Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process

A study on enhancement of wetting resistance in membrane distillation (MD) by engineering the physical morphology and modifying the surface energy through chemical fluorination

Department of Urban and Environmental Engineering (Environmental Science and Engineering)Currently, water treatment is required to provide clean water worldwide. Among many studies for water treatment, membrane distillation (MD) is one of the emerging technologies. The MD is a process that utilizes the temperature difference between the high-temperature feed solution and the low-temperature permeate solution, and the vapor generated due to this temperature difference passes through the membrane and then finally condenses to become a high-quality distillate. Less thermal energy is required for the generation of vapor because of the temperature difference, and almost 100% of non-volatile contaminants can be removed. However, if the generated vapor condenses inside of the pores, the membrane becomes wet. After the pores are wet, the feed solution can pass directly, reducing the removal rate and reducing the lifetime of the membrane. To solve this wetting problem, many studies are focus on the hydrophobicity of the membrane. For this, many types of hydrophobic polymers were applied. Mainly used hydrophobic polymers include polypropylene (PP), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and polyvinylidene fluoride (PVDF). However, there is a limit to the hydrophobicity a material can exhibit. Therefore, various studies have been conducted to improve the hydrophobicity of membranes to overcome the limitation. However, various previous studies still need improvement in performance decline, fouling, and wetting issues. First, coating methods that have been widely used to date have weaknesses such as poor performance and poor stability. Next, as a new method to increase hydrophobicity, the method of increasing hydrophobicity by applying a pattern on the surface is in the spotlight. However, this method has a problem in that it is difficult to make a pattern and fouling easily occurs due to an increase in roughness. To address the performance decline and poor stability in the coating method, the PVDF membrane was modified through four steps: pore expansion by a plasma treatment, hydroxylation of the membrane by the Fenton reaction, generation, and growth of nanoparticles (NPs) on the hydroxylated functional groups in pores, and hydrophobic modification using fluorine chemical. The membranes modified by the methods proposed in this study did not lose their hydrophobicity and maintained the flux over a significantly longer period MD test. The PVDF membrane modified by hydrophobic NPs attached inside enlarged pores exhibited a minimized flux reduction and significantly higher antiwetting stability. Sonication was also applied to test the stability of the NPs grown from the PVDF membrane. This result support that NPs grown from the hydroxyl functional group on PVDF enhance the stability. For overcoming the further performance change, a lot of research is being conducted on patterned membranes as a new technology, but it has the disadvantages such as difficulty to prepare a patterned membrane and fouling issue because of pattern on the surface. To overcome the issues in patterning studies, the template was used for easy fabrication of patterned membrane, and low surface energy was achieved polymerization of hydrophobic chemical on the membrane. To prepare the pattern surface, a polyvinylidene fluoride-co-chlorotrifluoroethylene (PVDF-CTFE) membrane was poured on a template having a specific structure. It has been found that patterned membranes with hierarchical microstructures are more hydrophobic than those with flat surfaces. It was also confirmed that the patterned membranes have high resistance in wetting in direct contact membrane distillation (DCMD) showing stable performance over a longer period compared to membranes with flat surfaces. However, the patterned membrane has the problem of rapid performance decline during fouling testing due to the deposition of foulants. In this study, the fouling issue was solved through polymerization with 1H, 1H-perfluorooctyl methacrylate (FOMA) which makes membrane have low surface energy. After surface polymerization with FOMA, it was confirmed that the superhydrophobic patterned membrane showed any performance decline in the DCMD process with foulants such as humic acid (HA), alginic acid (AA), and bovine serum albumin (BSA). In addition, it was confirmed that it did not get wet for more than 7 days in the actual DCMD process due to the higher hydrophobicity due to the lower surface energy as well as the rough surface due to the patterned surface. For the last, a new approach to prevent wetting of the membrane was investigated. As the reason for the wetting in the MD process, the vapor generated by the temperature difference between feed and permeate solution is condensed inside of the pores. To prevent this phenomenon, as a next-generation technology to prevent wetting, the internal temperature of the membrane increased by heating to prevent the vapor from condensing inside the pores. To achieve the heating membrane, a PVDF membrane was prepared using a copper mesh as a substrate which has good thermal conductivity, and it was possible to prevent wetting by transferring heat during the MD. Sweep gas MD (SGMD) was used to confirm the prevention of wetting through heating of the membrane. In the case of proceeding without applying heat, it was found that the membrane gets wetted so that feed solution passes through dramatically, whereas when the temperature of the membrane was increased by applying heat, it was confirmed that the membrane was not wetted over 2500 min. Furthermore, to enhance the thermal conductivity of the membrane, carbon nanofiber (CNF) was added into the dope solution before fabrication. With CNF, heat can be transferred more efficiently so that wetting could be prevented over 3500 min in SGMD.ope

PHYSICAL AND CHEMICAL MODIFICATION OF POLYMER TO IMPROVE HYDROPHOBICTY IN MEMBRANE DISTILLATION

Department of Urban and Environmental Engineering(Environmental Science and Engineering)As a water treatment technology, the membrane distillation (MD) method which can operate at lower temperature than reverse osmosis (RO) and can recover concentrated water generated from RO with high recovery rate has been studied. Membrane distillation is a technology that allows the vaporized water pass through membrane pores and collect pure water vapor so that many types of research has been studied. In this study, a hollow fiber was made using Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE), not usually used material, and compared with Poly(vinylidene fluoride) (PVDF). There is four type of membrane distillation methods. Among them, the vacuum membrane distillation method which has the highest flux method was studied in this research. In the case of PVDF-CTFE, it was confirmed that it has a higher flux because it has a macrovoids at lumen side than PVDF. However, since the mechanical strength is weak, there is breaking problem when operating long term experiment or high-pressure experiment. To solve the breaking problem, the mechanical strength was increased by using thermally induced phase separation (TIPs) method, which is a high-concentration spinning method, or by spinning hollow fibers into a dual-layer structure in which PVDF is spun out as a support layer. In the case of TIPS, the mechanical strength was increased, but the flux was found to be low. As can be seen from the cross-sectional images of the hollow fiber, in the case of the dual-layer membrane, the sponge-like structure at the middle was eliminated, and the flux was improved. Furthermore, to prevent the wetting phenomenon of membranes, physical and chemical modification experiments were carried out to increase hydrophobicity. First, a hydrophobic hollow fiber was prepared by physically blending with Polytetrafluoroethylene (PTFE), which is a highly hydrophobic material, in a dope solution, and physical properties were confirmed. PTFE was added as an additive to evaluate the structural change and performance. As a method of increasing the hydrophobicity using the chemical grafting method, attaching a hydrophobic chemical to the surface of the polymer using the atom transfer radical polymerization (ATRP) method was used. The chemical method was used to evaluate the performance by confirming the difference according to the length of the material with different reaction time. Both methods were confirmed physical property changes via FTIR and confirmed the increase in hydrophobicity through contact angle and liquid entry pressure (LEP). Through the physical blending method, it was confirmed that the contact angle was improved by the addition of the hydrophobic additive. Also, the cross-section of the hollow fiber was confirmed by SEM, confirming that the pores of the membrane became larger at lumen side. However, if more than 15 wt% of PTFE is contained, the sponge-like structure is formed again in the middle of the hollow fiber, and it becomes thicker depending on the PTFE content. As a result of this structural change, the flux was affected, and the flux was improved up to 10 wt%. Through ATRP, which is a chemical modification method, it was confirmed that a new peak of FTIR and XPS appeared on the polymer by grafting hydrophobic material. After grafting, membrane structural change was checked by SEM that there was no change of pore, the hydrophobicity was confirmed by the contact angle that there was increasing contact angle value, the flux was confirmed through the membrane distillation method that was no improvement. Increasing of hydrophobicity contributes to preventing the wetting problem that checked through LEP. With increasing reaction time, LEP value increased until 5 bars. Finally, the performance evaluation was carried out through the hollow fiber obtained by combining the two methods. The 10 wt% PTFE blended hollow fiber with the highest flux was prepared, and ATRP was carried out for 25 hours. In the case of the hollow fiber obtained, the flux is maintained as in the case of blending 10 wt% of PTFE, and the LEP is further increased which can be confirmed that the wetting phenomenon can be prevented.ope

Investigation of a Gas Hydrate Dissociation-Energy-Based Quick-Freezing Treatment for Sludge Cell Lysis and Dewatering

A gas Hydrate dissociation-energy-based Quick-Freezing treatment (HbQF) was applied for sewage sludge cell rupture and dewatering. Carbon dioxide (CO2) and water (H2O) molecules in sewage create CO2 gas hydrates, and subsequently the sludge rapidly freezes by releasing the applied pressure. Cell rupture was observed through a viability evaluation and leachate analysis. The decreased ratios of live cell to dead cells, increased osmotic pressure, and increased conductivity showed cell lysis and release of electrolytes via HbQF. The change in physicochemical properties of the samples resulting from HbQF was investigated via zeta potential measurement, rheological analysis, and particle size measurement. The HbQF treatment could not reduce the sludge water content when combined with membrane-based filtration post-treatment because of the pore blocking of fractured and lysed cells; however, it could achieve sludge microbial cell rupture, disinfection, and floc disintegration, causing enhanced reduction of water content and enhanced dewatering capability via a sedimentation post process. Furthermore, the organic-rich materials released by the cell rupture, investigated via the analysis of protein, polysaccharide, total organic carbon, and total nitrogen, may be returned to a biological treatment system or (an) aerobic digester to increase treatment efficiency

Designing Durable Anti-wetting and Anti-biofouling Membranes for Improved MD Performance

Stability of long-term anti-biofouling and superhydrophobicity features is doubtful because of wetting as well as poor adhesion of ultrahydrophobic agents onto the membrane. In this study, a simple approach is presented to fabricate reusable anti-biofouling and superhydrophobic PVDF membranes for membrane distillation (MD) application. Further, PVA-co-poly(MAA) chemical agent is utilized in order to enhance the stability of TiO2 nanoparticles. Poly(MAA), a functionalization agent that can chemically link TiO2 nanomaterials (n-TiO2) and polymer substrate, which has been further elaborated in terms of retained anti-biofouling property. The composite coating exhibits high resistance to wetting and biofouling during long term operation in MD process

Acid-catalyzed hydrolysis of semi-aromatic polyamide NF membrane and its application to water softening and antibiotics enrichment

The effect of post-treatment by acid-catalyzed hydrolysis of a commercial NE70 semi-aromatic polyamide (PA) membrane was systemically investigated to determine feasibility of use in water softening and antibiotic enrichment applications. The surface of a post-treated PA membrane was characterized using various analytical tools: SEM (Scanning Electron Microscopy) for surface morphology, ATR-FTIR (Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy) for chemical bonds, contact angle for hydrophilicity of membrane surface, and electrophoretic light scattering spectrophotometer for surface charge of membrane surface. Conversion of amide groups to amine and carboxyl groups by post-treatment increased hydrophilicity and absolute value of surface charge as well as pore size and molecular weight cut off (MWCO) value. Post-treated membrane under optimal condition showed enhancement of water flux (???10%) as well as ???20% ideal selectivity (Na+/Mg2+) for water softening using a single electrolyte solution. In addition, mixture selectivity (Na+/Mg2+) using a mixture solution at pH 3 was also improved ???2.6 times. Post-treated membranes at pH 0.25 for 7 and 14 days as optimization points were also applied to enrichment of antibiotics which are erythromycin (ERY) and vancomycin (Van). Optimized post-treatment membranes showed higher water flux and lower NaCl rejection as well as competitive rejection of antibiotics when compared to virgin NE70 membrane or other commercial/fabricated membranes. The approach to post-treatment of semi-aromatic membrane by the acid-catalyzed hydrolysis method can be utilized as a multipurpose usage in the future depending on characteristics of the target compound (e.g. surface charge (positive/negative) or size diversity)

Surface modification of polyvinylidene fluoride membrane for enhanced wetting resistance

Modifications of polyvinylidene fluoride (PVDF) membranes were carried out to improve both hydrophobicity and stability through four steps: pore expansion by a plasma treatment, hydroxylation of the membrane by the Fenton reaction, generation and growth of microparticles (MPs) on the hydroxylated functional groups in pores, and hydrophobic modification. The membranes modified by the methods proposed in this study did not lose their hydrophobicity and maintained the flux over a significantly longer period. The PVDF membrane modified by hydrophobic MPs attached inside enlarged pores exhibited a minimized flux reduction and significantly higher antiwetting stability

Degradation of full aromatic polyamide NF membrane by sulfuric acid and hydrogen halides: Change of the surface/permeability properties

We investigated the effects of both sulfuric acid (pH 0 to 2) and hydrogen halides (pH 0) on the physical, chemical, and performance properties of full aromatic nanofiltration (NF) polyamide (PA) NE90 membrane. Surface characterizations of the degraded membranes were conducted by Scanning Electron Microscopy (SEM), Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), goniometer, and zeta potential analyzer. No noticeable changes were observed in the surface properties of the membrane exposed to sulfuric acid; however, the permeable characteristics were changed due to the distortion of hydrogen bonding from additionally generated proton bridge for O-protonation and the formation of tetrahedral structure for N-protonation. On the other hand, the membrane's physico-chemical properties were much affected by hydrogen halides compared with sulfuric acid. Amide peaks N-H bending at 1541 cm ???1 and C=O stretching at 1663 cm ???1 in ATR-FTIR were reduced because of the reaction of the PA with halogens produced by oxidation of hydrogen halides. Molecular halogen gases (Cl 2 , Br 2 , I 2 ) generated in the reaction bottle were also analyzed by GC/MS during exposure of the membrane to various types of acids. The increment in halogenation on PA was in the order HCI, HBr, and HI, and it was the same as the order of temporary dipole moment from the effect of molecular size. Water flux after exposure to hydrogen halides was severely decreased due to broken hydrogen bonding by halogenation. Investigation of sulfuric acid and hydrogen halides on the change of the physico-chemical characteristics in the NE90 can be utilized when full aromatic NF membrane is applied to treat/recycle several industrial processes, which include sulfuric acid or hydrogen halides

The potential of CO2 HBID(Hydrate-Induced Ice Desalination) process for the pretreatment of RO(Reverse Osmosis) desalination

????????? ????????????????????? ???????????? CO2 ??????????????????????????? ????????? ??????????????????(HIID)????????? ????????? ????????? ???????????? ????????? ???????????????. CO2 ??????????????????????????? ????????? CO2????????? GUEST??? ???????????? ??????????????? ??????????????? ????????? ????????????, ??????, ????????? ????????? ???????????? ???????????? ?????? ??????????????? ???????????? ????????? ???????????? ??????. HIID????????? ??? ??????????????? ??????????????? ????????? ???????????????.??? ???????????? ????????? 225psi??? ??????????????? 6?????? ?????? ????????? ??????????????? 14LMH??????, ???????????? 98.5-99%??? ???????????????. ??? ???????????? HIID????????? ?????????????????? ???????????? ?????????????????? ??????????????????, ????????? ????????????????????? ???????????? ????????? ?????????????????? ???????????? ?????????. ?????? ?????? ??????????????? ??????????????? ???????????? ???????????????

Chemical and surface engineered superhydrophobic patterned membrane with enhanced wetting and fouling resistance for improved membrane distillation performance

Application of membrane distillation (MD) is still in its emerging stage due to membrane wetting and fouling issues. In this study, an anti-wetting and anti-fouling superhydrophobic patterned membrane was prepared utilizing patterned templet surface and subsequent chemical modifications with fluorine-based polymer. A uniform patterned polyvinylidene fluoride-co-chlorotrifluoroethylene (PVDF-CTFE) membrane was prepared using a template having a specific surface structure. It was found that the patterned membrane with a hierarchical microstructure was more hydrophobic than that with a flat surface. Long-term performance of the patterned membrane was determined through direct contact membrane distillation (DCMD). Results showed that such patterned membrane exhibited wetting resistance for a longer time compared to a pristine membrane. However, the patterned membrane showed rapid flux decline during a fouling test due to deposition of foulants such as humic acid (HA), alginate acid (AA), and bovine serum albumin (BSA). To overcome the fouling issue, a patterned membrane was chemically modified with 1H, 1H-perfluorooctyl methacrylate (FOMA) known to possess a low surface energy. After surface modification with FOMA, the superhydrophobic patterned membrane showed good stability in terms of water flux and salt rejection for more than 7 days in DCMD without wetting or fouling issue. Results of this study indicates the capability of a superhydrophobic patterned MD membrane for generating maximum water flux with excellent anti-fouling and wetting resistance properties