Sustainable Magnetic Cellulose–Alginate Beads for Selective Biosorption of Cd (II) and Ni (II) Ions from Contaminated Waters

Abstract The role of magnetic properties in metal ion adsorption remains unclear, particularly whether magnetite (Fe 3 O 4 ) enhances the uptake of paramagnetic ions such as Ni (II) compared to diamagnetic ions such as Cd (II). To address this, cellulose/alginate/magnetite (CEL/ALG/MAG) composites were synthesized via a green route and structurally characterized (FTIR-ATR, XRD, SEM, BET), confirming magnetite formation and revealed Fe²⁺/Fe³⁺ mediated crosslinking between components, which reduced cellulose crystallinity and created rough, cavity-rich surfaces despite low specific surface area. Batch adsorption experiments demonstrated removal efficiencies of ≥ 94.8% (1.34 mg g − 1 ) for Cd (II) and up to 94.4% (0.46 mg g − 1 ) for Ni (II) at pH 6. Kinetic and isothermal studies indicated that chemical mechanisms, such as complexation interactions, also play a relevant role in the process, complementing electrostatic interactions processes are involved in adsorption, as described by the pseudo-second-order Freundlich (n F > 1) and Dubinin-Radushkevich (E > 8 kJ mol − 1 ) models. Based on magnetic interactions, a preferential adsorption of Ni (II) was hypothesized; however, competitive adsorption experiments revealed the composites showed preferential Cd (II) adsorption (0.29 mg g − 1 – 92.9% vs. 0.22 mg g − 1 – 65% for Ni (II)). This unexpected selectivity is attributed to intrinsic physicochemical factors: larger ionic radius and smaller hydration sphere of Cd (II) promote stronger binding to biopolymer functional groups, whereas Ni (II) is hindered by extensive hydration, indicating that magnetic domains play a limited role in competitive adsorption. Beyond mechanistic advances, the magnetically recoverable materials combine sustainability with facile separation, highlighting their potential in water purification. Graphical Abstract