Multiscale modelling of transport in polymer-based reverse-osmosis/nanofiltration membranes: present and future.

Nanofiltration (NF) and reverse osmosis (RO) processes are physical separation technologies used to remove contaminants from liquid streams by employing dense polymer-based membranes with nanometric voids that confine fluids at the nanoscale. At this level, physical properties such as solvent and solute permeabilities are intricately linked to molecular interactions. Initially, numerous studies focused on developing macroscopic transport models to gain insights into separation properties at the nanometer scale. However, continuum-based models have limitations in nanoconfined situations that can be overcome by force field molecular simulations. Continuum-based models heavily rely on bulk properties, often neglecting critical factors like liquid structuring, pore geometry, and molecular/chemical specifics. Molecular/mesoscale simulations, while encompassing these details, often face limitations in time and spatial scales. Therefore, achieving a comprehensive understanding of transport requires a synergistic integration of both approaches through a multiscale approach that effectively combines and merges both scales. This review aims to provide a comprehensive overview of the state-of-the-art in multiscale modeling of transport through NF/RO membranes, spanning from the nanoscale to continuum media.

New insights into biochar ammoniacal nitrogen adsorption and its correlation to aerobic degradation ammonia emissions.

One of the technical barriers to the wider use of biochar in the composting practices is the lack of accurate quantification linking biochar properties to application outcomes. To address this issue, this paper investigates the use of ammonia nitrogen adsorption capacity by biochar as a predictor of ammonia emission during composting in the presence of biochar. With this in mind, this work investigated the use of ammonia nitrogen adsorption capacity of biochar when mixed with solid digestate, and the reduction in ammonia emissions resulting from the addition of biochar during aerobic degradation of solid digestate. A biochar synthesized at 900 °C, another synthesized at 450 °C, and two derivatives of the latter biochar, one chemically modified with nitric acid and the other with potassium hydroxide, were tested. This study concluded that the chemical characteristics of the biochar, including pH and oxygen/carbon atomic ratio, had a greater influence on the adsorption of ammonia nitrogen than physical attributes such as specific surface area. In this regard, nitric acid modification had superior performance compared to hydroxide potassium modification to increase biochar chemical attributes and reduce ammonia emissions when applied to aerobic degradation. Finally, a significant linear correlation (p-value < 0.05, r2 = 0.79) was found between biochar ammonia nitrogen adsorption capacity and ammonia emissions along composting, showing the potential of this variable as a predictive parameter. This study provides insights for future explorations aiming to develop predictive tests for biochar performance.

Molecular Dynamics Simulation Study of Organic Solvents Confined in PIM-1 and P84 Polyimide Membranes.

Organic solvent nanofiltration (OSN) has recently proved to be a promising separation process thanks to the development of membrane materials with suitable resistance toward organic solvents. Among those materials, P84 polyimide membranes are currently the most used in OSN while PIM-1 membranes have recently attracted attention due to their high permeance in apolar solvents and alcohols. Both P84 and PIM-1 membranes have nanosized free volumes, and their separation performance is finely connected to polymer/solvent interactions. Consequently, modeling OSN membranes at the molecular scale is highly desirable in order to rationalize experimental observations and gain a deeper insight into the molecular mechanisms ruling solvent and solute permeation. A prerequisite for understanding solvent transport through OSN membranes is therefore to characterize the membrane/solvent interactions at the molecular level. For that purpose, we carried out molecular simulations of three different solvents, acetone, methanol, and toluene in contact with P84 and PIM-1 membranes. The solvent uptake by both membranes was found to be correlated to the degree of confinement of the solvent, the polymer swelling ability and polymer/solvent interactions. The translational dynamics of the solvent molecules in the PIM-1 membrane was found to be correlated with the solvent viscosity due to the relatively large pores of this membrane. That was not the case with the P84 membrane, which has a much denser structure than the PIM-1 membrane and for which it was observed that the translational dynamics of the confined solvent molecules was directly correlated to the affinity between the P84 polymer and the solvent.

New Low-Cost Ceramic Microfiltration Membranes for Bacteria Removal.

Safe water provision in low-income countries is constrained by limited financial resources, and the problem is worsened during natural disasters. Thus, there is a need to develop efficient low-cost technologies for point-of-use water treatment. This work reports on the development of new ceramic microfiltration membranes made from mixtures of inexpensive raw materials available locally (kaolin, bentonite and limestone) and their efficiency in rejecting bacteria such as Escherichia coli and Staphylococcus aureus. Thermogravimetric analysis, differential scanning calorimetry, Fourier-transform infrared spectroscopy, X-ray diffraction, mercury intrusion porosimetry, flexural strength and water uptake were used to characterize the raw materials and membranes. The addition of limestone in the membrane fabrication increased the pore size, the porosity and, thus, the permeability of the membranes but at the expense of the rejection performance. Among the different compositions studied, the membrane made of 83% kaolin, 10% bentonite and 7% limestone showed the best performance compromise with water permeability of 566 L·h-1·m-2·bar-1 and 100% rejection of both Escherichia coli and Staphylococcus aureus. These new low-cost microfiltration membranes are expected to have potential applications in water treatment and household applications.

Modification Mechanism of Polyamide Reverse Osmosis Membrane by Persulfate: Roles of Hydroxyl and Sulfate Radicals.

Oxidative modification is a facile method to improve the desalination performance of thin-film composite membranes. In this study, we comparatively investigated the modification mechanisms induced by sulfate radical (SO4• -) and hydroxyl radical (HO•) for polyamide reverse osmosis (RO) membrane. The SO4• -- and HO•-based membrane modifications were manipulated by simply adjusting the pH of the thermal-activated persulfate solution. Although both of them improved the water permeability of the RO membrane under certain conditions, the SO4• --modified membrane notably prevailed over the HO•-modified one due to higher permeability, more consistent salt rejection rates over wide pH and salinity ranges, and better stability when exposed to high doses of chlorine. The differences of the membranes modified by the two radical species probably can be related to their distinct surface properties in terms of morphology, hydrophilicity, surface charge, and chemical composition. Further identification of the transformation products of a model polyamide monomer using high-resolution mass spectrometry demonstrated that SO4• - initiated polymerization reactions and produced hydroquinone/benzoquinone and polyaromatic structures; whereas the amide group of the monomer was degraded by HO•, generating hydroxyl, carboxyl, and nitro groups. The results will enlighten effective ways for practical modification of polyamide RO membranes to improve desalination performances and the development of sustainable oxidation-combined membrane processes.

Harvesting microalgal biomass using negatively charged polysulfone patterned membranes: Influence of pattern shapes and mechanism of fouling mitigation.

Membranes have a lot of potential for harvesting microalgae, but membrane fouling is hampering their breakthrough. In this study, the effects of charge and corrugated surface on membrane filtration performance were investigated. The clean water permeance (CWP), the microalgae harvesting efficiency and the membrane flux for a microalgal broth were determined using patterned polysulfone (PSf) membranes with different shapes of the surface patterns and containing different charge densities by blending sulfonated polysulfone (sPSf). The flow behavior near the patterned membrane surface, as well as the interaction energy between membrane and microalgae were investigated using computational fluid dynamics (CFD) simulation and the improved extended "Derjaguin, Landau, Verwey, Overbeek" (XDLVO) theory, respectively. Membrane charge and pattern shape significantly improve the membrane performance. The critical pressures of all sPSf blend patterned membranes were higher than 2.5 bar. A 4.5w% sPSf blend patterned membranes with wave patterns showed the highest CWP (2300 L/m2 h bar) and membrane flux in the microalgal broth (1000 L/m2 h bar) with 100% harvesting efficiency. XDLVO analysis showed that sPSf blend patterned membranes prepared obtained the lowest interaction energy and highest energy barrier for microalgal attachment. CFD simulation showed a higher velocity and wall shear on the pattern apexes.

Biofouling in membrane bioreactors: nexus between polyacrylonitrile surface charge and community composition.

The influence of membrane surface charge on biofouling community composition during activated sludge filtration in a membrane bioreactor was investigated in this study using polyacrylonitrile-based membranes. Membranes with different surface properties were synthesized by phase inversion followed by a layer-by-layer modification. Various characterization results showed that the membranes differed only in their surface chemical composition and charge, ie two of them were negative, one neutral and one positive. Membrane fouling experiments were performed for 40 days and the biofouling communities were analyzed. PCR-DGGE fingerprinting indicated selective enrichment of bacterial populations from the sludge suspension within the biofilms at any time point. The biofilm community composition seemed to change with time. However, no difference was observed between the biofilm community of differently charged membranes at specific time points. It could be concluded that membrane charges do not play a decisive role in the long-term selection of the key bacterial foulants.

High Water Flux with Ions Sieving in a Desalination 2D Sub-Nanoporous Boron Nitride Material.

Over the past decades, desalination by reverse osmosis (RO) membranes has attracted increasing attention. Although RO has proven its efficiency, it remains, however, relatively costly because of the use of high-pressure pumps and the low water permeability of conventional cross-linked polymer membranes. One route to improve the desalination performance consists of using membranes made from sub-nanoporous boron nitride (sNBN) monolayers. Indeed, by using molecular dynamics simulations, we report here that the water permeability of such sNBN membranes far exceeds that of conventional RO polymer membranes and is even higher than that of nanoporous graphene while the ion rejection remains close to 100%. At the same time, the molecular mechanism of water and ion transport through sNBN has been elucidated, with special attention paid to the impact of ions on water permeability through sNBN membranes.

High-performance membranes with full pH-stability.

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0-14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h-1 m-2 bar-1 while retaining NaCl, MgSO4 and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%.

Interactions of Organics within Hydrated Selective Layer of Reverse Osmosis Desalination Membrane: A Combined Experimental and Computational Study.

In this work we have examined a computational approach in predicting the interactions between uncharged organic solutes and polyamide membranes. We used three model organic molecules with identical molecular weights (100.1 g/mol), 4-aminopiperidine, 3,3-dimethyl-2-butanone (pinacolone) and methylisobutyl ketone for which we obtained experimental data on partitioning, diffusion and separation on a typical seawater reverse osmosis (RO) membrane. The interaction energy between the solutes and the membrane phase (fully aromatic polyamide) was computed from molecular dynamics (MD) simulations and the resulting sequence was found to correlate well with the experimental rejections and sorption data. Sorption of the different organic solutes within the membrane skin layer determined from attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) nicely agreed with interaction energies computed from molecular simulations. Qualitative information about solute diffusivity inside the membrane was also extracted from MD simulations while ATR-FTIR experiments indicated strongly hindered diffusion with diffusion coefficients in the membrane about 10-15 m2/s. The computational approach presented here could be a first step toward predicting rejections trends of, for example, hormones and pharmaceuticals by RO dense membranes.

Physics behind Water Transport through Nanoporous Boron Nitride and Graphene.

In this work, molecular dynamics simulations were used to determine the surface tension profile of water on graphene and boron nitride (BN) multilayers and to predict water permeation through nanoporous graphene and BN membranes. For both graphene and BN multilayers, a decrease in surface tension (γ) was evidenced as the number of layers increased. This lessening in γ was shown to result from a negative surface tension contribution due to long-range wetting of water, which also contributes to lower water permeation through a two-layer membrane with respect to permeation through a monolayer. We also showed that a decrease in water surface tension on a BN monolayer with regards to graphene was at the origin of an increase in water permeation through BN. Our findings suggest that nanoporous BN membranes could be attractive candidates for desalination applications.

Ultrafast diffusion of Ionic Liquids Confined in Carbon Nanotubes.

Over the past decade many works have focused on various aspects of the dynamics of liquids confined at the nanoscale such as e.g. water flow enhancement through carbon nanotubes (CNTs). Transport of room temperature ionic liquids (RTILs) through various nanochannels has also been explored and some conflicting findings about their translational dynamics have been reported. In this work, we focus on translational dynamics of RTILs confined in various CNTs. By means of molecular dynamics simulations we highlight a substantially enhanced diffusion of confined RTILs with an increase up to two orders of magnitude with respect to bulk-phase properties. This ultrafast diffusion of RTILs inside CNTs is shown to result from the combination of various factors such as low friction, molecular stacking, size, helicity, curvature and cooperative dynamics effects.

Superpermittivity of nanoconfined water.

Nowadays, it is well established that the physical properties of confined liquids strongly differ from those in bulk phase. While dynamical and structural properties were strongly explored, dielectric properties are poorly studied despite their importance in the understanding and the modelling of molecular mechanism in a number of nano-applications such as nanofluidics, nanofiltration, and nanomedicine. Among them, the dielectric permittivity is probably one of the most important. The lack of knowledge about it strongly limits our ability to model fluid-material interactions and more generally our understanding of the behaviour of confined fluids. Recently, the dielectric permittivity of confined water in silica, Metal Organic Frameworks, and graphene materials was found to be slightly higher than the permittivity of water in bulk phase. In this work, the permittivity of water and dichloromethane confined in carbon nanotubes was predicted by means of molecular dynamics simulations. The static dielectric constant was found to be 700, i.e., 10-fold higher than the bulk value. This superpermittivity has, for origin, the excluded volume and the presence of an unconfined direction leading to a pre-orientation of water molecules close to the pore wall and an increase in dipolar fluctuations.

Unravelling the anomalous dielectric permittivity of nanoconfined electrolyte solutions.

The dielectric properties of sodium chloride solutions confined in a hydrophilic nanocavity were investigated by means of molecular dynamics simulations. Unlike what is observed in the bulk phase, three dielectric regimes were evidenced, namely an anomalous increase in the dielectric permittivity at low concentrations (with respect to confined pure water), a dielectric plateau at intermediate concentrations and finally a bulk-like behavior for salt concentrations higher than a critical value. It was shown that this peculiar behavior results from the competition between dielectric saturation due to the electric field generated by ions (which tends to lower the dielectric permittivity) and the ion-induced perturbation of pre-oriented water molecules inside the nanocavity which gain some rotational degrees of freedom (entropic contribution) leading to an increase in dipolar fluctuations responsible for the increase in the dielectric permittivity.

Theoretical investigation of the ionic selectivity of polyelectrolyte multilayer membranes in nanofiltration.

Polyelectrolyte multilayer membranes have proven to be promising materials for ion nanofiltration. In this work, we implement a continuum mesoscopic transport model developed in previous works (Szymczyk, A.; Zhu, H.; Balannec, B. Langmuir 2010, 26, 1214; Szymczyk, A.; Zhu, H.; Balannec, B. J. Phys. Chem. B 2010, 114, 10143) to investigate the pressure-driven transport of electrolyte mixtures through this kind of membrane. The model accounts for an inhomogeneous distribution of the fixed charge through an arbitrary number of polyelectrolyte bilayers. We show that accounting for the multiple bipolar charge distribution resulting from the layer-by-layer assembly of polyelectrolytes with opposite charge is responsible for the increase in the Na(+)/Mg(2+) selectivity reported experimentally with respect to conventional nanofiltration membranes. The model also allows the rationalizing of the seemingly contradictory experimental results reported in the literature (i.e., the increase or decrease in the selectivity with the number of bilayers or the existence of an optimum number of bilayers). It is shown, however, that the nonmonotonous variation of the ionic selectivity does not originate from the multibipolar distribution of the fixed charge through polyelectrolyte multilayer membranes but from the existence of an optimum skin layer thickness.

Degradation of poly(ether sulfone)/polyvinylpyrrolidone membranes by sodium hypochlorite: insight from advanced electrokinetic characterizations.

Poly(ether sulfone) (PES)/polyvinylpyrrolidone (PVP) membranes are widely used in various industrial fields such as drinking water production and in the dairy industry. However, the use of oxidants to sanitize the processing equipment is known to impair the integrity and lifespan of polymer membranes. In this work we showed how thorough electrokinetic measurements can provide essential information regarding the mechanism of degradation of PES/PVP membranes by sodium hypochlorite. Tangential streaming current measurements were performed with ultrafiltration and nanofiltration PES/PVP membranes for various aging times. The electrokinetic characterization of membranes was complemented by FTIR-ATR spectroscopy. Results confirmed that sodium hypochlorite induces the degradation of both PES and PVP. This latter is easily oxidized by sodium hypochlorite, which leads to an increase in the negative charge density of the membrane due to the formation of carboxylic acid groups. The PVP was also found to be partly released from the membrane with aging time. Thanks to the advanced electrokinetic characterization implemented in this work it was possible for the first time to demonstrate that two different mechanisms are involved in the degradation of PES. Phenol groups were first formed as a result of the oxidation of PES aromatic rings by substitution of hydrogen by hydroxyl radicals. For more severe aging conditions, this membrane degradation mechanism was followed by the formation of sulfonic acid functions, thus indicating a second degradation process through scission of PES chains.

Concentration dependence of the dielectric permittivity, structure, and dynamics of aqueous NaCl solutions: comparison between the Drude oscillator and electronic continuum models.

We report molecular dynamics simulations of aqueous sodium chloride solutions at T = 298 K and p = 1 bar in order to investigate the salt concentration dependence of the dielectric permittivity, the structure, and the dynamical properties. Different models were applied up to 7 m salt concentration: the Drude oscillator model with a negative Drude particle (SWM4-NDP), the TIP4P/2005-Reif nonpolarizable model, and an electronic continuum polarizable model (MDEC). Both SWM4-NDP and MDEC polarizable models were able to quantitatively reproduce the concentration dependence of the dielectric permittivity of NaCl aqueous solutions. On the contrary, the nonpolarizable TIP4P/2005 water model failed to quantitatively predict this concentration dependence. In contrast with the SWM4-NDP model, the MDEC model was unable to capture the concentration dependence of the structure and the dynamics of NaCl solutions. The SWM4-NDP model proved to be the most efficient polarizable model to reproduce quantitatively the concentration dependence of the dielectric permittivity, the dynamics, and the structure of NaCl solutions.

Water confinement in nanoporous silica materials.

The influence of the surface polarity of cylindrical silica nanopores and the presence of Na(+) ions as compensating charges on the structure and dynamics of confined water has been investigated by molecular dynamics simulations. A comparison between three different matrixes has been included: a protonated nanopore (PP, with SiOH groups), a deprotonated material (DP, with negatively charged surface groups), and a compensated-charge framework (CC, with sodium cations compensating the negative surface charge). The structure of water inside the different pores shows significant differences in terms of layer organization and hydrogen bonding network. Inside the CC pore the innermost layer is lost to be replaced by a quasi bulk phase. The electrostatic field generated by the DP pore is felt from the surface to the centre of pore leading to a strong orientation of water molecules even in the central part of the pore. Water dynamics inside both the PP and DP pores shows significant differences with respect to the CC pore in which the sub-diffusive regime of water is lost for a superdiffusive regime.

Molecular simulations of confined liquids: an alternative to the grand canonical Monte Carlo simulations.

Commonly, the confinement effects are studied from the grand canonical Monte Carlo (GCMC) simulations from the computation of the density of liquid in the confined phase. The GCMC modeling and chemical potential (µ) calculations are based on the insertion/deletion of the real and ghost particle, respectively. At high density, i.e., at high pressure or low temperature, the insertions fail from the Widom insertions while the performing methods as expanded method or perturbation approach are not efficient to treat the large and complex molecules. To overcome this problem we use a simple and efficient method to compute the liquid's density in the confined medium. This method does not require the precalculation of µ and is an alternative to the GCMC simulations. From the isothermal-isosurface-isobaric statistical ensemble we consider the explicit framework/liquid external interface to model an explicit liquid's reservoir. In this procedure only the liquid molecules undergo the volume changes while the volume of the framework is kept constant. Therefore, this method is described in the Np(n)AV(f)T statistical ensemble, where N is the number of particles, p(n) is the normal pressure, V(f) is the volume of framework, A is the surface of the solid/fluid interface, and T is the temperature. This approach is applied and validated from the computation of the density of the methanol and water confined in the mesoporous cylindrical silica nanopores and the MIL-53(Cr) metal organic framework type, respectively.

Ion rejection properties of nanopores with bipolar fixed charge distributions.

Ion rejection properties of cylindrical nanopores with bipolar fixed charge distributions have been investigated theoretically by means of an approximate model based on the Poisson-Nernst-Planck (PNP) theory and accounting for the electroosmosis phenomenon. The approximate model has been shown to give results that are in good agreement with the full 2D PNP approach for the narrow and weakly charged pores considered in this work. Pressure-induced rectification of salt flux has been put in evidence as a result of the broken symmetry of the fixed charge distribution on the pore walls. The model also elucidates that pressure-induced transport is controlled by different pore regions depending on the magnitude of the pressure difference across the nanopore. The existence of an optimal pressure difference (i.e., leading to the highest salt rejection) has been put in evidence when there is a region within the nanopore that is more repulsive than the pore entrance with respect to a given electrolyte. For moderate pressure differences, our results show that nanopores with bipolar charge distributions can lead to close rejections for both 2-1 and 1-2 asymmetric electrolytes. This is a specific property of bipolar nanopores because these performances cannot be obtained with homogeneously charged nanopores, which strongly reject electrolytes with divalent co-ions but are much more permeable to electrolytes with divalent counterions. This work benefits the design of nanoporous systems with targeted distribution of ionizable surface groups for advanced membrane separations.