<p>The role of magnetic properties in metal ion adsorption remains unclear, particularly whether magnetite (Fe₃O₄) 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 &gt; 1) and Dubinin-Radushkevich (E &gt; 8&#xa0;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.</p> Graphical Abstract <p></p>