Novel Magnetic Chitosan@Slag/Fe-MNP Nanocomposites for Methyl Orange Removal and Circular Valorization

The pervasive contamination of water by synthetic dyes such as methyl orange (MO) presents a critical global challenge, compounded by the vast, underutilized volumes of industrial solid waste such as steel slag. This study addresses this dual problem via a circular-economy strategy that transforms waste into a functional material. A novel, magnetically separable adsorbent was developed via nickel-ion crosslinking of chitosan (CS) onto a composite of slag and slag-derived magnetic iron nanoparticles (Chitosan@Slag/Fe-MNPs). The material was characterized by FTIR, XRD, BET, and FESEM, and its adsorption performance was evaluated through systematic batch studies. The composite achieved a maximum monolayer adsorption capacity of 98.52 mg g−1 for MO at 25 °C, following the Elovich model and a Langmuir isotherm. MO removal is primarily driven by electrostatic attraction and surface complexation between the anionic dye and the protonated amine/metal-oxide sites of the composite, with pseudo-second-order-consistent kinetics indicating a possible partial contribution from surface-reaction-limited (chemisorption-type) steps. The composite showed high selectivity for anionic dyes at acidic pH, excellent magnetic recovery, and moderate reusability, retaining approximately 47% of its initial removal efficiency after four regeneration cycles. Beyond adsorption, the spent MO-saturated composite was successfully repurposed as an electrocatalyst for the urea oxidation reaction (UOR), achieving a current density of 234.30 mA cm−2 at 1.0 M urea, representing a result attributable to urea oxidation rather than MO redox activity, as confirmed by the urea-concentration-dependence of the response. This waste-to-adsorbent-to-electrocatalyst strategy supports a sequential remediation-to-energy conversion pathway consistent with circular economy principles.