Biomass-Derived Hydrochar Functionalized with Mg–Fe Layered Double Hydroxide for Bicomponent Cd(II)/Zn(II) Adsorption in Aqueous Systems

Toxic metal contamination in aquatic systems commonly occurs as multicomponent mixtures, making competitive adsorption assessment essential for realistic adsorbent evaluation. This study investigated corn stalk-derived hydrochar functionalized with Mg-Fe layered double hydroxide (Mg–Fe-LDH@HC) for simultaneous Cd(II) and Zn(II) adsorption in aqueous bicomponent systems. The material was evaluated through pH and dosage optimization, kinetic assays, bicomponent equilibrium modeling, thermodynamic assessment, mixture-design experiments, regeneration tests, and applicability assays with interfering ions and real water matrices. Under the selected conditions, pH 6.75, 4 g L−1 Mg–Fe-LDH@HC, 1 mM equimolar Cd(II)/Zn(II), 298.15 K, and 180 min, near-complete removal of both metals was achieved. Kinetic analysis showed rapid initial uptake followed by a slower approach to equilibrium. Bangham, Elovich, and Weber-Morris analyses supported a multistage adsorption process involving external surface uptake, diffusion-related resistance, and heterogeneous surface interactions, although intraparticle diffusion was not the sole rate-controlling step. Bicomponent equilibrium was better described by heterogeneous models, particularly the double-layer model and Extended Sips, indicating non-equivalent adsorption domains. Thermodynamic parameters showed favorable and mildly endothermic adsorption with limited temperature dependence. Mixture-design experiments demonstrated that metal proportion influenced adsorption more strongly than temperature, with increasing Cd(II) fractions reducing Zn(II) retention. Overall, Mg–Fe-LDH@HC showed promising performance for Cd(II)/Zn(II) removal under competitive conditions, although the adsorption pathway should be interpreted as an evidence-supported combined process rather than individually confirmed mechanisms.