Enhancing copper recycling through synergistic valence conversions in MnO_x-modified capacitive deionization electrodes

The copper present in industrial waste has significant value for environmental resource recovery. Nonetheless, in electrosorption employed for ion separation, electrode materials commonly exhibit limited adsorption capacities, inadequate cycling stabilities, and insufficient selectivities toward specific ions for their single adsorption mechanism. In this study, a strategy was developed for optimizing the design of electrode materials to substitute zirconium atoms in organometallic frameworks (UiO-66, abbreviated as ZO) for manganese atoms from manganese oxide precursors to form a robust three-dimensional conductive structure (MZO). MZO has a stable and sizable adsorption region for electric adsorption, and manganese atoms with a dot-like dispersion provide Faraday active sites. Notably, in an external power supply circuit, Mn3+ fluently transforms to Mn2+ and then facilitates the deposition of copper ions by capturing electrons from Mn2+. Manganese regains electrons by utilizing the power source, which facilitates the continuous transfer of electrons to copper ions, enhancing the pseudocapacitance process. The combined effects of electrodeposition, a double-layer reaction, and a pseudocapacitive reaction improve the adsorption performance of MZO for copper ions, with an adsorption capacity of 249.5 mg & sdot;g 1 (at 1.2 V, 40 mg & sdot;L 1 Cu2+). Meanwhile, MZO based on ZO has acceptable cyclic stability, with a capacity retention rate of over 75 % after 32 cycles and 3 times of performance recovery experiments. Moreover, the MZO electrode exhibits positive selectivity for copper ions in mixed multiple salt solutions (separation factor alpha = 0.68). Density functional theory (DFT) calculations confirmed the selectivity of the MZO electrode for Cu2+ by providing the lowest binding energy ( 2.4065 eV) for Cu2+.