Enhanced Selective Electrosorption of Pb2+ from Complex Water on Covalent Organic Framework−Reduced Graphene Oxide Nanocomposite

Selective capture of the toxic and ubiquitous heavy metal ions is crucially desired yet still significant challenge for water purification. Herein, a covalent organic framework-reduced graphene oxide nanocomposite (TpPa@rGO) was developed and employed as cathode for the first time to selectively remove Pb2+ from complex water via an electrosorption process. Specifically, the rGO provides the electrical conductivity and induces the electric double layer capacitance to attract cations approaching to the electrode surface, while the TpPa endows abundant highly selective redox-active sites to capture Pb2+. Thanks to the ingenious synergy of capacitive rGO and redox-active TpPa, more than 99.6% of Pb2+ can be effectively removed by the TpPa@rGO electrode in a mixed solution containing Pb(NO3)2 (50 mg L-1) and multicomponent nitrates (100 mg L-1 for each), as well as good cycling stability. Interestingly, the interfering ions can serve as additional electrolytes to accelerate the accumulation of Pb2+ around the TpPa@rGO electrode instead of competing for active sites, thereby increasing the absolute adsorption capacity of Pb2+ from 94.6 mg g-1 in single Pb(NO3)2 solution to 129.8 mg g-1 in the multicomponent solution. The enhanced selective electrosorption mechanism on Pb2+ removal by TpPa@rGO and their recyclability were also investigated. The capacitive matrix and additional electrolytes accelerate the ion transfer and enhance ion accessibility to the electrode, thereby anchoring the Pb2+ on the N/O groups of TpPa. This study may put forward a promising method for the selective capture of heavy metal ions in complex water.