Enhanced ammonium removal and recovery from municipal wastewater by asymmetric CDI cell equipped with oxygen functionalized carbon electrode

Following the concept of sustainable development and circular economy, the Capacitive Deionization (CDI) was examined for the removal and recovery of ammonium (NH₄⁺) ions from municipal wastewater towards prevention of eutrophication and obtaining a valuable component for fertilizers production. The electrosorption study was conducted in the symmetric CDI cell, composed of activated carbon (AC) electrodes, and an asymmetric CDI cell, equipped with oxygen functionalized AC (AC-ox) cathode. The asymmetric CDI cell was demonstrated the enhanced performance in terms of the removal efficiency towards ammonium ions (83%) and the related energy consumption (and 0.085 kWh/mmol NH₄⁺), compared to the symmetric one with the following results: 30% removal of NH₄⁺ ions and 0.215 kWh/mmol NH₄⁺. The beneficial performance of the AC-ox electrode also has been observed in terms of NH₄⁺ recovery: 20% recovered NH₄⁺ ions, compared to 9% NH₄⁺ recovered ions by AC pristine electrode. The presence of competitive ions hindered the electrosorption efficiency. Nevertheless, the time-dependent selectivity of the AC-ox electrode towards NH₄⁺ ions was observed. The specific capacities of the symmetric and asymmetric cells at 5 mV/s were calculated to be 10.4 F/g and 18.3 F/g, respectively. Material characterization of AC before and after oxidation explained the enhanced CDI performance of the AC-ox electrode due to the increased hydrophilicity and additional oxygen functionalities on the electrode surface.