Heterogeneous multilayer adsorption of methylene blue on MgFe-LDH/Fe₃O₄/biochar nanocomposite: CCD-based optimization, isotherm modeling, and thermodynamic insights

A magnetically separable MgFe-LDH/Fe₃O₄/Biochar nanocomposite was synthesized as an efficient adsorbent for methylene blue (MB) removal. The biochar precursor was derived from Hordeum murinum (wild barley). XRD, FTIR, Raman, BET, FESEM–EDS, and XPS analyses confirmed the successful integration of LDH nanosheets and Fe₃O₄ nanoparticles onto a porous carbon matrix, providing abundant active sites, surface heterogeneity, and magnetic recoverability. Process optimization using Central Composite Design (CCD) revealed significant effects of pH, adsorbent dosage, temperature, and contact time. The quadratic model (R 2 = 0.95) predicted optimum MB removal of 93.8%, experimentally validated at 94.2% under optimal conditions (dosage = 168.3 mg L −1 , pH = 5.7, time = 104 min, temperature = 35.05 °C). Kinetic data followed the pseudo-second-order model (R 2 > 0.99), while the Freundlich isotherm (R 2 = 0.91, n = 3.23) confirmed multilayer adsorption on a heterogeneous surface. Thermodynamic parameters (ΔH° = 2.18–13.89 kJ mol −1 , ΔS° = 9.9–64.9 J mol −1 K −1 , ΔG° < 0) indicated a spontaneous, endothermic, physisorption-driven process. The nanocomposite retained >80% of its removal efficiency after five cycles and achieved >81% removal for MB, Acid Red 14 (anionic dye), and Cd 2+ (heavy metal) in real groundwater. The estimated E -factor was ≈31, within the fine chemicals range (5–50). Overall, the synergistic interplay between MgFe-LDH, Fe₃O₄, and biochar results in a stable, recyclable, and versatile adsorbent for decentralized wastewater treatment. • MgFe-LDH/Fe₃O₄/Biochar synthesized via hydrothermal process. • High MB removal (94.2%) optimized by CCD-RSM. • Freundlich and PSO models confirmed multilayer adsorption. • Spontaneous, endothermic, physisorption-driven ( E -factor ≈31). • Magnetically recoverable (>80% after 5 cycles) and effective in real water.