Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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19.2 Structure and Synthesis of Carbon Nanotubes 619

Figure 19.3 Schematic representations of various processes

used to produce CNTs. (a) Electric - arc method apparatus.

Reproduced from Ref. [22] ; (b) Schematic representation of an

oven laser - vaporization apparatus. Reproduced from Ref. [26] ;

bed is ﬂ uidized. Reproduced from C.H. See, A.T. Harris, Ind.

Eng. Chem. Res. (2007), 46 , 997 – 1012.

620 19 Carbon Nanotubes as Adsorbents for the Removal of Surface Water Contaminants

19.3

Properties of Carbon Nanotubes

As described above, CNTs have unique structures, with remarkable properties that

can be grouped as electronic, mechanical, thermal, and optical. Most of the phys-

ico - mechanical properties of CNTs are dependent on the sp

bond network present

on their structure [36] and their diameter, length, and chirality. As these properties

have been discussed extensively elsewhere [37 – 39] , some of the most important

results in this area will be presented brieﬂ y, with emphasis placed on those proper-

ties that affect the adsorption capacity of these materials.

19.3.1

Mechanical, Thermal, Electrical, and Optical Properties of Carbon Nanotubes

The mechanical properties of a solid must ultimately depend on the strength of

its interatomic bonds. Both, experimental and theoretical, results have predicted

that CNTs have the highest Young ’ s modulus of all different types of nanostruc-

tures, with similar tubular forms such as BN, BC

, BC

N, C

, CN, and so on.

Furthermore, due to the high in - plane tensile strength of graphite, both single and

multiwalled CNTs, are expected to have large bending constants. The results of

experimental and theoretical studies have indeed indicated that CNTs can be very

ﬂ exible, able to elongate, twist, ﬂ atten, or bend into circles, before fracturing [37] .

The thermal and electrical properties of CNTs include conductivity. The speciﬁ c

heat and thermal conductivity are determined primarily by the nanotube ’ s elec-

tronic and phononic structures [38] , with theoretical and experimental results

demonstrating superior electrical properties for these materials. Carbon nano-

tubes have electric current - carrying capacities which are 1000 - fold higher than that

of copper wires [40] . In fact, theoretical calculations based on the tight - binding

model approximation within the zone folding scheme show that one - third of the

possible SWNT structures are metallic, while two thirds are semi - conducting

(Figure 19.4 ) [37, 41] .

19.3.2

Adsorption - Related Properties of Carbon Nanotubes

Early studies investigating the adsorption of nitrogen onto both MWNTs [42, 43]

and SWNTs [44] have highlighted the porous nature of these materials. Indeed,

due to their uniformity in size and surface properties, CNTs are considered as

ideal model sorbent systems to study the effect of nanopore size and surface mor-

phology on sorption and transport properties.

The surface area of a CNT has a very broad range, depending on the nanotube

number of walls, the diameter, length, wall defects and, in the case of SWNTs, the

number of nanotubes in a nanotube bundle [45] . An isolated SWNT with an open

end (this may be achieved through oxidation treatment) has a theoretical surface

area equal to that of a single, ﬂ at graphene sheet of 2700 m

− 1

[46] ; however,

reported experimental values indicate lower adsorption capacities [47] . In the case

19.3 Properties of Carbon Nanotubes 621

Table 19.1 Adsorption properties and type of adsorption sites in SWNT s and MWNT s .

Reproduced from Ref. [49] .

Type of nanotube

Porosity (cm

− 1

)

Surface area

− 1

)

Adsorption site Surface area per site

− 1

)

SWNT bundle Microporous

Vmicro: 0.15 – 0.3

400 – 900 Surface groove 483

Pore 783

Interstitial 45

MWNT Mesoporous 200 – 400 Surface pore;

aggregated pores

–

micro

= micropore volume.

of SWNTs, the diameters of the tubes and number of tubes in the bundle have

the strongest effects on the nanotube surface area. In the case of MWNTs, chemi-

cal treatments are reported to be useful for promoting microporosity. Surface areas

as high as 1050 m

− 1

have been reported for MWNT subjected to alkaline treat-

ment [48] . A two - step activation treatment (acid + CO

activation) has been also

reported to increase the speciﬁ c surface area of MWNT materials. It has been

proposed that these treatments open the ends of the nanotube structure, enabling

adsorption onto the nanotube inner openings [49] . Some representative results of

the surface area and pore volume of SWNTs and MWNTs are listed in Table 19.1 .

An important issue to address when considering adsorption onto nanotubes is

to identify the adsorption sites. For instance, the adsorption of gases into a SWNT

bundle can occur inside the nanotubes (pore), in the interstitial triangular chan-

nels between the tubes, on the outer surface of the bundle, or in the grooves

formed at the contacts between adjacent tubes outside of the bundle (Figure 19.5 ).

Figure 19.4 Band - gap values versus nanotube diameters

deﬁ ning SWNTs as metallic (

䊉

) or semi - conducting (

䊊

Reproduced from Ref. [41] .

622 19 Carbon Nanotubes as Adsorbents for the Removal of Surface Water Contaminants

In addition, the surface functionalization of CNTs by chemical methods has been

found to be a powerful tool for improving the adsorption capacity. Selective adsorp-

tion can be achieved through a controlled modiﬁ cation of the nanotube ’ s physical

and chemical properties, such as surface area, hydrophilicity, and permeability.

For instance, Vermisoglou and Georgakilas [50] have studied the sorption proper-

ties of pristine and chemically modiﬁ ed [functionalized with oleylamine and

poly(sodium 4 - styrene sulfonate)] nanotubes using adsorbates with different polar-

ities. Based on their measurements, these authors concluded that the sorption

behavior of the CNTs was greatly modiﬁ ed by chemical treatment. In fact, a chemi-

cal modiﬁ cation that increased the hydrophilicity of the nanotube walls enhanced

the adsorption selectivity for water over n - hexane. Chemical modiﬁ cation of the

nanotube wall was veriﬁ ed using infrared ( IR ) spectroscopy (Figure 19.6 ). When

comparing the IR spectra of pristine SWNTs with those of chemically treated

CNTs, new peaks corresponding to aliphatic chains were observed in the case of

hydrophobic nanotubes, whereas peaks corresponding to more polar bonds were

observed in the case of hydrophilic materials.

Yu et al. [51] have investigated the adsorptive performance on modiﬁ ed MWNTs

by using mechanical ball milling. For these materials, the adsorptive performance

for aniline in aqueous solution indicated that the adsorptive capacity of milled,

short open - ended MWNTs increased from 15 mg g

− 1

to 36 mg g

− 1

compared to the

unmilled MWNTs. The measurements of pore size distribution proved that the

inner pore diameter of 3 nm remained constant after milling, but the aggregated

pore diameter had decreased.

19.4

Carbon Nanotubes as Adsorbents

Carbon nanotubes have superior capabilities for the adsorption of a wide range of

toxic substances. The earliest reports of CNT use related to the removal of organic

pollutants, notably dioxins, from water [8] , though later they were reported also as

Figure 19.5 Sketch of the cross - sectional view of a SWNT

bundle, illustrating the four different adsorption sites.

Reproduced from B. Bhushan, Springer Handbook of

Nanotechnology , 2nd revision, Springer, Berlin, New York

(2007).

19.4 Carbon Nanotubes as Adsorbents 623

Figure 19.6 Infrared spectra of (a) pristine and (b,c) modiﬁ ed

CNTs, illustrating the characteristic peaks for hydrophilic

(b) and organophilic (c) samples. Reproduced from Ref. [51] .

having an exceptional ability to adsorb inorganic contaminants, such as ﬂ uoride

[52] . In both cases, the CNTs displayed a superior performance compared to “ tra-

ditional ” adsorbents such as activated carbon. These pioneering studies opened a

new ﬁ eld of CNT applications, with many subsequent reports noting CNTs to be

excellent adsorbents for the removal of other contaminants. For example, CNTs

624 19 Carbon Nanotubes as Adsorbents for the Removal of Surface Water Contaminants

were shown to adsorb up to 30 mg of a trihalomethane molecule per gram from

a 20 mg l

− 1

solution [11] . Other reports indicated that SWNTs could act as “ molecu-

lar sponges ” for small organic molecules such as CCl

[53] . A similar case was

demonstrated for inorganic contaminants, with CNTs again showing superior

performance; measurements of the adsorption capacity of a MWNT material

showed that it could adsorb 13.5 - fold more ﬂ uoride than a typical high - surface - area

alumina adsorbent (see below). These early results led to the suggestion that CNTs

might indeed serve as effective adsorbents for removing polluting agents from

water. Consequently during the past few years some extensive laboratory studies

have established the role of CNTs as effective adsorbents for common contami-

nants from water, including a wide variety of organic compounds and inorganic

ions. Although a large number of studies have been conducted into the use of

CNTs as adsorbents of gas contaminants [54 – 56] , this chapter will focus on the

removal of contaminants from surface water (the process being complementary

to the role of adsorbates in the gas phase). Consequently, below is presented a

discussion of recent results demonstrating the huge potential of CNTs for the

removal of contaminants from surface water.

19.4.1

Adsorption of Heavy Metal Ions

The effects of heavy metals such as lead, copper, zinc, nickel, and chromium on

human health have been widely studied, based on ﬁ ndings that the ingestion of

some of these species can cause accumulative poisoning, cancer, and nervous

system damage [57] . A variety of technologies exist for the removal of heavy metals,

including ﬁ ltration, surface complexation, chemical precipitation, ion exchange,

adsorption, electrode deposition, and membrane processing [58] . Among these

procedures, adsorption is considered one of the most attractive processes for heavy

metal removal from solution, as the adsorbents are generally easier to handle and

provide a greater operating ﬂ exibility [59] . Recent increasingly stringent standards

for the quality of drinking water have also catalyzed a growing effort in the devel-

opment of new, highly efﬁ cient adsorbents. The high surface area and chemical

stability of CNTs offer exciting possibilities for a new generation of adsorbents;

hence, some recently acquired data relating to the adsorption of a wide variety of

water contaminants are reviewed below. Here, emphasis is placed on the CNT

processing method, the adsorption capacities observed, and the prospects for their

successful application.

19.4.1.1 Adsorption of Lead ( II )

Li and collaborators [9] were the ﬁ rst to report experimental data on lead adsorp-

tion from water using CNTs. Lead is ubiquitous in the environment, and its inges-

tion is extremely hazardous; the consumption of drinking water containing high

levels of lead causes serious disorders, including anemia, kidney disease, and

mental retardation [60] . Li and colleagues have shown CNTs to show exceptional

adsorption capacities and a high adsorption efﬁ ciency of Pb(II) removal from

19.4 Carbon Nanotubes as Adsorbents 625

water. The CNTs used in these investigations were pretreated with nitric acid in

order to increase the adsorption capacities and to remove most of the catalyst

particles within the raw material. The amount of Pb(II) adsorbed onto the CNTs

was determined by the difference between the initial Pb(II) concentration and the

equilibrium Pb(II) concentration of the solution. The authors also monitored the

effect of varying the pH of the solution on lead adsorption.

The acid treatment was seen to have a major impact on the nanotubes ’ adsorp-

tion capacities. For example, whereas pristine as - grown CNTs had an adsorption

capacity for Pb(II) of 1 mg g

− 1

at pH 5, the capacity was increased remarkably (to

15.6 mg g

− 1

) when the CNTs were reﬂ uxed with concentrated nitric acid, again at

pH 5. It appears that acid oxidation of the CNTs leads to the introduction of many

functional groups, such as hydroxyl, carboxyl, and carbonyl onto the CNT surface

[61] , which in turn leads to improved adsorption capacity. Li also reported that the

removal of Pb(II) from water by acid - reﬂ uxed CNTs was highly dependent on the

solution pH, as this affects the surface charge of the adsorbents and the degree of

ionization and speciation of the adsorbates. The data in Figure 19.7 show that the

Pb(II) adsorption capacity of the CNTs was increased as the pH value was increased

from 3.0 to 7.0. It was proposed that, at low pH values, the adsorption of Pb(II)

was very weak due to the competition of H

with Pb(II) species for the adsorption

sites (Figure 19.7 , curve a). It was also proposed that, at pH 5, the adsorption

capability had increased due to role of functional groups present on the nanotube

surface (Figure 19.7 , curve b), and was further increased at pH 7 (Figure 19.7 ,

curve c). This might be the result of a combined effect of adsorption and a change

in the speciation of the lead ions. The results of other experiments have indicated

that, at pH 5 and room temperature, the amount of Pb(II) adsorbed onto the acid -

reﬂ uxed CNTs increased rapidly during the ﬁ rst 8 min (16.4 mg g

− 1

adsorbent,

81.6% removal), with equilibrium being reached after 40 min (17.5 mg g

− 1

, 87.8%

removal).

More recently, the same group [62] reported the adsorption thermodynamics

and kinetics of Pb(II) adsorption on CNTs, by evaluating various thermodynamic

parameters and employing a pseudo second - order kinetic model to describe the

Figure 19.7 Isotherms for Pb(II) adsorption by acid - reﬂ uxed

CNTs at different pH values. Curve a, pH = 3.0; curve b,

pH = 5.0; curve c, pH = 7.0. Reproduced from Ref. [9] .

626 19 Carbon Nanotubes as Adsorbents for the Removal of Surface Water Contaminants

adsorption processes. Based on their results, the authors concluded that the

adsorption of Pb(II) onto CNTs was endothermic; they also suggested that the

CNT material not only possessed a larger adsorption capacity but also showed a

good desorption rate. Such beneﬁ ts could result in a signiﬁ cant reduction in the

overall costs for adsorbent recycling. Desorption studies also revealed that Pb(II)

could be easily removed from the CNTs by altering the pH values of the solution

using both HCl and HNO

The results of another series of studies has further underlined the inﬂ uence of

the morphologies of CNTs on the removal of lead, in terms of speciﬁ c surface

area, particle size distribution, and type of functional group introduced to the CNT

wall [63] . The speciﬁ c surface area and pore volume of CNTs exposed to four dif-

ferent types of oxidation treatments were compared, and the pore and particle size

distributions of the CNTs evaluated. The results (as summarized in Table 19.2 )

indicated that the CNTs with the highest surface area, smallest particle size and

with a relative larger number of functional groups attached to their walls, showed

a maximum adsorption capability of 82.6 mg g

− 1

from a solution with an initial lead

concentration of 10 mg l

− 1

. Under the same conditions, the adsorption capacities

of samples with a lower number of functional groups attached, and with larger

wall defects, achieved adsorption capacities of only approximately 10 mg g

− 1

19.4.1.2 Adsorption of Chromium ( VI )

Chromium (VI) is one of the most toxic metals found in various industrial waste-

waters. Public health considerations of chromium are mostly related to Cr(VI)

compounds that are strong irritants due to their high solubility and diffusivity in

tissue; certain Cr(VI) compounds have also been shown to be carcinogenic and

mutagenic. The toxic effects of Cr(VI) ions in humans include liver damage, inter-

nal hemorrhages, respiratory disorders, dermatitis, skin ulceration, and chromo-

some aberrations [64] .

Di and collaborators [65] have investigated the suitability of CNTs for Cr(VI) (as

dichromate oxoanion) adsorption, and compared their behavior to that of activated

Table 19.2 Summary of results for lead adsorption in terms of

nanotube surface area, pore speciﬁ c volume, pore size

distribution, and particle size distribution. Adapted from

Ref. [63] .

BET

− 1

) V

(cm

− 1

) D

(cm

− 1

)

( μ m)

Sample 1 47 0.18 3.4 30 and 570

Sample 2 62 0.26 2.4 and 3.2 23 and 450

Sample 3 154 0.58 3.6 8 and 55

Sample 4 145 0.54 3.6 19 and 70

BET

− 1

) = BET surface area; V

(cm

− 1

) = pore speciﬁ c volume; D

(cm

− 1

) = mean pore

diameter: S

( μ m) = particle size.

19.4 Carbon Nanotubes as Adsorbents 627

Figure 19.8 Adsorption of Cr(VI) ions onto CNTs (

䉬

) and

activated carbon (

䉱

) as a function of pH. Cr(VI) ion

concentration = 5 mg g

− 1

CNT; carbon nanotube

concentration = 1 g l

− 1

; contact time = 12 h. Reproduced from

Ref. [65] .

carbon. In these studies, following pretreatment of both as - prepared CNTs and

activated carbon with nitric acid and hydroﬂ uoric acid, the CNTs were shown to

have superior adsorption capabilities and efﬁ ciencies for the removal of Cr(VI)

ions from water over the pH range 4.0 to 7.5 (see Figure 19.8 ). Activated carbon

was seen to adsorb chromate ions very rapidly at low pH values, but the adsorption

capacity declined very sharply at pH 3.5. In contrast, the adsorption efﬁ ciency of

the CNTs was maintained at over 90% over a wide pH range, although the Cr(VI)

ion adsorption capacity of CNTs fell sharply at pH 8. The authors attributed this

behavior to the inﬂ uence of the nanotube ’ s zeta potential. Subsequently, it was

shown experimentally that the isoelectric point of the CNT material was 7.7; at

higher pH values the surfaces of the CNTs became negatively charged and thus

inaccessible to the chromate anions. The authors also proposed competition of the

hydroxyl ions for the few adsorption sites available at these pH values. The data

in Figure 19.8 indicate that the highest Cr(VI) ion adsorption capacity was observed

over the pH range 4.0 to 7.5. At pH ≤ 3, the CNTs and activated carbon showed a

similar adsorption capacity, but at a higher pH the capacity of the CNTs was

greater. The maximum CNT adsorption capacity (20.56 mg g

− 1

) occurred at pH 7.5,

when the Cr(VI) concentration was 33.28 mg l

− 1

. The kinetic curves showed the

adsorption rate of Cr(VI) ions to be relatively high over the ﬁ rst 20 min, reaching

an adsorption capacity of 15 mg g

− 1

A few months later, improved values for chromium adsorption were reported

in a new study [66] . Here, a composite of aligned CNTs supported in ceria nano-

particles (CeO

/CNTs) was used as the adsorbent, the material being prepared by

the chemical reaction of CeCl

with NaOH in the presence of a CNT suspension,

followed by heat treatment. Scanning electron microscopy ( SEM ) images showed

628 19 Carbon Nanotubes as Adsorbents for the Removal of Surface Water Contaminants

that the CNT alignment was uniform, with lengths reaching 200 μ m and diameters

ranging from 20 to 80 nm. Transmission electron microscopy ( TEM ) analysis

indicated a homogeneous distribution on the ceria particles in the CNT network.

Overall, the study results indicated that highest capacity for Cr(VI) adsorption

occurred at pH values ranging from 3.0 to 7.4, with values of 30.3 mg Cr(VI) g

− 1

nanotube being observed at pH 7.0. The authors also compared the Cr(VI) adsorp-

tion isotherm obtained on the CeO

/CNT material with those obtained on activated

carbon and γ - Al

. As shown in Figure 19.9 , the adsorption capacity of the CeO

CNT material was 1.5 - fold higher than that observed for activated carbon, and

twofold larger than for Al

The authors proposed that the small size of the CeO

particles, and their uniform

distribution on the surface of the aligned nanotubes, contributed to the observed

high Cr(VI) adsorption. They also suggested that nanotube wall defects, produced

by the CVD synthesis process, could offer active sites for Cr(VI) adsorption on the

outer surfaces of the aligned nanotube array. The inner cavities and the opened

ends present in the inter - aligned nanotube space might also have contributed to

the effective adsorption of Cr(VI) ions.

19.4.1.3 Adsorption of Cadmium ( II )

Cadmium (II) represents a very high risk to human health due to its extremely

high toxicity, even in very small quantities. Drinking water with a cadmium content

in excess of permitted levels (0.005 mg l

− 1

) can cause nausea, salivation, diarrhea,

muscular cramps, renal degradation, lung insufﬁ ciency, bone lesions, cancer, and

hypertension [67] . Li et al. [68] have analyzed the suitability of CNT materials for

Cd(II) adsorption and its removal from water; the same group evaluated the efﬁ cacy

of several chemical treatments using three different oxidizing agents, namely H

KMnO

, and HNO

. The chemically treated CNTs were shown to have a larger

Figure 19.9 Adsorption isotherms of Cr(VI) on CeO

/CNT

materials compared with activated carbon, Al

and

ball - milled CNTs (at pH 5.0 and 25 ° C). Reproduced from

Ref. [66] .