**6. The effect of heteroatom doping on the ion exhange properties of carbon nanotubes**

Thus far it has been noted that the addition of oxygen-containing functional groups and oxygen as a heteroatom favours cation uptake by lowering the pHPZC. Likewise sulphurcontaining functional groups and sulphur as a heteroatom have been shown to have a similar effect for the uptake of Hg(II) [24]. However, the effect of other heterotoms such as the nitrogen atom have been poorly understood. Nitrogen –doped carbon nanotubes (N-CNTs) have been receiving much attention recently. Although the properties such as the semi-conducting properties of these materials are well understood [25] little is known about their function in metal ion exchange. Although Perez-Aguilar et al. [26] showed that oxidized nitrogen-doped carbon nanotubes were more effective for the uptake of Cd(II) and Pb(II) than undoped unoxidised MWCNTs the actual role played by the introduction of nitrogen as a heteroatom in this study was not clear. Pillay [14] investigated this further and found that the role played by the nitrogen atom depends largely on the form in which nitrogen occurs on the carbon nanotubes, For instance if quartenary nitrogen is present this can result in positively charged nitrogen centres which increase the pHPZC and favour anion uptake. On the other hand if nitrogen is occurring in conjunction with oxygen this can lower the pHPZC thereby favouring cation uptake.

Some emphasis has therefore been placed on the introduction of heteroatoms such as oxygen, sulphur and nitrogen atoms. The question of whether the introduction of other 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 specific target pollutants also needs to be addressed.
