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.
β-Zeolite Ion exchange Heavy metals Adsorption kinetics Adsorption isotherms
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The authors acknowledge the FAPERJ (Grant No. E-26/010.001842/2015) for financial support to this work. The support from CNPq, FAPES and CAPES is also gratefully acknowledged. The authors are thankful to MSc. Daniel F. Cipriano for his help with the NMR experiments and to Instituto Nacional de Tecnologia for the N2-physisorption and XRD experiments.
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