1 Introduction

The availability of clean, safe and healthy water is diminishing every day, which is projected to upsurge in future. To address this, numerous water decontamination methods and technologies being developed and adapted, and several new possibilities are in the way through extensive research. Among the numerous concepts proposed, carbon nanotube (CNT)-based water treatment technologies found to be one of the most promising methods due to their high surface area, excellent chemical reactivity, high aspect ratio, less chemical mass and low impact on the environment [1]. Therefore, the research, development and industrial interests of CNTs are growing globally to treat water contaminants, which expected to have huge impacts on the entire living species in the world.

To date, CNTs have emerged as an innovative and promising class of nanomaterial with unique optical, electrical, mechanical and thermal properties [2]. As reported by Iijima (1991) [3] and Bethune et al. [4], CNTs are seamless cylindrical-shaped nanomolecules with a radius as tiny as a few nanometres and up to several micrometres in length. The walls of these CNTs are constructed by a hexagonal lattice of carbon atoms parallel to atomic planes of graphite and capped at their ends by half of fullerene-like structures [5]. The structure of CNTs can be categorised into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The MWCNTs are composed of two or more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow area with spacing between the layers. In contrast, the SWCNTs composed of a single-cylinder graphite sheet held together by van der Waals bonds [6, 7].

Today, disinfection technique turned to be one of the powerful methods and heavily practised to remove contaminants from any freshwater sources. Generally, the water treatment plant is constructed based on few presumptions that the freshwater which fed into the treatment plant comprised of only natural occurring chemical and biological contaminants that can be removed easily by disinfection methods. However, for drinking water, higher quality of water which is free from pathogenic bacterial is required; this exerts a higher pressure on the treatment organisation to remove or filter out these pathogens from the freshwaters. The removal process of pathogens from water is quite difficult and challenging due to fluctuating concentration and type of pathogens present in the freshwater [8].

Studies have shown that CNTs have higher efficiency adsorption of these kinds of micro-organism as compared to other adsorbents [9]. Specifically, the SWCNTs have higher adsorption capacities for bacteria compared to other available adsorbents due to their fibrous structure and external surface accessibility of CNTs. The adsorption of a bacterial strain, such as Bacillus subtilis (B. subtilis) in SWCNTs, had shown up to 37 times greater than in other adsorbents (e.g. nanocream and activated carbon) [10, 11]. Besides, another distinguishing feature of CNTs adsorbents is the ability of selective adsorption of bacterial species. Liu et al. [12] have studied the toxic effects of pristine SWCNTs on both gram-positive, e.g. B. subtilis and Staphylococcus aureus (S. aureus), and gram-negative, e.g. Escherichia coli (E. coli) and Pseudomonas aeruginosa [13]. He detected that the dispersed individual CNTs puncture the cell membrane rapidly compared to aggregated CNTs. Soft and smooth cells such as gram-positive bacteria were more vulnerable and easily attacked by CNTs than gram-negative bacteria. The membrane-penetrating effect can be enhanced by increasing CNTs concentration, by controlling shaker speed augmentation during incubation and by using dispersed CNTs solution [12, 13]. Recently, both SWCNT- and MWCNT-based microfilters were also shown to be very effective for the complete removal of bacteria and multi-log removals of viruses. Apart from that, it has also been shown that the CNTs can be manipulated into micrometre-thick films for greater removal of viruses than conventional microfiltration [14].

2 Why CNT Does Is Special for Water Disinfection?

The ability of water disinfection treatment to eliminate the contaminants fully depends on the distinct sorption behaviour of novel sorbent. CNTs that possess high surface active site-to-volume ratio and optimally controlled pore size distribution provide an exceptional sorption capability and efficiency compared to conventional granular activated carbon and powdered activated carbon. Conventional activated carbons have some intrinsic restrictions such as the requirement of additional surface active sites and certain activation energy of sorption.

Apart from these advantages, CNTs also hold other essential characteristics including (1) high capacity to adsorb a broad range of contaminants, (2) fast reaction kinetics, (3) significant specific surface, and (4) selectivity towards aromatics [15, 16]. Since these vertically aligned CNTs with open tips (noted as an open-ended CNT membrane) have extremely high specific surface area and abundant membrane porous structures, they possess exceptional adsorption capabilities and efficiencies because of their frictionless or close frictionless flow characteristics [17, 18]. Recently, the scaffold function of both SWCNTs [19,20,21] and MWCNTs [22,23,24,25] have been aggressively employed to the low-cost, effective technologies to decontaminate and disinfect water. Up to date, the unique characteristics of the CNTs have been exploited in many ways in order to improve their adsorption and other useful properties (e.g. combined with other types of metals or supports). The functionalisation increases the amount of oxygen, nitrogen or other groups react on the surface of CNTs, enhancing their dispersibility and specific surface area. Some aspects of CNTs can be optimised to obtain better performance which fits their current applications and demands in water treatment technology.

In the twenty-first century, the application of CNTs membrane in disinfection and biofouling reduction for conventional water treatment turned out to be one of the focused research areas of nanotechnologies. The performances of CNTs in water treatment majorly depend on the diffusional water permeability across the outer wall of CNTs membrane. For the unique features of CNTs membrane, the ability of their membrane to discard finer impurities should be preserved as the pore size decreased for higher water permeability. By manipulating the structural characteristics of CNTs membrane, such as pore dimensions, granule shape and tortuosity, CNTs possess better membrane performance and render more suitability in water treatment application. Lee et al. [23] proved that by controlling the pore size from several tens of nanometre (nm) that correspond to the membrane with grown outer wall of 1–6.7 nm, the pore density increases as a result of the mechanical densification. The anti-biofouling capability of CNTs membrane is indeed able to inactivate bacteria based on the effect of bacterial adhesion. Confocal laser scanning microscopy (CLSM) was employed to examine the lives and dead cells on the anti-adhesion characteristic of the densified outer-wall CNTs membrane (Fig. 1). There are abundant bacteria adhered to the membrane, leading to the formation of a well-mature biofilm. The superior resistance to biofilm formation displayed by the CNT membranes is a prominent benefit.

Fig. 1
figure 1

CLSM images of the live (green) and dead (red) bacteria cells as a function of time where the longest time is 72 h with \( D_{f} \) corresponds to the densification factor and polysulfone (PSf) ultrafiltration (UF) membrane [23]

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The emerging use of membrane distillation (MD) driven by functionalised CNTs recently becomes a viable option for disinfection [26, 27]. A Functionalised CNTs membrane demonstrates high permeability without wetting due to the hydrophobic characteristic and its appropriate pore size and a tortuosity close to the one the system’s desired [28]. The open-ended structure of the CNTs permits the longitudinal movement of polymer molecules and solvent within the structure and gives rise to a well-aligned CNTs configuration as observed by Kyoungjin An et al., (Fig. 2) [29]. As the feed of direct contact, MD was increased to a certain level; the permeate flux gradually lowered in electrospun CNTs membrane. The flux dropped due to the salinity of the CNTs indicating that the CNT composites are less susceptible to salinity and had less concentration polarisation effect on their membrane surfaces. The CNTs facilitated the repulsion force for molecular diffusions, reduced the boundary layer effect in viscous flow and assisted surface diffusion allowing the fast vapour transport with anti-wetting. Therefore, low concentrations of CNTs (less than 0.5 wt%) are not favourable as they were insufficient to increase the permeate flux more than theoretical value while higher concentrations of CNTs (more than 1 wt%) suffice to increase the water flux above the real conditions.

Fig. 2
figure 2

A schematic illustration of the molecules’ transportation during solidification of CNT-embedded polymer nanofibres [29]

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A newly developed nanoporous membrane CNTs network which is super square (SS) CNTs has shown outstanding water disinfection performance. The nanopores in SS CNT network have demonstrated to efficiently sieve the NaCl from the water. The SS CNT networks are built from SWCNT arms (6,6). The nanostructure evolution of SS@(6,6) during desalination was compared with nanoporous graphene as shown in Fig. 3. Figure 3d showed that the sale ions can be strictly filtered by SS@(6,6), whereas the salt ions cannot be impeded by graphene due to its easier mechanical deformation (Fig. 3e). Therefore, better filtration capacity can be attained by optimising the pore size in the SS networks. The progress yields significant advance over existing technological method in water treatment [30]. On the same note, the continuing efforts in fabricating osmotic membrane of CNTs had faced major technical difficulties in a scope of quantifying the single-channel performance for forward osmosis (FO) and reverse osmosis (RO) functions in clean-water harvesting technologies. The optimised pore size is determined by a stable osmotic flow rate and salt rejection in typical FO and RO applications. CNTs are much indeed preferable as they possess longer osmotic flow rate than the one-atom thickness of porous graphene membrane; the osmotic strength drives similar osmotic flow across the entire membrane while the flow resistance in graphitic wall produces ultra-low flow resistance [31].

Fig. 3
figure 3

Snapshots of SS@(6,6) during water disinfection process under fix filtration velocity with the salt iron pass through deformed nanoporous graphene produced heavy ions’ filtration. The a corresponds to the water disinfection at stable pressure filtration stage, b is referring to the water disinfection at ascending pressure filtration stage, c indicates the visibility of water molecules, d shows the presence of salts ion, e shows that salts ion cannot be impeded due to the mechanical deformation of graphene under compression, and f indicates the heavy ions’ rejection [30]

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The role of CNTs in freeing the water from pathogenic bacteria and toxic metals is known for ages. The development of PGLa antimicrobial peptide and glutathione-conjugated CNTs connected with porous graphene oxide membrane has been manifested to be highly efficient in disinfecting the E. coli O157: H7 bacteria and elimination of As (III), As (V) and Pb (II) from the water (Fig. 4) [32]. The conjugated CNTs are capable of capturing and completely disinfecting the bacteria from water via the synergetic mechanism. Despite that, an alternative and safer water disinfection system consisting of silver nanoparticles/MWCNTs coated on polyacrylonitrile (PAN) hollow fibre membrane provides effective solutions in eliminating pathogens. Ag/MWCNTs were covalently coated on the external surface of a chemically modified PAN hollow fibre membrane to act as a disinfection barrier (Fig. 5) [33]. They reported that the relative flux drops over the Ag/MWCNTs/PAN for 6%, which significantly lower than that of pristine PAN under continuous filtration at 20 h. The distinctive disinfection properties of the composite membrane are majorly governed by the proper dispersion of Ag nanoparticles on the external surface of CNTs leading to a direct contact with bacterium cells. The same focus has been studied previously by Vecitis et al. [8] for the elimination and inactivation of bacteria (E. coli) and viruses (MS2) via anodic MWCNTs. The concomitant electrolysis during filtration process significantly enhanced the inactivation of influent bacteria and viruses in the effluent to under the limit of detection. The applied voltage (< 3 V) during post-filtration inactivated > 75% of the bacteria and > 99.6% of the adsorbed viruses.

Fig. 4
figure 4

Removal efficiency of As(III), As(V), Pb(II) and E. Coli from water by CNTs with the 3-D porous graphene oxide membrane of conjugated CNTs [32]

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Fig. 5
figure 5

Illustration of Ag/MWCNTs/PAN membrane for safer water disinfection system [33]

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The facile assembly of CNTs into novel hollow fibre membranes with tunable inner or outer diameters for superior performance for disinfection of water is indeed challenging. Wei et al. successfully demonstrated free-standing membranes built up entirely from CNTs (Fig. 6) [34]. The membranes feature an excellent porosity value (86%) with a permeation flux of about 460 Lm−2h−1 at a pressure differential of 0.04 MPa across the membrane. The adsorption capability of the hollow CNT membranes is necessary for removing small and trace contaminant molecules which are higher than that of commercial polyvinylidene fluoride hollow fibre membranes. On the same note, Fan et al. [35] recently developed a system composed of electropolished CNTs/ceramic membrane that possesses the high capability for fouling mitigation in water treatment. The free-standing electropolarised membranes have high antifouling abilities to remove coexisting pathogens resulting in an increase of flux times. The superior performance of the electropolished membrane was crucially depended on the synergistic effects of electrostatic repulsion, electrokinetic behaviours and electrochemical oxidation.

Fig. 6
figure 6

Manipulation of free-standing of CNTs through advanced membranes engineering for disinfection of water [34]

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3 Antibacterial Mechanism of CNTs

Various nanomaterials are now being used for antibacterial purposes including CNTs. An understanding of the mechanism involved in the antibacterial potential of CNTs towards bacterial cell is greatly needed to reduce the occurrence of antibiotic-resistant bacteria spread through contaminated water sources as clean water is still in high demand for improvement of overall human health.

Up to date, one of the mechanistic actions of CNT’s toxicity towards bacterial cells is suggested to be disruption of bacterial membrane integrity provided mainly through contact-dependent interactions between the bacterial cells and CNTs [36]. CNTs are associated to act as nanodarts in bacterial suspensions, loss of membrane integrity and ultimately cause the death of the cells through leakage of bacterial DNA and RNA contents which were found to be present in the medium of exposure [37]. The interaction mechanism differs between gram-positive and gram-negative bacteria mainly due to the difference in the thickness of peptidoglycan layer which is generally thicker in gram-positive bacteria than gram-negative bacteria (Fig. 7). The existence of an additional outer membrane layer in gram-negative bacteria is depicted in Fig. 7. Therefore, the outer membrane layer that consists of the lipid bilayer is responsible for resistance to the CNTs, whereas the exposed peptidoglycan layer readily interacts with CNTs and initiates puncturing of the gram-positive bacterial cells [14]. This phenomenon was clearly observed in another study where CNTs exposure seemed to be highly selective towards gram-positive S. aureus compared to gram-negative E. coli. Greater affinity towards CNT aggregates was observed with S. aureus with 100 times better adsorption rates for the gram-positive bacteria [38]. Additionally, existence of accessible surface area on the nanotubes and the presence of functional groups on the exposed layer of CNT aggregates facilitate interactions such as hydrogen binding and electrostatic absorption with the bacterial cells, which eventually would have caused the loss of membrane functions and ultimate cell death that is inevitable due to leakage of cytoplasmic content [39].

Fig. 7
figure 7

Representation of the difference observed in the interaction mechanism of CNTs towards a gram-positive S. aureus and b gram-negative E. coli. (1) Cytoplasmic membrane layer, (2) peptidoglycan layer and (3) outer membrane layer [14]

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Oxidative stress is another mechanism that is touted to be responsible for mortality of bacterial cells through exposure to CNTs. Production of detrimental reactive oxygen species (ROS) through CNT exposure was likely to cause toxic reactions while inducing damage to cellular components such as nucleic acid, protein and lipids [39]. The abundance of ROS could lead to the oxidation of fatty acids in the cell membrane and impairment of cell permeability, which would eventually affect important bacterial processes that cause the death of the cell [40]. Generation of ROS is carried out through photoinduced chemical reactions of CNTs especially in aqueous media where large amounts of singlet oxygen (1O2), hydroxyl radicals (•OH) and superoxide anions (O •−2 ) are continually produced. •OH is considered to be the most toxic as these radicals initiate lipid peroxidation process swiftly by mediating spontaneous reaction with the polyunsaturated fatty acids, sugars and proteins in biological components [41]. Lipid peroxidation involves disintegration of membrane integrity facilitated through oxidative stress mediated by hydroxyl groups and ultimately causes the mortality of the bacterial cells. Production of the •OH group through Fenton-like reactions stimulates peroxidation of unsaturated fatty acids without enzymatic processes which induce series of chain reactions that alter the structural makeup of the lipid bilayer in the outer membrane of bacterial cells [42].

Recent findings have acknowledged that physicochemical properties of CNTs are the major contributors of toxicity effects towards bacteria [43]. Interaction of CNTs with bacterial cells in the exposure medium is the initial step towards microbicidal effects. Toxicity of CNTs could not be exerted if no known direct contact mechanism exists between CNTs and bacterial cells [37]. Therefore, investigators are keen on refining existing experimental techniques to understand the interactions of carbonaceous nanoparticles with the lipid interferences found on the cell membrane as the complexity of biological barriers proves to be an obstacle in achieving this purpose [44]. In the light of this phenomenon, computer simulations is an alternate way of understanding the effects of carbon nanoparticles–cell membrane interactions through investigations on membrane properties and the functionality of membrane proteins upon contact with CNTs [45].

4 Factors Contributing to Antimicrobial Properties

  1. (a)

    CNT size

The most important criteria for a particle to be characterised under nanoscale is determined by the size of particles that falls into the nanometre (nm) scale. Although all CNTs are classified under nanomaterial, the difference in diameter further affects its role, especially in antibacterial study settings. The very first evidence on the importance of the size of CNTs demonstrated that SWCNTs with reduced diameter showed a better toxicity effect on E. coli compared to MWCNTs with a larger diameter [40]. The efficiency of SWCNTs remained constant in a liquid medium and layered configurations such as film and membrane [40]. Correspondingly, several articles were subsequently published emphasising on the role of SWCNTs in exhibiting greater bactericidal effects compared to MWCNTs [43, 46]. SWCNTs are associated by better contact with the bacterial cell membrane due to the existence of higher availability of surface area promoting interaction that enhances toxicity effects of SWCNTs [47].

SWCNTs are utilised as a water filter for removal of microbial pathogens from water, and E. coli is frequently used as a model bacterium. The bacterial cell was trapped on the SWCNT filter as size exclusion factor was included in the making of the filter. The gaps between immediate SWCNT bundle were 0.2 µm, whereas the size of E. coli cells is around 2 µm and the difference in size has prevented E. coli from passing through the filter. Additionally, application of SWCNT filter in the water disinfection system not only prevented bacterial cells from passing through but also further inactivated the E. coli on the SWCNT top layers. Scanning electron microscope (SEM) images revealed that the cells appeared flattened and morphology of the bacteria was significantly altered [48].

Besides the diameter of CNTs, the length of these materials often plays a role in determining the bacterial toxicity effects. Three types of length effects of SWCNTs were measured, and it was observed in SEM images that shorter SWCNTs formed aggregation among its type and did not attract a large number of bacterial cells to be adsorbed onto its surface. However, the longer SWCNTs attracted attachment of more bacterial cells compared to the shorter CNTs, and it tended to form aggregates with the bacterial cells, thus exerting its toxicity effects to the bacteria concurrently. The effects of SWCNTs length may be enhanced through an increase in the dose and time of exposure [49].

  1. (b)

    CNT Functionalisation

Pristine CNTs are hydrophobic in nature. However, their sidewall or tips’ surface can be modified through covalent and non-covalent binding of functional groups mainly with the hydroxyl and carboxylic groups. Although covalent bonds are stronger, non-covalent attachment of functional groups is rather preferred as covalent bonding may affect the pore textural of CNTs, resulting in a lasting effect on CNTs’ external surface area [9, 37]. CNTs are functionalised in order to improve water solubility for pharmacological applications. However, the presence of small carbon fragments with significant Raman characteristics of amorphous carbon species is greatly influencing cytotoxicity rate of CNTs hindering the use of this material in health-related applications [50]. The presence of industrial contaminants such as amorphous carbon species including CNTs is a contributory factor towards its toxicity to bacterial cells. However, filtrations of CNTs sample from industrial contaminants removed toxicity effects of the carboxyl group functionalised CNTs. Therefore, attachment of CNTs with polar functional groups enhances the dispersivity of CNTs which in turn determines the contact ratio with bacterial cells.

  1. (c)

    CNT aggregation and dispersivity

Strong π–π interactions between the nanotubes promote the CNTs to exist in a bundled state in aqueous and organic solutions [51]. The state of CNT aggregation during exposure to bacterial cells plays an important role in bactericidal effects. It was proven that partly bundled MWCNTs exhibited higher antibacterial activity value in comparison to bundled MWCNTs [37]. A large difference in the diameter of the tested MWCNTs has contributed to their antibacterial activity as MWCNTs with the partially debundled state have higher surface area to interact with bacterial cell promoting the contact between this bacterial cell and the nanomaterial. The larger diameter may have hindered sufficient bacterial interaction [37]. Similarly, agglomeration aspect was then investigated in SWCNTs through dispersion factor. It was demonstrated that individually dispersed SWCNTs were exceedingly efficient in bacterial toxicity in comparison to SWCNT aggregates. Bacterial death was associated with the destruction of the bacterial membrane potential discovered through SEM imaging techniques. The author further suggested that individual SWCNT could be likened to be nanodarts that are constantly on the move to initiate an attack on the bacterial cells in the exposure medium [52].

One of the key reasons for CNT functionalisation is to increase the solubility of CNTs in water in order to be applicable in a diverse range of applications. CNTs which are hydrophilic are proven to have better surface contact ratio compared to CNTs that are hydrophobic for better removal of biological contaminants in a water purification system [53]. Functionalisation of SWCNTs with –OH and –COOH groups promotes the dispersity of nanomaterial in the reaction medium, improving interaction rate between SWCNTs and bacterial cells. Formation of SWCNT aggregates in the reaction buffer enhances interaction, and their exerted toxicity effects are higher in DI water and saline [54]. Although complete solubility of CNTs in the water-based medium is highly preferred, partially or semi-dispersible CNTs have displayed a superior affinity with bacterial cell than the fully water-soluble CNTs. This notion was investigated in 2009, and the researchers found that balance in aggregation and dispersibility is better for the CNTs as these qualities provoke CNTs to exhibit higher bacterial toxicity levels [55].

  1. (d)

    Adsorption

Adsorption criteria play a significant role in the removal of biological contaminants in water disinfection systems. CNTs are well known to possess a remarkable bacterial adsorption characteristic. The popularity of the usage of CNTs in water purification system rises from the qualities of CNTs having an exceptionally high microbiological adsorption value, selective adsorption of bacterial cells and a rapid adsorption value [9]. Adsorption capabilities of pristine SWCNTs towards B. subtilis spores were 27–37 times higher than activated carbon and NanoCeram™, common components of water filter system. The author claimed that SWCNTs’ high adsorption capability is contributory to its fibrous nature where entrapment of bacterial cells have occurred [11]. Additionally, selective adsorption ability of SWCNTs was demonstrated through exposure of the nanomaterial in a mixed bacterial culture of E. coli and S. aureus, where the SWCNTs have targeted mainly to the gram-positive and coccus-shaped S. aureus contributed to the adsorption value of 100 times greater than the value measured for the gram-negative and rod-shaped E. coli. The Freundlich adsorption model proposed by the author also suggested that these two bacteria models did not compete for the adsorption onto SWCNTs but through discriminatory selection of SWCNTs [38]. However, further elucidation of this selective adsorption quality of SWCNTs is highly necessary as this application may prove to be useful in analysing the concentration of certain micro-organism from a co-culture, especially in clinical settings.

5 Future Trend

Worldwide commercial interest in CNTs has been increasing rapidly; the demand and production at present have exceeded several thousand tonnes per year. Even the research-related publications and patents issued for CNTs-related activities continue to grow exponentially [56]. Owing to their unique and tunable physical, structural and chemical properties, CNTs have exhibited high potentials for future water treatment [57]. Apart from that, the uniqueness of CNTs can inspire innovative technologies as water disinfection application [1]. The novel properties of CNTs will offer great promise for future application in water disinfection system.

One of the upcoming CNT-based materials for water disinfection is the tangled CNT sheets [57]. These tangled CNT sheets are expected to provide mechanical and electrochemical robust networks with controlled nanoscale porosity for advanced water disinfection. These structures have been used to oxidise the organic contaminants, bacteria and viruses electrochemically. Portable filters containing these CNT meshes have been shown to be effective for purification of contaminated drinking water. This improved permeability may enable lower energy cost for water disinfection by reverse osmosis in comparison to commercial polycarbonate membranes [56]. Development of CNTs towards commercial antibacterial applications also warrants the understanding of their interaction mechanism with bacterial cells [14].

Blending of CNTs with metal nanoparticles for water disinfection treatment is another focus being explored to improve the system. Seo and research team [58] have combined silver nanomaterials with MWCNTs and found an increase of synergistic in antibacterial activity against two types of bacteria, which are Methylobacterium spp. and Sphingomonas spp. Thus, we can expect that the development of CNTs as antibacterial nanostructures will move towards by combining various types of nanomaterials to improve their synergistic in antibacterial activity in the future. The integration between nanomaterials with CNTs can possibly provide maximal antibacterial properties and minimal bio-toxicity for the future technology.

Another potential direction of CNTs is to be used as nanomembrane for water treatment. The capability of CNTs to filter microscopic organisms including bacteria and viruses will contribute to the development of more efficient and low-cost water disinfection processes in future. For example, nanomembrane fabricated from CNTs and nanocomposite membranes formed by zeolite (crystalline aluminosilicate materials with uniform sub-nanometre) offer exciting new possibilities in offering an alternative way for water disinfection system [59, 60]. However, the preparation of zeolite membrane possesses significant challenges, especially in the aspect of economic feasibility. As such, CNT-based membranes would be more feasible and become an excellent candidate in water disinfection application. Thus, more focus on research and development by using CNTs-based membranes for advanced filtration can be expected as this new approach, and filtration techniques will become one of the efficient ways to filter and disinfect freshwater [61].

6 Challenges

The potentials of CNTs are endless due to their unique possibility of functionalisation with numerous types of exotic chemicals increasing their prospect to be utilised for a various range of applications including consumer electronics, drug delivery, water purification and antibacterial potentials [62]. In the light of providing clean water supply in some undeveloped countries in the world, CNTs hold a promising prospect in overcoming the limitations experienced with several water disinfections reagents such as chlorine and chloramine that are currently in use. Utilisation of CNT membrane for water disinfection reduces the production of carcinogenic compounds, sorption of chemical contaminants, and removal of micro-organisms in drinking water supplies [63]. Nevertheless, there are several challenging problems need to be addressed before large production scale of these nanomaterials.

  1. (a)

    Impact on environment and ecosystem

CNTs have significant safety and environmental effects, and their release into the environment can have broader impacts towards our ecosystem, especially to the drinking water treatment plant. One of the expected major problems is the loss of adsorbent media from the filters [9]. Thus, after constant usage, a portion of media might be lost, and an immediate effect might be stroked on the wastewater treatment plant. As CNTs being cytotoxic to microbial communities [9, 11, 53], this would disrupt the metabolic functions of microbes in the treatment plant [9]. Consequently, the transport and fate of CNTs in the environment (e.g. marine) might affect the aquatic life through which it enters the food chain of human beings.

  1. (b)

    Impact to mammalian cell system

CNTs have also shown multiple effects on mammalian cell systems. SWCNTs are bio-persistent and observed to induce pulmonary inflammation as well as lung cellular proliferation in rats [6]. The toxicity effect of CNTs on mammalian cells is greater than asbestos and quartz. For instance, the bio-persistence may limit the prospective usage of CNTs in systems having a direct impact on public health such as drinking water treatment [9]. The toxicity of CNTs on mammalian cells depends on several factors, including cell types, dose, size and length of CNTs [64]. Seemingly, due to their small size and unique physicochemical properties, CNT particles may interfere with the normal biological processes when absorbed in the body. Human health risks have been specially related to inhalation exposure, based on reports indicating pulmonary inflammation, fibrosis and asbestos-like responses induced by these fibre-shaped materials [65]. In addition, the diversity in the manufacturing process of CNTs production plays a pivotal role in determining its toxicity effects as these nanomaterials are being produced in such abundance with minor differences in diameter, shape, length, metallic catalyst content, functionalisation and automatic configuration. Therefore, toxicological evaluation of such heterogeneity in a single type of nanomaterial is challenging as established toxicity assessment procedure would not be suitable for a different assortment of nanomaterial [66].

  1. (c)

    Quality versus cost

Commercialisation of newly developed technology always depends on its overall material and system costs, reliability, and quality. Production of inexpensive CNTs without compromising its quality is economically viable for an industrialised-based water disinfection and purification technology. As such, producing CNTs using carbon feedstock and low-cost catalysts would enable the possibility to acquire MWCNTs at a reduced cost. However, the eminence of the produced CNTs could be less efficient for water purifications or disinfections applications. These are due to the inability of the method in controlling widespread variations in length, diameter, pore sizes and chirality of the tubes. Unsuitable chirality with inconstant pore sizes could mislead impurities adsorption and desalination [67]. Currently, SWCNTs obtained special consideration in water purification because of their narrow pore sizes, which are highly suitable for both brackish and seawater desalination. Also, other techniques such as laser ablation and arc discharge are not economically promising due to their high operation cost and expensive raw material required for obtaining high-grade SWCNTs. Therefore, novel or improved method to harvest in bulk, controlled and cost-effective SWCNTs is an urgent need to boost CNTs application in water decontamination technology [1].

  1. (d)

    Retention and Reusability

Retention and reusability of the CNTs are crucial due to their high-end cost in large production. Hence, when CNTs are employed, in the medium of slurry (a mixture of water and CNTs), an additional separation process is needed to retain these CNTs after usage. The membrane filtration could be one of the possible separation methods to retain these nanomaterials in which the immobilisation of nanomaterials on the membrane filters eases the separation process. Apart from that, coagulation technique could also be used to recollect the CNTs effectively. However, both methods involve additional processing step, time and cost. Also, the necessity in mixing various coagulants may cause them tough to be regenerated and reused. There have been numbers of studies proved that nanomaterials such as CNTs could be reused several times for water treatment which eventually ends up being the major key to save cost [68].

  1. (e)

    Highly reactive carbon atoms

In the desalination and disinfection technology, the principal challenge is to obtain a high-quality CNTs membrane and tuning CNTs for advanced processing and applications. Any anomalies in the membrane shape could jeopardise the water passage and impurity retention [69]. The other main barriers in this technology are functionalisation and selectivity of CNTs tip for a specific contaminant. However, highly reactive carbon atoms at CNTs tip can be functionalised (oxidisation) with different mediator treatments, which commonly used for CNTs decontamination and functionalisation. Such functionalisations are corrosive enough to break the tubes into smaller fragments leading to the membrane leakage. Also, the hydrophilic behaviour generated at CNTs tip stimulates quicker water molecules transport through the CNTs. These create steric blockage due to the molecules or ions that are saturated and attracted at the CNT tips [70]. The attraction between both functional groups and water molecules could cause the temporary pause of water molecules around the functionalised regions. Thus, the functionalisation becomes a rate-limiting step, which must be cautiously attuned to allow the CNT membrane permeability to water molecules and solutes [71].

7 Conclusion

Although physicochemical properties of CNTs contribute to the antibacterial potential of CNTs, the most important criteria for removal of bacterial contaminants from drinking water supply would be the exceptional adsorption property of the CNT that is highly selective and rapid concurrently. Another advantage of CNT usage in water disinfection systems would be the antibacterial effects of CNT in comparison to other conventional material that only succeeds in hindering the entry of bacterial cells into the clean water supply, while CNTs inactivate the bacterial cells upon contact. Numerous reports regarding pure and hybrid membranes composed of CNTs were explored in this chapter for the purpose of removing micro-organism from the water. The hybrid and flexible CNTs membrane reported having exceedingly strong adsorption capability towards pollutants from chemical and biological species. The performance is believed to be influenced mainly by the intrinsically excellent adsorption ability of the CNTs as well as the good dispersion of inorganic particles within the CNTs sheets. As the dispersion yield is high, the adsorption surface areas of the hybrid structure will also hugely increase. Therefore, the adsorption performance should greatly increase compared to their pure forms. Moreover, the pure CNTs membranes demonstrated short filtration time and excellent filtration efficiency which can be ascribed due to the porous structure formed during hybridisation process. The hybrid form of CNTs also has shown capabilities of adsorbing dyes, antibiotics and heavy metal ions efficiently. This might be due to the strong interaction between the functional hybrid elements and the antibiotics improving the effective adsorption capacities. The progress in the development of hybrid membranes consists of structurally diverse but complimentary carbonaceous elements that bring out a simple yet robust and scalable method for removal of antibiotics and other pathogen residues in water.

Although CNTs are an emerging nanotechnology-based material that offers operational and economically feasible solutions for the treatment of water, their unique characteristics have not been fully exploited for specific applications to address the environmental impacts or even to human health. Extra emphasis should be given to the design of CNTs and bactericidal effects of the nanomaterial as to provide toxic effects to bacterial cells only while causing no harm for human consumptions. Finally, the CNT-based membrane has tremendous achievements in water permeability, robustness, capable for water desalination purposes and conservation of energy which heightens CNT to a universal leader status in water technology fields.