Investigating the generation mechanism of nanobubbles and the ion-transport regulation on nanofiltration membranes for efficient salt-salt separation

Nanofoaming-assisted interfacial polymerization (IP) exhibits strong potential to regulate the structure and the separation performance of thin film composite (TFC) membranes. At present, it is recognized that when NaHCO 3 is employed as an aqueous foaming agent, the generation of nanobubbles is ascribed to the NaHCO 3 acidification and may be enhanced by increasing the NaHCO 3 dosage. However, to more accurately measure the generation of nanobubbles, it is necessary to analyze the content and composition of carbonates in the aqueous solution at the initial and final states of interfacial polymerization, which is currently lacking in research. In this work, based on the chemical equilibrium theory, the aqueous carbonate chemistry and the quantity of CO 2 released during IP was evaluated through monitoring the pH changes near the interface and calculating the carbonate distribution fraction. Results showed that the pH of the aqueous solution changed from 10.3 to 11.4 to 6.0–6.5 during interfacial polymerization when the NaHCO 3 dosage was 0.00–0.10 wt%. However, this variation in pH was subtle (from ∼9.3 to ∼8.8) when 1.00 wt% NaHCO 3 was added, leading to limited transformation of carbonate into H 2 CO 3 and, consequently, less generation of CO 2 . In the whole dosage range of NaHCO 3 , 0.10 wt% NaHCO 3 contributed to the maximum quantity of CO 2 during IP, which was also confirmed by the vesicle-like membrane morphology. The resultant membrane achieved a superior salt-salt separation, with selectivity for Mg 2+ and Li + /Na + /K + enhancements of more than 2 times compared to the membrane without NaHCO 3 dosage. This work proposed a new approach to evaluate the inner relationship between NaHCO 3 dosage and nanobubble quantity during IP, which provides an ion-transport regulation mechanism of nanofiltration membranes for efficient salt-salt separation.