Methylene blue adsorption by synergistic clay-chitosan-alginate hydrogel beads: Batch and fixed-bed column studies...

Methylene blue (MB), a persistent cationic dye widely detected in industrial effluents, requires efficient and scalable removal strategies. In this work, clay-chitosan-alginate (Clay-Cs@Alg) composite hydrogel beads were engineered to combine inorganic adsorption domains with biopolymer-derived functional groups, aiming to enhance structural stability and adsorption performance under both batch and continuous conditions. Structural and surface properties were characterized by SEM-EDS, XRD, FTIR, TGA, and zeta potential analyses, confirming the formation of a hybrid network with pH-responsive charge behavior. Batch adsorption experiments revealed a maximum capacity of 303.03 mg·g^-1 at 15 °C. Equilibrium data were best fitted by the Temkin model, indicating heterogeneous adsorption energies and significant adsorbate-surface interactions. Nonlinear kinetic modeling combined with multi-parameter statistical validation (R², RMSE, χ², AIC) demonstrated that the pseudo-first-order model most consistently describes the adsorption rate, while intraparticle diffusion contributes without being the sole rate-limiting step. Thermodynamic analysis confirmed a spontaneous and strongly exothermic process (ΔH° = -19.953 kJ·mol^-1). Fixed-bed column performance was successfully predicted by the Thomas model, highlighting applicability under dynamic flow conditions. The composite maintained high removal efficiency over five regeneration cycles, demonstrating operational stability and reusability.