A Comparative Study on the Adsorption Behavior of Heavy Metal Ions on Zeolites 13X and 4A in Neutral Solution

The present study aimed to investigate the adsorption behavior of heavy metal ions (Cu²⁺, Cd²⁺, and Pb²⁺) on zeolite 4A (26.9 m²/g) and zeolite 13X (550.1 m²/g) at neutral pH in batch experiments. Influencing parameters on the adsorption were studied, including equilibrium time, solution pH, and adsorbent dose. The metal removal efficiencies at neutral pH were considerably higher than those at acidic pH. Cu²⁺ and Cd²⁺ were more efficiently adsorbed on zeolite 4A, while the greater removal of Pb²⁺ was achieved with zeolite 13X. With an initial concentration of 300 ppm and an adsorbent-to-liquid ratio of 0.2% w/v, the highest removal efficiencies were 91.1% for Cu²⁺ and 95.2% for Cd²⁺ with zeolite 4A, and 92.9% for Pb²⁺ with zeolite 13X. The experimental data fitted well with the Langmuir isotherm and the pseudo-second-order models, indicating the monolayer formations and the dominance of chemisorption. Zeolite 4A showed higher pseudo-second-order rate constants than zeolite 13X. The intraparticle diffusion contributed more significantly to the adsorption of Cu²⁺ and Cd²⁺ than that of Pb²⁺ on the zeolites. The findings from this study provide valuable insights into the adsorption behavior of heavy metals on different kinds of zeolites in neutral solution.