High Capacitive Removal of Pb2+ from Wastewater and Mechanism Study over MoO2@N-Doped Hollow Carbon Sphere Anodes

Lead pollution in the water has become a serious environmental problem with the development of metallurgy, chemical manufacturing, and mining, especially battery industries. Highly efficient removal of Pb2+ is a big challenge to wastewater treatment. In this work, highly efficient removal of Pb2+ from wastewater over MoO2@N-doped hollow carbon sphere (MoO2@NHCS) anodes was developed by a capacitive deionization (CDI) process. The adsorption capacity of Pb2+ is as high as 202.14 mg/g in a 50 ppm Pb2+ solution (pH = 6 and U = 1.2 V). In addition, MoO2@NHCS electrodes show high selectivity toward Pb2+ under competition from Na+ and other heavy metal ions. Meanwhile, the MoO2@NHCS electrodes maintain an excellent recycling ability after 20 cycles for a unique hollow structure. Moreover, the density functional theory calculations show that MoO2 crystals adsorb Pb2+ in a tetrahedral mode with the highest adsorption energy (Eads = −1.61 eV). The selective removal of Pb2+ is attributed to the octahedral MoO2 transformed into tetrahedral [MoO4]2– and then could trap the Pb2+ to form PbMoO4. Hence, this work provides an innovative thinking for the effective removal of Pb2+ from wastewater.