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The capacitive deionization (CDI) studies on the ultra-low conductivity feeds for ultrapure water (UPW) production are limited. The Ca²⁺/Mg²⁺-selective removal from water has gained much interest for water softening applications while the driving forces of the ion exchange mechanism for ion-selective removal during the CDI process, named capacitive ion exchange (CIE), has been overlooked, deserving a systematic investigation. In this study, three non-modified activated carbon (AC) materials with various mesopore/total pore volume ratios 

(denoted as AC-18, AC-46, and AC-73) are designed to investigate the CIE mechanism for tap water desalination. The mesopore-rich AC-73 electrode presents a Ca²⁺/Na⁺ selectivity ratio of 2.5–5, which is 2–3 times greater than the micropore-rich AC-18 electrode. A significant exchange of Na^＋ and Ca²⁺ observed in the AC-73 electrode is attributable to the unconfined space of mesopores. CIE is majorly controlled by the electrostatic force instead of the concentration gradient as demonstrated by varying the cell voltage and influent Ca²⁺ concentration. At the feed water conductivity levels of 25 μS/cm, the AC-73 electrode shows an excellent deionization capacity to achieve a level ＜ 2 μS/cm. Our results provide insights into the CIE behavior, enabling the practical application of the CDI technology towards water softening and UPW production industries using the unmodified mesoporous AC electrodes.

Author：趙淑如,王鈞逸,林以萱,鐘琍菁,劉柏逸 (院內作者) 、蔡明瀚,許立強,黃弘逸,胡啟章 (院外作者)

★Article source: Chemical Engineering Journal