Sorption Of Divalent Metal Ions From Aqueous Solution By Oxidized carbon Nanotubes And Nanocages: A Review

Abstract

This is a scientific review defining the technical feasibility of surface oxidized carbon nanotubes (CNTs) for sorption of divalent metal ions (Cd 2+ , Cu 2+ , Ni 2+ , Pb 2+ , Zn 2+ ) from aqueous solution. By conducting the detailed literature review it was found that the adsorption capacities of CNTs remarkably increased after conducting their surface oxidization with NaOCl, HNO 3 and KMnO 4 solutions. Unlike many microporous adsorbents, CNTs possess fibrous shape with high aspect ratio, large accessible external surface area, and well developed mesopores, all contribute to the superior removal capacities of these ions. The adsorption mechanisms is majorly contributed by the chemical interactions between the metal ions and the surface functional groups of the CNTs. Larger the surface area greater will be the number of reducing groups hence more attributal to better CNT sorption performances. Effective process parameters defining CNT characterizations such as surface area pore size distribution, sorbent mass, and acidity at surface, solution properties (ionic strength, pH, initial adsorbate concentration and temperature) and competition for sorption sites by multiple metal ions, governs CNTs performances which are detailed in this review. The recovery of metal ions and the regeneration of CNTs can be achieved using acid elution with little effect on the CNT performance. Often during the production of CNTs a by-product is produced known as Carbon Nano Cages. These are hollow graphitic cages similar to fullerene structures having porous morphologies, but can be multilayered and have irregular shapes unlike the traditional fullerene spheres. The adsorption of divalent ions onto the surface of CNCs works as a function of solution pH. The increase in the adsorption of divalent ions with increasing pH for CNCs suggests that an ion exchange mechanism between the H+ ions and metal ions occurs at the oxygen-containing functional groups on the surface. Process parameters for CNC characterizations mainly elaborates upon zero point charge, dispersibility of sorbent materials in aqueous media, flowability of aqueous media through sorbent materials, variation of pH and agitation time. The kinetics of metal ion adsorption by the nanocages can be very well described by a pseudo-second-order kinetics model (detailed in this paper). The utilization of CNTs & CNCs for the treatment of water and wastewater containing divalent metal ions is gaining more attention as a simple and effective means of pollution control.

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