Comparative study of the structural characteristics and adsorption capacity of natural and synthetic zeolites

A comparative study was conducted on the structural and adsorption properties of natural zeolite (clinoptilolite), commercial dietary supplements based on it, subjected to mechanical and tribomechanical activation, and synthetic zeolites NaA and NaX. Structural characterization was carried out using XRD, FTIR spectroscopy, SEM, porometry, and low-temperature nitrogen adsorption. Langmuir, Freundlich and Temkin models were used to evaluate the adsorption mechanism. Clinoptilolite was identified as the dominant crystalline phase (up to 70 %) in natural and activated samples, whereas NaA and NaX exhibited LTA- and FAU-type frameworks, respectively. The SSA ranged from 13.5 m²/g for natural zeolite to 27.8 m²/g for the activated samples, and reached 460.7 m²/g for the synthetic zeolite NaX. Tribomechanically activated zeolites exhibited pronounced mesoporosity (up to 98.6 %) with an average pore radius of 5.4–6.6 nm. According to the Langmuir model (R² ≥ 0.997), the q_max for Cu²⁺ were 55.3–59.2 mg g⁻¹ for NaA and NaX, 10.1 mg g⁻¹ for natural zeolite, and 7.2 mg g⁻¹ for activated samples. In contrast, Methylene blue was more effectively adsorbed by natural and activated zeolites (q_max are from 13.2 to 34.3 mg g⁻¹, respectively), while NaA and NaX showed much lower uptake (6.9 mg g⁻¹). Congo red adsorption was less efficient for all samples. Adsorption of proteins occurred mainly on the external surfaces of zeolite particles, with q_max reaching 96.2 mg g⁻¹ (gelatin) and 112.4 mg g⁻¹ (ovalbumin) for activated zeolites. Thus, synthetic zeolites are more effective for metal ion removal, while modified natural zeolites are more effective for low-molecular-weight compounds and protein adsorption, supporting their potential use as multifunctional enterosorbents.