(2024):7:25 | Rebecca S. Roth, Liat Birnhack, Mor Avidar, Elizabeth A. Hjelvik, Anthony P. Straub & Razi Epsztein
The study investigates the ion-specific effects on the charge and performance of a loose polyamide nanofiltration (NF) membrane, which is commonly used for solute-solute separations. The researchers systematically evaluated the zeta potential (ZP) of the membrane under various conditions of pH, salinity, and ionic composition, and correlated these data with membrane performance. Key findings include:
1. **pKₐ of Carboxylic and Amines**: The pKₐ values of carboxylic and amine groups on the membrane surface were determined to be around 4.5 and 8.5, respectively. This suggests that the membrane's charge is influenced by the protonation of these functional groups.
2. **Chloride Ion Adsorption**: Chloride ions, which are highly polarizable, adsorb to the uncharged segments of the membrane, enhancing Donnan exclusion of anions. This adsorption increases the membrane's negative charge, leading to reduced ion permeability.
3. **Ion Specificity**: The "stickiness" of different ions (e.g., Cs⁺ vs. Li⁺) affects their ability to adsorb to the membrane surface. Cs⁺, with a lower hydration enthalpy, adsorbs more easily to the membrane compared to Li⁺, leading to a more negative membrane charge.
4. **Correlation with Permeability**: The ZP measurements were correlated with ion permeability, showing a clear inverse relationship. Higher ZP values (more negative charges) result in reduced ion permeability due to increased Donnan exclusion.
5. **Swelling Effects**: The study explains the discrepancy between the isoelectric point assumed from salt rejection curves and the actual isoelectric point measured by ZP analysis. Swelling of the polymer can outcompete the effect of charge exclusion at pH values below the pKₐ of carboxyl groups.
6. **X-ray Photoelectron Spectroscopy (XPS)**: XPS measurements confirmed the adsorption of chloride ions to the membrane surface, supporting the hypothesis of irreversible adsorption at high temperatures.
7. ** Ionic Strength Effects**: The effect of ionic strength on ZP was examined, showing that higher salt concentrations lead to better charge screening, reducing the absolute value of ZP. However, at low ionic strengths, increasing ionic strength leads to a decrease in ZP due to chloride adsorption.
The study provides insights into how solution ions affect the charge and performance of NF membranes, highlighting the importance of considering ion-specific interactions in the design of more efficient and customized NF membranes.The study investigates the ion-specific effects on the charge and performance of a loose polyamide nanofiltration (NF) membrane, which is commonly used for solute-solute separations. The researchers systematically evaluated the zeta potential (ZP) of the membrane under various conditions of pH, salinity, and ionic composition, and correlated these data with membrane performance. Key findings include:
1. **pKₐ of Carboxylic and Amines**: The pKₐ values of carboxylic and amine groups on the membrane surface were determined to be around 4.5 and 8.5, respectively. This suggests that the membrane's charge is influenced by the protonation of these functional groups.
2. **Chloride Ion Adsorption**: Chloride ions, which are highly polarizable, adsorb to the uncharged segments of the membrane, enhancing Donnan exclusion of anions. This adsorption increases the membrane's negative charge, leading to reduced ion permeability.
3. **Ion Specificity**: The "stickiness" of different ions (e.g., Cs⁺ vs. Li⁺) affects their ability to adsorb to the membrane surface. Cs⁺, with a lower hydration enthalpy, adsorbs more easily to the membrane compared to Li⁺, leading to a more negative membrane charge.
4. **Correlation with Permeability**: The ZP measurements were correlated with ion permeability, showing a clear inverse relationship. Higher ZP values (more negative charges) result in reduced ion permeability due to increased Donnan exclusion.
5. **Swelling Effects**: The study explains the discrepancy between the isoelectric point assumed from salt rejection curves and the actual isoelectric point measured by ZP analysis. Swelling of the polymer can outcompete the effect of charge exclusion at pH values below the pKₐ of carboxyl groups.
6. **X-ray Photoelectron Spectroscopy (XPS)**: XPS measurements confirmed the adsorption of chloride ions to the membrane surface, supporting the hypothesis of irreversible adsorption at high temperatures.
7. ** Ionic Strength Effects**: The effect of ionic strength on ZP was examined, showing that higher salt concentrations lead to better charge screening, reducing the absolute value of ZP. However, at low ionic strengths, increasing ionic strength leads to a decrease in ZP due to chloride adsorption.
The study provides insights into how solution ions affect the charge and performance of NF membranes, highlighting the importance of considering ion-specific interactions in the design of more efficient and customized NF membranes.