Three independent ways to obtain information on pore size distributions of nanofiltration membranes

Ice-confined synthesis of highly ionized 3D-quasilayered polyamide nanofiltration membranes

Existing polyamide (PA) membrane synthesis protocols are underpinned by controlling diffusion-dominant liquid-phase reactions that yield subpar spatial architectures and ionization behavior. We report an ice-confined interfacial polymerization strategy to enable the effective kinetic control of the interfacial reaction and thermodynamic manipulation of the hexagonal polytype (Ih) ice phase containing monomers to rationally synthesize a three-dimensional quasilayered PA membrane for nanofiltration. Experiments and molecular simulations confirmed the underlying membrane formation mechanism. Our ice-confined PA nanofiltration membrane features high-density ionized structure and exceptional transport channels, realizing superior water permeance and excellent ion selectivity.

Opportunities for Recycling PV Glass and Coal Fly Ash into Zeolite Materials Used for Removal of Heavy Metals (Cd, Cu, Pb) from Wastewater

This work shows the development and characterization of two zeolite structures by recycling PV glass and coal fly ash for the removal of cadmium, copper, and lead from synthetic solutions containing one or three cations. The materials were characterized in terms of crystalline structure (XRD), morphology (SEM, AFM), and specific surface. For increasing the heavy-metals removal efficiency, the adsorption conditions, such as substrate dosage, preliminary concentration, and contact time, were optimized. The pseudo-second-order kinetic model adsorption kinetics fit well to describe the activity of the zeolites ZFAGPV-A and ZFAGPV-S. The zeolite adsorption equilibrium data were expressed using Langmuir and Freundlich models. The highest adsorption capacities of the ZFAGPV-A zeolite are q_maxCd = 55.56 mg/g, q_maxCu = 60.11 mg/g, q_maxPb = 175.44 mg/g, and of ZFAGPV-S, are q_maxCd = 33.45 mg/g, q_maxCu = 54.95 mg/g, q_maxPb = 158.73 mg/g, respectively. This study demonstrated a new opportunity for waste recycling for applications in removing toxic heavy metals from wastewater.

Modeling micropollutant removal by nanofiltration and reverse osmosis membranes: considerations and challenges

Organic micropollutants (OMPs) in drinking water constitute a potential risk to human health; therefore, effective removal of these pollutants is required. Nanofiltration (NF) and reverse osmosis (RO) are promising membrane-based technologies to remove OMPs. In NF and RO, the rejection of OMPs depends on the properties and characteristics of the membrane, the solute, and the solution. In this review, we discuss how these properties can be included in models to study and predict the rejection of OMPs. Initially, an OMP classification is proposed to capture the relevant properties of 58 OMPs. Following the methodology described in this study, more and new OMPs can be easily included in this classification. The classification aims to increase the comprehension and mechanistic understanding of OMP removal. Based on the physicochemical principles used to classify the 58 OMPs, it is expected that other OMPs in the same groups will be similarly rejected. From this classification, we present an overview of the rejection mechanisms involved in the removal of specific OMP groups. For instance, we discuss the removal of OMPs classified as perfluoroalkyl substances (e.g., perfluorooctanoic acid, PFOA). These substances are highly relevant due to their human toxicity at extremely low concentration as well as their persistence and omnipresence in the environment. Finally, we discuss how the rejection of OMPs can be predicted by describing both the membrane-solution interface and calculating the transport of solutes inside the membrane. We illustrate the importance and impact of different rejection mechanisms and interfacial phenomena on OMP removal and propose an extended Nernst-Plank equation to calculate the transport of solutes across the membrane due to convection, diffusion, and electromigration. Finally, we show how the theory discussed in this review leads to improved predictions of OMP rejection by the membranes.

In Situ Characterization of Dehydration during Ion Transport in Polymeric Nanochannels

The transport of hydrated ions across nanochannels is central to biological systems and membrane-based applications, yet little is known about their hydrated structure during transport due to the absence of in situ characterization techniques. Herein, we report experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for a mechanistic reasoning. Notably, complete dehydration was not necessary for transport to occur across membranes with sub-nanometer pores. Partial shedding of water molecules from ion solvation shells, observed as a decrease in the average hydration number, allowed the alkali-metal ions studied here (lithium, sodium, and potassium) to permeate membranes with pores smaller than their solvated size. We find that ions generally cannot hold more than two water molecules during this sterically limited transport. In nanopores larger than the size of the solvation shell, we show that ionic mobility governs the ion hydration number distribution. Viscous effects, such as interactions with carboxyl groups inside the membrane, preferentially hinder the transport of the mono- and dihydrates. Our novel technique for studying ion solvation in situ represents a significant technological leap for the nanofluidics field and may enable important advances in ion separation, biosensing, and battery applications.

Advances in the Use of Nanocomposite Membranes for Carbon Capture Operations

The adoption of nanodoped membranes in the areas of gas stream separation, water, and wastewater treatments due to the physical and operational advantages of such membranes has significantly increased. The literature has shown that the surface structure and physicochemical properties of nanodoped membranes contribute significantly to the interaction and rejection characteristics when compared to bare membranes. This study reviews the recent developments on nanodoped membranes, and their hybrids for carbon capture and gas separation operations. Features such as the nanoparticles/materials and hybrids used for membrane doping and the effect of physicochemical properties and water vapour in nanodoped membrane performance for carbon capture are discussed. The highlights of this review show that nanodoped membrane is a facile modification technique which improves the membrane performance in most cases and holds a great potential for carbon capture. Membrane module design and material, thickness, structure, and configuration were identified as key factors that contribute directly, to nanodoped membrane performance. This study also affirms that the three core parameters satisfied before turning a microporous material into a membrane are as follows: high permeability and selectivity, ease of fabrication, and robust structure. From the findings, it is also observed that the application of smart models and knowledge-based systems have not been extensively studied in nanoparticle-/material-doped membranes. More studies are encouraged because technical improvements are needed in order to achieve high performance of carbon capture using nanodoped membranes, as well as improving their durability, permeability, and selectivity of the membrane.

Preparation of Thin-Film Composite Nanofiltration Membranes Doped with N- and Cl-Functionalized Graphene Oxide for Water Desalination

In the present work, chemically modified graphene oxide (GO) was incorporated as a crosslinking agent into thin-film composite (TFC) nanofiltration (NF) membranes for water desalination applications, which were prepared by the interfacial polymerization (IP) method, where the monomers were piperazine (PIP) and trimesoyl chloride (TMC). GO was functionalized with monomer-containing groups to promote covalent interactions with the polymeric film. The composite GO/polyamide (PA) was prepared by incorporating amine and acyl chloride groups into the structure of GO and then adding these chemical modified nanomaterial during IP. The effect of functionalized GO on membrane properties and performance was investigated. Chemical composition and surface morphology of the prepared GO and membranes were analyzed by thermogravimetric analysis (TGA), Raman spectroscopy, FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The fabricated composite membranes exhibited a significant increase in permeance (from 1.12 to 1.93 L m^-2 h^-1 bar^-1) and salt rejection for Na₂SO₄ (from 95.9 to 98.9%) and NaCl (from 46.2 to 61.7%) at 2000 ppm, when compared to non-modified membranes. The amine- and acyl chloride-functionalized GO showed improved dispersibility in the respective phase.

Fly Ash Waste Recycling by Pt/TiO2 Incorporation for Industrial Dye Removal

New materials are obtained by transforming fly ash wastes into a valuable composite, with tandem adsorption and photodegradation properties. Mild hydrothermal synthesis, from titanium dioxide, platinum nanoparticles and zeolite materials obtained from a waste, fly ash, as support, was involved in the composite preparation. The platinum nanoparticles extended the photocatalytic activity of the composite in visible range (Eg = 2.1 eV). The efficiency of tandem adsorption and photocatalytic activity of the new composite were determined to be 25% for Bemacid Blau and 43.89% for Bemacid Rot after 360 min, the irradiation time. The addition of H₂O₂ improves the process efficiency up to 80.70% and 93.43%, respectively. The Pt nanoparticle (PtNP) contribution led to the band gap energy change to Vis light (400 nm), thus suggesting the possibility of photocatalysis under the action of a fraction of natural light.

Nanofiltration for separation and purification of saccharides from biomass

Saccharide production is critical to the development of biotechnology in the field of food and biofuel. The extraction of saccharide from biomass-based hydrolysate mixtures has become a trend due to low cost and abundant biomass reserves. Compared to conventional methods of fractionation and recovery of saccharides, nanofiltration (NF) has received considerable attention in recent decades because of its high selectivity and low energy consumption and environmental impact. In this review the advantages and challenges of NF based technology in the separation of saccharides are critically evaluated. Hybrid membrane processes, i.e., combining NF with ultrafiltration, can complement each other to provide an efficient approach for removal of unwanted solutes to obtain higher purity saccharides. However, use of NF membrane separation technology is limited due to irreversible membrane fouling that results in high capital and operating costs. Future development of NF membrane technology should therefore focus on improving material stability, antifouling ability and saccharide targeting selectivity, as well as on engineering aspects such as process optimisation and membrane module design.

Tuning Cooperative Assembly with Bottlebrush Block Co-polymers for Porous Metal Oxide Films Using Solvent Mixtures

Block copolymer templating enables the generation of well-defined pore sizes and geometries in a wide variety of frameworks, typically through evaporation-induced self-assembly (EISA). Here, we systematically modulate the solvent quality with mixtures of tetrahydrofuran-ethanol (THF-EtOH) to manipulate the unimer/micelle ratio in the precursor solution to explore how the associated solution structure influences the final pore morphology. A bottlebrush block copolymer (BBCP) with poly(ethylene oxide) and poly(t-butyl acrylate) side chains was used as the template for pore formation. Irrespective of the solvent composition, a bimodal pore size distribution was obtained with mesopores templated by small aggregates of the BBCP unimers (potentially low aggregation number micelles) and macropores templated by large self-assembled BBCP micelles. The morphology and pore characteristics of the metal oxide films were dependent on the THF-EtOH composition. Interestingly, an intermediate solvent composition where the volume of micelles is approximately half the volume of unimers (in the precursor solution) leads to the best ordering of micelle-templated pores and also the maximum porosity in the films. The micelle/unimer ratios in the precursor solutions do not correspond directly to the bimodal pore distribution in the metal oxide films, which we attribute to kinetically trapped assembly of the BBCP at a low THF content. The increased critical micelle concentration at high THF composition leads to changes in the unimer/micelle ratio during solvent evaporation. These results appear to be universal for a number of metal oxides (cobalt, magnesium, and zinc) with the porosity maximized at a THF/EtOH ratio of 3:1. These results suggest the potential for enhancements in the porosity of block copolymer-templated films by EISA methods through judicious solvent selection.

Review on the Macro-Transport Processes Theory for Irregular Pores able to Perform Catalytic Reactions

We review and generalize a recent theoretical framework that provides a sound physicochemical basis to describe how volume and surface diffusion are affected by adsorption and desorption processes, as well as by catalytic conversion within the space defined by the irregular geometry of the pores in a material. The theory is based on two single-dimensional mass conservation equations for irregular domains deduced for the volumetric (bulk) and surface mass concentrations. It offers a powerful tool for analyzing and modeling mass transport across porous media like zeolites or artificially build materials, since it establishes how the microscopic quantities that refer to the internal details of the geometry, the flow and the interactions within the irregular pore can be translated into macroscopic variables that are currently measured in experiments. The use of the theory in mass uptake experiments is explained in terms of breakthrough curves and effective mass diffusion coefficients which are explicitly related to the internal geometry of the pores.

Application of a new dynamic transport model to predict the evolution of performances throughout the nanofiltration of single salt solutions in concentration and diafiltration modes

Although many knowledge models describing the rejection of ionic compounds by nanofiltration membranes are available in literature, they are all used in full recycling mode. Indeed, both permeate and retentate streams are recycled in order to maintain constant concentrations in the feed solution. However, nanofiltration of real effluents is implemented either in concentration or diafiltration modes, for which the permeate stream is collected. In these conditions, concentrations progressively evolve during filtration and classical models fail to predict performances. In this paper, an improvement of the so called "Donnan Steric Pore Model", which includes both volume and concentration variations over time is proposed. This dynamic model is used here to predict the evolution of volumes and concentrations in both permeate and retentate streams during the filtration of salt solutions. This model was found to predict accurately the filtration performances with various salts whether the filtration is performed in concentration or diafiltration modes. The parameters of the usual model can be easily assessed from full batch experiments before being used in the dynamic version. Nevertheless, it is also highlighted that the variation of the membrane charge due to the evolution of feed concentration over time has to be taken into account in the model through the use of adsorption isotherms.

Sintering process investigation during polytetrafluoroethylene hollow fibre membrane fabrication by extrusion method

In this study, the effect of sintering conditions including manner, temperature and duration on properties of polytetrafluoroethylene (PTFE) hollow fibre membrane fabricated by extrusion method was intensively investigated. Different from un-sintered and relaxed sintered, the fixed sintered PTFE hollow fibre membrane was observed to generate a uniform ‘fibril–node’ porous structure and a main crystal transformation to folded chain crystal with smaller size. Consequently, it was found that for fixed setting sintering, both temperature increase from 340°C to 400°C and duration prolongation obviously improved pore size, ethanol permeation performance and mechanical strength. Additionally, the test results revealed that the membrane sintered below virgin melting point (350°C) had a noticeable higher porosity but poorer ethanol permeation performance that could be primarily attributed to increased ratio of closed pore. The sintering condition exhibited evident influence on PTFE hollow fibre membrane thermal stability, though it showed no alteration to the thermal decomposition of PTFE. The obtained PTFE hollow fibre membrane was tested to evaluate their vacuum membrane distillation (VMD) performances. It was found that PTFE membrane from lower sintering temperature delivered a better salt rejection; on the other hand, the permeate flux was improved by increased vacuum pressure during VMD operation.

Enhanced hydrophilicity of a thermo-responsive PVDF/palygorskite-g-PNIPAAM hybrid ultrafiltration membrane via surface segregation induced by temperature

The hydrophilicity of a thermo-responsive PVDF/palygorskite-g-PNIPAAM hybrid ultrafiltration membrane was enhanced via surface segregation induced by coagulation bath temperature (CBT).

Fractionation of a galacto-oligosaccharides solution at low and high temperature using nanofiltration

Like in many applications, solutions of high sugar content can cause serious problems due to microorganism contaminations. Hence, the main aim of this work was to study a nanofiltration process for GOS purification at 5 °C and 60 °C that may circumvent or reduce potential microbial growth. Process performances and rejection behaviors of monosaccharide as well as individual GOS components were compared. Operating at 5 °C is more advantageous especially with respect to the oligosaccharide (OS) recovery yield. Using a NF membrane (NP030) at 45 bar, a product purity of 85% (based on monosaccharide content) and an OS recovery yield of 82% could be achieved. However, a low average permeate flux of 3 L/m² h had to be accepted. A diafiltration step improved product purity to 90% with 30% losses of OS. A qualitative theoretical discussion shows that a possible change of the pore radius distribution depending on temperature could play a role in solute rejection as well as selectivity.

Fly ash-TiO2 nanocomposite material for multi-pollutants wastewater treatment

This paper reports on the synthesis, characterization and adsorption properties of a novel nano-composite obtained using the hydrothermal method applied to a fly ash-TiO2 slurry and hexadecyltrimethyl-ammonium bromide, as surface controlling agent. The new adsorbent was investigated in terms of crystallinity (XRD), surface properties (AFM, SEM, and porosity and BET surface) and surface chemistry (EDX, FTIR). The nanocomposite's properties were sequentially tested in adsorption and photocatalysis processes applied to multi-pollutant synthetic wastewaters loaded with copper cations and two industrial dyes: the acid dye Bemacid Blau and the reactive dye Bemacid Rot; the nano-composite substrate allowed reaching high removal efficiencies, above 90%, both in adsorption and in photodegradation experiments, in optimised conditions.

Diffusion and filtration properties of self-assembled gold nanocrystal membranes

Close-packed nanoparticle monolayers have recently been shown to form mechanically robust, free-standing membranes. We report the first measurements of molecular transport through such ultrathin sheets, self-assembled from dodecanethiol-ligated gold nanocrystals. For aqueous solutions we find filtration coefficients 2 orders of magnitude larger than those observed in polymer-based filters, sieving of large solutes, and for smaller solutes a pronounced dependence of rejection on being charged. These results open up new possibilities for controlled delivery and separation of nano-objects.