Relationship between Si/Al ratio and the sorption of Cd(II) by natural and modified clinoptilolite-rich tuff with sulfuric acid

In this paper, the differences in the adsorption mechanisms and adsorption capacities of clinoptilolite materials with different Si/Al ratio (SAR) for Cd2+ ions from aqueous solutions are discussed. The adsorbents were characterized with respect to their phase composition, morphology, specific surface area, cation exchange capacity and point of zero charge. Batch adsorption experiments were performed considering the Cd initial ion concentration, contact time, adsorbent dose, SAR, and pH. The regression coefficient value revealed that the experimental data best fit to Pseudo-second-order model, whilst the kinetic rate constant k(2) (g/mg min(-1)) showed an exponential behavior as a function of adsorbent mass for all clinoptilolite materials. The equilibrium adsorption data were best described by the Langmuir adsorption isotherms with highest q(m) = 5.974 mg/g for Nat-CLI, whereas K-L parameter was found increased with increasing SAR. The adsorption capacities of the acid-modified clinoptilolite (high SAR) were lower than that of natural zeolite because of the dealumination of the zeolitic material and consequently the loss of the ion exchange sites.