Sulfonated polymer coating enhances selective removal of calcium in membrane capacitive deionization

There is a need for membranes and processes that can selectively separate target ions from other similar ionic species. Recent studies have shown that electrified processes for ion removal such as membrane capacitive deionization (MCDI) and electrodialysis (ED) are selective towards specific ionic species, but selectivities are generally limited. Here, we demonstrate that an ion-selective polymer coating can significantly enhance ion selectivities for MCDI processes. We focused on the preferential removal of Ca2+ over Na+ and used the conductive and sulfonated polymer poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a model selective ion-exchange coating. We first measured the permeability of Ca2+ and Na+ in freestanding PEDOT:PSS membranes of varying crosslink density and found that the permeability of Ca2+ was six times greater than that for Na+ in optimized membranes. Next, we used PEDOT:PSS in an MCDI process by depositing thin PEDOT:PSS coatings on top of composite electrodes. We found that the PEDOT:PSS coatings significantly enhanced the preferential permeability of Ca2+ over Na + relative to unmodified electrodes and produced a preferential removal as high as 8:1 on a molar basis. This work demonstrates a new approach to enhance selective ion removal in MCDI and other electro-driven ion separation processes.