Characterization of pore size distribution (PSD) in cellulose triacetate (CTA) and polyamide (PA) thin active layers by positron annihilation lifetime spectroscopy (PALS) and fractional rejection (FR) method

Feasibility of Forward Osmosis to Recover Textile Dyes Using Single Salts and Multicomponent Draw Solutions

The textile industry generates large volumes of water characterized mainly by an intense color coming from dyes that are difficult to process due to their synthetic base and the presence of aromatic components. Due to the stricter regulation on the discharge of these effluents, in order to reduce dye waste before discharge into natural channels, alternatives are being sought to manage this wastewater. In this work, the concentration of dyes in simulated wastewater from the textile industry was studied by forward osmosis (with a cellulose triacetate CTA membrane), with the aim of concentrating the dye for its future recovery and reincorporation into the production process. Two dyes of different nature were evaluated to study the efficiency of the proposed process, using NaCl and reverse osmosis brine from a model seawater desalination solution as extraction solutions. It was observed that dye type (reactive or direct) and their charge influence the color rejection with the forward osmosis membrane used. It was able to concentrate the dyes in the feed solution up to approximately 55% with the reverse osmosis brine from the model seawater desalination solution. Finally, the results demonstrate that the FO process is a promising option for concentrating dyes present in wastewater from the textile industry in order to reuse them in the dyeing process.

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique to investigate defects in solids, such as vacancy clusters and grain boundaries in metals and alloys, as well as lattice imperfections in semiconductors. Positron spectroscopy is able to reveal the size, structure and concentration of vacancies with a sensitivity of 10^-7. In the field of porous and amorphous systems, PALS can probe cavities in the range from a few tenths up to several tens of nm. In the case of polymers, PALS is one of the few techniques able to give information on the holes forming the free volume. This quantity, which cannot be measured with macroscopic techniques, is correlated to important mechanical, thermal, and transport properties of polymers. It can be deduced theoretically by applying suitable equations of state derived by cell models, and PALS supplies a quantitative measure of the free volume by probing the corresponding sub-nanometric holes. The system used is positronium (Ps), an unstable atom formed by a positron and an electron, whose lifetime can be related to the typical size of the holes. When analyzed in terms of continuous lifetimes, the positron annihilation spectrum allows one to gain insight into the distribution of the free volume holes, an almost unique feature of this technique. The present paper is an overview of PALS, addressed in particular to readers not familiar with this technique, with emphasis on the experimental aspects. After a general introduction on free volume, positronium, and the experimental apparatus needed to acquire the corresponding lifetime, some of the recent results obtained by various groups will be shown, highlighting the connections between the free volume as probed by PALS and structural properties of the investigated materials.

Novel Polyelectrolyte-Based Draw Solute That Overcomes the Trade-Off between Forward Osmosis Performance and Ease of Regeneration

Forward osmosis (FO) is an emerging technology for seawater and brackish desalination, wastewater treatment, and other applications, such as food processing, power generation, and protein and pharmaceutical enrichment. However, choosing a draw solute (DS) that provides an appropriate driving force and, at the same time, is easy to recover, is challenging. In this study, water-soluble poly(styrene sulfonate) (PSS) was modified by a high-electrical-conductivity 3,4-ethylenedioxythiophene (EDOT) monomer to fabricate a novel draw solute (mPSS). FO tests with the CTA membrane in the active layer facing the feed solution (AL-FS) orientation, using a 50 mS/cm aqueous solution of synthesized solute and distilled water as a feed solution exhibited a water flux of 4.2 L h^-1 m^-2 and a corresponding reverse solute flux of 0.19 g h^-1 m^-2. The FO tests with the same membrane, using a 50 mS/cm NaCl control draw solution, yielded a lower water flux of 3.6 L h^-1 m^-2 and a reverse solute flux of 4.13 g h^-1 m^-2, which was more than one order of magnitude greater. More importantly, the synthesized draw solute was easily regenerated using a commercial ultrafiltration membrane (PS35), which showed over 96% rejection.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Transparent oxygen barrier nanocellulose composite films with a sandwich structure

Transparent gas barrier materials have extensive applications in packaging, pharmaceutical preservation, and electronics. Herein, we designed transparent films with a symmetric sandwich structure using layer-by-layer assembly of biaxially oriented polypropylene (BOPP) and acrylic resin (AR) followed by a cellulose nanoparticle (CNP) layer. The BOPP as a substrate created a barrier to hinder the transmission of water molecules to the adhesive AR layer and gas barrier functional CNP layer. The aspect ratio of the CNPs was shown to affect the film microstructure, resulting in different values for the oxygen transmission rate (OTR). The well-organized CNP layer exhibited lower OTR when compared with the network layer. The thickness, density, and porosity of the CNP layer exhibited correlations with OTR. The water molecules were able to flow in through an additional pathway, thus increasing the water vapor transmission rate (WVTR). Moreover, these sandwiched cellulose composite films showed excellent light transmittance and tensile strength.

Unveiling the Molecular Mechanisms of Thickness-Dependent Water Dynamics in an Ultrathin Free-Standing Polyamide Membrane

Aromatic polyamide (PA) membranes fabricated from interfacial polymerization are widely used for desalination and water treatment. The fabrication of the high-flux PA membrane requires a fundamental understanding of the molecular mechanisms of water dynamics in the PA, which is still obscure due to the limited experimental methods. Herein, molecular dynamics (MD) simulations were employed to establish an atomic model of ultrathin free-standing PA membranes with various thickness and to explore the thickness-dependent dynamics of water molecules in the PA membrane. Simulation results illustrate that the simulated PA membrane has an average pore radius of 3 Å similar to the free volume size of the experimental PA membrane measured by PALS. The PA could be identified as the swelling layer (SL) and the confined layer (CL) based on their water diffusion rates. The diffusivity of water in the confined layer of PA membrane was much lower than that in the swelling layer and thus determined the water flux of the PA membrane. The water diffusivity in the sub-8 nm PA membrane is greatly enhanced due to a very thin confined layer thickness, illustrating the mechanism of the experimentally fabricated sub-8 nm PA membrane having the dramatically enhanced water permeability. Furthermore, results show that water molecules tend to transport rapidly in the free space inside the PA membrane. Our results provide some insights into the thickness-dependent water dynamics in the PA on a molecular level and may help to design the next generation of high-flux PA membranes.

Effect of Spacer Configuration on the Characteristics of FO Membranes: Alteration of Permeation Characteristics by Membrane Deformation and Concentration Polarization

Membrane deformation is a significant problem in osmotically driven membrane processes, as it restricts practical operating conditions and reduces overall process performance due to unfavorable alteration of membrane permeation characteristics. In this respect, a spacer plays a crucial role, as it dictates the form and extent of membrane deformation in association with concentration polarization (CP), which is also influenced by spacer-induced hydrodynamic behavior near the membrane surface. These two roles of spacers on membrane permeation characteristics are inherently inseparable with the coexistence of hydraulic and osmotic pressures. Here, we suggest a novel analytical method to differentially quantify the proportions of effective osmotic pressure drop caused by membrane deformation and CP. Furthermore, we tested two different FO membranes with three different spacer configurations to define and discuss different forms of membrane deformation and their effects on membrane permeation characteristics. The differential analysis revealed the effect of spacer configuration on effective osmotic pressure drop in membrane deformation (up to ∼201% of variation) is much greater than that in CP (up to ∼20.1% of variation). In addition, a combined configuration of a feed spacer and tricot spacer demonstrated its ability of mitigating membrane deformation with lower selectivity loss and channel pressure drop under pressurization.

Electro-Forward Osmosis

The impact of ion migration induced by an electrical field on water flux in a forward osmosis (FO) process was examined using a thin-film composite (TFC) membrane, held between two cation exchange membranes. An applied fixed current of 100 mA (1.7 mA cm^-2) was sustained by the proton flux through the TFC-BW membrane using a feed of 34 mM NaCl, and a 257 mM NaCl draw solution. Protons generated at the anode were transported through the cation exchange membrane and into the draw solution, lowering the pH of the draw solution. Additional proton transport through the TFC-BW membrane also lowered the pH of the feed solution. The localized accumulation of the protons on the draw side of the TFC-BW membrane resulted in high concentration polarization modulus of 1.41 × 10⁵, which enhanced the water flux into the draw solution (5.56 LMH at 100 mA), compared to the control (1.10 LMH with no current). These results using this electro-forward osmosis (EFO) process demonstrated that enhanced water flux into the draw solution could be achieved using ion accumulation induced by an electrical field. The EFO system could be used for FO applications where a limited use of draw solute is necessary.

Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na⁺ and Cl⁻ ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.

Enhancement in Proton Conductivity and Thermal Stability in Nafion Membranes Induced by Incorporation of Sulfonated Carbon Nanotubes

Proton exchange membrane fuel cell (PEMFC) is one of the most promising green power sources, in which perfluorinated sulfonic acid ionomer-based membranes (e.g., Nafion) are widely used. However, the widespread application of PEMFCs is greatly limited by the sharp degradation in electrochemical properties of the proton exchange membranes under high temperature and low humidity conditions. In this work, the high-performance sulfonated carbon nanotubes/Nafion composite membranes (Su-CNTs/Nafion) for the PEMFCs were prepared and the mechanism of the microstructures on the macroscopic properties of membranes was intensively studied. Microstructure evolution in Nafion membranes during water uptake was investigated by positron annihilation lifetime spectroscopy, and results strongly showed that the Su-CNTs or CNTs in Nafion composite membranes significantly reinforced Nafion matrices, which influenced the development of ionic-water clusters in them. Proton conductivities in Su-CNTs/Nafion composite membranes were remarkably enhanced due to the mass formation of proton-conducting pathways (water channels) along the Su-CNTs. In particular, these pathways along Su-CNTs in Su-CNTs/Nafion membranes interconnected the isolated ionic-water clusters at low humidity and resulted in less tortuosity of the water channel network for proton transportation at high humidity. At a high temperature of 135 °C, Su-CNTs/Nafion membranes maintained high proton conductivity because the reinforcement of Su-CNTs on Nafion matrices reduced the evaporation of water molecules from membranes as well as the hydrophilic Su-CNTs were helpful for binding water molecules.

Forward Osmosis Membranes under Null-Pressure Condition: Do Hydraulic and Osmotic Pressures Have Identical Nature?

Forward osmosis (FO) membranes fall into the category of nonporous membranes, based on the assumption that water and solute transport occur solely based on diffusion. The solution-diffusion (S-D) model has been widely used in predicting their performances in the coexistence of hydraulic and osmotic driving forces, a model that postulates the hydraulic and osmotic driving forces have identical nature. It was suggested, however, such membranes may have pores and mass transport could occur both by convection (i.e., volumetric flow) as well as by diffusion assuming that the dense active layer of the membranes is composed of a nonporous structure with defects which induce volumetric flow through the membranes. In addition, the positron annihilation technique has revealed that the active layers can involve relatively uniform porous structures. As such, the assumption of a nonporous active layer in association with hydraulic pressure is questionable. To validate this assumption, we have tested FO membranes under the conditions where hydraulic and osmotic pressures are equivalent yet in opposite directions for water transport, namely the null-pressure condition. We have also established a practically valid characterization method which quantifies the vulnerability of the FO membranes to hydraulic pressure.

Robust water desalination membranes against degradation using high loads of carbon nanotubes

Chlorine resistant reverse osmosis (RO) membranes were fabricated using a multi-walled carbon nanotube-polyamide (MWCNT-PA) nanocomposite. The separation performance of these membranes after chlorine exposure (4800 ppm·h) remained unchanged (99.9%) but was drastically reduced to 82% in the absence of MWCNT. It was observed that the surface roughness of the membranes changed significantly by adding MWCNT. Moreover, membranes containing MWCNT fractions above 12.5 wt.% clearly improved degradation resistance against chlorine exposure, with an increase in water flux while maintaining salt rejection performance. Molecular dynamics and quantum chemical calculations were performed in order to understand the high chemical stability of the MWCNT-PA nanocomposite membranes, and revealed that high activation energies are required for the chlorination of PA. The results presented here confirm the unique potential of carbon nanomaterials embedded in polymeric composite membranes for efficient RO water desalination technologies.