Selective removal of Na+ by NaTi2(PO4)3-MWCNT composite hollow-fiber membrane electrode in capacitive deionization

Abstract

In this study, NaTi2(PO4)3 (NTP) nanoparticles were incorporated into a carbon nanotube hollow-fiber (CHF) electrode for the selective removal of Na+ during the capacitive deionization (CDI) of salty water. Due to the Na+-selective electro-sorption of NTP nanoparticles, NTP-CHF achieved 1.5 ~2 times higher Na+ adsorption capacity than that of CHF only electrodes at a range of 1.7 mM~17.1 mM (100 mg L−1~1000 mg L−1) NaCl solution. Moreover NTP-CHF sustained the Na+ adsorption capacity even at the presence of Ca2+ ions, whereas the dramatic decrease in Na+ removal was observed in CHF with an increase of Ca2+ concentration. As the result, NTP-CHF can retain 52% of the initial Na+ removal capacity after three times of capture and release cycles at the presence of Ca2+, while CHF showed <10% of the initial Na+ removal capacity due to the irreversible adsorption of Ca2+ ions onto the MWCNT surface. Thus, the incorporation of NTP into the conductive carbon nanomaterial network can greatly improve both the adsorption capacity and selectivity of Na+ compared to the conventional carbon-based electrode during the CDI of salty water containing multivalent cations such as Ca2+.