An electron-scale comparative study on the adsorption of six divalent heavy metal cations on MnFe2O4@CAC hybrid: Experimental and DFT investigations

Heavy metal is a serious environmental issue mostly because of their toxicity and bioaccumulation. The recovering and recycling of the adsorbents became a bottleneck in heavy metal removal through adsorption. A hybrid adsorbent MnFe2O4@CAC showed great application potential because of its easy regeneration. However, as for other commonly used adsorbents, the removal performance of different types of heavy metals are distinctively different, but the mechanism is still unclear. To address this knowledge gap, the electron-scale behavior and adsorption mechanism of six divalent cations (Cu(II), Cd(II), Pb(II), Co(II), Ni(II), Hg(II)) on MnFe2O4@CAC were investigated in-depth combining experimental analysis and density functional theory (DFT) calculations. The experimental results showed the order of adsorption: Ni(II) > Cu(II) > Co(II) > Pb (II) > Cd(II) > Hg(II). The reaction from transition state to the final product was the rate-determining step for the Cu(II), Pb(II), Co(II) and Ni(II) adsorption, while the reaction from reactant to intermediate controlled the adsorption rate of Cd(II) and Hg(II). The main adsorption mechanisms were surface complexation and ionic exchange, which were further confirmed by the spectroscopic (Fourier Transform infrared spectroscopy, FT-IR and X-ray photoelectron spectroscopy, XPS) characterizations and DFT calculations. In addition, the electron density of state (DOS) results indicated the 4s orbit of the cations most easily to be occupied by the d electrons of MnFe2O4. The order of adsorption affinity agreed well with the apparent adsorption, and was decided by the electronic empty orbit and ionic electron cloud size of the cations. The results of the current study provide an in-depth understanding on the adsorption mechanism of heavy metals and offer a theoretical guidance for improving their removal efficiency.