Ion exchange enabled selective separation from decontamination to desalination to decarbonization: recent advances and opportunities

Abstract

Fundamentals of ion exchange selective separation based on electrostatic interactions and hydrated ionic radius have been well established, but until the later part of the 20th century, their primary applications were limited to softening and demineralization. With the surge of the environmental movement during the last five decades, novel ion exchange applications based on various selectivity mechanisms have emerged as major contributors toward critical environmental separations. While interests are emerging recently in transforming conventional bulk separation processes (e.g., membranes) to be more selective, a revisit to the selectivity mechanisms of conventional selective processes (e.g., ion exchange) may provide critical insights to facilitate selective separation design. The primary objectives of this review are to present some major developments and progress in using ion exchange selective separation for decontamination, desalination, and decarbonization. Specifically, we present selectivity principles and applications of conventional ion exchangers in softening, novel hybrid anion exchangers in decontaminating oxyanions, weak acid cation exchangers in desalinating and reusing wastewater, and hybrid ligand exchangers in direct air capture of CO₂. Besides selectivity, we highlight the critical role of material regenerability and regeneration process design in transforming ion exchange selective separation from chemical-driven to CO₂ or electricity-driven for better adapting to the carbon-neutral era. We envision this review to inform future selective ion exchange designs to address emerging environmental separation challenges.