Mechanistic insights into heavy metals affinity in magnetic MnO₂@Fe₃O₄/poly(m-phenylenediamine) core-shell adsorbent

Adsorption represents an attractive mean to remediate polluted water. Unfortunately, the surface positive charges, low surface area and complicated separation procedures inhibit the usability of poly (m-phenylenediamine) (PmPD) as an adsorbent for heavy metal removing. To overcome these drawbacks, a magnetic MnO₂@Fe₃O₄/PmPD core-shell adsorbent was designed to remove heavy metals from water. The MnO₂ shell, came from the redox reaction between KMnO₄ and PmPD, increased the surface area and changed the surface electronegativity. MnO₂@Fe₃O₄/PmPD could be easily separated from water. It showed a significant increase in heavy metals removal efficiency, with maximum capacities of 438.6 mg/g for Pb(II) and 121.5 mg/g for Cd(II), respectively. The affinity between heavy metals and MnO₂@Fe₃O₄/PmPD were mainly due to electrostatic attraction, ion exchanges and coordinated interaction. Density functional theory (DFT) calculations further confirmed that Pb and Cd were bonded with O atoms. The calculated adsorption energy indicated that the (111) MnO₂ facet presented stronger adsorption affinity toward Pb(II) than Cd(II). Additionally, FM150 (150 mg) could regenerate 22 L Pb(II) wastewater upon single passage through the filterable column with a flux of 20 mL/min. Thus, the present work demonstrates the promising potential of using MnO₂@Fe₃O₄/PmPD for efficiently removing heavy metals from wastewater.