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 (MoO 2 /C) electrodes was developed by an environment-friendly asymmetric capacitive deionization (CDI) method. The MoO 2 /C spheres act as cathodes of an asymmetric CDI device and effectively reduce the concentration of Pb 2+ from 50 ppm to <0.21 ppb. Moreover, the SCR efficiency of lead ions over redox-active MoO 2 /C electrodes is >99% in mixtures of 100 ppm Pb(NO 3 ) 2 and 100 ppm NaCl solutions. In addition, the electrodes exhibit high regeneration performance in mixtures of NaCl and Pb(NO 3 ) 2 and high SCR efficiency for lead ions from mixtures of heavy metal ions. The tetrahedral structure of the [MoO 4 ] 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 [MoO 6 ] and [MoO 4 ] cluster lattice could be electrochemically controlled during SCR. Therefore, this study provides a new direction for the SCR of lead ions from wastewater.