has been and is still used for drinking water disinfection. Silver nanoparticles can be manufactured by inert gas condensation (IGC) [99], where temperatures above 2000 • C are used. The temperature can be varied to change the size of the nanoparticles.
Another method used to manufacture silver nanoparticles is co-condensation, where the use of higher temperatures leads to larger particle sizes and a narrower distribution [100]. When grown using the ICG method, the average particle size is 75 nm, whereas an average particle size of 15 nm can be achieved when the co-condensation method is applied [101]. As the co-condensation method produces smaller particles (with an increased surface area), it is a viable synthesis route, as smaller particle sizes possess better adsorption properties. In addition to these methods, chemical reduction, microemulsion, UV-initiated photoreduction and micro-assisted synthesis can be applied for the synthesis of silver nanoparticles. The interested reader may refer to the extensive review by Iravani et al. [102], which discusses several synthesis methods. In the study by Baker et al. [101], the antimi-crobial effect of silver nanoparticles was tested using E. coli. The nanoparticles were mixed with E. coli and spread on agar plates. Plates containing silver nanoparticles were also used, and the E. coli was spread on these plates. Figure 5 shows the colony-forming unit (CFU) on each of the plates using different methods. Increasing the concentration of silver led to increasing antibacterial behaviour. Similar antimicrobial properties have been reported in the following studies [103,104].
Environments 2022, 9, x FOR PEER REVIEW 14 of 28 Figure 5. Antimicrobial effect of silver nanoparticles on bacteria spread on plates containing silver nanoparticles and on bacteria mixed with silver nanoparticles and then spread on a plate. The IGC method was used to produce these nanoparticles [101].
Silver nanoparticles have been demonstrated to be effective in eliminating over 700 microorganisms found in WWTPs. Silver nanoparticles target microorganisms through more than three mechanisms. This implies a reduced possibility of mutation resulting in resistance. Even at very low concentrations, silver nanoparticles are very effective. The silver ions bind to the DNA of the microorganisms, preventing them from taking up salt and phosphorous, which are necessary transport mechanisms for the bacteria [105]. This also prevents the necessary respiratory mechanisms required for the cell's survival. Silver nanoparticles have also been used as an effective larvicidal agent. They are also used as antifouling membranes in UF processes. Silver nanoparticles have been shown to be effective against drug-resistant bacteria and biofilm-forming bacteria, as they prevent the biofilm from forming [106]. Madeła [107] examined the influence of 2 mg/L AgNPs on the biological wastewater treatment process in an SBR reactor. The AgNPs showed enhancing effects on the efficiency of the treatment process, as determined by the TOC removal (Figure 6). A similar study [108] by the same author using CuNPs, as subsequently presented, showed the opposite effect. Recently, silver-loaded magnetic nanoparticles were utilised for the removal of coliform bacteria and heterotrophic bacteria, as well as the reduction in the COD of wastewater treatment plants [109,110]. The smaller size of these nanoparticles provided an increased surface area for the effective adsorption of organic matter in the water sample. Another key and recent advancement in the application of AgNPs is the hybrid application of nano-silver and other polymers, which has been demonstrated to be an effective method for removing heavy metals from wastewater [110]. For, example, the combination of AgNP and polyvinyl alcohol/aminopropyltriethoxysilane is an effective route for the removal of Mn 2+ ions, as well an effective antifungal agent [111]. In addition, AgNP complexed with the Schiff base N-(4-hydroxy-3-methoxybenzylidine)-biphenyl-4amine is effective for the removal of Cu 2+ ions [112]. Silver nanoparticles have been demonstrated to be effective in eliminating over 700 microorganisms found in WWTPs. Silver nanoparticles target microorganisms through more than three mechanisms. This implies a reduced possibility of mutation resulting in resistance. Even at very low concentrations, silver nanoparticles are very effective. The silver ions bind to the DNA of the microorganisms, preventing them from taking up salt and phosphorous, which are necessary transport mechanisms for the bacteria [105]. This also prevents the necessary respiratory mechanisms required for the cell's survival. Silver nanoparticles have also been used as an effective larvicidal agent. They are also used as antifouling membranes in UF processes. Silver nanoparticles have been shown to be effective against drug-resistant bacteria and biofilm-forming bacteria, as they prevent the biofilm from forming [106]. Madeła [107] examined the influence of 2 mg/L AgNPs on the biological wastewater treatment process in an SBR reactor. The AgNPs showed enhancing effects on the efficiency of the treatment process, as determined by the TOC removal ( Figure 6). A similar study [108] by the same author using CuNPs, as subsequently presented, showed the opposite effect. Recently, silver-loaded magnetic nanoparticles were utilised for the removal of coliform bacteria and heterotrophic bacteria, as well as the reduction in the COD of wastewater treatment plants [109,110]. The smaller size of these nanoparticles provided an increased surface area for the effective adsorption of organic matter in the water sample.
Another key and recent advancement in the application of AgNPs is the hybrid application of nano-silver and other polymers, which has been demonstrated to be an effective method for removing heavy metals from wastewater [110]. For, example, the combination of AgNP and polyvinyl alcohol/aminopropyltriethoxysilane is an effective route for the removal of Mn 2+ ions, as well an effective antifungal agent [111]. In addition, AgNP complexed with the Schiff base N-(4-hydroxy-3-methoxybenzylidine)-biphenyl-4-amine is effective for the removal of Cu 2+ ions [112]. Environments 2022, 9, x FOR PEER REVIEW 15 of 28

Copper Nanoparticles
Copper nanoparticles also possess exceptional antimicrobial qualities against Grampositive and Gram-negative bacteria. The antimicrobial impacts of silver and copper particles on E. coli, S. aureus and Bacillus subtilis were examined in the work of Ruparelia et al. [113]. Copper showed a higher antibacterial activity in the inactivation of B. subtilis, whereas the silver nanoparticles outperformed copper in ensuring the inaction of E. coli and S. aureus. Thus, copper nanoparticles have a great affinity towards B. subtilis and are a good choice of nanomaterials for the purpose of inactivation. A study by Suleiman et al. [114] investigated water decontamination by synthesised copper oxide (CuO) nanoparticles alone and CuO nanoparticles stabilised with a surfactant. The nanoparticles were formed using a precipitation method (which is environmentally friendly, safe and simple). The average size of the rod-shaped nanoparticles was between 7 and 12 nm. Parameters such as the nanoparticle size, the concentration of nanoparticles, pH, temperature of wastewater and contact were considered. The antimicrobial effects of CuO were observed when the concentration reached 100 µ g/mL. However, the antimicrobial effects of the CuO nanoparticles stabilised with a surfactant were observed when the concentration was only 10 µ g/mL. Of the three different sizes (9.1, 11.4 and 12.4 nm) of nanoparticles produced, the largest 12.4 nm particles had the least significant antibacterial activity, whereas the 11.4 nm particles showed the best antibacterial effects. The bacteria considered were total coliforms (TC), faecal coliforms (FC) and E. faecalis. Figure 7 also illustrates the impact of the contact times applied on the observed antibacterial activity.

Copper Nanoparticles
Copper nanoparticles also possess exceptional antimicrobial qualities against Grampositive and Gram-negative bacteria. The antimicrobial impacts of silver and copper particles on E. coli, S. aureus and Bacillus subtilis were examined in the work of Ruparelia et al. [113]. Copper showed a higher antibacterial activity in the inactivation of B. subtilis, whereas the silver nanoparticles outperformed copper in ensuring the inaction of E. coli and S. aureus. Thus, copper nanoparticles have a great affinity towards B. subtilis and are a good choice of nanomaterials for the purpose of inactivation. A study by Suleiman et al. [114] investigated water decontamination by synthesised copper oxide (CuO) nanoparticles alone and CuO nanoparticles stabilised with a surfactant. The nanoparticles were formed using a precipitation method (which is environmentally friendly, safe and simple). The average size of the rod-shaped nanoparticles was between 7 and 12 nm. Parameters such as the nanoparticle size, the concentration of nanoparticles, pH, temperature of wastewater and contact were considered. The antimicrobial effects of CuO were observed when the concentration reached 100 µg/mL. However, the antimicrobial effects of the CuO nanoparticles stabilised with a surfactant were observed when the concentration was only 10 µg/mL. Of the three different sizes (9.1, 11.4 and 12.4 nm) of nanoparticles produced, the largest 12.4 nm particles had the least significant antibacterial activity, whereas the 11.4 nm particles showed the best antibacterial effects. The bacteria considered were total coliforms (TC), faecal coliforms (FC) and E. faecalis. Figure 7 also illustrates the impact of the contact times applied on the observed antibacterial activity.

Copper Nanoparticles
Copper nanoparticles also possess exceptional antimicrobial qualities against Grampositive and Gram-negative bacteria. The antimicrobial impacts of silver and copper particles on E. coli, S. aureus and Bacillus subtilis were examined in the work of Ruparelia et al. [113]. Copper showed a higher antibacterial activity in the inactivation of B. subtilis, whereas the silver nanoparticles outperformed copper in ensuring the inaction of E. coli and S. aureus. Thus, copper nanoparticles have a great affinity towards B. subtilis and are a good choice of nanomaterials for the purpose of inactivation. A study by Suleiman et al. [114] investigated water decontamination by synthesised copper oxide (CuO) nanoparticles alone and CuO nanoparticles stabilised with a surfactant. The nanoparticles were formed using a precipitation method (which is environmentally friendly, safe and simple). The average size of the rod-shaped nanoparticles was between 7 and 12 nm. Parameters such as the nanoparticle size, the concentration of nanoparticles, pH, temperature of wastewater and contact were considered. The antimicrobial effects of CuO were observed when the concentration reached 100 µ g/mL. However, the antimicrobial effects of the CuO nanoparticles stabilised with a surfactant were observed when the concentration was only 10 µ g/mL. Of the three different sizes (9.1, 11.4 and 12.4 nm) of nanoparticles produced, the largest 12.4 nm particles had the least significant antibacterial activity, whereas the 11.4 nm particles showed the best antibacterial effects. The bacteria considered were total coliforms (TC), faecal coliforms (FC) and E. faecalis. Figure 7 also illustrates the impact of the contact times applied on the observed antibacterial activity.  It was also realised that a lower temperature (25 • C) was required for inactivation when the surfactant stabilised CuO nanoparticles were used, compared to the 30 • C requirement of the ordinary CuO nanoparticles. The typical average temperature of wastewater is between 10 and 20 • C but can be higher because of warm effluents originating from households and businesses. The pH also has an impact on the antibacterial effect of CuO nanoparticles with and without the surfactant. Table 4 shows the results of the antibacterial effects on TC, FC and E. faecalis using CuO nanoparticles alone and those modified with the surfactant tetra-octylammonium bromide (TOAB (3)). Cu nanoparticles can also be synthesized via one-pot synthesis using underwater plasma [115]. The one-pot synthesis method has the advantages of an improved reaction time and high throughput. A schematic of the one-pot method is presented in Figure 8. It was also realised that a lower temperature (25 °C) was required for inactivation when the surfactant stabilised CuO nanoparticles were used, compared to the 30 °C requirement of the ordinary CuO nanoparticles. The typical average temperature of wastewater is between 10 and 20 °C but can be higher because of warm effluents originating from households and businesses. The pH also has an impact on the antibacterial effect of CuO nanoparticles with and without the surfactant. Table 4 shows the results of the antibacterial effects on TC, FC and E. faecalis using CuO nanoparticles alone and those modified with the surfactant tetra-octylammonium bromide (TOAB (3)). Cu nanoparticles can also be synthesized via one-pot synthesis using underwater plasma [115]. The one-pot synthesis method has the advantages of an improved reaction time and high throughput. A schematic of the one-pot method is presented in Figure 8. In addition to the antimicrobial properties of Cu (although it is not as effective as Ag), Cu nanoparticles also act as surface area and pore volume enhancers of polymeric beads (the commonly applied substrate for nanoparticle utilisation during large-scale applications) [116]. Thus, they can provide the Ag nanoparticles (in the beads) with better access to different bacteria when Cu and Ag nanoparticles are simultaneously applied for wastewater treatment. As in the case of Silver, the combination of Cu nanoparticles with the -SH groups of key microbial enzymes is the probable inactivation mechanism [117]. However, further studies are required to validate this mechanism. It has been found that Cu nanoparticles (CuNP), particularly at high concentrations, can alter the In addition to the antimicrobial properties of Cu (although it is not as effective as Ag), Cu nanoparticles also act as surface area and pore volume enhancers of polymeric beads (the commonly applied substrate for nanoparticle utilisation during large-scale applications) [116]. Thus, they can provide the Ag nanoparticles (in the beads) with better access to different bacteria when Cu and Ag nanoparticles are simultaneously applied for wastewater treatment. As in the case of Silver, the combination of Cu nanoparticles with the -SH groups of key microbial enzymes is the probable inactivation mechanism [117]. However, further studies are required to validate this mechanism. It has been found that Cu nanoparticles (CuNP), particularly at high concentrations, can alter the physicochemical  [118]. However, at a lower concentration of 0.1-10 mg/L, there was no observable impact on the activated sludge ( Figure 9) and the consequent removal of nitrogen from the wastewater, as nearly all the CuNPs were absorbed by the activated sludge [119]. Conversely, Madeła [108] showed that a CuNP concentration of 3 mg/L resulted in the decreased effectiveness of wastewater treatment from 92.17% to 71.9% (based on the TOC values). Similar observations were also reported by Chen et al. [120], where phosphorus removal was studied. Thus, CuNPs may negatively impact the activity of activated sludge in sequencing batch reactors. This was attributed to the changes in the microorganism concentration in the activated sludge. These observations demonstrate the impact of the CuNP concentration on the activated sludge, and it can thus be argued that CuNPs may be better utilised for the subsequent disinfection stages of the treatment process. Other copper-containing nanoparticles, such as CuFe 2 SO 4 NPs, have also found numerous applications for water treatment, particularly when they are combined with other materials (as a surface coating) to enhance their adsorption capacities, as well as for photodegradation [121].  [118]. However, at a lower concentration of 0.1-10 mg/L, there was no observable impact on the activated sludge ( Figure 9) and the consequent removal of nitrogen from the wastewater, as nearly all the CuNPs were absorbed by the activated sludge [119]. Conversely, Madeła [108] showed that a CuNP concentration of 3 mg/L resulted in the decreased effectiveness of wastewater treatment from 92.17% to 71.9% (based on the TOC values). Similar observations were also reported by Chen et al. [120], where phosphorus removal was studied. Thus, CuNPs may negatively impact the activity of activated sludge in sequencing batch reactors. This was attributed to the changes in the microorganism concentration in the activated sludge. These observations demonstrate the impact of the CuNP concentration on the activated sludge, and it can thus be argued that CuNPs may be better utilised for the subsequent disinfection stages of the treatment process. Other copper-containing nanoparticles, such as CuFe2SO4 NPs, have also found numerous applications for water treatment, particularly when they are combined with other materials (as a surface coating) to enhance their adsorption capacities, as well as for photodegradation [121].  [119]. The surfaces of the cells did not seem to be damaged by the concentrations investigated. However, the toxic effects of CuNPs (at higher concentrations) are illustrated in another study by the authors [118].

Iron Nanoparticles
The study by Daniel et al. [106] illustrated the antibacterial properties of iron nanoparticles, particularly zero-valent iron nanoparticles. These particles were shown to inactivate Gram-negative E. coli., as well as Pseudomonas fluorescents and B. subtilis. Iron oxide nanoparticles are also promising candidates for contaminant removal in WWTPs due to their easy separation, low cost, magnetic properties, robust adsorption capacity and improved stability. Iron oxide nanomaterials have also successfully been used as an adsorbent of heavy metals (Pb, Zn, Hg, Ni, Cd, Cr), which are increasingly problematic, even at low levels, due to their toxicity to humans, animals and plants [122,123]. Iron oxide nanoparticles were also shown to eliminate mercuric ions, cadmium ions and copper ions in  [119]. The surfaces of the cells did not seem to be damaged by the concentrations investigated. However, the toxic effects of CuNPs (at higher concentrations) are illustrated in another study by the authors [118].

Iron Nanoparticles
The study by Daniel et al. [106] illustrated the antibacterial properties of iron nanoparticles, particularly zero-valent iron nanoparticles. These particles were shown to inactivate Gram-negative E. coli., as well as Pseudomonas fluorescents and B. subtilis. Iron oxide nanoparticles are also promising candidates for contaminant removal in WWTPs due to their easy separation, low cost, magnetic properties, robust adsorption capacity and improved stability. Iron oxide nanomaterials have also successfully been used as an adsorbent of heavy metals (Pb, Zn, Hg, Ni, Cd, Cr), which are increasingly problematic, even at low levels, due to their toxicity to humans, animals and plants [122,123]. Iron oxide nanoparticles were also shown to eliminate mercuric ions, cadmium ions and copper ions in the work of Xu et al. [37]. Zero-valent iron particles were also used to remove methyl blue from water and from a water-ethanol mixture (50:50) in a study by Sawafta and Shahwan [124].
They realised that water solutions achieved the best removal of methyl blue compared to the water-ethanol solution. Table 5 presents some of the bacteria that can be removed by silver, copper and iron oxide nanoparticles. Recent advances in the application of these nanomaterials have featured the development of spinel ferrite nanoparticles (SFNPs) and their derivative composites (SFNCs) (Figure 10), which are sometimes used as photocatalysts [121]. Some of the commonly applied spinel ferrites include Fe 3 [125]. Recently, SFNPs were utilised for the degradation of dyes in textile effluents under visible light [126]. A 99% degradation of methylene blue was observed using Fe 3 O 4 @TiO 2 with H 2 O 2 [127]. The removal of phenols and chlorophenols (one of the largest groups of environmental pollutants) was also effectively demonstrated using SFNPs [128,129]. Graphitic carbon sand composite and bentonite-supported MnFe 2 SO 4 were applied for the degradation of ampicillin (AMP) and oxytetracycline (OCT) antibiotics under visible light; 96% and 99% degradations of AMP and OTC were observed after the treatment [130]. It is important to mention that the use of these NPs has been complemented by several oxidation-based treatment techniques, such as ozone, hydrogen peroxide and UV-based treatment methods. Catalytic ozonation, which involves the hybrid application of ozone and NPs, has been effectively utilised for the degradation of phenacetin (PNT) [131]. This process results in the generation of critical intermediates, such as H 2 O 2 and OH • radicals, which further accelerate the decontamination process. Table 5. Silver, copper and iron oxide nanoparticles and some of the pathogens they are effective against (adapted from [35,106,113] the work of Xu et al. [37]. Zero-valent iron particles were also used to remove methyl blue from water and from a water-ethanol mixture (50:50) in a study by Sawafta and Shahwan [124]. They realised that water solutions achieved the best removal of methyl blue compared to the water-ethanol solution. Table 5 presents some of the bacteria that can be removed by silver, copper and iron oxide nanoparticles. Recent advances in the application of these nanomaterials have featured the development of spinel ferrite nanoparticles (SFNPs) and their derivative composites (SFNCs) (Figure 10), which are sometimes used as photocatalysts [121]. Some of the commonly applied spinel ferrites include Fe3O4, CuFe2O4, MnFe2O4, ZnFe2O4, NiFe2O4 and CoFe2O4 [125]. Recently, SFNPs were utilised for the degradation of dyes in textile effluents under visible light [126]. A 99% degradation of methylene blue was observed using Fe3O4@TiO2 with H2O2 [127]. The removal of phenols and chlorophenols (one of the largest groups of environmental pollutants) was also effectively demonstrated using SFNPs [128,129]. Graphitic carbon sand composite and bentonite-supported MnFe2SO4 were applied for the degradation of ampicillin (AMP) and oxytetracycline (OCT) antibiotics under visible light; 96% and 99% degradations of AMP and OTC were observed after the treatment [130]. It is important to mention that the use of these NPs has been complemented by several oxidation-based treatment techniques, such as ozone, hydrogen peroxide and UV-based treatment methods. Catalytic ozonation, which involves the hybrid application of ozone and NPs, has been effectively utilised for the degradation of phenacetin (PNT) [131]. This process results in the generation of critical intermediates, such as H2O2 and OH • radicals, which further accelerate the decontamination process. Figure 10. Schematic representation of wastewater treatment using SFNPs/SFNCs, showing their potential for recovery and reuse [125]. Table 5. Silver, copper and iron oxide nanoparticles and some of the pathogens they are effective against (adapted from [35,106,113]).

Nanoparticle Pathogen
Silver

Challenges of Nanoparticle Application
In addition to surfactant stabilisers, more investigations are required to fully comprehend and improve the stability of nanoparticles, as well as the development of a new understanding of the mechanisms of surface energy reduction. There are also concerns that other compounds (apart from the target contaminant) may be adsorbed during the nanoparticle treatment of wastewater, leading to a reduction in the general efficiency of the decontamination process. A typical example of this is the adsorption of phosphates instead of heavy metals, which are usually the constituents of interest. To combat this limitation, chelating ligands have been grafted onto the surfaces of nanoparticles to aid in the uptake of heavy metals from the water. The environmental fate of engineered nanomaterials and their possible effects on human health are also a growing concern and have not been adequately investigated in the literature. Thus, further investigations are required regarding the toxicity and pathology of nanomaterials and their impacts on the environment. Upon entry into the environment, there are many mechanisms of exposure to humans, including dermal contact, the inhalation of water aerosols and ingestion of contaminated drinking water. Predictions of the possible physical and biological effects have proven difficult, but they are necessary [37]. Of particular difficulty is the generalization of the materials' toxicity, as different behaviours are often observed for the different nanomaterial types. The adsorption of nanoparticles has been observed in the gastrointestinal tract, enabling them to be further distributed around the body. The data collected on the health effects of nanoparticles is too generic to form a concrete conclusion.
There are various sources from which nanomaterials can leach into the environment, such as point sources including WWTPs and manufacturing plants or nonpoint sources such as runoffs. Thus, future studies must analyse both sources to ascertain the presence and concentrations of nanoparticles. Samples from WWTP effluents, surface waters and soil ought to be considered. Drinking water usually undergoes many treatment steps to ensure the removal of nanomaterials, such as flocculation, filtration coagulation and sedimentation. Alum coagulants have been proven to remove various amounts of nanoparticles from water, but the amount depends on the water chemistry and the amount of alum coagulant used [132,133]. Metal oxide nanoparticles have been removed from water using a 0.45 µm filter [132]. However, further research is required on the effectiveness of nanoparticle removal by filtration. Additionally, new developments on the techniques and methods used for nanoparticle detection and characterization are needed. These methods must be suitable for use in complex matrices such as surface waters, and they must be highly sensitive, robust and cost-effective [134]. The specific challenges affecting the nanomaterials of interest in this study (carbon nanotubes, graphene-based nanomaterials and silver, copper and iron nanoparticles) are presented below.

Carbon Nanotubes
Although CNTs are promising materials for wastewater purification, there are some limitations. The synthesis and application of these membranes are still at an early stage. Furthermore, the cost of synthesis, improving the scale of manufacturing, and the environmental impacts and commercial readiness are the current considerations limiting their extensive large-scale application. Their release into the environment is a source of concern, as they constitute an occupational inhalation exposure risk. They also have the potential to negatively affect aquatic life if leached from WWTPs into the environment. Studies on rats showed that they had pulmonary inflammation and lung cellular propagation after exposure to CNTs [135,136]. The toxicity of CNTs also depends on factors such as their physical state, the way they are manufactured and the presence of impurities [43]. Functionalisation with carboxyl groups has been identified as a viable route for mitigating the toxicity of CNTs (particularly MWCNTs). Allegri et al. [137] demonstrated that the nanoparticle size (a consequence of the production route) affects the toxicity levels.

Graphene-Based Nanomaterials
In a similar way to CNTs, GBNs can be released from WWTPs into the environment and can pose a potential risk. As graphene is a relatively new material, evidence of its positive or negative biological impacts on humans is scarce. A recent study performed on animals demonstrated that the inhalation of graphene and graphene oxide induced lung damage [61]. However, there are preventative measures which can be taken to avoid the release and transportation of GBNs into the environment. They can be fixed to 2D membranes, which allow for the selective separation of certain molecules. They can also be collected and added to 3D aerogels or hydrogels, which makes their handling much easier, and they can be recycled and reused, thus reducing waste [69].

Fullerenes
An in vitro cytotoxicity investigation of SWNT, MWNT and fullerene in alveolar macrophages revealed an order of cytotoxicity based on mass of SWNT > MWNT > C60. At low concentrations of C60 of up to 226.00 µg/cm 2 , no significant cytotoxicity was observed. High concentrations of C60 induced cell injury [138]. Another study revealed the severe harmful effects of C60 materials (concentrations of 50 mg/kg) on mouse embryos in vivo and in vitro [93]. There is still no known toxicity to humans; however, more investigations are required, considering that these nanomaterials can leach into underground water or persist after the treatment of drinking water. The influences of C 60 's interactions with micro-contaminants on the toxicity of river biofilms were investigated. The studied microcontaminants were triclosan, diuron and venlafaxine. The results showed no toxic effects on the river biofilms by the C60. Moreover, the exposure of the contaminants with C 60 at low concentrations revealed antagonistic effects in the case of diuron (decreased toxicity) and a synergistic effect (toxicity effects increase) in the case of triclosan [139]. The molecular structure of the contaminants plays a vital role in the mechanism of interaction with C 60 .

Silver Nanoparticles
One of the limitations of using silver nanoparticles for WWT is the potentially high large-scale implementation costs, despite their relatively cheap preparation methods on the lab scale. Thus, it is implied that the economic viability of the materials' large-scale production may constitute a source of concern, depending on the intended application [101]. Despite the widely claimed low toxicity of silver nanoparticles [140], there are new reports on the adverse effects of silver nanoparticles on the reproduction of some experimental animals. The lungs and liver may also be at risk, as well as potential neurotoxic effects when inhalation occurs above the maximum admissible concentration [141,142]. However, the grafting of AgNPs onto selected polymers is a potential solution that could be used to mitigate the eco-safety concerns, as well as the prevalent challenge of nanoparticle coalescence [143].

Iron Nanoparticles
Zero-valent iron nanoparticles are prone to oxidation, aggregation and difficulty in their separation from aqueous solutions. However, coating these nanoparticles with an inert material, prevents their aggregation and improves their diffusion. Favela-Camacho et al. demonstrated the potential of sodium citrate, sodium metasilicate and colloidal silica from tetraethyl orthosilicate to stabilise suspensions of magnetite nanoparticles [144]. Encapsulation in a matrix, conjugation with supports and emulsification have been proposed as viable methods for enhancing their performance in wastewater treatment [145,146].

Copper Nanoparticles
The possible toxic effects of copper nanoparticles are unknown and remain to be a subject of ongoing investigation. A study by Chen et al. [118] examined the impact of copper nanoparticles (20-40 nm; 99% purity) on the physical-chemical properties of sludge. Properties such as dewatering, the surface charge, hydrophobicity, settleability, flocculation and extracellular polymer substance content were assessed using different concentrations of copper nanoparticles. At a concentration of 5 ppm, no observable effect on the tested parameters was observed. However, at concentrations of 30 ppm and 50 ppm, the hydrophobicity, flocculation ability and phosphorus removal efficiency decreased, whereas the surface charge and extracellular polymer substance content increased. This led to the conclusion that high concentrations of copper nanoparticles can alter the physical and chemical properties of sludge, which in turn affect the efficiency of wastewater treatment. This concentration threshold effect and the nonuniform impact on the key parameters of the water quality present optimisation challenges when working with these nanomaterials.

Discussion
As water scarcity is a current and ongoing global problem, the search for improved methods of treating wastewater more efficiently for the purpose of reuse is important and necessary, particularly with climate change and population growth being the two main drivers of water scarcity. Conventional treatment technologies (preliminary, primary, secondary and tertiary), which mainly remove the bulk of the contaminants in wastewater, are essential for water purification. Considering the pitfalls of each treatment step, nanotechnology has been discussed as a potential solution that addresses these limitations. Some of the advantages of NP deployment include the removal of pathogens, pharmaceuticals, CECs and organic and inorganic contaminants in the nanoscale range. Most nanotechnology-based processes are environmentally friendly and recyclable. Adsorption is the widely adopted nano-based separation process, although membrane filtration is also effective and considered to have similar performance. However, membrane filtration processes are often plagued by the possibility of fouling. In this review, the use of CNTs was also presented, emphasizing their main advantage of low running/operational costs, as no external energy is required to propel the wastewater through the CNT. The hydrophilic and hydrophobic elements of CNTs enable the continuous flow of water while the contaminants are caught and removed. CNTs are a good choice for the removal of organic and inorganic contaminants, as well as PAHs. Their strength and antifouling properties also make them desirable for WWTPs.
Many structural properties of GBNs make them an attractive technology for contaminant removal. The chips and defined edges increase the surface area of GO and rGO, providing many binding sites on their surfaces and thus enhancing pollutant removal. The final structural properties of the produced graphene depend on the production method adopted. Liquid exfoliation can produce graphene on a large scale but yields a less pure form compared to production by chemical exfoliation. The chemical stability of GBNs in WWTPs is desirable, as they can withstand the extreme acidic and alkaline conditions that are commonly found in WWTPs. For the removal of PAHs, dyes and antibiotics, GO and rGO are more effective. The hydrophobic and hydrophilic properties of GOs also make them an attractive choice for water purification. It is crucial to emphasize that pH plays a key role in the adsorption capacity, as decreasing the pH of an aqueous solution increases the adsorption capacity of GOs.
Overall, silver nanoparticles are a fitting choice for pathogen removal, as they remove more than 700 different microorganisms and have excellent antifouling qualities. In WWTPs, biofilms cause problems by building up in pipes and inducing blockages. The use of silver nanoparticles for treating wastewater can ease this problem. As they possess a very low toxicity to animal cells, their exposure to the environment poses low risks. The applied temperature during their synthesis is a key factor affecting the size of the produced nanoparticles. Furthermore, iron oxide nanoparticles are particularly effective in removing lead contamination. The stability and adsorption capacity of iron nanoparticles make them extremely attractive. The development of spinel ferrite nanoparticles represents a key advancement in the application of iron-based nanoparticles for WWT. Furthermore, lower concentrations of CuO nanoparticles can be used if they are stabilised with a surfactant. This is crucial for determining their effective concentration levels for WWT applications and the corresponding synthesis costs. The smaller the size of the CuO nanoparticles, the better the antibacterial properties. Copper nanoparticles would be a good choice for the removal of Bacillus subtilis, as they have a strong affinity toward this microorganism. Temperatures between 25 • C and 30 • C would be optimal when applying CuO nanoparticles for WWT. Overall, modifying these nanoparticles with a surfactant provides additional benefits compared to the independent application of the nanoparticles. However, CuNPs (at certain thresholds) may have negative effect on activated sludge; thus posing a threat to the wastewater treatment process.
Membrane fabrication costs, their purification and the large-scale production of these nanomaterials are the key challenges affecting their translation to industrial settings/commercial implementation [147]. In addition to the potential toxicity of the nanomaterials, unknown by-products that form (via interactions between chemicals and pollutants) during wastewater treatment also pose an environmental concern. Furthermore, toxicity information on several of these nanomaterials is limited, despite the increasing research attention they have received (particularly in the case of carbon nanotubes). Thus, further investigations are required in this regard. The progressive reduction in the efficiency of nanoparticles over repeated treatment cycles also requires further investigation. This may be attributable to factors such as nanoparticle erosion and the accumulation of decomposition products on the active sites. In addition, the application of nanoparticles and ozone nanobubbles has tremendous potential for micropollutant removal, and is deserving of further investigation for the improvement of wastewater treatment efficiencies [148].

Conclusions
It is evident that the current large-scale WWT technologies have efficiency limitations, which result in the discharge of harmful compounds into the environment. The main risk is the ingestion of pathogen-laden water, which causes diverse illnesses, particularly in developing countries. Nanotechnology was discussed as a potential solution that can be used to address these limitations and a viable means of meeting the United Nations' sustainable development goals for clean water and sanitisation. The efficient removal of contaminants in the nanoscale range relies on the combination of current WWT technologies and nanotechnologies. The use of nanoparticles is promising, as some of them can be costeffective, environmentally friendly and recyclable, particularly on a small scale. The ease of adaptability to existing treatment plants is also noteworthy, as major infrastructure may not be required. For the removal of organic and inorganic pollutants and PAHs, CNTs appear to be commonly used. GBNs are a popular choice for the decontamination of PAHs, dyes and antibiotics, whereas for pathogen removal, silver, copper and iron nanoparticles are most desirable. Silver/copper/iron nanoparticles can be coated onto CNTs or GBNs to enhance the removal of organic and inorganic pollutants, PAHs, dyes, antibiotics and pathogens. Surfactant modification, particularly for CuO nanoparticles, increases the contaminant removal efficiency in wastewater and should be further investigated using other classes of nanomaterials. The use of nanoparticles for WWT is still in its early stage, and more lab-scale and pilot-scale research and testing must be performed before their full-scale implementation in WWTPs. Further investigations are also required to determine the environmental effects of these nanomaterials, as well as their viability, via techno-economic assessments.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
BSA Bovine serum albumin CEC Contaminants of emerging concern CFU Colony forming unit