Biofunctionalized hierarchical mesoporous zeolite 5A–maghemite hybrids for Cd(II) adsorption: Surface chemistry and interfacial binding mechanisms

Cadmium contamination of water requires adsorbents that combine high removal efficiency, rapid kinetics, and easy recovery after treatment. In this work, two families of biofunctional magnetic nanohybrids, MCZO and MCZ1, were prepared by combining zeolite 5A with γ-Fe 2 O 3 nanoprecursors in the presence of Citrus reticulata peel extract and evaluated for the Cd(II) adsorption capacity in aqueous solution. Multi-technique characterization confirmed the coexistence of zeolite 5A and γ-Fe 2 O 3 , the preservation of Fe(III), and the absence of a new crystalline Cd-containing phase after adsorption. The results also showed that both hybrids contain oxygenated functional groups on their surface, while the MCZ1 nanoadsorbent exhibits a higher degree of plant-derived organic functionalization and a more chemically heterogeneous surface. Adsorption experiments revealed a strong pH dependence, with equilibrium being reached in approximately 20–30 min and maximum removal being recorded in near-neutral environments. The kinetic data were best described by the pseudo-second-order model, which is consistent with the relevant participation of specific surface interactions, although kinetic fitting alone cannot be considered direct proof of chemisorption. Equilibrium data were better fitted by Langmuir/Sips-type models, with theoretical maximum capacities of approximately 180–200 mg g −1 for the MCZO and 185–213 mg g −1 for the MCZ1 nanoadsorbent. XPS and ATR-FTIR/Raman analyses provided evidence compatible with a cooperative uptake process involving zeolitic Al–O–Si and silanol sites, Fe–O surface groups, and plant-derived oxygenated functions, with a stronger contribution of the latter in MCZ1. Reuse experiments also demonstrated the superior cyclic stability of the MCZ1 nanoadsorbent, which retained 82% removal after five cycles, compared with 62% for the MCZO nanoadsorbent. Overall, the results identified the MCZ1 sample as the more promising magnetically recoverable hybrid adsorbent for the Cd(II) removal from water.