Highly efficient removal of bivalent heavy metals from aqueous systems by magnetic porous Fe₃O₄-MnO₂: Adsorption behavior and process study

In this study, magnetic Fe₃O₄-MnO₂ nanoplates were successfully synthesized by a facile hydrothermal process in order to remove bivalent heavy metals from water. The Fe₃O₄ part facilitated the reclamation of the adsorbent. Meanwhile, the modification by amorphous MnO₂ could significantly enhance the specific surface area of Fe₃O₄-MnO₂ and decrease the point of zero charge, thus ensured the good adsorption capacity for metal cations. The adsorption isotherms and kinetics of the five metal ions on Fe₃O₄-MnO₂ and the pH effect on removal efficiency were detailedly investigated. Sips isotherm was the most suitable model to describe Pb²⁺, Cu²⁺, Cd²⁺ and Zn²⁺ adsorption on Fe₃O₄-MnO₂, while Temkin, Redlich-Peterson and Langmuir models showed a higher fitting degree for Ni²⁺. The adsorption capacities of Pb²⁺, Cu²⁺, Cd²⁺, Zn²⁺ and Ni²⁺ on Fe₃O₄-MnO₂ were 208.17, 111.90, 169.90 100.24 and 55.63 mg/g, respectively. Kinetic data were better fitted to pseudo-second-order model (R² > 0.99), indicating a dominant role of chemisorption. Further analysis of infrared spectra demonstrated that the adsorption process was achieved mainly by ion exchange and complexation. The adsorption rates and preference ranking of metal ions on Fe₃O₄-MnO₂ were in the same order: Pb²⁺ > Cu²⁺ > Cd²⁺ > Zn²⁺ > Ni²⁺. Moreover, the adsorbent showed good recyclability and excellent ability of neutralizing the solutions by surface protonation and deprotonation. Overall, the reported results here have significant implications for removing metal cations from water in practice.