Efficient fluoride removal in hybrid capacitive deionization enabled by Ce-Zn-MOF-derived CeO2@C and birnessite electrodes

Abstract

Fluorine pollution has become a public health issue of global concern. Capacitive deionization (CDI) presents the benefits of simple operation, high energy efficiency, and good sustainability. The development of high adsorption performance, economical and environmentally friendly defluorination electrodes is the key direction of capacitive deionization. Here, a spherical metal–organic framework (MOF) Ce/Zn-BTC was prepared by solvothermal method utilizing benzene-tricarboxylic (BTC) acid as solvent, and then it was transformed into CeO2@C composites through pyrolysis. The spherical structure of the original MOFs is preserved by CeO2@C, leading to significant enhancements in specific surface area (275.48 m2/g), pore volume (0.3359 cm3/g), and electrochemical performance. In this work, the anode material CeO2@C was employed for the first time, while birnessite-type MnO2 (δ-MnO2) was utilized as a cathode material for the fabrication of a hybrid CDI (HCDI) device for the treatment of fluorinated wastewater. The CeO2@C/δ-MnO2 cell shows excellent fluoride removal performance with a removal capacity of 22.03 mg/g for 50 mg/L fluoride solutions under a 1.2 V applied voltage, which is better than most carbon-based materials. Importantly, the CeO2@C/δ-MnO2 cell has favorable fluoride selectivity and regeneration performance (>75 %) even after undergoing ten cycles. Fluoride is mainly removed by intercalation into the carbon skeleton, coordination with Ce, and electrostatic interaction. This study shows that the CeO2@C/δ-MnO2 cell has excellent fluoride removal performance and practical application prospects.