Feed-membrane Interface Research Articles

Improved in-situ characterization for the scaling-induced wetting in membrane distillation: Unraveling the role of crystalline morphology

Despite being recognized as a promising technique for treating high salinity water, membrane distillation (MD) has been plagued by the scaling of sparingly soluble salts. The growth of crystals can not only create additional resistance to evaporating water at the feed-membrane interface, but also alter the hydrophobic network to bridge the feed and distillate (i.e., result in the phenomenon of wetting). When recognizing the uncertain behaviors of calcium sulfate (CaSO4) scaling in MD, this study was motivated to ascertain whether the crystal-membrane interactions could be dependent on the variation in crystalline morphology. In particular, optical coherence tomography (OCT) was employed to characterize the scaling-induced wetting via a direct-observation-through-the-membrane (DOTM) mode, which mitigated the effects of developing an external scaling layer on resolving the crystal-membrane interactions. The improved in-situ characterization suggests that the crystalline morphology of CaSO4 could be effectively regulated by varying the stoichiometry of crystallizing ions; the richness of calcium in the aqueous environment for crystallization would be in favor of weakening the crystal-membrane interactions. The stoichiometry-dependent growth of CaSO4 crystals can be exploited to develop an effective strategy for preventing the hydrophobic network from being wetted or irreversibly damaged.

Evaluation of microplastic particle transmission in a microfiltration process using fluorescence measurements: Effect of pore size and flux

Microplastic (MP) pollution in the aquatic environment is widespread, with a significant fraction of these particles originating from municipal wastewater treatment plant (WWTP) effluents. Whereas membrane filtration processes are touted as an effective MP removal strategy, the rejection of irregularly-shaped plastic particles, similar to those found in WWTPs, is poorly understood. Here, we characterize the filtration of irregularly-shaped MP particles (∼10 μm) through Durapore® microfiltration membranes (0.45 and 5 μm pore sizes). These particles were produced via ball-milling/sieving processes from a fluorescent polyethylene feedstock, enabling particle concentrations to be quantified using a standard fluorometric plate reader. Permeate samples from the 0.45 μm membrane exhibited low fluorescent intensities relative to feed samples, implying minimal MP transmission. Conversely, appreciable MP transmission through the 5 μm membrane was noted, with sizable MPs (∼2–7 μm) found in the permeate. This transmission was exacerbated at higher fluxes which emphasizes how operating conditions can govern MP retention. Post-filtration analyses demonstrated that particle capture occurred largely at the feed-membrane interface, where greater MP intrusion into the membrane was seen at the larger pore size. These results reaffirm the importance of choosing an appropriate membrane/membrane pore size and operating conditions to maximize MP retention in WWTPs.

Experimental and theoretical analysis of scaling mitigation for corrugated PVDF membranes in direct contact membrane distillation

To combat membrane scaling in membrane distillation (MD), corrugated surface patterned PVDF membrane have been designed for controlling the surface hydrodynamic properties as an engineering solution for preventing scaling. This paper attempted to correlate the CaSO4 scaling behavior of tailor-made flat PVDF (F-PVDF) and corrugated PVDF membrane (C-PVDF) in DCMD. We evaluated two feed flow modes: flow in parallel to the ridge(C-PVDF-p), and perpendicular to the ridge(C-PVDF-v). The experimental results demonstrated that C-PVDF-p exhibits the best of scaling resistance compared other two cases as well as flux. A 3D computational fluid dynamics (CFD) multiphysics model of DCMD was built by encompassing the conservation equations for mass, species, momentum, and energy. The hydrodynamic and thermal characteristics were analyzed. Modeling showed that C-PVDF-p exhibits higher flow velocity in the bulk and near the feed-membrane interface due to instabilities from feed turbulence; this enhances heat and mass transfer, feed mixing, and reduces residence time. Conversely, C-PVDF-v exhibited flow confinement within the grooves, resulting in a limited contribution to heat and mass transfer with the bulk solution. This flow behavior, coupled with the presence of recirculation vortices within the grooves, leads to an increased occurrence of CaSO4 scaling on the membrane surface. Additionally, C-PVDF-p exhibited high thermal efficiency, which explains a significant 48% increase in experiment, despite a modest 30% increase in the membrane surface area. This work demonstrated the successful module-scale CFD simulation for DCMD, and we expect future reliable modeling of large systems which provides tremendous savings in both time and cost for DCMD design for realistic applications.

Plasmonic Phenomena in Membrane Distillation.

Water scarcity raises important concerns with respect to human sustainability and the preservation of important ecosystem functions. To satisfy water requirements, seawater desalination represents one of the most sustainable solutions. In recent decades, membrane distillation has emerged as a promising thermal desalination process that may help to overcome the drawbacks of traditional desalination processes. Nevertheless, in membrane distillation, the temperature at the feed membrane interface is significantly lower than that of the bulk feed water, due to the latent heat flux associated with water evaporation. This phenomenon, known as temperature polarization, in membrane distillation is a crucial issue that could be responsible for a decay of about 50% in the initial transmembrane water flux. The use of plasmonic nanostructures, acting as thermal hotspots in the conventional membranes, may improve the performance of membrane distillation units by reducing or eliminating the temperature polarization problem. Furthermore, an efficient conversion of light into heat offers new opportunities for the use of solar energy in membrane distillation. This work summarizes recent developments in the field of plasmonic-enhanced solar evaporation with a particular focus on solar-driven membrane distillation applications and its potential prospects.

Laser-induced graphene Janus membrane for electrothermal membrane distillation

To improve the energy efficiency of membrane distillation (MD), localized heating of the feed-membrane interface is advantageous relative to heating of the bulk feed. This motivated the current study, which targeted at developing a simple two-step way of fabricating Laser-Induced Graphene (LIG) Janus membrane for electrothermal membrane distillation. Specifically, the Janus membrane had an electrically conductive graphene side and a polydimethylsiloxane (PDMS) coated polyethersulfone (PES) side. The PES membrane was first coated with PDMS in limonene (a green solvent) and the active layer exposed to low-powered laser. Joule heating was carried out on the LIG surface by connecting two electrodes to the opposite ends of the LIG surface and providing power with an alternating current power supply at regular household frequency. This Janus membrane can be heated on the LIG surface while insulated from heat loss on the PDMS-coated PES surface, resulting in a marked improvement in the MD performance. Relative to pre-heated feed at the same power input, Joule heating of the Janus membranes gave a flux increase of up to 119.5%, specific heating energy decrease of up to 53.9% and single-pass heat utilization efficiency increase of up to 119.5%, while maintaining excellent material stability.

Internal membrane fouling by proteins during microfiltration

The current study aimed to understand both external and internal membrane fouling by three proteins with different net charges, namely, negatively charged pepsin and bovine serum albumin (BSA), as well as positively charged lysozyme. Polycarbonate track-etched (PCTE) membranes were used. Per electrostatic attraction, the flux decline was the worst for lysozyme, which is attributed by the fouling model to the greatest pore blockage (α) and pore constriction (β), and by field-emission scanning electron microscope (FESEM) and optical coherence tomography (OCT) to the most extensive external fouling. Between pepsin and BSA, BSA gave worse flux decline despite its more negative net charge. The fouling model indicates that BSA gave greater pore blockage (α) and denser internal cake (Rc/Rm), while the quartz crystal microbalance with dissipation (QCM-D) indicates a rigid cake structure. Notably, despite monotonic flux decline with filtration, the OCT fouling voxel trends show significant fluctuations, which has not been reported before and thus signify the unique behavior of protein foulants in straight-through pores. Specifically, the trends below and above the −4.5 μm layer (i.e., 4.5 μm below the feed-membrane interface) are perfectly opposite, indicating the non-uniform protein deposits slipping downwards in the membrane pores as filtration progressed. The dynamic movements of the protein cakes unveiled here warrant more understanding in future studies.

Europium(III) permeation through a flat sheet supported liquid membrane containing CMPO with iso-decanol phase modifier: Experimental and modeling studies

The permeation of Eu3+ ion across a microporous polymeric membrane impregnated with n-octyl(phenyl)-N,N-di-isobutylcarbamoyl methyl phosphine oxide (CMPO) was investigated from a nitric acid feed into a receiver phase of distilled water. A combination of 0.2M CMPO in 5% iso-decanol+95% n-dodecane was chosen as the carrier solvent. Here, iso-decanol has a dual role; acts as a phase modifier to avoid third phase formation, and minimizes the acid co-transport. The transport of Eu3+ was mainly driven by its distribution ratio by CMPO at the feed–membrane interface (extraction) and membrane–receiver interface (stripping). The effect of various experimental parameters such as feed acidity, nitrate ion concentration, and ligand concentration on permeation of Eu3+ was investigated. Effect of various stripping solutions was also studied to optimize the permeation efficiency of Eu3+ ions. A mathematical model was derived for predicting the mass transfer from feed phase to the receiver phase through the membrane. The results obtained by the calculations were subsequently validated through the experimental results.

Plasmonic Titanium Nitride Nano-enabled Membranes with High Structural Stability for Efficient Photothermal Desalination.

Herein, we demonstrate the desalination performance of a solar-driven membrane distillation (MD) process, where upon light illumination, a highly localized heating of plasmonic titanium nitride nanoparticles (TiN NPs) immobilized on a hydrophobic membrane provides the thermal driving force for the MD operation. The engineered TiN photothermal membrane induces vapor generation directly at the feed-membrane interface upon solar irradiation, thereby eliminating the need to heat the entire bulk feed water. The results indicate that the average vapor flux through the TiN photothermal membrane without any auxiliary feed heating was recorded as 1.01 L m-2 h-1, which corresponds to the solar-thermal efficiency of 66.7% under 1 sun solar irradiance. The superior performance of the photothermal MD process is attributed to the broadband optical absorption and excellent light-to-heat conversion properties of the plasmonic TiN NP layer, which enabled efficient interfacial water heating at the membrane surface and increased the net driving force for vapor transport. Results also reveal the high mechanical stability of the TiN photothermal coating layer during long-term photothermal MD operations. We believe that the TiN photothermal membranes fabricated using a relatively inexpensive and nontoxic material via the simple technique with high stability and photothermal conversion efficiency will provide a path forward for developing the solar-driven MD applications.

Superior membrane distillation by induction heating of 3D rGO/Nafion/Ni foam for water treatment

Membrane distillation (MD) is a promising green technology that can harness waste or solar heat to treat water. Since such heat sources suffer from temporal variations, a complementary, efficient means is necessary. Induction heating is attractive in providing localized heating at the feed-membrane interface where the heat matters, but the typical conductive materials (e.g., metal, alloy) corrode readily in the high-salinity, high-temperature environment. In this work, a superior induction material was proposed, namely, reduced graphene oxides (rGO) coated onto a porous Nickel foam by Nafion (rGO/Nafion/Ni), which exhibited superior and sustained performance for membrane distillation. Results indicate that rGO/Nafion/Ni achieved up to 28.1% higher water flux (6.42 ± 0.36 Lm−2h−1) and 37.5% lower energy consumption (3.13 kWhL−1) compared to other carbon materials coated on Ni or the bare Ni foam. The mechanism underlying the enhanced MD performance was the higher absorption of the electromagnetic waves via multiple internal reflections and larger eddy currents generated by rGO, leading to higher temperatures at the feed-membrane interface that increased the driving force for distillation and thereby the energy efficiency of MD. Our work demonstrated that the induction heating of rGO has high potential for augmenting MD performance in water treatment.

Transport of As (III) from Acidic Feed Solution through Supported Liquid Membrane Impregnated with Tri-n-dodecylamine and Recovery as As (V)

The transport of As (III) via supported liquid membrane containing tri-n-dodecylamine (TDDA) as carrier in acidic medium has been studied. Various factors affecting the transport of As (III) were evaluated. The effects of acid concentration (HCl), metal ion concentration, carrier, and strippant (NaOH) concentration on transport of As (III) were thoroughly examined. Mathematical transport model was presented considering the formation of complex between TDDA, acid, and As (III) at feed membrane interface, then the diffusion of this complex through membrane, and finally dissociation of the complex in basic medium at strip membrane interface. The predicted results of this model strongly agree with experimental ones. The stoichiometry of complex was determined on the basis of slope analysis of plots. The mechanism of transport was determined by coupled co-ion transport mechanism, with H+ and Cl− coupled ions. The optimized SLM was efficiently used for transport of As (III) from acidic feed solution into basic strippant NaOH and oxidized to less toxic form As (V) by adding H2O2.

Critical flux and fouling mechanism in cross flow microfiltration of oil emulsion: Effect of viscosity and bidispersity

Membrane-based filtration is promising for applications in the food industry, for example, for processing dairy products, but membrane fouling restricts the efficiency of the process. The deposition of the main constituents in skim milk of casein, lactose and milk fat globules onto the membrane due to the permeate drag is inevitable, which necessitates a mechanistic study of the fouling phenomena. The Direct Observation through the Membrane (DOTM) technique was used to understand the effects of viscosity and bidispersity on the microfiltration of oil emulsions. Deposits at the feed-membrane interface clearly slowed down as the feed viscosity increased. The higher drag to the crossflow as a result of the higher viscosity caused more oil droplets to foul the membrane. However, although both casein and lactose increased the feed and permeate viscosity, the presence of lactose did not alter the fouling phenomena of a moving cake layer of oil droplets much, but the presence of casein caused the oil droplets to deposit instead as clusters. For casein, other than the viscosity effect, it also resulted in a bidisperse suspension, with the smaller particulates promoting membrane fouling. Agreement between the experimental data and the shear-induced diffusion model was good at the lower cross-flow velocities (CFVs) but poorer at higher CFVs, which indicate that the effect of viscosity appeared negligible at the lower CFVs but more dominant at the higher CFVs. The DLVO model agreed with the Jcrit and TMP results. This study sheds light on the membrane fouling phenomena for viscous feeds, and are expected to be useful particularly for food applications.

Analyzing external and internal membrane fouling by oil emulsions via 3D optical coherence tomography

Membrane-based filtration is an emerging technique for processing oily wastewater. Improving the efficiency of oil-water separation via membranes entails novel techniques for studying the complex interactions between the oil droplets and membrane underlying fouling. This paper presents the first study that applies optical coherence tomography (OCT) to the characterization of membrane fouling by oil emulsions. A series of dead-end filtration experiments was performed to characterize the rejection of oil droplets (~ 10µm; hexadecane) by the membrane structure (0.45µm PVDF) via three-dimensional (3D) OCT scanning in real time. The experimental results were compared with the control experiments with ~ 10µm glass beads to identify the optical artifacts. Both the external and internal fouling by the oil droplets were successfully revealed by analyzing the variation in OCT intensity at various layers that were mathematically defined in terms of the coordinate surfaces parallel to, and above and below, the feed-membrane interface. The evolution of membrane fouling was quantified by evaluating the fraction of fouling voxels as a function of time at varied depths. This study demonstrates that the OCT-based characterization has the potential to shed light on the complex interactions occurring in oil-water separations via membrane filtration, particularly by providing real-time non-invasive monitoring of internal fouling, the understanding of which is valuable for both fundamental research and practical applications.

Experimental and theoretical analyses of temperature polarization effect in vacuum membrane distillation

This paper discusses the effect of temperature polarization in Vacuum Membrane Distillation (VMD). The main motivation for using VMD in this work is that this module configuration is much simpler and more suitable for this kind of investigation than the other MD configurations such as Direct Contact Membrane Distillation (DCMD). The coupling between heat and mass transfer mechanisms at the feed-membrane interface is presented from a theoretical point of view. In addition, a new simple graphical method and a mathematical model for determining VMD flux are presented. The two methods used in evaluating the extent of temperature polarization effect on water vapor flux (flux sensitivity factors and temperature polarization coefficient (TPC)) are also analyzed and compared. The effect of integrating a heat recovery system in a large scale module on the TPC coefficient has also been studied and presented in this paper.

A Reaction Flux Model for Extraction of Cu(II) with LIX84I in HFSLM

Hollow fiber supported liquid membrane (HFSLM) is a favorable method to extract both valuable compounds and heavy metal pollutants such as chromium, copper, and nickel at a very low concentration. In this work, the extraction of Cu(II) by LIX84I dissolved in kerosene was theoretically and experimentally investigated. A model to estimate the percentage of extraction of copper ions from synthetic water considering the effect of reaction flux in membrane phase of the HFSLM system was studied. H2SO4 solution was used as the stripping solution. The facilitated transport mechanism of the chemical reaction at the feed-membrane interface was taken into account in the model equations. The percentage of copper ion extraction was plotted against its initial concentration in feed and also feed flow rate. Subsequently, the separation time and separation cycle were determined in accordance with the simulated values of copper ion concentration and the feed flow rate from the model. The modeled results were in good agreement with the experimental data at the average percentage of deviation about 2%.

Transport of Thorium(IV) Across a Supported Liquid Membrane Containing N,N,N′,N′-Tetraoctyl-3-oxapentanediamide (TODGA) as the Extractant

The transport behavior of Th4+ was investigated from a feed containing 3.0 M HNO3 into a receiver phase containing 0.1 M oxalic acid across a PTFE flat sheet supported liquid membrane (SLM) which contained TODGA (N,N,N′,N′-Tetraoctyl-3-oxapentanediamide) in n-dodecane as the extractant. Effects of the nature of the strippant, extractant concentration, Th concentration in the feed, and feed acidity on the transport rates were investigated. The transport behavior apparently depended on the rate of extraction of the metal ion at the feed-membrane interface and was not diffusion controlled. Influence of Th concentration on flux was also investigated. Transport mechanism was elucidated and the diffusion coefficient was calculated to be 2.13 × 10−7 cm2/s. Solvent extraction studies at varying feed acidity and TODGA concentration were also carried out.

Mathematical modelling and simulation of cotransport phenomena through flat sheet-supported liquid membranes

A mathematical model of cotransport phenomena of metal ions across flat sheet-supported liquid membranes (FSSLM) is developed in the transient. The initial conditions of the FSSLM system, the physical characteristics of the membrane (porosity, tortuosity and thickness), the kinetics of the chemical reaction in the feed–membrane interface and the diffusion in the membrane phase are taken into account by the model. By using the electroneutrality principle, an expression of the concentration of cotransport species with the metal is deduced in terms of the temporal variation of the metal concentration in the feed phase. From the proposed model, the maximum percentage of metal transported is derived. Finally, a simulation is performed for the case of the transport of metal anions in alkaline solution and is checked against experimental data corresponding to the transport of Au(CN) 2 − in alkaline solution by LIX 79. The proposed model reproduces this transport phenomenon with great accuracy.

Extraction of cadmium with trilaurylamine–kerosine through a flat-sheet-supported liquid membrane

Cadmium has been extracted as a chloride complex through a flat-sheet-supported liquid membrane (SLM), using the tertiary amine Alamine 304-1 (mainly trilaurylamine or TLA) in kerosine. The typical permeability of the membrane was 1.1×10 −6 m s −1. The rate limiting step is diffusion through the membrane. The cadmium loading of the extractant at the feed–membrane interface is high. Trilaurylammonium chloride crystallizes at the surface of the membrane above 0.2 M TLA. This salt blocks the pores and lowers the extraction rate up to a factor of 3. Apart from this blocking effect, the permeability through the membrane is described well with the presented model, using physically realistic parameter values. When the precipitation can be minimized, the system has good potentials for the extraction of cadmium chloride complexes.

Analysis of the transport rates of europium(III) across an organophosphinic acid supported liquid membrane

The transport rates of europium(III) through a supported liquid membrane (SLM) containing di(2,4,4-trimethylpentyl)phosphinic acid as a mobile carrier were analyzed. First, the batch extraction equilibrium of the title system was investigated and the equilibrium constants for the formation of the extracted complexes were determined. Second, an SLM transport model was presented considering diffusion of metal across the feed stagnant layer, chemical reaction of metal and carrier at the feed-membrane interface, and diffusion of carrier and its metal complex in the membrane phase. The kinetic parameters of the complex formation reaction were evaluated from the SLM transport data. Moreover, the transport mechanism was identified quantitatively by calculating the fractional resistance of each step during transport.

Rate-Controlling Mechanism of Cobalt Transport through Supported Liquid Membranes Containing Di(2-ethylhexyl)phosphoric Acid

Abstract In this paper the permeation process of trace cobalt(II) through supported liquid membranes using di(2-ethylhexyl)phosphoric acid as a mobile carrier was modeled and the rate-controlling mechanism was examined. The permeation rate equations were derived considering diffusion of metal ions across the aqueous stagnant layers toward the membrane, chemical reaction at the feed-membrane interface, and diffusion of extractant and its metal complex in the membrane phase. The application of the measured permeation rates for determining the kinetic parameters of the solvent extraction reaction was also investigated. It was found that the calculated rates were in good agreement with the measured ones (average standard error, 12%).

Pervaporation of dilute organic-waters mixtures. A literature review on modelling studies and applications to aroma compound recovery

Pervaporation as a tool for the removal of organic substances from dilute aqueous solutions has been reviewed. During pervaporation the membrane governs the performance of the process. The removal of organics from water is accomplished with membranes made of elastomeric or hydrophobic polymer materials. The performance of these membranes, measured as fluxes and enrichment factors, differs considerably. Total fluxes vary between 24 and 2728 g/m 2-hr while the enrichment factor varies between 0.6 and 372. Modelling of the process involves four successive steps, a mass transfer from the bulk of the feed to the feed-membrane interface, partition of the penetrants between the feed and the membrane, diffusion in the membrane and desorption at the membrane-permeate interface. All these steps are crucial for the overall performance of the process. Several models with different approaches and validities exist for each step. The possible of pervaporation for aroma compound recovery has been verified in a number of studies. These studies indicate that aroma compounds, especially volatile aroma compounds, can be enriched several hundreds of times with pervaporation and that the process could be profitable.