Sorption of divalent metal ions from aqueous solution by carbon nanotubes: A review

In this article, the technical feasibility of various kinds of raw and surface oxidized carbon nanotubes (CNTs) for sorption of divalent metal ions (Cd2+, Cu2+, Ni2+, Pb2+, Zn2+) from aqueous solution is reviewed. The sorption mechanisms appear mainly attributable to chemical interactions between the metal ions and the surface functional groups of the CNTs. The sorption capacities of CNTs remarkably increased after oxidized by NaOCl, HNO3 and KMnO4 solutions and such surface oxidized CNTs show great potential as superior sorbents for environmental protection applications. Effects of process parameters, such as CNT characterizations (surface area, pore size distribution, sorbent mass, and surface total acidity), solution properties (ionic strength, pH, initial sorbate concentration and temperature) and competition for sorption sites by multiple metal ions, on the performance of CNTs are addressed in some detail. The recovery of metal ions and the regeneration of CNTs can be achieved using acid elution with little effect on the CNT performance. The utilization of CNTs for the treatment of water and wastewater containing divalent metal ions is gaining more attention as a simple and effective means of pollution control. Future research works on developing a cost-effective way of CNT production and testing the toxicity of CNTs and CNT-related materials are recommended.