What makes aromatic polyamide membranes superior: New insights into ion transport and membrane structure

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m-2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m-2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

Tailoring properties and performance of thin-film composite membranes by salt additives for water treatment: A critical review

During the fabrication of thin film composite (TFC) membranes by interfacial polymerization (IP), the utilization of salt additives is one of the effective methods to regulate membrane properties and performance. Despite gradually receiving widespread attention for membrane preparation, the strategies, effects and underlying mechanisms of using salt additives have not yet been systematically summarized. This review for the first time provides an overview of various salt additives used to tailor properties and performance of TFC membranes for water treatment. By classifying salt additives into organic and inorganic salts, the roles of added salt additives in the IP process and the induced changes in membrane structure and properties are discussed in detail, and the different mechanisms of salt additives affecting membrane formation are summarized. Based on these mechanisms, the salt-based regulation strategies have shown great potential for improving the performance and application competitiveness of TFC membranes, including overcoming the trade-off relationship between water permeability and salt selectivity, tailoring membrane pore size distribution for precise solute-solute separation, and enhancing membrane antifouling performance. Finally, future research directions are suggested to focus on the long-term stability assessment of salt-modified membranes, the combined use of different salt additives, and the integration of salt regulation with other membrane design or modification strategies.

Research on Membranes and Their Associated Processes at the Université Paris-Est Créteil: Progress Report, Perspectives, and National and International Collaborations

Research on membranes and their associated processes was initiated in 1970 at the University of Paris XII/IUT de Créteil, which became in 2010 the University Paris-Est Créteil (UPEC). This research initially focused on the development and applications of pervaporation membranes, then concerned the metrology of ion-exchange membranes, then expanded to dialysis processes using these membranes, and recently opened to composite membranes and their applications in production or purification processes. Both experimental and fundamental aspects have been developed in parallel. This evolution has been reinforced by an opening to the French and European industries, and to the international scene, especially to the Krasnodar Membrane Institute (Kuban State University-Russia) and to the Department of Chemistry, (Qassim University-Saudi Arabia). Here, we first presented the history of this research activity, then developed the main research axes carried out at UPEC over the 2012-2022 period; then, we gave the main results obtained, and finally, showed the cross contribution of the developed collaborations. We avoided a chronological presentation of these activities and grouped them by theme: composite membranes and ion-exchange membranes. For composite membranes, we have detailed three applications: highly selective lithium-ion extraction, bleach production, and water and industrial effluent treatments. For ion-exchange membranes, we focused on their characterization methods, their use in Neutralization Dialysis for brackish water demineralization, and their fouling and antifouling processes. It appears that the research activities on membranes within UPEC are very dynamic and fruitful, and benefit from scientific exchanges with our Russian partners, which contributed to the development of strong membrane activity on water treatment within Qassim University. Finally, four main perspectives of this research activity were given: the design of autonomous and energy self-sufficient processes, refinement of characterization by Electrochemical Scanning Microscopy, functional membrane separators, and green membrane preparation and use.

Proteoliposome-Incorporated Seawater Reverse Osmosis Polyamide Membrane: Is the Aquaporin Water Channel Effect in Improving Membrane Performance Overestimated?

The water channel feature of the aquaporin (AQP) is considered to be the key in improving the permselectivity of AQP-based thin-film composite (TFC) polyamide (PA) membranes, yet much less attention has been paid to the physicochemical property changes of the PA layer induced by AQP-reconstituted proteoliposomes. This study systematically investigated the roles of proteoliposome constituents (liposome/detergent/AQP) in affecting the physicochemical properties and performance of the membranes. For the first time, we demonstrated that the constituents in the proteoliposome could facilitate the formation of a PA layer with enlarged protuberances and thinner crumples, resulting in a 79% increase in effective surface area and lowering of hydraulic resistance for filtration. These PA structural changes of the AQP-based membrane were found to contribute over 70% to the water permeability increase via comparing the separation performance of the membranes prepared with liposome, detergent, and proteoliposome, respectively, and one proteoliposome-ruptured membrane. The contribution from the AQP water channel feature was about 27% of water permeability increase in the current study, attributed to only ∼20% vesicle coverage in the PA matrix, and this contribution may be easily lost as a result of vesicle rupture during the real seawater reverse osmosis process. This study reveals that the changed morphology dominates the performance improvement of the AQP-based PA membrane and well explains why the actual AQP-based PA membranes cannot acquire the theoretical water/salt selectivity of a biomimetic AQP membrane, deepening our understanding of the AQP-based membranes.

Elucidating the Effect of Aliphatic Molecular Plugs on Ion-Rejecting Properties of Polyamide Membranes

Polyamide RO membranes are widely used for seawater desalination owing to their high salt rejection and water permeability; however, improved selectivity-permeability trade-off is still desired. "Molecular plugs," small molecules immobilized within the polyamide structure, offer an attractive approach; however, their overall effect on polyamide physicochemical properties poses many questions. Here, we analyze the effect of decylamine, a promising plug, and a few charged and uncharged mimics on polyamide films using several in situ techniques. Electrochemical impedance spectroscopy (EIS) reveals a complex pH-dependent response, whereby, upon exposure to amine solution, conductivity first rapidly drops; however, under alkaline conditions, when amine is uncharged, the trend subsequently slowly reverses, and conductivity increases. This slow reversal was observed for noncharged alcohols of similar size as well, but not for larger surfactant molecules. The reversal was assigned to the uptake of plug molecules within polyamide, as opposed to the fast initial drop assigned to surface adsorption. EIS and quartz-crystal microbalance (QCM) results showed that exposure to decylamine under alkaline conditions ultimately led to an irreversible decrease in conductivity, that is, stronger ion rejection, remaining after re-exposure of polyamide to amine-free buffer. This suggests that plug uptake within polyamide resulted in polymer stress, indeed observed in surface stress measurements, and subsequent relaxation. The results indicate that the moderate size of decylamine and conditions minimizing its charge were optimal for irreversible change; however, charge interactions helped maximize its binding within polymer and induce the desired sustained change in selectivity. The results have many potential implications for improving current membrane desalination technology and increasing inherent membrane selectivity toward hard-to-remove species.

A Review on Ion-Exchange Membranes Fouling during Electrodialysis Process in Food Industry, Part 2: Influence on Transport Properties and Electrochemical Characteristics, Cleaning and Its Consequences

Ion-exchange membranes (IEMs) are increasingly used in dialysis and electrodialysis processes for the extraction, fractionation and concentration of valuable components, as well as reagent-free control of liquid media pH in the food industry. Fouling of IEMs is specific compared to that observed in the case of reverse or direct osmosis, ultrafiltration, microfiltration, and other membrane processes. This specificity is determined by the high concentration of fixed groups in IEMs, as well as by the phenomena inherent only in electromembrane processes, i.e., induced by an electric field. This review analyzes modern scientific publications on the effect of foulants (mainly typical for the dairy, wine and fruit juice industries) on the structural, transport, mass transfer, and electrochemical characteristics of cation-exchange and anion-exchange membranes. The relationship between the nature of the foulant and the structure, physicochemical, transport properties and behavior of ion-exchange membranes in an electric field is analyzed using experimental data (ion exchange capacity, water content, conductivity, diffusion permeability, limiting current density, water splitting, electroconvection, etc.) and modern mathematical models. The implications of traditional chemical cleaning are taken into account in this analysis and modern non-destructive membrane cleaning methods are discussed. Finally, challenges for the near future were identified.

Influence of Solute Molecular Diameter on Permeability-Selectivity Tradeoff of Thin-Film Composite Polyamide Membranes in Aqueous Separations

Fundamental understanding of the reverse osmosis (RO) transport phenomena is necessary for quantitative prediction of contaminant rejection and development of more selective membranes. The solution-diffusion (S-D) model predicts a tradeoff relationship between permeability and selectivity, and this tradeoff trend was recently reported for RO. But the first principles governing the relationship are not well understood for aqueous separation membranes. This study presents a framework to elucidate the underlying factors of the permeability-selectivity tradeoff relationship in thin-film composite polyamide (TFC-PA) membranes. Water and solute permeabilities of membranes with a range of selectivities are examined using six nonelectrolyte solutes of various sizes and dimensions. The permeability-selectivity tradeoff trend, as defined by S-D, was observed for all six solutes. Crucially, the slopes of the tradeoff lines, λ, are found to be related to the solute and solvent (i.e., water) diameters, d_s and d_w, respectively, by λ = (d_s/d_w)² - 1, consistent with the S-D framework established for gas separation membranes. Additionally, the intercepts of the tradeoff lines are shown to be also influenced by d_s. These results highlight that solute molecular diameter is a primary influence on the permeability-selectivity tradeoff for the permeants investigated in this study. Furthermore, a transport regime where solute permeation is only very weakly coupled to water transport, in addition to the conventional S-D, is identified for the first time. We demonstrate that the boundary delineating the two transport regimes can be determined by the solute diameter. The relationship between characteristic features of the "additional regime" and solute dimensions are analyzed. The study shows that the general principles of the S-D framework are applicable to TFC-PA membranes and the analysis quantified the principal role of solute size in governing RO transport. The experimental and analytical evidence suggest that nonelectrolyte solute transport can, in principle, be a priori predicted using molecular diameter. Findings of this investigation provide new insights for understanding the transport mechanisms in osmotic membrane processes.

Ionization behavior of nanoporous polyamide membranes

Escalating global water scarcity necessitates high-performance desalination membranes, for which fundamental understanding of structure-property-performance relationships is required. In this study, we comprehensively assess the ionization behavior of nanoporous polyamide selective layers in state-of-the-art nanofiltration (NF) membranes. In these films, residual carboxylic acids and amines influence permeability and selectivity by imparting hydrophilicity and ionizable moieties that can exclude coions. We utilize layered interfacial polymerization to prepare physically and chemically similar selective layers of controlled thickness. We then demonstrate location-dependent ionization of carboxyl groups in NF polyamide films. Specifically, only surface carboxyl groups ionize under neutral pH, whereas interior carboxyl ionization requires pH >9. Conversely, amine ionization behaves invariably across the film. First-principles simulations reveal that the low permittivity of nanoconfined water drives the anomalous carboxyl ionization behavior. Furthermore, we report that interior carboxyl ionization could improve the water-salt permselectivity of NF membranes over fourfold, suggesting that interior charge density could be an important tool to enhance the selectivity of polyamide membranes. Our findings highlight the influence of nanoconfinement on membrane transport properties and provide enhanced fundamental understanding of ionization that could enable novel membrane design.

Separation of Volatile Fatty Acids from Model Anaerobic Effluents Using Various Membrane Technologies

Effluents of anaerobic processes still contain valuable components, among which volatile fatty acids (VFAs) can be regarded and should be recovered and/or used further in applications such as microbial electrochemical technology to generate energy/energy carriers. To accomplish the separation of VFAs from waste liquors, various membrane-based solutions applying different transport mechanisms and traits are available, including pressure-driven nanofiltration (NF) and reverse osmosis (RO) which are capable to clarify, fractionate and concentrate salts and organics. Besides, emerging techniques using a membrane such as forward osmosis (FO) and supported liquid membrane (SILM) technology can be taken into consideration for VFA separation. In this work, we evaluate these four various downstream methods (NF, RO, FO and SILM) to determine the best one, comparatively, for enriching VFAs from pH-varied model solutions composed of acetic, butyric and propionic acids in different concentrations. The assessment of the separation experiments was supported by statistical examination to draw more solid conclusions. Accordingly, it turned out that all methods can separate VFAs from the model solution. The highest average retention was achieved by RO (84% at the applied transmembrane pressure of 6 bar), while NF provided the highest permeance (6.5 L/m2hbar) and a high selectivity between different VFAs.

Ion partitioning and permeation in charged low-T* membranes

Understanding ion transport in membrane materials is key to engineering and development of desalination and water purification technologies as well as electro-membrane applications. To date, modeling of ion transport has mainly relied on mean-field approaches, originally intended for weak inter-ionic interactions, i.e., high reduced temperature T*. This condition is violated in many membranes, which could explain disagreement between predicted trends and experiments. The paper highlights observed discrepancies and develops a new approach based on the concept of ion association, more adequate in the low-T^⁎ limit. The new model addresses ion binding and mobility consistently within the same physical picture, applied to different types of single and mixed salts. The resulting relations show a significantly weaker connection between ion partitioning and permeability than the standard ones. Estimates using primitive model (PM) of ions in a homogeneous dielectric suggest that non-PM mechanisms, originating from the molecular structure of the ion-solvating environment, might enhance ion association in membranes. PM analysis also predicts that ion solvation and association must be rigidly related, yet non-PM effects may decouple these phenomena and allow a crossover to non-trivial regimes consistent with experiments and simulations. Despite the crude nature of the presented approach and some questions remaining open, it appears to explain most available experimental data and presents a step towards predictive modeling of ion-selective membrane separations in water-, environment- and energy-related applications.

Degradation of Polyamide Membranes Exposed to Chlorine: An Impedance Spectroscopy Study

Polyamide is the key material in modern membrane desalination; however, its well-known and incompletely understood drawback is its low tolerance to chlorine, the most efficient in-line disinfectant. Here we report a first investigation of the mechanism and kinetics of chlorine attack using electrochemical impedance spectroscopy (EIS) that directly probes changes in ion permeation upon chlorination at different pH values, focusing on its early stages and low chlorine concentrations (15-197 ppm). EIS results partly conform to an established two-stage mechanism that proceeds as N-chlorination followed by either C-chlorination in acidic conditions or amide bond scission in alkaline conditions. However, early time kinetics in acidic conditions shows inconsistencies with this model, explained by possible effects of direct ring chlorination and finite polymer relaxation rates. The findings indicate that (a) N-chlorination reduces membrane polarity and ion permeability, while C-chlorination has an opposite effect; (b) chlorination in acidic conditions must involve other reactions, such as direct ring chlorination, in addition to N-chlorination and Orton rearrangement; and (c) the ultimate chemical transformations (C-chlorination or amide bond scission) result in an irreversible increase in membrane polarity and loss of ion rejection. The results highlight the potential of EIS as a powerful and sensitive tool for studying chemical degradation of ion-selective materials that may assist in developing new chlorine-resistant membranes.

Selectivity and polarization in water channel membranes: lessons learned from polymeric membranes and CNTs

The aspects of ion exclusion and concentration polarization are highlighted as critical for achieving high selectivity in an artificial water channel.