A comprehensive review on the removal of antibiotics from water and wastewater using carbon nanotubes: synthesis, performance, and future challenges

Abstract

Recently, various emerging pollutants have been detected in water and wastewater, among which antibiotics can be considered a real threat to human life and environment because of their unique properties (bio-accumulation, bio-magnification, resistance, and bio-transformation). Therefore, it is urgent to apply methods for the detection and removal of these pollutants. Given that antibiotics cannot be completely removed by conventional methods, developing alternative methods to enhance their removal efficiency should be the focus of researchers. Over the past decades, carbon nanotubes (CNTs) have gained considerable attention owing to their remarkable merits (high surface area, low density and high thermal conductivity). However, despite the numerous advantages of CNTs, they suffer from some limitations, which can be solved by surface modification. Hence, this comprehensive review aims to help readers to improve their fundamental knowledge regarding the main characteristics of common antibiotics and their environmental and health effects, their fate in the environment, common methods for their removal, physicochemical methods for the modification of CNTs, identifying the effects of various parameters (pH, dosage, initial concentration, and time) on the removal process, and the reusability and major mechanism for the adsorption of antibiotics by CNTs. Finally, we summarize the isotherm and kinetic models for the removal of antibiotics. Our results exhibit that all the data fitted the Langmuir isotherm model, which reveals that the adsorption process on modified-CNTs is homogenous and monolayer. Furthermore, herein, the adsorption kinetics is well described by the pseudo-second-order model. Overall, this review provides useful insight for young researchers to understand the adsorption of antibiotics and achieve the future goal of using CNTs as promising adsorbents.

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Water impact

This review aimed for the first time to fill the knowledge gap on the efficiency of modified-CNTs for the removal of various common antibiotics as one of the major environmental pollutants. It is hoped that this review will provide useful approaches for the development and wide application of CNTs and their modification in the environmental engineering field in the future on a large scale. Herein, we focused on the structure of the common antibiotics, their health and environmental effects, the conventional methods (advantages and disadvantages) for their removal, the methods for the synthesis and preparation of pristine and modified CNTs, the effect of various experimental conditions on the system, the adsorption mechanism and the isotherm-kinetic models.

1 Introduction

Antibiotics are one of the most important drugs used for humans and veterinary medicine.¹ It has been reported that the consumption of antibiotics has increased significantly from 100 000 to 200 000 tons per year due to the global economic and population growth.^2,3 More than 50 million pounds of antibiotics (40% for animal farming and 60% for humans) are produced yearly in the United States.⁴ Moreover, based on statistical data, China is considered one the countries with the largest antibiotic consumption rate from 2009 to 2013 (248 000 tons).⁵

Antibiotics are classified based on their chemical structure and mechanism of action, and therefore, they can be subdivided into different subgroups with complex molecules such as β-lactams, tetracyclines (TC), quinolones, sulphonamides, glycopeptides, macrolides, and aminoglycosides, which may have different properties.^6–8

Medicinal chemicals enter the water through two main routes, i.e., industrial and domestic activities. The most medicinal chemicals come from the pharmaceutical industry through fermentation, chemical synthesis, extraction and formulation, which contain a high concentration of antibiotics, while domestic route includes the metabolised and non-metabolised forms of antibiotics, which are excreted by patients into wastewater.⁹ Studies have shown that conventional wastewater treatment plants (WWTPs) only remove certain drugs by about 20–90%, indicating that antibiotics are not completely removed during biological treatment, and consequently enter the receiving waters. Moreover, because of their low biodegradability, it has reported that the concentration of antibiotics in raw domestic wastewater varies in the range from 100 ng L⁻¹ to 6 mg L⁻¹.¹⁰

Thus, the removal of antibiotics from wastewaters before their discharge into receiving water systems is important.^11,12 It has been confirmed that the presence of antibiotics in waters is a global concern for suppling water to industries (focusing on water reuse) because of their severe ability to disrupt the microbial ecology, increase the proliferation of antibiotic-resistant pathogens, and endanger human health.¹³

Nowadays, researchers have used a wide variety of technologies (physical, chemical, and biological) such as ozone and UV,¹⁴ Fenton and photo-Fenton,¹⁵ coagulation and flocculation,¹⁶ nanofiltration,¹⁷ and adsorption¹⁸ for the removal of antibiotics and pharmaceutical residues from water and wastewater. Based on the results of previous research, conventional water and wastewater treatment methods cannot remove antibiotics according to the established standards.¹⁹ Therefore, researchers in the field of environmental chemistry need to develop alternative methods to overcome the shortcomings of the traditional methods.

Among the aforementioned methods, the adsorption process can be widely applied in the elimination of pollutants because of its merits such as high selectivity, low energy consumption, high reliability, low cost, simple design, design flexibility, non-toxic nature, and simple operation.^20,21 However, it also has some drawbacks such as the adsorption efficiency can be restricted by various factors such as mass transport resistance, it is difficult to separate the adsorbent, the production of adsorbents is expensive and the spent adsorbents need to be disposed.^22–24 Hence, researchers have attempted to develop new adsorbents, namely, nano adsorbents, to improve the efficiency of conventional adsorbents and overcome their shortcomings.²⁵ In recent decades, nanostructured materials (dimensions of less than 100 nm) have gradually become a hot research topic. Among the new and novel materials, carbonaceous and porous materials especially carbon nanotubes (CNTs) have attracted attention from environmental scientists.²⁶ CNTs as emerging carbonaceous materials are one of the most promising carbon adsorbents used in water and wastewater treatment for the removal of antibiotics.²⁷ Generally, carbon nanotubes are a one-dimensional structures with a micron axial length and nanometer diameter, which includes one or more concentric graphite-like layers (0.4–10 nm in terms of diameter).^28,29 Therefore, based on the number of graphite layers, CNTs can be mainly categorized into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).³⁰ Basically, only a single hexagonal sheet of carbon atoms is rolled into a tube (with a diameter of 0.4–3 nm) in SWCNTs, whereas several hexagonal sheets of carbon atoms are rolled up and each hexagonal sheet of carbon atoms is separated by ∼0.34 nm in MWCNTs.³¹ Due to the outstanding properties of these promising materials such as good electrical and thermal conductivity, high specific surface area, high strength, large pore capacity,³² specific structural and functional properties (e.g., high aspect ratio, high electron emission, high optical absorption, and low energy loss),³³ high adsorption performance because of the presence of oxygen-containing functional groups on their surface,³⁴ and strong interaction with organic molecules via non-covalent forces³⁵ play a very useful role in the field of environmental engineering. For example, CNT-activated carbon (as a high-performance suspension electrode for the electrochemical desalination of wastewater),³⁶ ZnO/CNT nanocomposite as an electrochemical sensor,³⁷ ruthenium/MWCNT as a catalyst,³⁸ heavy metal uptake by MWCNTs,³⁹ and hydrogen storage using SWCNTs.⁴⁰ The results of previous studies confirmed that CNTs exhibit certain limitations such as weak dispersion in aqueous solutions,^41,42 few active sites, difficult recycling,⁴³ hydrophobicity and high production cost for their total synthesizes,²⁷ prompting extensive research on the modification of the structure of CNTs. For instance, the poor dispersion of pristine CNTs can be solved by surface modification with the different functional groups such as hydroxide (OH), amine (NH₂), and thiol (SH),^41,42 and therefore functionalized or modified CNTs can lead to increase adsorption efficiency.⁴⁴

Besides, it is interesting to mention that various CNTs integrated with the other types of treatment systems have been investigated recently. Briefly, some of these integration processes include pre-coagulation by adsorptive filtration of pharmaceutical and personal care products using carbon nanotube membranes,⁴⁵ degradation of the common aqueous antibiotic tetracycline using a CNT electrochemical filter,⁴⁶ bacterial filtration using CNT/antibiotic buckypaper membranes,⁴⁷ covalent binding of SWCNTs to polyamide membranes for antimicrobial surface properties,⁴⁸ improvement of antifouling and antimicrobial abilities on silver-CNT-based membranes under electrochemical assistance,⁴⁹ study on the mechanism of tetracycline removal in electrocoagulation coupled with electro-Fenton reaction system with Fe anode and carbon nanotube cathode,⁵⁰ Al-doped carbon nanotubes with adsorption and coagulation,⁵¹ and integration of coagulation/flocculation and sedimentation with carbon nanotubes ‘sandwiched’ in a granular filter bed.⁵² To date, several studies have been conducted on the modification of CNTs in various ways for the removal of antibiotics from aqueous solution. Balarak et al. (2016)¹⁶³ applied MWCNTs modified with alumina (Al₂O₃) for the removal of tetracycline from aqueous solution in batch systems. Their results showed that the modified CNTs have potential to remove the tetracycline antibiotic.⁵³ Shao et al. (2010)⁷³ showed that the adsorption capacity of CNTs could be increased by modifying their surface with KOH, which improved the removal of sulfamethoxazole, tetracycline, and aromatic compounds compared to graphene sheets.⁴⁴ Modification of the surface of CNTs by physical adsorption or chemical bonding, in addition to increasing the removal efficiency, can enable their use as a suitable adsorbent for water treatment.⁵⁴

To the best of our knowledge, there is no comprehensive review on this hot topic; hence, this review aims to fill the knowledge gap for the first time on the efficiency of modified CNTs for the removal of various common antibiotics as major environmental pollutants. It is hoped that this review will provide a useful approach for the development and wide application of CNTs and their modifications for environmental engineering in the future on a large scale. Herein, the different sections are organized as follows:

1) Reviewing the structure of the common antibiotics.

2) Describing the health and environmental effects of antibiotics.

3) Summarizing the conventional methods (advantages and disadvantages) for the removal of antibiotics.

4) Investigating the methods for the synthesis and preparation of pristine CNTs and their modification process.

5) Focusing on the removal of various antibiotics using CNTs.

6) Identifying the effect of experimental conditions (dosage, initial concentration, contact time, and pH) on the system.

7) Describing the adsorption isotherm and kinetic models.

8) Characterization of the physical and chemical properties of CNTs.

9) Finally, suggesting the major points for the future use of CNTs and their derivatives for large-scale application in water and wastewater treatment.

2 Structure and classification of antibiotics

Antibiotics are chemical compounds that are completely or partially synthesized by living microorganisms, which inhibit or kill the growth of other microorganisms.¹⁵ In particular, they prevent or treat infections caused by bacteria or fungi, which lead to diseases in humans and animals, with different action mechanisms such as inhibition of nucleic acid and protein synthesis, cell membrane replacement, and suppression of the cell wall.⁵⁵ They are divided into various subgroups including β-lactams, quinolones, tetracyclines (TCs), macrolides, sulfonamides (SAs),³ aminoglycosides⁷ and glycopeptides⁸ (Table 1). Among the aforementioned classes, the β-lactam group (specifically amoxicillin (AMX)) is the most widely used (more than 65%) antibiotics in the world,⁵⁶ where β-lactam antibiotics, including the sub-groups of penicillins, cephalosporins, and carbapenems, are the most widely used antibiotics by humans. They are a group of antibiotics that have a typical four-membered β-lactam ring. However, the presence of this ring in their structure causes unstable thermal property. Also, these antibiotics are easily affected by light, extreme pH, metal ions, reducing and oxidizing agents, and solvents such as water and methanol.^3,55 Besides, the presence of β-lactam residues in aqueous solution leads to various issues such as antibiotic resistance and public health and environmental problems even at very low concentrations.⁵⁷

Table 1 The main classes and properties of common antibiotics with their applications

Class	Applications	Compound	Molecular formula	Chemical structure	Molecular weight (g mol^−1)	Ref.
Tetracycline	Acne and rosacea, infections of respiratory tract, urinary tract and intestinal tract, and treatment of chlamydia	Chlortetracycline	C22H24Cl2N2O8		515.3	59 and 62
		Oxytetracycline	C22H25Cl N2O9		496.9
β-Lactams	Severe sepsis and septic shock, genitourinary tract infections, infections of ear, nose, and throat, lower respiratory tract infections, Helicobacter pylori infections, pharyngitis, and tonsillitis	Amoxicillin	C16H19N3O5S		365.4	63–66
		Piperacillin	C23H27N5O7S		517.6
Aminoglycoside	Treatment and prevention of severe enterococcal Gram-negative bacterial infections, and mycobacterial infections, which is the principal drug in the treatment of tuberculosis	Gentamicin	C21H43N5O7		477.596	65 and 67
		Rifamycin	C43H58N4O12		822.94
Macrolides	To treat upper and lower respiratory infection caused by Gram-positive bacteria, feed additives in livestock and poultry farming to promote animals' growth	Erythromycin	C37H67O13N		733.9	68–70
		Pikromycin	C28H47O8N		525.32
Glycopeptide	The treatment of aerobic Gram-positive infections, the treatment of methicillin-resistant Staphylococcus aureus, and the treatment of infections in patients intolerant of β-lactam antibiotics	Vancomycin	C66H75Cl2N9O24		1449.25	71 and 72
Sulfonamides	The treatment of urinary tract infections, sinusitis, and toxoplasmosis, and to promote livestock growth	Sulfamethoxazole	C10H11N3O3S		253.279	73
Quinolones	The treatment of pneumonia, urinary tract infections, sexually transmitted diseases (gonorrhea and chancroid), skin, bone, and joint infections, gastrointestinal infections, prostatitis, anthrax, typhoid fever, plague, and salmonellosis	Ciprofloxacin	C17H18FN3O3		331.346	74 and 75

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The tetracycline (TC) group is another commonly used antibiotic, which has been widely applied in the livestock industry and treatment of humans, but its metabolism is difficult and approximately 72% of TC can be excreted by humans and animals in its non-metabolized form.⁵³ The TC group includes oxytetracycline hydrochloride (OTC), tetracycline hydrochloride (TCN) and chlortetracycline hydrochloride (CTC), which are obtained from a common hydro-naphthalene core with four fused rings to prevent infection.^55,58 This type of antibiotics has unique properties such as high polarity and solubility in water, insoluble nature in saturated hydrocarbons, and degradation in bases, acids, and polar organic solvents especially alcohols.⁵⁵

Quinolones, especially fluoroquinolones (FQs) are commonly used in human and veterinary medicine with high antimicrobial activity because of their two-ring nucleus structure, which is related to 4-quinolone.^55,59 FQs as one of the largest groups of antibiotics, have strong chemical stability, and thus they are not fully metabolized in the body.⁵⁸ It is estimated that due to this, more than 80% of FQs in their original form or as metabolites is eventually released into the soil and water, and consequently may cause resistance in pathogens or toxicity.⁶⁰ FQs are soluble in lipids, with high polarity and poor water solubility at pH 6–8 and can be destroyed by UV light but are resistant to acid and high temperature.⁵⁵ Another type of antibiotics is sulfonamides, which consist of a p-amino-benzene sulfonamide functional group in which the acidic sulfonamide group (–SO₂NH–) is attached to a basic amine group (–NH₂) and can be classified as polar molecules with high water solubility, low volatility and mobility properties, making them easily transported and dispersed in water.^55,61

3 The fate of antibiotics in the environment

In recent decades, the rapid development of the pharmaceutical industry has led to the production of a high level of contamination especially antibiotics, causing severe environmental concerns. Because of their persistence and off-site migration,²⁶ antibiotics have become severe pollutants of various environmental compartments such as surface and groundwater, municipal sewage, soil, vegetables, sludge sewage effluent, ground water, and even drinking water (to the level of g L⁻¹) even at low concentrations,^12,53 resulting in negative ecological effects and antibiotic resistance.⁷⁶ Antibiotics enter the environment in two main ways (Fig. 1), as follows: 1) they are discharged into the sewage after human use (studies in Germany indicated that three-quarters of the volume of used drugs are disposed through sewage) and 2) they are used as growth stimulants or for veterinary purposes, which are excreted by animals, and eventually converted into fertilizer.³

Fig. 1 Main sources of antibiotics contamination in the environment.⁸³

Based on the structure of different antibiotics, their extend in animal farming causes them to be partially absorbed in the animal gut and poorly metabolized, and consequently antibiotic compounds are transferred to liquid manure (as soil fertilization) and feces, eventually leading to their presence in soil samples.⁷⁷ Consequently, the presence of residual antibiotics in agricultural soils has significant negative effects on the farming process such as delaying germination or reducing the content of biomass, eventually having a negative effect on the yield in fertilized farmland.⁷⁸ Therefore, the residual antibiotics in environmental compartments have different transportation pathways, for instance, they can be present in different concentrations in agricultural soil due to their leaching, sorption, and degradation.⁷⁹ Tetracyclines have the soil adsorption coefficient (K_d) of 400 to 1147 (1 kg⁻¹) in clay loam, where the highest adsorption ability in clay loam is related to chlortetracycline with a K_d in the range of 1280 to 2386 (1 kg⁻¹), leading to its longer degradation time in manure or soil (9 to 180 days).⁷⁹ As stated earlier, antibiotics react with soil minerals to various degrees, where it has been proven that OTC is stable in alkaline clays, whereas chlortetracycline is unstable.⁸⁰ Moreover, several parameters affect the adsorption of antibiotics including cation exchange capacity (CEC), clay content, organic carbon (OC), and soil pH.⁸¹

Although the treatment of wastewaters reduces the concentration of antibiotics, it does not completely eliminate them, and consequently the residual concentration of antibiotics in the discharged effluent of WWTPs enters the receiving surface water.⁸² Generally, each type of antibiotics has a different fate in the environment. For example, among the groups of antibiotics, sulfonamide, fluoroquinolone, and macrolide antibiotics exhibit the highest half-life, while aminoglycoside antibiotics and β-lactam have the lowest.¹³ Furthermore, the rate of adsorption of antibiotics in soil, sediment, and/or sewage sludge varies, which can be classified into different groups. For instance, TCs and FQs are easily adsorbed, macrolides are moderately adsorbed, sulfonamides are moderately to poorly adsorbed, and aminoglycosides and β-lactams are poorly adsorbed in the aforementioned environments.⁴

4 The effects of antibiotics on human health and the environment

Recently, the environmental pollution by antibiotics compounds due to their overuse has become a significant concern because of their properties including low absorption after consumption and high solubility in water.⁸⁴ The presence of antibiotics and their byproducts in natural ecosystems poses a serious risk to human health and the environment even at low concentrations.^6,85 As stated previously, high exposure to antibiotics can cause allergic reactions in people with hypersensitivity, which is one of the adverse drug reactions. The main symptoms of allergies include angioedema, urticaria, aplastic anemia, gastrointestinal reactions, severe shock, and even death if serious.^86,87 However, assessing the behavior of antibiotics in the environment is usually difficult due to their complex chemical structures. The release of antibiotics in the aquatic environment can disrupt the aquatic ecosystem, create antibiotic resistance, and make water reuse impossible. In fact, they may reach drinking water and even affect non-target and target organisms in the environment.³ For example, high concentrations of antibiotics in the aquatic environment can cause disturbances in the fish population, body deformities, DNA damage, microbiota dysfunction, suppressed immunity, and the extinction of endangered species.⁸⁴ Also, the excretion of some antibiotics through urine and animal feces, such as fluoroquinolones, has toxic effects on microbial activity, and thus often exhibit poor biodegradability in biological treatment systems.²⁶ Some antibiotics such as TCs can cause liver damage, gastrointestinal disorders, and yellowing of the teeth, even at low concentrations.¹

Besides, antibiotic resistance is becoming another significant threat to human health (e.g., can cause prolonged morbidity and mortality) and the environment.^88,89 Based on statistical data, it has been reported that antibiotic resistance will result in 10 million deaths by 2050, with a financial burden of approximately $100 trillion.⁸⁹ As an example, a 50% detection frequency has been observed for Klebsiella pneumoniae, indicating its resistance to third-generation carbapenems and cephalosporins, in Southeast Asia.⁸⁸

5 Removal of antibiotics from environmental waters

Conventional wastewater treatment systems cannot properly treat wastewater containing antibiotics, and thus antibiotics eventually leach into receiving water such as surface water resources.⁹⁰ Therefore, it is essential to apply new, effective, and environmentally friendly approaches to remove antibiotics. To date, several water and wastewater treatment technologies including reverse osmosis (RO),⁹¹ coagulation–flotation,⁹² advanced oxidation processes (AOPs),^90,93 ozonation (O₃),^94,95 coagulation–flocculation,⁹⁶ and adsorption⁹⁷ as chemical processes and moving bed biofilm reactor (MBBR),⁹⁸ microbial fuel cell (MFC),⁹⁹ constructed wetlands (CW),¹⁰⁰ and conventional activated sludge (CAS)¹⁰¹ as the most prevalent biological treatment processes have been developed. Besides, physical processes such as membrane filtration have also been widely used for the reduction or purification of antibiotic compounds.^61,102 However, although some of them have attracted attention because of their effectiveness, their applications are restricted due to some issues, as will be discussed below.

Advanced oxidation processes (AOPs) as a one of the chemical and emerging technologies for the removal of antibiotics can be broadly divided into various groups such as photochemical (photo-Fenton, sonochemical, UV/H₂O₂, UV/O₃, photocatalysis, sono-photocatalysis, and sonocatalysis) and non-photochemical (chlorination, ozonation, Fenton, and electrochemical), which have attracted interest as promising and reliable treatment methods for the removal of various antibiotics.^24,103 Specifically, these processes are highly effective for the mineralization and degradation of organic pollutants and their conversion into harmless compounds,^90,93 and some of them are summarized as follows. Fenton reactions as a type of AOP process is based on hydroxyl radicals (˙OH), which react with organics and destroy them. In this process, hydroxyl radicals are formed by the catalytic decomposition of hydrogen peroxide (H₂O₂) upon reaction with iron ions (Fe²⁺).¹⁰⁴ The Fenton reaction is an economical and environmentally friendly method for the treatment of water and wastewater with remarkable advantages such as highly oxidative and non-selective nature,⁷² but this process requires acidic conditions (pH range between 2–4) and produces a large amount of iron sludge.¹⁰⁵ Ozone oxidation is a complex process, which takes place through direct reaction with soluble ozone (O₃) or indirect oxidation through the formation of radicals. Both methods depend on factors such as the nature of the pollutant, the pH of the medium and the dose of ozone. For example, the production of hydroxyl radicals can be increased by increasing the pH or by combining ozone with hydrogen peroxide or UV-irradiation.^94,95 O₃ can be produced in situ, requiring the storage of H₂O₂ solution, iron salt or other chemicals. However, the main problem associated with this method is its high energy consumption and the relatively high cost of equipment and maintenance.⁹⁴ Furthermore, during AOP processes, some harmful metabolites may be generated, which must be removed. Therefore, the main goal of treatment processes should be the production of non-harmful products, not just the removal of the main compounds.^53,106

Another promising method for removing antibiotics is membrane filtration via reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). However, little information is available on the rejection of antibiotics by RO and NF membranes, especially in wastewater treatment systems, where different species may interact on the membrane surface and in solution.⁶¹ Also, membranes require expensive equipment and significant energy.⁶⁸

Among these processes, the adsorption process is the most recommended process for water and wastewater treatment, which is very effective and usually less expensive than other removal methods.^71,107 Among the known adsorbents, activated carbon (AC) has been widely used in various fields due to its excellent properties such as high adsorption capacity.¹⁰⁸ However, despite the widespread use of AC in the adsorption process, it has some limitation such as difficult production and regeneration, and hence researchers need to find new adsorbents with high adsorption capacity.¹⁰⁹ Among the different types of carbon materials, CNTs (from the carbon family) have been widely used in water and wastewater treatment, nanotechnology, electronics, optics and other fields of materials sciences. Besides, the adsorption capacity of CNTs, as a special adsorbent for organic pollutants, can be enhanced by coating or modifying them. This modification can increase the reaction between organic pollutants and CNTs, thus increasing the adsorption rate on the surface of CNTs.^110,111 However, despite the remarkable merits of all the aforementioned methods, they suffer from some limitations. In Table 2, we summarize the advantages and disadvantages of the different methods.

Table 2 Advantages and disadvantages of common antibiotics removal technologies

Methods	Advantages	Disadvantages	Ref.
Fenton reaction	Wide application range, strong anti-interference ability, simple operation, rapid degradation and mineralization	Iron sludge production, and limited pH range	112 and 113
Heterogeneous Fenton reaction	Low iron ion leaching, efficient cycling of Fe^3+ and Fe^2+, low iron sludge production, wide working pH range, reusability and long-term stability of the catalysts	Relatively harsh synthetic conditions, complicated synthesis routes, high costs for the synthesis of heterogeneous Fenton catalysts and reactor design	114 and 115
Ozone	Elimination of many organic compounds, volume of effluent remains constant throughout the process, no sludge formation, simple installation, and usability in flow fluctuations or compounds	Low stability of ozone in the reaction medium, low solubility, low stability in water, and high costs	94, 95 and 116
Membrane filtration	The removal of a multitude of emerging contaminants, continuous operation, no formation of by-products or metabolites, the high adaptability to conventional water treatments	Reduction of removal efficiencies due to adsorption capacity saturation, the high cost, problem of water loss, fouling, and high energy consumption	56, 61 and 102
Adsorption	Low operational and maintenance costs, great efficiency, no production of relevant by-products	Difficult production, difficult reconstruction of some adsorbents	102 and 109
CF	Low energy consumption, relatively simple design and easy operation	Sludge production	117
RO	High removal rate, effective, easy to use, easy operation, high purity of water quality	High cost, high energy, relatively high pressure, electrical necessity, repeated erosion	118 and 119
Electrochemical	Effective, versatile, cost-effective, easy and clean technology, suitable for the treatment of toxic wastewater that contains high-concentration antibiotics	Low flow rates, and high operating costs	120
MFC	High antibiotic removal, it generates electricity, produce less sludge than conventional treatment process, can reduce the costs and challenges associated with sludge treatment and disposal, promising method for renewable energy production, simultaneous removal of nitrogen components and carbon	Operation problems such as short life span, high cost, low production rates, limited efficiencies, membrane fouling, instability, and inconvenience in maintaining microbe-based systems, many factors affect the performance of MFCs, including the chemical substrate, ionic concentration, proton exchange material, catalyst, internal resistance, electrode spacing, and electrode material	121–125
Constructed wetlands	Low cost and nature-based treatment technologies, can be used for the removal of organic matter and nutrients (nitrogen and phosphorus) as well as pharmaceuticals	Requirement of large land area, the unpleasant odor, requires a shallow basin for the collection of effluent, a water control structure to sustain a shallow water depth, and a water surface above the substrate	126

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6 Methods for the synthesis of carbon nanotubes

CNTs are carbon allotropes composed of sp²-hybridized carbons such as graphene, fullerene and graphite. Carbon nanotubes were discovered by Iijima (1991), which based on their applications can be classified as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT).¹²⁷ The typical diameter of SWCNT and MWCNT is 0.8–2 nm and 10–40 nm, respectively. These graphite structures are formed after carbon combustion and placement processes in the form of a tube.^66,67

CNTs as highly efficient carbonaceous materials can be produced through various routes including arc discharge (Fig. 2), laser ablation, chemical vapor deposition (CVD), and flame treatment for their industrial development.⁶⁵ Among them, the synthesis of CNTs via CVD (Fig. 3) of the preferred carbon precursor on an appropriate catalyst surface has been successfully examined.¹²⁸ It should be noted that one of the critical parameters that can impact the growth of CNTs is the selection an appropriate catalyst because the composition and morphology of the catalyst nanoparticles influence the formation of CNTs.¹²⁹ Iron (Fe), cobalt (Co), nickel (Ni), and yttrium (Y), either alone or incorporated on suitable supporting materials (alumina, zeolite, silica, and other metal oxides) have been shown to be suitable for the growth of CNTs.¹³⁰ It is interested to note that in the process for the synthesis of SWCNTs, various catalysts as precursors can be employed including Gd, Co, Ni, Fe, Ag, Pt, and Pd, or mixtures of the aforementioned precursors such as Co, Ni, and Fe with different components including Co–Pt, Co–Ru, Ni–Y, Fe–Ni, Co–Ni, Co–Cu, Ni–Cu, Fe–No, Ni–Ti, and Ni–Y, whereas for the synthesis of MWNTs, it is not necessary to apply catalyst precursors.¹³¹ Moreover, the use of other CVD techniques has been reported, such as radiofrequency CVD (RF-CVD), microwave plasma (MPECVD) water-assisted CVD, oxygen-assisted CVD, and hot-filament CVD (HFCVD).¹²⁹ The attractive use of the CVD method is due to its more controllable and cost-efficient process than other methods (arc-discharge and laser ablation), which allows the tailored control of the CNT architecture for application in highly advanced electronics and optoelectronics, while being easily scalable.^128,132 In the second type of CNT synthesis (laser ablation technique, Fig. 4), a laser is used to vaporize graphite inside a quartz reactor, which was used for the first time in 1996 by Thess et al.¹³³ Another method for the preparation of CNTs is using a flame to simultaneously provide chemical species and energy, where various fuels such as acetylene (C₂H₂), ethylene (C₂H₄), and methane (CH₄) can be used with an oxidizer to generate a mixture of gases such as carbon dioxide (CO₂), water vapor (H₂O), carbon monoxide (CO), and hydrogen (H₂) as well as saturated and unsaturated hydrocarbons (C₂H₂, C₂H₄, and C₂H₆).¹³⁴Table 3 lists the advantages, disadvantages, and operating conditions of each method.

Fig. 2 Schematic representation of the typical arc-discharge apparatus. Reproduced from¹³⁵ with permission from AIP Publishing.

Fig. 3 Schematic representation of CVD apparatus. Reproduced from¹³⁴ with permission from IntechOpen, the world's leading publisher of open access books built by scientists, for scientists.

Fig. 4 Laser ablation apparatus for the production of CNTs. Reproduced from¹³⁴ with permission from IntechOpen, the world's leading publisher of open access books built by scientists, for scientists.

Table 3 Comparison of the operational conditions, advantages, and disadvantages of the most common CNT synthesis methods

Method	Operational conditions	Advantages	Disadvantages	Ref.
CVD	Decomposition of hydrocarbon (on cobalt, methane and ethylene) with metal catalysts at 300 °C to 1200 °C	Large-scale usage, producing high-quality MWCNTs, relatively low cost, low power input, lower temperature range, control of CNT structural characteristics, and production of nanotubes with a controlled diameter, length and number of layers	High density of defects in CNT structure and generation of by-products, graphitic debris, catalyst particles and fullerenes	8, 59, 62, 65 and 136
Arc discharge	Direct current passing through a gaseous atmosphere (inert gas) between two electrodes (at least one electrode is made of graphite) and uses high temperature (>3000 °C) for evaporating carbon atoms into plasma	Production of nanotubes with low defects and high quality, with CNT yield exceeding 50%	Existence of by-products, graphitic debris, catalyst particles and fullerenes and the reaction product always contains a mixture of different types of CNTs	8, 59, 62 and 65
Laser ablation	Use of intense laser beams for the ablation/vaporization of a target consisting of a mixture of graphite and metal catalysts in an inert gas stream and involves the vaporization of graphite in an electrical furnace heated at 1200 °C	Production of nanotubes with low defects and high quality, environmentally benign method, producing large volumes of uniform, high-yield single-wall nanotubes SWNTs with minimal post processing, no catalyst needed, and fair control of size and morphology	Existence of by-products and graphitic debris, small yield of ablated nanoparticles, and large particle aggregations	8, 59, 62–64 and 137

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7 Modified CNTs

CNTs are known as superior adsorbents because they have a unique one-dimensional tubular structure, high surface area, and tendency to create strong π–π electrostatic interactions.⁷ They also have various advantages such as flexibility, high strength, light weight, and high conductivity due to the arrangement of the graphite sheets, including carbon atoms in a hexagon template, which are wound and overlapped.⁶⁶ However, the hydrophobicity and chemically neutral nature of the outer wall of the raw nanotubes are the main disadvantages of CNTs, which hinder their wide application.¹³⁸ Accordingly, on strategy to address this problem is to coat the carbon nanotubes. The coating of CNTs extends their application to various fields and improves their properties.¹³⁹ The modification of CNTs can be roughly classified into two categories including covalent and non-covalent methods.¹³⁸ Between them, the non-covalent modification of CNTs is highly recommended because the structures and properties of the nanotubes can be preserved after the modification reactions.¹⁴⁰ One of the sub-methods of non-covalent modification is the bonding of polymers, which occurs through the weak interactions between the polymer and the sp² hybrid network of CNTs. This method is also recommended by researchers because it can preserve the sp² hybridization of carbon atoms, preserving their mechanical and electronic properties, and consequently can has effect on their covalent performance.¹⁴¹ However, despite the significant merits of non-covalent modification, one of its major disadvantages is related to the weak forces between the nanotube and the wrapping molecule, resulting in a low charge transfer efficiency.¹⁴²

Besides, various functional groups can be incorporated on the surface of CNTs according to their application and the chemical nature of the target contaminants, where non-polar organic contaminants have high affinity to CNTs due to their interactions with the aromatic rings of the CNTs.¹⁴³

Covalent coating involves the use of a combination of modifying molecules and CNTs, or in other words, functional groups such as carboxyl attached by covalent bonds to the outer side walls and ends of CNTs.¹²⁷ In this method, by changing the carbon atoms from sp² to sp³, the translation symmetry of CNTs is disturbed, and consequently their properties such as electronics and transport properties are affected.¹⁴² The covalent coating methods include oxidation, cycloaddition, electrophilic addition, fluorination, nucleophilic addition, and radical addition.¹²⁷

Coating CNTs with polymer molecules (polymer bonding) is of particular importance because it causes CNTs to dissolve in a wide range of solvents, even at low levels of functionality, depending on the nature of the bonded polymer, and increases the affinity of CNT-g-polymer in polymer matrices.^141,144,145 There are two methods for the covalent bonding of polymers to CNTs, i.e., “bonding to” and “bonding from”. Polymer brushes can be made by “bonding to” via covalent tethering of preformed or end-coated polymer chains to a coated surface, and “bonding from” via polymerization on a substrate previously modified with originators to provide polymerizable sites.¹⁴⁶ However, a problem encountered with covalent coating is that during functionalization, many defects are created on the wall of the carbon nanotubes and the nanotubes split into smaller portions. These defects result in the degradation of the mechanical properties and disruption of the π electrons of carbon nanotubes.¹⁴⁷

7.1 Chemical coating

Chemical coating has attracted much attention as an important method for modifying the physical and chemical properties of CNTs.⁴⁴ This method is based on covalent bonding, which involves the combination of hydrophilic groups in the outer CNT side walls.¹⁴⁸ In fact, the goal of chemical methods is to create a surface coating on CNTs, which improves their chemical reaction with a solvent or polymer as well as further dispersion.¹⁴⁹

There are various methods for the chemical coating of CNTs, including sidewall fluorination, chemical oxidation, defect coating, cycloaddition, Diels–Alder reaction, chlorination, addition of carbene and nitrene, use of azomethine ylides, bromination and hydrogenation.^2,138 Fluorination is the starting point in many processes for the modification of SWCNTs, making fluorite tubes (F-SWCNTs) one of the basic precursors in the broad field of CNT chemistry.¹⁵⁰ Some properties of the F2 molecule, such as high reactivity, oxidation resistance, and maximum electronegativity, result in the formation of very strong bonds between fluorine and many elements in the periodic table. Therefore, fluorine, fluorinated gases and fluoridated plasma are very useful for modifying the surface properties of materials.¹⁵¹

Chemical oxidation is another method for the covalent coating of CNTs. In this method, the ends and walls of the nanotubes are covered with oxygen-containing groups such as ether groups and carboxyl groups.¹⁵² Wet chemical methods, photo-oxidation, oxygen plasma and gas phase treatment are some of the chemical oxidation methods that have received significant attention due to the increased chemical reaction of the graphic network.¹³⁸ Oxidation processes are divided into two stages of defect production and defect consumption. In the production of defects, oxidants attack the graphene structure with electrophilic reactions, producing active sites such as hydroxyl (–OH) and carbonyl (–C=O). In the defect consumption stage, the graphene structure of the tube is destroyed due to the oxidation of the active sites produced in the previous stage (defect production). This step depends on the ability of the oxidant to degrade the graphite structure around –C=O and the previously formed surrounding groups.¹⁵²

Defect coating is also one of the methods of chemical coating. The defects produced by oxidants in CNTs are stabilized by binding to –OH or carboxylic acid (–COOH) groups. Because these functional groups have rich chemistry, the CNTs coated with them can be used as precursors for subsequent chemical reactions such as esterification, silanization, polymer bonding, arylation, alkylation, and even with some biological molecules.² Defects can be heptagons in the hexagonal graphene frame, open ends or side wall holes, and oxygenated sites.¹⁴⁷

Chemical coatings are divided into two categories, as follows: (1) aggressive method, especially oxidation process, which causes structural defects due to the use of concentrated acids and disturbs the intrinsic properties of CNTs. (2) Although some milder coating processes, such as plasma and UV/ozone treatment, have been developed, followed by silane, amine or fluorine treatment, the CNT-surface active sites are limited (mostly at the end caps and defects), causing low efficiency, and thus altering the dispersability of the CNTs in the polymer to a certain extent.¹⁴⁹ This type of method causes the coated CNTs to dissolve in many organic solvents because the hydrophobic nature of the CNTs changes to hydrophilic (due to the bonding of polar groups). Furthermore, these methods are often time-consuming and involve several steps. Therefore, it is necessary to develop simple and cost-effective chemical methods in terms of basic and practical research.²

7.2 Physical coating

The non-covalent coating (physical) method is based on weak interactions such as π–π stacking interactions, van der Waals forces, and hydrophobic interactions between CNTs and modifying molecules (organic molecules).¹²⁷ Non-covalent treatments, such as the use of polymer coatings and surfactants, followed by surfactant treatment, rarely damage the structure of CNTs because they are based on physical adsorption, i.e., they do not disrupt the intrinsic π-bonds of CNTs and do not interfere with their electrical properties.¹⁴⁹ They change the properties of CNTs by adsorption on their surface.¹²⁷

The suspension of CNTs in the presence of polymers (such as polystyrene and polyphenylene vinylene) results in the wrapping of the polymer around the CNTs and the formation of supramolecular complexes.¹⁴⁷ The polymer wrapping method preserves the intrinsic sp² composition and electronic structure of CNTs.¹⁵³ Natural polymers, including DNA, proteins, and some polysaccharides, have been used for the modification of SWCNTs, but their use is limited due to their high cost and limited availability.¹⁵⁴ One of the important problems related to polymer/CNT systems is the relative incompatibility between CNTs and polymer matrices, which causes problems in the homogeneous dispersion, phase separation and accumulation of CNTs at high loadings.¹⁵⁵

In addition to polymers, surfactants have also been used for the coating of CNTs. Surfactants can be divided into three categories, i.e., non-ionic, anionic and cationic surfactants.¹⁵⁶ Surfactants can overcome the problem of CNT instability in polar solvents such as water by creating non-covalent bonds.¹⁵⁷ Also, the physical adsorption of surfactants on the surface of CNTs overcomes the van der Waals absorption (using electrostatic/steric repulsion forces), reducing the surface tension and preventing the formation of aggregates.¹⁴⁹ However, surfactants also exhibit disadvantages. For example, some surfactant bonds may break at high temperatures, reducing the efficiency of the suspension.¹⁵⁷

Another non-covalent method for coating CNTs is the endohedral method. In this method, the nanotubes are opened initially, and then filled with special materials. The simplest method is to use nitric acid, which is very selective in attacking the carbon pentagon at the end of the tube.¹⁵⁸ Metal nanoparticles, salts and organic compounds (dyes such as β-carotene and phthalocyanines, metallocenes, coronene and carbon nanostructures such as graphene and C₆₀ derivatives) are among the materials that have been successfully trapped in CNTs through various pathways.¹⁵⁹ Recently, successful methods have been discovered to fill open multi-walled CNTs with fullerenes and other organic molecules, enabling endohedral inclusion in a soft low-temperature manner.¹⁵⁸

8 Antibiotic removal using coated CNTs

Table 4 shows the coated CNTs in various forms of SWCNTs and MWCNTs used to remove antibiotics. Despite the unique properties of CNTs, the high removal efficiency of antibiotics by coated CNTs compared to other adsorbents has caused researchers to use CNTs in the coated form to achieve the optimal removal of contaminants. Although the coating process is complex, in some cases, the use of these complex processes to eliminate some antibiotics and reduce their risks may be necessary, and thus can be justified from an environmental and health perspective.¹⁶⁰

Table 4 Summary of the results of literature reviews for the removal of antibiotics using coated CNTs

Antibiotics	Coated-CNTs	Results	Ref.
SMZ	Fe3O4/MWCNTs	✓ Adsorption of organic pollutants and ozone in Fe3O4/MWCNTs is an effective step for the degradation of SMZ	75
		✓ Acidic conditions are more suitable for the degradation and mineralization of SMZ
		✓ Catalytic ozonation of SMZ by Fe3O4/MWCNTs is influenced by its adsorption, ozone decomposition rate, and Fe3O4 state
	Cu–TiO2@SWCNT	✓ SMZ can be completely destroyed with Cu content of 4%, SWCNT content of 10%, pH of 7, catalyst loading of 0.9 g L^−1, and initial sulfamethazine concentration of 30 mg L^−1 after 135 min solar irradiation	70
TC	MWCNT/TiO2	✓ Completely removal of TC was observed at a TC concentration of 10 mg L^−1, pH 5, photocatalyst dose of 0.2 g L^−1, and irradiation time of 100 minutes	161
		✓ Real drug wastewater was efficiently treated by UVC/MWCNT–TiO2 photocatalytic process
	Graphitized-MWCNTs, OH–MWCNTs, COOH–MWCNTs, and NH2–MWCNTs	✓ Adsorption dependence followed the order of G–MWCNTs > OH–MWCNTs > COOH–MWCNTs > NH2–MWCNTs, showing that the adsorbent interactions between MWCNT and TC could be influenced by the surface properties of MWCNTs	162
	Magnetic CNTs/C@Fe/chitosan	✓ CNTs/C@Fe/CS had good adsorption capacity for TC (104 mg g^−1)	111
		✓ The reusability of the adsorbent was excellent, where after 10 times use the sorption capacity was still 99.3%
	Al2O3–MWCNT	✓ Langmuir adsorption isotherms best matched the experimental data (maximum adsorption capacity: 855.6 mg g^−1)	163
		✓ The adsorption kinetics followed the pseudo-second-order kinetics
	MWCNT/MIL-53(Fe)	✓ Maximum adsorption capacity of TCN, CTC and OTC on MWCNT/MIL-53(Fe) was 364.37, 180.68, 325.59 mg g^−1 at 25 °C, which was 1.25, 3.34, and 8.28 times than that of a single MWCNT, respectively (pH = 7.0)	74
		✓ Excellent reusability and great water stability indicated the potential use of MWCNT/MIL-53 (Fe) for the removal of TC antibiotics from aqueous solutions
CIP	CNTs/FeS	✓ The Fenton-like catalyst CNTs/FeS has good catalytic activity to remove antibiotics (due to the synergistic effect of adsorption- advanced oxidation)	72
		✓ The CNTs/FeS catalyst was recoverable with a high catalytic capacity (>75%) even after four uses
AMX	8-Hydroxyquinoline–MWCNT and COOH–MWCNT	✓ Maximum AMX removal efficiencies were obtained at pH = 5 for virgin-MWCNT and 8-HQ–MWCNT as well as pH = 3 for COOH–MWCNT	164
		✓ The adsorption equilibrium time was 4 hours for virgin-MWCNT and 2 hours for coated CNTs
		✓ The maximum adsorption capacity of 8-HQ–MWCNT was nearly 50% higher than that of virgin-MWCNT

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According to the study by Xiong et al., a new adsorbent composite was synthesized by combining amino-functionalized MIL-53(Fe) with MWCNTs, which was used to adsorb chlortetracycline hydrochloride (CTC) and tetracycline hydrochloride (TCN). The maximum adsorption capacities of CTC and TCN over MWCNT/NH₂-MIL-53(Fe) were 254.04 and 368.49 mg g⁻¹, which are 8.37 and 1.79 times higher than that of chaff biochar, respectively. Upon the combination of MWCNT and NH₂-MIL-53 (Fe), the mesoporosity of MWCNT/NH₂-MIL-53 (Fe) increased significantly, which is suitable for the production of active adsorption sites. The hydrogen bonding between the hydroxyl functional groups in TCN or CTC and amino functional groups in MWCNT/NH₂-MIL-53 (Fe) caused an increase in adsorption capacity. Also, the π–π interaction between the adsorbate and adsorbent is the main reason for the adsorption of these two antibiotics.¹⁰ In another study, the catalytic ozonation of sulfamethazine (SMT) by Fe₃O₄/MWCNTs was investigated. The results showed that Fe₃O₄/MWCNT-catalyzed ozonation could increase the degradation and mineralization of SMT compared to ozonation alone.⁷⁵ Other results regarding the removal of antibiotics by coated CNTs are described in Table 4.

9 Characteristics of modified CNTs

Herein, we summarize the major characteristics of modified CNT adsorbents for the removal of common antibiotics based on various analyses such as BET, SEM or FESEM, FTIR, and XRD. Generally, XRD, as a crystallographic technique, can be used to identify the main details about the nanocrystal structure, size, and particle shape.¹⁶⁵ Another important analysis that can be applied for the characterization of CNTs is SEM, where using this technique, the surface morphological properties and cross-sections of nanocomposites containing CNTs can be determined by getting electron pictures of the sample at the nanoscale level.¹⁶⁶ Also, BET analysis is very useful for predicting the specific surface area of the synthesized CNTs.²⁰ Furthermore, the major functional groups of the adsorbents before and after the adsorption process can be determined by FTIR analysis.⁹⁷ The chemical and physical characteristics of different coated-CNTs are presented in Table 5. It can be observed that the surface area of the prepared adsorbents is in the range of 60 to 1228 m² g⁻¹,^74,167 where this high surface area of the synthesized CNTs demonstrate that these adsorbents are preferable and environmentally friendly for the removal of various common antibiotics. According to the XRD results, it can be concluded that structure of the prepared CNTs had crystalline phases, and also the coating agents successfully covered the CNTs.^43,167 The results of SEM analysis indicated that coated-CNTs had varying morphological properties depending on the modifying agents, as can be seen in Table 5. Furthermore, should be noted that different functional groups were observed on the surface of CNTs. In brief, the main functional groups can be summarized as follows: 1) different peaks at 3278–3444 cm⁻¹, 1549–1570 cm⁻¹, 1372–1376 cm⁻¹, 3465, 2350, and 1400 cm^{− 1} associated with the hydroxyl group (O–H) and 2) peaks located at 2856–2922 and 2800–3100 cm⁻¹ due to asymmetric C–H vibrations.^34,43,70,167

Table 5 The main characteristics of coated CNTs

Coated-CNTs	Antibiotics	BET (m^2 g^−1)	SEM	FTIR	XRD	Ref.
CNTs/β-cyclodextrin/MnFe2O4	TC	166.13	Particles with high size and density	3278–3444 cm^−1, 1549–1570 cm^−1, and 1372–1376 cm^−1: –OH vibrations and 2856–2922 cm^−1: asymmetric C–H vibrations.	Crystalline structure	43
MnFe2O4/MWCNT	TC	220.62	The surface of the composites was successfully covered with iron oxide nanoparticles, and the new MMWCNTs were about 10–50 nm in size	3465, 2350, and 1400 cm^−1: O–H, 2800–3100 cm^−1: C–H, and 1600–1650 cm^−1: C=C	Purity and crystallinity of modified CNT structure	34
MWCNT/MIL-53(Fe)	TC	60.17	Intimate contact between the original MOFs and MWCNT (attached on the surface and in the original MOF structures)	After incorporation of MWCNT, two peaks of O–H (2360 cm^−1) and O–H (stretch from H-bonded-COOH, 3450 cm^−1)	The characteristic diffraction peaks of both MWCNT and MIL-53(Fe), attesting the presence of MWCNT and MIL-53(Fe)	74
MIL-100(Fe)-CNTs	OTC	1228	Relatively smoother than MIL-100(Fe)	3460 cm^−1: stretching vibration of the O–H group, 1720 cm^−1 may be attributed to C=O stretching, 1618 cm^−1: frame stretching vibration of benzene ring, and vibration at 1378 cm^−1 assigned to the COO– group stretch of carbohydrates	The successful preparation of the material	167
MWCNTs-AC	CIP	708.16	MWCNTs are attached to the surface of AC sheets, and the agglomeration of MWCNTs is detectable on AC	2922.39 and 1720.53 cm^−1: C–H band and C=C stretching of AC, respectively, and 462.60 cm^−1: Mn–O band in MnO4	—	168
Magnetic-MWCNTs	CIP	—	Iron oxide nanoparticles were effectively coated on the surface of MWCNTs	580 cm^−1: the vibrational properties of Fe–O bond and 3300–3500 cm^−1: –OH group	Good crystallinity of Fe phases	169
Magnetic-graphene/CNT	OTC	394	Scrolled and hierarchical architecture of thin graphene layers	1500–1600 cm^−1: skeletal vibration of aromatic ring, 580 cm^−1: Fe–O vibration, 1710 cm^−1: C=O, 1300 cm^−1: C–O, 3600 cm^−1: O–H, and 910 cm^−1: epoxy C–O	—	170
Cu–TiO2@functionalized SWCNT	SMZ	138.82	Uniform distribution of Cu-doped TiO2 particles on the surface of SWCNTs	3000 to 3800 cm^−1: –OH stretching of surface-adsorbed water molecules and carboxylic bands, 1580 and 1344 cm^−1: –OH deformation of –COOH functional groups and adsorbed water molecules, respectively, 1699 to 1651 cm^−1: stretching vibrations of carbonyl, carboxyl, and aldehyde functional groups, and 1625 and 3300−3400 cm^−1: hydroxyl functional groups and H2O molecules, respectively	Crystallinity phases	70

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10 Adsorption mechanisms of antibiotics on CNTs

Among the separation technologies, adsorption is a method in which the separation of some special components or pollutants from the fluid phase on the surface of a solid phase (adsorbent) can occur.¹⁷¹ The steps predicted for the adsorption process for any adsorbent material are as follows: 1) contaminants are dispersed or accumulated on the external surface of the adsorbent, 2) with sufficient time, the pollutants move deep in the interlayers of the adsorbent by internal diffusion (the transfer of contaminated mass to the interior surfaces), 3) and finally, the process continues until a layer is formed on the surface of the active pores, which is deemed as the end of the adsorption process.¹⁷² Furthermore, it should be noted that the adsorption efficiency depends on the major characteristics of the applied adsorbents including porosity (macro or micro porosity), pore diameter, specific surface area (SSA), and functional groups.¹⁷³ The adsorption of pollutants in CNT materials can be predicted based on the four regions in CNTs, i.e., hollow interiors of nanotubes, which are open ended, between the tube bundles at the interstitial pore spaces, at the groves present at the boundary of the nanotube bundles and the outermost CNT external surface.^174,175 Moreover, one of the critical steps in the design of adsorbents based on carbon materials is determining the reaction mechanism of pollutants (with focus on antibiosis) on their surface because it can provide significant information on their adsorptive capacity, regeneration efficiency, and finally their economic viability.¹⁷⁶

Based on previous studies, it has been proven that the adsorption of antibiotics on carbon-based materials such as activated carbon and CNTs can be accomplished via various mechanisms such as pore filling, surface complexation, covalent bonding, electrostatic interactions (cation and anion attractions), hydrophobic effects (hydrophobic interaction), π–π electron donor–acceptor interactions (EDA mechanism), hydrogen bonding, and partition into uncarbonized fractions, and other processes (surface precipitation).^176–178 As previously mentioned, various adsorption mechanisms can be dominant in the adsorption of organic compounds given that the internal characteristics of adsorbents play a significant role in this process because of their unique properties (heterogeneous surface area due to the co-existence of carbonized and non-carbonized fractions).¹⁷⁹ For instance, electrostatic interactions greatly control the adsorption of ionic compounds, which highly depend on the pH, where at pH values lower than pH_zpc (zero point of charge), the surface of all CNTs will be positively charged, whereas at pH values higher than pH_zpc, the surface if CNTs is negatively charged.¹⁷⁷ Besides, because antibiotic compounds differ based on their hydrophobicity (such as polar and nonpolar), the dominant adsorption mechanism of antibiotics vary.¹⁷⁹ Previous works have demonstrated that the adsorption of highly hydrophilic antibiotics such as SMX and lincomycine on SWCNTs may be related to various mechanisms including electrostatic interactions, hydrogen bonds, and hydrophobic interactions.¹⁸⁰ Also, considering the simultaneous effects of the properties of antibiotics and CNTs, together with the environmental conditions (temperature, solution pH, and matrix) is very helpful to better understand the dominant adsorption mechanism of antibiotics on CNTs.¹⁸¹

11 Effective parameters for the removal of antibiotics

11.1 Effect of pH

pH is an important and fundamental parameter in the adsorption process, which has a significant effect on the physicochemical properties of the adsorbent and adsorbate and determines the effect of the sorption process.^74,182 CNTs are often coated with a variety of materials, presenting an array of surface properties such as carboxyl, hydroxyl, and amino groups. Most of these surface properties depend on the pH, where at a relatively low pH, the amino group has a stronger catalytic surface yield than carboxyl and phenolic groups.¹⁸³ Alternatively, at a solution pH below pH_pzc, the functional groups of the surface are deprotonated due to the presence of OH⁻ ions in solution, whereas at a solution pH above pH_pzc, the functional groups on the surface of the adsorbent are protonated by additional H⁺ ions.¹⁶⁴

Ma et al. (2015) found that the removal of ciprofloxacin from aqueous solution by FeS/CNTs increased with an increase in pH and remained stable at high pH levels from 3.0 to 7.0. When the pH was <3.0, the adsorption of ciprofloxacin increased with an increase in the pH of the solution, because when the pH of the solution was below 3.0, the surface of FeS/CNTs was positively charged, thus repelling ciprofloxacin (cationic at pH < 6.0), consequently resulting in less adsorption. Also, when the pH of the solution was above 7.0, the surface of FeS/CNTs was negatively charged, and thus ciprofloxacin (zwitterionic at 6.0 < pH < 8.7) was strongly ionized, resulting in less adsorption.⁷² In the study by Bazregari et al. (2018), maximum amoxicillin removal efficiencies were obtained for virgin-MWCNT and 8-HQ-MWCNT at pH = 5 and for COOH–MWCNT at pH = 2. The decrease in amoxicillin removal by increasing the pH to more than 5 can be due to two reasons, as follows: 1) increased formation of OH– in solution and 2) its competition with COO⁻ anions on amoxicillin molecules to occupy the CNT adsorption sites. Alternatively, in very acidic conditions, the removal of amoxicillin was reduced due to interference from H⁺. In this study, the pH_pzc for virgin-MWCNT, 8-HQ–MWCNT and COOH–MWCNT was 6, 5.2 and 3, respectively, which is greater than the pH and caused a positive charge on their surface. Thus, in acidic solution (pH = 2–6), the COOH group in the amoxicillin molecule is isolated in the form of COO⁻. Consequently, electrostatic adsorption was predominant between the amoxicillin (anionic) molecules via their COO⁻ functional group and the CNT surface.¹⁶⁴ According to Payan et al. (2019), the sorption of sulfamethazine on Cu–TiO₂–SWCNT depends on the pH. At pH 7, sulfamethazine was almost completely eliminated. At this pH, the surface of the catalyst had a negative charge and the neutral form of sulfamethazine was detected as the dominant species. However, at pH values above 7.4, the removal efficiency decreased due to the electrostatic driving force occurring between the surface of the catalyst and the main form of sulfamethazine.⁷⁰

11.2 Effect of initial antibiotic concentration

The effect of initial concentration has been investigated in studies as an important parameter affecting the antibiotic removal efficiency. High concentrations of antibiotics produce many intermediates compounds, which compete with the active radicals. Consequently, the number of active species is significantly reduced, decreasing the rate of antibiotic removal.⁷⁰

In the study by Balarak et al. (2016), they described the effect of the tetracycline initial concentration on its percentage removal by Al₂O₃–MWCNTs. The results showed that the removal percentage of tetracycline using Al₂O₃–MWCNTs decreased with an increase in its initial concentration from 10 to 100 mg L⁻¹. This may be ascribed to the filling of the empty spaces, preventing further adsorption from occurring.¹⁶³ In another study by Kang et al. (2018), Ni@NCNTs was used as an adsorbent and catalyst to activate persulfate to remove sulfachloropyridazine as an emerging contaminant. According to the results, high concentrations of antibiotics reduced the degradation efficiency. This is because the sulfachloropyridazine molecules and intermediates occupied most of the active sites of Ni@NCNTs and inhibited the activation of persulfate. Also, an insufficient amount of persulfate can be a limiting factor for the degradation of high concentrations of sulfachloropyridazine.⁹³

11.3 Effect of coated CNT dosage

The dosage of coated CNTs is another factor affecting the removal of antibiotics. Increasing the absorbent dosage leads to more active sites and increase in surface area. Consequently, antibiotics are more efficiently adsorbed on the adsorbent surface.⁷⁰ According to Balarak et al. (2016), increasing the dosage of Al₂O₃–MWCNTs increased the adsorption percentage of TC, which is due to the increase in adsorption sites and adsorbent surface area. However, a further increase in adsorbent dosage did not result in the complete adsorption of tetracycline because the excessive condensation of the adsorbent particles caused the adsorption sites to overlap.¹⁶³ In another study conducted by Ahmadi et al. (2017), the effect of different dosages of MWCNT/TiO₂ (0.1–0.4 g L⁻¹) on the photocatalytic degradation of tetracycline at a concentration of 5 mg L⁻¹, pH 5 and irradiation time of 120 min was investigated. According to the obtained results, with an increase in the adsorbent dosage, the amount of antibiotic removed increased, and at a dosage of 0.2 g L⁻¹ TC, it was completely removed after 100 min. By increasing the adsorbent dosage to 0.4 g L⁻¹, no improvement in the removal process was achieved. At high dosages, the particles accumulated, which interfered with the homogeneous structure of the suspension and reduced the number of active sites.¹⁶¹ In the study by Payan et al. (2019), the effect of different dosages of Cu–TiO₂/SWCNT (0.1–1.2 g L⁻¹) on the photodecomposition of SMZ at pH 7, initial SMZ concentration of 30 mg L⁻¹, copper content of 4%, SWCNT content of 10%, and contact time of 135 min was checked. The results showed that with an increase in the adsorbent dose from 0.1 to 0.6 g L⁻¹ and 0.9 g L⁻¹, the removal rate increased from 51% to 79% and 100%, respectively.⁷⁰

11.4 Effect of contact time

One of the most important parameters in the adsorption process is the contact time between the adsorbate and adsorbent. According to studies, the amount of adsorption is very high early in the adsorption process, and then it reaches equilibrium. After the equilibrium phase, the adsorption rate decreases. The contact times for different adsorbents vary, which may be due to the differences in their chemical structure, availability of active surface sites, surface area and adsorption binding constants.^164,184

Balarak et al. (2016) investigated the effect of contact time for the removal of TC by Al₂O₃–MWCNTs. Based on the results at C_o = 10 to 100 mg L⁻¹ for an adsorbent dosage of 1 g L⁻¹, tetracycline was removed rapidly in the first 30 min due to the large number of empty surface sites for adsorption. However, the adsorption rate gradually decreased and the adsorption reached equilibrium within about 60 min. The repulsive force of the TC molecules on the solid surface and in the bulk phase made it difficult to occupy the empty space of the remaining surface.¹⁶³ Bazregari et al. (2018) investigated the removal of AMX using coated CNTs, where the time required to reach equilibrium in the adsorption process on the virgin and coated CNTs (8-HQ–MWCNT and COOH–MWCNT) was approximately 240 and 120 min, respectively. The coated CNTs showed an increase in adsorption kinetics due to the presence of functional groups on their surface. Also, the amount of adsorbed antibiotics in the coated CNTs increased, which can be attributed to the high rate of adsorption at the beginning of the process due to the presence of more adsorption sites on the adsorbent.¹⁶⁴

12 Reusability of CNTs

In the adsorption process, the reusability or recovery of adsorbents saturated by pollutants is one of the critical parameters that must be considered for the commercial application of the selected adsorbents.¹⁰ Generally, three main thermal methods are employed for the regeneration of carbonaceous materials (such as AC and CNT) including pyrolysis, pyrolysis–gasification, and gasification.^179,185 For example, previous works were conducted on the regeneration of AC by the pyrolysis and gasification techniques using air or CO₂ (200 cm³ min⁻¹) under an N₂ flow. It was shown that between these two methods, the best result was obtained by air gasification with the minimum time contact of 3 h.^179,185 Besides the thermal regeneration of the adsorbents, it was recently demonstrated that they can be regenerated by wet air oxidation, ultrasonication, chemical treatment, and washing with a solvent such as sodium hydroxide (NaOH) and hydrochloric acid (HCl), and also sodium chloride (NaCl).¹⁸⁶ Therefore, in recycling or estimating the reusability of the selected adsorbents, two steps (regenerating the adsorbent and maintain its stable adsorption capacity) are very important, where recycling of CNTs can be better performed at a stable temperature below 400 °C because of their thermal stability.¹⁸⁷ Various studies have been conducted on the performance and regeneration of CNTs and modified-CNTs. Xiong et al. (2018) used MWCNT/NH2-MIL-53(Fe), and after it was dried at 100 °C in a vacuum oven, there was no significant decrease in adsorption for up to four recycling cycles.¹⁰ Wang et al. (2015) used CNTs/CoFe₂O₄ as a promising adsorbent for the removal of SMZ. The obtained data from the regeneration process under heating at 300 °C demonstrated that this adsorbent can be reused for five consecutive cycles.¹⁸⁷ Sharifpour et al. (2020) performed the recovery of MWCNTs-AC for the removal of CIP using three different washing methods (NaOH, distilled water, and H₂SO₄), and found that the highest (96.7%) adsorbent recovery was obtained using NaOH followed by distilled water (94.1%), and H₂SO₄ (86.5%).¹⁶⁸

13 Management and disposal of CNTs

As mentioned before, nanotechnology is deemed as remarkable and fascinating technology, which can be widely applied in many fields, but the severe risk related to the use of nanoparticles and their disposal have not been well studied, and thus this topic is increasingly becoming a public concern.¹⁸⁸ According to the available data, the production rate of nanoparticles (NPs) increased (from 2000 tons to 58 000 tons) from 2004 to 2020, and thus it is urgent to comprehensively investigate the hazardous effects of various NPs, and consequently applying the appropriate management to prevent the aforementioned effects.¹⁸⁹ Various studies have been reported on the synthesis of several types of NPs based on carbonaceous materials, but there is little information on applying feasible and professional approaches for the management of nanowaste streams generated in different phases of nanotechnology-based products and the life cycle of these materials.¹⁹⁰ Thus, new guidelines and protocols for the safe handling and disposal of nanowastes must be further developed in the future.¹⁹⁰

14 Adsorption modeling: kinetics and isotherms

14.1 Adsorption kinetics

To design an adsorption system, it is necessary to have sufficient knowledge about the equilibrium and kinetics of adsorption. The theoretical complexity of adsorption mechanisms is recognized as a limiting factor in the development of kinetic understanding.¹⁹¹ Kinetics is the basis for determining the efficiency of a fixed bed or any other flow-through system, determining the solute uptake rate (which determines the residence time to complete the adsorption reaction), and knowing the scale of the adsorption device.¹⁹² For the kinetic study of the adsorption of antibiotics on coated-CNTs, recently, various kinetic models such as pseudo-first-order (PFO) based on solid capacity, pseudo-second-order (PSO) based on solid-phase adsorption, intra-particle diffusion model (IPD) describing the diffusion mechanism, and Elovich model¹⁹³ have been applied. Also, the correlation coefficient (R²) is used to measure the agreement between the measured data and the proposed model.¹¹¹ The intraparticle diffusion equation is used to identify the adsorption mechanism and predict the rate of adsorption kinetic control.¹⁶³ Among them, PFO and PSO have been widely used for the adsorption of antibiotics using coated-CNTs in previous works. Furthermore, the equations and major application of the aforementioned kinetic models are summarized in Table 6.

Table 6 Kinetic models, equations, and applications

Kinetic model	Model equations	Constant parameters	Application	Ref.
PFO		qt is the amount of adsorbed at any time (mg g^−1)	To estimate the initial stage of the adsorption mechanism	194 and 195
		K 1 is the adsorption rate constant
PSO			Suggests chemisorption process	196 and 197
		K 2 is the adsorption rate constant
		k i is the intraparticle diffusion rate constant
IPD			To detect the pathway involved in the adsorption system	194 and 198
		α is the initial sorption rate (mg g^−1 min^−1)
Elovich			To examine chemisorption	193 and 197
		β is the de-sorption constant (g mg^−1)

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Ma et al. (2015) noted that CNT/C@Fe/CS had a good adsorption capacity (q_e) for TC (104 mg g⁻¹). The results of their adsorption experiments showed that the adsorption kinetics in this study followed the PSO model (q_e = 117.6 mg g⁻¹, K₂ = 0.004 min⁻¹, and R² = 0.99).¹¹¹ In the study by Bazregari et al. (2018) on the removal of AMX with coated and virgin CNTs, it was observed that the PSO model was the most compatible with the adsorption kinetics data (R² > 0.97). Also, the results showed that the kinetic constant (k) was greater for the coated CNTs than their virgin form (k₂ virgin-MWCNT = 0.0000196 g mg⁻¹ min⁻¹, k₂ 8-HQ–MWCNT = 0.0000223 g mg⁻¹ min⁻¹ and k₂ COOH–MWCNT = 0.0000207 g mg⁻¹ min⁻¹) and led to the adsorption of more pollutants in less time.¹⁶⁴Table 7 shows the parameters of the adsorption kinetic models in studies on the removal of antibiotics with coated CNTs.

Table 7 Parameters of adsorption kinetic models

Antibiotic	Coated-CNTs	Kinetic model
		PFO			PSO			IPD			Ref.
		q e	K 1	R ^2	q e	K 2	R ^2	k id	C	R ^2
TC	CNTs/C@Fe/CS	69.4	0.004	0.88	117.6	0.004	0.99	8.2	58.9	0.97	111
	Al2O3–MWCNTs	56.24	0.094	0.841	94.46	0.0041	0.997	0.06	0.80	0.73	163
		214.7	0.042	0.823	249.8	0.0059	0.998	0.12	1.24	0.75
		402.8	0.035	0.796	474.2	0.0074	0.998	0.17	1.51	0.78
		745.2	0.019	0.803	889.5	0.0088	0.997	0.25	1.98	0.80
TCH	MWCNT/MIL-53(Fe)	83.16	0.0098	0.98	89.21	0.00015	0.99	—	—	—	74
OTC		55.96	0.0057	0.94	60.83	0.00015	0.98	—	—	—
CTC		39.54	0.0063	0.91	43.08	0.00018	0.98	—	—	—
TCH	NH2-MIL-53(Fe)	69.86	0.0095	0.96	76.92	0.00019	0.99	—	—	—	10
	MWCNT/NH2-MIL-53(Fe)	85.62	0.0097	0.97	90.09	0.00018	0.99	—	—	—
CTC	NH2-MIL-53(Fe)	34.43	0.0064	0.90	35.97	0.00033	0.98	—	—	—
	MWCNT/NH2-MIL-53(Fe)	41.99	0.0062	0.91	44.31	0.00023	0.99	—	—	—
AMX	8-HQ–MWCNT	1145	0.358	0.99	556	0.0000223	0.99	—	—	—	164
	COOH–MWCNT	825	0.272	0.96	526	0.0000207	0.98	—	—	—
CIP	CNTs/FeS	—	0.06	0.93	—	—	—	—	—	—	72

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14.2 Adsorption isotherms

An adsorption isotherm is an important tool for evaluating the adsorbent distribution on solid/liquid boundaries and can be used to estimate the adsorption capacity.¹⁹⁹ Previous research works confirmed that the interaction between the adsorbate and adsorbent can be determined using adsorption isotherm models.²⁰⁰ Applying isotherms (commonly named equilibrium relationships) to the adsorption processes can be useful for several purposes, as follows: (1) to represent the physicochemical characteristics (especially surface properties) and capacities of adsorbents; (2) to optimize the adsorption process pathways; and (3) to develop efficient designs of adsorption systems.²⁰¹ To date, several isotherm models including Langmuir,^199,202 Freundlich,^203,204 Temkin,¹⁹⁶ Sips,²⁰⁵ and Dubinin–Radushkevich (D–R)²⁰⁶ models have been applied to describe the equilibrium relations. Notably, among the aforementioned isotherm models, the two well-known isotherm models of Freundlich (Freundlich, 1906) and Langmuir (Langmuir, 1918) were applied to the equilibrium data of coated-CNTs.²⁰³ Furthermore, the derivation of the Temkin isotherm assumes that the adsorption heat loss is linear rather than logarithmic.²⁰⁷ This isotherm shows a linear decrease in the adsorption energy with an increase in the number of adsorption centers in an adsorbent. When an adsorbent is coated, the adsorption heat of all the molecules in the layer is reduced due to the adsorbent–adsorbate interaction. In this isotherm, the adsorption is determined by the uniform distribution of the binding energies.¹⁶³ This model is suitable for chemical adsorption based on strong electrostatic reaction between negative and positive loading.¹¹¹ The different isotherm models and their main applications are listed in Table 8.

Table 8 Isotherms models, equations, and applications

Isotherms models	Equations models	Constant parameters	Application	Ref.
Langmuir		q e is the amount of adsorbed at equilibrium (mg g^−1)	To express the single-layer adsorption process	199, 202 and 208
		C e is the metal concentration in the aqueous phase (mg L^−1)
Freundlich		b is Langmuir equilibrium constant (L g adsorbent^−1)	To suggest the heterogeneous structure of the adsorbent surface	203, 204 and 209
		q 0 is maximum monolayer adsorption (mg g adsorbent^−1)
Temkin	q e = B ln A × Ce	k L is the Langmuir constant (L mg^−1)	Assumes the heat of adsorption and effects of indirect interaction between adsorbent molecules have predictive power in a wide concentration range	196, 204 and 210
		K F and n are Freundlich coefficients
	q e = B ln A + B ln Ce	Temkin constant is B = RT/b and b (J mol^−1) is the adsorption heat
		A (L g^−1) is Temkin isotherm constant
D–R	ln Qe = ln QD–R – βε^2	R (3314 J Mol^−1 K^−1) is the global gas constant in which T (K) is an absolute	Is used for the isotherm analysis with a high degree of regularity	206 and 211
		Q D–R (lg/g) is the theoretical saturation capacity of the adsorbent
Sips			Is an alternative empirical approach with the characteristics of the Freundlich and Langmuir isotherms	205
		b is a constant related to the mean free energy of adsorption per mole of adsorbate
		ε is a logarithmic function of concentration called Polanyi potential. R and T are gas constant and temperature in Kelvin scale, respectively

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The results from the study by Ma et al. (2015) on the adsorption of TC at 25 °C by CNTs/C@Fe/CS showed that the adsorption followed the Freundlich isotherm (R² = 0.98). The high value of K_F (30.30 L g⁻¹) indicates the good affinity between TC and CNTs/C@Fe/CS and its high adsorption capacity. Alternatively, because the value of 1/n is less than 1 (0.78), the adsorption is desirable.¹¹¹ Also, based on the results from the study by Xiao et al. (2018) on the removal of TC antibiotics using MWCNT/NH₂-MIL-53(Fe), the R² of the Langmuir model was higher than that of the Freundlich and Temkin isotherm models, which indicated the adsorption of a monolayer on its surface. Alternatively, low the R_L values showed the strong interaction between the adsorbates and NH₂–MWCNT/MIL-53 (Fe).¹⁰Table 9 shows the parameters of the adsorption isotherm models in studies on the removal of antibiotics with coated CNTs.

Table 9 Parameters of adsorption isotherm models

Antibiotic	Coated-CNTs	Isotherm model									Ref.
		Langmuir			Freundlich			Temkin
		q m	B	R ^2	K F	n	R ^2	A	B	R ^2
TC	CNTs/C@Fe/CS	103	0.05	0.82	4.55	2.88	0.96	2.49	18.65	0.69	111
		104	0.21	0.95	30.30	1.28	0.98	4.29	23.35	0.84
		117	0.35	0.94	33.87	4.04	0.96	22.09	14.86	0.89
	Al2O3–MWCNTs	833.4	0.012	0.998	59.36	2.81	0.84	12.84	47.69	0.79	163
		855.6	0.031	0.997	72.11	3.17	0.85	9.424	53.46	0.80
		876.8	0.056	0.999	84.48	3.64	0.81	7.436	61.78	0.82
TCH	MWCNT/MIL-53(Fe)	364.37	0.044	0.98	47.74	0.40	0.97	—	—	—	74
OTC		325.59	0.011	0.99	9.16	0.62	0.97	—	—	—
CTC		180.68	0.042	0.97	24.82	0.38	0.98	—	—	—
TCH	NH2-MIL-53(Fe)	302.63	0.042	0.99	36.03	2.38	0.96	49.12	1.13	0.97	10
	MWCNT/NH2-MIL-53(Fe)	368.49	0.064	0.98	60.73	2.70	0.96	49.01	3.95	0.90
CTC	NH2-MIL-53(Fe)	152.29	0.024	0.99	11.74	2.12	0.94	25.03	0.56	0.91
	MWCNT/NH2-MIL-53(Fe)	254.04	0.044	0.99	33.58	2.56	0.95	39.97	1.34	0.91
AMX	8-HQ–MWCNT	784	0.006	0.99	10.62	1.33	0.99	—	—	—	164
	COOH–MWCNT	647	0.007	0.97	8.23	1.27	0.98	—	—	—

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15 Future challenges and perspectives

As mentioned above, recently many experimental studies have been performed regarding the use of carbon-based NPs for the removal of antibiotics. Among the carbonaceous martials, CNTs have gained considerable attention in the environmental engineering field (adsorbents, sensors, membrane modifiers, catalysts, gene therapy, and cancer treatments) due to their unique physicochemical, mechanical, thermal, and electrical properties. However, it worth mentioning that despite their numerous advantages, the use of CNTs on a large scale is hindered because of some limitations such as their high synthesis cost, low hydrophilicity, complex interactions with environmental colloids and microorganisms, toxicity and effect on the eco-system, and difficult recovery and regeneration. Therefore, it is urgent that researchers design new CNTs that are safe and non-toxic (synthesized through green methods), with fewer impurities, easy regeneration, and also lower costs. Herein, based on the above overview and lack of important information on the use of CNTs, we propose some points that should be considered for their future research and real-world applications, as follows:

• Study of the feasibility of CNTs on an industrial and large scale.

• Study of the optimization of various parameters affecting the synthesis route of CNTs.

• Study of the accurate human health, environmental, and eco-toxicological risk assessments related to the toxicity of CNTs.

• The development of new CNT-based remediation technologies.

• Evaluating the cost-effective synthesis of CNTs.

• Study of proper, economic, safe, and environmentally friendly options for the disposal of CNTs.

• Study of the by-products produced after using CNTs and their derivatives during the removal of antibiotics.

• Integrating CNTs with the other systems.

16 Conclusions

Examination of the literature showed that the surface of single-walled and multi-walled CNTs can be modified with different materials to overcome the hydrophobicity and chemical neutrality of the outer wall of conventional CNTs for the removal of antibiotics from aqueous solutions. Similar to many other adsorbents, the adsorption capacity of coated CNTs depends on factors such as adsorbent dosage, antibiotic initial concentration, pH, and contact time. In terms of equilibrium study, in most of the studies mentioned in the present study, the adsorption follows the Langmuir model, which represents monolayer adsorption. Studies have used the intra-particle diffusion, pseudo-first-order and pseudo-second-order models to model the adsorption kinetics, where in most of them, the pseudo-second-order model was in full agreement with the experimental data. Furthermore, this comprehensive review highlighted that the major mechanisms for the adsorption of antibiotics on coated-CNTs are π–π interaction, electrostatic interaction, hydrophobic interaction, and hydrogen bonding.

Abbreviation

AMX	Amoxicillin
CIP	Ciprofloxacin
TC	Tetracycline
SA	Sulfonamide
FQ	Fluoroquinolone
OTC	Oxytetracycline
SMX	Sulfamethoxazole
SMZ	Sulfamethazine
TCN	Tetracycline hydrochloride
CTC	Chlortetracycline hydrochloride
AC	Activated carbon
CNT	Carbon nanotube
SWCNT	Single-wall CNT
MWCNT	Multi-wall CNT
CVD	Chemical vapor deposition
MMWCNT	Magnetic multi-wall CNT
RO	Reverse osmosis
MFC	Microbial fuel cell
CW	Constructed wetland
CF	Coagulation-flocculation
MBBR	Moving bed biofilm reactor
AOP	Advance oxidation process
XRD	X-Ray diffraction
SEM	Scanning electron microscopy
EDX	Energy dispersive X-ray analysis
FTIR	Fourier transform infrared spectroscopy
WWTP	Wastewater treatment plant
PFO	Pseudo-first-order
PSO	Pseudo-second-order
IPD	Intra particle diffusion
D-R	Dubinin-Radushkevich

Author contributions

P. Darvishi: writing first draft, review, editing; S. A. Mousavi: supervision, conceptualization, investigation, and writing—review, editing, and project administration; A. Mahmmoudi: writing first draft, review, editing; D. Nayeri: writing–original draft preparation, reviewing, and editing.

Conflicts of interest

The authors have declared no conflict of interest.

Acknowledgements

The authors gratefully acknowledge the Research Council of Kermanshah University of Medical Sciences for the financial support (Grant number: 4010455) under ethics approval number: IR.KUMS.REC.1401.081.

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