MD Membranes Research Articles

Biofouling control of thermophilic bacteria in membrane distillation

Previously, fouling of MD membranes was perceived to be minimal due to the operational conditions preventing bacterial growth. However, recent studies presented fouling of MD membranes, largely due to various halophilic and thermophilic bacterial. The current study presented potential of antibacterial cellulose nanocrystals (CNCs) capped silver nanoparticles (AgNPs) towards biofouling control in MD water desalination processes. Interestingly, the CNCs were used as capping and reducing agent in a microwave mediated synthesis of AgNPs. The green synthesis approach of AgNPs and its successful use in MD processes was reported for the first. The MD experiments were carried out using a series of modified PVDF membranes and feed solution spiked with a thermophilic G. stearothermophilus. Upon evaluation of fouled membranes, particularly PVDF, PVP-equipped PVDF, and f-CNTs-modified PVDF, the surface energy of interactions were −131 mJ/m2, −31.5 mJ/m2, −28.6 mJ/m2, suggesting favorable interaction between the membranes and bacterial foulant. The findings of this study were supported by fouling layer and bacterial growth as seen from scanning electron microscopy, atomic force microscopy and Fourier transform infrared results. Upon membrane surface modification using CNCs-capped AgNPs, bacterial growth on membrane surface was reduced. Specifically, the positive cohesion energy between the CNCs-capped AgNPs-modified PVDF membrane and thermophilic bacterial foulant was + 63.2 mJ/m2, distinctly proving reduced fouling. The findings were supported by 5.5 % flux decay of the modified membrane compared to 79.3 % flux decay of virgin PVDF membrane. This suggested a long-term membrane integrity in thermophilic bacterial contaminated feed solutions during the MD water desalination.

Beyond the surface: Understanding temperature polarisation in DCMD membranes through varied operational parameters

Conventionally, a single Temperature Polarisation Coefficient (TPC) value is calculated to quantify Temperature Polarisation (TP). In this research, the extent of polarisation is investigated by capturing temperature profiles at specific points along a MD membrane using miniature thermocouples, eliminating the need for TPC calculations. The extent of polarisation at a point is affected by two contributory factors, namely the proximity of flow inlets and the difference in vapour pressure across the membrane at that point. Under this direction, this work examined the influence of permeate temperature, feed salinity, and flow direction on the development of the temperature profiles. Our analysis revealed an elevation in TP on both sides of the membrane when the permeate temperature was increased. In addition, changes in feed salinity had a very minute impact on the development of the temperature profiles. By comparing the cocurrent and counter-current flow, the influence of the two contributory factors was further proved, with counter-current flow working better for long membrane modules. Furthermore, an investigation on the symmetricity of polarisation across the membrane revealed asymmetricity depends on the operating conditions, especially direction of flow. The asymmetricity was infinitesimal at low inlet temperature differences for cocurrent flow.

Investigating mechanism of enhanced anti-fouling performance of MXene modified Janus MD membrane by XDLVO theory combined with surface elemental integration method

Organic membrane fouling is a major obstacle to the application of membrane distillation in low surface tension wastewater treatment. Membrane modification might be an effective way to mitigate membrane fouling. This study mainly explored the mechanism of enhanced anti-fouling performance of the MXene modified PVDF membrane by applying XDLVO theory combined with surface elemental integration (SEI) method. The rough membrane surfaces were first reconstructed using fractal function. Then, the interaction energy between low surface tension substances and the membranes before and after modification was calculated. The results showed that oil droplets and surfactant fouling were mainly caused by hydrophobic attraction due to Lewis acid-base (AB) interaction energy. By modifying the virgin PVDF membrane with MXene nanoparticles, the polar component of membrane surface tension (γAB) increased significantly, so that the interaction energy between all contaminants and membrane surface changed from attraction to repulsion. In addition, roughness weakened the interaction energy between contaminants and membrane surface but significantly lengthened the range of interaction distance, which together determined the ability of the membrane to attract or repel the contaminants. These findings may improve the understanding of the structure-effect relationship between membrane modification and anti-fouling ability, and provide new insights into the membrane modification for MD.

Enhancing water recovery through integrated graphene oxide-modified forward osmosis and membrane distillation for real textile wastewater treatment

The textile industry is a highly water-intensive sector, emphasizing the increasing importance of reclaiming clean water from textile wastewater and advancing water recovery towards zero liquid discharge. This study proposes and assesses an integrated forward osmosis and membrane distillation (FO-MD) process for treating real textile wastewater, aiming for enhanced water recovery and ensure robustness towards zero liquid discharge. Firstly, a newly developed graphene oxide (GO)-enhanced FO membrane (GFO) was fabricated by adding GO into the polyamide layer during the interfacial polymerization process. Furthermore, the real textile wastewater treatment performance of the FO-MD process based on the GFO membrane was compared to that of a commercial FO membrane (CFO). The results indicated the GFO-based FO-MD process outperformed due to enhanced antifouling properties and stable water flux. Remarkably, the GFO membrane-based FO-MD system achieved over 80% water recovery and nearly 100% rejections for COD, Na+, K+, Cl-, and S2- during continuous 24-hour filtration. The findings highlighted FO-MD as a viable solution, surpassing the individual FO and MD processes by overcoming the limitations of FO draw solution dilution and MD membrane wetting challenges. Moreover, a preliminary techno-economic study for a 200 m3 day−1 textile wastewater estimated water production costs of $6.48 m−3 and $5.73 m−3 for CFO-MD and GFO-MD systems. The analysis highlighted maintaining the water flux above 10 L m−2 h−1 is crucial for economic viability. This work demonstrates the potential of the FO-MD system as a sustainable solution to address the textile wastewater treatment challenges, providing insights for FO-MD system design and optimization.

Gas equilibrium membrane inlet mass spectrometry (GE-MIMS) for water at high pressure

Abstract. Gas species are widely used as natural or artificial tracers to study fluid dynamics in environmental and geological systems. The recently developed gas equilibrium membrane inlet mass spectrometry (GE-MIMS) method is most useful for accurate and autonomous on-site quantification of dissolved gases in aquatic systems. GE-MIMS works by pumping water through a gas equilibrator module containing a gas headspace, which is separated from the water by a gas-permeable membrane. The partial pressures of the gas species in the headspace equilibrate with the gas concentrations in the water according to Henry's Law and are quantified with a mass spectrometer optimized for low gas consumption (miniRUEDI or similar). However, the fragile membrane structures of the commonly used equilibrator modules break down at water pressures ≳3 bar. These modules are therefore not suitable for use in deep geological systems or other environments with high water pressures. To this end, the SysMoG® MD membrane module (Solexperts AG, Switzerland; “SOMM”) was developed to withstand water pressures of up to 100 bar. Compared to the conventionally used GE-MIMS equilibrator modules, the mechanically robust construction of the SOMM module entails slow and potentially incomplete gas–water equilibration. We tested the gas equilibration efficiency of the SOMM and developed an adapted protocol that allows correct operation of the SOMM for GE-MIMS analysis at high water pressures. This adapted SOMM GE-MIMS technique exhibits a very low gas consumption from the SOMM to maintain the gas–water equilibrium according to Henry's Law and provides the same analytical accuracy and precision as the conventional GE-MIMS technique. The analytical potential of the adapted SOMM GE-MIMS technique was demonstrated in a high-pressure fluid migration experiment in an underground rock laboratory. The new technique overcomes the pressure limitations of conventional gas equilibrators and thereby opens new opportunities for efficient and autonomous on-site quantification of dissolved gases in high-pressure environments, such as in research and monitoring of underground storage of CO2 and waste deposits or in the exploration of natural resources.

Optimizing electrocoagulation pre-treatment efficiency during simultaneous treatment of different produced water streams in a FO-MD hybrid system

In this study, the electrocoagulation (EC) process was explored and optimized for the pre-treatment of key contaminants present in hyper-saline produced water (PW) streams before their treatment through forward osmosis-membrane distillation (FO-MD) hybrid system. Due to their hyper-saline nature and the associated high temperatures, PW streams can be treated cost-effectively and sustainably using low-pressure dual membrane filtration systems like FO-MD hybrids. Desalter effluent (DE) stream was used as the FO feed solution, and WOSEP outlet (WO) stream served simultaneously as the FO draw solution and MD feed solution. The key foulants were identified in the WO and DE streams. Silicon, calcium, and sulfur were found as the key elements contributing towards FO and MD membrane fouling in the form of CaSO4, SiO2, and CaSiO3. Applying EC for 10 min with 100 mA current (7 mA/cm2 current density) at a neutral pH of 7 demonstrated the highest removal efficiencies for silicon (97 %), calcium (3.6 %), and sulfur (12.2 %) with only 0.023 kWh/m3 electric energy requirement. FO-MD hybrid efficiency was re-assessed with pre-treated WO and DE streams. The FO flux showed more stability and increased by 32.5 % and MD by 28.9 %, with a 10 % reduction in specific reverse solute flux. EC is thus a robust and effective process for hyper-saline hazardous PW pre-treatment and can substantially improve the FO-MD hybrid system efficiency.

Development of Non–heated Concentration Membrane System using FO and MD Membrane

Development of Non–heated Concentration Membrane System using FO and MD Membrane

A hybrid anaerobic osmotic membrane bioreactor - membrane distillation for water reclamation: aquatic toxicity, membrane fouling characterization, and economic assessment

The incomplete removal of pharmaceutically active compounds (PhACs) by conventional wastewater treatment has been challenging due to their ecological implications. Therefore, advanced technologies are necessary to remove PhACs efficiently. In the present study, an anaerobic osmotic membrane bioreactor - membrane distillation (AnOMBR-MD), aiming to remove seven PhACs, was evaluated by an integrated approach, considering the environmental toxicity, fouling characterization, and economic assessment. PhACs removal was evaluated using liquid chromatography coupled with mass spectrometry. Besides, organic matter and nutrient removal were evaluated. The FO and MD membrane fouling was characterized with scanning electron microscopy and energy dispersive spectroscopy. Toxic effects were analyzed for Aliivibrio fischeri, Daphnia similis, and Raphidocelis subcapitata. Furthermore, a preliminary economic assessment was carried out using capital and operating expenditures. The results showed a DOC, N-NH4+, and P-PO43- removal of 92%, 97%, and 99.9%, respectively, while removal of PhACs was >96%. However, the distillate was classified as very toxic forR. subcapitata, and a strong correlation was found between toxicity and the Mg2+ originating from the draw solution (DS). This result showed the importance of including toxicity tests in the selection of DS. The economic evaluation indicated that the higher cost is related to the membrane replacement due to the decrease in the permeate flux, pointing out the need for future studies that address cleaning strategies to solve the organic and inorganic fouling on the membrane's surface. In this sense, this study promoted an improvement in understanding the AnOMBR-MD technical-economic viability aiming at full-scale application.

Nanomaterial‐Incorporated Membrane Distillation Membranes: Characteristics, Fabrication Techniques, and Applications

AbstractMembrane distillation (MD), a temperature‐driven membrane separation process, is used for various applications due to its less complicated design. MD operations encounter major issues such as permeate flux decrease, membrane fouling, and wetting. A lot of research has been conducted in the past years on the modification of MD membranes by incorporating nanomaterials to overcome these obstacles and considerably increase their performance. Nanomaterials incorporated into the membranes improve the water permeability, mechanical strength, and fouling. The incorporation of next‐generation nanomaterials like metal oxide nanoparticles, carbon‐based nanomaterials, graphene‐based membranes, quantum dots, and metal‐organic frameworks in the MD membranes is investigated. Essential membrane properties for MD operations are comprehensively studied, including higher liquid entry pressure, permeability, porosity, hydrophobicity, thermal stability, mean pore size, and low fouling rate. Significant advances in the application of nanomaterials to the modification of MD membranes as well as other membrane fabrication techniques adopted for the incorporation of nanoparticles like surface grafting, interfacial polymerization, plasma polymerization, and dip coating are reviewed. Important future aspects are discussed.

Hybrid Technology Seawater Desalination Based on Reverse Osmosis and Membrane Distillation Methods

The article presents the results of a computational and analytical study of hybrid RO–MD (Reverse Osmosis–Membrane Distillation) technologies for desalination of the Caspian Sea, providing for the production of an additional amount of desalinated water by the MD method from RO concentrates heated to 50–80°C due to waste heat of fuel combustion products in steam boilers. Two options for solving the problem of the formation of CaCO3 and CaSO4 precipitates on membranes were studied: with preliminary nanofiltration (NF) or Na-cationation (Na) of sea water, as an alternative to the use of antiscalants (AS) and acid. The negative environmental effect of most plants (eutrophication of water bodies) and their low efficiency at high concentrations of desalinated water are taken into account. The Langelier index (СаСО3) and the degree of concentrate saturation (СаSO4) were used as criteria for precipitation of deposits on the membranes. The NF and RO processes were studied using the ROSA computer program, and the MD and Na processes were studied by computer simulation of the corresponding calculation models. It was found that at a 70% permeate yield at the NF and RO stages, the possibility of calcium precipitation on the RO and MD membranes is prevented, but their precipitation on the NF membranes is predicted, which makes the use of AS forced. At the same time, additional permeate production at the MD stage from RO concentrates reaches 40% of the amount of permeate from RO stadium, and the electricity consumption in general according to the scheme is 1.88 kWh/m3. Reducing the calcium hardness of sea water to 50 µg-eq/dm3 by the Na-cationization method makes it possible to refuse both the use of AS and acidification with sulfuric acid with additional production of MD permeate – 27% relative to RO permeate. Electricity consumption rises to 2.5 kWh/m3. To use the known advantages of NF without the use of AS, a hybrid Na–NF–RO–MD scheme is proposed. It has been established that at 80% yields of NF and RO permeates, to prevent the formation of CaSO4 precipitates at all stages of treatment, it is sufficient to reduce the hardness of sea water from 16 to 5.5 m-eq/dm3, and by acidifying the softened water to exclude the formation of CaCO3 precipitates.

Thin‐Film Composite Membrane with a Hydrophobic Substrate for Robust Membrane Distillation

AbstractMembrane distillation (MD) is a promising thermal desalination technology, but its application is largely limited by membrane wetting. This study introduces a thin‐film composite (TFC) membrane comprising a polyamide (PA) surface layer, a polydopamine (PDA) interlayer, and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate for anti‐wetting MD operations. With a PDA reaction time of 1 h, a compact and hydrophilic PDA interlayer forms on the PVDF membrane substrate, facilitating the subsequent formation of the PA surface layer. With 2 cycles of interfacial polymerization process, a dense PA surface layer forms on the PDA interlayer, rendering a TFC membrane with robust wetting resistance. From diffusion experiments and breakthrough pressure measurements, the TFC membrane exhibits excellent wetting resistance because the dense PA surface layer can prevent the wetting contaminants from reaching the hydrophobic membrane substrate due to the combination of the size exclusion effect and capillary effect. The results from this study shed lights on a new approach for the fabrication of robust MD membranes for practical applications.

Forward osmosis, reverse osmosis, and distillation membranes evaluation for ethanol extraction in osmotic and thermic equilibrium

One of the significant challenges of the industry is the economic extraction of ethanol from aqueous solutions of interest to the food industry. Specifically, in the alcoholic beverages industry, the non-alcoholic beverage market has gained importance, demanding more non-alcoholic drinks with higher organoleptic quality. In this context, the technology related to the use of membranes has shown great potential. Our research aims to determine the performance of forward osmosis using FO, RO, and MD membranes as an option in the extraction of alcohol in an osmotic (Δπ = 0) and thermal (ΔT° = 0) equilibrium process. The results showed that all the membranes have the potential to dealcoholize a 12% v.v. synthetic ethanol solution. The results showed that Membrane Distillation is a viable strategy as a complement in a hybrid FO-MD approach, achieving ethanol flux values close to those offered by FO membranes and much higher than those shown by RO membranes. We also proposed other technical criteria for membrane performance evaluation, such as maintaining a low reverse salt flux and a low or no water flux to keep the organoleptic properties of a hypothetical alcoholic beverage subjected to the FO-MD hybrid process. For a high Ethanol/Salt flux ratio, the RO-82V and FO-CTA membranes presented the best performance. Furthermore, in an Ethanol/Water flux ratio, RO membranes perform best when working in an osmotic equilibrium process with a water flux equal to zero. Regarding the FO membranes, in an Ethanol/Water flux ratio, the FO-TFC membrane presented the best performance.

Performance and implications of forward osmosis-membrane distillation hybrid system for simultaneous treatment of different real produced water streams

This study investigates the simultaneous treatment of different real produced water streams using forward osmosis-membrane distillation (FO-MD) hybrid system. The water–oil separator outlet (WO) was used simultaneously as FO draw solution (DS) and MD feed solution (FS). The FO process generated stable average fluxes of 13.6 L/m2/h and 15.8 L/m2/h with desalter effluent (DE) and wash water (WW) as FO feed streams, while MD produced 13.2 L/m2/h and 11 L/m2/h, respectively. Monovalent ions induced internal concentration polarization and CaSiO3 colloidal scaling on the support layer reduced the FO flux. Further FO flux reduction was caused by CaSO4 and NaCl crystals on the active layer. Moreover, CaSO4 created partial pore covering while oil and grease depicted pore clogging of the MD membrane, which decreased the MD flux. Volatile fatty acids and organic nano pollutants reached MD permeate and FO FS. Humic acid scaling was fully recovered by ethylenediaminetetraacetic acid by masking calcium ions and reducing the pH. No membrane wetting was observed in the system. No bacteria were found in real streams as analyzed with Epifluorescence microscopy, eliminating the potential of microbial fouling. The hybrid system showed > 93 % removal of oil, inorganics and organics, making the permeate quality excellent for re-injection or industrial reuse. The FS streams were concentrated by 77–84 % resulting in smaller disposal volumes. No utilization of chemicals and water makes this hybrid concept practical and sustainable. The findings of this study can serve as design criteria for future onsite pilot plant applications.

Scale-up of membrane distillation systems using bench-scale data

A procedure to design full-scale air gap membrane distillation (AGMD) processes is presented. A mathematical model was then developed for both direct contact membrane distillation (DCMD) and AGMD. The model is centered on solving local mass and energy balances using a finite difference approach. The full-scale model was calibrated by utilizing the membrane distillation coefficient (MDC) determined by DCMD bench-scale experiments, as the sole adjustable parameter. The MDC was then used to model the water production and energy efficiency of a spiral-wound AGMD full-scale element. The model yields accurate representation of full-scale AGMD elements using polytetrafluoroethylene (PTFE) and polyethylene (PE) membranes. Full-scale experimental results obtained over a wide range of feed flow rates (2 to 4.5 L/min), temperatures (40 to 80 °C), and salinities (0 to 200 g/L NaCl) confirmed that the developed procedure can be applied to model and design large-scale AGMD elements. Furthermore, the model guides the selection of specific temperature and flow conditions at a given salinity and element geometry to maximize water production and energy efficiency. This methodology is suitable for rapid evaluation of novel MD membranes performance in field AGMD applications.

Assessment of anaerobic membrane distillation bioreactor hybrid system at mesophilic and thermophilic temperatures treating textile wastewater

The performance of a lab-scale anaerobic membrane distillation bioreactor (An-MDBR) system was evaluated treating high-strength synthetic textile wastewater with a Chemical Oxygen Demand (COD) concentration of 3000 ± 130 mg/L. A novel hybrid process incorporating membrane distillation in a submerged anaerobic membrane bioreactor was developed at mesophilic and thermophilic temperatures and instigated. The An-MDBR process attained 99.99% inorganic salt rejection irrespective of the operating temperatures and high initial flux between 5.9 and 11.5 L/m2·h (LMH) at 35–50 °C. Removal efficiencies (%) of COD and color were in the range of 40–69% and 43–74%, respectively in an anaerobic bioreactor, whereas overall MD performance was in the range of 99 ± 0.9% for COD as well as color. Among the four temperatures, 40 °C exhibited the optimum performance for 47 days with the average COD and color removal efficiencies of 99 ± 0.9%, respectively with steady biogas production. Furthermore, the conductivity of An-MDBR was maintained at 4.0 ± 0.5 mS/cm using low-cost Woven Fiber Microfiltration (WF-MF) coupled with An-MDBR and the flux was optimized at 2 LMH. The removal efficiency of NH4+1-N was greater in the range of 89 to 93% at mesophilic temperatures while smaller in the range of 79 to 86% at thermophilic temperatures by maintaining pH between 6.8 and 7.2. The concentration of PO4−3-P in the permeate water was near zero, showing that PO4−3 can be rejected completely by the MD membrane. Due to membrane biofouling and scaling, wetting was observed for different durations at different temperatures.

An innovative hollow fiber vacuum membrane distillation-crystallization (VMDC) coupling process for dye house effluent separation to reclaim fresh water and salts

The discharge of dyeing wastewater has become one of the important restrictive factors for the green manufacturing and sustainable development of textile industry. Realizing the maximum reuse of dyeing wastewater resources is an efficient solution. Vacuum membrane distillation crystallization (VMDC) coupling process can separate and recover the components such as fresh water and salts (mordants) in dyeing wastewater as well as achieve the concentration of the valuable substances (dyes) simultaneously. In this study, hollow fiber VMDC coupling process was developed for the treatment of saline dye solution to recover the inorganic salt (mordant Al(NO3)3) and fresh water while realize the high-multiple concentration of dyes (Congo red, CR). High-performance hydrophobic PVDF hollow fiber porous membrane was prepared by the dry-jet wet-spinning technique and used in the VMDC process. The VMDC process in one cycle was optimized, including MD section, crystallization and membrane cleaning process. Variations of MD separation performance including permeate water flux and solute rejection of PVDF hollow fiber membrane for CR and Al(NO3)3 with operating time was determined by the vacuum membrane distillation (VMD) experiment. A three-cycle VMDC process was developed and the variations of MD separation performance and crystal yield in each VMDC cycle were investigated. In each VMDC process, the rejection of CR and Al (NO3) 3 by hollow fiber MD can be maintained above 99%. With the extension of VMDC operating time, the gradually decreased water flux can be recovered by around 90% through the combined cleaning method of NaOH and absolute ethanol (abs). In the three-cycle of VMDC process, 42.5, 32.5 and 9.8% of water were recovered from the feed solution with a total recovery ratio of 84.8%. In the second and third VMDC cycles, 48.0 and 21.5% of the salts in the saline dye solution were obtained with a total salt recovery ratio of 69.5%. An estimation of economic cost was also made to evaluate the economic feasibility of the proposed MDC process for its practical application. Hollow fiber VMDC coupling process was proved to be a simultaneously efficient separation method of saline dye solution.

Influence of multi-component composition of dyeing bath in the membrane distillation performance

The influence of the multi-component composition of cotton and polyester dyeing baths was investigated in a DCMD process using a PTFE membrane, aiming to water reclamation from textile wastewater. The process was evaluated with each auxiliary, solutions combining auxiliary and dye, and residual water from pilot-scale dyeing baths. The salts used in the cotton dyeing bath did not influence the DCMD process, resulting in a similar rejection rate and permeate flux to those in dye solutions. The formic acid employed in the polyester dyeing bath resulted in high permeate fluxes but decreased the permeate quality by almost 33%. The surfactants (detergent and dispersant) demonstrated the most harmful behavior in the process, evidenced by a complete membrane wetting. An absence of synergy of the dyes and auxiliaries influencing the DCMD process was verified for the textile dyeing wastewater. This work enabled identifying the influence of each dyeing bath component in the rejection rate and permeate flux, allowing to find alternatives in mitigating the remaining gaps involving DCMD operation and the MD membranes. This work contributes to consolidating this promising technique in water reclamation from textile wastewater, encouraging research on membrane development and modification or integrated operations involving MD.

Scaling control of forward osmosis-membrane distillation (FO-MD) integrated process for pre-treated landfill leachate treatment

Membrane fouling is a major challenge for membrane-based wastewater treatment. In this study, we investigated the integrated forward osmosis-membrane distillation (FO-MD) process for landfill leachate treatment. However, severe scaling induced by the formation of calcium sulfate happened at the FO membrane selective layer. Herein, an organic phosphonic acid scale inhibitor, hexamethylene diamine tetra (methylene phosphonic acid) (HDTMPA), was added in draw solution (DS) to control the scaling behavior. HDTMPA chelated with Ca2+ to prevented the nucleation and growth of gypsum scale crystals and alleviated membrane scaling. For FO membrane cleaning, the water flux recovery rate was greater than 90% after three scaling-cleaning cycles owing to the formation of deformed crystals and loose scaling layer via addition of HDTMPA. The mechanism of scaling with the existence of HDTMPA were further explored. In addition, we conducted long-term FO-MD experiment using NaCl DS with HDTMPA. The efficiency of the integrated system was much superior to that of the FO alone system with an extreme high contaminant rejection of 99%. HDTMPA not only alleviated FO scaling, but also prevented wetting of the MD membrane, and then promoted the efficiency and operation stability of the integrated FO-MD system.

Superhydrophobic modification of electrospun nanofibrous Si@PVDF membranes for desalination application in vacuum membrane distillation

Superhydrophobic nanofibers have received prominent attention owing to their exceptional properties and researchers are focused on developing high-performing MD membranes. Herein, we fabricate superhydrophobic electrospun nanofibrous membranes using polyvinylidene fluoride (PVDF) solutions with silica nanoparticles (0 wt% to 6 wt%) to create multiscale (or hierarchical) surface roughness. For superhydrophobicity, the composite membranes (Si@PVDF) were subjected to a two-step modification that included acid pre-treatment and silanization with fluoroalkylsilane (FAS) compound of low surface energy. The acid pre-treatment enhances the hydroxyl group of SiO2 nanoparticles and create active sites in abundance for silanization. The modified membranes (FAS-Si@PVDF-A) having 6 wt% SiO2 showed excellent wetting resistance with water contact angle (WCA) up to 154.6 ± 2.2°, smaller average pore size of 0.27 ± 0.3 μm, and high liquid entry pressure (LEP) of 143 ± 4 kPa. It was observed, increasing silica content decreased the fiber diameter and average pore size and increased WCA and LEP of modified membranes. The modified superhydrophobic membranes gave stable permeate flux, exhibited strong wetting resistance and excellent salt rejection in vacuum membrane distillation (VMD) test. The optimal FAS-Si@PVDF-A membrane (6 wt% SiO2) of thickness 98 ± 5 μm produced a stable permeate flux of more than 11.5 kg m−2 h−1 and salt rejection as high as 99.9% after 22 h of continuous operation using NaCl solution (3.5 wt%) as feed. Therefore, this modification provided superhydrophobic membranes possessing robust anti-wetting properties with significant permeability and has encouraging application in membrane distillation for desalination.

Analysis of Polyvinylidene Fluoride Membranes Fabricated for Membrane Distillation.

The optimization of the properties for MD membranes is challenging due to the trade-off between water productivity and wetting tendency. Herein, this study presents a novel methodology to examine the properties of MD membranes. Seven polyvinylidene fluoride (PVDF) membranes were synthesized under different conditions by the phase inversion method and characterized to measure flux, rejection, contact angle (CA), liquid entry pressure (LEP), and pore sizes. Then, water vapor permeability (Bw), salt leakage ratio (Lw), and fiber radius (Rf) were calculated for the in-depth analysis. Results showed that the water vapor permeability and salt leakage ratio of the prepared membranes ranged from 7.76 × 10−8 s/m to 20.19 × 10−8 s/m and from 0.0020 to 0.0151, respectively. The Rf calculated using the Purcell model was in the range from 0.598 μm to 1.690 μm. Since the Rf was relatively small, the prepared membranes can have high LEP (more than 1.13 bar) even at low CA (less than 90.8°). The trade-off relations between the water vapor permeability and the other properties could be confirmed from the results of the prepared membranes. Based on these results, the properties of an efficient MD membrane were suggested as a guideline for the membrane development.