Effective removal of Pb2+ from water by a novel magnetic Fe3O4-MnO2 composite prepared from steel pickling waste liquid: adsorption behavior and mechanism

The high-value utilization of steel pickling waste liquid (SPWL) is an important direction in the resourceful utilization of hazardous waste. In this study, nano-Fe₃O₄ prepared from SPWL (S-Fe₃O₄) was hydrothermally compounded with MnO₂ as Fe₃O₄-MnO₂ (SFM), which were compared with Fe₃O₄-MnO₂ (CFM) prepared by chemical reagents. The synthetic materials were evaluated by SEM, VSM, BET, XRD, and zeta potential, the results show that the spherical S-Fe₃O₄ was encapsulated by spiny spherical shell-like MnO₂, and the SFM has a high saturation magnetization and specific surface area (159.91 m²/g). The adsorption experiments demonstrated that the adsorption effect of SFM on Pb²⁺ is highly similar to that of CFM. The maximal adsorption capacity of SFM reached 129.74 mg/g under ideal composite ratio and pH conditions, and the adsorption process was unaffected by competing heavy metal ions coexisting in the environment. Moreover, the high saturation magnetization strength of the SFM makes it simple to separate materials under a magnetic field. The pseudo-second-order kinetics and Langmuir isotherm models provided a good description of the Pb²⁺ adsorption on SFM. XPS and FTIR analysis provided evidence that the adsorbent surface has a high concentration of -OH functional groups, and that O atoms serve as Pb²⁺ binding sites, suggesting that ion exchange occurs during the SFM adsorption process. In this paper, we developed a cheap SFM with high adsorption performance and provided a novel approach to the high-value application of SPWL.