nano tube vs organik nano

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Review Adsorption of synthetic organic contaminants by carbon nanotubes: A critical review Onur Guven Apul, Tanju Karanfil * Department of Environmental Engineering and Earth Sciences, Clemson University, 342 Computer Court, Anderson, SC 29625, United States article info Article history: Received 27 May 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 5 October 2014 Keywords: Adsorption Carbon nanotube Nanoparticle Predictive model Sorption Synthetic organic compound abstract In last ten years, a large number (80 þ ) of articles regarding aqueous phase adsorption of a variety of synthetic organic compound (SOC) by CNTs were published in peer-reviewed journals. Adsorption depends upon the physicochemical properties of the adsorbates and CNTs as well as the background water chemistry. Among all properties reported in the literature, no parameter was reported as solely controlling SOC adsorption by CNTs. In this article, these contributing parameters were reviewed and the associated explanations were discussed. This comprehensive literature survey provides (i) a thorough CNT character- ization summary, (ii) a discussion of adsorption mechanisms of SOCs by CNTs and (iii) a summary of the statistical adsorption model development efforts. It also includes dis- cussions of agreements and differences in the literature, and identifies some research needs. © 2014 Elsevier Ltd. All rights reserved. Contents 1. Introduction ................................................................................................. 35 2. General features of SOC adsorption by CNTs .................................................................... 35 3. Characterization of CNTs ..................................................................................... 36 4. Adsorption of SOCs by CNTs .................................................................................. 42 4.1. Influence of CNT properties ............................................................................. 42 4.2. Influence of SOC properties ............................................................................. 42 4.3. Influence of background solution properties .............................................................. 45 5. Predictive models for adsorption of SOCs by CNTs .............................................................. 47 6. Comparison of carbon nanotubes with activated carbon ......................................................... 49 7. Future research needs ........................................................................................ 50 8. Conclusions .................................................................................................. 51 Acknowledgments ............................................................................................ 51 * Corresponding author. Tel.: þ1 864 656 1005; fax: þ1 864 656 0672. E-mail address: [email protected] (T. Karanfil). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research 68 (2015) 34 e55 http://dx.doi.org/10.1016/j.watres.2014.09.032 0043-1354/© 2014 Elsevier Ltd. All rights reserved.

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Page 1: Nano Tube vs Organik nano

ww.sciencedirect.com

wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5

Available online at w

ScienceDirect

journal homepage: www.elsevier .com/locate /watres

Review

Adsorption of synthetic organic contaminants bycarbon nanotubes: A critical review

Onur Guven Apul, Tanju Karanfil*

Department of Environmental Engineering and Earth Sciences, Clemson University, 342 Computer Court, Anderson,

SC 29625, United States

a r t i c l e i n f o

Article history:

Received 27 May 2014

Received in revised form

23 September 2014

Accepted 24 September 2014

Available online 5 October 2014

Keywords:

Adsorption

Carbon nanotube

Nanoparticle

Predictive model

Sorption

Synthetic organic compound

* Corresponding author. Tel.: þ1 864 656 100E-mail address: [email protected] (T.

http://dx.doi.org/10.1016/j.watres.2014.09.0320043-1354/© 2014 Elsevier Ltd. All rights rese

a b s t r a c t

In last ten years, a large number (80þ) of articles regarding aqueous phase adsorption of a

variety of synthetic organic compound (SOC) by CNTs were published in peer-reviewed

journals. Adsorption depends upon the physicochemical properties of the adsorbates

and CNTs as well as the background water chemistry. Among all properties reported in the

literature, no parameter was reported as solely controlling SOC adsorption by CNTs. In this

article, these contributing parameters were reviewed and the associated explanations were

discussed. This comprehensive literature survey provides (i) a thorough CNT character-

ization summary, (ii) a discussion of adsorption mechanisms of SOCs by CNTs and (iii) a

summary of the statistical adsorption model development efforts. It also includes dis-

cussions of agreements and differences in the literature, and identifies some research

needs.

© 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2. General features of SOC adsorption by CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3. Characterization of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4. Adsorption of SOCs by CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.1. Influence of CNT properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2. Influence of SOC properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.3. Influence of background solution properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5. Predictive models for adsorption of SOCs by CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

6. Comparison of carbon nanotubes with activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

7. Future research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5; fax: þ1 864 656 0672.Karanfil).

rved.

Page 2: Nano Tube vs Organik nano

wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 35

Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

1. Introduction

Graphitic carbon is sp2 hybridized solid phase of pure carbon

where three of the four valance electrons are covalently

shared in a two-dimensional plane and the fourth valance

electron is delocalized among all atoms present as a weak p

bond in the third dimension (Ajayan, 1999; Terrones, 2003,

2004). Carbon nanotubes (CNTs) can be visualized as

graphitic carbon sheets rolled into hollow cylinders with

nanometer scale diameters and micrometer scale lengths

(Terrones, 2003, 2004; Iijima, 1991). There are two types of

CNTs: single-walled (SWCNT) and multi-walled (MWCNT)

(Ajayan, 1999). Owing to their unique properties, the produc-

tion and use of CNTs has been growing rapidly (Lam et al.,

2006). The CNT market estimates were approximately 90.5

million dollars in 2010, and global revenues are projected to

exceed 1 billion dollars by 2015 (The Global Market for Car,

2010). CNTs are now synthesized at larger scales (i.e. more

than 3000 metric tons/year), and used in many electronic,

medical, space and military applications (Klaine et al., 2008;

Mauter and Elimelech, 2008; Keller et al., 2013). In addition,

superior hydrophobicity, high specific surface area, and hol-

low and layered structures of CNTs make them also particu-

larly promising adsorbents (Upadhyayula et al., 2009; Zhang

et al., 2011). Adsorption of many compounds in water such

as various classes of synthetic organic contaminants (Gotovac

et al., 2006, 2007a, 2007b; Yang et al., 2006a; Yang et al., 2006b;

Cho et al., 2008; Wang et al., 2008; Chen et al., 2007; Lin and

Xing, 2008a; Oleszczuk et al., 2009; Ji et al., 2009a; Gupta

et al., 2013; Pyrzynska et al., 2007) (see Table S1 in

Supplementary Information), natural organic matter (NOM)

(Su and Lu, 2007; Yang and Xing, 2009; Ajmani et al., 2014;

Smith et al., 2012), and metallic ions (Li et al., 2005, 2003; Rao

et al., 2007) by CNTs has been widely reported.

From a natural system perspective, the remarkable in-

crease in production and use of CNTs raises health and envi-

ronmental concerns, particularly upon release into the

environment (Lam et al., 2006; Powell and Kanarek, 2006;

Johnston et al., 2010; Petosa et al., 2010). CNTs may enter the

environment through either intentional or unintentional re-

leases (e.g. atmospheric emissions and solid or liquid waste

streams) from production facilities, causing damage to plant

and animal life at the cellular level (Lam et al., 2006; Klaine

et al., 2008; Petosa et al., 2010). The toxicity of CNTs may

also be enhanced by the adsorbed organic contaminants in

the environment (Ferguson et al., 2008; Xia et al., 2010a; Yang

and Xing, 2007). Therefore, it is of critical importance to un-

derstand adsorption of SOCs by CNTs to adequately assess

their environmental impact. Froman engineering perspective,

understanding the adsorption of SOC by CNTs is also impor-

tant for evaluating the feasibility of using CNTs as alternative

adsorbents to conventional activated carbons in water and

wastewater treatment systems.

The main objective of this article was to critically review

the published peer-reviewed literature, and to articulate some

future research needs on the aqueous phase adsorption of

SOCs by CNTs with (i) a thorough CNT characterization sum-

mary, (ii) a discussion of adsorption mechanisms of SOCs by

CNTs and (iii) a summary of the statistical adsorption model

development efforts.

2. General features of SOC adsorption byCNTs

Adsorption of SOCs from water by activated carbon and other

porous carbon materials has been studied for several decades

(Dobbs and Cohen, 1980; Giusti et al., 1974; Moreno-Castilla,

2004; Kutics and Suzuki, 1993). On the other hand, adsorp-

tion of SOCs by CNTs has been rapidly growing (Iijima, 1991;

Yang et al., 2006b). More than 80 articles have been pub-

lished in peer-reviewed journals on SOC adsorption by CNTs

over the last ten years. These studies included the adsorption

of a multitude of organic contaminants by CNTs: polycyclic

aromatic hydrocarbons (PAHs) (Yang et al., 2006a, 2006b; Yang

and Xing, 2007; Zhang et al., 2010a), benzene derivatives

(Wang et al., 2008; Chen et al., 2007, 2008a; Peng et al., 2003),

phenolic compounds (Lin and Xing, 2008a; Shen et al., 2009;

Yang et al., 2008; Salam and Burk, 2008; Chen et al., 2009a),

pharmaceuticals (Chen et al., 2011; Ji et al., 2010a; Peng et al.,

2012; Yang et al., 2012; Zhang et al., 2010b; Fu et al., 2011; Ji

et al., 2010b; Cai and Larese-Casanova, 2014; Li et al., 2014;

Yu et al., 2014), polychlorinated biphenyls (Velzeboer et al.,

2014; Beless et al., 2014), dialkyl phthalate esters (Wang

et al., 2014), proteins (Kharlamova et al., 2013), insecticides

(Peng et al., 2009), herbicides (Pyrzynska et al., 2007; Chen

et al., 2009b), organic dyes (Gupta et al., 2013), aliphatics (Lu

et al., 2006; Wang et al., 2010a) and dioxin (Long and Yang,

2001).

Multiplemechanisms, of varying relative importance, have

been proposed to control the adsorption of SOCs by CNTs. The

quantification of these individual contributions is a chal-

lenging task, and it has yet to be addressed in any significant

fashion in the current CNT adsorption literature. However,

some previous studies indicated that certain parameters are

more predominant in controlling adsorption than others.

According to Yang et al. (2006b), Kow of SOCs were strongly

correlated with adsorption capacity of CNTs. Evidently, hy-

drophobic driving forces play important roles; however they

cannot completely explain adsorption. Electrostatic in-

teractions, pep interactions and hydrogen bonding also in-

fluence adsorption interactions considerably (Chen et al.,

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 536

2007; Wang et al., 2014). According to the mechanism pro-

posed by Chen et al. (2007), p-electron donor and acceptor

interactions influence adsorption of aromatic SOCs by CNTs.

Similarly, among CNT parameters, specific surface area was

reported to be a controlling parameter of adsorption capacity,

though not the only controlling factor (Zhang et al., 2010a).

Pore volume, pore size distribution and functional groups of

CNTs were also influential on adsorption. As a result,

describing SOC adsorption by CNTs may require considering

multiple adsorption mechanisms and factors (Yang et al.,

2006b; Cho et al., 2008; Wang et al., 2008; Chen et al., 2007;

Lin and Xing, 2008a).

In addition to the already complicated nature of SOC

adsorption, CNTs are prone to aggregation. Aggregation

(homo- and hetero-) is a characteristic that differentiate CNTs

from other carbonaceous adsorbents, further complicating

the adsorption properties. According to Saleh et al. (2008),

CNTs exhibited DerjaguineLandaueVerweyeOverbeek

(DLVO) interactions that are predominantly similar to most

other aqueous colloidal particles. Aggregation may reduce the

surface area during the formation of interstitial channels be-

tweennanotubes and grooves on the periphery of the bundles.

The outermost surface, interstitial channels, inner cavities

and grooves are the four proposed adsorption sites for CNT

bundles (Fig. 1) (Agnihotri et al., 2006, 2008). The SOCs first

attach to high-energy adsorption sites and then spread to

lower energy sites among all available sorption sites

(Agnihotri et al., 2008).

In natural waters, the behavior of CNTs and the in-

teractions between organic compounds and CNTs are further

complicated by the presence of natural organic matter (NOM)

(Zhang et al., 2011). The suspension stability of CNTs in

aqueous solutions may be improved by the presence of NOM

due to the increased electrostatic and steric repulsion among

NOM-coated nanotubes, while NOM molecules compete with

Fig. 1 e Schematic representation of a typical CNT bundle

and its adsorption sites (1) inner cavities, (2) interstitial

channels, (3) external grooves, (4) outermost surface

(Agnihotri et al., 2006).

SOC molecules for adsorption sites (Long and Yang, 2001; Liu

et al., 2014).

The aqueous phase adsorption of SOCs by CNTs depends

on the physicochemical properties of the adsorbate and CNTs

as well as the background water chemistry (Ma et al., 2011). In

order to simplify the complicated nature of intermolecular

adsorption interactions, to gain amore comprehensive insight

into the adsorption mechanism, understand the interactions

of CNTs and SOCs and systemize the available literature, the

influence of adsorbent (CNT), adsorbate (SOC) and background

solution were investigated separately, as detailed in the

following sections. A literature review of the characteristics of

CNTs is also provided below prior to the review of adsorption

mechanisms.

3. Characterization of CNTs

Adsorption of organic contaminants is influenced by both CNT

morphology and surface chemistry. To examine the CNT

properties, a comprehensive literature survey was conducted

and tabulated in Table 1. The compiled information indicated

that the properties of two types of CNTs (i.e. SWCNT and

MWCNT) varied significantly in literature. In the 57 journal

articles reviewed, adsorption of SOCs was investigated using

18 SWCNTs and 81 MWCNTs as adsorbents. Five studies,

however, did not report the type of CNT used. The abundance

of MWCNT in these adsorption studies was likely due to the

higher market prices of SWCNTs than MWCNTs. A compila-

tion of prices in 2013 from 44 commercially available lab grade

CNT products revealed that the average market price for

MWCNTs was approximately 10 $/g, and the average price for

SWCNTswas higher than 100 $/g. However, it should be noted

that obtaining cheaper SWCNTs andMWCNTs (e.g., 45 $/g and

1.5 $/g, respectively) is possible depending on the purity and

properties sought (outer diameter, oxygen content etc.). In

2008, Cho et al. reported the average price of MWCNT as 140

$/g, which shows the remarkable decrease of its price

recently. In the articles reviewed, approximately 30 different

CNTswere synthesized in the lab, whereas remaining 75 CNTs

were obtained frommanufacturers, indicating thewidespread

use of commercially available CNTs. It should be noted that

the activated carbon is available for less than 5 $/kg (Babel and

Kurniawan, 2003), thus the price of CNTs should significantly

decrease to be competitive with activated carbons.

The most common CNT characteristics reported in the

adsorption literature were specific surface area (SSA), pore

volume (PV), pore size distribution (PSD), purity, elemental

analysis (EA) and morphological information such as length,

the inner and outer diameter and the number of walls and

types of functional groups. The mean SSA of SWCNTs and

MWCNTswere 373 ± 146m2/g and 216 ± 159m2/g, respectively

(calculated from the data tabulated in Table 1). Considering

various manufacturing, purification, and surface modification

techniques, high standard deviations of SSA are reasonable.

The theoretical SSA of SWCNTs with open ends was calcu-

lated as 2630 m2/g (Peigney et al., 2001). The theoretical SSA

calculations assumed that CNTs were composed of perfect

sheets of carbons that covalently formhexagonal arrays. If the

ends were closed, the inner cavity of the tube would be

Page 4: Nano Tube vs Organik nano

Table 1 e Summary of CNTs employed for SOC adsorption in the literature, their characteristics and characterization methods.

Source Adsorbentname

Supplier Pretreat. Surface area Pore volume Inner diameter Outer diameter Length Purity

Value(m2/g)

Mthd Value(cm3/g)

Mthd Value(nm)

Mthd Value(nm)

Mthd Value(mm)

Mthd Value (%) Mthd

Yang et al. (2006b); Yang

and Xing (2009)

MWCNT 8 Chengdu, China HNO3, H2SO4 348 BET 0.816 BET 2e3 TEM <8 TEM 10e50 TEM 95þ TGA

Yang et al. (2006a); Yang

et al. (2006b); Yang and

Xing (2009); Shen et al.

(2009); Wang et al. (2014);

Tournus et al. (2005)

MWCNT 15 Chengdu, China HNO3, H2SO4 174 BET 0.665 BET 3e5 TEM 8e15 TEM 10e50 TEM 95þ TGA

Yang et al. (2006b); Yang

and Xing (2009); Wang

et al. (2014)

MWCNT 30 Chengdu, China HNO3, H2SO4 107 BET 0.317 BET 5e15 TEM 15e30 TEM 10e50 TEM 95þ TGA

Yang et al. (2006b); Yang

and Xing (2009); Wang

et al. (2014); Yan et al.

(2008)

MWCNT 50 Chengdu, China HNO3, H2SO4 95 BET 0.253 BET 5e15 TEM 30e50 TEM 10e50 TEM 95þ TGA

Lin and Xing (2008a);

Oleszczuk et al. (2009);

Peng et al. (2009); Yan

et al. (2008)

MWCNT 10 Shenzen, China HNO3, H2SO4 357 BET 1.093 BET NR e 9.4 (10) TEM (MC) 1e2 TEM 95þ MC

Oleszczuk et al. (2009); Peng

et al. (2009); Yan et al.

(2008); Wu (2007a)

MWCNT 20 Shenzen, China HNO3, H2SO4 126 BET 0.415 BET NR e 21 (20) TEM (MC) 1e2 TEM 95þ MC

Wang et al. (2008);

Oleszczuk et al. (2009);

Peng et al. (2009); Yan

et al. (2008); Wu (2007a);

Apul et al. (2013b)

MWCNT 40 Shenzen, China HNO3, H2SO4 86 BET 0.319 BET NR e 28 (40) TEM (MC) 1e2 TEM 95þ MC

Oleszczuk et al. (2009); Yan

et al. (2008); Wu (2007a)

MWCNT 60 Shenzen, China HNO3, H2SO4 73 BET 0.191 BET NR e 43 (60) TEM (MC) 1e2 TEM 95þ MC

Lin and Xing (2008a);

Oleszczuk et al. (2009);

Agnihotri et al. (2008); Wu

(2007a)

MWCNT 100 Shenzen, China HNO3, H2SO4 58 BET 0.137 BET NR e 70 (100) TEM (MC) 1e2 TEM 95þ MC

Peigney et al. (2001) MWCNT-P NAM, USA e 164 BET 0.664 BET 3e5 MC 8e15 MC 10e50 MC 89.9 TGA

Kilduff et al. (1996) MWCNT-SD NAM, USA e 178 BET 0.848 BET 5e10 MC 10e20 MC 10e30 MC 99.1 TGA

Kilduff et al. (1996) MWCNT-MD NAM, USA e 127 BET 0.722 BET 5e10 MC 20e30 MC 10e30 MC 97.1 TGA

Kilduff et al. (1996) MWCNT-LD NAM, USA e 157 BET 0.702 BET 5e15 MC 30e50 MC 10e20 MC 98.6 TGA

Kilduff et al. (1996) MWCNT-SL Nanoshel, USA e 163 BET 0.728 BET NR e 4e12 MC 3e10 MC 95.0 TGA

Kilduff et al. (1996) MWCNT-ML Nanoshel, USA e 80 BET 0.367 BET NR e 4e12 MC 5e15 MC 99.2 TGA

Kilduff et al. (1996) MWCNT-LL Nanoshel, USA e 301 BET 0.978 BET NR e 4e12 MC 15e30 MC 95.3 TGA

Peigney et al. (2001) MWCNTeOH NAM, USA e 192 BET 0.765 BET 3e5 MC 8e15 MC 10e50 MC 95þ MC

Peigney et al. (2001) MWCNT

eCOOH

NAM, USA e 134 BET 0.589 BET 3e5 MC 8e15 MC 10e50 MC 95þ MC

(continued on next page)

water

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Table 1 e (continued )

Source Adsorbentname

Supplier Pretreat. Surface area Pore volume Inner diameter Outer diameter Length Purity

Value(m2/g)

Mthd Value(cm3/g)

Mthd Value(nm)

Mthd Value(nm)

Mthd Value(mm)

Mthd Value (%) Mthd

Kilduff et al. (1996) MWCNT-S Cheap Tube, USA e 192 BET 0.925 BET 3e5 MC 8e15 MC 0.5e2 MC 97.8 TGA

Kharlamova et al. (2013) MWCNT Shenzen, China HNO3,H2SO,

Ultrasound

295 BET NR e 5e10 MC 10e30 MC 0.5e500 MC 98.9 TGA

Chen et al. (2007); Ji et al.

(2009a); Ward and Getzen

(1970)

MWCNT 1030 Shenzen, China Heat, NaClO,

Ultrasound

148 BET 0.240 BET NR e 10e30 MC 5e15 MC 95 MC

Chen et al. (2007) MWCNT 4060 Shenzen, China Heat, NaClO,

Ultrasound

74 BET 0.104 BET NR e 40e60 MC 5e15 MC 95 MC

Yu et al. (2007) r-MWCNT Shenzen, China e 300 BET 0.793 BJH NR e 20e40 MC <10 MC 97.9 MC

Wang et al. (2008) MWCNT 3.3 Nanolab, USA e 283 BET NR e 7 MC 15 MC 1e5 AFM 95þ MC

Wang et al. (2008) MWCNT Ox. Lab Modified e 287 BET NR e 7 MC 15 MC 1e5 AFM 95 MC

Pyrzynska et al. (2007) MWCNT Aldrich 40e600 MC NR e NR e 10e30 MC NR e 95þ MC

Salam and Burk (2008) MWCNT 8 Chengdu, China HNO3 559 BET 1.775 BET 2e5 TEM <8 TEM 10e30 TEM 95þ NR

Salam and Burk (2008) MWCNT 15 Chengdu, China HNO3 181 BET 0.426 BET 5e8 TEM 10e15 TEM 10e30 TEM 90þ NR

Salam and Burk (2008) MWCNT 20 Chengdu, China HNO3 167 BET 0.566 BET 5e10 TEM 10e20 TEM 10e30 TEM 95þ NR

Salam and Burk (2008) MWCNT 30 Chengdu, China HNO3 91 BET 0.300 BET 5e15 TEM 30e50 TEM 10e20 TEM 95þ NR

Salam and Burk (2008) MWCNT 50 Chengdu, China HNO3 68 BET 0.160 BET 5e10 TEM >50 TEM 10e20 TEM 95þ NR

Wang et al. (2014) MWCNT 0.85 Chengdu, China e 167 BET NR e NR e 10e20 MC NR e NR e

Wang et al. (2014) MWCNT 2.16 Chengdu, China e 185 BET NR e NR e 10e20 MC NR e NR e

Wang et al. (2014) MWCNT 7.07 Chengdu, China e 178 BET NR e NR e 10e20 MC NR e NR e

Chen et al. (2008a) MWCNT Lab Synthesized e 160 BET 0.652 BJH NR e NR e NR e 93 EA

Chen et al. (2008a) MWCNT Ox. Lab Synthesized e 274 BET 0.382 BJH NR e NR e NR e 93 EA

Yang et al. (2008) MWCNT Sun Nanotech, China e 148 BET NR e NR e 100e200 SEM NR e NR e

Yang et al. (2008) MWCNT H2O2 Lab Synthesized 144 BET NR e NR e NR e NR e NR e

Yang et al. (2008) MWCNT

HNO3

Lab Synthesized 140 BET NR e NR e NR e NR e NR e

Coughlin and Ezra (1968) MWCNT

PD15L1-5

Nanolab, USA e 200 MC NR e NR e 15 MC 1e5 MC 95 TGA

Coughlin and Ezra (1968) MWCNT

PD30L1-5

Nanolab, USA e 200 MC NR e NR e 30 MC 1e5 MC 95 TGA

Coughlin and Ezra (1968) MWCNT

PD30L5-20

Nanolab, USA e 200 MC NR e NR e 30 MC 5e20 MC 95 TGA

Chen et al. (2008b) MWCNT Chengdu, China HCl 167 BET 1.08 BET 5e10 MC 10e20 MC 10e30 MC 95þChen et al. (2008b) MWCNT

eCOOH

Chengdu, China HCl 187 BET 1.08 BET 5e10 MC 10e20 MC 10e30 MC 95þ

Apul et al. (2012) MWCNT 3.84 Chengdu, China HCl 563 BET 0.780 BET <5 MC <8 MC 10e30 TEM NR e

Apul et al. (2012) MWCNT 10.08 Lab Modified e 520 BET 0.568 BET NR e NR e <3 TEM NR e

Apul et al. (2012) MWCNT 18.01 Lab Modified e 384 BET 0.333 BET NR e NR e <0.5 TEM NR e

Apul et al. (2012) MWCNT 22.8 Lab Modified e 143 BET 0.131 BET RN e NR e <0.3 TEM NR e

Ji et al. (2009b) O-MWCNT Shenzen, China HCl 114 BET NR e NR e 10e30 MC 5e15 MC 95þ MC

Datsyuk et al. (2008) MWCNT 10 Chengdu, China e 176 BET 1.748 BJH NR e 10e20 MC NR e 95þ MC

Datsyuk et al. (2008) MWCNT 20 Chengdu, China e 130 BET 1.169 BJH NR e 20e30 MC NR e 95þ MC

Datsyuk et al. (2008) MWCNT 50 Chengdu, China e 77 BET 0.696 BJH NR e >50 MC NR e 95þ MC

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Datsyuk et al. (2008) F-MWCNT 10 Chengdu, China e 174 BET 1.366 BJH NR e 10e20 MC NR e 95þ MC

Datsyuk et al. (2008) F-MWCNT 20 Chengdu, China e 123 BET 1.060 BJH NR e 20e30 MC NR e 95þ MC

Datsyuk et al. (2008) F-MWCNT 50 Chengdu, China e 77 BET 0.587 BJH NR e >50 MC NR e 95þ MC

Ji et al. (2009c) MWCNT Shenzen, China e 52 BET 0.16 BET NR e 20e40 MC NR e NR e

Wang et al. (2010b) MWCNT 2.0 Lab Synthesized Heat, Air,

HNO3

471 BET 0.64 BET NR e NR e NR e NR e

Wang et al. (2010b) MWCNT 3.2 Lab Synthesized Heat, Air,

HNO3

381 BET 0.58 BET NR e NR e NR e NR e

Wang et al. (2010b) MWCNT 4.7 Lab Synthesized Heat, Air,

HNO3

382 BET 0.58 BET NR e NR e NR e NR e

Wang et al. (2010b) MWCNT 5.9 Lab Synthesized Heat, Air,

HNO3

327 BET 0.49 BET NR e NR e NR e NR e

Ward and Getzen (1970) MWCNT in Nanothinx, Greece Ultrasound,

HNO3

525 BET NR e NR e 12e31 MC >10 MC 97 MC

Ward and Getzen (1970) MWCNT 500 Lab Modified e 320 BET NR e NR e NR e NR e NR e

Wang et al. (2007) MWCNT MH Chengdu, China Hydoxylized 228 BET NR e NR e NR e NR e 94 EA

Wang et al. (2007) MWCNT MC Chengdu, China Carboxylized 164 BET NR e NR e NR e NR e 97 EA

Wang et al. (2007) MWCNT MG Chengdu, China Graphitized 117 BET NR e NR e NR e NR e 98 EA

Zhang et al. (2012) MWCNT 800 Lab Modified e 263 BET NR e NR e NR e NR e NR e

Zhang et al. (2012) MWCNT 1200 Lab Modified e 245 BET NR e NR e NR e NR e NR e

Material Genome Initiativ,

2014)

MWCNT Shenzen, China HCl NR e NR e NR e 20e30 MC NR e NR e

Ghaedi et al. (2012a) MWCNT R.I.P.I., Iran e 280 NR NR e NR e <10 MC 5e15 MC 95þ MC

Yang et al. (2010); Chen et al.

(2008c)

MWCNT Lab Synthesized e 82 BET 1.07 BET 5e10 TEM 20e80 TEM 5e15 98þ

Rambabu et al. (2012);

Oleszczuk and Xing (2011)

MWCNT Merck, Germany HCl NR e NR e NR e NR e NR e NR e

Shi et al. (2010) MWCNT

eCOOH

Lab Modified HCl 97 NR NR e NR e NR e NR e NR e

Kotel et al. (2009); Liu et al.

(2008)

MWCNT Lab Synthesized HCl 181 BET 0.345 BET NR e 3e40 TEM NR e 95þ FTIR

Ghaedi et al. (2011) f-MWCNT Lab Synthesized Air, Acid 92 BET 0.22 BET 5e10 TEM, SEM 40e50 TEM, SEM NR e NR e

Wu (2007b) MWCNT Sun Nanotech,

China

HCl, HNO3 162 BET NR e NR e 20e50 SEM 20e50 SEM NR e

Ghaedi et al. (2012b) MWCNT Lab Synthesized e NR e NR e NR e 20 NR NR e NR e

Ghaedi et al. (2012b) O-MWCNT Lab Synthesized NR e NR e NR e 20 NR NR e NR e

Chen et al. (2011) P-SWCNT Shenzen, China e 411 BET NR e NR e <2 MC 5e15 MC 90þ MC

Chen et al. (2011) P-MWCNT Shenzen, China e 157 BET NR e NR e 10e30 MC 5e15 MC 95þ MC

Chen et al. (2011) K-SWCNT Shenzen, China e 653 BET NR e NR e NR e NR e NR e

Chen et al. (2011) K-MWCNT Shenzen, China e 423 BET NR e NR e NR e NR e NR e

Machado et al. (2012) MWCNT NR e 157 BET NR e NR e NR e NR e NR e

Machado et al. (2012) NH3-MWCNT Lab Modified e 195 BET NR e NR e NR e NR e NR e

Machado et al. (2012) HNO3-

MWCNT

Lab Modified e 152 BET NR e NR e NR e NR e NR e

Yang et al. (2006b); Wang

et al. (2014); Peng et al.

(2009)

SWCNT Chengdu, China HNO3, H2SO4 541 BET 1.021 BET 0.8e1.6 TEM 1e2 TEM 10e50 TEM 90þ TGA

Peigney et al. (2001)) SWCNT-P Cheap Tube, USA e 486 BET 0.722 BET 0.8e1.6 MC 1e2 MC 5e30 MC 88.6 TGA

(continued on next page)

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Table 1 e (continued )

Source Adsorbentname

Supplier Pretreat. Surface area Pore volume Inner diameter Outer diameter Length Purity

Value(m2/g)

Mthd Value(cm3/g)

Mthd Value(nm)

Mthd Value(nm)

Mthd Value(mm)

Mthd Value (%) Mthd

Kilduff et al. (1996) SWCNT-L Cheap Tube, USA e 442 BET 0.917 BET 0.8e1.6 MC 1e2 MC 5e30 MC 96.0 TGA

Kilduff et al. (1996) SWCNT-S Chengdu, China e 413 BET 0.613 BET 0.8e1.6 MC 1e2 MC 0.5e2 MC 97.3 TGA

Peigney et al. (2001) SWCNTeOH Chengdu, China e 420 BET 0.739 BET 0.8e1.6 MC 1e2 MC 5e30 MC 91.1 TGA

Peigney et al. (2001) SWCNT

eCOOH

Chengdu, China e 386 BET 0.680 BET 0.8e1.6 MC 1e2 MC 5e30 MC 95.9 TGA

Chen et al. (2007); Ji et al.

(2009a); Peng et al. (2003)

SWCNT Shenzen, China Heat, NaClO,

Ultrasound

370 BET 0.425 BET NR e <2 MC 5e15 MC 90 MC

Yu et al. (2007) SWCNT 20 Shenzen, China e 167 BET 0.417 BJH NR e 10e20 MC 1e2 MC 95þ MC

Yan et al. (2008) SWCNT Shenzen, China HNO3, H2SO4 541 BET 1.021 BET NR e 1.4 (1e2) TEM (MC) 5e15 TEM 90 þ 50þ MC

Kato et al. (2008) SWCNT Shenzen, China e 447 BET 0.974 BET NR e NR e NR e 87 EA

Toth et al. (2012)) SWCNT Aldrich, USA e 248 BET NR e NR e 1.2e1.5 SEM 2e5 SEM 50e70 NR

Toth et al. (2012) SWCNT Ox. Lab Synthesized HNO3 284 BET NR e NR e 1.2e1.5 SEM 2e5 SEM 50e70 NR

Coughlin and Ezra (1968) SWCNT 1 Cheap Tube, USA e 407 MC NR e NR e 1e2 MC 5e30 MC 90 Raman, XRD

Coughlin and Ezra (1968) SWCNT 2 Carbon Solutions,

USA

e 355 Calculated NR e NR e 1.55 MC 1e5 MC 40e60 NIR

Coughlin and Ezra (1968) SWCNT

Carbolex AP

Aldrich, USA e 240 Calculated NR e NR e 1.4 MC 3e5 MC 66 TGA

Coughlin and Ezra (1968) SWCNT

HipCo

Carbon

Nanotechnologies,

USA

e 633 BET NR e NR e 1e2 0.4e0.7 MC NC 85 TGA

Ji et al. (2009c) SWCNT Shenzen, China e 406 BET 1.11 BET NR e <2 MC NR e NR e

Liu et al. (2008) SWCNT Lab Synthesized HCl 388 BET 0.662 BET NR e 1e2 TEM NR e 99þ FTIR

Zhang et al. (2010a) As grown

CNT

Lab Synthesized e 134 NR NR e NR e NR e NR e NR e

Zhang et al. (2010a) Graphitized

CNT

Lab Synthesized Heat, N2 126 NR NR e NR e NR e NR e NR e

Su and Lu (2007) CNT Shenzen, China e 550 BET 1.150 BET 4.5 TEM <10 MC 5e15 MC 95þ MC

Kuo et al. (2008) CNT Lab Synthesized Heat 145 BET 0.398 BET NR e 30 TEM NR e NR e

Xie et al. (1997) CNT Shenzen, China e 189 BET NR e NR e 20e40 MC 5e10 MC 97 NR

Machado et al. (2011) CNT Lab Synthesized Acid Wash,

Air

NR e NR e NR e NR e NR e NR e

Adsorbent Name: Adsorbent names were assigned by their associated authors in the original articles; Supplier: The short names for manufacturer companies and their countries (full names of

companies can be found in Table S2 in Supplementary Information); Pretreat: The pretreatment techniques that were applied to CNTs after purchase if reported by the authors.

Acronyms used in the table: MC: Manufacturer claimed, NR: Not reported, Mthd: Analytical method employed for obtaining the morphological information, BET: BrunauereEmmetteTeller, BJH:

BarretteJoynereHalenda, FTIR: Fourier transform infrared spectroscopy, TEM: Transmission electron microscopy, SEM: Scanning electron microscopy, TGA: Thermogravimetric analysis, EA:

Elemental Analysis, AFM: Atomic force microscopy, XRD: x-ray diffraction, NIR: near-infrared spectroscopy.

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 41

unavailable and the theoretical SSA would decrease to

1315 m2/g. The reported SSAs of SWCNTs were significantly

lower than the calculated theoretical surface areas, a decrease

attributed to the tight aggregation of SWCNTs (Babel and

Kurniawan, 2003). Zhang et al. (2009) reported 19e37 individ-

ual tubes were forming bundles with a diameter 5e7 times

larger than an individual SWCNTs.MWCNTswere assumed to

be concentric tubular sheets with a distance of 0.45 nm be-

tween walls; therefore, the theoretical SSA is a function of

number of walls. The measured and theoretical SSA of

MWCNTs were reported to be in better agreement, which was

attributed to the looser aggregation state ofMWCNTs allowing

the tubes to behave like isolated ones (Zhang et al., 2009).

The PV of SWCNTs was slightly higher than the PV of

MWCNTs. The mean total PVs were 0.78 ± 0.23 cm3/g and

0.64 ± 0.39 cm3/g for SWCNT and MWCNT, respectively.

Theoretically, the pore volume occupied by the inner pores

of SWCNTs is considerably higher when compared to

MWCNT inner pore volume because the inner walls of

MWCNTs occupy space whereas the SWCNTs have open

inner channels. Zhang et al. (2009) reported that the avail-

ability of inner pores significantly affected the pore volume

especially for SWCNTs.

SSA and PV are similar parameters that are expected to

show an association with each other. However, the data

compiled for all CNTs from the literature showed no correla-

tion between SSA and the total PV (R2 ¼ 0.18). Investigating the

CNT types separately yielded higher correlations (R2 ¼ 0.50 for

SWCNT; R2 ¼ 0.27 for MWCNT) supporting that SWCNTs and

MWCNTs have different aggregation states. It should be also

noted that the SSA and PV of CNTs are determinedwith N2 gas

adsorption in their bulk phases. Given the aggregating nature

of CNTs in water, to better interpret adsorption experiments

performed in aqueous systems, it is important to develop

techniques to characterize the SSA and PV distribution of CNT

aggregates in water. The aggregation state of CNTs in aquatic

systems using time-resolved dynamic scattering and UVevis

spectrometer (Saleh et al., 2008, 2010; Khan et al., 2013; Yu

et al., 2007); however, no study has correlated the aggrega-

tion state of CNTs with their SSA and PVs in aquatic medium

to the best of our knowledge.

The inner and outer diameter ranges of MWCNTs were

2e15 nm and 4e200 nm, respectively. The wide range of the

outer diameter was attributed to variations in the number of

concentric tubular layers (walls) of CNTs. The diameters of

SWCNTswere in the range of 0.8e2 nmwith one exception, in

which the SWCNT diameter range was reported as 10e20 nm

as claimed by the manufacturer (Yan et al., 2008). The length

of MWCNT and SWCNT were in the ranges of 0.3e500 mm and

0.4e50 mm, respectively. Almost all reported morphological

properties (i.e. inner and outer diameter, length) were either

claimed by the manufacturer and/or measured by electron

microscopy (transmission, scanning electron microscopy)

(e.g., Oleszczuk et al., 2009, Lin and Xing, 2008b).

Approximately 50% of the CNTs reported were subject to

pretreatment for purification or targeted functionalization

prior to use (Table 1). The characterization of CNTs after pre-

treatment, and not relying only on the information provided

by the manufacturers for virgin CNTs, is critical to avoid

misinterpretation of the adsorption results. The most

common pretreatment techniques used have been acid wash

(HNO3, HCl, H2SO4), heat treatment and ultrasonication.

Several studies oxidized the CNTs to increase the surface

polarity (Salam and Burk, 2008; Chen et al., 2009b; Zhang et al.,

2010c). Functionalization of the CNTs has occurred at the

sidewalls, and either at ends of the tubes or the defect sites

through the covalent and non-covalent attachment of func-

tional groups. Non-covalent functionalization may be benefi-

cial since it has been shown not changing the CNT pore

texture (Ma et al., 2011; Datsyuk et al., 2008). Adding oxygen

containing functional groups on the CNT surface is the most

commonly used functionalization approach. The amount and

form of oxygen-containing functional groups (eOH, eCOOH,

>C]O) depend upon the type of oxidation technique and the

acid utilized for purification (Ma et al., 2011). Oxidation gives

CNTs hydrophilic moieties and removes impurities, amor-

phous carbon and hemispherical caps (Datsyuk et al., 2008).

The chemical modification of CNTs may also alter their

physical properties. There are different reports regarding the

impact of surface oxidation on the CNTmorphology. Cho et al.

(2008) oxidized MWCNTs by refluxing varying strengths

(10e70% w/w) of HNO3, KMnO4 and H2O2 solutions at elevated

temperatures (80e140 �C). The controlled oxidation produced

an array (1 pristine and 8 oxidized) of CNTs with surface ox-

ygen ranging from 3.3 to 14%. Authors reported no change in

SSAs after oxidation. Salam and Burk (2008) studied CNT

surface oxidation by 18% H2O2 and 8 M HNO3 at elevated

temperatures (80e140 �C). Attachment of ~1 mmol/g acidic

functional groups was detected; however, there was no

change in SSA. Wang et al. (2010b) also reported no SSA dif-

ference between the raw (0.2e1.0%) and oxidized (2.2e4.3%)

CNTs. In their studies, raw and oxidized CNTswere purchased

from a supplier and the oxidation techniques were not re-

ported. On the other hand, Ji et al. (2010a) reported a

‘remarkable’ increase in SSA from 410 to 650 m2/g and 160 to

420 m2/g for SWCNTs and MWCNTs, respectively, while using

dry KOH etching to functionalize SWCNTs and MWCNTs. The

dry KOH etching was described as contacting CNTs with KOH

powder and heating it to 800 �C under N2 stream yielding ~10%

oxygen containing functional groups. The increase in SSA

after oxidation was attributed to the removal of amorphous

carbon during functionalization (Shen et al., 2009; Chen et al.,

2009b). In contrast, Yu et al. (2012) reported a notable decrease

in SSA (from 471 m2/g to 327 m2/g) when the total surface

oxygen content was increased from 2.0% to 5.9%. The re-

searchers used NaClO solution at ambient temperature for

12 h to oxidize the pristine MWCNTs. Salam and Burk (2008)

also reported a 26% decrease (from 144 to 106 m2/g) in SSA

when 1MKMnO4 is applied to CNTs at 80 �C for 4 h followed by

a H2SO4 treatment. This decrease was attributed to the

strength of the oxidizing agent that damaged the MWCNTs.

These different findings indicate the importance of CNT type

as well as the oxidizing agent and its strength on alteration of

CNT surface chemistry. In addition to SSA, Cho et al. (2008)

reported no change in structure or length distribution of

CNTs after oxidation. However, Wu (2007a) reported a

decrease in the diameter of MWCNTs after oxidation and

attributed the modification of this diameter to the removal of

the amorphous carbon from the surface. These results clearly

demonstrate the necessity of extensive characterization of

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 542

CNTs after pretreatment and/or functionalization for more

accurate interpretation of their adsorption behaviors.

4. Adsorption of SOCs by CNTs

4.1. Influence of CNT properties

The physical properties of CNTs play an important role in the

adsorption of organic contaminants. Yang et al. (2006b)

experimentally estimated that adsorption capacity of hydro-

phobic SOCs (i.e. PAHs with log Kow ranging 3.36e5.18) on

CNTs increases with SSA and PV. Similarly Oleszczuk et al.

(2009) showed a positive correlation of SSA, and micro- and

mesopore volumes with the adsorption capacity of MWCNTs

for two rather hydrophilic pharmaceuticals (i.e. carbamaze-

pine, log Kow �1.22 and oxytetracycline, log Kow 2.45). Both

studies presented strong linear relationships (R2 � 0.97) be-

tween the adsorption capacity of SOCs on MWCNTs and their

SSAs. Zhang et al. (2009) also reported that SSA and PV were

influential for the adsorption of hydrophobic SOCs, but they

were not the exclusive factors determining adsorption ca-

pacities. Li et al. (2013a) reported higher methylene blue

adsorption by activated carbon than CNT per unit mass, yet

the order was reversed after surface area normalization.

Similarly, Beless et al. (2014) presented single point sor-

bentewater partitioning coefficients of eleven PCB congeners,

and reported similar CNT adsorption affinities with activated

carbon and black carbon per unit area. On the other hand,

Wang et al. (2009) observed SSA dependency for adsorption of

macromolecular humic acids by MWCNTs only at higher

concentrations because at low concentrations the sorption

site availability was not a limiting factor (thus not controlling

adsorption).

Sorption sites such as inner pores and interstitial channels

can impede the penetration of especially large organic mac-

romolecules. Known as the size exclusion phenomenon, this

remarkably decreases the adsorption of organics by CNTs

(Wang et al., 2009), a similar observation has also been re-

ported on activated carbons (e.g., Kilduff et al., 1996). The ac-

cess of SOCs to the inner regions of CNTs or CNTs bundles

may also be hindered due to amorphous carbon or metal

catalysts that were introduced during synthesis (Gotovac

et al., 2007c), and water cluster formation around the oxygen

containing functional groups (Zhang et al., 2009).

The nanocurvature and diameter of CNTs also influence

SOC adsorption. In their study, Gotovac et al. (2007a) reported

that the alignment between PAH molecules and CNT surface

affected adsorption. They observed that tetracene molecules,

which have four benzene rings aligning with the SWCNT sur-

face, while phenanthrene had only 2.5 rings in alignment,

resulting in a six-fold greater increase of tetracene adsorption

over phenanthrene. Also the increasing strength of pep

interaction causedmore benzene rings to align on the surface.

Apul et al. (2012) also reported a positive correlation between

the outer diameter of CNTs and surface area normalized

phenanthrene adsorption, which was attributed to the better

alignment of adsorbed planar phenanthrenemolecules due to

the decreasing surface curvature with increasing CNT diam-

eter. Li et al. (2013b), investigated the diameter effect on the

adsorption of perfluorooctanesulfonate by SWCNT using mo-

lecular dynamics simulations. Authors reported diameter de-

pendency of binding energies for the attachment of molecules

to the inner sites of CNTs. A sharp increase in the binding en-

ergy fromþ60 to�60 kcal/mol was observed with the increase

of diameter from 0.8 to 1.0 nm. The sorption became less

favorable (�60 to�30 kcal/mol) as thediameter increased from

1.0 to 1.6 nm. A relatively steady increasing trend (�11 to

�15 kcal/mol) was observed for the binding energies at the

outer surface of SWCNTs with increasing diameters. Consid-

ering that SWCNTs have typically narrow range of diameters

(0.8e2.0 nm), similar simulations should also be investigated

for MWCNTs with larger diameter ranges (2e200 nm).

To the best our knowledge, the affect of CNT length, and

chirality (a parameter related to the angle between graphene

plane and the tube axis) on SOC adsorption have yet to be

reported in the literature.

In addition to physical characteristics, CNT surface

chemistry can also influence the SOC adsorption. The unin-

tended oxidation of the surface during manufacturing and/or

in the environment, and the intentional oxidation with

treatment are some possible causes of CNT surface oxidation.

Several studies have reported an overall decrease in the SOC

adsorption with an increase in surface oxygen content (Cho

et al., 2008; Shen et al., 2009; Salam and Burk, 2008; Yu et al.,

2014; Chen et al., 2009b; Zhang et al., 2009; Li et al., 2011).

Two mechanisms have been proposed to explain these ob-

servations: (i) the presence of oxygen on the CNT surface

making adsorption of water molecules energetically more

favorable relative to SOC adsorption, which results in water

clusters that deplete the available surface area for SOCs; (ii)

the presence of oxygen on CNT surface localizes the p elec-

trons, which reduces the pep interactions between the CNT

graphitic surface and benzene rings of aromatic SOCs. Similar

findings were presented and discussed for activated carbon

(Yu et al., 2014; Kato et al., 2008; Coughlin and Ezra, 1968). On

the other hand, in their investigation of the adsorption of

hydroxyl- and amino-substituted aromatics on oxidized

CNTs, Chen et al. (2008b) reported higher removal of amino

and hydroxyl containing SOCs (2-naphtol, 2,4-dichlorophenol

and 1-naphtylamine) than structurally similar 1,3-

dichlorobenzene and naphthalene by oxidized CNTs, which

was attributed to stronger nonhydrophobic attraction due to

hydrogen bonding and p-hydrogen bonding interactions be-

tween eOH containing adsorbates and oxidized CNT surface.

Yu et al. (2012) reported a remarkable increase (~100%) in

adsorption capacities of toluene, ethylbenzene and m-xylene

with increasing surface oxygen content per SSA, up to ~8%, for

MWCNTs. The increase was attributed to the increase in the

dispersion of CNTs, also increasing the available adsorption

sites. However, a further increase in the oxygen content per

SSA (up to 18%) showed a decreasing trend in the adsorption

capacity. The authors attribute this opposing behavior to

water cluster formation effect dominating over the CNT

dispersion for SOC adsorption.

4.2. Influence of SOC properties

The driving forces of SOC adsorption are attraction to the CNT

surface and repulsion from the background solution (i.e.

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 43

water). The predominant repulsive force that repels SOCs

from water onto the CNT surface are hydrophobic forces that

are either associatedwith the size and polarity of themolecule

or with the protonation state of ionizable compounds. There

are also physical and chemical attractive forces between the

SOCs and CNT surface. Nonspecific interactions are major

contributors of overall attractive forces, which result from the

affinity of electron-deficient and electron-rich regions of un-

charged molecules. These nonspecific attractions are gener-

ally referred as van der Waals (vdW) interactions. Though

nonspecific attractive forces contribute to the overall inter-

molecular attraction regardless of the SOC properties, the in-

tensity of these forces depends upon the molecular size,

electric charge and polarizability. Other conditional attractive

forcesmay contribute to adsorption, such as pep interactions

between the resonating p electrons of the graphitic structure

of the CNT surface and the p electrons of aromatic SOCs.

Hydrogen bonding is a polarity dependent electrostatic

attraction between certain SOCs with functional groups (such

as eOH) and functionalized surfaces of CNTs. Electrostatic

interactions also occur between the charges of ionizable SOCs

and charged surfaces of CNTs.

Hydrophobicity, represented mostly by octanolewater

partitioning coefficient (Kow) or aqueous solubility (Cw), was

reported as the most predominant adsorbate property con-

trolling adsorption of several SOCs by MWCNTs (Yang et al.,

2006a, 2006b; Wang et al., 2008; Wang et al., 2009) and by

SWCNTs (Brooks et al., 2012). After hydrophobicity normali-

zation, MWCNT and SWCNT adsorption capacities were

relatively comparable for two SOCs (phenanthrene and

biphenyl) (Zhang et al., 2009). On the other hand, Wang et al.

(2009) reported hydrophobicity normalized adsorption ca-

pacities following the order of molecular sizes for tested SOCs

(phenanthrene > naphthalene > 1-naphtol) indicating that

larger molecules have more affinities provided that SOCs had

no hydrophobic differences. On the other hand, Yang et al.

(2006b) reported a negative correlation between the adsorp-

tion of naphthalene, phenanthrene and pyrene to molecular

size, which they attributed to the poor access of large mole-

cules to themicropores. Themolecular size directly influences

adsorption because the hydrophobic affinity cannot overcome

the steric hindrances, a finding that was confirmed by Chen

et al. in which the determined that the molecular sieving ef-

fect prevent bulky 1,2,4,5-tetrachlorobenzene molecules from

accessing the innermost surfaces (Chen et al., 2007). These

findings show the importance of the bundle structure of CNTs

and the availability of adsorption sites to SOCs. According to

Wang et al. (2010b), the primary adsorption sites of SOCs were

the external curved surfaces of CNTs since the probe mole-

cules (naphthalene) failed to penetrate into the hollow spaces

within the nanotubes. Toth et al. (2012) also concluded that

CNT adsorption occurred on the external surfaces exclusively.

Chen et al. (2007) supported the concept that sorption sites of

MWCNT were located on the innermost and outermost sur-

faces because the interlayer spacing between coaxial tubes are

impenetrable to SOCs. Kinetic experiments conducted by

Shen et al. (2009) suggest that the MWCNT pore-filling

mechanism of nitroaromatic organics has a relatively low

contribution to overall adsorption, indicating sorption to

external surfaces rather than interstitial channel spaces.

Zhang et al. (2009) reported that rigid and planar phenan-

threne molecules can mainly attach to the external surface

area of interstitial channels of SWCNTs. Unlike phenan-

threne, nonplanar and flexible biphenyl and 2-phenylphenol

molecules have adjustable molecular configurations for bet-

ter packing in the MWCNT bundles. Similarly, Pan and Xing

(2008) reported that the easily rotating structure of

bisphenol-A allowed the molecule wedging into the groove

regions of CNTs unlike rigid 17a-ethinyl estradial molecules.

Oleszczuk et al. (2009) applied ultrasonication to disperse CNT

bundles and quantified the desorption of carbamazepine and

oxytetracycline from CNTs. After ultrasonic pretreatment (i.e.

disintegration of CNT bundles) oxytetracycline molecules

were released more from CNTs when compared to non-

sonicated CNTs, indicating the presence of molecules trap-

ped in the interstitial areas of CNTs. However, the dispersion

state of CNTs did not influence carbamazapine adsorption,

due to the attraction of adsorbed carbamazepine molecules to

free molecules that cause stacking of carbamazepine regard-

less of the aggregation state. Overall, these reports indicate

that the hydrophobicity itself may not explain the adsorption

affinity of SOCs on CNTs, and the accessibility of SOCs to

different regions of CNT bundles and pores can be important

for the adsorption of SOC molecules.

Another principle SOC property influencing adsorption by

CNTs is the p electron density of the compounds resonating in

aromatic rings (also possibly for some aliphatic chains with

double bonds). In that the graphitic surfaces of CNTs have

regions with rich and poor p electrons, the interaction of p

electrons influence adsorption (Gotovac et al., 2006; Oleszczuk

et al., 2009; Chen et al., 2009b; Long and Yang, 2001; Zhang

et al., 2009; Wang et al., 2010b). In their comparison of hy-

drophobicity normalized adsorption affinities of lindane (i.e. a

cyclic chlorinated aliphatic molecule) with atrazine and

phenanthrene (i.e. two aromatic SOCs), Wang et al. (2010b)

concluded that pep interactions influence adsorption posi-

tively. Chin et al. (2007) reported a 60% increase in the

adsorption of o-xylene onto SWCNT after purification by 3 M

nitric acid at 120�C for 3 h at pH 3. At pH 5 and 7, no notable

increase was observed. This increase was attributed to pep

attraction of xylene molecules and CNT surface. The methyl

groups on o-xylene were protonated and pushing electrons

towards the benzene ring increasing the electron density of

resonating p electrons enhancing the attraction between the

partially positive SWCNT surfaces, which becomes more

profound at pH 3. At pH 5 and 7, no notable increase was

observed because at pH 3, themethyl groups on o-xylene were

protonated and pushing electrons towards the benzene ring

increasing the electron density of resonating p electrons. Ac-

cording to Lin and Xing (Lin and Xing, 2008a), the sorption

affinity of cyclohexanol was lower than phenol, and this dif-

ference was attributed to the pep interaction because cyclo-

hexanol is missing p electrons and has comparable

hydrophobicity with even a lower solubility than phenol. Ac-

cording to Chen et al. (2007), the adsorption affinity of the two-

ring 2-naphtol was stronger than one ring 2,4-dichlorophenol.

The higher adsorption of 2-naphtol was attributed to stronger

conjugation potential of two rings resultingwith stronger pep

interactions. This study is supported by a comparison of two

aromatic compounds: phenanthrene is adsorbed more than

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 544

naphthalene because it has onemore benzene ring that allows

more polarization and higher dispersive forces with highly

polarizable CNT surfaces (Wang et al., 2009). Additionally, the

electron induction effect of chlorine atoms reduces the elec-

tron density of 2,4-dichlorophenol in the benzene ring, which

suppresses its adsorption affinity. The pep stacking is ob-

tained when the benzene rings of SOCs align with the CNT

surface as demonstrated by Gotovac et al. (2007a); therefore,

the contribution of pep interaction between CNTs and the p

electrons in aliphatic compounds depends on the molecular

arrangement of molecules. Pan and Xing (2008) support the

importance of molecular configuration by demonstrating the

attachment of bisphenol-A with two benzene rings on CNTs

along the circumference, suggesting that the pep electron

donor acceptor complexes are stronger than pairs of donor or

pairs of acceptor. The thermodynamic calculations indicated

that adsorption of bisphenol-A along the outer circumference

of CNTs is very unlikely due to high energy requirement for

the steric conformation. Therefore, the presence ofpep bonds

may not always promote adsorption, and molecular configu-

ration or surface conformation may also play an important

role. Chen et al. (2007) propose pep electron donor acceptor

interactions between the p electron rich aromatic rings of

adsorbates and p electron depleted regions of CNT surface.

The ground-state hybrid structure of the pep electron donor

acceptor system consisted of electrostatic forces between sep

quadrupoles of opposing benzene rings. This mechanism in-

volves one-electron transfer from the highest occupied mo-

lecular orbital to the lowest unoccupiedmolecular orbital. The

resulting bond is a weak covalent bond formed by unpaired

electrons (Lin and Xing, 2008a). Chen et al. (2007) also reported

an “extremely strong adsorption of 1-naphthylamine on

CNTs”. This finding was attributed to the presence of the un-

shared pair of electrons of nitrogen on the amino group

(eNH2) making the benzene ring electron rich, which strongly

(stronger thaneOH) interacts with electron poor groups of the

CNT surface. Since electron depleting regions are likely to be

limited, strong adsorption affinity is expected to be more

notable at low concentrations. Similarly, Chin et al. (2007)

concluded that adsorption of xylenes depends upon the po-

sition of the methyl group on the benzene ring resulting from

the repulsive impact of the methyl group on the p electron

density of the xylene molecule. Wang et al. (2010a) tested the

adsorption of dialkyl phthalate esters and pep electron donor

acceptor interactions were proposed after hydrophobicity

normalization because of the p electron-accepting ester

functional group. Another study (Lin and Xing, 2008a) inves-

tigated the adsorption of chlorophenols onto pristine and

functionalized MWCNTs and reported a reduced capacity on

oxidized CNT surfaces, because the pep dispersion was

weakened by the oxygen containing functional groups on the

surface. The substituent on the benzene ring was found to

influence the resonance and time-dependent electron density

of the aromatic SOCs, thus influencing the pep interactions.

Adsorption simulations supported the coupling of p electrons

between CNT surface and aromatic molecules (such as ben-

zene, azulene, DDQ, pyrene and nitrotyrosine) using density

functional theory optimizing the binding energies (Zhao et al.,

2003; Tournus et al., 2005).

Hydrogen bonding is another principal dipoleedipole

attraction between a hydrogen atom and an electronegative

atom such as nitrogen or oxygen; however; it cannot be eval-

uated completely independent from pep interactions (Chen

et al., 2008b). Several CNT adsorption studies have empha-

sized the contribution of hydrogen bonding to overall adsorp-

tion (Lin and Xing, 2008a; Yang and Xing, 2009; Wang et al.,

2010a; Li et al., 2011). According to Wang et al. (2008) 1-

naphtol molecules may form hydrogen bonds with the oxy-

gen containing functional groups of CNT surface or benzene

rings of 1-naphtol can be aligned to the CNT surface, which

leaves theeOH functional group facing the aqueous phase that

forms new hydrogen bonding sites for free 1-naphtol mole-

cules in water. Li et al. (2013c) tested the adsorption of phen-

anthrene and phenol, in the range of ~2e1000 ppb and

100e1000 ppb, respectively, onto pristine (3.4% O) and oxidized

(11.3% O) MWCNTs. Adsorption of phenol by oxidizedMWCNT

was higher than pristine MWCNT. Opposite trend was

observed for adsorption of phenanthrene. Authors attributed

the difference to the hydrogen bonding between phenol mol-

ecules and the oxidized surface of MWCNTs. Yang et al. (2010)

also proposed hydrophobic attraction between 2,4-

dichlorophenol, 4-chloroaniline and oxygen containing

groups of MWCNTs, they emphasized the dependency of pH,

because once the molecules are dissociated, the hydrogen

bonding ability disappears. Yang et al. (2010) also found that

nitro-, chloride- and methyl-functional groups attached to

phenols or anilines enhance the affinity toMWCNTs. The order

of their influence followed the nitro group > chloride

group > methyl group. They also reported the dependency of

phenol substitution pattern by observing higher adsorption

when the hydroxyl group is attached in meta-position rather

than ortho- and para-position. However, the authors avoided

conclusive statements about the influence of group positions

because different functional groups had different influences

on adsorption affinity (Shen et al., 2009). On the other hand,

Chen et al. (2007) found that the contribution of hydrogen

bonding on overall CNT adsorption of nitroaromatics was not

significant. After testing the adsorption of 2,4-dinitrotoluene

onto oxidized SWCNTs (17% oxygen) at a pH range of 2.8e7.3,

they observed a slight decrease in adsorption capacity with

decreasing pH. On the contrary, an increase in adsorption ca-

pacity was expected as the pH decreases if the hydrogen

bonding was the controlling adsorption mechanism. In such a

case, the eCOO� functional groups on the CNT surface would

be protonated and eCOOH groups would act as H-bond donor

and form hydrogen bonds with H-bond accepting nitro groups.

Lin and Xing (2008b) and Oleszczuk et al. (2009) also reported

that hydrogen bonding might not be significant in that there

was no proportional increase with the number of eOH groups

of their adsorbates; the CNTs they used for adsorption, how-

ever, had a very low hydrogen and oxygen content. Yang and

Xing (2009) also determined that fulvic acid can act as a

hydrogen bond donor due to carboxylic and phenolic moieties,

while the CNT surface may act as hydrogen bond acceptors.

They also reported electrostatic interactions between the sur-

face charges of fulvic acid and CNTs, indicating that these in-

teractions would strongly depend on the pH. In demonstrating

the adsorption of 2,4,6-trichlorophenol at different pH values

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 45

Chen et al. (2009a) found that increasing the pH values

increased the dissociated (i.e. negatively charged) fraction of

the compound, which decreased the adsorption from repul-

sion between the negatively charged surface of CNT and

anionic 2,4,6-trichlorophenol. In addition, the increased solu-

bility of the ionized form of the adsorbate reduced the

adsorption. Similarly, decreasing uptake of organic acids onto

activated carbon was reported with increasing pH (ranging

from 1 to 11) (Ward and Getzen, 1970). Shen et al. (2009) pre-

sented stronger adsorption affinities of aromatic compounds

with more nitro groups because these groups are strong elec-

tron acceptors interacting with highly polarizable electron-

donating graphitic surfaces of MWCNTs. At lower pH values,

adsorption was favored because of less ionization of contam-

inants such as acidic herbicides (Pyrzynska et al., 2007), direct

dyes (Kuo et al., 2008) or sulfonamide antibiotics (Ji et al.,

2009b). At pH levels around pKa, both electrostatic and hy-

drophobic sorption mechanisms are anticipated; however, it

was not possible to distinguish between those two mecha-

nisms (Pyrzynska et al., 2007).

4.3. Influence of background solution properties

NOM is ubiquitous in natural waters and adsorbs on CNT

surfaces influencing its SOC adsorption (Su and Lu, 2007; Yang

and Xing, 2009; Wang et al., 2007). The net influence of NOM

on SOC adsorption by CNTs is a tradeoff between two opposite

effects (Zhang et al., 2010c): (i) the competition by NOM

depleting the sorption sites for SOC adsorption (Hou et al.,

2013), and (ii) coating of NOM dispersing CNTs, thus

exposing more adsorption sites for SOC (Pan et al., 2013). Ac-

cording to Hou et al. (2013), though humic acid coated

MWCNTs were better dispersed in water forming a loosely

coiled network of tubes, the coverage of MWCNT adsorption

sites reduced the adsorption affinity of phenanthrene, 2-

naphtol and 1-naphtylamine noticeably. The reduction was

attributed to the decrease in surface area due to effective

humic acid coating of individual tubes. Chen et al. (2008a)

reported a moderate reduction of naphthalene, 1,3-

dinitrobenzene and 1,3,5-trinitrobenzene adsorption from

humic acid competition. In addition to the direct competition

for sorption sites, molecular sieving in micropores due to

steric hindrance was proposed because the suppression of

SOC adsorption was proportional to molecular sizes of tested

SOCs. Wang et al. (2008) explored the adsorption of phenan-

threne, naphthalene and 1-naphtol by MWCNTs coated with

humic acid, a-phenylalanine and peptone. Peptone coating

substantially reduced the surface area (from87 to 35m2/g) and

shifted the pore size distribution from micro- to meso- and

macro-ranges, indicating that the peptone coating was

depleting adsorption sites and blocking micropores. The au-

thors also reported the introduction of polar moieties to CNT

surfaces through peptone adsorption further reduced the

adsorption of hydrophobic contaminants. On the other hand,

the humic acid coating showed a much lower suppression of

adsorption, which was attributed to the negatively charged

polar functionalities of humic-acid dispersing MWCNTs that

increased the effective surface area. According to Pan et al.

(2013), humic acid suspended CNTs exhibited up to two or-

ders of magnitude greater adsorption capacity for

sulfamethoxazole than aggregated CNTs. The formation of a

stable CNT suspension in tannic acid solution was also pre-

viously demonstrated by Lin and Xing (2008b). The increasing

dispersion in water leading to higher surface area is likely to

counterbalance the depletion of surface area due to humic

acid coating. Another study by the same group (Wang et al.,

2009) entailed an investigation of the influence of humic

acid concentrations in competition with SOCs. At higher NOM

concentrations, the competitionwas less pronounced because

CNTs have limited number of high-energy adsorption sites

and once these high-energy sorption sites are depleted, the

competition for low-energy sorption sites was lower. Zhang

et al. (2010c) also reported competition for high-energy sorp-

tion sites that was indicated by increasing surface heteroge-

neity represented by Freundlich n values. The study also

showed a greater NOM-to-SOC ratio and a longer contact-time

that reduced the SOC adsorption capacity. The comparison of

simultaneous SOC andNOMadsorptionwith the preloading of

CNTs with NOM indicated that NOM preloading further

decreased the adsorption capacities, suggesting a slower

adsorption rate of NOM than SOCs. Wang et al. (2014) tested

competitive adsorption of dialkyl phthalate esters and humic

acid under various loading sequences such as simultaneous

loading of humic acid and SOCs, preloading of CNTs with

humic acid, preloading of CNTs with SOCs and pre-

equilibration of SOCs and humic acid prior to injection. Au-

thors observed better SOC adsorption under simultaneous

injection due to faster occupation of high energy sorption sites

by smaller and more hydrophobic SOCs than humic acid.

However, the loading sequences did not make an “obvious”

difference in the competitive sorption indicating that the

system containing SOC, CNT and NOM would equilibrate

under ambient conditions. However, more compounds with

different structural and chemical properties and different

NOM should be tested to validate this finding.

The pH of the background solution is another major factor

controlling adsorption. For organic acids, if pH < pKa, the non-

dissociated species for organic bases dominate the solution

and vice versa. Therefore, the influence of background solution

pH and ionic strength depends upon the ionizability and the

electron donor acceptor ability of SOCs. The pH change also

influences the protonation/deprotonation state of the func-

tional groups on CNT surfaces. Deprotonation of acidic func-

tional groups may increase the density of negatively charged

functional groups that may create repulsive forces between

negatively charged SOCs or may promote p-electron donor

ability of CNT surface and enhance pep electron donor in-

teractions between CNTs and SOCs. The formation of water

clusters decreasing hydrophobicity and reduction of hydrogen

bond formation decreasing adsorption affinity are other

possiblemechanisms for this increase in either repulsive forces

or the promotion of electron donor ability (Zhang et al., 2010c;

Pan and Xing, 2008). In their comparison of the adsorption of

nonionic phenanthrene and ionizable 2-phenylphenol byCNTs

under varying pH values, Zhang et al. (2010c) found that if the

background solution had pH values ranging from 4 to 10,

phenanthrene adsorption remained unaffected. However,

there was an observable decrease in adsorption of ionizable 2-

phenylphenol when the pH of the solution was over the pKa

of 2-phenylphenol. This decrease was attributed to the

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increased ionization and decreased hydrophobicity. Lin and

Xing (2008a) tested adsorption of three phenolic adsorbates

(phenol, pyrogallol and 1-naphtol) and one apolar adsorbate

(naphthalene) over a pH range of 2.2e11. The decrease of

adsorption affinities for phenolic compounds over their pKa

values were attributed to the increased electrostatic repulsion

between dissociated adsorbates and negatively charged

MWCNTs and the increase in hydrophilicities. In addition, the

authors also noted that the dissociation of eOH groups on

phenolic compounds might be inhibiting the formation of

hydrogen bonding. The adsorption affinities of phenolic com-

pounds increased up to their pKa values (from pH 2e6), indi-

cating the p-electron donating properties of these compounds

were altered. The change in adsorption affinitywith increasing

pH was not attributed to changing CNT properties because the

adsorption affinity of nonionic naphthalene to the same CNTs

remained constant in the same pH range. Similar results were

demonstrated byChenet al. (2008b) in thepH range of 3e11. For

two nonionic aromatics (1,3-dichlorobenzene and naphtha-

lene) adsorption was minimally affected. However, 2,4-

dichlorophenol showed a significant decrease above the com-

pound's pKa value. The authors also reported an increase in the

adsorption capacity of 2-naphtol above its pKa value due to

adsorption enhancing interactions counterbalancing the

decrease in hydrophobicity. It should be noted that single point

adsorption data were presented at pH 11 and the increase of

adsorption was relatively small. In another study, Chen et al.

(2007) tested adsorption of nonionic 1,2,4-trichlorobenzene

and ionic 2,4-dinitrotoluene by varying the pH from 2.8 to 7.4.

Nonionic adsorbate was independent of pH. Ionic 2,4-

dichlorotoluene showed a very slight increase as the pH

increased. Li et al. (2011) reported a decrease in adsorption of

perfluorooctanic acid (pKa ¼ �0.5) onto MWCNTs with

increasingpHfrom2to10.Thiswasattributed to the increaseof

electrostatic repulsion between deprotonated perfluorooctanic

acid and negatively charged functional groups of MWCNT at

elevatedpHvalues. In thesamestudy,Li et al. (2011) reportedan

increase in adsorption of 4-nitrophenol (pKa ¼ 10.7) with

increasing pH. In the pH range tested (2e10) 4-nitrophenol was

protonatedhence the increasewas attributed to the increase in

hydrogen bonding between the eOH group of 4-nitrophenol

and oxygen containing functional groups of MWCNTs

inducing strongerattraction. Similarly, Smithetal. (2012) tested

the adsorptionofNOMontopristine (2.2%oxygen) andoxidized

(8.1% oxygen)MWCNTs at pH 4,7, 6.0 and 9.0. The adsorption of

NOM by pristine MWCNT remained largely unaffected by pH,

however oxidized MWCNTs showed a remarkable decrease

with increasing pH. This decrease was also attributed to in-

crease of electrostatic repulsion. Yao and Strauss (1992)

observed an increasing attraction between cationic dyes and

negatively charged CNT surfaces as the pH of the solution was

increased. Among all tested ionizable compounds, the

adsorption of dissociated compounds was remarkably lower

than their non-dissociated forms.

The variance of the ionic strength of natural waters can be

another factor thatmay influence adsorption of SOCs. Organic

compounds are less soluble in aqueous salt solutions, which

are known as the salting-out effect. Salting-out may enhance

the hydrophobic interactions of SOCs with CNTs (Zhang et al.,

2010c; Xie et al., 1997). Only a limited number of studies have

been undertaken to determine the influence of ionic strength

on SOC adsorption by CNTs. Further research is needed to

examine the influence of ionic strength on CNT adsorption.

Kuo et al. (2008) reported the aggregation of dye molecules at

higher salt ion concentrations and the promotion of adsorp-

tion. Ions may penetrate into the diffuse double layer and

eliminate the repulsive energy between CNTs, however,

which in turn forms a more compact aggregation structure

that is unfavorable for SOC adsorption, which is known as the

squeezing-out effect (Zhang et al., 2010c). According to Zhang

et al. (2010c), the ionic strength (in the range of 0.001e0.1 M)

had negligible impact on adsorption of phenanthrene and 2-

phenylphenol on CNTs because the contribution of salting-

out effect was equivalent to that of the squeezing-out from

CNTs due to tighter aggregation or both the salting-out effect

and squeezing-out effect were too weak to exert any changes.

Chen et al. (2008a) also observed negligible influence of ionic

strength on SOC adsorption, which they attributed to a rela-

tively narrow range of ionic strength (0.02e0.1 M). Clearly,

tested ionic strengths exhibited no significant difference on

nonionic SOCs, which have low electronic coordination abili-

ties. Kuo et al. (2008) reported the aggregation of dye mole-

cules at higher salt ion concentrations that promoted

adsorption. In addition to ionic strength, the presence ofmetal

ions has also been the subject of separate investigations.

Wang et al. (2007) reported a significant increase in fulvic acid

adsorption on CNTs with increasing Ca2þ/Mg2þ ions. They

attributed this increase to (i) the compression of the diffuse

double layer and/or charge neutralization that decreases the

repulsive forces between NOM and CNTs; (ii) bridging of cat-

ions with negatively charged functional groups of NOM mol-

ecules; and (iii) bridging of cations between the NOM and

functional groups of the CNT surface. Similarly, Chen et al.

(2008a) found that the presence of 50 mg/l of Cu2þ reduced

adsorption of SOCs up to 20%. This reductionwas attributed to

the complexation of metal ions with surface oxygen func-

tionalities and formation of hydration shells of dense water

that competes with SOCs. Chen et al. (2009a) observed the

suppression of 2,4,6-trichlorophenol adsorption by MWCNTs

in the presence of 6.5e65 mg/L Cu2þ especially for oxidized

MWCNTs. In a separate study, Chen et al. (2008c) reported a

reduction in adsorption capacities in the presence of copper,

lead and cadmium, which they attributed to the large hydra-

tion spheres around the copper complexes. Additionally, the

cross-bridging effect of cations between the anionic func-

tional groups of CNT surfaces was suggested to form tighter

CNT bundles, which in turn shielded the sorption sites.

Adsorption of SOCs by CNTs is predominantly a

temperature-dependent process in which physical sorption

occursmostly as an exothermic process releasing energy. Shen

et al. (2009), reported negative DH values for adsorption of 1,3-

dinitrobenzene, m-nitrotoluene, p-nitrophenol and nitroben-

zene, suggesting that adsorption of nitroaromatics by

MWCNTs and oxidized MWCNTs is thermodynamically

spontaneous and exothermic. The reported absolute DH values

for adsorption of all were less than 40 kJ/mol implying physical

reaction (i.e. physisorption) except adsorption of p-nitrophenol

onto oxidizedMWCNTs (�42 kJ/mol). The higher enthalpy of p-

nitrophenol adsorption onto the oxidizedMWCNT surface was

attributed to chemical bonding via hydrogen bonding.

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 47

However, it should be noted that DH > 40 kJ/mol is not a

definitive boundary between physisorption and chemisorp-

tion. Lu et al. (2006) reported negative DH values ranging from

�1.9 to �2.7 kJ/mol for adsorption of four trihalomethane

species (CHCl3, CHCl2Br, CHClBr2 and CHBr3) indicating

exothermic reaction. Authors also reported higher first order

rate constants for adsorption with increasing temperatures

and attributed the finding to the rise in the diffusion rate of

trihalomethanes at higher temperatures. Much smaller DH

values for trihalomethanes may be attributed to weaker

adsorption tendencies of trihalomethanes than hydrophobic

aromatic contaminants. Thermodynamic investigations for

the adsorption of nitroaromatics (Shen et al., 2009), trihalo-

methane (Lu et al., 2006), atrazine (Chen et al., 2009b; Yan et al.,

2008; Rambabu et al., 2012), NOM (Su and Lu, 2007; Wang et al.,

2007), and TEX (toluene, ethylbenzene andm-xylene) (Yu et al.,

2012) revealed that the CNT adsorption capacity decreases as

the temperature increases. Su and Lu (2007) observed faster

NOM adsorption kinetics when they increased the tempera-

ture, which in turn increased the diffusion rate. On the other

hand, the adsorption of 1,2-dichlorobenzene (Peng et al., 2003)

and pentachlorophenol (Salam and Burk, 2008), dyes (Kuo

et al., 2008; Wu, 2007b; Ghaedi et al., 2012a; Rodriguez et al.,

2010), methylene blue (Shahryari et al., 2010) exhibited

product-favored (endothermic) reactions, which again resulted

in an increase in adsorption capacity with an increase in

temperature. The commonality of all reported endothermic

sorption behavior was the potential electrostatic attraction

between the surface and the adsorbates, indicating that the

adsorption thermodynamics depends upon the nature of the

predominant sorption mechanism.

5. Predictive models for adsorption of SOCsby CNTs

Even many adsorption studies have been undertaken, these

cover only small portion of the approximately 40,000 anthro-

pogenic pollutants that are known to us (Nirmalakhandan and

Speece, 1990). While it is possible to expand conventional

adsorption studies on CNTs to amass data, these adsorption

isotherm experiments are time consuming, costly and labo-

rious. Therefore, predictive models are useful in rapidly

gathering adsorption data and more thoroughly elucidate the

mechanism of SOC adsorption on CNTs.

Of the many studies undertaken to predict the adsorption

of contaminants on activated carbon, some involved the use

of physical properties (e.g. molecular refraction, aromaticity,

parachor and the number of hydrophilic functional groups) to

predict the adsorption of chemicals on activated carbon (Abe

et al., 1983, 1981a, 1981b). Fundamental thermodynamic con-

cepts have also been used utilized to explain the adsorption

phenomena, the most ubiquitous of these being the Polanyi

Theory (proposed by Manes, 1978), the Net Adsorption Energy

Concept (proposed by Suffet and McGuire, 1978) and the Sol-

vophobic Approach (by Belfort, 1980).

Similar to those methods, the quantitative structur-

eeactivity relationship (QSAR) is a statistical model develop-

ment tool that has been employed to predict the adsorption of

organic chemicals by activated carbon (Blum et al., 1994). In

QSAR modeling, chemical properties are related to molecular

structures through molecular connectivity indices (MCI or c

index). MCIs are simple computational descriptors that do not

necessarily possess any chemical or physical meaning.

Another common predictive model development approach,

known as the Linear Solvation Energy Relationship (LSER),

involves the use of solvatochromic parameters predicting the

adsorption by activated carbon. In the LSER model, these pa-

rameters are used to relate chemical properties to solvation

energies such as cavity formation, dipolar interactions and

the hydrogen-bonding energies (Hickey and Passinoreader,

1991). First introduced by Kamlet et al. (1985), LSER has also

been used to predict the activated adsorption of carbon

(Luehrs et al., 1996; Shih and Gschwend, 2009; Dickenson and

Drewes, 2010). Solvatochromic descriptors contain chemical/

physical information about the organic molecules used to

explain the interactions between the adsorbate, the adsorbent

and the solvent by five independent descriptors. The LSER

model is expressed as follows:

log K ¼ aAþ bBþ vV þ pPþ rRþ c (1)

where A is the hydrogen bond donating ability, B is the

hydrogen bond accepting ability, V is the molecular volume or

McGowan's volume, P is the polarizability/dipolarity, R is the

excessmolar refraction, c is the regression constant and the a,

b, v, p and r are the regression coefficients.

There are several statistical predictive CNT models with

dependent and independent variables, all of which are listed

in Table 2. Yang et al. (2006b) presented single parameter

linear relationships between the adsorbed volume capacity

and the specific surface area, which is the monolayer N2

adsorption volume capacity, also known as the micropore

volume of CNTs. In all of their generated equations for five

CNTs, all coefficients of determination (r2) were above 0.99.

Similarly in their correlations between adsorption coefficients

with specific surface areas, mesopore volume, and micropore

volume of five CNTs, Oleszczuk et al. (2009) obtained a r2 > 0.96

coefficient. Yang et al. (2008) reported a single parameter

linear relationship between the hydrogen bond that donated

properties of anilines and phenols, with the rest of the pa-

rameters statistically insignificant at a ¼ 0.95. In addition, the

effort of a stepwise variable selection technique provided the

P term to the LSER equation, indicating that like hydrogen

bonding, p-electron polarizability is necessary for adsorption.

Yang and Xing (2010) reviewed Polanyi theory based Dubinin

Ashtakov model to estimate adsorption affinity and capacity

using empirical information, and they obtained a linear rela-

tionship between the E term also known as “the correlating

divisor”, that captures the adsorption energy of all responsible

adsorption forces and the b term, which is a fitting parameter.

Xia et al. (2010b) developed an LSER equation to depict

relative contributions of molecular interactions on CNT

adsorption, which (Eq. (2)) is expressed as follows:

log K ¼ �0:37A� 2:78Bþ 4:18V þ 1:75Pþ 0:043R� 1:33�n

¼ 28; r2 ¼ 0:93�

(2)

The authors employed a mixture of 28 aromatic SOCs

assuming that the SOC concentrations are low enough to

Page 15: Nano Tube vs Organik nano

Table 2 e Summary of correlative equations between CNT adsorption descriptors and independent variables.

Source Dependent variable Independent variables Numberof data

points e n

Coefficient ofdetermination e r2

Notes

Yang et al. (2006b) PMM adsorption capacity descriptor-(Q0) Specific surface area (SSA) 5 0.99 Phenantherene on 5 different MWCNTs

Yang et al. (2006b) PMM adsorption capacity descriptor-(Q0) Micropore volume (PVmicro) 5 0.99 Phenantherene on 5 different MWCNTs

Oleszczuk et al. (2009) Adsorption coefficient (log K at Ce ¼ 0.01 Cs) Specific surface area (SSA) 5 0.97 Two aromatic pharmaceuticals on 5 different

MWCNTs

Oleszczuk et al. (2009) Adsorption coefficient (log K at Ce ¼ 0.01 Cs) Micropore volume (PVmicro) 5 0.97 Two aromatic pharmaceuticals on 5 different

MWCNTs

Oleszczuk et al. (2009) Adsorption coefficient (log K at Ce ¼ 0.01 Cs) Mesopore volume (PVmeso) 5 0.99 Two aromatic pharmaceuticals on 5 different

MWCNTs

Yang et al. (2008) Dubinin-Ashtakhov “correlating divisor” (E) Acidity (hydrogen bond

donating ability e a)

13 0.68 13 anilines/phenolic compounds on MWNCT; single

parameter correlation

Yang et al. (2008) Dubinin-Ashtakhov “correlating divisor” (E) Acidity and polarizability

(hydrogen bond donating

ability e a and polarizability e p)

13 0.68 13 aniline and phenolic compounds onMWNCT; poly

parameter correlation using stepwise parameter

selection indicated two significant parameters at 95%

level of significance

Yang and Xing (2010) Dubinin-Ashtakhov “correlating divisor” (E) b e Dubinin-Ashtakhov

fitting coefficient

39 0.62 Dubinin-Ashtakhov fitting coefficient e b was

speculated to contain information regarding the

adsorption affinity capturing sorption interactions

Xia et al. (2010b) Adsorption constant (k ¼ Cad/Ce) LSER solvatochromic

parameters (a, b, V and p)

28 0.93 Mixture of 28 aromatic compounds on MWCNT, poly

parameter correlation indicated V and B as the most

predominant contributors

Apul et al. (2013a) Adsorption descriptors (K∞ at Ce ¼ 0.002 Cs;

K0.1 at Ce ¼ 0.01 Cs; K0.01 at Ce ¼ 0.1 Cs)

QSAR parameters (c indices) 29 0.88 29 aromatic compounds were compiled from

different sources all adsorbed by MWCNTs; three

significant c indices were influential

Apul et al. (2013a) Adsorption descriptors (K∞ at Ce ¼ 0.002 Cs;

K0.1 at Ce ¼ 0.01 Cs; K0.01 at Ce ¼ 0.1 Cs)

LSER solvatochromic

parameters (a, b, V and p)

29 0.83 29 aromatic compounds were compiled from

different sources all adsorbed by MWCNTs; the V

term was the sole significant parameter with

increasing concentration A and B terms that were

also significant.

water

research

68

(2015)34e55

48

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 49

prevent compoundecompound interactions. According to

their LSER derivative (Eq. (2)) the strongest contributor to

adsorption is hydrophobicity, which is represented by the V

term. The second most predominant factor is the hydrogen-

bond donating ability with a negative correlation, which in-

dicates an increase in the adsorbateewater interactions with

an increase in B values.

In an earlier study, we compiled MWCNT adsorption data

on 29 aromatic SOCs from the literature, including some

experimental results obtained in our laboratory (Apul et al.,

2013a) to develop predictive models using QSAR and LSER

techniques. The obtained QSAR and LSER equations are pre-

sented in equations (3) and (4), respectively (Apul et al., 2013a):

log K∞ ¼��2:98±0:52

�þ�0:18±0:12

�0cþ

�0:17±0:15

�0cv

þ�0:55±0:20

�3cp

�n ¼ 29; r2 ¼ 0:88

� (3)

log K∞ ¼� �4:34±0:56

�þ �0:05±0:32

�A� �

0:48±0:86�B

þ �4:55±0:56

�V þ �

0:61±0:34�P

�n ¼ 29; r2 ¼ 0:83

(4)

The molecular volume term (V) of the LSER model was the

most influential descriptor controlling adsorption at all con-

centrations, and the molecular size dependency was due to

the increasing hydrophobicity and increasing non-specific

attraction of larger molecules. Through this study we deter-

mined that QSAR and LSER techniques were effective in

developing effective models for predicting the adsorption of

organic compounds by CNTs. We validated our models were

externally using the adsorption data developed Xia et al.

(2010b). Experimentally measured CNT adsorption de-

scriptors were plotted against the predicted values by QSAR

and LSER modeling approaches, the results of which are pro-

vided in Fig. 2.

Our survey of predictive modeling efforts indicated that

both CNT (SSA, PV) and SOC properties (solvatochromic pa-

rameters) can be used to correlate CNT adsorption of SOCs.

Both single parameter and poly-parameter linear regression

studies were reported. Poly-parameter linear regression

studies were either conducted by employing a predefined set

of independent variables (solvatochromic parameters) or

through the use of a parameter selection technique (e.g.

stepwise parameter selection (Yang et al., 2008) or least

Fig. 2 e Experimentally measured adsorption descriptors vs. pre

based on Eq. (3)); and LSER models (to the right based on Eq. (4

2013a).

absolute shrinkage and selection operator) (Apul et al., 2013a).

The increasing number of data points decreased the linearity

of correlations (smaller r2) which reflected the ease fitting a

straight line to a fewer number of data points. As such, a

higher coefficient of determination (r2) need not necessarily

indicate a successful predictive model. An external validation

may be required to enhance the reliability and accuracy of the

model, however. In that modeling studies have only been

undertaken to elucidate the adsorption of aromatic organics

by MWCNTs, further research is required to model various

classes of organics and other CNT types, most particularly

SWCNTs.

6. Comparison of carbon nanotubes withactivated carbon

Both activated carbons and CNTs consist of graphitic carbon

backbone. They possess similar chemical properties (highly

hydrophobic, delocalized p-electron rich surfaces etc…),

while the pore structures of activated carbons differ greatly

from carbon nanotubes (Zhang et al., 2010a;Wang et al., 2014).

Activated carbons have a rigid pore structure with a network

of micro- and meso- and macropores where the majority of

adsorption surface area is located. On the other hand, indi-

vidual CNTs form random aggregates in water, driven by hy-

drophobic repulsion, non-specific attractive forces (van der

Waals) and pep stacking creating an ill-defined pore compo-

sition. The aggregation can also be influenced from the sur-

face chemistry of CNTs, pH, ionic strength and the presence of

NOM. For CNTs, four possible adsorption sites are external

surface of the outmost CNTs, peripheral grooves at the

boundary of CNT bundles, interstitial pore spaces between the

tubes, and inner cavities of open-ended CNTs (Zhang et al.,

2009).

Carbon nanotubes were reported to exhibit higher or

comparable adsorption capacity than activated carbons in

several studies (Ji et al., 2009a; Su and Lu, 2007; Lu et al., 2006;

Long and Yang, 2001; Pan et al., 2008; Apul et al., 2013b) while

opposite findings were reported in others (Lu et al., 2006; Ji

et al., 2009c). Ji et al. (2009c) reported higher adsorption ca-

pacity of a SWCNT and aMWCNT than an activated carbon for

tetracycline, whereas an opposite observation was made for

dicted adsorption descriptors from QSAR models (to the left

)) for training and external validation datasets (Apul et al.,

Page 17: Nano Tube vs Organik nano

wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 550

naphthalene in the same study. This was attributed to the

bulky tetracycline molecules being subject to size exclusion

resulting in less adsorption by the microporous activated

carbon. Similarly, Lu et al. (2006) reported higher CHCl3 uptake

of a MWCNT than a powdered activated carbons, despite the

lower specific surface area of the former (295 m2/g) than the

latter (900 m2/g). The authors speculated that functional

groups on the CNT surface made it more favorable for the

adsorption of polar CHCl3 molecules. However, this could be

also attributed to better dispersion of functionalized CNTs and

exposing more sorption sites to the CHCl3 molecules. Su and

Lu (2007) reported higher sorption affinity of NOM molecules

by CNTs than a granular activated carbon. Long and Yang also

reported superior sorption of dioxin to a MWCNT than an

activated carbon. However, in these two reports, detailed

structural information about the adsorbents has not

been provided to assess the factors controlling the

differences in the adsorption results of activated carbons and

CNTs. Wang et al. (2014) tested adsorption of three aromatic

(4-chloronitrobenzene, 1,4-dichlorobenzene and 1,2-

dichlorobenzene) and one nonplanar aliphatic (trans-1,2-

dichlorocyclohexane) SOCs by a MWCNT, a SWCNT and

two activated carbons. Aromatic SOCs showed better

adsorption capacities onto CNTs, which was attributed to the

pep stacking with benzene rings of the aromatics and the

higher graphitized surfaces of CNTs than activated carbons.

On the other hand, better adsorption of trans-1,2-

dichlorocyclohexane onto activated carbons than CNTs in

the same study was attributed to better micropore filling. Ji

et al. (2009c) compared the adsorption of phenol, 1,3-

dichlorobenzene, 1,3-dinitrobenzene onto SWCNTs and acti-

vated carbons. Adsorption capacities were roughly correlated

with the surface areas of adsorbents. The better sorption ca-

pacity of activated carbons was attributed to the micropore

filling mechanism. Pore filling mechanism may enhance the

adsorption affinity of activated carbons for SOCs especially

when the pore and adsorbate molecule sizes are comparable.

Adsorption kinetics on CNTs have been reported to be faster

than on activated carbons (Zhang et al., 2012), although some

studies suggest that more studies are needed examining the

roles of adsorbate properties (molecular size, structure) on the

adsorption kinetics by CNTs and activated carbons. Further-

more, it has been shown that although in distilled and

deionized water, the adsorption capacity of a granular acti-

vated carbon was higher than a SWCNT and MWCNT, in the

presence of NOM, the SWCNT showed similar adsorption ca-

pacities to those of the granular activated carbon (Zhang et al.,

2012). It was also noted in the same study that NOM compe-

tition was more significant on the adsorption of a nonplanar

and hydrophilic SOC than a planar and hydrophobic SOC

indicating also the importance of adsorbate characteristics.

7. Future research needs

“System heterogeneity” plays an important role on the

adsorption process and it is manifested in several ways. The

organic (e.g., SOC and NOM) and inorganic (e.g., pH and ionic

strength) characteristics of each individual water may vary

substantially. Moreover, the inherent characteristics (e.g.,

type, dimensions, and functional groups) and aggregation

state of CNTs in water may vary greatly depending on their

preparation and water matrix. Each of these variables com-

plicates the development of universal adsorption models,

necessitating their individual consideration for further un-

derstanding and research. The greatest challenge remains to

understand andmodel adsorption of SOCs by CNTs accurately

in heterogeneous and complex systems.

The quantification of individual contributions to overall

adsorption still remains as a challenging task. Examining

adsorption of structurally related small molecular weight

SOCs (e.g., similar molecular structures with different mo-

lecular weights, isomers with different molecular configura-

tions, varying Lewis acidebase properties) may provide

further insights to the significance of different intermolecular

adsorption interactions. Testing SOC adsorption in nonpolar

organic solvents such as hexadecane may be useful to

examine intermolecular interactions in the absence of hy-

drophobicity effect. Understanding the influence and role of

heterogeneous CNT characteristics (type, dimensions, and

surface chemistry) in the SOC adsorption interactions needs

further systematic investigations. Another challengewill be to

use this knowledge in the generation of improved or new

(mechanistic and/or statistical) adsorption models including

the contribution of the individual interactions to overall

adsorption. Developing models that successfully predict the

adsorption of some classes of SOCs (e.g., aromatic and

aliphatic) on some types of CNTs (e.g., SWCNTs andMWCNTs)

will still present important advances. Furthermore, the ability

of such models predicting adsorption in natural waters in the

presence of NOM and complex backgroundwatermatrices is a

major challenge due to site specific, heterogeneous, water

chemistry (pH, calcium/ionic strength levels) dependent na-

ture of NOM components (e.g., hydrophobic/hydrophilic, low/

highmolecular weight, aromatic/aliphatic) thatmay influence

both CNT aggregation and SOC adsorption. A fundamental

understanding of SOC adsorption kinetics and desorption

behaviors in such complex systems is equally important, and

have been much less investigated as compared to adsorption

equilibrium in literature.

Detailed characterization of CNTs, especially after pre-

treatment and/or functionalization, should be included with

the research articles (e.g., in the supplementary information

sections of most journals). Furthermore, providing the

adsorption data is also important. In the “Big Data” age and

initiatives such “Materials Genome Initiative” (Material

Genome Initiativ, 2014), there is an abundance of data pro-

duction including adsorption of SOCs by CNTs, therefore

detailed reporting of the data in peer-reviewed articles,

especially in the electronic domains, can facilitate the data

mining, in-depth analysis and the development of advanced

adsorption models. There are more data available for

adsorption of SOCs byMWCNT than SWCNTs in the literature.

Therefore, developing adsorption data on SWCNTs can still be

useful. The SSA and PV have been influential for the adsorp-

tion of hydrophobic SOCs, but they were not the exclusive

factors determining the adsorption capacities. The SSA and PV

of CNTs are determined with N2 gas adsorption in their bulk

phases. Given the aggregating nature of CNTs in water, it is

important to develop novel techniques to characterize the

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wat e r r e s e a r c h 6 8 ( 2 0 1 5 ) 3 4e5 5 51

accessible SSA and PV for adsorption on CNT aggregates in

water. The impact of CNT characteristics on its aggregation

behavior and impact on SOC adsorption need further

investigation.

8. Conclusions

Over the last ten years, more than 80 articles have been pub-

lished in peer-reviewed journals on adsorption of synthetic

organic contaminants (SOCs) (including polycyclic aromatic

hydrocarbons, benzene derivatives, phenolic compounds,

pharmaceuticals, insecticides, herbicides, organic dyes and

polychlorinated biphenyls) by CNTs. In these articles,

adsorption of SOCs was investigated using 18 single-walled

CNTs and 81 multi-walled CNTs as adsorbents. The abun-

dance of multi-walled CNT in these studies was likely due to

the higher market prices of single-walled CNTs. The average

market price formulti-walled CNTswas approximately 10 $/g,

and the average price for single-walled CNTs was higher than

100 $/g. Themean specific surface areas (±standard deviation)

of all reviewed single-walled CNTs and multi-walled CNTs

were 373 ± 146 m2/g and 216 ± 159 m2/g, respectively.

Both specific surface area and pore volume have been

found to be influential parameters for adsorption of hydro-

phobic SOCs, but they were not the exclusive factors deter-

mining adsorption capacities. Severalmechanisms, of varying

relative importance, have been proposed for controlling the

SOC adsorption by CNTs. The predominant driving force that

repels SOCs from water onto the CNT surface is hydrophobic

forces. Other principle SOC properties that positively in-

fluences adsorption is the p electron density of compounds

resulting in pep interaction with the p electron rich graphitic

surface of CNTs and polar moieties of SOCs enhancing dipo-

leedipole attraction. Size exclusionwas found to be important

especially for large organic macromolecules. In terms of CNT

surface chemistry, oxygen content was often investigated,

and two mechanisms were proposed influencing the SOC

adsorption: (i) the presence of oxygen on the CNT surface

making adsorption of water molecules energetically more

favorable relative to SOC adsorption, reducing the available

surface area for SOCs; and (ii) the presence of oxygen on CNT

surface localizes the p electrons, which reduces the pep in-

teractions between the CNT graphitic surface and benzene

rings of aromatic SOCs. On the other hand, the increase in

adsorption capacities as a result of surface oxidation was

attributed to better dispersion of CNTs, exposing more

adsorption sites, and increase in the ability to form hydrogen

bonds with some SOCs.

Natural organicmatter is ubiquitous in natural waters. The

presence of natural organic matter, in general, has been re-

ported to decrease the adsorption of SOCs by CNT. However,

natural organic matter coatings may disperse CNTs. revealing

more sorption sites for SOC adsorption.

The influences of pH, temperature, and ionic strength have

been examined less frequently. In general, pH and ionic

strengths exhibited no significant difference on nonionic

SOCs, which have low electronic coordination abilities. Some

dependence to pH and ionic strength were observed for

adsorption of ionizable SOCs (e.g. phenolic compounds,

anilines etc.) especially onto CNTs with ionizable functional

groups. Two counter-balancing forces influencing adsorption

were discussed; (i) salting-out effect: the decrease of SOC

solubility due to ionic strength that is favorable for adsorption

and (ii) squeezing-out effect: elimination of repulsive energies

between CNTs, which in turn forms a more compact aggre-

gation structure that is unfavorable for SOC adsorption. In

terms of temperature effect, adsorption of SOCs by CNTs was

predominantly reported as an exothermic process (i.e.

releasing energy); therefore, increase in temperature

decreased adsorption capacity.

Even several adsorption studies have been undertaken in

the past decade, they cover only small portion of the

approximately 40,000 anthropogenic pollutants that are

known to us. Statistical SOC adsorptionmodels have emerged

in recent years to minimize time consuming, costly and

laborious adsorption isotherm experiments. Our survey of

predictive modeling efforts indicated that both CNT and SOC

properties (solvatochromic parameters) can be used to corre-

late CNT adsorption of SOCs. Several single parameter and

poly-parameter linear regression studies were reported in the

literature. Poly-parameter linear regression studies were

either conducted by employing a predefined set of indepen-

dent variables (i.e. solvatochromic parameters) or through the

use of parameter selection techniques (e.g. stepwise param-

eter selection or least absolute shrinkage and selection

operator).

Overall, well-controlled laboratory studies have provided a

good insight to adsorption mechanisms of SOCs by CNTs, and

the greatest challenge still remains to further understand and

model adsorption of SOCs by CNTs accurately in heteroge-

neous and complex systems.

Acknowledgments

This review is a part of the work that was supported by a

research grant from the National Science Foundation (CBET

0967425). However the manuscript has not been subjected to

the peer and policy review of the agency and therefore does

not necessarily reflect its views.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.watres.2014.09.032.

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