Science of the Total Environment 482–483 (2014) 241–251

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Aqueous adsorption and removal of organic contaminants by carbon nanotubes Jin-Gang Yu a,d,e,⁎, Xiu-Hui Zhao a,e, Hua Yang a,e, Xiao-Hong Chen b, Qiaoqin Yang c, Lin-Yan Yu a,e, Jian-Hui Jiang d, Xiao-Qing Chen a,e,⁎ a

College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China Collaborative Innovation Center of Resource-conserving & Environment-friendly Society and Ecological Civilization, Changsha, Hunan 410083, China Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada d College of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, China e Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha, Hunan 410083, China b c

H I G H L I G H T S • We summarize the most recent research progress of CNTs for removal of organics. • Adsorption mechanisms between CNTs and organics were elucidated in detail. • The developing trends and prospects of CNTs for removal of organics were discussed.

a r t i c l e

i n f o

Article history: Received 16 September 2013 Received in revised form 26 February 2014 Accepted 27 February 2014 Available online 18 March 2014 Keywords: Adsorption Application Carbon nanotubes Organic pollutants Removal

a b s t r a c t Organic contaminants have become one of the most serious environmental problems, and the removal of organic contaminants (e.g., dyes, pesticides, and pharmaceuticals/drugs) and common industrial organic wastes (e.g., phenols and aromatic amines) from aqueous solutions is of special concern because they are recalcitrant and persistent in the environment. In recent years, carbon nanotubes (CNTs) have been gradually applied to the removal of organic contaminants from wastewater through adsorption processes. This paper reviews recent progress (145 studies published from 2010 to 2013) in the application of CNTs and their composites for the removal of toxic organic pollutants from contaminated water. The paper discusses removal efficiencies and adsorption mechanisms as well as thermodynamics and reaction kinetics. CNTs are predicted to have considerable prospects for wider application to wastewater treatment in the future. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4. 5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of CNTs for adsorption . . . . . . . . . . . . . . . . . 2.1. Removal of organic dyes . . . . . . . . . . . . . . . . . . 2.2. Removal of pharmaceuticals . . . . . . . . . . . . . . . . 2.3. Removal of pesticides . . . . . . . . . . . . . . . . . . . 2.4. Removal of phenols, aromatic amines, and other toxic organics 2.5. Removal of organic contaminants using CNT composites . . . Solid-phase extraction (SPE) based on CNTs . . . . . . . . . . . . The adsorption mechanism . . . . . . . . . . . . . . . . . . . . Kinetic, isothermic, and thermodynamic studies . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China. Tel./fax: +86 731 8887 9616. E-mail addresses: [email protected] (J.-G. Yu), [email protected] (X.-Q. Chen).

http://dx.doi.org/10.1016/j.scitotenv.2014.02.129 0048-9697/© 2014 Elsevier B.V. All rights reserved.

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Conflicts of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The disposal of large amounts of wastewater that contain potentially toxic organic solutes is a problem shared by many companies. The removal of toxic components in wastewater (e.g., metal ions, organic poisons, and harmful germs) prior to its disposal is crucial (Fu and Wang, 2011; Jagtap et al., 2012) because of general safety concerns and environmental consequences. Many technologies have been established for such removal, including chemical oxidation/reduction, biological treatment, coagulation/flocculation, adsorption, membrane separation, and ion exchange. Novel technologies are continuously being developed through the constant efforts of researchers (Ma et al., 2012a). One of the most effective of these methods is adsorption because processes based on this concept are simple, highly efficient, and easy to operate; therefore, adsorption processes are widely used (Jia et al., 2013; Liu et al., 2013; Sun et al., 2012). Various adsorbents have been developed for the removal of organic pollutants (e.g., dyes, pesticides, pharmaceuticals/drugs, and phenols) from water (Alexander et al., 2012; Atul et al., 2013; Bond et al., 2012; Delgado et al., 2012). Activated carbon (AC) is the most commonly used commercial adsorbent because of its excellent adsorption capacity for organic contaminants (Demirbas, 2009; Hosseini et al., 2011). However, it has certain shortcomings that include limited availability, low adsorption capacity, and difficult recovery. Recently, a great deal of attention has been focused on the application of nano-structured materials as adsorbents to remove toxic and harmful organic substances from wastewater (Aditya et al., 2011; Mohmood et al., 2013). Carbon nanotubes (CNTs), which were discovered by Iijima in 1991 (Iijima, 1991), are one of the most widely studied carbon nanomaterials and can serve as excellent adsorbents (Brar et al., 2010; Herrero Latorre et al., 2012; Pyrzynska, 2010) because of their hollow and layered structure and large specific surface area, which is why CNTs are the most commonly used nano-materials for adsorbing toxic material (Sui et al., 2012b; Sweetman et al., 2012; Tan et al., 2012). CNT adsorbents can be classified into three types: single-walled CNTs (SWCNTs), multi-walled CNTs (MWCNTs), and functionalized CNTs (f-CNTs) (Bahgat et al., 2011; Ma et al., 2011b; Ruelle et al., 2012). Such materials have already played an important role in the effective removal of several organic contaminants from water (Ma et al., 2011b; Ren et al., 2011). For example, (MWCNTs) are much more effective in the removal of methyl orange (MO) (Hosseini et al., 2011), Eriodirome Cyanine R (ECR) (Ghaedi et al., 2011b), arsenazo(III), and methyl red (MR) (Ghaedi et al., 2011c) from wastewater than activated carbon (AC). This paper reviews the recent progress in the application of CNTs in the removal of toxic organic pollutants from contaminated water. A total of 145 published studies (2010–2013) are reviewed, the current applications of various types of organic pollutants are presented, and several possible adsorption mechanisms and thermodynamics/kinetics are discussed. We conclude with a glimpse into future challenges for the wider application of CNTs and related materials in the field of environmental engineering. 2. Application of CNTs for adsorption 2.1. Removal of organic dyes Organic dyes are one of the most hazardous materials in industrial effluents that are discharged from various industries (e.g., textiles, leather, cosmetics, and paper) and act as contaminants to the

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environment in general and water sources in particular. Many organic dyes have high levels of biotoxicity that cause potential mutagenic and carcinogenic effects in humans. For example, dyes such as Sudan red I, II, III, and VI, whose use in food is prohibited as a result of their toxicity or carcinogenicity at even low concentrations, are widely used in other industries. Most dye compounds contain complex aromatic structures (Fig. 1) that make them highly resistant to biodegradation and recalcitrant to conventional biological and physical oxidation treatments. Therefore, the targeted removal of such compounds has attracted a growing amount of attention (Jiao et al., 2013; Yang et al., 2013b). A wide range of materials has been used for the removal of organic dyes from wastewaters, including AC, zeolite, clay, and polymers, to name but a few. The current priority is to develop novel adsorbent materials with high adsorption capacities and removal efficiencies to realize effective control of these environmental pollutants. CNTs could be one of the most promising absorbents for this purpose because of their large adsorption capacity for organic dyes. Indeed, MWCNTs have been shown to outperform cadmium hydroxide nanowireloaded AC (Cd(OH)2-NW-AC) with respect to their efficient removal of safranine O (SO) from wastewater (Ghaedi et al., 2012a). However, only a few reports on the application of CNTs for dye removal from aqueous solution have been published until now (Bahgat et al., 2011; Ghaedi et al., 2011a; Ghaedi et al., 2011b; Ghaedi et al., 2011c; Hu et al., 2011b; Machado et al., 2011) and the CNTs were typically directly used without further treatment (Ghaedi et al., 2011a; Ghaedi et al., 2011b; Ghaedi et al., 2011c; Machado et al., 2011) (Table 1). Functionalization of CNTs has been undertaken because the introduction of various functional groups can provide new adsorption sites for organic dyes. Among such modifications, oxidation is an easy method of introducing hydroxyl and carbonyl groups to the sidewalls of CNTs. Oxidized MWCNTs have been shown to be effective in the removal of MR (Ghaedi and Kokhdan, 2012) and methylene blue (MB) from aqueous solutions (Ghaedi et al., 2012b). Another work has focused on the development of CNT-impregnated chitosan hydrogel beads (CSBs) for the removal of Congo red (CR). In Langmuir adsorption modeling, CSBs demonstrated a higher maximum adsorption capacity than normal chitosan CBs (450.4 vs. 200.0 mg·g−1; Chatterjee et al., 2010). A new generation of CSBs prepared by using sodium dodecyl sulfate (SDS) and MWCNTs to improve upon their mechanical properties has also demonstrated a high maximum adsorption capacity for CR (375.94 mg·g−1; Chatterjee et al., 2011). Compared to MWCNTs and hybrid CNTs (HCNTs), SWCNTs can demonstrate better adsorption properties for organic contaminants because of their higher specific surface area. Indeed, SWCNTs are more efficient at removing benzene and toluene, and has shown maximum adsorption capacities of 9.98 and 9.96 mg·g− 1, respectively (Bina et al., 2012). A maximum adsorption capacity of 496 mg·g−1 was achieved when reactive blue 29 (RB29) was removed from aqueous solution using SWCNTs (Nadafi et al., 2011). Recently, a novel self-assembled cylindrical graphene-CNT (G-CNT) hybrid was developed, and it achieved a maximum adsorption capacity of 81.97 mg·g−1 for the removal of MB from aqueous solution, and the removal efficiency reached 97% for low (10 mg·L−1) initial MB concentrations (Ai and Jiang, 2012). Lastly, Zeng et al. (2013) proposed a new concept of using entangled CNTs as porous frameworks to enhance the adsorption of organic dyes. The composites obtained through polymerization with polyaniline (PANI) possessed large surface areas. At an initial malachite green (MG) concentration of 16 mg·L−1, the CNT/PANI composites exhibited a 15% higher equilibrium adsorption capacity of 13.95 mg·g−1 compared to neat PANI.

J.-G. Yu et al. / Science of the Total Environment 482–483 (2014) 241–251

HO

243

HO

N N

H 3C

N N CH3 Sudan II

Sudan I CH3

HO N N

CH3

N N

N N

Sudan III

H3C

HO

N N

Sudan IV

CH3 N

N

H2N

N S

NH2 N

H3C

O ONa S O

N

N

N

CH3 Cl

O S O ONa

MB

CR

N Cl N

N

N

N N H

N

N H

S

Janus green

N H

H

Thionine

N HO

O

Cl

SO3Na

HO

O OH

OH N

N Crystal violet

O

OH

Alizarin Red S (ARS)

OH

OH O Morin

Fig. 1. Chemical structures of organic dyes targeted for removal by CNTs.

The research on the removal and adsorption of organic dyes using CNTs is summarized in Table 1. Overall, the CNT composites are being and will continue to be pursued as better adsorbents for this purpose. 2.2. Removal of pharmaceuticals In light of their environmental persistence, bioaccumulation, and potential toxicity, all currently used human or animal drugs can exert potential negative effects on the environment. For example, steroid hormones, including estradiol, estrone (Fig. 2), ethinylestradiol, etc., are known carcinogens, and both natural and synthetic steroid hormones have been found in water systems at ng·L−1 levels. Olaquindox (OLA; Fig. 2) is a well-known food additive that is highly phototoxic, mutagenic, genotoxic, and carcinogenic. Tetracycline (TC; Fig. 2) is one of the most widely used antibiotics in the world but has serious side effects on human health and potential negative effects on the environment when it accumulates in water systems. Thus, effectively removing

pharmaceuticals/drugs from contaminated water is of great importance and has attracted significant attention; however, only a few recent reports on their removal by adsorption have been published. As a result of their highly specific surface area and large micropore volume, CNTs are considered superior adsorbents for the removal of pharmaceuticals (Heo et al., 2011; Joseph et al., 2011a; Joseph et al., 2011b). In particular, the removal of endocrine disrupting compounds (EDCs) from drinking water has attracted considerable attention because of their interference with human reproductive systems by blocking or mimicking the activity of natural hormones. Joseph et al. (2011a,b) investigated the adsorption of EDC from artificial seawater, brackish water, or the combination thereof using SWCNTs and reported a higher removal efficiency for 17α-ethinyl estradiol (EE2; 95–98%) than for bisphenol A (BPA; 75–80%). As a result of the excellent adsorption capacity of SWCNTs, a strong linear correlation between the retention and adsorption of BPA and 17β-estradiol (E2) in ultrafiltration (UF) membrane systems blended with SWCNTs (SWCNT–UF) has also been

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Table 1 Removal of different organic dyes using CNTs. Type of CNT Modification method

Type of dye removed

Adsorption effect

Ref.

MWCNTs

Toluidine blue, MB, methyl green (MG), and bromopyrogallol red ECR Arsenazo(III) Alizarin red S (ARS), morin Reactive red M-2BE (RRM) textile dye MO RB29

Not provided

Bahgat et al. (2011)

MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs SWCNTs

Refluxed pristine MWCNTs in concentrated HNO3/H2SO4 mixture for 4 h Untreated Untreated Untreated Untreated Oxidized using concentrated nitric acid Untreated

73.18 mg·g−1 Not provided ARS: 161.290 mg·g−1; morin: 26.247 mg·g−1 335.7 mg·g−1 Not provided 496 mg·g−1

MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs MWCNTs

Untreated Untreated Alkali-activated Untreated Untreated Untreated Untreated

MWCNTs

Fabricated magnetic MWCNTs by Fenton's reagent method (M-MWCNTs) Pristine and oxidized Oxidized

CR MO MB, MO Acid red 18 (Azo-Dye) MB, acid dye (acid red 183, AR183) Acid blue 161 (AB 161) Reactive blue 4 (RB4) and acid red 183 (AR183) MO

Ghaedi et al. (2011b) Ghaedi et al. (2011c) Ghaedi et al. (2011a) Machado et al. (2011) Hu et al. (2011b) Jahangiri-Rad et al. (2013) and Nadafi et al. (2011) Ramazani et al. (2011) Yao et al. (2011) Ma et al. (2012b) Shirmardi et al. (2012) Wang et al. (2012a) Geyikci (2013) Wang et al. (2012b)

SWCNTs MWCNTs MWCNTs

MWCNTs SWCNTs

Produced by Ni nanoparticle catalyzed pyrolysis of methane in a hydrogen and nitrogen flow at 650 °C Untreated Untreated

Not provided Not provided MO: 149 mg·g−1; MB: 399 mg·g−1 166.67 mg·g−1 MB: 59.7 mg·g−1; AR183: 5.2 mg·g−1 91.68% RB4: 69 mg·g−1; AR183: 45 mg·g−1 28 mg·g−1

Yu et al. (2012a) −1

−1

; oxidized: 49.45 mg·g

Moradi (2013) Ghaedi et al. (2013)

Basic red 46 (BR 46) 5-(4-Dimethyl benzylidene amino) rhodanine (DMBAR) MB

SWCNTs: 38.35 mg·g 15.52 mg·g−1 188.68 mg·g−1

Li et al. (2013)

Triclosan Reactive red 120 (RR-120)

153.1 mg·g−1 426.49 mg·g−1

Zhou et al. (2013) Bazrafshan et al. (2013)

observed (Heo et al., 2012a). The addition of SWCNTs to the UF process did not significantly exacerbate the permeate flux decline and total membrane resistance, which is important from a practical standpoint; however, the use of SWCNT–UF reduced the membrane resistance because the SWCNTs offered additional binding sites for the adsorbates (Heo et al., 2012b). To uncover the possible relationship between adsorbates and CNTs, three functionalized MWCNTs (hydroxylated, carboxylated, and graphitized MWCNTs), three p-MWCNTs with different diameters (15 nm, 30 nm, and 50 nm), and three functionalized SWCNTs (hydroxylated, carboxylated, and purified SWCNTs) were used to remove ofloxacin and norfloxacin (NOR) (Fig. 2) from aqueous solutions (Peng et al., 2012). The results indicated that site-specific adsorption might be less important and that the hydrophobic effect contributes more to the adsorption effect. Adsorption of the antibiotics onto the CNTs was described as a structurally controlled process, which is different from that reported for hyper-crosslinked resin MN-202, aminated polystyrene resin MN-150, or macroporous resin XAD-4 (Yang et al., 2012). Importantly, the effective reuse of CNTs will affect their utility in long-term applications. A systematic investigation into the removal of two representative sulfonamide antibiotics demonstrated that small amounts of sulfamethoxazole (SMX; 3%) and sulfapyridine (SPY; 9%) were irreversibly bonded to a CNT/sand porous media during fixedbed regeneration, thus reducing the column capacity (Tian et al., 2013). However, Shi et al. (2013) reported on a new magnetic mesoporous carbon nanocomposite (Fe3O4/C) for the removal of ciprofloxacin (CIP) that retained over 85% of its adsorption capacity even after 10 recycle runs. A newly developed CNTs/Al2O3 hybrid adsorbent demonstrated high maximum sorption capacities of 157.4 and 106.5 μmol·g−1 for diclofenac sodium (DS) and carbamazepine (CBZ), respectively; more importantly, the adsorbent could be thermally regenerated at 400 °C in air (Wei et al., 2013). Research on the removal and adsorption of pharmaceuticals using CNTs is summarized in Table 2. Many challenges remain to be addressed, and further research is required with respect to the design and preparation of novel absorbents with good sorption and regeneration properties.

2.3. Removal of pesticides The application of pesticides in agricultural practices can increase yields and improve the quality and quantity of products; however, both soils and water sources can become contaminated with these compounds. The removal of residual pesticides from drinking water is of particular importance with respect to human health. Chen et al. (2011) reported the removal of two herbicides (diuron and dichlobenil; Fig. 3) from contaminated water using MWCNTs. The adsorption of diuron onto as-prepared and oxidized MWCNTs was studied by Deng et al. (2012a), with the oxidation treatment increasing the surface area and pore volume and resulting in an increased adsorption. SWCNTs have been demonstrated to have a higher adsorption capacity for 4-chloro-2-methylphenoxyacetic acid (MCPA), a phenoxy acid herbicide, than three kinds of MWCNTs (with average outer diameters of 15, 30, and 50 nm) and several nanoscale metal oxides (Al2O3, TiO2, and ZnO); the adsorption kinetics usually followed pseudo-second-order kinetics, with the adsorption process being spontaneous and exothermic (De Martino et al., 2012). The adsorption of atrazine (ATZ) from aqueous solution by granular AC (GAC) and CNTs also occurred via the same kinetic and thermodynamic processes (Rambabu et al., 2012). Overall, the use of CNTs for pesticide removal appears to be less studied than for other organic contaminants. Considering the widespread use of pesticides, more efforts should be directed to investigate the removal of pesticide residues from drinking water and wastewater.

2.4. Removal of phenols, aromatic amines, and other toxic organics The most common pollutants found in effluents are anilines and phenols, which are common constituents of many industrial wastewaters and have led to a series of serious environmental problems. MWCNTs have been shown to remove a series of phenols and anilines from aqueous solutions, and the adsorption was strongly dependent on the solution pH and number and type of solute groups (Yang et al., 2008). Two kinds of pristine MWCNTs (short MWCNTs with average diameters of 10–20 nm and 40–60 nm, respectively) have been evaluated as absorbents for alanine from aqueous solution (Al-Johani and Salam, 2011). This

J.-G. Yu et al. / Science of the Total Environment 482–483 (2014) 241–251

245

3

3

3

3

2

3

3

3

Fig. 2. Chemical structures of pharmaceuticals/drugs targeted for removal by CNTs.

in-depth thermodynamic study indicated that the adsorption is exothermic and follows a pseudo-second order process over two steps: the diffusion of aniline from the aqueous solution to the outer surface of MWCNTs and a rate-determining step in which the aniline molecules diffused through the nanotubes. A new method proposes the synergistic removal of aniline using CNTs and enzymes from Delftia sp. XYJ6, a newly isolated bacterial strain (Yan et al., 2011). In the presence of the enzyme, more aniline was removed by capillary electrophoresis (CE) in the presence of SWCNTs than MWCNTs, and an efficient reaction on the SWCNT surface appeared to play a vital role in the degradation of this compound. By treating MWCNTs with ball milling (to modify their surface properties), the adsorptive capacity of short open-ended MWCNTs for aniline in aqueous solution was increased from 15 to 36% (Sun et al., 2007). SWCNTs have also been applied to the removal of aromatic organics. Rogers et al. (2011) reported the time-dependent aqueous adsorption of 1pyrenebutyric acid (PBA) using SWCNTs and found a maximum adsorption of 0.27 mg·mg−1 at very low PBA concentrations (b1 μg·mL−1). A greater adsorption of organic contaminants has also been demonstrated for oxidized SWCNTs (Moradi et al., 2013a). As a result of their excellent performance with respect to the removal of phenols, amines, and other toxic organics, CNTs are already being considered for broad ranging applications for wastewater treatment (Table 3). The number of studies in this area indicates their potential utility for the removal of possible industrial organic wastes from wastewater. 2.5. Removal of organic contaminants using CNT composites The interstitial spacing formed between bundles of nanotubes and the sidewalls of CNTs are good sites for adsorption. However, the interior of CNTs is not suitable for adsorption because of its smaller size and

close gaps. Grafting of chemical groups onto CNTs can provide additional potential sites for adsorption (Table 4). For example, constructing tailor-made binding sites for the specific adsorption of target molecules via molecular imprinting is becoming an attractive technique in the field of adsorption science. Complicated, costly, and time-consuming filtration and/or centrifugation are usually required to recover CNT adsorbents; however, their recovery from aqueous solution is crucial for applications that remove organic pollutants. Magnetic CNT/iron oxide composites have received considerable attention because they can be easily recovered from aqueous solution by magnetic separation. In particular, magnetiteloaded MWCNTs (M-MWCNTs) have received particular attention, with many studies demonstrating their promise (Hu et al., 2011b). Using M-MWCNTs, MB was successfully removed from aqueous solution in a pH-dependent fashion with a maximum monolayer adsorption capacity of 48.06–48.1 mg·g−1 (Ai et al., 2011; Madrakian et al., 2011; Song et al., 2011). Using M-MWCNTs, high adsorption capacities of 227.7, 36.4, and 250.0 mg·g− 1 were obtained for the removal from aqueous samples of the cationic dyes crystal violet (CV), thionine (Th), and Janus green B (JGB), respectively (Madrakian et al., 2011). Recently, novel M-MWCNT composites were created by functionalizing them with β-cyclodextrin (β-CD), and a maximum adsorption capacity of 200.0 mg·g−1 was demonstrated for the removal of 1-naphthylamine from aqueous solution (Hu et al., 2011a). A novel and facile one-pot method to produce magnetic CNTs (M-CNTs) by modification of as-prepared CNTs using NaOCl resulted in M-CNT hybrids with excellent adsorption properties; maximum adsorption capacities of 107.53 mg·g−1 for neutral red, 101.63 mg·g−1 for MB, 94.34 mg·g−1 for malachite green, 67.57 mg·g−1 for Congo red, and 46.08 mg·g−1 for rhodamine B were demonstrated (Yu et al., 2012b). As a result of their unique physicochemical properties, core–shell nano-structured

Table 2 Removal of pharmaceuticals/drugs using CNTs. Type of CNT Modification method

Type of pharmaceuticals/drugs removed Adsorption effect

SWCNTs

Contribution of SWCNTs to BPA and EE2 ultrafiltration membrane

EE2 adsorption was greater than BPA

MWCNTs MWCNTs MWCNTs MWCNTs

Untreated Untreated Untreated Untreated

Ranged from 95.2 to 97.6% Maximum adsorption capacity of 269.54 mg·g−1 Adsorption efficiency of 99.7% Not provided

MWCNTs

Untreated

Cephalexin Tetracycline (TC) Olaquindox Atenolol, caffeine, diclofenac, and isoproturon Bovine serum albumin (BSA)

Not provided

Ref. Heo et al. (2011), Joseph et al. (2013), Joseph et al. (2011a), Joseph et al. (2011b), and Zaib et al. (2012) Jafari and Aghamiri (2011) Zhang et al. (2011a) Zhang et al. (2011b) Sotelo et al. (2012) Kharlamova et al. (2013) and Moradi et al. (2013b)

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Fig. 3. Chemical structures of diuron and dichlobenil.

imprinted polymers (MIPs) based on magnetic nanoparticles combine the advantageous properties of both materials. Novel core–shell nanostructured estrone-imprinted MWCNT-Est-MIP composites were applied to the removal of low concentrations of estrone from water samples, which showed removal efficiencies ranging from 96.14 to 98.03% (Gao et al., 2011). CNTs and M-CNT composites are both oleophilic and hydrophobic. To improve the dispersion of M-MWCNTs in aqueous solution and increase the adsorption of organics such as dyes by M-MWCNTs, soluble starch covalently grafted MWCNTs have been employed as templates for the preparation of iron oxide nanoparticles (Chang et al., 2011). The prepared M-MWCNT-starch composites exhibited a better second-order rate constant and adsorption for MO and MB at equilibrium than did the M-MWCNTs. Methyl methacrylate (MMA) or chitosan (CS) grafted MWCNT composites have also demonstrated excellent removal efficiency for 4,4′-dichlorinated biphenyl (4,4′-DCB) (Shao et al., 2011a; Shao et al., 2011b). Recently, a facile functionalization process was investigated to modify M-MWCNTs with 3-aminopropyltriethoxysilane (APTS); the MWCNT/Fe3 O4 –NH 2 composites demonstrated better adsorption properties for tetrabromobisphenol A (TBBPA) than for the asprepared M-MWCNTs (Ji et al., 2012). A wet spinning technique was used to produce MWCNT–calcium alginate (CA) composite fibers that demonstrated increased adsorption capacities for MB and MO ionic dyes (Sui et al., 2012a). Because the adsorption performance of CNT composites largely depends on functionalization methods and introduced molecules, additional research should be focused towards achieving effective adsorption for a variety of specific contaminants.

3. Solid-phase extraction (SPE) based on CNTs The high adsorption capacity of CNTs is attributed to their large surface areas (~150–1500 m2·g−1), and their affinity towards both volatile and semi-volatile organic compounds can be achieved through hydrogen bonding interactions, π–π stacking interactions, Van der Waals forces, and hydrophobic interactions (Scida et al., 2011). The adsorption interactions usually occur on the CNT walls and in the interstitial spaces between the tubes. Although some contaminants may become irreversibly bonded to CNT adsorbents (Section 2.2), most of the adsorbed organic compounds can be easily desorbed by a solvent rinse. Thus, CNTs have been employed as an SPE sorbent for preconcentrations of organic compounds. The separation and enrichment of trace organic dyes have been realized with CNTs. For example, tartrazine can be eluted and analyzed with a detection limit of 3.4 g·L−1 using MWCNTs as the stationary phase (Soylak and Cihan, 2013). Carbamate pesticides in foods have been detected at limits ranging from 0.09 to 6.00 ng·g−1 using MWCNTreinforced hollow fiber solid-phase microextraction (MWCNT-HFSPME) combined with high performance liquid chromatographyphotodiode array detection (HPLC-DAD) (Song et al., 2013). Two important plant auxins, indole-3-butyric acid (IBA) and 1-naphthylacetic acid (NAA), could be detected to limits of 0.0030 mg·L−1 and 0.0012 mg·L−1, respectively, using MWCNTs as the adsorbent (Wang et al., 2013). The

combination of CNT-coated SPME fibers with gas chromatography (GC) was used to detect three phthalate esters (PAEs) down to limits of 0.005 μg·L−1 and eight polycyclic aromatic hydrocarbons (PAHs) to limits of ~ 0.001–0.003 μg·L− 1 from aqueous samples (Sun et al., 2013a). Coupling CNT–TiO2 composite grafted stainless steel with GC improved the sensitivity limits (~0.002–0.004 μg·L−1) for seven PAHs (Sun et al., 2013b). Using gatifloxacin (GTFX) as a template, methacrylic acid as a functional monomer, and ethylene glycol dimethacrylate as a cross-linker, MWCNT-MIPs displayed a high adsorption capacity of 192.7 μg·mg−1 towards GTFX. Lastly, a magnetic SPE method combined with HPLC (MSPE-HPLC) was developed and could detect GTFX to a limit of 999.9 μg·mL−1 (Xiao et al., 2013a). This method also exhibited highly efficient isolation, enrichment, and detection of fluoroquinolones (FQs) and sulfonamides with detection limits of ~0.25–0.40 ng·g−1 (Xiao et al., 2013b) and ~1.4–2.8 ng·g−1 (Xu et al., 2013b), respectively. The methods developed since 2010 are fast, sensitive, and effective, making them useful for determining low concentrations of organic compounds in environmental water samples (Pyrzynska, 2011). These characteristics also suggest that CNTs could find widespread use as SPE or SPME sorbents in analytical methods aimed at detecting trace pollutants in water and food. 4. The adsorption mechanism The major mechanisms by which CNTs adsorb organic compounds differ depending on the properties of the compound of interest (e.g., polar vs. nonpolar). The prediction of adsorption of organics on CNTs may not be straightforward (Pan and Xing, 2008), and several possible interactions between organics and CNTs have been proposed. Hydrophobic interactions, π–π stacking interactions, van der Waals forces, electrostatic interactions, and hydrogen bonding interactions might act individually or simultaneously. A study employing an ultrafiltration (UF) membrane coupled with CNTs attributed the greater adsorption of EE2 vs. BPA to its higher hydrophobicity (Heo et al., 2011). Other studies have confirmed electrostatic attraction and π–π stacking interactions between CNTs and adsorbates (Ai et al., 2011). The adsorption of six perfluorinated compounds (PFCs) on CNTs increased with C–F chain length but decreased overall when the CNTs were oxidized, which indicated that hydrophobic interactions play a major role; as the pH increased, lower sorption was attributed to the suppression of electrostatic repulsion (Deng et al., 2012b). The surface functional groups resulting from oxidative modification (hydroxyl and carbonyl groups) can also increase the hydrophilicity of CNTs and result in the increased rejection of hydrophobic organics (Kim et al., 2013). Oxidization would also decrease the aromatic properties of CNTs and, therefore, decrease π–π stacking interactions. Contributions to adsorption are also made by π-electron polarizability and van der Waals forces. Such interactions would be enhanced for phenols or anilines substituted with functional groups such as \NO2, \Cl, or \CH3. Adsorption affinity in the order \NO2 N \Cl N \CH3 indicates that the solutes with hydrogen bonding donors have higher adsorption affinity than those without such donors (Yang et al., 2008). Along these lines, an alkali-activated method was used to prepare activated MWCNTs and resulted in increased specific surface area and pore volume as well as introduced oxygen-containing functional groups on the surface that contributed to the excellent adsorption affinity observed for MO (149 mg·g−1) and MB (399 mg·g−1) (Ma et al., 2012b). Surface-associated metal catalysts might also influence the adsorption ability of CNTs. The adsorption of perfluorooctanoic acid (PFOA) was reported for two commercial MWCNTs: one containing Co/Mn/ Mg/Al catalysts on its outer surface and inner pores and one containing a Fe-based catalyst typically within the tubes (Li et al., 2011a). The adsorption affinity and capacity of the MWCNTs containing the Co/Mn/ Mg/Al catalysts were significantly higher and indicated that the catalyst on the outer surface likely changed the electrophoretic mobility from a negative to positive value, thus increasing the adsorption of PFOA.

Table 3 Removal of phenols, aromatic amines, and other toxic organics using CNTs. Modification method

Type of toxic organic removed

Adsorption effect

Ref.

SWCNTs MWCNTs

Pristine and oxidized Functionalized with iron oxides and β-cyclodextrin (β-CD) Electrochemical assistant

Ethidium bromide 1-Naphthylamine

SWCNTs: 36.10%; oxidized SWCNTs: 38.42% 200.0 mg·g−1

Moradi et al. (2013a) Hu et al. (2011a)

Maximum electrosorption capacity increased 150-fold for PFOA and 94-fold for PFOS Adsorption capacity decreases dramatically with increasing oxygen content

Li et al. (2011a)

26.1–20.8 mg·g−1 Thermally treated MWCNTs showed increased sorption capacity for these compounds (39–97%) Not provided Not provided Not provided 7.643 mg·g−1 1.44 and 4.42 mg·g−1 by SWCNTs and MWCNTs, respectively The presence of surface functional groups affected the adsorption 96.5%

Lou et al. (2011) Ma et al. (2011a)

1-Naphthol

Solution chemistry, such as pH and surface oxygen-containing groups on CNTs, plays a significant role in the adsorption

Wu et al. (2012)

Toluene, ethylbenzene, xylene isomers (TEX)

3% NaOCl-oxidized MWCNTs show the greatest enhancement in TEX adsorption Toluene: 87.12 mg·g−1; ethylbenzene: 322.05 mg·g−1; m-xylene: 247.83 mg·g−1 73.3–98.9%

Yu et al. (2012c) Yu et al. (2012d)

85.1% Up to 95.2%

Mamba et al. (2013) Xu et al. (2013a)

MWCNTs

SWCNTs MWCNTs

Oxidized by concentrated HNO3/H2SO4 (1:3) or 70% sodium hypochlorite solution Untreated Heat-treated

SWCNTs MWCNTs MWCNTs MWCNTs MWCNTs; SWCNTs

Untreated Untreated Untreated Untreated Untreated

Perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS) (Fig. 4) 4-n-Nonylphenol (4-NP), PFOS, PFOA, perfluorooctane sulfonamide (PFOSA), 2,4-dichlorophenoxyacetic acid (2,4-DCPA) (Fig. 4) Dissolved organic matter (DOM) Trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), and 1,3,5-trichlorobenzene (1,3,5-TCB) Polychlorinated biphenyls (PCBs) Trichloroethylene (TCE) Oleic acid Roxarsone 4-Chloro-2-nitrophenol (4C2NP)

SWCNTs

Oxidized

Naphthalene and p-nitrophenol

MWCNTs

Physical mixing MWCNTs and ferrite (NiFe2O4) Purified (P-MWCNTs), graphitized (G-MWCNTs), carboxylated (C-MWCNTs), and hydroxylated (H-MWCNTs). Oxidized using different concentrations of NaOCl solution KOH activated

Aniline

MWCNTs

MWCNTs

MWCNTs MWCNTs MWCNTs

MWCNTs MWCNTs

Fe3O4 nanoparticles grafted carboxyl groups of MWCNTs with poly (dimethyldiallylammonium chloride) (MWCNT-COO–/PDDA-Fe3O4) 4-Aminophenyl methylphosphonate grafted Introduced in nanoscale zero-valent iron (nZVI) synthesis

Toluene, ethylbenzene, m-xylene Six kinds of major toxic polychlorinated biphenyls (PCBs)

4-Chlorophenol 2,4-Dichlorophenol (2,4-DCP)

Li et al. (2011b)

Mahdavian (2011) Naghizadeh et al. (2011) Vadi and Maleki (2011) Hu et al. (2012) Mehrizad et al. (2012) Moradi et al. (2012) Salam et al. (2012)

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Type of CNT

Zeng et al. (2012)

247

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2 7

3

3

2 6

3

2 7

2

2

2

Fig. 4. Chemical structures of PFOS, PFOA, PFOSA, 4-NP, and 2,4-DCPA.

Kubota et al. (2011) studied the adsorption of 1-aminopyrene onto metallic and semiconducting SWCNTs and used the fluorescence properties of the adsorbed 1-aminopyrene to determine the adsorption mechanisms. The 1-aminopyrene could be adsorbed on the oxidized surfaces of SWCNTs via three types of interactions: a π–π interaction (aromatic ring), an interaction of the amino group (N lone pair or H atom of NH2) with the graphene sidewalls of SWCNTs, and a hydrogen bonding interaction with the carboxyl groups on the SWCNTs. Adsorption to the metallic SWCNTs was attributed to the interaction with the amino group and the graphene surface and hydrogen bonding with the carboxyl group. Adsorption to the semiconducting SWNTs was attributed to the π–π interaction and hydrogen bonding with the carboxyl groups. Overall, the metallic SWNTs had a higher affinity for 1-aminopyrene. Chemical functionalization can also introduce hydrophilic/ hydrophobic properties to CNTs. To improve dispersion in aqueous solution, Chang et al. (2011) used soluble starch to modify M-MWCNTs. The increased contact surface between M-MWCNTs and dyes reduced MWCNT aggregates and facilitated the diffusion of dye molecules to MWCNT surfaces. Chen et al. (2011) investigated the effect of lead on the adsorption of diuron and dichlobenil on MWCNTs. Increasing the oxygen content of the MWCNTs resulted in decreased adsorption of diuron and dichlobenil but increased adsorption of lead. The main mechanism identified on the diuron and dichlobenil adsorption was hydrogen bonding, whereas carboxyl groups facilitated the adsorption of lead through inner- and outer-sphere complexes. The presence of lead complexed with oxygen-containing groups resulted in partial occupation of the surface of oxidized MWCNTs and, therefore, reduced diuron and dichlobenil adsorption. Computer simulations have also been employed to determine the mechanisms and thermodynamics of adsorption onto CNTs. Results based on computing the adsorption of cyclohexane, benzene derivatives, and polycyclic aromatic hydrocarbons (PAHs) on SWCNTs using M05-2X density functional theory implicate π–π interactions and an enhancing effect of an \NO2 substituent (Zou et al., 2012). Such simulations may be of increased value in the future for investigating adsorption mechanisms and predicting the adsorption affinity of CNTs for organic compounds. Overall, the adsorption of organic chemicals onto CNTs may involve one or more mechanisms, such as π–π interactions, hydrophobic interactions, hydrogen bonding, electrostatic interactions, and Lewis acid– base interactions. However, there are no standard methods to address the relative contributions of these mechanisms for a given adsorption, which reveals this as an area in need of further investigation.

5. Kinetic, isothermic, and thermodynamic studies Factors affecting the effective adsorption and removal of organic contaminants from CNTs include the pH, temperature, adsorbate concentration, amount of adsorbent, adsorbent particle size, contact time, etc. The nature of adsorption can be characterized by calculating changes in the free energy of adsorption (ΔGθ), enthalpy (ΔHθ), and entropy (ΔSθ).

To understand the surface chemical characteristics and thermodynamics of organic adsorption, Xie et al. (2007) treated CNTs with citric acid, potassium permanganate, or nitric acid prior to evaluating their ability to adsorb aniline. The CNTs treated with citric acid performed favorably at high temperatures. Adsorption isotherm data were fitted using Langmuir and Freundlich models to evaluate the orientation and feasibility of physicochemical adsorptive reactions. Overall, the results showed that both the presence of functional groups and changes in surface area play important roles in aniline adsorption. A wealth of equilibrium data are available in the literature with respect to the adsorption of organic compounds by CNTs. Hosseini et al. (2011) studied the kinetics of the adsorption of MO by MWCNTs using pseudo-first order, pseudo-second order, Elovich, and intraparticle diffusion kinetic models and concluded that the pseudo-second-order kinetic equation was the most appropriate. The removal of arsenazo(ΙΙΙ) (Ghaedi et al., 2011c), ECR (Ghaedi et al., 2011b), MR, ARS (Ghaedi et al., 2011a), and morin (Yao et al., 2011) from aqueous solution using CNTs also adhered to pseudo-second-order kinetics. Recently, a novel carbonbased hybrid, G-CNT, was developed as a new adsorbent for removing MB from aqueous solution, and the kinetics of adsorption also followed the pseudo-second-order kinetic model (Ai and Jiang, 2012). Overall, the current research indicates that the adsorption process is spontaneous and endothermic and follows a pseudo-second-order model with the involvement of an intraparticle diffusion mechanism (Moradi et al., 2013a; Xu et al., 2013a; Yang et al., 2012; Yu et al., 2012d; Zhang et al., 2012). 6. Conclusions SWCNTs exhibit a strong affinity towards many organic compounds because of their very large specific surface areas. However, a disadvantage with respect to the application of SWCNTs in adsorption technologies is their high cost. MWCNTs are less expensive, but their relatively low adsorption capacity limits their potential applications. The modification of MWCNTs can enhance their adsorption capacity for various organics, and they can also be further tuned by selective functionalization. Moreover, surface chemistry, solution chemistry, and properties of adsorbates influence the adsorption process. An improved understanding of the interactive nature of adsorption will provide useful information for the design and preparation of “smart” f-CNTs. The preparation of CNT composites with varying degrees of specificity could also lead to valuable properties such as quick adsorption of targeted pollutants and predictable release of absorbed products. Overall, further studies on a number of basic aspects are required to advance the application of CNTs for the adsorption and removal of organic contaminants. Beyond the direct application of CNTs or CNT composites in adsorption technology is the possibility of coupling techniques for a variety of applications. For example, by dispersing β-CD and silver nanoparticles (Ag NPs) on nitrogen-doped CNTs (N-CNTs), a novel three-component hybrid water purification system was developed and used repeatedly without any leaching or degradation of the components (Mhlanga et al., 2013). Gui et al. (2010) reported on CNTs that self-assembled into a three-dimensional interconnected framework. The CNT sponges were very light, highly porous, and hydrophobic and could be elastically and reversibly deformed into any shape, suggesting that they could be floated on the water surface to adsorb oil films spreading over large areas (Gui et al., 2011). The studies reviewed addressed the inter-relationships between surface properties of the CNTs, chemical structures of the targeted organic adsorbates, and important consequential effects on the adsorption process. To apply CNTs as efficient adsorbents in real environmental applications, the kinetics and thermodynamics of the adsorption must also be well understood. Research to date indicates that the adsorption of organic compounds by CNTs is a spontaneous and physical process. Further evaluation of the adsorption kinetics is required via the application of thermodynamic and kinetic models.

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Table 4 Removal of organic contaminants using CNT-composites. Type of CNT

Modification method

Organic contaminants removed

Adsorption effect

Ref.

MWCNTs MWCNTs MWCNTs MWCNTs

Impregnation with chitosan hydrogel beads Grafted with methyl methacrylate (MMA) Grafted with chitosan (CS) M-MWCNTs modified with APTS to obtain MWCNT/Fe3O4–NH2 M-MWCNT–Fe3C nanocomposite (M-MWCNT-ICN) Copper and silver nanoparticles were immobilized on CNTs, which were then embedded in water-insoluble CD polyurethane polymers MWCNT-activated carbon fabric composites Grafted with guar gum (GG) and then iron oxide nanoparticles were synthesized on the GG-MWCNTs (GG-MWCNT–Fe3O4) Grafted with polyacrylamide (PAAM) Decorated with CoFe2O4 nanoparticles

Congo red 4,4′-DCB 4,4′-DCB TBBPA

Chatterjee et al. (2010) Shao et al. (2011a) Shao et al. (2011b) Ji et al. (2012)

Direct red 23 para-Nitrophenol

450.4 mg·g−1 Maximum adsorption N90% Not provided MWCNT/Fe3O4–NH2 performed better than M-MWCNTs for adsorption of TBBPA 85.5 mg·g−1 Maximum adsorption of 55%

Phenol Neutral red (NR), MB

High regeneration efficiency of phenol (~79.1%) MB: 61.92 mg·g−1; NR: 89.85 mg·g−1

Wang and Yang (2012) Yan et al. (2012)

Humic acid, HA MB

Not provided 11.8 mg·g−1

Yang et al. (2011) Farghali et al. (2012)

MWCNTs MWCNTs

MWCNTs MWCNTs

MWCNTs MWCNTs

Lastly, disposal and penetration of CNTs into the environment are also causes for increasing concerns (Weng et al., 2013). Inconsistent data on the cytotoxicity and environmental toxicity of CNTs limits their wider application (Beg et al., 2011; Dolatabadi et al., 2011; Hwang et al., 2013; Voge and Stegemann, 2011; Wu et al., 2010; Zhang et al., 2010), and coordinated research on effective recycling technologies should be pursued. Heat treatment and desorption treatment of adsorbing materials are two methods with considerable promise. CNT-based membranes, such as the ultra-long version exploited by Yang et al. (2013a) for specific adsorption of organic pollutants and metal ion contaminants, represent a third potential choice. Indeed, CNT-based membranes are expected to be the next-generation of reusable and eco-friendly adsorbing materials because of their relative ease of cleaning and recycling. Conflicts of interest None. Acknowledgments The financial support received from the National Natural Science Foundation of China (Nos. 21201181, 21276282), Research Fund for the Doctoral Program of Higher Education of China (No. 20110162120070), China Postdoctoral Science Foundation (No. 2013M542107), Hunan Provincial Natural Science Foundation of China (No. 14JJ2006), Open Fund of Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education (No. 2012-KF-04) and Key Project of Philosophy and Social Sciences Research, Ministry of Education, PRC (No. 13JZD0016, Research on Institutional Building of Ecological Civilization) is greatly appreciated. References Aditya D, Rohan P, Suresh G. Nano-adsorbents for wastewater treatment: a review. Res J Chem Environ 2011;15:1033–40. Ai LH, Jiang J. Removal of methylene blue from aqueous solution with self-assembled cylindrical graphene-carbon nanotube hybrid. Chem Eng J 2012;192:156–63. Ai LH, Zhang CY, Liao F, Wang Y, Li M, Meng LY, et al. Removal of methylene blue from aqueous solution with magnetite loaded multi-wall carbon nanotube: kinetic, isotherm and mechanism analysis. J Hazard Mater 2011;198:282–90. Alexander JT, Hai FI, Al-aboud TM. Chemical coagulation-based processes for trace organic contaminant removal: current state and future potential. J Environ Manage 2012; 111:195–207. Al-Johani H, Salam MA. Kinetics and thermodynamic study of aniline adsorption by multiwalled carbon nanotubes from aqueous solution. J Colloid Interface Sci 2011;360:760–7. Atul WV, Gaikwad GS, Dhonde MG, Khaty NT, Thakare SR. Removal of organic pollutant from water by heterogenous photocatalysis: a review. Res J Chem Environ 2013;17: 84–94. Bahgat M, Farghali AA, El Rouby WMA, Khedr MH. Synthesis and modification of multiwalled carbon nano-tubes (MWCNTs) for water treatment applications. J Anal Appl Pyrol 2011;92:307–13.

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