Adsorption of hexavalent chromium on carbon nanotubes: influences of surface chemistry and pore structure

Oxygen containing functional groups were introduced onto the surface of carbon nanotubes (CNTs) by oxidation with ammonium persulphate and nitric acid respectively. The capacities of oxidised CNTs to absorb Cr(VI) from simulating drinking water at 25 degrees C were found to be influenced by surface chemistry and pore structure. The characteristic (002) X-ray diffraction peaks of oxidised CNTs were intensified relative to the unmodified CNTs, and their crystallite sizes were increased. The distribution range of the pore size of oxidised CNTs became narrower. The amounts of oxygen containing functional groups, such as carboxylic, phenolic and lactonic groups, were greatly enhanced by oxidation. The point of zero charge was greatly decreased from 5.08 of the original CNTs to 3.35 and 3.30 for the CNTs oxidised with (NH4)(2)S2O8 and HNO3 respectively. The results showed that the saturation adsorption of Cr(VI) on the oxidised CNTs was increased due to the oxygen containing functional groups despite the fact that the surface areas and pore volumes were decreased, especially on the HNO3 oxidised CNTs.