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

Abstract

Water hardness, predominantly due to the presence of Ca²⁺ and Mg²⁺ 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 CaCl₂ 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 Mg²⁺ and Ca²⁺, respectively. The composite electrode exhibited high selectivity for Mg²⁺ and Ca²⁺ 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.