Electrosorption of cadmium ions in aqueous solutions using a copper-gallate metal-organic framework

Recently, there is a recognized need for green technologies for the effective decontamination of toxic heavy metal ions in wastewater. This study demonstrates the electrochemically assisted uptake and release of cadmium ions (Cd2+) using a redox-active Cu-based metal-organic framework (MOF) electrode. Copper gallate (CuGA), which was synthesized in an aqueous solution, is a water-stable and cost-effective MOF adsorbent in which naturally abundant gallic acid is used as a linker. This work utilized copper within the CuGA structure as a redox center to attract Cd2+ by means of Cu2+/Cu+ reduction, exhibiting rapid uptake kinetics and a much higher capacity (>60 mg g−1) compared to the case without electrochemical assistance (~15 mg g−1). In addition, by applying an opposite overpotential to induce the re-oxidation of copper, the facile recovery of Cd2+ and the regeneration of the electrode were possible without regenerants. Physicochemical characterizations including XPS were conducted to investigate the chemical oxidation states and stability of the electrode after the effective electrosorption-regeneration process. This work presents the feasibility of a Cu-based MOF electrode as a reusable platform for the efficient removal of Cd2+, supporting the continued discovery and development of new Faradaic electrodes for electrochemical wastewater treatments.