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

Abstract

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