Capacitive deionization (CDI) is an electrochemical method of removing ions from aqueous solutions. The primary application of CDI is the desalination of brackish water. As a result, most research has focused on removing inorganic ions, namely Na<sup>+</sup> and Cl<sup>-</sup>. CDI may also be useful to remove organic ions, including pollutants (e.g., per- and polyfluoroalkyl substances) and high-value chemicals (e.g., volatile fatty acids [VFAs]). Removal of many organic ions is not as straightforward as small, inorganic ions, especially when activated carbon (AC) electrodes are used. Some organic ions can physically and/or chemically adsorb to the AC in the absence of an applied potential. Toward understanding the complex interplay of different adsorptive processes and fundamental factors controlling organic ion removal, we investigated four organic ions that varied only by carbon chain length in flow-through CDI cells using AC cloth (ACC) electrodes. Formate, acetate, butyrate, and hexanoate (VFAs generated in many anaerobic microbial processes) were selected as representative monovalent organics and chloride as an inorganic ion. To date, we found that a pristine electrode held under a constant applied voltage adsorbed more hexanoate than formate (20.18 ± 0.39 mg/g vs 17.20 ± 0.25 mg/g), with physisorption/chemisorption contributed as much as electrosorption. In contrast, for the electrodes which were first conditioned through multiple charge/discharge cycles, total adsorption decreased for all ions with electrosorption now becoming the predominant mechanism of removal. No significant difference in adsorption capacity was observed between formate and chloride ions (both comparable in size and ionic mobility), indicating that the size and C-H bond length of hexanoate played a significant role. Open circuit saturation of the ACC electrodes with hexanoate ions affected electrosorption by reducing the availability of micropores presumably utilized for electrical double layer formation, while no significant change in micropores was observed with formate or chloride. Physisorption/chemisorption of hexanoate ions also altered the electrical properties of ACC and shifted the potential distribution for electrosorption of hexanoate to ~0.2 V smaller than for formate and chloride, resulting in lower adsorption capacity. These results highlight that the physical and chemical adsorptive behavior of ions can have a strong influence on the electrosorption of ions in CDI.

Elucidating the contributions of physical, chemical, and electrical adsorption processes during the removal of organic ions in capacitive deionization

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