Abstract
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−1, the AC-73 electrode shows an excellent deionization capacity to achieve a level < 2 μS cm−1. 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.
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•The capacitive ion exchange (CIE) mechanism was systematically investigated.•CIE mechanism was observed only in mesopore-rich AC electrode.•CIE was affected mainly by cell voltage, Ca2+ concentration, and pore saturation.•Capacitive deionization was able to produce pure water from tap water.