The increasing global demand for both energy and water necessitates the development of sustainable and efficient water remediation technologies. Traditional methods, such as biological nitrification, reverse osmosis, and various distillation techniques, often require substantial energy input and capital investment. Ammonia, a common contaminant in wastewater, is particularly problematic due to its toxicity and the environmental damage caused by its release into aquatic ecosystems. The Clean Water Act mandates its removal, but existing methods are often cumbersome, slow, and wasteful. This research explores capacitive deionization (CDI) as an alternative, focusing on flow electrode CDI (FE-CDI) to address the limitations of conventional CDI, including cross-contamination and discontinuous operation. FE-CDI offers the advantage of uninterrupted cycles and prevents cross-contamination, but often suffers from low conductivity in electrodes. This study proposes using Ti3C2Tx MXene, a high-conductivity material with excellent electrochemical properties, to overcome this limitation and improve FE-CDI performance for ammonia removal and recovery.

The literature highlights the energy-water nexus and the environmental and economic benefits of water reclamation. Several water remediation technologies are reviewed, including aerobic and anaerobic bio-treatment, various distillation methods, and physical-chemical treatments, all of which have high energy demands (up to 6.60 kWh kg⁻¹). Reverse osmosis, while common, consumes a significant amount of energy (30–40 kWh kg⁻¹). Capacitive deionization (CDI) emerges as a promising alternative, using low voltage to induce charge separation and store ions in the electric double layer of high surface area electrodes. However, the availability of suitable electrode materials remains a bottleneck. Various CDI cell architectures are discussed, with FE-CDI highlighted for its continuous operation and prevention of cross-contamination. Carbon-based electrodes, while possessing high surface area and electrochemical stability, suffer from insufficient conductivity at high mass loadings, leading to clogged flow channels. MXenes, a class of 2D transition metal carbides, nitrides, and carbonitrides, are presented as a potential solution due to their high conductivity, hydrophilicity, and scalability. Previous research on MXenes in conventional CDI systems has shown promising results, but their application in FE-CDI systems remained unexplored.

The study employed both activated carbon (AC) and Ti3C2Tx MXene as flow electrodes in an FE-CDI system. The AC slurry (10 wt%) served as a control. The Ti3C2Tx MXene was synthesized using the minimally intensive layer decontamination (MILD) method, characterized by its reduced toxicity and production of low-defect MXene flakes. The prepared slurries underwent characterization using XRD, SEM, TEM, DLS, and Raman spectroscopy to analyze their structure, morphology, particle size, and surface chemistry. Rheological properties, including viscosity and zeta potential, were measured to assess flowability and stability. Deionization performance was evaluated using a batch-mode FE-CDI system with a feed solution of 500 mg L⁻¹ NH4Cl, applying a constant voltage of ±1.20 V. Electrochemical properties were analyzed using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) profiles. Key parameters, such as conductivity ratio, adsorption capacity, adsorption rate, charge efficiency, energy consumption, and regeneration efficiency, were determined and compared between the AC and Ti3C2Tx electrodes.

Despite the low loading (1 mg mL⁻¹), Ti3C2Tx flow electrodes exhibited significantly improved performance compared to AC electrodes. They achieved 60% ion removal efficiency in 115 min with an adsorption capacity of 460 mg g⁻¹, which is more than two orders of magnitude higher than the AC electrodes (4.20 mg g⁻¹). The Ti3C2Tx electrodes showed high regeneration efficiency (92%), indicating suitability for long-term use. The system displayed energy efficiency, with charge efficiency ranging from 58% to 70% and energy consumption of 0.45 kWh kg⁻¹, significantly lower than traditional wastewater treatment plants (4.60 kWh kg⁻¹). Characterizations revealed that the high performance of Ti3C2Tx is attributable to its high conductivity, hydrophilic surface terminations, expanded interlayer spacing (facilitating ion intercalation), and unique surface chemistry. Cyclic voltammetry demonstrated the pseudocapacitive behavior of Ti3C2Tx electrodes, confirming the combined contribution of surface redox reactions, electrical double layer capacitance, and ion intercalation to the overall charge storage mechanism. XRD analysis showed that the interlayer spacing increased after the FE-CDI operation, further supporting the ion intercalation mechanism.

The findings demonstrate the potential of Ti3C2Tx MXene as a superior electrode material for FE-CDI systems, leading to energy-efficient ammonia removal and recovery from wastewater. The exceptional performance of Ti3C2Tx compared to AC is attributed to its superior electrochemical properties and structural features, which facilitate enhanced ion adsorption and intercalation. The high regeneration efficiency and low energy consumption highlight the economic and environmental viability of this technology. The results address the limitations of conventional CDI systems and provide a compelling case for the use of MXenes in large-scale, sustainable water treatment applications. The combination of high adsorption capacity, regeneration efficiency, and low energy consumption surpasses the performance of previously reported materials and systems.

This study successfully demonstrated the high performance of Ti3C2Tx MXene flow electrodes in an FE-CDI system for ammonia removal from simulated wastewater. The superior adsorption capacity, high regeneration efficiency, and low energy consumption make this technology a promising alternative to traditional methods. Future research should focus on testing with real wastewater, optimizing system parameters, and exploring the application of other MXenes for broader pollutant removal.

The study utilized simulated wastewater, which may not perfectly represent the complexity of real wastewater. The influence of other ions and organic matter present in real wastewater on the performance of the FE-CDI system warrants further investigation. Additionally, long-term stability and scalability of the system under continuous operation need to be thoroughly evaluated before widespread industrial application.