Development of a capacitive deionization stack with highly porous oxygen-doped carbon electrodes for brackish water desalination in remote coastal areas

Abstract

This study demonstrates the development and application of a capacitive deionization (CDI) stack in treating saline water to provide safe drinking water to remote coastal areas. The commercial coconut shell-derived electrode (CAC) was chosen by its favorable characteristics, such as high porosity and specific surface area of 1742 m2 g−1, and a rich abundance of oxygen functional groups. CAC also displays exceptional capacitive properties, as evidenced by a specific capacitance of 72.7 F g−1, suggesting the potential for high-performance capacitive storage applications. Desalination experiments were conducted in both laboratory and field settings to assess the system's efficacy. On a laboratory scale, the CDI cell demonstrated a high desalination capacity of 14.95 mg g−1 and low energy requirements of 0.088 kWh m−3. The 40-pair CDI stack was successfully upscaled for desalination in the field, showing impressive capability for desalination and removal of unwanted ions in various water sources, including surface water, shallow groundwater, and deep groundwater. The CDI-treated effluent from the commonly used shallow groundwater source had a low electrical conductivity of 57 μS cm−1. Notably, the system maintained excellent desalination performance (98.4% Na+ and 96.9% Cl− removal) with steady repetition in the charging-discharging processes of the purified effluent over a 5-week operation, while also demonstrating impressive softening efficiency with a 97.6% reduction in CaCO3 hardness. Furthermore, the ion selectivity of the system was evaluated, with the following order observed: Na+ < K+ < Mg2+ < Ca2+ for cations, and As < Cl− < NO3− < SO42− for anions.