(320j) Acid Activated Bamboo-Type Carbon Nanotubes and Cup-Stacked-Type Carbon Nanostructures as Adsorbent Materials: Cadmium Removal from Water

Considerable amounts of heavy metals have been discharged to the environment since the beginning of the industrialization. Consequently, the concentration of toxic metals in rivers, lakes and oceans has increased notably. Cadmium, arsenic, copper, zinc, lead and other metals have been listed by the USEPA as the most common contaminants in wastewaters. Cadmium, in particular, has been identified as one of the most toxic contaminants that causes severs health problems to human beings, and harms flora and fauna. A wide variety of adsorbents materials, such as activated carbons, clays, zeolites, etc., have been utilized to remove heavy metals from aqueous solutions, although in recent years nanostructured materials have taken special attention due to their excellent physicochemical characteristics such as high surface area and strength, and have already had an ample application in electronic devices and composite materials. The applications of carbon nanostructures as adsorbent materials of contaminants present in solution have just begun to be explored; and recent reported results appear very promising despite the fact that these materials are not yet produced in large amounts. In this investigation bamboo-type carbon nanotubes and cup-stacked-type carbon nanostructures were modified with nitric acid at boiling point (about 84 oC) for one to three hours. The carbon nanostructures were characterized before and after oxidation by SEM, EDX, FTIR, and acid/base titration. Plus cadmium adsorption experiments were carried out at an initial concentration of 60 mg/L, 25 oC and pH 6 in batch reactors. Preliminary results have shown that the three hours oxidized carbon nanostructures have less Fe (catalyst) and the oxygen content increased about 2%, which was also reflected in a high amount of acid sites (about 2 mmol/g). These last findings were also supported by FTIR; treated materials showed more accentuated energy absorption peaks at wavelengths between 1750 and 1700 cm-1, which is attributed to higher concentration of carboxylic acids (COOH). Untreated and oxidized carbon nanostructures adsorbed about 2 and 14 mg of cadmium per g of material, respectively, which demonstrates that the introduction of oxygen-containing surface groups in the nanostructures enhanced the adsorption capacity. These preliminary findings indicate that the modified nanostructure materials studied herein have a potential application in the removal of contaminants and/or recovery of precious metals present in aqueous phase.