Adsorption uptake of mordenite-type zeolites with varying Si/Al ratio on Zn2+ ions in aqueous solution

Abstract

The adsorption uptake of synthetic mordenite-type zeolites with varying Si/Al ratio was investigated for Zn2+ ions in aqueous solution to determine the influence of Si/Al ratio in removing heavy metals. Synthetic mordenites were hydrothermally synthesized from gel solutions with Si/Al ratio of 10, 15 and 20 as denoted by samples SAR10, SAR15 and SAR20 respectively. The samples were characterized using XRD, SEM, XRF, and TG-DTA. The adsorption kinetic and thermodynamic behaviour of the synthetic mordenites were examined. From the kinetic study, the pseudo-second-order kinetic model best fit the kinetic data among other models (i.e. pseudo-first order and intraparticle-diffusion models). It was found that cation-exchange was the most dominant adsorption mechanism. Further, it was observed that the pH level significantly affects the sorption of Zn2+ ions. The uptake of crystalline mordenite increases more than threefold from 7.19 mg Zn2+/g at pH = 3 to 24.27 mg Zn2+/g at pH = 7 using feed solution of 100 mg Zn2+/L. With regards to the equilibrium data, the adsorption isotherm generated from Langmuir model fits better than that of Freundlich model. Accordingly, the theoretical maximum adsorption capacities of SAR10, SAR15 and SAR20 are 39.97, 48.90, 32.48 mg Zn2+/g respectively. The lower the Si/Al ratio, the higher the negative charge density of zeolites resulting to a higher cation exchange capacity (CEC). However, lower Si/Al ratio do not automatically mean higher maximum adsorption capacity of mordenites to heavy metal ions especially Zn2+, as demonstrated in this study.