Effects of textural and chemical properties of β-zeolites on their performance as adsorbents for heavy metals removal

The adsorption behavior of β-zeolite prepared with different Si/Al ratio from synthesis gel and the effect of their chemical and textural properties on the zinc, copper and cadmium ions removal from aqueous solutions are studied in this work. The samples were characterized by N₂ physisorption, X-ray diffraction, X-ray fluorescence, scanning electron microscopy and solid-state ²⁷Al nuclear magnetic resonance spectroscopy. The chemical composition, structure, and textural properties of the β-zeolites were found to depend on the initial Al content in the synthesis gel. It was verified that the solid with higher content of framework aluminum also presented higher surface-to-volume ratio. This suggests that a higher initial Al content directly affects the nucleation kinetics of the β-structure, ensuring a solid with high density of accessible ion-exchange sites for metal uptake. Consequently, the amount of metal adsorbed onto β-zeolites was influenced by their textural and chemical properties. It was found that the monolayer capacity follows the order: Cd²⁺ (29.5 mg g⁻¹) ≈ Cu²⁺ (29.5 mg g⁻¹) > Zn²⁺ (17.3 mg g⁻¹) and Cd²⁺ (23.1 mg g⁻¹) > Cu²⁺ (14.1 mg g⁻¹) > Zn²⁺ (11.1 mg g⁻¹), for BETA 40 and BETA 60, respectively. The uptake selectivity order depends on the hydration energy, hydrated ion diameter and electronegativity of the metals. Kinetic studies followed the pseudo-second order model, while the adsorption isotherms were well described by the Langmuir model, suggesting that the adsorption process took place by ion exchange on the zeolite monolayer surface, since all values of mean free energy of adsorption are in the range 8–16 kJ mol⁻¹.