**4. Functionalisation of carbon nanotubes**

A number of functional groups have been added to the surface of CNTs. These have included mostly oxygen-containing functional groups which take on the form of both –OH and carboxyl groups. This type of functionalisation has been primarily via oxidation of the CNTs in acid which results in the introduction of carboxyl groups. Consequently, these types of CNTs have been extensively used in the uptake of a number cations. These studies are summarized in **Table 1**.

Most of the above results conclusively support the fact that interaction between the functional groups and the cations are the primary means by which the cations are retained. The fact that an ion exchange mechanism predominates is also supported by the fact the

uptake of cations has been pH dependent in some cases. In general it has been observed that cation uptake is low at low pH and high at high pH. This suggests that electrostatic repulsions between the cations and excess H+ ions at low pH hinder the uptake of cations whereas electrostatic attractions between the cations and excess OH ion at high pH enhances the uptake of the cations.



TYPE OF

sodium hypochlorite solution

Oxidised MWCNTs

As grown and oxidised MWCNTs

SWCNTs and MWCNTs purified with a

enhances the uptake of the cations.

NANOTUBE POLLUTANT MAIN

uptake of cations has been pH dependent in some cases. In general it has been observed that cation uptake is low at low pH and high at high pH. This suggests that electrostatic repulsions between the cations and excess H+ ions at low pH hinder the uptake of cations

OBSERVATIONS COMMENTS REFERENCE

1. The concentration range studied was 10-80 mg L-1.

These workers also neglected to study the effect of competing cations and did not conduct any desorption experiments.

ion at high pH

Lu and Chiu[12]

Chen and Wang[15]

Kandeh and Meunier[16]

whereas electrostatic attractions between the cations and excess OH-

CNTs more favourable for the uptake of Zn(II) by making the CNTs more hydrophilic. 2. The adsorption was also pH dependent. 3. Both types of CNTs

were better adsorbents than powdered activated carbon (PAC)

could be desorbed from the oxidised MWCNTs at pH< 2 using distilled water as the desorbing

2. Lack of dependence on ionic strength. 3. Equilibrium is attained within 40 min of contact time

capacity of oxidised MWCNTs was superior.

2. A short time was required to attain equilibrium and noted that the

adsorption capability

Ni(II) Up to 93% Ni(II)

agent.

Ni(II) 1. Adsorption

method rendered the

Zn(II). 1. This purification


**Table 1.** Some Cation Uptake Studies by Oxidised CNTs (from Pillay [14])

Another interesting feature of the above results has been the general observation that surface modified CNTs show superior adsorption capabilities than that of conventional adsorbents like activated carbon and other low cost adsorbents. Furthermore, the methods of functional group introduction have varied from acid-treatment to purification with sodium hypochlorite. The primary atom that seems to be governing the surface charge hence the pHPZC seems to be the oxygen atom. Such an electron rich atom would impart a negative surface charge to these materials. However, it is by no means the only atom which can achieve this.

Recently Cech et al. [23] showed that sidewall thiolation of MWCNTs can be achieved by treatment with P2S5. and Pillay et. al [24] modified this method to produce sulphur containing MWCNTs which showed improved and selective uptake of Hg(II). Thus, both functional group and heteroatom introduction can be controlled by well-defined chemical treatment of CNTs. Such a method has been identified as acid and base treatment of the CNTs.

### **5. Acid and base treatment of carbon nanotubes**

96 Ion Exchange Technologies

NANOTUBE POLLUTANT MAIN

MWCNTs Cu(II) Adsorption was

Co(II) and Cu(II).

OBSERVATIONS COMMENTS REFERENCE

containing

at pH < 7.5.

The carbon

functional groups.

Adversely affected

nanomaterials only lost 5-11% of their adsorption

efficiency in saline environments where competing ions exist while activated carbon lost 30-50% of its adsorption efficiency.

Sheng et. al [21]

Pyrzynska and Bystrzejewski[22]

with PAA adsorbed onto the MWCNTs was higher than the strength between Ni(II) and oxygen

(PAA) enhanced the uptake at low pH. 2. That the adsorption was independent of the addition sequence of PAA and Ni(II). 3. The presence of foreign ions had no effect at low pH.

positively affected by the presence of humic and fulvic acids at

Both types of carbon nanomaterials had a superior adsorption capability to that of activated carbon.

Another interesting feature of the above results has been the general observation that surface modified CNTs show superior adsorption capabilities than that of conventional adsorbents like activated carbon and other low cost adsorbents. Furthermore, the methods of functional group introduction have varied from acid-treatment to purification with sodium hypochlorite. The primary atom that seems to be governing the surface charge hence the pHPZC seems to be the oxygen atom. Such an electron rich atom would impart a negative surface charge to these materials. However, it is by no means the only atom which

Recently Cech et al. [23] showed that sidewall thiolation of MWCNTs can be achieved by treatment with P2S5. and Pillay et. al [24] modified this method to produce sulphur containing MWCNTs which showed improved and selective uptake of Hg(II). Thus, both functional group and heteroatom introduction can be controlled by well-defined chemical

pH>7.5

**Table 1.** Some Cation Uptake Studies by Oxidised CNTs (from Pillay [14])

TYPE OF

Activated carbon, carbon nanotubes and

carbon encapsulated nanoparticles

can achieve this.

As discussed above acid treatment of CNTs leads primarily to the introduction of oxygen containing functional groups such as hydroxyl and carboxyl groups. The introduction of these functional groups subsequently leads to a negative surface charge which favours cation uptake. However, very little knowledge on the effect of base treatment is available. Pillay [14] attempted studying the effect of strong and weak acid mixtures on the uptake of both cations and anions. Here it was observed that treatment in both strong acid and base mixtures resulted in the introduction of oxygen-containing functional groups which favoured cation uptake by lowering the pHPZC of these materials. Conversely, treatment in a weak base like NH3 resulted in the introduction of nitrogen-containing functional groups which favoured anion uptake by increasing the pHPZC. Thus treatment in acid and base depends on the strength of the acid or base and the types of heterotoms which can be introduced. The ion exchange properties of the CNTs have therefore become dependent ultimately on heteroatom doping. The methods of heteroatom include functional group addition but the effects of individual heteroatoms depends on the individual atoms added the method of introduction.
