**3. Carbon nanomaterials as anion and cation exchangers**

92 Ion Exchange Technologies

treatment.

future research.

**2. Problem statement** 

modifications to these materials improve the ion exchange capacities. Such modifications

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

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

**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]

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

include heteroatom doping and functionalisation of the surfaces of these materials.

In some recent studies conducted it has been shown that surface modifications to carbon nanotubes have played a key role influencing their ion exchange properties. It has been said that oxidized CNT's show a better potential for cation uptake than unoxidised CNTs [4]. Conversely, it has also been shown that unoxidised MWCNTs are more effective for uptake of anions such as dichromate than oxidized MWCNTs [13]. In such observations it has been argued that oxidation has an impact on the point of zero charge (pHPZC) of these materials which ultimately governs the overall surface charge [4,13,14].

Typically, oxidation lowers the pHPZC of these materials thereby resulting in a predominantly negatively charged surface which is more favorable for cation uptake. Likewise, unmodified or unoxidised CNTs have a higher pHPZC which results in a predominantly positively charged surface which is more favorable for anion uptake. This is supported conclusively by experimental evidence where Rao et al. [4] have reported that oxidized SWCNTs showed a better uptake for Ni(II) and Zn(II) ions than unmodified MWCNTs. Further, Pillay et al. [13] have also shown conclusively that unmodified MWCNTs were more effective for removal of Cr(VI) than oxidized MWCNTs due to a higher (pHPZC).

The question of how these surface modifications can be achieved so as to control the ionexchange properties of these materials has thus arisen. This can be achieved in two ways. One way includes the addition of surface functional groups and the other is via heteroatom doping. These methods are discussed below.
