**8. Conclusion**

From the information reviewed in this chapter it is evident that carbon nanomaterials are a new form of ion-exchangers based on the diverse number of metal ion uptake studies which have been conducted using these materials. An interesting feature of these materials has been the fact that they can be modified to function as both cation and anion exchangers. This is carefully controlled by both the introduction of functional groups and heteroatoms which ultimately impact on the surface charge. However, the selectivity of these materials for specific ions still needs improvement. Furthermore, the heteroatoms introduced have been restricted to oxygen, sulphur and nitrogen. The effect of other heteroatoms also need to be explored. Even acid and base treatments contribute mainly to the introduction of oxygen and nitrogen atoms. Such topics now lend themselves to future research perspectives.

## **Nomenclature:**



## **Author details**

98 Ion Exchange Technologies

**8. Conclusion** 

**Nomenclature:** 

CNTs Carbon Nanotubes

SWCNTs Single-walled Carbon Nanotubes

specific target pollutants also needs to be addressed.

**7. Carbon nanotubes as selective ion exchangers** 

heteroatoms particularly less electronegative atoms can also influence the ion-exchange properties of CNTs has arisen. Again this is an area which has not been extensively explored. Although Aguiar et al [27] showed the SWCNTs doped with iron were effective for the removal of benzonitrile, the question of how this would affect metal ion exchange still needs to be investigated. Furthermore the introduction of other electronegative atoms such as the halogens needs to be explored. Tan et al [28] also showed the introduction of surfactants to these materials also influence the ion-exchange properties where such a system provided a counterion system for the uptake of Ni(II). The issue of whether the introduction of such atoms or groups of atoms can also impact on the selectivity of CNTs for

Carbon nanotubes are therefore a new form of ion-exchangers which have demonstrated potential to extract a number of cations and anions based on the functional groups and heteroatoms which are present. However, the selectivity of these materials for specific pollutants is still questionable and limited studies in this area have been conducted. Li et al. [29] attempted addressing this issue by showing that MWCNTs are able to adsorb Pb(II) more efficiently in the presence of other competing cations such as Cd(II) and Cu(II). However, complete selectivity was not achieved. Pillay et al. [13] also showed that unfunctionalised MWCNTs were not selective to the uptake of Cr(VI) in the presence of competing anions such as chloride and sulphate ions. This also applied to industrial effluents where sulphites are present [14]. In fact, the only study to date which been the study in which sulphur-doped MWCNTs showed selectivity for Hg(II) in the presence of

competing ions [24]. Thus the selectivity of these ion-exchangers needs improvement.

and nitrogen atoms. Such topics now lend themselves to future research perspectives.

From the information reviewed in this chapter it is evident that carbon nanomaterials are a new form of ion-exchangers based on the diverse number of metal ion uptake studies which have been conducted using these materials. An interesting feature of these materials has been the fact that they can be modified to function as both cation and anion exchangers. This is carefully controlled by both the introduction of functional groups and heteroatoms which ultimately impact on the surface charge. However, the selectivity of these materials for specific ions still needs improvement. Furthermore, the heteroatoms introduced have been restricted to oxygen, sulphur and nitrogen. The effect of other heteroatoms also need to be explored. Even acid and base treatments contribute mainly to the introduction of oxygen Kriveshini Pillay

*Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa* 
