Ammonia Removal from Simulated Wastewater Using Ti₃C₂T _x Mxene in Flow Electrode Capacitive Deionization

The energy-water nexus poses an integrated research challenge with crucial global implications. The nexus highlights the need to adopt a proactive approach by focusing on the conservation and management of both resources. As global freshwater supply continues to deplete, a larger number of people in the world will need to rely on recycled water. One of the ways to address this concern is by focusing efforts on the development and deployment of energy efficient water remediation technologies. Ammonia is one of the most common contaminants found in domestic and industrial wastewater. Concurrently, it is also one of the most liberally produced chemicals because of its widespread use in the agricultural and textile industries. In 2020, approximately 144 million metric tons of ammonia were produced globally (1). Despite that, biological nitrification remains the most widespread method for treating ammonia wastewater (2). Aside from being cumbersome and slow, it also fails to recover ammonia, which is otherwise a valuable product. In pursuit of sustainability, product conservation directly translates to energy conservation and there remains a need for a water purification and ammonia recovery technology that is energy, water, and resource efficient. Capacitive deionization (CDI) is a separation technology that removes ions from water by applying a small voltage below the dissociation potential of water (1.23 V). The ions are temporarily stored in the electrodes and later released to a separate concentrated stream upon voltage removal or reversal. Flowing electrode CDI systems (FE-CDI) have proven advantageous because of their ability to prevent cross contamination, and to run in uninterrupted cycles ad infinitum . The performance of FE-CDI systems relies on the selection of suitable electrode materials with desired properties of high electrical conductivity, high surface area, colloidal stability, and surface sensitivity (3). MXenes are an emerging class of two-dimensional (2D) materials that are highly conductive, hydrophilic, tunable, and possess attractive charge storage and ion transport properties (4,5). In this work, we developed a flowing electrode capacitive deionization (FE-CDI) system using Ti 3 C 2 T x MXene suspension electrodes for the removal and recovery of ammonia from synthetic wastewater. A custom-built FE-CDI unit using titanium current collectors with an effective transfer area of 10 cm 2 was used to test system efficiency. The electrode performance was evaluated by operating the CDI system with a feed solution of 500 mg/L NH 4 Cl running in batch mode at a constant voltage of 1.2 and -1.2 V in charging and discharging stages respectively. Despite low loading (1 mg/ml), Ti 3 C 2 T x flow electrodes showed a markedly improved performance by achieving 60% ion removal efficiency (Fig. 1a) in a short saturation time of 115 minutes, and an unprecedented ion adsorption capacity of 460 mg/g (Fig. 1b). This is ~ 7× higher than the currently reported value using Ti 3 C 2 in a conventional CDI system targeted at sodium chloride (NaCl) desalination (6). Post-mortem characterization of the suspension electrodes by X-ray diffraction revealed that Ti 3 C 2 T x MXene sequestered high amounts of pollutants due to its ability to intercalate ions between its 2D layers. The electrodes possessed favorable electrochemical properties with a high specific capacitance of 148 F/g. The system proved to be a green technology by exhibiting satisfactory charge efficiency of 58-70% while operating at a relatively low energy consumption of 0.45 kWh/kg (Fig. 1c), which is about an order of magnitude lower than the 4.6 kWh/kg used by commercial wastewater treatment plants (7). The results demonstrate that the Ti 3 C 2 T x MXene electrodes have the potential to improve the FE-CDI process for energy-efficient removal and recovery of ammonia. IEA, Ammonia Technology Roadmap , International Energy Agency, Paris, (2021). F. Jaramillo, M. Orchard, C. Muñoz, M. Zamorano, and C. Antileo, J. Environ. Manage. , 218 , 154–164 (2018). Y. Oren, Desalination , 228 , 10–29 (2008). Y. Gogotsi and B. Anasori, ACS Nano , 13 , 8491–8494 (2019). P. Singh et al., J. Power Sources , 506 (2021). J. Ma, Y. Cheng, L. Wang, X. Dai, and F. Yu, Chem. Eng. J. , 384 (2020). M. Ekman, B. Björlenius, and M. Andersson, Water Res. , 40 , 1668–1676 (2006). Figure 1