Separated removal of ions by ion exchange membrane: investigating the effect of ion transport mechanisms

Water separation technologies with a high ion selectivity between like-charged ions are desirable for preferentially removing hazardous oxyanions (e.g. nitrate) from water while leaving non-toxic ions behind [1]. Meanwhile, water streams are also one of the major resources for ions of economic value, such as lithium (Li+), which often exist in seawater and brine at concentrations of orders of magnitude lower than common ions (e.g. sodium). Therefore, improving ion-ion selectivity is not only important for decontamination to improve water quality but also for extracting valuable ions from water for green energy [2-4]. Especially, if we can separate and recover sodium (Na+) and potassium (K+) from wastewater, we can use them for various purposes. For example, in agriculture, they can be turned into custom fertilizers [5]. Industries can use them in manufacturing processes, contributing to more sustainable production. Recovered sodium can be used for water softening [6], and potassium holds the potential for eco-friendly batteries [7]. Additionally, these elements can find applications in the mining industry, healthcare, and more. Recovering and reusing them not only conserves resources but also reduces the environmental impact of wastewater [8]. Ion exchange membranes (IEMs) play a crucial role in separating ions from water in various processes such as electrodialysis (ED) [9], membrane capacitive deionization [10], and Donnan dialysis [11, 12]. While commercial IEMs are designed to have high permselectivity for ions with opposite charges, they lack selectivity for ions with like charges (such as nitrate (NO3-) vs chloride (Cl-) and K+ vs Na+) [13]. These dense polymeric membranes contain fixed charges in the polymer matrix, allowing the passage of counter-ions while blocking co-ions [14].