Efficient adsorption of hardness ions by a mordenite-loaded, nitrogen-doped porous carbon nanofiber cathode in capacitive deionization

Water hardness, predominantly due to the presence of Ca 2+ and Mg 2+ ions, presents significant challenges to water quality and public health. Addressing this issue necessitates effective water softening, which remains a pivotal task in water treatment. Capacitive deionization (CDI) has emerged as a promising technology for selective hardness removal, leveraging the low-cost, non-toxic and environmentally friendly selective electrode materials. Electrospun nanofibers, characterized by their three-dimensional porous structure, offer good flexibility, high specific surface area and excellent electrical conductivity. Their components can be tailored to meet the specific requirements. In this study, we incorporated mordenite (MOR), noted for its excellent ion-exchange capacity, into self-supporting nitrogen-doped carbon nanofibers (N–CNF) via electrospinning a blend of polyacrylonitrile (PAN), urea, and MOR, followed by carbonization. The resulting mordenite-loaded N–CNF composite (MOR@N–CNF) exhibited good flexibility and high conductivity. Scanning electron microscopy and X-ray diffraction analysis confirmed the presence and uniform distribution of MOR within the CNF matrix. X-ray photo spectroscopy demonstrated an increase in nitrogen content in MOR@N–CNF. In addition, the MOR@N–CNF composite displayed enhanced hydrophilicity and an increased specific surface area. When used as a self-supporting electrode, MOR@N–CNF exhibited the electrochemical specific capacitance of 162.7 ​F/g, with the specific capacitance retention of 60% in a CaCl 2 solution. In an asymmetric CDI setup with activated carbon (AC) as the anode, the MOR@N–CNF cathode demonstrated outstanding adsorption capacities of 1501 and 1416 ​μmol/g for Mg 2+ and Ca 2+ , respectively. The composite electrode exhibited high selectivity for Mg 2+ and Ca 2+ over Na + with a selectivity factor of 9.7 and 8.9, respectively. These attributes endow the material with exceptional ability to discriminate between divalent and monovalent ions, thereby enhancing its potential for hardness removal. Furthermore, the electrode retained 78% of its adsorption capacity after 40 cycles, demonstrating robust cyclic stability, and ensuring long-term CDI operation. This work provides a new strategy for preparing ion-exchange material-based composite electrodes and highlights the potential of CDI technology in hard water softening. A novel composite electrode of mordenite-loaded nitrogen-doped carbon nanofibers (MOR@N–CNF) is developed for capacitive deionization (CDI). The MOR@N–CNF electrode exhibits high selectivity and cyclic stability for Mg 2+ and Ca 2+ removal in hard water softening applications. • Self-supported MOR@N–CNF is prepared by electrospun mordenite/urea-containing fiber. • Conductivity and hydrophilicity of MOR@N–CNF are improved. • Adsorption capacities to Mg 2+ (1501 ​μmol/g) and Ca 2+ (1416 ​μmol/g) is superior in MOR@N–CNF-based CDI. • The MOR@N–CNF cathode is highly selective to the adsorption of Ca 2+ , Mg 2+ . • High adsorption amount with good stability is achieved in multi-salt hard water.