A comprehensive review of the application of modified carbon nanotubes (CNTs) for the removal of metals from wastewater

Carbon nanotubes (CNTs), a type of carbonaceous material, have extremely distinctive qualities in terms of tensile strength, heat stability, electrical conductivity, catalysis, and adsorption. These properties rely on structure, length, and thickness. Carbon nanotube and metal oxide combination have been successfully used over the past few decades to create carbon nanomaterials with extraordinary features. The current study offers an outline of the developments in the theory, procedures, and chemical modification of CNT with metals or polymers. This review presents different synthesis methods of functionalized CNTs along with their properties and factors affecting their adsorption capacity. In addition, it explains the role of different functionalized CNTs in removing different metals like Pb2+, Cd2+, Cr3+, Cr6+, Ni2+, Tl+3, and Hg2+ from wastewater. The adsorption capacity of these modified CNTs is in the range of 130–180 mg g−1. This review offers an essential understanding of the methods for creating multifunctional nano-hybrids for various applications and prospects of using nanomaterials for environmental remediation.


Introduction 1.Nanotechnology
Nanotechnology is the handling of atoms and molecules of matter with sizes ranging between 1-100 nm [1,2].Nanotechnology includes the study of a wide range of materials such as nanomaterials, nanostructures, and nanoparticles [3].As depicted in Figure 1, nanotechnology is a diverse discipline that necessitates the assistance of scientists to comprehend and address a variety of scientific challenges.Nanotechnology studies the pattern, fabrication, characterization, and uses of components, methods, and techniques while managing the dimension and form at the nanoscale [4].Due to their small size, higher surface area, and enhanced mechanical properties, nanomaterials are advantageous in numerous applications, including solar cells [5], supercapacitors [6], wastewater treatments [7], biomedical [8], and photocatalysis [9].

Types and classification of nanomaterials
Nanostructures have distinctive properties that may not be present in chemical substances with similar larger dimensions.The International Organization for Standardization (ISO) describes a nanostructured material as a material that possesses fundamental nanoscale surface morphology or any foreign nanoscale aspects [10].Table 1 shows the types and examples of some common nanomaterials.

Carbon nanotubes (CNTs)
Carbon nanotubes (CNTs) are tubular molecules containing layers of carbon-spiral carbon atoms, such as graphene.Based on their structure and physical properties, CNTs are divided into two types: multi-walled (MWCNT), which has dimensions greater than 100 nm and is composed of several spatially interlinked nanotubes, and single-walled (SWCNT), which has a diameter in the range of 0.4 to 3 nm.Similar to its primary component, CNTs are covalently bonded together through sp 2 hybridization, a highly potent type of chemical contact [25].CNTs exhibit greater metal elimination efficiency than activated carbon and biochar [26].CNTs' usefulness is constrained by their hydrophobic character, so they are unable to be utilized solely in aqueous solutions [27].Modification can be used to get around this restriction, which will increase the range of applications for CNTs [28].Figure 2 shows the structure of single and multi-wall carbon nanotubes.

Single walled CNTs
Identical to fullerenes, single-walled carbon nanotubes are an allotrope of sp2-bonded carbon.The framework can be visualized as a hollow tube made of graphite-like 6-membered carbon bands.The outermost portion of the buckyball or fullerene structure can surround one or both edges of the hollow tubes [30].A SWCNT can be depicted as a tube-shaped layer of graphite that is a single atom wide.The axis and size at which the sheet is rolled are both described by chirality.Single SWCNTs are much more powerful than steel.The tensile strengths of SWCNTs have been calculated to be about 100 times higher than those of steel at one-sixteenth of its mass [31].SWCNT has been produced using many techniques.These involve methods like laser ablation, carbon arc, and chemical vapor deposition that either use a supported catalyst or a gaseous catalyst [32].

Multi walled CNTs
By nature, MWCNTs are more complicated than SWCNTs.Numerous arrangements can be created using the fundamental graphene-building component.The most basic configuration uses a concentric design in which the diameter of the tubes is increased over time.Within, smaller tubes bigger ones are transformed [33].The number of walls in MWCNT can range from two to an infinite maximum.Thereby, MWCNT diameters could  reach as large as 30 nm, as compared to 0.7 to 2.0 nm for conventional SWCNTs [34].MWCNTs are high aspect ratio materials with dimensions that are usually 100 times or higher than the SWCNTs diameter [35].Their use and effectiveness rely not merely on aspect ratio as well as on how straight and twisted the tubes are, which in response depends on the size and severity of any imperfections in the tubes [33].Concerning the adsorption and consequent elimination of contaminants, the modification of MWCNTs increases their hydrophobicity [36].
Usually, the altered surface composition shows better qualities [37].More functional/adsorption groups are added to the MWCNTs by changing from a wide range of metal nanoparticles [38].

Functionalized carbon nanotubes
The most common modifications to CNTs are oxidation, magnetism, and functional group addition.Powerful oxidizing agents (K 2 Cr 2 O 7 /H 2 SO 4 , KMnO 4 /H 2 SO 4 ), which are used in the oxidation process to make the CNTs more hydrophilic, open the door for heavy metal binding.Acid oxidation, which involves a solution of HNO 3 and H 2 SO 4 , is one of the frequently used techniques for oxidizing CNTs [39].These acids enhance the efficacy of the heavy metals adsorption by increasing the amount of functional groups (−OH, −COOH) having oxygen on the surface of CNTs [40].
Combining the carbon nanotubes with transition metals forms the functionalized/modified CNTs.Metals are reinforced with CNT metal matrix composites, which have outstanding electrical characteristics and are utilized to improve the conduction characteristics of metals.Due to their extraordinarily high thermal conductivity, carbon nanotubes in metal matrix can be employed for heat management [41].MWCNTs are more frequently applied for different uses than single-walled carbon nanotubes.The most common method for creating magnetic nanoparticles incorporated with MWCNTs is to coat MWCNTs with chemically altered iron salts to form iron oxide molecules.However, there are several alternative methods for producing them.Due to their special ability to separate solids from liquids in an aqueous matrix using an exterior magnetic field, magnetic nanomaterials have great importance for the elimination of heavy metals [42,43].Table 2 shows some functionalized CNTs and their uses for the removal of heavy metals.

Synthesis methods to modify CNTs
Functionalized CNT can be synthesized in a variety of techniques.Two basic methods for creating CNT-based hybrid materials have been described in this review: There are two methods: (i) In-situ technique, which involves growing CNTs concurrently [67].(ii) Ex-situ technique, which involves adding decoration after CNT synthesis.Functionalized nanomaterials (NMs) are made up of multiphase NPs with a single nanoscale phase.They can either link NPs together in chains or organize NPs with heavier or larger components (like hybrid nanofibers) or more intricate configurations, like metalorganic frameworks [68].There are numerous ways to functionalize CNTs, however, the ones that are most frequently used are automated mixing, sol-gel, electrospinning techniques, atomic layer deposition, and chemical vapor deposition (CVD).Figure 3 represents some methods used for the synthesis of functionalized CNTs.Although CVD and electrospinning technologies can provide a homogeneous coating of MO on CNTs, these processes are difficult and demand specialized tools [69].

Sol-gel method
The production of various nanomaterials, particularly metal oxide nanoparticles (MON), can be achieved via the sol-gel process, which is a chemical process.This process involves dissolving the chemical source (typically metal alkoxide) in either water or alcohol, heating it, and stirring it until it is converted into gel [70].Although MON typically results in a non-uniform coating of CNTs, the sol-gel technique is the easiest and least expensive.The use of the sol-gel technique in the synthesis of composites has made significant headway, including its adaptability and the ability to produce materials with good homogeneity, high stability because of the chemical bonding between the glaze and the support, and the ability to regulate the dispersion [71,72].

Hydrothermal method
The technique for creating nanomaterials that are most frequently utilized is hydrothermal synthesis.It employs a mixed-method design.In hydrothermal synthesis, nanomaterials can be synthesized at a range of temperatures, including very high temperatures and ambient temperatures.Reducing or oxidizing parameters can affect the geometries of the substances for the synthesis based on the vapor pressure of the key component in the reaction.This approach is being successfully applied to synthesize a variety of nanomaterials.The hydrothermal synthesis method offers several benefits over other approaches.The hydrothermal method minimizes material loss while enabling the fabrication of high-vapor-pressure nanomaterials.The structures of the nanomaterials to be created can be finely regulated during hydrothermal synthesis using solvent phase or multistage chemical methods [73].

Co-precipitation method
The coprecipitation method enables the precipitation of metal which is base-assisted and provides hydroxide ions from a salt precursor.The controlled delivery of anions and cations aids in the production of nanomaterials that are dispersed homogeneously by regulating the nucleation and particle growth kinetics.Typically, a nucleation forms in solution when a material concentration reaches super-saturation.Diffusion occurring on the surface of the nucleation will cause it to expand, resulting in the formation of nanomaterial.To produce homogenous nanomaterial, the nucleation must be slowed down during the growth process [74].

Chemical vapor deposition (CVD)
The most prevalent method for thin-film deposition utilized to modify CNTs is CVD.Compared to other methods of CNT production, CVD is a unique technique.Due to its large productivity of nanotubes and reduced temperature requisite (550 °C-1000 °C), the CVD approach for the synthesis of CNTs is more affordable and suitable to be synthesized in laboratories.Moreover, the CNTs that are made can have their morphology and design changed using the CVD method, and paired nanotubes can expand in any intended way.To produce CNTs, CVD is thought to be the least expensive method [75].For the synthesis of CNTs/MO, a quartz tube is used to mount the metal catalyst using this technique.The catalytic metal particles are detected in the CVD reaction furnace at a temperature of 700 °C.Generally, under high-temperature conditions, the synthesis of carbon nanotubes promotes the breakdown of hydrocarbons which results in the growth of MWCNTs upon cooling [76].Once the carbon source has been turned on by the automated process, the doping procedure for the carbon nanotubes is carried out [77].The doping apparatus contains a nebulizer that vaporizes the metal and lets nitrogen flow through while doing so.This procedure is accomplished by pulsating the metal solution's vaporization every 30 s over 20 min, which is the necessary amount of time for CNTs to grow [78].

Applications of functionalized CNTs
Functionalized CNTs and their diverse uses are both rapidly growing.During the next 10 years, production in the following regions will exceed 600,000 tons.CNTs find their uses in contaminant adsorption and treatment, energy storage, microelectronics, and medicinal devices.Numerous studies have shown that modified CNTs are capable of efficiently adsorbing phenols, crimson dye, heavy metals, and other organic substances.Furthermore, modified CNTs are employed as an adsorbent in removing personal care items and endocrine-disrupting substances in water and wastewater purification processes [79].The powerful adsorption ability of nanomaterials is primarily a result of the surface's unsaturation, which makes it simple for other atoms in the aqueous solution to create stable bonds with the nanomaterial surface [80].
According to reports, roughly 40% of the population lacks access to clean water resources.In emerging nations, fresh water is the main issue.Freshwater is continuously in short supply due to increasing industrial development.The need for water for infrastructure and the maintenance of human resources rises as cities grow.Overuse of the resources is caused by population growth.There are numerous issues with freshwater availability and a persistent requirement for high-quality water.Researchers and scientists are seeking a way to purify salty water and wastewater.Water problems are caused by the contamination of water supplies [81].Many water purification techniques have been used to address this issue [82] and among all, the most potential water treatment techniques are based on carbon nanotubes (CNTs) [83].It can rid water of biological, inorganic, and organic impurities [42].

Metals in wastewater
In terms of potential heavy metal elimination adsorbents, carbon nanotube-based adsorbents have generated a lot of interest.However, there are still a lot of unresolved issues, including the interface of CNTs with heavy metal ions, the quantitative impact of functional groups, and the recycling of CNT-based adsorbents [84].Throughout the environment, including industries, the air, land, groundwater, agriculture, and people, the heavy metal contamination chain almost invariably follows a cyclical pattern.Detrimental consequences are brought on by low-level chronic exposure to heavy metals and metalloids [29].Many health problems are connected to heavy metal toxicity, which is seen as a severe threat.They may occasionally mimic bodily functions while also having the potential to disrupt key metabolic processes [85].

Chromium (Cr)
Chromium is used in many sectors involving the manufacture of steel, tanneries, metalworking, fabric dyeing, and timber conservation [86].Due to its extremely toxic effects, resistance to biodegradation, and accumulating features, hexavalent Cr has been regarded as a priority pollutant.Cr(VI) readily disseminates into freshwater lakes and has a high penetration into biofilms, where it may change into a range of radicals that are reactive and harmful to human health [87].To avoid damaging effects on human health, a suitable technology for removing Cr(VI) must be used before industrial effluent is released [88].Table 3 shows some modified CNTs used for the removal of chromium from water.Kumar et al used an oxidation polymerization technique to create MWCNTpolyaniline (Pani) composites, which were further treated with para-toluene sulfonic acid (pTSA).The pTSA-Pani-CNT composite was utilized to remove Cr 6+ adsorptively, and the composite demonstrated increased Cr 6+ adsorption.At an acidic pH, the maximum removal amount was discovered [89].Huo et al used a hydrothermal synthesis to create a new graphite-C 3 N 4 -BiFeO 3 -CNTs ternary magnetic composite (CNBT) was created.The elimination of Cr (VI) from effluent using this substance involved simultaneous adsorption and photocatalysis.The efficacy of CNBT as a photocatalyst was examined concerning pH, time, and pollutant concentration.The findings demonstrated that CNTs could significantly improve the photocatalytic efficiency of the prepared composite by reducing the rate of electron-hole pair recombination.The rates of elimination of Cr 6+ after 5 h of adsorption by CNBT were 98% [90].Zhu et al prepared nano nickel particles implanted in Nitrogen-doped CNTs with porous biochar support (Ni-N-K-C).The findings show that stimulation by KOH can enhance pore characteristics, encourage later Ni and N doping, and accelerate the development of CNTs.The specific surface area of prepared composites improved considerably to 604.62 m 2 g −1 after KOH pretreatment.The Ni-N-K-C can reduce Cr and the removal capacity for Cr 6+ is 824.4 mg g −1 [88].

Cadmium (Cd)
Cr is ranked as the seventh most risky substance.The environment can be exposed to cadmium in several ways, including through runoff from metal refineries, erosion of natural deposits, and electronic waste.The lungs of people are affected by high exposure to cadmium, which can be lethal [113].Kidney failure and weak bones are made more likely over time by low levels of cadmium in the environment, such as through tobacco smoke.According to widespread opinion, cadmium causes cancer [114].Table 3 shows some modified CNTs used for the removal of cadmium from water.The carboxylate carbon nanotubes (CNTs-COOH) were added to aloe vera leaf powder (OAMP), and the resulting nanomaterial is considered a novel adsorbent for cadmium ion removal from industrial wastewater [115].Fard et al prepared SWCNTs with an ethylenediamine modification that is utilized to examine the Pd and Cd adsorption in wastewater (EDA-SWCNTs).The effects of lead and cadmium ion concentration, carbon nanotube dosage, pH, temperature, time, and interfering ions were also considered.For Pb 2+ and Cd 2+ ions, the optimal adsorption capacity was measured to be 96.41% and 93.47%, respectively [116].
Mahmoodi et al prepared CNTs-COOH and added it to Aloe Vera leaf powder (OAMP), and the resulting nanomaterial was utilized as a new, reasonably priced adsorbent for the Cd 2+ elimination from wastewater.The highest adsorption capacity, which was 46.95 mg g1, indicated that the prepared nanomaterial was highly effective in the adsorption of Cd 2+ .Elovich's kinetic model was used to track the Cd 2+ adsorption process' kinetics.According to the reported thermodynamic values, Cd 2+ was adsorbed spontaneously and endothermically on OAMP/CNTs-COOH.The findings showed that in contrast to Aloe Vera which has not been treated, OAMP/CNTs-COOH exhibits improved adsorption performance [115].Rakhtshah et al removed cadmium from water samples using a quick and easy method based on functionalized CNTs-DHSP.The SFM-SPE method was used in this research to extract Cd 2+ ions.About 152.6 mg g −1 of Cd 2+ could be adsorbed using CNTs-DHSP [117].

Arsenic (As)
Arsenic is a metalloid of group IV, but it is referred to most frequently as a metal.It has a variety of negative consequences for living things.There are several distinct types and toxicities of arsenic.Arsenic can enter water supplies naturally or because of human activity [95].The WHO states that the upper boundary for arsenic in drinking water is 10 mg l −1 .Many deadly illnesses, including bladder, skin, and urinary tract malignancies, have been related to arsenic exposure [39].Table 3 shows some modified CNTs used for the removal of arsenic from water.Luan et al prepared Cu/CNT NC membranes which demonstrated durability in eradicating As 3+ at pH ranges 5-9.According to a mechanical study, As 3+ is directly adsorbing onto Cu/CNT composites during the filtration process.Additionally, As 5+ species were created as a result of As 3+ oxidation by DO which was catalyzed by Cu and adsorbed by mixtures of Cu and CNT [52].Egbosiuba et al removed Pb 2+ , As 5+ , and Cd 2+ from industrial effluent by using NiNPs-MWCNTs as an adsorbent.The outcome showed that the addition of NiNPs enhanced the hydrophilic surface of MWCNTs-KOH, allowing greater surface area, functional groups, and pore distribution.The highest adsorption capacities for Pb 2+ , As 5+ , and Cd 2+ respectively, were found to be 481.0,440.9, and 415.8 mg g −1 .For 8 rounds of adsorption, the modified CNTs performed exceptionally well [95].Lee et al presented a highly effective technique for the adsorption of arsenic on N-doped CNTs (NCNTs) with agglomeration-free Fe 3 O 4 nanoparticles (NPs), where the size-controlled NPs were produced through the N-mediated nucleation of NCNTs.The size-controlled NP-NCNT hybrid, which was developed to explore the benefits of purification that is effective in constantly moving water, has a high aspect ratio of >3600 [97].

Copper (Cu)
At higher concentrations, copper is a dangerous heavy metal over the allowable limit, it is categorized as a chemical that causes cancer [101], causes learning impairments, and can build up in the kidneys.It must be taken out of industrial wastewater for these reasons [114].Table 3 shows some modified CNTs used for the removal of copper from water.Eldeeb et al used chitosan and carbon nanotubes to produce a magnetic adsorbent, which was then easily sonochemically cross-linked with citric acid.then checked for the elimination of Cu 2+ ions from the water solution.In almost 20 minutes of contact time, the findings demonstrated that it has excellent Cu 2+ ion adsorption efficiency [99].Ghanavati et al prepared a tetrahydrofuran-modified carbon nanotube.Discontinuous copper metal adsorption tests were run on the adsorbent after it had been prepared.The findings revealed that at PH around 5, the greatest removal effectiveness was attained.The maximum absorption number after these variables were optimized was 96.33%.Kinetic analyses of the elimination of copper using a synthetic adsorbent were carried out [101].Dou et al prepared a new one-pot method for making composites of carbon nanotubes (CNTs) covered with chitosan.The samples were used to extract Cu 2+ from wastewater to assess the adsorption properties.The chitosan-modified CNT composites displayed high affinity and quick rates for the adsorption of Cu 2+ ions, and it was discovered that the composites' adsorption capacity was two times greater than that of unaltered CNTs.The maximum adsorption capacity of 115.84 mg g −1 , could best explain equilibrium results [102].

Nickel (Ni)
Among the highest-risk heavy metals, nickel is known as a poisonous heavy metal ion that is non-biodegradable and causes some problems for the human body [118].More than 40% of nickel is utilized in the manufacturing process of steel, certain alloys, and nickel batteries [114] which are both common uses for the metal in industry and sources of nickel effluent.Several separation techniques have been widely used to remove nickel.however, because it is very effective and inexpensive, the adsorption approach has proven to be more successful in removing nickel [114].Table 3 shows some modified CNTs used for the removal of nickel from water.Raygan et al prepared a new magnetic adsorbent, nickel zinc ferrite CNT using the inverse co-precipitation technique.The effectiveness of the adsorbents in removing arsenic(V)-contaminated anions from a model industrial effluent was then evaluated.CNZF were found to have maximum adsorption capacities of 56 mg g −1 and 66 mg g −1 , respectively [119].Lin et al prepared the magnetic MWCNTs which were covalently grafted with cyclodextrin to create a new composite material known as CD-Fe 3 O 4 /MWCNT.Nickel ions were adsorbed on CD-Fe 3 O 4 /MWCNT, and pH, temperature, and adsorption duration all had an impact.The adsorption mechanism was exothermic and spontaneous, according to thermodynamic measurements, with a maximal nickel ion (Ni 2+ ) adsorption amount of 103 mg g −1 at room temperature on the CD-Fe 3 O 4 /MWCNT.Additionally, regeneration tests revealed that even after 5 rounds, the adsorbent still had a high adsorption capacity [103].Rahmati et al removed Ni +2 ions from water using an adsorption method based on a novel adsorbent material.MWCNTs were functionalized to create the absorbent using a tetra-n-butyl-ammoniumbromide-based deep eutectic solvent (DES).Using the D-optimal design technique, the adsorbent efficiency was assessed, and the adsorption process conditions were optimized.An adsorbent dosage of 0.011 g with a contact duration of 68 min resulted in a maximum removal rate of 93% and a maximum adsorption capacity of 115.8 mg g −1 .Additionally, the adsorbent's reusability of roughly 7 times demonstrated its successful removal of nickel [104].

Lead (Pb)
Wastewater containing lead (Pb) can cause major health issues.Both humans and animals are poisoned by lead (Pb).Brain and nerve system diseases can result from lead exposure [42].Lead is present in the water supply from industrial waste as well as corrosion of plumbing materials.The primary cause of Pb absorption in the human body is drinking water [39].Table 3 shows some modified CNTs used for the removal of lead from water.Gugushe, Mpupa et al created MWCNT-Fe 3 O 4 /Zeolite using a solvothermal technique.For the preconcentration of residual Pb and Tl, MWCNT-Fe 3 O 4 /Zeolite was utilized as an adsorbent in UA-MSPE.For Pb and Tl, the highest adsorption capacities were 37.8 mg g −1 and 44.5 mg g −1 , respectively.Pb and Tl were simultaneously preconcentrated and determined in environmental samples using the developed UA-MSPE/ ICP-OES technique [120].Zondo et al investigated the adsorptive elimination of lead from wastewater using functionalized MWCNTs embellished with gold-iron oxide nanoparticles.Commercially available CNTs were produced and then functionalized using a solution of H 2 SO 4 and HNO 3 acids.To improve the binding of heavy metals, Au/Fe 3 O 4 was coated on the CNTs.At 298 K, the adsorption capacity of Pb 2+ ranges from 1.233 to 7.266 mg g −1 .When compared to MWCNT-COOH, MWCNT-Au/Fe 3 O 4 had a higher sorption capability.By increasing the dosage of MWCNT-Au/Fe 3 O 4 from 0.02 g to 0.1 g, the proportion of Pb 2+ removed rises from 50% to 78%.Heavy metal removal from wastewater has excellent potential for the synthesized MWCNT-Au/Fe 3 O 4 [106].Salem et al successfully prepared nanostructured CoBi-LDH/Cr-CNT composites by chemical synthesis method.Experiments were performed to test the adsorption of Pb +2 and two organic pigments, Congo red (CR) and Rose Bengal (RB), by prepared nanomaterial.Under optimum circumstances, the adsorption capacity of prepared nanomaterials for Pb 2+ pollutants was 503.2 mg g −1 and the percentage removal was 100% [105].

Mercury (Hg)
Mercury is the most hazardous heavy element found in nature.It can appear as a liquid or a vapor.The body systems primarily impacted by mercury exposure include the nervous and renal systems.There are three different types of mercury (Hg): metallic element, inorganic salt, and organic salt [107].These substances can be found in soil, freshwater, and ocean, and industrial waste processes and products all contain mercury [121].Due to the harmful consequences of mercury, even at very low concentrations, WHO has set a maximum acceptable mercury concentration in water at 1 mg l −1 [39].Table 3 shows some modified CNTs used for the removal of mercury from water.Fayazi et al created a mixture of S-M-MWCNT; a thin Sulphur layer was applied to M-MWCNTs using an easy heating procedure.Hg 2+ was drawn from wastewater using the produced superparamagnetic adsorbent.The maximum adsorption capacity of the prepared adsorbent was calculated as 62.11 mg g −1 , and the isotherm data complied with the Langmuir isotherm model [107].Ebrahimi et al used Ni-MWCNTs as a new sorbent to remove mercury vapor from the air.Mercury was combined with Ni-MWCNTs using the solid-gas phase removal (SGPR) technique.25 mg of the adsorbent ranging in size from 30-100 nm were used under optimal circumstances.The sorbents' respective absorption capacities were 194 mg g −1 and 64 mg g −1 .The ideal circumstances led to an efficient recovery.Ni-MWCNTs can be utilized as a cheap, effective adsorbent and have excellent potential for removing mercury vapor from the air [108].
4.8.Thallium (Tl) Thallium (Tl) is a highly toxic heavy metal that poses significant risks when present in wastewater.It is a byproduct of certain industrial processes, such as smelting, electronics manufacturing, and coal burning, and can also enter water bodies from natural sources like volcanic activity [122].The fatal dose of Tl for adults is about 8-10 mg.kg −1 but the uptake of even lower doses can cause poisoning with various symptoms [123].Thallium's harmful effects on human health are well-documented.Even at low concentrations, thallium exposure can lead to severe health issues, including neurological, gastrointestinal, and cardiovascular problems [124].Efficient wastewater treatment is crucial to mitigate thallium's harmful effects.Precipitation, adsorption, and ion exchange are common treatment methods used to remove thallium from wastewater.Extensive research has been conducted on the adsorption processes involving diverse pollutants, such as heavy metals, and CNTs, but the exploration of interaction mechanisms between Tl (I) and CNTs remains limited.Table 3 shows some modified CNTs used for the removal of thallium from water.It is crucial to recognize that altering the surface properties of CNTs through modification can profoundly influence their colloidal behavior, potentially leading to consequential impacts on their adsorption capabilities.Consequently, the behavior of Tl (I) ions, specifically with the modified surfaces of CNTs, might exhibit noteworthy changes within the environmental context.Saeed et al, subjected MWCNTs to distinct oxidative treatments using H 2 SO 4 , KMnO 4 , and HNO 3 , to harness as effective sorbents for assessing their capacity to adsorb Tl (III) ions.The acidic oxidation procedures significantly augmented the abundance of functional groups on MWCNTs, inducing a marked escalation in Tl (III) adsorption efficiency.Optimal Tl (III) adsorption was attained at a pH of 7. Noteworthy disparities in maximum Tl(III) sorption capacities were unveiled: unmodified MWCNTs exhibited 3.0 mg g −1 , H 2 SO 4 -oxidized counterparts reached 11.7 mg g −1 , KMnO 4 -oxidized variants achieved 21.6 mg g −1 , and pretreated with HNO 3 , MWCNTs demonstrated the highest proficiency at 31.5 mg g −1 [111].

Mechanism of heavy metal removal by adsorption
The processes of heavy metal ion adsorption on CNTs are extremely complex, and they seem to involve the following processes [112].
1. Physical adsorption: During this process, the heavy metals accumulate on the adsorbent's surface and within its holes.The shape, pore size, and surface area of the adsorbent substantially impact the adsorption capacity because of the surface adhesion of the metal ions [125].For instance, more micropores and mesopores promote and facilitate contaminant diffusion, which speeds up the adsorption rates [28,126].Figure 4 describes the key variables that influence the adsorption method.
2. Electrostatic interaction: Electrostatic interaction occurs when cations and anions build up on the adsorbent surface because most carbon-based compounds have charged surfaces.Previous studies using activated carbon, carbon nanotubes, and graphene have verified the role of electrostatic forces in the adsorption of heavy metals [127].
3. π-π Stacking and Van der Waals interactions: CNTs possess π-electron-rich surfaces, and some metal ions can undergo π-π stacking interactions with the graphene walls of the nanotubes.Van der Waals interactions between metal ions and the CNT surface also contribute to adsorption.
4. Ion exchange: Cation exchange capacity, which measures the effectiveness of ion exchange, typically happens between the H + of -COOH or -OH functional groups and M 2+ ions.Numerous studies have discovered that this process significantly influences adsorption capacity [128].
5. Surface complexes: Multi-atom compounds are formed on the surface of CNTs.These complexes typically develop when M 2+ ions react with functional groups (such as -COOH, and -OH).
6. Precipitation: Sometimes, the ions or groups on the adsorbent surface react with the heavy metals to cause them to separate from water and form solid precipitates [129].
Heavy metals can occasionally be eliminated through reduction.Before being eliminated by complexation and ion exchange, the higher oxidation state of the heavy metals is converted into a lower oxidation state.For instance, before removal, the decrease of Cr 6+ to Cr 3+ .Except for precipitation, which may or may not be involved in the adsorption process and is thus not regarded as the primary mechanism for heavy metal adsorption, the other processes can all be referred to as universal figure 5 shows the mechanism of removal of metals from wastewater [130].
Fayazi et al prepared sulfur-coated magnetic MWCNT for mercury removal from wastewater.To create a mixture of S-M-MWCNT, a thin S layer was applied to M-MWCNTs using an easy heating procedure.Mercury (II) (Hg (II)) was drawn from a water solution using the superparamagnetic adsorbent, which was then magnetically isolated without the assistance of centrifugation or filtration.With increasing pH, Hg (II) adsorption increased until it hit a peak in the pH range of 4.5 to 8.0.The S-M-MWCNT adsorbent's maximal adsorption capacity was measured as 62.11 mg g −1 .Studies showed that the S-M-MWCNT composite can be used again without suffering any loss of adsorption capability after multiple removals of Hg(II) [107].Eldeeb et al prepared multi-walled carbon nanotubes encapsulated in polyvinyl alcohol/chitosan hydrogel (Cs/MWCNT/ PVA) and then cross-linked with glutaraldehyde to create a unique hybrid adsorbent.The positions and  intensities of the infrared bands were impacted by the chemical interactions between the constituents.This polymer is very effective at adsorbing Cr 6+ ions.The group equilibrium tests showed that 1.5 pH was the best value for chromium adsorption.The Langmuir model calculated that the hydrogel had a maximal adsorption capacity of 217.4 mg g −1 [45].Di-t-butyl selenophosphoryl groups functionalized multiwalled carbon nanotubes were employed to extract Pb 2+ ions from both artificial and natural water systems.At pH 5.0 and 313 K, the best Pb 2+ removal rate was found.

Conclusion
This review explains the role of different modified CNTs and how they can remove different heavy metals from wastewater.The adsorption method of removing the toxic metals is discussed in detail and provides a mechanism for heavy metal adsorption on the CNT surface.The adsorption capacity of these functionalized CNTs is in the range of 130-180 mg g −1 .The forces involved in the adsorption of metals from these modified CNTs are electrostatic interaction, Vander Waal's forces, π-π interactions, and surface complexation.Thus, modified CNTs possibly be used as novel adsorbents for the remediation of heavy metals like lead, cadmium, chromium, arsenic, and copper from wastewater.

Future perspectives
Because of their distinct physical, chemical, and electrical characteristics, CNTs can be used for pollution prevention technology.Modified CNTs have the potential to be very efficient filters for turning wastewater into pure, recyclable water when used with the right functionalization agent.Overall, from the standpoints of both natural and human health, the remediation of heavy metals in wastewater is significant.We now have a potentially effective replacement for the conventional adsorbents utilized to remove metals through the growth of modified CNTs.However, most studies on these nanomaterials' removal abilities take place in stimulated water using comparatively basic components.Information on their use in real-world effluent is rare and urgently needed.To effectively use modified CNTs for metal water treatment, much work remains to be done, especially when taking into account all of their characteristics, including their capacity for removal, renewability, isolation, production, and expense.

Figure 1 .
Figure 1.Several fields where nanotechnology is being used.

Figure 5 .
Figure 5. Mechanism of heavy metal removal by modified CNTs.Reproduced from [131], with permission from Springer Nature.

Table 1 .
Some types of nanomaterials.

Table 2 .
Functionalized carbon nanotubes and metal removed.

Table 3 .
Removal efficiency and adsorption capacity of functionalized CNTs for metal removal.