The Performance of Functionalized Multi-Walled Carbon Nanotube-Based Filters for Water Treatment Applications

Abstract

Water contamination is a crucial environmental issue, and various methods and processes have been implemented for water treatment and decontamination. Various methods have been developed for handling issues relevant to water quality. In environmental applications, particularly wastewater treatment, carbon-based nanomaterials, particularly multi-walled carbon nanotubes (MWNTs), have attracted significant interest because of their large specific surface area and associated adsorption sites. Despite having previously mentioned attractive characteristics, their natural chemical structure causes them to aggregate, which restricts their practical applications. It necessitates surface modification or functionalization to reduce agglomeration and improve the dispersibility. For the purpose of purifying water, several studies have focused on covalent and non-covalent functionalization. Different functionalization procedures of MWNTs are employed to enhance the adsorption potential applications. According to several studies, functionalized MWNTs may remove up to 98% of organic contaminants and heavy metals when performing under ideal conditions. Because of their high adsorption capacity, functionalized MWNTs have been shown to be promising nanomaterials for the purification of waterways. Nevertheless, most functional carbon nanotube applications are restricted to laboratory-based research. Further research is required to determine the viability of their adsorption methods in large-scale and industrial applications.

1. Introduction

One of the biggest issues facing the world today is the scarcity of pure and fresh water. Efficient technology for saltwater desalination and wastewater reclamation is necessary due to the scarcity of water resources. Anthropogenic activities are introducing a variety of contaminants into water resources, ranging from established pollutants such as distillates and heavy metals to newly discovered micropollutants, such as antibiotics [1]. Fish and other safe-to-eat species absorb heavy metals as they move up the food chain. The most prevalent harmful ions in aqueous solutions that cause specific problems include radionuclides, heavy metallic ions, oils, dyes, and organic pesticides [2, 3, 4, 5, 6]. Conventional water treatment techniques were unable to effectively remove some of these contaminants from the water [7, 8].

Because of their tunable structural, electrical, chemical, and physical characteristics, carbon nanotubes (CNTs) can spur the development of novel solutions to solve issues with water pollution and scarcity [9]. However, understanding the full potential of CNTs for a range of applications has been restricted by their chemical inertness and amphiphilic character [10]. Surface modification of carbon nanotubes (CNTs) is required to fully exploit their special features by changing their surface properties in order to get over these limitations [11]. Several research works have investigated surface modification methods, including acid oxidation treatment [12], coating with surfactant [13], and ozone oxidation [10].

CNTs extract heavy metals through complexation or by attracting metal ions electrostatically to different functional groups on their surface that include oxygen [14]. Even though CNTs are now frequently modified by various chemical processes, the possible effects of these modifications have not yet been well investigated, particularly with regard to water purification. Functionalization makes CNTs more reactive, more soluble and opens possibilities for additional chemical alterations such as metal deposition, grafting reactions, and ion adsorption [15, 16, 17].

Because of their toxicity, heavy metal wastes can be dangerous to people and the environment even at relatively low concentrations [18]. Basic techniques for eliminating metals from water, for example, particle trade, turn-around assimilation, and electrodialysis have been demonstrated to be either excessively costly or wasteful to eliminate substantial metal particles from fluid solutions [19, 20]. Regardless of having smaller pores, CNTs have a high penetrability and a lower pressure is needed to siphon water through the channel, perhaps because of the smooth inside of the CNT [21]. Multi-walled carbon nanotubes (MWNTs) have recently been utilized to eliminate heavy metal particles, for example, lead, copper, cadmium, silver, and nickel [22]. Kandah and Meunier have found that the adsorption of Ni²⁺ by oxidized CNTs is 1.24 times higher than that by activated carbon [23, 24].

Sumio Iijima discovered carbon nanotubes (CNTs) in 1991 using the arc-discharge process [25]. Carbon atoms arranged in hexagonal sheets within CNTs form a hollow tube-like structure that has different helicities of graphitic sheets. Because CNTs have stronger sp² bonds than diamonds, which have sp³ bonds, they possess unique hardness. Single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) are the two major categories of carbon nanotubes. Because of their structural variations, SWNTs and MWNTs exhibit remarkably distinct physical characteristics from one another [26]. A single graphene cylinder, with a diameter ranging from 0.4 to 2 nm, has been identified in the case of SWNTs. However, MWNTs consist of two or more coaxial cylinders with a single graphene sheet around a hollow core in each layer [27]. With a length of several 𝜇m, the outside diameter ranges from 2 to 100 nm, while the inner diameter is in the range of 3 nm [28]. CNT may be classified into two zones: the tips and the sidewalls according on its chemical structure [29]. Certain key features of MWNTs and SWNTs are shown in Table 1.

Table 1.

Presents the differences between carbon nanotubes with one wall and those with multiple walls.

Table 2 displays a few key physical characteristics of MWNTs [30, 31]. Due to their fascinating physical and chemical characteristics, such as their high mechanical strength, high aspect ratio, high thermal conductivity, high electrical conductivity, electron emission, very good optical absorption, and minimal energy loss, CNTs are promising nanostructured filters for wastewater decontamination [32].

MWNT properties	BET surface area; (m^2/g)	Diameter; (nm)	Length; (μm)	Density; (g/cm^3)	Tensile strength (GPa)
Average value	95–350	20–110	2–5	1.5–1.7	100–150

Table 2.

Physical characteristics of MWNTs.

In this chapter, we present the different functionalization routes and the application for water treatment. Here, the removal of heavy metals and eventual environmental cleanup were suggested along with the intriguing research potentials and prospects of CNT-based materials. Therefore, the CNTs are promising for the decontamination of water pollution, including heavy metals from wastewater, while considering adsorption efficiency as well as process cost.

2. Functionalization of carbon nanotubes: methods and advantages

The process of creating functional groups on CNT surfaces is known as functionalization. These functional groups contribute to the solubilization of CNTs by increasing the CNT-matrix/solvent interaction and reducing the long-range van der Waals forces of attraction [33]. There are several functionalization techniques that have been thoroughly outlined, as shown in Table 3. These methods of interacting with CNTs may be simply classified into two categories: physical (non-covalent) and chemical (covalent) functionalization. Direct attachment of functional groups, such as fluorination [34] and hydrogenation [35], can be used to carry out covalent functionalization. Moreover, using carboxylic or hydroxyl functional groups bonded to nanotubes for chemical reactions and further derivatization (amidation or esterification) [36, 37, 38]. However, the adsorption of different groups on the surface of carbon nanotubes (CNTs) without affecting the π system of the graphene sheets makes non-covalent treatment extremely desirable [39]. Non-covalent techniques involve the hydrophobic part of the adsorbed molecules, associating with the sidewalls of the nanotubes via van der Waals, π–π, CH–π, and other interactions, while the hydrophilic portion of the molecules provides water solubility [40]. Therefore, functionalization increases the solubility and reactivity and opens up new chemical modification possibilities for CNTs, including grafting reactions, metal deposition, and ion adsorption [16]. Moreover, the functional groups can act as anchor groups to fuse two moieties together and carry out additional derivatization through chemical interactions with other functional groups [41].

Functionalization approaches of CNTs	Advantages	Disadvantages	Ref.
Covalent	It has no impact with CNTs’ main structure. Covalent bonds serve as functionalization substitutes. Functionalization results in increased solubility in organic solvents and water.	Reduction of the p-conjugation system and transition from sp2 to sp3 hybridization on the graphene layer. Sometimes, oxidative consumption of CNTs occurs during the process.	[33, 34, 35, 36, 37, 38]
Non-covalent	Adsorbing different groups on the surface of CNTs without affecting the graphene sheets’ π system. Utilizing p-p stacking interactions, a non-covalent method attaches carboxylic functional groups to reduce cytotoxicity and provide stable aqueous dispersions.	Dispersions’ stability is also influenced by the kind and concentration of surfactants. The solvent determines the polymer wrapping across the CNT surface.	[16, 39, 40, 41]

Table 3.

The positive and negative aspects of different CNT functionalization techniques.

3. Chemical functionalization of MWNTs and removal of heavy metal ions

Two distinct procedures were used to purify and functionalize the MWNTs: the first involved altering the pristine sample with HCl and H₂O₂ mixture, and the second involved oxidizing the purified MWNTs with nitric acid. Figure 1 illustrates the exploration of the oxidation mechanism by nitric acid [42]. The surface chemistry and charge of CNTs are altered by the carboxylic function groups. Figure 2 shows the steps for filter design for the adsorption process [32]. The filtration results demonstrated that Ni (II) removal efficiency is highly pH dependent with removal efficiency of 85% at pH = 8. Functionalized MWNT filters can also be repurposed with excellent performance through numerous regeneration cycles.

Figure 1.

The adsorption mechanism between functionalized MWNTs and the heavy metal ions [42].

Figure 2.

The procedures involved in designing and creating an MWNT filter [42].

For the purpose of removing heavy metals from wastewater, functionalized MWNT filters could be a good adsorbent choice. In another study, commercial MWNTs were functionalized and employed as a filter to remove Cr (VI) ions from aqueous solutions and purify water [9]. Following this investigation, it was observed that low pH and low initial concentration are the main parameters that promote removal efficiency; at pH = 2 and a concentration of 10 ppm, the effectiveness of removal could increase to 97%. The data obtained in this report showed that the nanotube samples with lower diameters have more ability for functionalization. Functionalized CNTs offer higher specific surface area, which provides more adsorption sites, and its increased presence of oxygen-containing functional groups [21].

4. Heavy metal adsorption techniques using MWNT adsorbents

Chemicals could create physical interactions, such as π bonds, van der Waals forces, or electrostatic forces, with organic materials that attach to MWNT without changing their own chemical or physical properties [43]. Additionally, functional groups on the innermost layer of CNT, such as carboxyl or hydroxyl groups, or suspended bonds may be utilized to chemically link the material, Figure 3. Although chemical bonding seems stronger than physical adsorption, the strength of these interactions depends on the quantity of functional groups and the adsorption surface. When it comes to surface properties, pure sp²-hybridized carbon is hydrophobic and promotes the highly effective adsorption of hydrocarbons [44].

Figure 3.

Heavy metal adsorption procedure employing CNT.

CNT agglomerates’ adsorption capacity may be significantly increased by stacking their pore configurations [45]. MWNTs have an essential role in the process of treating wastewater. In their work, Elsehly et al. exploited commercial MWNTs to eliminate heavy metal ions, such as Mn and Fe, from a water-based solution. They found that oxidized CNTs and both metal ions interacted more favorably than raw CNTs [14]. Oxidation produces oxygen-containing groups, reduces the carbon tubes’ diameter, and eliminates contaminants, all of which serve to highlight this behavior. It was discovered that the adsorption capabilities of Mn and Fe were 71.5% and 52%, correspondingly. At pH = 3 for manganese and pH = 8 for iron, there was a noticeable improvement in removal efficiency. Energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) methods were used to analyze the impact of oxidation on the structural characteristics of MWNTs and to examine the outer diameter distribution and functionalization featuring oxygen.

Another report showed that MWNTs significantly improved the adsorption of heavy metal ions from an aqueous solution, such as antimony Sb (III) [46]. Using 200 mg of MWNTs at 298 K and pH 7.0, almost all of the Sb (III) ions were eradicated from the solution in less than 30 minutes. Utilizing functionalized MWNT, Rahmati et al. investigated the removal of Ni ions from aqueous environments [47]. Under particular ideal circumstances, such as a temperature of 55°C, an adsorbent dose of 0.011 g, a pH of 7.21, and a contact period of 68 min, a maximum capacity of adsorption of 115.8 mg/g was achieved. The enhancement in Ni (II) ion removal by functionalized MWNTs may be attributed to the hydrophilicity of MWNTs, which increases the force of electrostatic and the chemical reactions involving Ni (II) molecules and the adsorbent’s surface.

5. Ozone-induced surface functionalization of MWNTs and their potential applications in water purification

An alternative approach for curing difficulties caused by chemical oxidation involves ozone oxidation [48]. In addition to being cheap to operate and sustainable, ozone oxidation has been demonstrated to oxidize nanotubes at room temperature and can be executed on particular nanotubes. The ozonolysis procedure most likely causes the caps on the ends to break off and holes to be introduced into tube side walls, Figure 4 [49]. Elsehly et al. investigated the performance of ozone functionalization on the structure of MWNTs and how to enhance their adsorption ability [10]. In their investigation, a quartz U-type reactor containing a tablet of MWNTs was subjected to streams of 12 l/h of an ozone-oxygen mixture, with an ozone concentration that was around 90 mg/l, for a duration of 7 hours at ambient temperature. Ozone functionalized CNT (O₃-MWNT)-based filters were employed to effectively remove benzene from polluted water. The adsorption capacity for benzene at a concentration of 500 mg/l may be approximated as 162.5 mg/g for raw-MWNTs and 193.2 mg/g for O₃-MWNTs, taking into account the saturation concentration, 50 ml of benzene solution at pH = 7, and 0.3 g filter mass.

Figure 4.

The process via which benzene is adsorbed on ozone functionalized MWNTs [3].

Figure 4 suggests that the absorption of benzene by O3-MWNTs is caused by the π - π electron-donor-acceptor process, which involves the carboxyl oxygen-atom of the MWNT interface as electron-donor and the aromatic chain of benzene as electron acceptor [50]. Moreover, the discovery of high benzene adsorption via the O3-MWNTs might potentially be explained by the electrostatic contact between the benzene molecules and the MCNT surfaces. Because benzene molecules have a positive charge, adsorbents with a negative surface charge are better at attracting and binding benzene molecules. Additionally, the adsorption of benzene on the modified MWNTs is significantly influenced by its physical adsorption. The development of more active adsorption sites justifies the selection for O3-MWNT filters. In addition to its increased efficiency, the O3 process has the added benefit of being a “dry” method, making it more environmentally friendly than HNO₃. O₃-MWNT filters are less expensive economically than HNO₃-treated filters. O₃-MWNTs are, therefore, an inexpensive adsorbent and a potential method for cleaning up wastewater from industries.

On the other hand, another study examined both raw and ozone-treated MWNTs were employed to eliminate heavy metals, such as Mn from contaminated water [51]. Low concentration and low pH have been found to be the main factors responsible for supporting elimination capability. A notable enhancement in the removal performance of O₃-MWNT filters, which achieved 99.5% for a pH of 4 and an Mn concentration of 20 ppm. As seen in Figure 5, one potential method for eliminating of the manganese from the liquid solution is the chemical reaction between Mn ion and the carbon molecules through carboxyl group by chemisorption of carboxylate ions. Future research on nanostructures is expected to benefit from CNTs’ controllable ozone functionalization.

Figure 5.

Ion exchange and complexation (a), physical adsorption (b), and the ozone functionalization of MWNT-based filters are depicted in the schematic along with the uptake of Mn processes. When functionalized MWNTs are present, the filtered solution becomes clearer [29].

6. Functionalized aligned carbon nanotube membranes for potential seawater desalination

Over the last few years, investigations and theoretical simulations have demonstrated that water moves through CNTs many orders of magnitude quicker than it does through other porous materials of similar size [52, 53]. The inner walls of CNTs are smooth and hydrophobic, which is responsible for the high fluid velocity. High-density, vertically aligned CNT membranes were created in order to completely use the interior pores of CNTs, as previously described in [54]. The capacity to exclude ions can be acquired by CNT membranes by tip functionalization. On the basis of CNTs with specifically organized carbonyl oxygen atoms changed inside the nanopores, Gong et al. created a programmable ion-selective nanopore [55].

7. The challenges of removing heavy metals from wastewater media

There were still restrictions on the functionalized CNTs’ ability to remove complex organics and heavy metals despite their ability to effectively deal with certain organic contaminants and heavy metals that are present. The following are the particular challenges and restrictions related to using carbon nanostructures for heavy metal removal:

1. Because carbon nanomaterials typically cluster together and form bundles, which reduces their surface area and adsorption capacity, aggregation and dispersion are thought to be among the most important problems [56].

2. Specificity and selectivity: It might be difficult to extract a particular heavy metal ion from a complicated mixture because of the great attraction of carbon nanotubes toward a variety of heavy metals [57].

3. Reusability and regeneration: In practical applications, carbon nanomaterials’ ability to regenerate and repurpose themselves is essential. It can also be difficult to desorb heavy metal ions that have absorbed onto carbon nanomaterials without causing damage to the materials [58]. In our opinion, the first issue could be solved by conducting more research and simulations about the functionalization methods to control the aggregation and dispersion of CNTs. Also, it should be more practical techniques to face the problem of selectivity, for example take real wastewater samples and analyze, then examine the effect of CNT on the removal of different ions. Regarding the last issue, the solutions used in regeneration of CNTs should be controlled through their pH and concentrations. Summarizing, extensive studies should be done on enhancing the manufacturing processes used to produce more applicable nanotubes.

8. Conclusions

Wastewater treatment is a major worldwide challenge; hence, the development of sustainable adsorbents that can remove hazardous heavy metal pollutants efficiently is necessary. Carbon nanoparticles, or CNTs, have a lot of promise for treating wastewater due to the progress of environmental and community demands. However, amorphous carbon and metal nanoparticles are contaminants found in raw carbon nanotubes that must be eliminated by processing them with powerful acids and oxidizing chemicals. In order to improve dispersion and decrease aggregation, functionalization is necessary. Many covalent and non-covalent functionalization techniques are used to improve their functional characteristics. In addition to being extensively investigated, functionalized carbon nanotubes have been effective as adsorbents for the removal of liquid pollutants, including oils and organic solvents, as well as heavy hazardous substances such as zinc, arsenic, and chromium. Both functionalized and pure carbon nanotubes (CNTs) possess outstanding features for purifying wastewater. Their huge pore volume, diameter, and specific surface area make them highly advantageous for the adsorption of heavy metals. This chapter primarily highlights the most recent advancements in the methods of CNT functionalization for new generation of carbon nanomaterials to facilitate the adsorption of heavy metal ions, as well as organic compounds from aqueous environments. CNT filters can eliminate a variety of both organic and inorganic contaminants. Compared to other materials, CNTs have a higher adsorption capacity, better adsorption selectivity, a shorter equilibrium period, and easier regeneration. However, before widespread use, CNT filters should be thoroughly analyzed for the possibility of CNT leakage into drinking water.

Acknowledgments

Thanks to the Faculty of Science, Physics Department, Damanhour University, Egypt. The work was supported by the project #122081700088-9 “Nuclear-physical methods and physical properties of nanostructures” at Skobeltsyn Institute of Nuclear Physics Lomonosov Moscow State University.

Conflict of interest

The authors declare no conflict of interest.