Highly selective and ultrafast removal of cadmium and copper from water by magnetic core-shell microsphere

Advanced treatment of heavy metals from various types of water is challenging but of great urgency due to their serious threat to water quality security. In this study, a new magnetic hybrid adsorbent with the core-shell structure, which is denoted as Fe3O4@SiO2@MO, was prepared by successively covering SiO2 and manganese oxide (MO) on Fe3O4. The covered SiO2 was designed to protect the magnetic core Fe3O4 from acid corrosion in practical application. The Fe3O4@SiO2@MO can be separated from solution within 10 s using an external magnetic field and very quickly remove Cd(II) and Cu(II) with equilibrium time below 10 min due to its special coating structure, which sharply reduces the diffusion distance of two metals before adsorption. Their effective coefficients of intraparticle diffusion (D) were determined to be 2.0 × 10−10 and 8.5 × 10−10 cm2 s−1. High levels of Ca(II) and Mg(II) inconspicuously affected the Cd(II) and Cu(II) retention onto Fe3O4@SiO2@MO, and only a decrease in removal below 20 and 12% for Cd(II) and Cu(II) occurred, respectively, due to the specific inner-sphere complexation between MO and the targeted metals. We preliminarily uncovered this interaction using XPS and FTIR analyses. Furthermore, in cyclic batch experiments, 1 kg Fe3O4@SiO2@MO can effectively treat 0.5 and 3.25 m3 of Cd(II)- and Cu(II)-contaminated wastewater to satisfy their corresponding discharge standards, respectively, which is equivalent to 500 and 3250 folds of its own weight. All results validate that Fe3O4@SiO2@MO is a promising engineered magnetic adsorbent to decontaminate heavy-metal-polluted wastewater.