Nitrite desorption from activated carbon fiber during capacitive deionization (CDI) and membrane capacitive deionization (MCDI)

Abstract

In this study, nitrite desorption from activated carbon fiber (ACF) in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) was examined and the effects of the operation parameters (pH, voltage, temperature and flow rate) and co-existing matters were deeply investigated. Desorption mechanisms were analyzed via Brunauer–Emmett–Teller (BET) and Fourier transform infrared spectroscopy (FTIR). Results showed that the final desorption ratio increased from 0 to 100% in MCDI and the enhancement was also observed in CDI that the ratio increased from 18.7 to 83.5% when solution pH increased from 2 to 10. Increasing the voltage and solution temperature also contributed to the ion desorption both in CDI and MCDI, while the effect of flow rate was negligible. Generally, MCDI showed greater desorption performance than CDI due to the elimination of co-ions effect. However, it was interesting to find that when the voltage was in the range of 0.4-0.6 V, the desorption ratio increased from 38.4% to 50.8% in MCDI which was lower than that in CDI (45.4% to 55.8%). One possible explanation was that the presence of membranes would inevitably introduce additional resistance into the system and decrease effective voltage especially at lower voltage. Compared to the desorption performance in MCDI when the solution pH was 2, the greater desorption performance was observed in CDI which was mainly attributed to the site competition. As for the influence of coexisting matters, the presence of bovine serum albumin (BSA) posed an adverse effect for the ion desorption both in CDI and MCDI. The inhibition effect was more serious in CDI, and pore blockage caused by BSA attachment onto ACF was the main mechanism. Therefore, this study would provide some referential advice for the investigation of ion desorption in CDI and MCDI.