Determining the dielectric constant inside pores of nanofiltration membranes from membrane potential measurements

Electrochemical Characterization of Charged Membranes from Different Materials and Structures via Membrane Potential Analysis

Electrochemical characterization of positively and negatively charged membranes is performed by analyzing membrane potential values on the basis of the Teorell-Meyer-Sievers (TMS) model. This analysis allows the separate estimation of Donnan (interfacial effects) and diffusion (differences in ions transport through the membrane) contributions, and it permits the evaluation of the membrane's effective fixed charge concentration and the transport number of the ions in the membrane. Typical ion-exchange commercial membranes (AMX, Ionics or Nafion) are analyzed, though other experimental and commercial membranes, which are derived from different materials and have diverse structures (dense, swollen or nanoporous structures), are also considered. Moreover, for some membranes, changes associated with different modifications and other effects (concentration gradient or level, solution stirring, etc.) are also analyzed.

Effect of ion fluxes on regulating the light-induced transthylakoid electric potential difference

The light-induced transthylakoid membrane potential (ΔΨ) can not only drive the ATP synthesis through the ATP-synthase in chloroplasts but serve as an essential modifier in the acclimation of photosynthesis to fluctuating light conditions. It has been manifested that during photosynthesis, the light-induced ΔΨ is responsive to multiple factors among which the ion channels/transporters (e.g., V-K⁺, VCCN1, and KEA3) are key to adjust the ion distribution on the two sides of the thylakoid membrane and hence shape the kinetics of ΔΨ. However, an in-depth mechanistic understanding of ion fluxes on adjusting the transthylakoid electric potentials is still unclear. This lack of a mechanistic understanding is due to the experimental difficulty of closely observing ion fluxes in vivo and also hacking the evolution of parameters in a highly intertwined photosynthetic network. In this work, a computer model was applied to investigate the roles of ion fluxes on adjusting transthylakoid electric potentials upon a temporal cycle of a period of high illumination followed by a dark-adapted phase. The computing data revealed that, firstly, upon illumination, the dissipation of the steady-ΔΨ by ∼10 mV is contributed from the V-K⁺-driven K⁺ flux whilst ∼8 mV of the steady-ΔΨ is dissipated by the VCCN1-pumped Cl^- flux, but there were no appreciable KEA3-evoked variations on ΔΨ; secondly, on transition from high light to darkness, V-K⁺ and KEA3 are serving as major contributors whereas VCCN1 taking a counterbalancing part in shaping a standard trace of ECS (electrochromic shift), which commonly shows a sharp fall to a minimum before returning to the baseline in darkness. Besides, the functional consequences on components of ΔΨ adjusted by every particular ion channel/transporter were also explored. By employing the model, we bring evidence that particular thylakoid-harbored proteins, namely V-K⁺, VCCN1, and KEA3, function by distinct mechanisms in the dynamic adjustment of electric potential, which might be mainly importnat under fluctuating light conditions.

Influence of Chemical Cleaning on Physicochemical Characteristics and Ion Rejection by Thin Film Composite Nanofiltration Membranes

The impact of membrane cleaning with NaOH and HCl on the characteristics and associated changes in ion rejection was investigated in this study. NaOH affected the zeta potential of membranes with a greater concentration of carboxylic groups so that it was negative across the entire pH range investigated. Exposure to NaOH led to swelling of the active layer after each cleaning, especially for poly(piperazineamide) membranes. A 23% increase in the effective pore radii for these membranes after NaOH cleaning for 18 h led to 25, 36, 53 and 62% decrease in the rejection of magnesium, calcium, sodium, and chloride ions, respectively. Sulfate rejection decreased only slightly even for poly(piperazineamide) membranes (i.e., 7%) despite an appreciable increase in pore radii, which can be explained by the impact of charge exclusion on ion rejection that was enhanced by the 16% reduction in zeta potential. On the other hand, cleaning with HCl had a negligible impact on the zeta potential and performance of all membranes evaluated in this study. The increase in permeability after chemical cleaning was in agreement with the decrease in rejection of inorganic ions and correlated well with the effective pore radii measured using the membrane potential technique. The importance of charge exclusion in the rejection of inorganic ions was highlighted by the observed differences in rejection and permeability values when testing membranes after NaOH cleaning.

Influence of Active Layer on Separation Potentials of Nanofiltration Membranes for Inorganic Ions

Active layers of two fully aromatic and two semi-aromatic nanofiltration membranes were studied along with surface charge at different electrolyte composition and effective pore size to elucidate their influence on separation mechanisms for inorganic ions by steric, charge, and dielectric exclusion. The membrane potential method used for pore size measurement is underlined as the most appropriate measurement technique for this application owing to its dependence on the diffusional potentials of inorganic ions. Crossflow rejection experiments with dilute feed composition indicate that both fully aromatic membranes achieved similar rejection despite the differences in surface charge, which suggests that rejection by these membranes is exclusively dependent on size exclusion and the contribution of charge exclusion is weak. Rejection experiments with higher ionic strength and different composition of the feed solution confirmed this hypothesis. On the other hand, increase in the ionic strength of feed solution when the charge exclusion effects are negligible due to charge screening strongly influenced ion rejection by semi-aromatic membranes. The experimental results confirmed that charge exclusion contributes significantly to the performance of semi-aromatic membranes in addition to size exclusion. The contribution of dielectric exclusion to overall ion rejection would be more significant for fully aromatic membranes.

When Salt-Rejecting Polymers Meet Protons: An Electrochemical Impedance Spectroscopy Investigation

Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport in polymers relevant to desalination, dense aromatic polyamide Nomex and cellulose acetate (CA), using impedance spectroscopy, focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low scaling of conductivity with salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have far-reaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes.

Deviations from Electroneutrality in Membrane Barrier Layers: A Possible Mechanism Underlying High Salt Rejections

Reverse osmosis and nanofiltration (NF) employ composite membranes whose ultrathin barrier layers are significantly more permeable to water than to salts. Although solution-diffusion models of salt transport through barrier layers typically assume ubiquitous electroneutrality, in the case of ultrathin selective skins and low ion partition coefficients, space-charge regions may occupy a significant fraction of the membrane barrier layer. This work investigates the implications of these deviations from electroneutrality on salt transport. Both immobile external surface charge and unequal cation and anion solvation energies in the barrier layer lead to regions with excess mobile charge, and the size of these regions increases with decreasing values of either feed concentrations or ion partition coefficients. Moreover, the low concentration of the more excluded ion in the space-charge region can greatly increase resistance to salt transport to enhance salt rejection during NF. These effects are especially pronounced for membranes with a fixed external surface charge density whose sign is the same as that of the more excluded ion in a salt. Because of the space-charge regions, the barrier-layer resistance to salt transport initially rises rapidly with increasing barrier thickness and then plateaus or even declines within a certain thickness range. This trend in resistance implies that thin, defect-free barrier layers will exhibit higher salt rejections than thicker layers during NF at a fixed transmembrane pressure. Deviations from electroneutrality are consistent with both changes in NF salt rejections that occur upon changing the sign of the membrane fixed external surface charge, and CaCl2 rejections that in some cases may first decrease, then increase and then decrease again with increasing CaCl2 concentrations in NF feed solutions.

Predicting the Rejection of Major Seawater Ions by Spiral-Wound Nanofiltration Membranes

Seawater nanofiltration (SWNF) generates a softened permeate stream and a retentate stream in which the multivalent ions accumulate, offering opportunities for practical utilization of both streams. This study presents an approach to simulation of SWNF including all major seawater ions (Na(+), Cl(-), Ca(2+), Mg(2+), and SO4(2-)) based on the Nernst-Planck equation, and uses it for permeate and retentate streams composition prediction. The number of degrees of freedom in the system was reduced by assuming a very high ionic permeability for Na(+), which only weakly affected the other parameters in the system. Two alternatives were examined to analyze the importance of concentration dependence of ion permeabilities: The assumption of constant ion permeabilities resulted in a reasonable fit with experimental data. However, for the permeate composition the overall fit was significantly improved (P < 0.0001) when the permeabilities of Ca(2+) and Mg(2+) were allowed to depend on the ratio of their total concentration to Na(+). This type of dependence emphasizes the strong interaction of divalent ions with the membrane and its effect on the membrane fixed charge through screening or charge reversal. When this effect was included, model predictions closely matched the experimental results obtained, corroborating the phenomenological approach proposed in this study.

Theory of the formation of the electric double layer at the ion exchange membrane-solution interface

This work aims to extend the study of the formation of the electric double layer at the interface defined by a solution and an ion-exchange membrane on the basis of the Nernst-Planck and Poisson equations, including different values of the counter-ion diffusion coefficient and the dielectric constant in the solution and membrane phases. The network simulation method is used to obtain the time evolution of the electric potential, the displacement electric vector, the electric charge density and the ionic concentrations at the interface between a binary electrolyte solution and a cation-exchange membrane with total co-ion exclusion. The numerical results for the temporal evolution of the interfacial electric potential and the surface electric charge are compared with analytical solutions derived in the limit of the shortest times by considering the Poisson equation for a simple cationic diffusion process. The steady-state results are justified from the Gouy-Chapman theory for the diffuse double layer in the limits of similar and high bathing ionic concentrations with respect to the fixed-charge concentration inside the membrane. Interesting new physical insights arise from the interpretation of the process of the formation of the electric double layer at the ion exchange membrane-solution interface on the basis of a membrane model with total co-ion exclusion.