Adsorption of cadmium on carbonaceous adsorbents developed from used tire rubber

Carbonaceous adsorbents (CAs) are developed from used tire rubber (UTR) and tested as adsorbents of Cd2+ in aqueous solution. In the preparation of the CAs, UTR was treated thermally at 400-900 degrees C for 2 h in N-2 and at 850 degrees C for 2 h in steam. Concentrated NaOH, HCl, H2SO4, HNO3 and H2O2 solutions were also used. UTR and H900 (i.e. UTR pyrolyzed at 900 degrees C) were treated with O-3 at 25 degrees C for 1 h and with air at 250 degrees C for 1 and 24 h. CAs were characterized texturally by N-2 adsorption at -196 degrees C, mercury porosimetry, and density measurements. The surface groups were analyzed by FT-IR spectroscopy. Using the batch method, the adsorption process of Cd2+ was studied mainly from the kinetic standpoint at various pH values of the adsorptive solution. Significant porosity developments are achieved only when UTR is heat-treated, in particular in steam. However, the variety and concentration of surface groups are low in CAs. This is so even for CAs prepared using oxidizing agents as strong as O-3 and H2O2, which has been associated with a lack of available or accessible surface active sites for oxidation in UTR and H900, respectively. Thermal and thermal-chemical treatments are usually more effective than chemical treatments to increase the adsorption of Cd2+ in aqueous solution. The adsorption process of Cd2+ is first fast and then much slower. Adsorption-time data fit better to a pseudo-second order kinetic equation than to a pseudo-first order kinetic equation. The extent to which the adsorption process occurs is strongly dependent on the pH of the Cd2+ solution, being larger at pH 4.6 or 7.0 according to the adsorbent. (C) 2011 Elsevier Ltd. All rights reserved.