Selective Capacitive Removal of Pb2+ from Wastewater over Redox-Active Electrodes

Water pollution has become an environmental hazard. Diverse metal cations exist in wastewater; lead is the most common heavy metal pollutant among them. Selective removal of highly toxic and ultradiluted lead ions from wastewater is a major challenge for water purification. Here, selective capacitive removal (SCR) of lead ions from wastewater over redox-active molybdenum dioxide/carbon (MoO2/C) electrodes was developed by an environment-friendly asymmetric capacitive deionization (CDI) method. The MoO2/C spheres act as cathodes of an asymmetric CDI device and effectively reduce the concentration of Pb2+ from 50 ppm to <0.21 ppb. Moreover, the SCR efficiency of lead ions over redox-active MoO2/C electrodes is >99% in mixtures of 100 ppm Pb(NO3)2 and 100 ppm NaCl solutions. In addition, the electrodes exhibit high regeneration performance in mixtures of NaCl and Pb(NO3)2 and high SCR efficiency for lead ions from mixtures of heavy metal ions. The tetrahedral structure of the [MoO4] lattice is shown to be more favorable for the intercalation of lead ions. In situ Raman spectroscopy further shows that the transition of the crystal interface between [MoO6] and [MoO4] cluster lattice could be electrochemically controlled during SCR. Therefore, this study provides a new direction for the SCR of lead ions from wastewater.