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Effect of solution ions on the charge and performance of nanofiltration membranes

Engineering and Technology

Effect of solution ions on the charge and performance of nanofiltration membranes

R. S. Roth, L. Birnhack, et al.

Discover the intriguing ion-specific effects on loose polyamide nanofiltration membranes, as explored by Rebecca S. Roth, Liat Birnhack, Mor Avidar, Elizabeth A. Hjelvik, Anthony P. Straub, and Razi Epsztein. This research unveils how varying pH, salinity, and ionic composition impact membrane charge and performance, revealing significant insights into the interplay of solution ions and Donnan exclusion.

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Playback language: English
Introduction
Nanofiltration (NF) membranes are increasingly important in water treatment and various industries, enabling selective solute separation based on size exclusion and other molecular-level interactions including Donnan exclusion, dielectric effects, and van der Waals forces. Donnan exclusion, stemming from the charge of amine and carboxyl groups on the polyamide active layer, plays a crucial role in NF membrane separation capabilities. However, the impact of solution ions on membrane charge and subsequent performance remains incompletely understood. This is particularly relevant given efforts to achieve separation between similar ions. A molecular-level understanding of solution-ion/membrane interactions is vital for improving NF membrane efficiency and customization. Existing research utilizing zeta potential (ZP) measurements and salt rejection versus pH trends to characterize membrane surface charge has yielded inconsistent results. Discrepancies exist in interpreting ZP measurements and the effect of salt concentration on membrane ZP. Some studies show an increase in negative charge with higher salt concentrations, while others report the opposite. These inconsistencies suggest NF membrane charge at a given pH isn't solely an intrinsic property but is significantly influenced by solution composition. Different ions, even those with similar charge, valency, and hydrated radius, may exhibit varying 'stickiness'—their ability to adsorb to the membrane surface—affecting membrane charge. This study systematically investigates the ZP and ion permeability of a polyamide NF membrane across a wide range of pH values, ionic compositions, and ionic strengths to elucidate the influence of solution ions on membrane charge and performance. By correlating ZP, ionization behavior of fixed groups, and ion rejection/permeability, the research aims to resolve existing discrepancies and demonstrate the significant contribution of adsorbed solution ions to NF membrane behavior.
Literature Review
The literature shows significant inconsistencies in the understanding and interpretation of nanofiltration (NF) membrane behavior, particularly regarding the influence of solution ions on membrane charge and performance. While Donnan exclusion, driven by the fixed charges on the membrane surface (amine and carboxyl groups), is acknowledged as a critical factor in solute separation, the specific effects of different ions in solution remain debated. Studies using zeta potential (ZP) measurements to assess membrane surface charge have yielded contradictory results, with some suggesting an increase in negative charge at higher salt concentrations, and others observing the opposite. This discrepancy highlights the need for a more nuanced understanding of the interplay between solution ions and the membrane. Furthermore, the relationship between salt rejection curves (typically showing minimum rejection at a specific pH) and membrane charge is not always straightforward. The pH at minimum rejection is often assumed to be the isoelectric point, but this often contradicts ZP measurements. Existing models often fail to account for the ion-specific effects that can significantly influence membrane performance. The 'stickiness' or adsorption capacity of different ions to the membrane surface, which depends on factors like hydration enthalpy and polarizability, remains largely under-investigated in the context of NF membrane behavior.
Methodology
This research employed a combination of experimental techniques to comprehensively investigate the effects of solution ions on a polyamide nanofiltration (NF) membrane (NF270, Dow Filmtec). The membrane was characterized under a wide range of conditions, systematically varying pH, ionic strength, and ionic composition. **Zeta Potential Measurements:** Streaming potential measurements using a SurPASS3 electrokinetic analyzer (Anton Paar GmbH, Austria) were employed to determine the zeta potential (ZP) of the membrane surface. The Helmholtz-Smoluchowski equation was used to convert streaming potential to zeta potential. Measurements were conducted with single-salt solutions of LiCl, NaCl, KCl, and CsCl at various concentrations (0.1-75 mM) and pH values (adjusted using HCl and NaOH), ensuring reliable results through verification of linearity, asymmetry, and static resistance. **X-ray Photoelectron Spectroscopy (XPS):** XPS measurements were performed using a Kratos Supra X-ray photoelectron spectrometer to probe the adsorption of ions onto the membrane surface. Membranes were soaked in DI water or NaCl solutions at different concentrations (0 mM, 1 mM, 10 mM) and temperatures. High-resolution spectra provided information on elemental chemical states, enabling the calculation of atomic percent. **Permeability Experiments:** Permeability experiments were carried out using a crossflow filtration system with custom-made stainless steel cells accommodating 15.3 cm² flat-sheet membrane coupons. Single-salt solutions were used, and the permeate and reject streams were continuously recycled to the feed tank. Water and salt fluxes were calculated based on weight and electrical conductivity measurements. Concentration polarization was accounted for using the film theory and Sherwood correlation. Water permeability (Pw) and salt permeability (Ps) were calculated using the solution-diffusion model. Different conditions of pH, ionic strength, and temperature were assessed using the same membrane coupon to observe irreversible changes in chloride adsorption. The specific ions used (Li⁺, Na⁺, K⁺, Cs⁺, Cl⁻, H₃O⁺, OH⁻, HCO₃⁻, CO₃²⁻) were selected to systematically vary properties like crystal radius, hydrated radius, hydration enthalpy, and polarizability, allowing investigation of ion-specific effects. The data analysis involved correlating ZP measurements, obtained under the various conditions, with membrane permeability data to uncover the interplay between solution ions, membrane charge, and transport properties.
Key Findings
This study yielded several key findings regarding the influence of solution ions on the charge and performance of nanofiltration membranes: 1. **pKa Determination:** The pKa values of carboxyl and amine groups on the NF270 membrane surface were determined to be approximately 4.5 and 8.5, respectively. These values provide a crucial baseline for understanding membrane ionization behavior. 2. **Chloride Adsorption:** Chloride anions were found to adsorb to the polyamide membrane surface, even on uncharged segments, due to their high polarizability. This adsorption significantly increases the overall negative charge of the membrane, enhancing Donnan exclusion of anions. 3. **Cation-Specific Effects:** Monovalent cations exhibited varying degrees of influence on membrane charge, influenced by their 'stickiness.' Cs⁺, with a softer hydration shell, showed a stronger tendency to adsorb to the membrane than Li⁺, leading to observable differences in zeta potential at low ionic strength. 4. **Ionic Strength Effects:** At high ionic strength (>1 mM), charge screening by solution cations predominated, reducing the absolute value of zeta potential regardless of cation type. At low ionic strength (<1 mM), chloride adsorption became the dominant factor affecting zeta potential. 5. **Correlation between Zeta Potential and Permeability:** A clear inverse correlation was observed between zeta potential (absolute value) and salt permeability. This demonstrates that zeta potential serves as a good indicator of Donnan exclusion, reflecting the effective membrane charge influenced by both fixed charges and adsorbed solution ions. Differences in the permeability of LiCl and CsCl were noticeable at low ionic strength (1 mM) and high pH (7.5), where the negative membrane charge was high and charge screening was minimal. 6. **Irreversible Chloride Adsorption:** Experiments demonstrated irreversible chloride adsorption at elevated temperatures (40 °C), further supporting the significant role of chloride adsorption in influencing membrane charge and consequently, salt permeability.
Discussion
The findings of this study significantly advance our understanding of the complex interplay between solution ions and nanofiltration membrane performance. The determination of the pKa values for carboxyl and amine groups on the membrane surface provides a critical foundation for modeling membrane charge behavior. The discovery of significant chloride adsorption onto the membrane, even on uncharged regions, resolves previous discrepancies in the literature concerning the effect of salt concentration on membrane charge. This adsorption mechanism is shown to significantly impact Donnan exclusion and subsequent salt rejection. The observation of cation-specific effects based on 'stickiness' further highlights the need for a more sophisticated understanding of ion-membrane interactions than simple size and charge exclusion models. The correlation between zeta potential and permeability underscores the value of using zeta potential as a predictive tool for Donnan exclusion in NF membranes, thereby improving the design and optimization of NF processes. The findings demonstrate the importance of considering ion-specific interactions beyond simple electrostatic effects to predict and control membrane behavior.
Conclusion
This study provides a comprehensive examination of how solution ions affect the charge and performance of nanofiltration membranes. By integrating zeta potential measurements with permeability experiments under a wide array of conditions, the research clarifies previously conflicting observations regarding the influence of solution concentration and ionic composition. The key finding of significant chloride adsorption, coupled with the demonstrable impact of cation 'stickiness,' provides crucial insights for designing improved NF membranes with enhanced selectivity. Future research should explore the applicability of these findings to other membrane types and investigate the long-term stability of adsorbed ions under varying operational conditions.
Limitations
While this study provides valuable insights, some limitations should be noted. The study focused on a single type of commercially available polyamide NF membrane (NF270). The results might not be directly generalizable to all NF membranes, as membrane properties like pore size distribution and surface chemistry can vary significantly. The experiments were conducted under controlled laboratory conditions; the influence of other factors, such as organic fouling, might differ under real-world conditions. Further investigation is needed to explore the long-term effects of ion adsorption and the potential for ion desorption or exchange over extended periods of operation.
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