Exfoliated Bi2Te3-enabled membranes for new concept water desalination: Freshwater production meets new routes.

Water scarcity forces the science to find the most environmentally friendly propulsion technology for supplying plentiful freshwater at low energy costs. Membrane Distillation well meets criteria of eco-friendly management of natural resources, but it is not yet competitive on scale. Herein, we use a dichalchogenide compound (Bi2Te3) as a conceivable source to accelerate the redesign of advanced membranes technologies such as thermally driven membrane distillation. A procedure based on assisted dispersant liquid phase exfoliation is used to fill PVDF membranes. Key insights are gained in the crucial role of this topological material confined in hydrophobic membranes dedicated to recovery of freshwater from synthetic seawater. Intensified water flux together with reduced energy consumption is obtained into one pot, thereby gathering ultrafast production and thermal efficiency in a single device. Bi2Te3-enabled membranes show ability to reduce the resistance to mass transfer while high resistance to heat loss is opposite. Permeate flux is kept stable and salt rejection is higher than 99.99% during 23 h-MD test. Our results confirm the effectiveness of chalcogenides as frontier materials for new-concept water desalination through breakthrough thermally-driven membrane distillation, which is regarded as a new low-energy and sustainable solution to address the growing demand for access to freshwater.

A few-layer graphene for advanced composite PVDF membranes dedicated to water desalination: a comparative study.

Membrane distillation is envisaged to be a promising best practice to recover freshwater from seawater with the prospect of building low energy-consuming devices powered by natural and renewable energy sources in remote and less accessible areas. Moreover, there is an additional benefit of integrating this green technology with other well-established operations dedicated to desalination. Today, the development of membrane distillation depends on the productivity-efficiency ratio on a large scale. Despite hydrophobic commercial membranes being widely used, no membrane with suitable morphological and chemical feature is readily available in the market. Thus, there is a real need to identify best practices for developing new efficient membranes for more productive and eco-sustainable membrane distillation devices. Here, we propose engineered few-layer graphene membranes, showing enhanced trans-membrane fluxes and total barrier action against NaCl ions. The obtained performances are linked with filling polymeric membranes with few-layer graphene of 490 nm in lateral size, produced by the wet-jet milling technology. The experimental evidence, together with comparative analyses, confirmed that the use of more largely sized few-layer graphene leads to superior productivity related efficiency trade-off for the membrane distillation process. Herein, it was demonstrated that the quality of exfoliation is a crucial factor for addressing the few-layer graphene supporting the separation capability of the host membranes designed for water desalination.

On the use of ultrasonic dental scaler tips as cleaning technique of microfiltration ceramic membranes.

The use of ultrasonic dental scaler (UDS) tips has been investigated for cleaning ceramic membranes fouled when filtering cactus juice. Thin and long tips having a larger coverage exhibited the best performance for removing the cake layer deposited on the membrane surface. Such tips cleaned an area equivalent to almost one third of total area of the membrane surface. However, the cleaned area could be increased notoriously if the membrane were placed in rotatory disc holder. The resistance-in series model and atomic force microscopy (AFM) technique helped to reveal the effect of the UDS tips as cleaning process of ceramic membranes. The reversible resistances estimated for UDS tips were 58% and 17% lower than the ones obtained by chemical cleaning at transmembrane pressures of 0.3 bar and 0.5 bar, respectively. This was corroborated by microscope images, which showed the detachment of cake layer of the membrane surface. Results of this work showed that UDS tips are an innovative option as cleaning strategy for filtration membranes.

Adsorption-assisted transport of water vapour in super-hydrophobic membranes filled with multilayer graphene platelets.

The effects of confinement of multilayer graphene platelets in hydrophobic microporous polymeric membranes are here examined. Intermolecular interactions between water vapour molecules and nanocomposite membranes are envisaged to originate assisted transport of water vapour in membrane distillation processes when a suitable filler-polymer ratio is reached. Mass transport coefficients are estimated under different working conditions, suggesting a strong dependence of the transport on molecular interactions. Remarkably, no thermal polarization is observed, although the filler exhibits ultrahigh thermal conductivity. In contrast, enhanced resistance to wetting as well as outstanding mechanical and chemical stability meets the basic requirements of water purification via membrane distillation. As a result, a significant improvement of the productivity-efficiency trade-off is achieved with respect to the pristine polymeric membrane when low amounts of platelets are confined in spherulitic-like PVDF networks.

Integrated carboxylic carbon nanotube pathways with membranes for voltage-activated humidity detection and microclimate regulation.

This work describes some single walled carboxylic carbon nanotubes with outstanding transport properties when assembled in a 3D microarray working like a humidity membrane-sensor and an adjustable moisture regulator. Combined nano-assembly approaches are used to build up a better quality pathway through which assisted-charge and mass transport synchronically takes place. The structure-electrical response relationship is found, while controllable and tunable donor-acceptor interactions established at material interfaces are regarded as key factors for the accomplishment of charge transportation, enhanced electrical responses and adjustable moisture exchange. Raman and infrared spectroscopy provides indications about the fine structural and chemical features of the hybrid-composite membranes, resulting in perfect agreement with related morphology and electrical properties. Enhanced and modular electrical response to changes in the surrounding atmosphere is concerned with doping events, while assisted moisture regulation is discussed in relation to swelling and hopping actions. The electro-activated hybrid-composite membrane proposed in this work can be regarded as an attractive 'sense-to-act' precursor for smart long-distance monitoring systems with capability to adapt itself and provide local comfortable microenvironments.

Micro and nano polycaprolactone particles preparation by pulsed back-and-forward cross-flow batch membrane emulsification for parenteral administration.

In the pharmaceutical field, manufacturing processes which are able to make products with tailored size at suitable shear stress conditions and high productivity are important requirements for their industrial application. Cross-flow and premix membrane emulsification are the membrane-based processes generally used for particles preparation at large scale, however some disadvantages still limit their applicability for the production of fragile products. In this work, we investigated, for the first time, the preparation of micro and nano polymeric particles in a size range between 2.35 (±0.14)µm and 210 (±10)nm by using pulsed back-and-forward membrane emulsification for the application in pharmaceutical field. The suitability of the method to produce tailored particles by applying mild shear conditions has been demonstrated. The optimized fluid-dynamic conditions studied allowed the production of particles with target size by selecting the appropriate pore size of the membrane (1 µm and 0.1 µm). The uniformity of the particles could be obtained with an axial velocity of 0.5 ms(-1) (corresponding to a shear stress of 4.1 Pa) that is 9 times lower than the maximum cross flow velocity reported in literature (4.5 ms(-1)).

On the cause of controlling affinity to small molecules of imprinted polymeric membranes prepared by noncovalent approach: a computational and experimental investigation.

Imprinting technique applied to membrane preparation via phase inversion methods yields membranes with enhanced affinity toward target molecules. In the imprinted membranes prepared by noncovalent approach hydrogen bond and electrostatic interactions can play a crucial role in determining the performance of these membranes. In this work, quantum mechanical calculations and experiments were performed to understand the physical-chemical causes of the affinity increase in imprinted polymeric membranes to 4,4'-methylendianiline (MDA), dissolved in an organic solvent. An ad hoc synthesized copolymer of acrylonitrile and acrylic acid was used to prepare the membranes. The calculated binding energies show that the hydrogen bonds and electrostatic interactions among polymeric chains are comparable to the strength of the same interactions occurring between polymer and MDA. Using this result and correlated experimental data, this work concluded that one of the causes responsible for the increased affinity of the imprinted membranes is the augmented availability of free carboxylic groups in the nanocavities of the membranes. However, along with this reason, the membrane pore sizes must evermore be taken into account. The knowledge acquired in this study helps us to better understand the mechanisms of molecular recognition and hence to optimize the design of new imprinted membranes.

Comparison of the performance of UF membranes in olive mill wastewaters treatment.

Olives are known to contain an appreciate amount of phenols with good antioxidant properties which are lost in large part in olive mill wastewaters (OMWs) during olive oil production. Membrane technology offers several advantages (low energy consumption, no additive requirements, no phase change) over traditional techniques to recover phenolic compounds from OMWs. The aim of this work was to evaluate the performance of different UF membranes in the treatment of OMWs finalized to the recovery of polyphenols. For this purpose, OMWs were processed, in selected operating conditions, with four flat-sheet UF membranes characterized by different molecular weight cut-off (MWCO) (4, 5 and 10 kDa) and polymeric material (regenerated cellulose and polyethersulphone). Permeate fluxes, fouling index and retention coefficients towards phenolic compounds, total antioxidant activity (TAA) and total organic carbon (TOC) were evaluated. Regenerated cellulose membranes exhibited lower rejections towards phenolic compounds, higher permeate fluxes and lower fouling index if compared with PES membranes.

Recovery and concentration of polyphenols from olive mill wastewaters by integrated membrane system.

The purpose of this work was to analyse the potentialities of an integrated membrane system for the recovery, purification and concentration of polyphenols from olive mill wastewater (OMW). The proposed system included some well-known membrane operations such as microfiltration (MF) and nanofiltration (NF), as well as others not yet investigated for this specific application, such as osmotic distillation (OD) and vacuum membrane distillation (VMD). The OMW was directly submitted to a MF operation without preliminary centrifugation. This step allowed to achieve a 91% and 26% reduction of suspended solids and total organic carbon (TOC), respectively. Moreover, 78% of the initial content of polyphenols was recovered in the permeate stream. The MF permeate was then submitted to a NF treatment. Almost all polyphenols were recovered in the produced permeate solution, while TOC was reduced from 15 g/L to 5.6 g/L. A concentrated solution enriched in polyphenols was obtained by treating the NF permeate by OD. In particular, a solution containing about 0.5 g/L of free low molecular weight polyphenols, with hydroxytyrosol representing 56% of the total, was produced by using a calcium chloride dihydrate solution as brine. The obtained solution is of interest for preparing formulations to be used in food, cosmetic and pharmaceutical industry. Besides the OD process, VMD was applied as another way for concentrating the NF permeate and the performance of both processes was compared in terms of evaporation fluxes.

Performance of PDMS membranes in pervaporation: effect of silicalite fillers and comparison with SBS membranes.

Laboratory-made silicalite filled PDMS membranes were tested by means of concentration and temperature influence on the membrane performance in removal of ethanol from ethanol/water mixtures. This allowed studying the applicability of solution-diffusion model in the transport mechanism description. Experiments were performed by varying the ethanol concentration in the feed and temperature. Two types of fillers were incorporated into the PDMS network: commercial zeolite silicalite (CBV 3002) and laboratory-made colloidal silicalite-1. Obtained results were then compared with data gathered for unfilled PDMS membranes to examine the effect of fillers incorporation. Moreover, the comparison with novel block co-polymer based porous and dense SBS membranes was done. It was found that the solution-diffusion model was a good representation of ethanol transport through both filled and unfilled PDMS membranes, whereas the water flux did not obey this model due to the swelling effects. Incorporation of the fillers increased membrane stability and improved the selectivity. Performance of the SBS membranes characterized by a dense structure was found to be similar to the performance of filled PDMS membranes.

Competitive hydrogen-bonding interactions in modified polymer membranes: a density functional theory investigation.

The subject of this work is the density functional theory (DFT) investigation of competitive hydrogen-bonding interactions that occur in modified block poly(ether/amide) (PEBAX) membranes. Previously, an evaluation of hydrogen-bonding interactions occurring between N-ethyl-o,p-toluensulfonamide (KET) modifiers was performed to establish the role of these interactions in affinity processes when the modifier is dissolved in PEBAX matrixes. However, some issues related to polymer-polymer (host-host) and modifier-polymer (host-guest) interactions were not analyzed from a theoretical point of view in the previous analysis. Here, a comparative computational analysis of these intermolecular interactions is discussed. New insights into the role of hydrogen bonding in domino processes are provided. Calculations in solvent and in vacuum have been done, yielding indications about the change in the availability of the polar groups of the polymer, which is considered to be partially responsible for the enhanced hydrophilicity of the membranes. This study can open the way to the construction of new predictive quantum modeling approaches for designing improved modifiers, enabling the optimization of polymer membrane performance.

Supercritical gel drying: a powerful tool for tailoring symmetric porous PVDF-HFP membranes.

In this work, poly(vinylidene fluoride) copolymer with hexafluoropropylene (PVDF-HFP) membrane-like aerogels have been generated for the first time. PVDF-HFP gels have been prepared from polymer-acetone solutions by adding various amounts of ethanol. A series of supercritical drying experiments have been performed at different pressures (from 100 to 200 bar) and temperatures (from 35 to 45 degrees C) and at various polymer concentrations (from 5 to 12 wt %). The effects of the process conditions on the membrane morphology have been evaluated, and structure-property relationships have been found. In all cases, the membranes exhibit interconnected structures with nanosized pores and high porosity, leading to reduced resistance to the gas mass transfer and high hydrophobic character of the surfaces. These membrane-like aerogels promise to form a new class of highly hydrophobic porous interfaces, potentially suitable to be used in membrane operations based, for example, on the contactor technology.

Treatment of dye solutions by vacuum membrane distillation.

In this work, the vacuum membrane distillation (VMD) process has been applied to treat water containing different types of dyes. The influence of operating parameters, as feed temperature, feed flow rate, feed concentration, on the permeate flux and on rejection has been investigated. In all experimental tests, a complete rejection has been achieved and pure water has been recovered at the permeate side. Furthermore, experiments with water as feed have been carried out before and after the tests with dyes, in order to analyze the effect of fouling on the performance of the VMD. The water vapor fluxes immediately after the tests with dyes were higher than the values registered before the tests, probably due to an interaction with the polymeric membrane material which promotes a swelling of the membrane when in contact with the dye solutions. However, initial fluxes are recovered after prolonged cleaning with only water.

Combined emulsion and phase inversion techniques for the preparation of catalytic PVDF microcapsules.

In this work, polyvinilydene fluoride (PVDF) microcapsules were prepared by using combined emulsion and phase inversion techniques. With this method, microcapsules with different diameters and porosities have been obtained by just controlling the diameter of the membrane used during the preparation. Using a PVDF solution containing the oxidation catalyst ammonium molybdate (20 wt %), catalytic polymeric microcapsules with diameters ranging from 600 to 1,200 microm have been obtained. Characterization of catalytic microcapsules by means of SEM, BSE, and EDX analyses showed a uniform ammonium molybdate dispersion in the polymeric matrix. Catalytic microcapsules have been tested in the oxidation of aromatic primary alcohols to corresponding aldehydes. In the range 600-1,200 microm, the microcapsule diameter influences the formation of oxidation products: in particular, microcapsule diameters >900 microm slightly diminish the formation of aldehyde due to a beginning diffusion limitation. An interesting structure-reactivity behavior, induced by the interaction between the polymeric membrane and the substituted aromatic alcohol, has been observed.

New PVDF membranes: The effect of plasma surface modification on retention in nanofiltration of aqueous solution containing organic compounds.

New nanofiltration membranes were prepared by non-solvent-induced phase inversion from a PVDF/DMF/water system. The effect of exposure time before coagulation on the membrane characteristics (morphology, thickness, overall porosity, tensile strength) was investigated. PVDF membrane prepared at a fixed exposure time of 45s (PF45) was further plasma surface modified (RF 13.56 MHz) (PF45psm), introducing amino groups on the membrane. The performances of PF45, PF45psm and of a commercial nanofiltration membrane (N30F) were tested in the removal of two dyes from aqueous solution, characterized by different charge and molecular weight (congo red and methylene blue). The observed rejections depended more on the charge of the compound than on their molecular weights and results were optimized for the plasma modified membrane (PF45psm) with respect to unmodified (PF45) and commercial N30F membranes. In particular, methylene blue was retained for 100% by PF45psm with a relative flux of 65% compared to 38% of rejection and 59% of relative flux observed for N30F.

Intermolecular interactions as controlling factor for water sorption into polymer membranes.

A multidisciplinary approach was used for delineating the mechanisms controlling water sorption into modified block co-poly-(ether/amide) (PEBAX) membranes. In particular, incorporation of aromatic sulfonamide (KET) into the polymer matrix led to a nonlinear increase of water sorption in the membrane. The modification in sorption was accompanied by a nonlinear behavior in membrane surface energies. Infrared analysis revealed a different availability and accessibility of free polar groups supporting the formation of hydrogen bonding as a function of modifier concentration. A combination of both experimental and theoretical procedures was used to analyze the molecular processes of water sorption on PEBAX membranes. Molecular dynamics (MD) and quantum chemical (QC) calculations demonstrated that the formation of KET-KET dimers in the polymeric matrix led to a decrease in the interaction energy between water and modifiers. In addition, no variations in the dipole moments of water-dimer structures were found in comparison to a single KET and water-KET molecule. The formation of water-dimer complexes at higher concentration of modifier decreases the number of the dipole moment, thus preventing the polarization of polymer chains.

Study of the surface character as responsible for controlling interfacial forces at membrane-feed interface.

The role of the interfacial forces was emphasized in interactive processes, involving membrane surface and penetrating molecules. The surface character controlling the dissolution process of some species (CO2, H2O, C3H6O2, C4H8O2, C5H10O2) was evaluated in relation to the supra-molecular chemistry of membranes based on 80PTMO/PA12. Infrared analyses combined with the estimation of the hydrophilic and hydrophobic domains of the membrane surface yielded useful information about the distribution, availability and accessibility of the polar moieties responsible for the penetrant sorption. At the interface, attractive Lewis acid/base interactions such as H-bonding directed the sorption of vapor species into the membranes, whereas quadrupolar CO2 participated in specific Lifshitz-van der Waals interactions with the modified polymers. In both the cases, the presence of additional polar moieties such as carbonyl, sulfonamide, and hydroxyl groups enhanced the affinity of the Pebax-based membranes for the penetrating species considered in this work. As a result, the quantification of the reactivity of a membrane surface for specific molecules may allow predictive models to be constructed and selective membranes to be designed.

Chromium (III) removal by supported liquid membranes: a comparison among D2HEPA, DNNSA, and a novel extractant as carriers.

A facilitated transport study in supported liquid membranes (SLM) using the extraction reagents di-2-ethylexilfosphoric acid (D2HEPA), dinonylnaftalene sulfuric acid (DNNSA), and a novel complexing agent, trimethyl cis,cis-1,3,5-tripropyl-1,3,5 cyclohexenetricarboxilic acid (TTCHTCA) as carriers has been carried out. Organic solvents with different dielectric constants as diluents have been used to obtain the highest extraction and transport values. The results obtained have shown that, by using different organic phases (carrier and/or diluent), SLMs with different ion flux and transport ability can be obtained. The carrier concentration in the membrane and the chromium (III) [Cr(III)] ions concentration in aqueous phase have been varied to see the effect on transport of Cr(III) ions across the membrane. For the carriers D2HEPA and TTCHTCA, the transport of Cr(IlI), both in batch and in recirculation operation mode, has been studied. Very good results in terms of flux and transport have been obtained using TTCHTCA.

Force balance conditions for droplet formation in cross-flow membrane emulsifications.

The membrane emulsification process is becoming of growing interest in many industrial fields. For monitoring the influence of process and membrane parameters on the droplet formation, quantitative information on the droplet detachment, during this process, is important. Until now, droplet formation has theoretically been described using computational fluid dynamics and global force equations. Unlike computational fluid dynamics methods, the global force models are less accurate but easier to handle and instructive. On this basis, in the present paper we present new force balance equations to describe droplet detachment during cross-flow membrane emulsification. In a first approximation, the droplet is supposed to grow leaning on the pore border as long as a force balance exists along the contact line located on the membrane surface. During this phase the base of the droplet, still stuck on the pore border, begins to bend on the membrane surface until its final detachment. Using force balance equations made along the contact lines, we obtain the minimum and maximum sizes that a droplet could have during cross-flow membrane emulsification. This approach is tested with different continuous phase velocities, membrane pore sizes and interfacial tensions. The results are compared to various experimental data, reported in literature. In particular, based on the experimentally found linear correlation between droplet and pore sizes, we show how the proposed balance equations can be used to obtain a satisfactory evaluation of the slopes of this linear correlation and how these force balances could be used to obtain an estimation of the interfacial tensions during the droplet formation.

Hybrid membrane operations in water desalination and industrial process rationalisation.

Membrane science and technology are recognized today as powerful tools in resolving some important global problems, and developing newer industrial processes, needed from the imperative of sustainable industrial growth. In seawater desalination, for resolving the dramatic increase of freshwater demand in many regions of the world, membrane unitary operations or the combination of some of them in integrated systems are already a real means for producing water from the sea, at lower costs and minimum environmental impact, with a very interesting prospective in particular for poor economy countries. However, membranes are used or are becoming used in some important industrial fields, for developing more efficient productive cycles, with reduced waste of raw-material, reducing the polluting charge by controlling byproduct generation, and reducing overall costs. In the present paper, other than for seawater desalination applications, some industrial applications where membrane technology has led already to match the goal of process intensification are discussed.