Selective adsorption of Pb2+, Cr3+ and Cd2+ mixtures on activated carbons prepared from waste tires

Heavy metal cation mixtures constitute a critical issue facing water treatment. Among available technologies, adsorption is here presented as a selective and effective technology. With this purpose, two adsorbents have been prepared by chemical activation of discarded tires, which in turn, constitutes the valorization of a residue from the automotive industry. Two different activating agent:tire ratios by weight (1:1 and 4:1) were employed, and the activated carbons obtained, namely AC-1 and AC-4, were characterized with Scanning Electron Microscopy (SEM), N2 adsorption-desorption, acid-base titrations, determination of pHpzc and Temperature Programmed Decomposition (TPD). Different textural and acid-base properties were revealed upon different activation conditions, obtaining higher surface area, microporosity and surface acidity for AC-4. The sample AC-1, with lower surface area, consistently led to higher adsorption values for the three cations (Pb2+, Cr3+, Cd2+), indicating that the amount adsorbed was rather governed by type of surface groups present in the adsorbent under the conditions tested. Regarding the amount adsorbed, the sequence Pb2+ > Cr3+ > Cd2+ was measured in individual and mixing conditions, independently of the carbon, and adsorption equilibrium and kinetics were fitted to different models, being Langmuir and pseudo-second order equations the models that better reproduced the equilibrium and kinetic data, respectively.