Electrodeposition nanofabrication of carboxylated carbon nanotubes/ -MnO₂ nanorods/ polypyrrole composites as high hybrid capacitance electrodes for efficient U(VI) electrosorption

The uranium-containing wastewater resourced from the nuclear fuel cycle presents serious harm to human health and ecological environment. Herein, carboxylated carbon nanotubes/alpha-MnO2 nanorods/polypyrrole (CMP) composite as high hybrid capacitance (EDL and pseudocapacitance) electrodes with good electrosorption performance for U(VI) were nanofabricated by the electrodeposition of polypyrrole (PPy) on the surface of carboxylated carbon nanotubes/alpha-MnO2 nanorods composites using potentiostatic method. The morphology and structure characteristics, electrochemical properties and electrosorption U(VI) properties of the CNT/alpha-MnO2/PPy composites were systematically investigated. The results show that among the CMP composites obtained at different pyrrole concentrations, CMP-0.10 has the highest specific capacitance (366.97F/g), and its electrosorption performance is also the best, which is due to its well balanced porous structure and high hybrid capacitance which favors U(VI) ions transfer and storage. Cyclic voltammetry (CV) curves with obvious redox peaks were observed at low sweep speeds, indicating its hybrid capacitance behavior of CMP composites in which the capacitance contribution can be quantified. The electrosorption experiments were performed in a capacitive deionaiztion (CDI) system in which different factors (voltage, pH, U(VI) concentration and contact time, etc.) on the electrosorption of U(VI) were investigated, which shows that high U(VI) removal could be achieved at a wide pH range. The theoretical simulation using different models shows the best fit of the Langmuir model for the electrosorption isotherms and PFO model for the kinetics. The U(VI) electrosorption capacity reaches 339.45 mg/g at 0.9 V and pH 4.5 for CMP-0.10, which is much higher that achieved at 0.0 V. The facile fabrication, excellent electrochemical and electrosorption performance as well as good cycling stability for the CMP composites highlight their potential application in radioactive wastewater treatment.