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

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.
