Mechanistic study on pH-related behavior in rocking-chair capacitive deionization

Rocking-chair capacitive deionization (RCDI) is a semi-continuous desalination technology, however, suffering from issues pertaining to effluent salinity and pH. We thoroughly investigated the composition evolution and pH-related behavior in RCDI and disclosed the underlying causes for quality decay. RCDI concomitantly produces two streams: the desalted cathodic effluent becomes basic, whereas the brine anodic stream becomes acidic. These pH-related species deteriorate the outflow quality and distort conductivity profiles. Under variation in dissolved oxygen and potential distribution, we verified the carbon oxidation and oxygen evolution reactions as causes for the acidic anode and oxygen reduction as the cause for the basic cathode. Faradaic reactions and resultant pH fluctuation were exacerbated with increasing charging voltages, even at 0.9 V, whereas they were negligibly affected by the resembling potential distribution at different flowrates. To improve pH stability and water recovery, we integrated the ion exchange and flowrate adjustment procedures; consequently, water recovery increased to 71.43% and pH was stabilized at 5–6.5 for over 80 cycles, with significant adsorption capacity enhancement owing to the ion exchange with pH-related species. Thus, with a better understanding of pH-related behaviors, RCDI can be optimized and extended more specifically for demands such as acid/base production and resource enrichment.