**1. Introduction**

Rapid urbanization and industrialization has significantly increased the clean water demands in the domestic, industrial, and agricultural sectors [1–3]. Meanwhile, large quantities of pollutants including organic, inorganic, and biological contaminants are being released into the water bodies from these sectors [4–6]. Eccentric waters such as brackish, storm, and wastewater are being used depending upon the purposes [7,8]. Increasingly, use of these waters has also increased the urgent concern about the burden of negative impacts on the surrounding environment; one of the tremendous challenges confronting mankind is the exploration of green and sustainable methods to overcome these shortcomings [9–13]. Keeping in mind the current situation of water and wastewater treatment status, the technologies are not sustainable to meet healthy requirements for surrounding environment and community health [14,15].

**Citation:** Aslam, M.M.-A.; Kuo, H.-W.; Den, W.; Usman, M.; Sultan, M.; Ashraf, H. Functionalized Carbon Nanotubes (CNTs) for Water and Wastewater Treatment: Preparation to Application. *Sustainability* **2021**, *13*, 5717. https://doi.org/10.3390/ su13105717

Academic Editors: Muhammad Sultan, Yuguang Zhou, Redmond R. Shamshiri and Aitazaz A. Farooque

Received: 24 April 2021 Accepted: 10 May 2021 Published: 19 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Historically, numerous techniques and methods have been investigated for advanced treatment of water and wastewater [16]. The most common, adsorption, was proven to be the improved technique to remove a variety of pollutants including organic and inorganic contaminants present in water and wastewater [17,18]. Limited treatment efficiency was reported by using conventional adsorbents due their small surface area, limited number of active sites, deficiency in selectivity, and low adsorption kinetics [19]. These shortcomings of conventional adsorbents have been addressed in recent advancements of nano-adsorbents owing to their high surface area coupled with a higher number of active sites, tunable pore size, fast kinetics, and improved surface chemistry [10,20–24].

The nanomaterials can be used for treatment of water and desalination as well and also reveal properties including electron affinity, mechanical strength, and flexibility during functionalization [25–27]. Carbon nanomaterials (CNs) such as carbon nanotubes (CNTs) are supposed to be a promising material to break down the tradeoff concerning selectivity and adsorption, resulting in an increase of the economics of adsorption technology [25]. As a result, CNTs, as an adsorbent for treatment of water, have attained the focus of countless scholars over the previous few decades who are projected to carry on the exploration and developments in the field of CNs [28].

Numerous significant articles have been published on nanomaterials applied for the treatment of water and wastewater in previous few years [29–37]. Despite rapid developments, innovations, and applications of CNT-based nanomaterials, there is an increasing need for an across-the-board review of the synthesis of CNTs, functionalization of surface modifications, and finally their application to remove aqueous contaminants and to identify potential directions. This is the main motivation of the current review article.

This review attempts to address a brief history of CNTs, synthesis, purification, and functionalization, followed by the application of these nanomaterials for eliminating organics, inorganics, and microorganisms present in water and wastewater samples.

### *1.1. Historical Background*

The discovery of CNTs was reviewed in 2006 by Monthioux and Kuznetsov, showing that the science has seemed to remain controversial [38]. Most literature mentions that nanotubes were discovered by Sumio Iijima [38]. However, Radushkevich and Lukyanovich explained the synthesis process of CNTs with 50 nm diameter [39]. Oberlin et al. explained the vapor phase growth technique for the synthesis of carbon fibers; the synthesized tubes consisted of turbostratic stacks of carbon layers (i.e., describing a crystal structure in which basal planes have slipped out of alignment) [40]. In addition, Abrahamson et al. [41] described the arc discharge method for carbon fiber synthesis using carbon anodes. Later on, scientists described the thermal catalytic disproportionation of CO for the synthesis of CNs. Transmission electron microscopy (TEM) and X-ray diffraction pattern (XRD) were used to characterize the synthesized CNs, and as a result they believed that CNTs can be formed by a graphene layer turning into a tubular shape. They also concluded that two types of promising arrangements, such as a helix-shaped spiral and circular arrangements in the form of a graphene hexagonal network, can result by turning the graphene layer into a tabular shape [42]. Later, a US patent was issued in 1987 on carbon nanofibers synthesis, the diameter ranging from 3.5 to 70 nm and five times greater in length than the diameter [43].

Back to the dates in 1950s, after the disclosure of CNTs by Iijima, projection of surprising properties of single-walled carbon nanotubes (SWCNTs) made by Dunlap and colleagues also attracted the attention of researchers around the world. At this time, after the discoveries and exploration of SWCNTs by Bethune and Iijima independently at IBM (Shiba, Minato) and Nippon Electric Co., Ltd. (Tokyo, Japan), respectively, the research on CNs and their specific methods of production was extended [44,45].

The above findings seem to be the extension of Fullerence's discovery. Arc discharge technology had previously been applied for the production of laboratory-scale Buckminster

fullerenes [46,47]. CNTs are being studied after the report published in 1991 by Iijima [45] was fundamental, because it put CNTs in the limelight [38].

### *1.2. Types of CNTs and Structure*

CNTs are composed of carbon atoms organized in a progression of fused benzene rings, which are pleated into a cylindrical shape. This new sort of man-made nano-material has a place with fullerene family and is treated as carbon's third allotrope as well as sp<sup>2</sup> and sp<sup>3</sup> forms of graphite and diamond, respectively [42,48,49].

Generally, there are two types of CNTs [50] on the bases of number of layers shown in Figure 1:


**Figure 1.** Types of typical dimensions of CNTs, SWCNTs (**left**) and MWCNTs (**right**).

CNTs are made by a sheet of graphene when rolled into a cylindrical shape, which may have a capped or open end, usually in a hexagonal form close packed with a diameter at a small scale of 1 nm, and a few microns long. SWCNTs (Figure 1) with a diameter as small as 0.4 to 2 nm are made by the single sheet of graphene rolled into a cylindrical shape, while MWCNTs (Figure 1) with an outer and inner diameter ranging from 2–100 nm and 1–3 nm, respectively, and a several microns in length are made up of two or more sheets of graphene incasing a hollow core in the same way as in SWCNTs [49,51,52].

Based on the chemistry, there are two zones of CNTs: the sidewall and tip. A significant aspect in controlling these distinctive properties emanates after the change in the tubelike structure due to entrapment of graphene layers into a cylindrical shape. Figure 2 shows different structures of rolled SWCNTs based on graphene sheets. Depending on alignment of the cylinder axis relative to the hexagonal matrix, the CNTs structure can be stipulated by chiral carrier in three ways, armchair, chiral, and zigzag, illustrated by their chirality index (n,m). Geometric arrangement of carbon atoms present at the layer of nanotubes is responsible for the foundation of zigzag (m = 0) and armchair (n = m) CNTs, whereas the structure of the nanotube with the two enantiomorphs on the right side is chiral (n 6= m) [53,54].

**Figure 2.** Roll-up of graphene sheet into different types of CNTs [54,55].

Recent reviews are good to find detailed elucidations of the structure of CNTs [48,49,56–61]. Here, Table 1 summarizes a comparison of the properties of SWCNTs and MWCNTs.



### **2. Synthesis of CNTs**

Typically, there are three extensive methods for the synthesis of CNTs as given bellow:


CNTs are produced by using energy and carbon sources in all the synthesis methods. A carbon electrode or a gas and an electric current or heat is used as a carbon and energy source, while using arc discharge or CVD methods, respectively, for the synthesis of CNTs,

whereas a laser beam is used as an energy source during the laser ablation method. Table 2 presents a detailed summary of the efficiencies of the CNT synthesis methods. These methods are based on the formation of a single or a consortium of carbon atoms that sack to recombine into CNTs.

CNT synthesis mechanisms have been debated in detail by Cassell et al. [64] and Sinnott et al. [65]. It is believed that by using the metallic catalyst in the CVD method for CNT synthesis, the cylindrical shaped graphene tube is formed by initial deposition of carbon atoms on the used catalyst surface [66]. It was also concluded that the size of particles of the used catalyst also play an important role in the CNTs diameter, as the catalyst particles in a smaller size produce SWCNTs with a diameter of a few nanometers, while the larger particles tend to produce MWCNTs [67].

The arc discharge method between graphite electrodes is the first method of producing CNTs [68]. Briefly, in this method, direct current of 50 to 100 A and about 20 volts of potential difference is established between a graphite electrode pair in the presence of one of the inert gases containing helium or argon with a pressure of 500 Torr [69,70]. The carbon electrode surface evaporates and forms a cylindrical-shaped tube structure because of the high temperature generated due to the discharge of electric current in low pressure, inert gas, and catalyst [44,70,71]. MWCNTs can be synthesized via an arc discharge method without a metallic catalyst; on the other hand, mixed-metal catalysts, for example iron, cobalt, and nickel, are necessary for SWCNTs fabrication [72]. In general, higher levels of structural precision are noted in CNTs produced via the arc discharge method [73]; however, different variables such as chamber temperature, concentration and type of catalyst, hydrogen presence, etc., may affect the structure and size of synthesized CNTs [74]. Recently, nickel-filled CNTs were synthesized via a local arc discharge method in liquid ethanol [75], nitrogen-doped CNTs via vaporization of boron nitride [76], low-cost SWCNTs via an arc discharge method in open air [77], and SWCNTs and MWCNTs via a hot plasma arc discharge method [78].

The use of laser ablation to synthesize CNTs was first reported by Guo and colleagues in 1995 [79,80]. Briefly, a graphite object is targeted by a laser beam with high energy in the presence of argon at 800–1200 ◦C temperature and 500 Torr pressure [69,70,81,82]. In this method, a laser pulse provides an energy source, and a graphite object serves as a source of carbon. Soot deposition of carbon can be avoided by uniform evaporation of the target resulting because of continuous applications of laser pulses. The larger size particles after the first laser beam are broken down into smaller ones by the successive beams. Later, the smaller size particles are produced into the CNT structure. Commonly, transition metals are used as catalysts in this method. Rope-shaped CNTs can be found by using a laser ablation method with the diameter ranging from 10 to 20 nm and about 100 mm long [83].

CVD is a well-liked method for bulk fabrication of CNTs around the globe. Typically, in this method, carbon monoxide or hydrocarbons gases are used as a source of carbon, while 500 to 1100 ◦C of temperature is used as the source of energy. The carbon atoms are deposited by the decomposition of the used carbon source and shaped into CNTs [84,85]. Briefly, the decomposition of gas (carbon source) occurs because of high temperature when transferred into reaction chamber together with the carrier gas and generates a substrate of carbon atoms on the surface of catalyst to form CNTs [86]. When compared to other methods of synthesis of CNTs, the CVD is the most common route for relatively large-scale production of CNTs as it is simple in operation, higher in yield, and economic and has a high rate of deposition and good control over the morphology of tubes during the synthesis process [87–89] Cassell et al. [64] studied that CNTs in bulk can be produced, especially SWCNTs via the CVD method, by using acetylene as a source of carbon deposition in the presence of iron and cobalt and zeolite or silica as a carrier support material. They also concluded that SWCNTs can be produced on a largescale when a mixture of H<sup>2</sup> and CH<sup>4</sup> is deposited on the catalyst (Co or Ni), and MgO as carrier support material is used.




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### **3. CNT Purifications**

Some of the impurities include an amorphous phase of carbon, particles of particular metals, or any other carrier material associated with CNTs that will have an effect on their execution performance [115]. Some typical purification technologies and their characteristics are discussed in Table 3. On average, the CNTs synthesized via the CVD method showed a purity level ranging from 5% to 10% [116]. Therefore, a broad purification of CNTs is necessary before being used for different applications. The detection and identification of different impurities associated with CNTs using different techniques have been discussed in Table 4. It is believed that the CNT structure may be affected to some extent when removing impurities, so there is always need for a compromise with the final structure after purification process [116]. The common CNTs purification methods are discussed below:


Oxidation is a decent manner to remove carbon [117–122] and metal [117,121,123–126] impurities associated with CNTs. One of the main shortcomings that occur using this process of purification is the oxidation of CNTs themselves along with the impurities, but fortunately, the loss of CNTs is smaller than the impurities [114]. The reason to oxidize these impurities is more defects or open structures associated with them. This is another reason that the attachment of these impurities is often observed with a used metallic catalyst, and this metal catalyst may also play a role in oxidation [117,118,123,124,127]. There are some factors, such as type of oxidant, time of oxidation, temperature, metal contents, and environment, which can affect the oxidation efficiency and final yield.

Typically, the method of acid treatment is used to eliminate the metallic impurities associated with CNTs. First of all, by oxidizing or sonication of the CNTs, the surface of associated impurities (metals) is made apparent to acid until the solvation and finally CNTs collect in suspension. It has been observed in a number of studies that by using HNO<sup>3</sup> for the purpose of acid treatment, it only affects the metallic impurities rather than the CNTs or other carbon containing impurities [117,118,122,125]. By using HCl for this purpose, the impacts on CNTs and carbon impurities are also observed to a small extent [117,123,126]. Acid treatment for purification of CNTs in diluted form (4M HCl) can show same results as by the HNO3, but the metal surface must be apparent to the applied acid to make the solvation [128].

Although purified CNTs are produced relatively by acid reflux, the nanotubes amalgamate, and the impurities that they capture are very hard or sometime impossible to remove by filtration [129]. Therefore, a surfactant-based sonication process is implemented generally by dissolving sodium dodecyl benzene sulfate (SDBS) in ethyl or methyl alcohol solution for this purpose. Since after sonication CNTs took longer to settle down, ultrafiltration is required and then annealed at a high temperature (about 1000 ◦C) in the presence of N<sup>2</sup> for 4 h. Annealing of CNTs is performed to optimize their structure. Surfactant-based sonication has been presented to be an effective method for removing tangled impurities associated with amalgamated CNTs [116].

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## **4. Functionalization of CNTs**

The non-polar nature of graphene layers makes the CNTs naturally hydrophobic. The hydrophobic property of CNTs is indispensable for the adsorption of aromatic contaminants like benzene and anthracene. A very strong complexion is formed due to π electrons present on the graphene layer making CNTs, between aromatic contaminants and the tube surface [132]. The surface affinity of CNTs can be modulated to a variety of contaminants in water and wastewater after the purification and surface functionalization. Higher adsorption of benzene was obtained by using CNTs as compared to activated carbon (AC) because of strong interaction between benzene rings and the surface of CNTs due its hydrophobicity [133]. Figure 3 shows the different routes and schemes of CNT functionalization to increase their affinity for water and wastewater contaminates, which can be subsequently captured on the surface of CNTs used. Moreover, the functionalization of CNTs can be divided into three categories, shown in Figure 4 [134]:


**Figure 3.** Functionalization routes of CNTs and associated functional groups [132].

**Figure 4.** Methods of functionalization of carbon nanotubes.

CNTs are unique because of their distinctive properties such as adsorption capability, permeability, morphology, and physicochemical properties. There are several disadvantages that are also associated with raw CNTs, such as their low dispersion in solutions and low adsorption capacity for bulk fabrication of CNTs with organic and inorganic composites. In fact, aggregation is the main problem for low contaminant adsorption efficiencies by the original CNT samples [33,53,135–137] and also obscures the process of membrane preparation [138]. The π–π interactions and van der Waals force between CNTs are responsible for the less dispersion, which results in tight fit bundles and aggregation of CNTs [139]. As the number of graphite layers of CNTs decreases from MWCNTs to SWCNTs, the tendency to bunch increases [140]. Therefore, to overcome these drawbacks, the chemical reactivity and contaminant adsorption capacity of CNTs must be improved by increasing their dispersion rate, and this is done by functionalizing the nanotube [53,135–137,139,141]. Furthermore, solubility of CNTs can be increased by their functionalization, which causes them to repel each other [50]. Table 5 provides a compression between adoption capacities and the corresponding surface area of pristine and oxidized CNTs treated with different acids [142].


**Table 5.** Adsorption capacity of CNTs and corresponding surface area [142].

Different physical or chemical processes like oxidation, impregnation, or grafting (Figure 3) are used for the functionalization of CNTs [143,144]. During the process of functionalization, the covalent or non-covalent bonds of particular functional groups result on the end or sidewall of CNTs. Functionalization is preferred over covalent bonding, because non-covalent bonding does not influence the structure and surface area of CNTs [135]. Advantages and the limitation of covalent and non-covalent functionalization of CNTs are discussed in Table 6. Functionalization is generally done to increase the dispersibility as well as contaminant removal efficiency of CNTs, thereby improving the water or wastewater treatment application capability of the CNTs or promoting membrane fabrication [145]. Higher dispersibility in polar solvent (water) was found by covalent functionalization of CNTs with the phenolic group by 1,3 dypolar cyclo addition; covalent functionalization of CNTs with phenol groups by 1, 3-dipolar cyclo-addition was found, which facilitated

CNTs' amalgamation into the polymer matrix [139]. Oyetade et al. [141] found that MWC-NTs efficiently absorbed Pb2+ and Zn2+ from aqueous samples after functionalizing with nitrogen. The increase in adsorption was due to higher surface area and more adsorption sites linked to CNTs.

**Table 6.** Benefits and limitations of covalent and non-covalent functionalization [134].


Oxidation of raw CNTs with HNO3, H2SO4, HCl, H2O2, KMnO4, and NaOCl, or sometimes a combination of some of these (Figure 5), has often been exercised to introduce oxidized functional groups [146,147]. Generally, oxidation improves the dispersibility and enhances the ability to adsorb certain harmful contaminants in water and wastewater at the expense of fractional damage to the surface of CNTs, as described earlier. Furthermore, the surface of CNTs can also be modified with the addition of oxygen containing functional groups by performing oxygen–plasma action. In addition to oxidation and plasma action for surface modification, the CNTs can also be successfully modified with the addition of metal oxides like Al2O<sup>3</sup> [148], MnO<sup>2</sup> [149], and Fe3O<sup>4</sup> [150], which deliver another way to coat the surface of CNTs, thereby increasing the contaminant removal efficiencies of CNTs [53,135–137]. The potential surface modification schemes of CNTs used for targeted contaminants are shown in Figure 5.

**Figure 5.** Schematic representation of surface modification schemes of CNTs used for targeted contaminants (green arrows refer to decrease in final effluent concentration).

### **5. Characterization of CNTs**

Intrinsic properties of CNTs make them fascinating and desirable candidates for diverse remediation. Characterization of CNTs plays an important role due to their distinctive properties. Numerous comments and debates have been published in past decades on

different techniques and strategies used to evaluate CNTs [151–154]. Techniques used for the characterization of CNTs are divided into four groups: microscopy and diffraction, thermal, spectroscopic, and separation techniques [155]. Sometimes research includes one more group, the magnetic measurement technique. Table 7 shows different characterization techniques used for the evaluation of CNTs. It must be noted more than one technique is prominent for the characterization of CNTs; techniques employed alone are not fully characterized, nor they are absolutely quantitative. Even though qualitative analysis of CNTs can be done by electron microscopy, scanning electron microscopy (SEM) evaluates the nanostructure of the tubes, and transmission electron microscopy (TEM) is used for further precise inspection, generally detecting the defects in CNTs [129,156].


**Table 7.** Different analytical techniques used for the characterization of carbon nanotubes [155].

The ultrastructure of different types of species including organic, inorganic, and biological species can be evaluated by using very popular techniques known as SEM and TEM. The scanning of the sample in SEM generates an image when the targeted electron ray interacts with sample of CNTs. Generally, the technique is used to analysis the morphology (length and diameter) of nanomaterials [123,155,165] to assess the quality of prepared CNTs. For example, Figure 6a shows a SEM image of the as-prepared CNTs [148], and Figure 6b

shows alignment of CNTs synthesized by using a horizontal quartz tube housed in muffle furnace. Average length of the tubes was 70 µm measured by using SEM [166]. Sometimes, it is also used to validate the surface modifications in terms of functional reactions that occur on the surface of CNTs [167]. In the case where the required measurement exceeds 1 to 20 nm resolution while using SEM, then TEM is used [156,167]. Small dimensions in CNTs such as interlayer distance, diameter, and number of graphene sheets can be easily examined (Figure 6c,d) by targeting the sample with a high energy electron beam of up to 300 keV [168,169]. It should be noted that the functional groups (organic and inorganic) that modify the surface of CNTs can also be evaluated by using TEM [161,170,171]. Moreover, structural integrity, surface functionalization, and defect son the surface of CNTs caused by the oxidation (acidic or basic) to introduce oxygen containing functional groups like hydroxyl, carbonyl, and carboxylic acid groups have also been studied by using both SEM and TEM techniques [155,172].

The image of the atomic structure and crystal structure information can be obtained by using high resolution transmission electron microscopy (HR-TEM) [173]. A high phase contrast image as small as the crystal unit can be obtained. This technology is widely used for advanced characterization of materials, allowing access to information on just-in-time defects, stacking faults, deposits, and grain boundaries. In addition to the morphology of the MWCNTs in the HR-TEM image, the direct measurements can be made on the MWCNT walls. For example, the number of walls constituting the nanotubes can be directly counted and recorded as control parameters in subsequent experiments in case the number of such walls needs to be changed. In addition, the interplanar distance between the walls can be accurately measured and compared with the crystal structure data table of the carbon structure and its respective diffraction pattern.

**Figure 6.** (**a**) SEM image of a bulk sample of multi-walled carbon nanotubes [155], (**b**) SEM image showing vertical aligned CNTs [174], (**c**) high-magnified TEM images of CNTs grown on unreduced catalyst [168] and, (**d**) TEM image of a bulk sample of multi-walled carbon nanotubes [155].

The chemical state and structure of CNTs can be obtained by using an X-ray photoelectron spectroscopy (XPS) technique [114]. However, the data obtained from this technique are used to examine the structural modification of CNTs before and after the chemical interactions with organics, inorganics, or gaseous adsorption. The investigation of CNTs by using XPS is done after the incorporation of nitrogen into the nanotube [175]. Due to the polar nature of the carbon–nitrogen bond, the peak shift before and after the modification is evidence of nitrogen incorporation [176,177]. Furthermore, the technology demonstrates that carbon nanofibers are more similar to carbon oxides than various graphites [178]. Fluorinated functionalization of SWCNTs was also studied by using XPS; the results concluded that three peaks of sp<sup>2</sup> , sp<sup>3</sup> , and carboxyl groups (284.3, 285, and 288.5 eV, respectively) were associated with C1s of un-doped nanotubes [114]. The observed carboxyl group (288.5 eV) was similar to in nanofibers [179].

Crystallinity of CNTs can be obtained by using an X-ray diffraction (XRD) technique [90,176]. X-ray diffraction patterns of graphite and CNTs are very close to each other because of their inherent properties. The XRD pattern of CVD-synthesized MWCNTs is shown in Figure 7, illustrating a peak similar to graphite (002), and the measured layer spacing can be obtained from Bragg's law, while the other peaks (family of (hk 0) peaks)) can be obtained because the mono-graphene layer makes the honeycomb matrix [114]. Therefore, the curve obtained by XRD does not distinguish between the microstructure information of CNTs and graphite; nevertheless, it is helpful for purity analysis of the sample [102,103]. The XRD pattern revealed that straight CNTs with a good alignment on the surface of the substrate did not show the peak, i.e., 022 [114]. For CNTs aligned vertically to their substrate surface, the XRD pattern is not collected because of the scattering of the beams inside the sample. Therefore, the 002 peak is always lowered for better-aligned CNTs [180]. In addition, several other types of parameters such as bundle size, mean diameter, and diameter dispersion can also be studied by using the XRD technique. The peak 10 is greatly influenced by these parameters in terms of its location and thickness [181,182].

**Figure 7.** XRD pattern of MWCNTs synthesized by CVD [114].

Qualitative evaluation for nanotubes' surface can be done by using Fourier transform infrared spectroscopy (FT-IR) [183]. The sample is characterized by passing the infrared radiations through it, and the part of radiations absorbed by the sample at specific energy is determined. Each functional group is identified by a particular range of frequency with associated absorption peaks. The infrared spectrum of the original CNTs (Figure 8) shows a characteristic band of about 1600 cm−<sup>1</sup> associated with aromatic rings (C=C bond) of

rolled graphene layers. Sometimes, the peaks of 3800 to 3200 cm−<sup>1</sup> (O-H stretch) and 1700 particles that absorb moisture into the atmosphere or due to certain purification processes [184]. In addition, a band of 2910 to 2940 cm−<sup>1</sup> was also detected for CNTs, which is related to the vibration of C-H stretching methylene (CH2) [185]. The FTIR spectra can also suggest surface modification of CNTs. For example, in Figure 8b, a new peak near 1450 cm−<sup>1</sup> appeared that was assigned to asymmetric CH<sup>2</sup> bending. This peak is typically interpreted as evidence of defects in the structure of CNTs. In Figure 8c, a new peak near 1735 cm−<sup>1</sup> suggested a carbonyl stretch of the carboxylic acid group. In addition, a double peak at approximately 2900 cm−<sup>1</sup> was attributed to the loss of aromaticity due to the oxygen functional groups.The thermal stability and proportion of volatile compounds of nanotubes can be analyzed by an analytical technique called thermogravimetric analysis (TGA). The analysis is made by heating the sample directly in the air or inert gases (He/Ar) while recording the change in its weight with respect to elevated temperature [176,186,187]. In some cases, the analysis is made in the presence of N<sup>2</sup> or He with poor oxygen atmosphere (1% to 5% O2) to slow oxidation [188]. During the TGA analysis of the CNT sample in the air atmosphere, the weight loss of the sample (Figure 9) is usually due to carbon oxidation to CO2, while the solid oxides after the oxidation of metallic catalyst are responsible for the superposition of the sample [165,189,190]. Generally, the percent yields of carbon deposits are determined by using TGA. Usually, during the oxidation of the sample the weight occurs in the temperatures ranging from 200 to 680 ◦C [190]. The contents of carbon can be calculated by obtaining the percentage of (m<sup>1</sup> − m2)/m1, where m<sup>1</sup> is the weight of the sample before oxidation and m<sup>2</sup> is the weight of the sample after the oxidation [11].

**Figure 8.** FT-IR spectra of CNTs: (a) pristine CNTs (CNTs-WT), (**b**) CNTs treated with HCl (CNT-H), and (**c**) CNTs treated with mixture of H3SO<sup>4</sup> and HNO<sup>3</sup> (CNTs-AC) [191].

**Figure 9.** TGA analysis of the CNT sample [190].

The structural features of CNTs can be defined by their crystalline structure, chiral carrier, and single- or multi-walled features [192]. Crystalline arrangement of nanotubes can be characterized by the ID/IG ratio determined from the RAMAN spectroscopy. ID/IG indicates the ratio between the organized and unstructured carbon in the CNTs and uses the intensity of defective and graphitic carbon (D and G bands) at the high wavenumbers in the RAMAN absorption spectrum (Figure 10) [193]. ID/IG is a good quality indicator for CNTs, and the low ID/IG ratio is characteristic of highly graphitized structures; the laboratory reported mass of MWCNT is 0.65, and industrial grade MWCNT is 2.04 [90,194].

**Figure 10.** Raman spectra of pristine (MWCNTs) and graphitized MWCNTs (g-MWCNTs) [193].

### **6. Applications of CNTs**

### *6.1. Removal of Heavy Metals*

Removal of heavy metals from water and wastewater by using surface modified CNTs has been studied extensively [143,195–197]. Contaminant adsorption mainly occurs at four possible types of sites that are available on the CNTs such as outer and inner grooves and interstitial channels mas shown in Figure 11, but the inner region of the nanotubes is less adsorbed [53,135–137,198]. Bahgat et al. [143] used functionalized MWCNTs for the adsorption of heavy metals and concluded that adsorption of the metals occurs because of a number of adsorption spots available on the tubes' surfaces. In another study, higher adsorption of Zn2+ ions was observed by the plasma-treated CNTs due to the availability of more oxygen-containing functional groups. The mechanism of surface complexation was responsible for the adsorption of cationic ions, as the adsorption sites increased due to deprotonating functional groups [197].

Surface modification of CNTs improved their affinity for heavy metal ions and metalloid adsorption [199,200] by incorporating iron oxide and oxidation [16,201–203], coating with oxides of nonmagnetic metals [204], introducing a thiol functional group [82], and functionalizing with sulfur [205]. According to Addo Ntim and Mitra [203], the different oxides of iron such as magnetite (Fe3O4), maghemite (γ-Fe2O3), hematite (α-Fe2O3), and goethite (α-FeO) are very useful for the removal of trace heavy metals present in water and wastewater [170].

Generally, the process of surface modification of functionalization of CNTs is responsible for the adsorption mechanisms of heavy metals [53,191,206]. Table 8 shows an explanation of previously used functionalized CNTs and their adsorption capacities and removal mechanisms. Surface area, porous structure, functional groups, and interaction mechanisms between the absorbate and absorbents are the main attributes of CNTs for their heavy metal adsorption capability [53,136,137,143]. In addition, these properties enhance the heavy metal removal efficiencies onto the polymer film incorporated by functional CNTs [135].

Physical and chemisorption mechanisms of adsorption have been observed between the CNTs modified with metal oxides and heavy metal ions [203]. In addition, the contaminant with particular hydration energy, ionic radius, and potential of hydrolysis also affects the adsorption capacities of CNTs. A study conducted by Hu et al. [207] confirms the above statement, when higher removal of Pb+2 was observed than Cu+2 by using MWCNTs modified with iron oxide under the same experimental conditions. The adsorption performance of CNTs for heavy metals is also affected by the presence of other organics. For example, enhanced removal of Cd+2 ion was observed by using oxidized MWCNTs in the presence of 1–naphthol, while there was no effect recorded during the presence of Cd+2 ion on the removal of 1–naphthol in similar conditions [208]. These types of results mainly occur because of the distinct interaction mechanisms that are responsible for the adsorption of different types of pollutants [170].

Solution pH also plays an important role in the adsorption capacity and mechanisms of CNTs. For example, higher pH is favorable for endospheric interactions, while the lower pH facilitates extracellular interaction and/or ion exchange of targeted metal ions and surface functional groups of CNTs [208]. Moreover, as the pH increases (alkaline conditions), the charge density of functionalized CNTs moves towards more negativity, which is efficient for the adsorption of cationics, while, at lower pH (acidic conditions), because of protonating functional groups, the positive charge density increases, which repels the cationic metals, resulting in the lower efficiency of CNTs [53,136,137]. In a study, oxidized and ethylenediamine-doped MWCNTs were used for the adsorption of

Cd+2 ions from aqueous samples. The results concluded that both types of absorbents removed Cd+2 ions strongly depending on pH ranging from 8 to 9 [209]. According to Rao and coworkers [146], the best adsorption capacity of nanotubes was observed in the pH ranging from 7 to 10. In addition to this particular range of pH, the ionization and competition between different species may also happen [210–213]. For example, an effective adsorption of Pb+2 ions on functionalized CNTs was reported during the co-existence of sodium dodecylbenzenesulfonate (DBS), while the adsorption of lead was significantly reduced in the presence of benzalkonium chloride [214]. The higher adsorption of lead ions might be due to anionic surfactant formation between Pb+2 and DBS, which are very complex compounds. In addition, the charge density (negative or positive) on the surface of CNTs makes the different interaction mechanisms of metal ions. For example, chemical interaction occurred between the N-dopped magnetic CNTs and Cr(III), while an electrostatic interaction was observed between the acid oxidized CNTs and Cr(III) [215].

Regardless of the costs, the CNTs are more efficient in terms of their adsorption and desorption phases than the AC, as reported in many studies [146,216]. Lu and coworkers [216] reported a slight decrease in adsorption and desorption of Ni+2 while using CNTs, and on the other hand, a sharp decrease was observed in the case of AC [216].

The adsorption of heavy metals by the functional CNTs depends on the interactions between available functional groups on the surface of CNTs and particular contaminants rather than the size of nanotubes. For example, higher adsorption of As(III) and As(V) was reported by Addo Ntim and Mitra [203] by using zirconia-modified CNTs of the same diameter (20 to 40 nm) than by using the CNTs coated with iron oxide. These results demonstrate higher arsenic adsorption by zirconia nanocomposites than the CNTs modified with iron oxide with similar range of diameters. Therefore, based on these results, further investigations on the adsorption capacities of functionalized CNTs are needed in terms of their surface area rather than the size CNTs.







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### *6.2. Removal of Organics*

Organics due to human actions [259], animals, or plant deterioration [260] are present in water and wastewater, usually in the form of dissolved and/or suspended particulate matters [135]. Zare et al. [261] reported that the adsorption efficiencies of CNTs for organic dyes can be enhanced after the functionalization. Methylene blue and orange were effectively removed from the water matrix by using oxidized MWCNTs [262,263] as compared to other types of modified adsorbents. Duman et al. [264] successfully used a novel nanocomposite, MWCNTs/carrageenan/Fe3O4, for effective removal of crystal violet. New nanocomposites can also be used for CNT modification to increase the adsorption capacity of cationic dyes. Sadegh and coworkers [265] reported that the adsorption of amide black can be significantly enhanced by using MWCNT-COOH-cysteamine. The results showed a maximum adsorption capacity of 131 mg g−<sup>1</sup> for MWCNT-COOH-cysteamine, while it was 90 mg g−<sup>1</sup> for MWCNT-COOH [265].

The adsorption capacities of CNT composites can be reduced effectively because of competition that occurred between several types of organic contaminants present in water and wastewater at the same time [266], similarly to as described for heavy metal adsorption. However, as reported by Ali et al. [135], the initial concentration did not have any significant effect on the removal capacities of CNTs for the removal of inorganic contaminants. As a result, customization and modifications of the surface of CNTs make them significant for selective adsorption of organic contaminants. Moreover, Wang et al. showed about 95% efficiency of the CNT composite for pharmaceutical and personal care product (PPCP) removal by increasing the specific surface area and aromatic ring of CNTs [267,268]. Jahangiri-Rad et al. [269] showed an adsorption capacity of 496 mg g−<sup>1</sup> by using oxidized SWCNTs with large specific surface for the removal of blue 29 dye.

Adsorption capacities and mechanisms responsible for the interaction between organic contaminants and functionalized CNTs at different experimental conditions are described in Table 9. The increase in oxygen-containing functional groups on the surface of nanotubes results in a decrease in natural organic maters (NOM), because the π–π interaction decreases due to more electrostatic repulsion [270]. The same type of results were shown in the case of higher pH [270]. Therefore, π–π interactions are responsible between the large specific surface area of functionalized CNTs and NOM for the adsorption area of CNT [271,272]. Yang et al. [208] also reported similar results, as organics may be absorbed by π–π interactions occurring between the surface of CNTs and aromatic rings of 1–naphthol.

CNTs have also been effectively used for the removal of pesticides [170]. Oxidized and as prepared MWCNTs showed excellent removal efficiency for diuron at pH ≥ 7.0 [273]. Oxidation of CNTs leads to the increase of their surface area, and pore size results in the higher adsorption of diuron [170]. Hamdi et al. [274] reported a reduction in uptake of chlordane and p,p0 -dichlorodiphenyldichloroethylene from 78% to 23% in the roots of a lettuce crop by the addition of CNTs functionalized with amino group. The pesticides (1 pyrenebutyric, 2,4-dichlorophenoxyacetic, and diquat dibromide) were absorbed more (up to 70.6%) on semiconductor CNTs than metallic CNTs [275]. The lower density of electrons on the surface of semiconductor CNTs were found to facilitate higher adsorption [275]. On the other hand, during the batch modes, the pesticide removal was found to be limited [276]. SWCNTs and MWCNTs were used for the removal of diquat dibromide in the fixed bed system. The results were not higher than those of the batch system, but the pesticide absorbed completely in the fixed bed system by increasing the time [276].

The CNTs are also being used for nano-filtration of contaminants from aqueous solutions [162,277,278]. For adsorbents, the selectivity of nano-filters can be controlled by attaching different functional groups to the surface of CNTs [279]. Beside the hydrophobicity of CNTs, molecular dynamics simulations showed a weak interaction between water and nanotubes [280]. Hummer [281] explained that friction-free water flow was caused by the nano-scale pore size, which makes interaction energy smaller and finally lowers the interactions with water [281].



increased





### *6.3. Removal of Microorganisms*

Bacteriological contaminants, deteriorating the assimilative capacity of water bodies leading to diverse impacts on surrounding environment, are a major challenge [297]. Bacteriological contaminants are often found in surface waters, wastewaters, and respective treatment plants [145,298]. CNTs, with their diverse range of surface and functional characteristics, have a high-affinity interaction with biological contaminants. CNTs have been proven for their higher adsorption capacities, inactivation efficiencies of viral or bacterial spores, and greater antimicrobial potential than conventional sorbents because of their greater surface area [170,299,300].

Previous studies that have reported that CNTs can inactivate or remove a variety of microorganisms, including bacteria, are shown in Table 10. The inactivation of *E. coli* was observed by using SWCNTs because of its penetration into the bacterial cell wall [301]. In addition, surface-modified CNTs with different chemical groups destroy the cell wall of microorganisms more strongly than the raw and polymer-grafting CNT membranes [135]. Furthermore, the straight interaction of microbes (i.e., *E. coli*) and functionalized CNTs can cause adverse effects on the metabolisms and morphological structure of the cell wall of bacteria [135]. The penetration of CNTs into the cell wall of microorganisms is the main reason for their higher inactivation affinity [301].

CNTs functionalized with silver nanoparticles showed excellent ability to inactivate microorganisms. For example, Ihsanullah and his coworkers [302] synthesized and successfully used silver-doped CNTs for the inactivation of *E. coli* bacteria. The results proved that 100% of the bacteria were killed due to toxicity of synthesized silver doped nanomaterial [302]. In addition, many researchers reported that the diameter of nanotubes can be an important factor of concern for inactivation pathogenic microorganisms (Table 10) in water and wastewater.

The single kinetics of CNTs also reveal the CNTs' ability to eliminate pathogens in water and wastewater treatment, and microbes remain on the CNT surface based on deep filtration mechanisms [303,304]. Brady and coworkers [299] used a poly vinylidene fluoride-based SWCNT filter for excellent removal of *E. coli* bacteria at low pressure. The results show that the cells were completely captured by the filter [299]. In addition to filtrations, an excellent removal efficiency of MS2 virus was observed by filtering the sample through a controlled nano porous CNT based filter at a pressure of about 8 to 11 bar [304]. Beside the filtrations, CNTs are widely used in water and wastewater treatment as an antimicrobial agent, as described earlier. This behavior makes them a replacement for chemical disinfectants as a new method for controlling pathogens [301,305–308]. The applications of CNTs for disinfection treatment in water and wastewater avoid the materialization of unsafe disinfectant by-products like trihalomethanes, aldehydes, and haloacetic acids due to their low oxidation state and solubility in water. Therefore, it is necessary to promote the dispersibility of these compounds; a surfactant or a polymer such as sodium dodecylbenzenesulfonate, polyvinylpyrrolidone is used [309]. Due to the excellent mechanical properties on CNTs, they act as scaffolds for antimicrobial agents, such as silver nanoparticles [310,311] and antibacterial lysozyme [307,309].



### **7. Conclusions**

Purifying water from assorted contaminants is challenging, and carbon nanotube-based nanocomposites can provide simple as well as effective water decontamination/disinfection. This review shows that functionalized CNTs are a new generation of pollution management materials. These materials have excellent adsorption capacities and work effectively in removing organics, inorganics, and biological species. Various sorption mechanisms include physical adsorption, electrostatic interaction, surface complexation, and chemical interactions between surface functional groups and metal ions. The effects of pH, CNT dosage, time, ionic strength, temperature, and surface charge on the adsorption of heavy metal ions on carbon nanotube surface were also discussed. Even functionalized CNTs also has antibacterial efficacy against Gram-positive and Gram-negative bacteria.

Almost all the studies show effective removal of contaminants and have been performed using deionized water, but the potential of functionalized CNT nanomaterials needs to be verified under real water conditions. In wastewater, carbonates, phosphates, and silicates can successfully compete with target metals/organics for adsorption sites in nanostructures. Other important components of source water and wastewater are natural organic materials, such as fulvic acid and humic acid, which will occupy the surface of CNTs and thus affect the adsorption of contaminants on the nanostructures. The effectiveness of CNT-based nanotechnology should be evaluated under real water conditions.

Many researchers have also focused on evaluating the adsorption–desorption capacity to make it a cost-effective adsorbent for use in wastewater treatment. However, more studies are encouraged to check the feasibility of reuse.

**Author Contributions:** Conceptualization, W.D. and M.M.-A.A.; resources, W.D. and H.-W.K.; writing—original draft preparation, M.M.-A.A.; writing—review and editing, H.-W.K., W.D., M.S., M.U., M.M.-A.A.; visualization, H.A.; supervision, H.-W.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** This study was done at the Department of Environmental Science and Engineering, Tunghai University, Taiwan. The authors gratefully acknowledge the funding support by the Research Council of Texas A&M University San Antonio, and the Ministry of Science and Technology of R.O.C. (Taiwan), 107-2221-E-029-001-MY2. The authors would also like to thank Yu-Ling Wei, Chiung-Fen Chang, and Meng-Hau Sung for providing financial and analytical support to M.M.A.A. We acknowledge support of the Hamburg University of Technology (TUHH) by enabling open access publishing through funding programme Open Access Publishing.

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

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