Fabrication of a chitosan-grafted-4‑vinylpyridine/thiol-amine-HZSM-5 nanocomposite via casting method in adsorption of heavy cations from water systems: an evaluation of adsorption mechanism

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Abstract
This study presents the synthesis of low-silica HZSM-5 zeolite through a hydrothermal process. Subsequently, chitosan-grafted-4‑vinylpyridine/thiol-amine-HZSM-5 nanocomposites were fabricated using casting method for the effective removal of copper (Cu2+) and zinc (Zn2+) cations from aqueous systems. The fabricated cast nanocomposites were characterized using XRD, BET, XPS, FESEM, EDX, CHNS, FTIR, and TGA analyses. The simultaneous roles of amine (–NH2) and thiol (–SH) groups in enhancing the adsorption efficiency of Cu2+ and Zn2+ were thoroughly investigated. Additionally, the influence of key factors, including solution pH, contact time, adsorption temperature, and cation concentration, was systematically assessed. Equilibrium data fitting revealed the dominance of monolayer adsorption, as evidenced by the excellent fit of the Redlich–Peterson (R-P) and Langmuir isotherm models for both Cu2+ and Zn2+ cations. Examination of the kinetic experimental data indicated a close correspondence with the double-exponential model. The maximum adsorption capacity of the fabricated cast nanocomposite was determined to be 328.05 mg/g for Cu2+ and 107.96 mg/g for Zn2+ cations. Additionally, the fabricated cast nanocomposite demonstrated satisfactory regeneration capabilities after 9 cycles of desorption. In both synthetic binary and ternary systems, as well as in real wastewater, the adsorption process exhibited antagonistic behavior, indicating that the presence of one type of cation interfered with the adsorption of the other. The nanocomposite displayed a higher affinity for Cu2⁺ compared to Zn2⁺ cations, in both synthetic and real systems, demonstrating its potential for selective heavy metal removal.

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