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 °C for 2 h in N2 and at 850 °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 °C) were treated with O3 at 25 °C for 1 h and with air at 250 °C for 1 and 24 h. CAs were characterized texturally by N2 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 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 O3 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.