**1. Introduction**

Carbon nanomaterials are a new form of carbon based materials which have been receiving much attention for their interesting properties over the past three decades. The materials were initially discovered in 1952 [1] but were only scientifically recognized in 1991 when Iijima submitted a report [2]. An interesting feature of these allotropes of carbon has been the different forms in which they can exist. These include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). These structures are depicted in **Figure 1**. The fundamental difference between these unique forms of carbon lies in their structural properties. SWCNTs and MWCNTs differ with respect to number of graphene carbon sheets.

Nevertheless, these carbon based materials have demonstrated a vast array of physical and chemical properties. Such properties include outstanding mechanical properties, electrical properties, chemical and thermal stability and large specific surface areas [4]. Consequently, much emphasis has been placed on synthesizing these materials and using them in a number of applications which include nanodevices [5], field emissions [6], plasma apheresis [7], catalyst supports [8], biosensors [9] and chemosensors [10,11]. However, one application of much interest has been their use in environmental remediation. Such an application has arisen from their well-defined porosity and functionality [12]. This property has enabled them to demonstrate superior adsorption capabilities to that of conventional adsorbents like activated carbon [12]. Among, the numerous adsorption applications examined, metal ion uptake by these materials has featured quite prominently. However, the actual mechanism of metal ion adsorption by these materials has been poorly understood.

This chapter therefore serves to examine the ion-exchange properties of carbon nanomaterials. The ability of wide range of nanomaterials namely carbon nanotubes to act as anion and cation exchangers are discussed. Such a discussion includes potential

© 2012 Pillay, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Pillay, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

modifications to these materials improve the ion exchange capacities. Such modifications include heteroatom doping and functionalisation of the surfaces of these materials.

With regards to functionalisation acid-base treatment is discussed. The focus in this section of the chapter is mainly on the how the introduction of functional groups alters the surface of the nanomaterials thereby improving the ion exchange capacities. The influence of various functional groups on the point of zero charge is examined. Such effects are then extended to the effect of heteroatom doping such as sulphur-doping and acid-base treatment.

Lastly, the ability of carbon nanomaterials to act as selective ion-exchangers is reviewed. This includes a discussion on how these materials can function as ion-exchangers in complex matrices. The use of these materials as ion-exchangers in real industrial effluents is also discussed. The way forward with respect to using carbon nanomaterials in analytical ion-exchange applications is discussed with some conclusions and recommendations for future research.

**Figure 1.** The two main types of carbon nanotubes a) Single –walled carbon nanotubes ( b) Multiwalled carbon nanotubes (from: http://www.cnano-rhone-alpes.org/spip.php?article57&lang=en [3]

### **2. Problem statement**

Both SWCNTs and MWCNTs have been extensively used in a number of environmental applications. These include the removal of both organic and inorganic contaminants by adsorption. However, as far as the uptake of ions (especially metal ions) is concerned, ionexchange has been identified as one of the key mechanisms governing this process. The key question to be addressed here is how can these carbon nanotubes act as ion-exchangers? What properties of these materials enable them to possess ion-exchange characteristics? A number of researchers have attempted addressing these questions and the findings of the research conducted to date are reviewed below.
