Magnetically controlled alginate-encapsulated aluminosilicates: highly effective sorbents for the target removal of...

A series of alginate-encapsulated composites (AECs) based on zeolite (NaX) and montmorillonite (MMT) and their magnetite-functionalized derivatives (Alg/NaX/Fe₃O₄ and Alg/MMT/Fe₃O₄) were synthesized using a natural polysaccharide, sodium alginate, cross-linked with calcium ions. The composites were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The synthesized AECs showed good adsorption properties for metal ions. The maximum Cd(II) adsorption reached 0.74-0.76 mmol/g for NaX- and MMT-containing AECs and 0.69-0.70 mmol/g for magnetite-incorporated AECs. Fe(II) uptake showed a similar trend. The sorption capacity was 0.43-0.41 mmol/g for AECs without magnetite and 0.38 and 0.28 mmol/g for Alg/NaX/Fe₃O₄ and Alg/MMT/Fe₃O₄, respectively. The time dependence of ion sorption was investigated, and the sorption mechanism was analyzed. The experimental adsorption kinetics aligned with the pseudo-second-order model, indicating that chemisorption is the main mechanism for the interaction of the ions with the surface groups of the sorbents. The AECs achieved 96-99% removal of Cd(II) and 83-97% removal of Fe(II) ions, confirming their high efficiency in extracting metal ions. Thermal analysis showed that the composites exhibit high stability, with decomposition starting above 150 °C. This makes them suitable for use in hot aqueous solutions and enables efficient magnetic separation.