Exploring the electrosorption selectivity and recovery of indium ions with capacitive deionization in acidic solution.

High-efficiency recycling technology for endangered elements effectively mitigates the risk of resource shortages and promises the sustainability of supply chains, which is significant to the industry. In this study, an activated carbon (AC)-based capacitive deionization (CDI) for the selective electrosorption and recovery of indium ions (In) from acidic aqueous solution is proposed. The effects of applied voltage, pH, and initial concentration of indium were investigated to optimize the operation parameters for In electrosorption. The results of cyclic voltammetry and the galvanostatic charge/discharge measurements indicate that the AC electrode shows good capability for the electrosorption of In based on electrical double-layer capacitance. As demonstrated, In can be successfully removed by CDI without deposition when the pH < 4, as confirmed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The deionization capacity of In is 7.95 mg/g with an energy consumption of 0.84 kWh/mol in single-pass mode CDI with an initial concentration of 50 mg/L (pH = 3) at 1.2 V. However, the removal of In is affected by the solution pH since hydrogen ions (H) compete for electrosorption. Note that In ions with high valence are preferentially electrosorbed on the electrode surface over H ions, exhibiting a selectivity coefficient of 2.12. Herein, in the charging step, a large number of H ions in solution are rapidly electrosorbed onto the electrode, while these H ions are gradually replaced by a small number of In ions in solution. Therefore, this electrosorption process shows great potential for effectively recovering indium ions from acidic aqueous solutions.