Adsorption of cadmium on carbonaceous adsorbents developed from used tire rubber

Abstract

Carbonaceous adsorbents (CAs) are developed from used tire rubber (UTR) and tested as adsorbents of Cd 2+ in aqueous solution. In the preparation of the CAs, UTR was treated thermally at 400–900 °C for 2 h in N 2 and at 850 °C for 2 h in steam. Concentrated NaOH, HCl, H 2SO 4, HNO 3 and H 2O 2 solutions were also used. UTR and H900 (i.e. UTR pyrolyzed at 900 °C) were treated with O 3 at 25 °C for 1 h and with air at 250 °C for 1 and 24 h. CAs were characterized texturally by N 2 adsorption at −196 °C, mercury porosimetry, and density measurements. The surface groups were analyzed by FT-IR spectroscopy. Using the batch method, the adsorption process of Cd 2+ 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 H 2O 2, 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 Cd 2+ in aqueous solution. The adsorption process of Cd 2+ 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 Cd 2+ solution, being larger at pH 4.6 or 7.0 according to the adsorbent.