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

Abstract

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 acidbase 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.