Adsorption of Mn(II) on green-synthesized magnetite-maghemite nanocomposite adsorbents: kinetic, equilibrium, and thermodynamic studies

Green-synthesized magnetic nanocomposite adsorbents can remove heavy metals, like Mn(II), from drinking water. This study aimed to investigate the equilibrium, kinetic, and thermodynamic analysis of the green-synthesized magnetite-maghemite nanocomposite adsorbents (GSMMNs). UV–vis, FT-IR, XRD, TEM, SEM, EDS, TGA, DLS, BET, XPS, zeta potential, and VSM studies have been employed to characterize the nanocomposite. The average particle size of the GSMMN was 14.34 nm, according to the results of the TEM investigation. The highest removal efficiency and adsorption capacity have been determined to be 93% and 79.36 mg/g, respectively, at an optimal initial pH of 6.5, Mn(II) ion concentration of 10 mg/L, and adsorbent dose of 1.5 g/L. Adsorption kinetics better fit the pseudo-second-order model, and the Langmuir model was associated more closely than the Freundlich model. The thermodynamic parameters, such as enthalpy (ΔH0), entropy changes (ΔS0), and free energy changes (ΔG0), validated the adsorption nature. The Mn(II) ion adsorption process is endothermic, spontaneous, and physisorption-controlled, as evidenced by the thermodynamic parameters ΔH0 (36.5 kJ/mol), $$\Delta$$ S0 (127 J/mol K), and ΔG0 (− 1.3 to − 4.5 kJ/mol). Incorporating organic moieties into GSMMN enhanced Mn(II) adsorption capacity due to additional functional groups. GSMMNs nanocomposite showed strong Mn(II) selectivity over other divalent cations. The wastes from regeneration cycles can be safely disposed of by precipitating as solid waste. The study demonstrated the potential of the GSMMNs adsorbent to eliminate Mn(II) ions from contaminated aqueous solutions because of its stability, reusability, and high adsorption capacity.