Abstract
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 NaHCO3 is employed as an aqueous foaming agent, the generation of nanobubbles is ascribed to the NaHCO3 acidification and may be enhanced by increasing the NaHCO3 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 CO2 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 NaHCO3 dosage was 0.00–0.10 wt%. However, this variation in pH was subtle (from ∼9.3 to ∼8.8) when 1.00 wt% NaHCO3 was added, leading to limited transformation of carbonate into H2CO3 and, consequently, less generation of CO2. In the whole dosage range of NaHCO3, 0.10 wt% NaHCO3 contributed to the maximum quantity of CO2 during IP, which was also confirmed by the vesicle-like membrane morphology. The resultant membrane achieved a superior salt-salt separation, with selectivity for Mg2+ and Li+/Na+/K+ enhancements of more than 2 times compared to the membrane without NaHCO3 dosage. This work proposed a new approach to evaluate the inner relationship between NaHCO3 dosage and nanobubble quantity during IP, which provides an ion-transport regulation mechanism of nanofiltration membranes for efficient salt-salt separation.
Published Version
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