Direct Contact Membrane Distillation Experiments Research Articles

Advancing Sustainable Direct Contact Membrane Distillation: Performance and Stability of Novel Polymer Inclusion Membranes

Novel hydrophobic membranes for direct contact membrane distillation (DCMD) were developed using poly(1,1-difluoroethylene) (PVDF) loaded with the ionic liquid methyltrioctylammonium bis(2-ethylhexyl) phosphate (MTOA-DEHP). These polymer inclusion membranes (PIMs) were fabricated via non-solvent induced phase separation (NIPS). Detailed characterization revealed that MTOA-DEHP significantly enhances the morphological and physicochemical properties of the PIMs. Through a series of DCMD experiments, the membrane’s flux, salt rejection, and stability under varying conditions, including different feed solutions (i.e., synthetic NaCl solution and real seawater from the Atlantic Ocean), were assessed. Our results demonstrate that the PVDF/20 %MTOA-DEHP membrane exhibits superior performance, maintaining consistent transmembrane flux values (8.98 ± 0.61 kg·m−2·h−1) over multiple cycles and achieving high salt rejection rates (>99.9 %). Furthermore, the membrane demonstrates excellent stability, with minimal degradation observed even after prolonged operation. The results affirm the viability of PIMs as a promising avenue for achieving sustainable and efficient desalination via DCMD, particularly in real-world operational scenarios.

Maximizing the wetting resistance of fluorine-free omniphobic membranes for hypersaline wastewater desalination

Membrane distillation (MD) equipped with omniphobic (non-wetting) membranes has found a niche in water reclamation from hypersaline industrial wastewater. Here, we examined the efficacy of non-fluorinated materials as surface coating agents for omniphobic MD membrane fabrication, and identified necessary mechanisms to attain a maximized wetting resistance using fluorine-free materials. We first prepared MD membranes with different surface chemistries using a series of linear alkylsilanes and polydimethylsiloxane (PDMS) as representative fluorine-free, low surface energy materials. Membranes modified with a longer chain alkylsilane exhibited a lower surface energy and demonstrated a greater wetting resistance in direct contact MD experiments using feedwaters of various surface tensions. Despite the nearly identical surface energy measured for the longest alkylsilane and PDMS, PDMS-modified membrane exhibited an extended antiwetting performance as compared to the membrane treated with the longest alkylsilane. To elucidate the source of the distinctive wetting resistance, we examined the nucleation and condensation kinetics on the surfaces with the different surface chemistries via environmental scanning electron microscopy. Our analysis suggests that the membranes treated with long chain alkylsilanes contain surface defects (i.e., hydrophilic regions) whereas the high mobility of the PDMS effectively minimizes the defect exposure, slowing down the condensation and subsequent surface wetting.

Predicting the flux of various volatile fatty acids mixtures in a membrane distillation

This study aims to predict the initial flux of various volatile fatty acids (VFAs) mixtures in direct contact membrane distillation (DCMD), focusing on understanding the interaction between the acids and improving a previous model to better suit the VFAs mixtures. The DCMD experiments were conducted under conditions of feed temperatures of 40, 50, and 60 °C, pH conditions of 3, 4.8, and 6, and VFA concentrations within the feed of 1000–4000 mg/L. The experiments investigated the influence of acids in the VFAs mixture. It was verified whether the ionization fraction of VFAs, even in multi-acid solutions, is proportionate to the flux. The correction coefficient and the overall mass transfer coefficient were introduced to enhance the model that previously used an empirical equation. The correction coefficients were derived, reflecting the interaction between acids and the hydrophobic membrane in the VFAs mixtures. A random factor experiment was conducted to verify the accuracy of the prediction model. Additionally, the sum of each VFA transport can be estimated and expressed using a common water quality measure, chemical oxygen demand. The modeling work can provide further insights into the relationship among the flux, ionization, hydrophobicity, and interactions among acids.

Facile design of amphiphobic PVDF membranes utilizing synergy of pore structure and surface fluorination: A demonstration in membrane distillation

Amphiphobic membranes have attracted much attention in membrane distillation (MD) applications, due to its potential in simultaneously resisting membrane wetting and fouling when treating complex wastewater. The current amphiphobic membranes often require non-trivial design of multilevel re-entrant structure. Here we proposed a new class of amphiphobic membranes without multilevel re-entrant surface, by creating a composite poly(vinylidene fluoride)/polyvinyl alcohol/SiO2 membrane with asymmetric structure and well-controlled small surface pores via the NTIPS method, followed by direct fluorination via chemically binding the –OH of polyvinyl alcohol and low surface energy moieties 1H,1H,2H,2H-perfluorodecyl-triethoxysilane, assisted by 3-aminopropyltriethoxysilane and trimesoyl chloride. The amphiphobic membrane showed superior liquid entry pressure above 5 bar with various simulated saline feeds containing surfactants and organic pollutants. The amphiphobic membrane demonstrated robust anti-wetting and anti-fouling performance through a series of 24-h direct contact MD experiments using the above feeds, presenting stable water flux and high rejection, e.g., flux of 50.6 kg/(m2·h) and rejection >99.9 % with feed containing 3.5 wt% NaCl, 1 mM FA, and 2 mM SDS. The results demonstrated the fabrication of robust amphiphobic membrane through simple procedure utilizing the synergy between pore structure and surface chemistry, inspiring more efforts to design adequate membranes for treating complex wastewater.

Experimental Investigation of the Desalination Process for Direct Contact Membrane Distillation Using Plate and Frame Membrane Module

Through experiments, the effect of membrane material selection and operating conditions on permeate fluxes in direct contact membrane distillation (DCMD) desalination was investigated. The experiment used a plate and frame membrane module, and with nine different hydrophobic porous membranes, a comparative analysis of the desalination performance of 3 wt% NaCl solution was performed. The results of this experiment were compared to find out the effect of different materials, pore sizes and membrane thicknesses on the permeate flux under same operating conditions. Further, a three-factor, three-level orthogonal experiment was designed. The effects of hot-side temperature, hot-side inlet flow and cold-side inlet flow on the permeate flux of PTFE membranes with a pore size of 0.22 μm were investigated when the temperature on the cold side was set at 20 °C. The results showed that in the DCMD experiments, both PTFE and PVDF membranes performed well, and that hot-side inlet temperatures and cold-side inlet flow rates had significant effects on the permeate flux.

Mechanistic insights to the reversibility of membrane wetting in membrane distillation

Membrane wetting is an inevitable challenge in membrane distillation (MD). To overcome this challenge, a few dewetting techniques have been developed, but the reversibility of membrane wetting induced by various contaminants has not been well understood. Herein, we evaluate the reversibility of membrane wetting induced by different types of contaminants and elucidate the underlying mechanisms. From direct contact MD experiments and membrane wettability characterizations, membrane wetting induced by amphiphilic molecules, low-surface-tension liquids, and highly soluble salts can be easily reversed by 20-min DI water rinsing and oven drying, while to reverse membrane wetting induced by sparingly soluble salts, 3-h DI water rinsing and oven drying are required. Based on microscopic imaging and elemental mapping, the membrane wetting processes did not induce severe damage to membrane structure, and thus the hydrophobicity of the membrane can be restored after the contaminants were removed from the membrane. Furthermore, the mechanisms of the removal of different contaminants are elucidated. Specifically, after membrane rinsing and drying, the amphiphilic molecules automatically desorb from the dried membrane due to the disappearance of the underwater hydrophobic-hydrophobic interaction, while the low-surface-tension liquids leave the membrane pores because of evaporation. Additionally, during the membrane rinsing process, the highly soluble salts inside the membrane pores are dissolved, while the sparingly soluble salts detach from the membrane due to the hydrodynamic disturbance. The insights from this study shed lights on the development of effective dewetting techniques for hydrophobic membranes, which benefits not only MD but also membrane contactor applications.

Behavior and removal of microplastics during desalination in a lab-scale direct contact membrane distillation system

The behavior of microplastic particles (MPPs) is explored in a lab-scale direct contact membrane distillation (DCMD) system used to produce drinking water from seawater, while also analyzing the impact of their presence on the process performance. Commercial MPPs (LDPE, PET and uPVC) were first studied by temperature stress (TS) tests (using amber bottles) to mimic the exposure to the high temperatures commonly used in DCMD (ca. 80 °C), which led to uPVC being selected to be tested under different loads and aging degrees in the DCMD system. By analyzing uPVC MPPs samples before and after the TS and DCMD experiments by ATR-FTIR, UV–Visible spectroscopy and SEM, it was concluded that minor aging is expected to occur. High loads (>0.1 g L−1) of uPVC MPPs decreased the DCMD performance in terms of permeate flux (from 32.8 ± 0.3 with filtered seawater to 28.7 ± 0.3 kg m−2 h−1 with a load of 0.2 g L−1), although no signs of inferior treated water quality were found based on the conductivity and salinity results. The removal of MPPs (size > 1.2 μm) in DCMD applications with seawater was analyzed by μRaman, with all the results suggesting very high removal efficiencies (≥99 %).

Patterned dense Janus membranes with simultaneously robust fouling, wetting and scaling resistance for membrane distillation

Membrane fouling, wetting and scaling are three prominent challenges that severely hinder the practical applications of membrane distillation (MD). Herein, polyamide/polyvinylidene fluoride (PA/PVDF) Janus membrane comprising a hydrophobic PVDF substrate and a patterned dense PA layer by reverse interfacial polymerization (R-IP) was developed. Direct contact MD experiments demonstrated that PA/PVDF Janus membrane could exhibit simultaneously superior resistance towards surfactant-induced wetting, oil-induced fouling and gypsum-induced scaling without compromising flux. Importantly, the size-sieving effect, rather than the breakthrough pressure of the membrane, was revealed as the critical factor that probably endowed its resistance to wetting. Furthermore, a unique possible anti-scaling mechanism was unveiled. The superhydrophilic patterned dense PA layer with strong salt rejection capability not only prevented scale-precursor ions from intruding the substrate but also resulted in the high surface interfacial energy that inhibited the adhesion and growth of gypsum on the membrane surface, while its relatively low surface -COOH density benefited from R-IP process further ensured the membrane with a low scaling propensity. This study shall provide new insights and novel strategies in designing high-performance MD membranes and enable robust applications of MD facing the challenges of membrane fouling, wetting and scaling.

Polymer Inclusion membranes with long term-stability in desalination via membrane distillation

This work explored the feasibility of implement of polymer inclusion membranes (PIMs) in the desalination industry. The methyltrioctylammonium oleate (MTOAO) was imbedded in a blend of polyvinylidene fluoride (PVDF) and polysulfone (PSU) prepared via Non-Solvent Induced Phase Separation (NIPS). The investigation encompassed the analysis of the chemical compositions and morphologies of the novel PIMs, with the objective of identifying the optimal MTOAO concentration within the PVDF/PSU matrix. The membrane characterizations revealed that addition of 33% of MTOAO in PVDF/PSU significantly increased the porosity (from 73.4% to 89.8%) and conferred a highly-hydrophobic character (WCA=123°). In direct contact membrane distillation experiments, PVDF/PSU doped with 33% of MTOAO resulted in a flux of 21.9 kg·m−2·h−1 and a 99.98% NaCl rejection where the salinity and temperature were 30 g/L and 70 °C, respectively. Remarkably, the inclusion of the MTOAO led 625% in transmembrane flux of. Moreover, PVDF/PSU load with 33% of MTOAO demonstrated a long-term stable performance over 10 weeks, whereas the PVDF/PSU membrane exhibited a poor stability in terms of salt rejection because of pore wetting caused by the poor hydrophobic character. Lastly, the performance of the process was positively assessed by desalting seawater from Atlantic Ocean.

Fouling characterization and treatment of water reuse concentrate with membrane distillation: Do organics really matter

Membrane distillation (MD) for the treatment of concentrated brines has been limited in part by membrane fouling, resulting in subsequent flux decline and membrane wetting. This study provides new insight into the identification of fouling and scaling mechanisms and pretreatment strategies for mitigating flux decline with MD treatment of water reuse reverse osmosis concentrate (ROC). Bench-scale direct contact MD experiments were performed with untreated and pretreated ROC. Biological activated carbon (BAC), chemical water softening, or fluidized bed crystallization reactor coupled with ion exchange (FBCR-IX) were selected as pretreatment strategies to isolate the effects of organic fouling and calcium scaling. Organic and inorganic compounds were analyzed by high-performance liquid chromatography (HPLC) and inductively coupled plasma mass spectrometry (ICP-MS). Calcium ions were found to be the major contributor to flux decline despite the high organic content in the ROC. Minimal organic fouling is likely because the organic matter in the ROC is hydrophilic, limiting hydrophobic-hydrophobic interactions between the organics and the membrane. Furthermore, the water flux declined by 63 % after removing organic compounds by BAC pretreatment, with 60 % of the calcium mass precipitating from the solution. Whereas, the water flux remained constant after removing multivalent ions with fluidized bed crystallization. Cleaning the membrane by acid washing and temperature reversal recovered 73 % and 12 % of the water flux, respectively. The analyses outlined in this study can assist in selecting appropriate fouling and scaling mitigation strategies for water reuse ROC and a wide range of feed solutions used in MD applications.

UiO-66-NH2/PVA composite Janus membrane with a dense hydrophilic surface layer for strong resistance to fouling and wettability in membrane distillation

Polytetrafluoroethylene (PTFE) hydrophobic membranes used in direct contact membrane distillation (DCMD) processes were prone to fouling and wetting by oil and surfactants, which hampered their broad industrial application. In this work, a new Janus composite membrane was developed by modifying UiO-66-NH2 with polyvinyl alcohol (PVA) as a cross-linking agent and immobilizing the modified UiO-66-NH2 on the PTFE membrane by vacuum filtration. By tuning the ratio of PVA to MOF, an optimally proportioned Janus membrane was obtained, and its performance was tested in DCMD experiments through the use of a sodium chloride feed solution containing hexadecane and sodium dodecyl sulfate. The top layer of this Janus membrane exhibited underwater oleophobicity (130.1 ± 0.96°) and hydrophilicity in the air (54.0 ± 0.92°), while the bottom layer is hydrophobic (132.3 ± 0.97°). Compared to the pristine PTFE membrane, the modified Janus membrane demonstrated promising resistance to both membrane fouling and wetting of oils and surfactants. The desalination rate of 99.99% was still achieved within 24 h with the conductivity remaining lower than 10 μS/cm and the flux maintained at 21.3 L/(m2·h). As a result, the Janus composite membrane developed in this study was shown to be a prospective alternative for treating wastewater containing oily substances and surfactants in a resourceful manner.

Omniphobic surface modification of silica sand ceramic hollow fiber membrane for desalination via direct contact membrane distillation

Omniphobic silica sand ceramic hollow fiber membrane (SCHFM) was successfully fabricated for the reduction of the wetting and fouling in direct contact membrane distillation (DCMD). Surface functionalization of the laboratory spun SCHFM was performed following a series of steps 1) deposition of spherical silica nanoparticles (SiNPs), 2) fluorination by 1H,1H,2H,2H perfluorodecyltriethoxysilane (FAS17) and 3) poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) coating to glue SiNPs to the SCHFM surface. Initially, the pristine SCHFM was hydrophilic, but when the surface modification was fully applied the hollow fiber became omniphobic, exhibiting high contact angles for all tested liquids of different surface tensions, such as distilled water, red palm oil and ethanol. For example, one of the surface modified hollow fibers, called SiNPs-FAS-PVDF/FAS, was superomniphobic toward water (167°) and red palm oil (157°), and near superomniphobic toward ethanol (146.1°). As for the DCMD experiments with the feed solution containing 35 g/L NaCl and 10 mg/L humic acid, two of the surface modified hollow fibers, called SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, showed high anti-fouling and anti-wetting resistance during the entire DCMD operation period of 400 min with little flux reduction and no observable increase in the conductivity of the permeate stream. The anti-fouling resistance of these hollow fibers was further evidenced by the cleanness of the surface in the micrographic picture taken after the DCMD run. The omniphobicity of these hollow fibers is ascribed to the combined effects of surface tension decrease by the FAS17 fluorination and the formation of the re-entrant structure by the densely packed SiNPs. With reasonably high fluxes of 35.1 kg/m2∙h and 43.6 kg/m2∙h, for SiNPs-PVDF/FAS and SiNPs-FAS-PVDF/FAS, respectively, and 100% of salt rejection, these hollow fibers have potential for seawater desalination by DCMD.

Performance and membrane fouling mitigation for bio-treated coking wastewater treatment via membrane distillation: Effect of pre-treatment

A large amount of coking wastewater is generated from the coal coking industry in China. As the coking wastewater containing high concentration of hazardous substances is harmful to the environment and health, it is necessary to be properly treated before being discharged. In this work, direct contact membrane distillation (DCMD) is explored to deeply treat the real bio-treated coking wastewater (BCW). DCMD system performance and membrane fouling are evaluated. Results showed that the serious membrane fouling caused by the brown fulvic and humic acid-like organics in the BCW occurred, resulting in permeate flux decline sharply. Various pre-treatment approaches including coagulation, ozonation and electrocoagulation (EC) were selected to alleviate membrane fouling. It was noteworthy that the long-time steady flux and low permeate conductivity was kept during DCMD filtration of the BCW pre-treated by EC. Moreover, the membrane surface remained clean at the conclusion of the DCMD experiment. In contrast, permeate flux declined quickly due to membrane pores plugging during DCMD treatment of the BCW pre-treated via ozonation. More pollutants penetrated into the permeate, which was caused by the removal of humic acid from the fouling layer during DCMD treatment of the BCW pre-treated via coagulation. Our work suggested that DCMD process combined with EC can be used as an advanced treatment to treat the BCW effectively.

Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance

Although membrane distillation (MD) has been identified as a promising technology to treat hypersaline wastewaters, its practical applications face two prominent challenges: membrane wetting and fouling. Herein, we report a facile and scalable approach for fabricating a Janus MD membrane comprising a dense polyvinyl alcohol (PVA) surface layer and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate. By testing the Janus membrane in direct contact MD experiments using feeds containing a sodium dodecyl sulfate (SDS) surfactant or/and mineral oil, we demonstrated that the dense Janus membrane can simultaneously resist wetting and fouling. This method represents the simplest approach to date for fabricating MD membranes with simultaneous wetting and fouling resistance. Importantly, we also unveil the mechanism of wetting resistance by measuring the breakthrough pressure and surfactant permeation (through the PVA layer) and found that wetting resistance imparted by a dense hydrophilic layer is attributable to capillary pressure. This new insight will potentially change the paradigm of fabricating wetting-resistant membranes and enable robust applications of MD and other membrane contactor processes facing challenges of pore wetting or/and membrane fouling.

Novel MIL101(Fe) impregnated poly(vinylidene fluoride-co-hexafluoropropylene) mixed matrix membranes for dye removal from textile industry wastewater

In this work, the potential of direct contact membrane distillation (DCMD) technology using novel MIL101(Fe) impregnated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) mixed matrix membranes has been explored for dye removal from the textile industry wastewater. The influence of MIL101(Fe) impregnation on membrane performance was investigated by various morphological (SEM, AFM) and spectral techniques (XRD, FT-IR) along with transmembrane flux and dye rejection efficiency. Membrane performance over the range of feed temperature and the flow rate has also been explored for its applicability and suitability. The permeate flux grows exponentially with the feed temperature, whereas the dye rejection decreased with increased feed temperature. A decrease in boundary layer thickness at the membrane surface resulted in a linear increase in permeate flux with flow rate. The rejection was negligibly affected by the increase in flow rate. Among the fabricated membranes, M-M(0.5) (20% PVDF-HFP/0.5% MIL101(Fe)) membrane was adjudged the most suitable. Response surface methodology was used to investigate the influence of feed temperature and flow rate on permeate flux and dye rejection to optimize membrane performance. The optimized process parameters obtained were feed temperature: 353 K, flow rate: 40 l h−1. For these optimized input process parameters, the predicted responses were permeate flux: 6.75 l m−2 h−1 and dye rejection: 98.14%. Using the optimized parameters obtained from the RSM study, 24 h long run DCMD experiment was done to study membrane fouling on membranes using SEM and FT-IR analysis. The fabricated membranes showed great potential to treat real textile industry wastewater.

Photocatalytic TiO2 incorporated PVDF-co-HFP UV-cleaning mixed matrix membranes for effective removal of dyes from synthetic wastewater system via membrane distillation

We report membranes with UV-cleaning properties by incorporating photocatalytic porous TiO2 sheets (PTS) (0–5 wt%) in the PVDF-co-HFP matrix. PTS were synthesized using controllable large-scale synthesis protocols by spray drying followed by calcination. These membranes were assessed for the removal of Congo red (CR) and methylene blue (MB) from synthetic textile industry wastewater system (CR: 100 mg l−1 + MB: 100 mg l−1 + 4% NaCl) via direct contact membrane distillation (DCMD) configuration and their UV-cleaning properties. The PTS were characterized by FE-SEM, TEM and different spectral techniques, while membranes were assessed by surface morphology (FE-SEM, AFM), thermal stability, hydrophobicity, permeate flux, and dye rejection performance. The surface morphology study of PTS revealed its micro-sized sheet with the porous surface. With increased PTS concentration in the membrane matrix, the flux and dye rejection of mixed matrix membranes improved. Suitably optimized PT3 (3 wt% PTS in PVDF-co-HFP matrix) membrane showed ~100% dye removal efficiency (MB and CR) and 6.1 kg m−2 h−1 vapour flux. Long run (5 days) DCMD experiments showed >91% flux recovery ratio (FRR) after UV-cleaning. This study suggests the suitability of the prepared membranes to treat textile wastewater and their UV-cleaning properties.

Improvement of nanostructured electrospun membranes for desalination by membrane distillation technology

A systematic study is carried out to determine the optimum electrospinning preparation condition to prepare an adequate electrospun nanofibrous membrane (ENM) for direct contact membrane distillation (DCMD). A structural properties investigation of ENM was carried out because of the significant impact of its architectural structure, nanofiber diameter, inter-fiber space and ENM thickness, on DCMD performance. The morphology, hydrophobicity, mechanical properties, crystallinity and DCMD desalination were investigated. A long-term DCMD experiment (100h) was carried out using 30 g/L NaCl aqueous solution, both in the feed and permeate side of the optimum ENM membrane to evaluate its potential to produce drinkable water in case of lack of distilled water, for instance in a remote area, emergency situation, and/or portable system. In this case, drinkable water could be produced after 28 h with a permeate flux of 57.5 kg/m2.h and a salt rejection factor greater than 99.9%. Another long-term DCMD experiment (65 h) was conducted using 30 g/L NaCl aquesous solution as feed but at a higher temperature and distilled water as permeate to evaluate the desalination stability, wettability and scaling of the optimum ENM. A permeate flux of 58.5 kg/m2.h was obtained with a salt rejection factor greater than 99.9%.

Evaluation of direct contact membrane distillation coupled with fractionation and ozonation for the treatment of textile effluent

Direct contact membrane distillation (DCMD) coupled with fractionation or ozonation as a pretreatment method was investigated for the treatment of combined textile effluent. The stand-alone DCMD process has difficulty in directly treating the effluent due to the wetting issue, in particular at higher feed temperatures. Fractionation was conducted to 21 L combined effluent for 24 h. After the pretreatment, total organic carbon (TOC) and conductivity decreased by 47 % and 28 %, respectively, but an additional 5% concentrate was also generated. A long-term DCMD experiment was conducted to treat the fractionated effluent for 50 h and a high water recovery of 98 % was achieved. The optimum recovery of 95 % was identified to maintain stable performance of membrane. Ozonation was used to treat 550 mL combined eluent in batch mode. The TOC removal increased with time and reached the plateau after 1 min. The TOC was reduced by 38 %, while the colour of the combined effluent became lighter after 1 min treatment. As the temperature increased, longer ozonation time was required to inhibit the membrane wetting. For the feed inlet temperatures of 50 °C and 70 °C, the ozonation times of 40 s and 60 s were able to suppress the membrane wetting in the short-term tests. For the fractionated effluent, near zero discharge (water recovery of 98 %) could be achieved without serious wetting issue. To avoid membrane fouling issues, 95 % water recovery is recommended. For the 40 s ozonation pretreated effluent, DCMD could only achieve 38 % of the feed recovery before occurrence of wetting.

Elucidating the Trade-off between Membrane Wetting Resistance and Water Vapor Flux in Membrane Distillation.

Membrane distillation (MD) has been receiving considerable attention as a promising technology for desalinating industrial wastewaters. While hydrophobic membranes are essential for the process, increasing membrane surface hydrophobicity generally leads to the reduction of water vapor flux. In this study, we investigate the mechanisms responsible for this trade-off relation in MD. We prepared hydrophobic membranes with different degrees of wetting resistance through coating quartz fiber membranes with a series of alkylsilane molecules while preserving the fiber structures. A trade-off between wetting resistance and water vapor flux was observed in direct-contact MD experiments, with the least-wetting-resistant membrane exhibiting twice as high vapor flux as the most wetting-resistant membrane. Electrochemical impedance analysis, combined with fluorescence microscopy, elucidated that a lower wetting resistance (still water-repelling) allows deeper penetration of the liquid-air interfaces into the membrane, resulting in an increased interfacial area and therefore a larger evaporative vapor flux. Finally, we performed osmotic distillation experiments employing anodized alumina membranes that possess straight nanopores with different degrees of wetting resistance, observed no trade-off, and substantiated this proposed mechanism. Our study provides a guideline to tailor the membrane surface wettability to ensure stable MD operations while maximizing the water recovery rate.

Novel Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process for produced water desalination

In this study, an innovative Janus composite hollow fiber membrane-based direct contact membrane distillation (DCMD) process was boosted for desalination of actual produced water with elevated total dissolved solids (TDS) of 154,220 mg/L. The Janus composite hollow fiber membrane (Janus-HFM) was characterized with a thin and hydrophobic polyvinylidene difluoride (PVDF) and superhydrophobic silica nanoparticles (Si-R) outer layer, and a thick porous and hydrophilic PVDF and polyethylene glycol (PEG) inner layer. Compared to the neat PVDF hollow fiber membrane, the Janus-HFM showed both increased permeate water flux and improved energy efficiency with more than 99.99% salt rejection. The long-term desalination performance of the Janus-HFM was investigated via 200 h of continuous DCMD experiment. The results showed that the permeate water flux declined from 25.41 kg/m2h to 15.21 kg/m2h, and the salt rejection was 98.4% at the end of the operation. It was found that both scales and dissolved organic matters induced foulants accumulated at the feed side of the membrane due to the complex composition of the produced water. Nonetheless, the desalination performance decline can be effectively inhibited from 40% to 9.7% with a simple physical cleaning process by using the permeate water that recovered in the DCMD process.