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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.31 No.1 pp.16-34
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2021.31.1.16

Graphene Oxide Incorporated Antifouling Thin Film Composite Membrane for Application in Desalination and Clean Energy Harvesting Processes

Daewon Lee, Rajkumar Patel†
Energy and Environmental Science and Engineering (EESE), Integrated Science and Engineering Division (ISED), Underwood International College, Yonsei University, Songdogwahak-ro, Yeonsu-gu, Incheon 21983, South Korea
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr, http://orcid.org/0000-0002-3820-141X)
January 13, 2021 ; January 17, 2021 ; January 21, 2021

Abstract


Water supplies are decreasing in comparison to increasing clean water demands. Using nanofiltration is one of the most effective and economical methods to meet the need for clean water. Common methods for desalination are reverse osmosis and nanofiltration. However, pristine membranes lack the essential features which are, stability, economic efficiency, antibacterial and antifouling performances. To enhance the properties of the pristine membranes, graphene oxide (GO) is a promising and widely researched material for thin film composites (TFC) membrane due to their characteristics that help improve the hydrophilicity and anti-fouling properties. Modification of the membrane can be done on different layers. The thin film composite membranes are composed of three different layers, the top filtering active thin polyamide (PA) layer, supporting porous layer, and supporting fabric. Forward osmosis (FO) process is yet another energy efficient desalination process, but its efficiency is affected due to biofouling. Incorporation of GO enhance antibacterial properties leading to reduction of biofilm formation on the membrane surface. Pressure retarded osmosis (PRO) is an excellent process to generate clean energy from sea water and the biofouling of membrane is reduced by introduction of GO into the active layer of the TFC membrane. Different modifications on the membranes are being researched, each modification with its own advantages and disadvantages. In this review, modifications of nanofiltration membranes and their composites, characterization, and performances are discussed.


nanofiltration , pressure retarded osmosis , desalination , antifouling

해수담수화와 청정 에너지 하베스팅을 위한 산화 그래핀 결합 합성 폴리머 방오 멤브레인

이 대 원, 라즈쿠마 파텔†
연세대학교 언더우드국제대학 융합과학공학부 에너지환경과학공학

초록


물 공급은 늘어나는 담수 수요와 다르게 줄어들고 있다. 담수의 수요를 충당하기 위해서 나노여과법은 가장 효율 적이고 경제적인 방법이라고 할 수 있다. 해수담수화를 위한 나노여과법의 일반적인 방법으로는 나노여과 멤브레인을 이용한 역삼투압 방식이다. 하지만 기존의 멤브레인들은 주요 특성인 안정성, 경제성, 그리고 살균 및 방오특성이 부족하다. 기존의 나노여과 멤브레인을 향상시키기 위해서 친수성과 방오성이 높은 흑연 산화물이 가장 향상성이 높으며 널리 연구되고 있는 재료이다. 멤브레인 변형은 다른 레이어에 적용될 수 있다. 얇은 막으로 이루어진 멤브레인은 다른 세 레이어로 구성되어 있 다, 표면의 폴리아미드 레이어, 기공 레이어, 그리고 전체적인 구조를 구성하는 지원 직물이다. 정삼투압 토한 에너지 효율적 인 해수담수화 방식이지만 효율이 생물 오염 때문에 떨어진다. 산화그래핀 결합은 향균 기능을 향상할 수 있으며 멤브레인 표면에 바이오필름 생성을 억제할 수 있다. 압력지연삼투는 해수에서 청정에너지를 발전시키는 최고의 방법 중 하나이다. 멤 브레인의 생물 오염은 합성 폴리머 멤브레인의 합성 레이어에 산화 그래핀을 합성하여 줄일 수 있다. 나노여과 멤브레인을 개량하는 여러 연구가 각자의 장단점을 가지고 이루어지고 있다. 이 보고서는 나노여과 멤브레인의 개량, 성질, 그리고 성능 에 대해 논의한다.


    1. Introduction

    Water resources are limited compared to the growing global demand due to exponentially growing population and increasing industrial activities. In recent years, antifouling and desalination techniques of wastewater and seawater are the most important methods of fresh water supply in water scarce regions. Currently, many different methods of water desalination techniques are being researched[1-19]. Among the water desalination techniques, using RO, NF, and FO are simple, economical, and energy effective. Currently, number of technologies are being researched such as distillation and advanced oxidization processes. However, the technologies mentioned suffer from the complex equipment needed with the high energy consumption and high operation costs. The membrane filtration process is the new generation of water purification technology. There are inherent properties of membrane separation such as high selective separation, continuity, automatic operation, chemical free, easy scale-up, low space requirement, and low energy consumption. RO, NF, and FO techniques are also environmentally friendly as they remove foulants and bacteria effectively. Various technologies are being combined for enhanced properties for the membrane. However, the membrane fouling is the setback for the efficiency restrictions reducing water flux rate, deteriorating water quality, and increasing energy consumption. The membrane fouling can take place by pore blocking, bio film formation, organic adsorption, inorganic precipitation and cake formation. There are different types of methods to improve the membrane fouling. The pretreatment of raw water, optimizing the operating conditions, membrane cleaning and developing antifouling membranes. However, the existing methods had some setbacks in their own fields. Therefore, the passive and active antifouling is both incorporated into the membrane structure. Graphene oxide is one of the most promising material that can be incorporated to the RO, NF, and FO membranes. Graphene oxide with its unique properties, shows excellent antifouling properties while increasing the water flux of the membrane. Due to the possible enhancements by graphene oxide related modification on the membranes, graphene oxide is widely researched. There are different types of filtering and different types of modification methods for the membrane. In this review graphene oxide incorporated composite membrane are discussed in detail. Desalination and PRO process is explained in Fig. 1 and the review is summarized in Table 1.

    2. Graphene Oxide

    2.1. Reverse osmosis

    Reverse Osmosis shows high energy efficiency in desalination. While PA-TFC RO membrane are used for their high-water permeability and high salt rejection rate, it can be improved in permselectivity, anti-fouling, and chlorine resistance[20]. Chlorine is used for membrane cleaning, but PA layer is vulnerable to the chlorine. Therefore, instead of coating GO on top of PA layer, GO was embedded into the PA layer by adding it to aqueous solution of m-phenylenediamine (MPD) before polymerization. GO was prepared by chemical exfoliation of graphite by Hummers method. After graphite being oxidized, it was converted into graphitic oxide. Then the aqueous solution was neutralized then sonicated to convert into GO. Only small sized GO was used to prepare GO-TFC membrane. Go was polymerized onto the PSF UF membrane. GO was characterized by scanning probe microscope and SPM measurements. TEM and XPS were also used. FTIR spectrums were also measured. GO-TFC membranes were examined by a Raman spectrometer. The water contact angle, surface zeta potential, and surface average roughness were measured. Anti-biofouling properties were also measured by tagged microbials. Chlorine resistance was measured by soaking in chlorite solution then rinsed to measure water flux and salt rejection rate. GO showed different sizes and the sizes indicated the single-double layer GOs. GO FTIR spectrum showed O-H groups, C=O carboxyl groups, C=C bonds, epoxy group, C-O alkoxy group. GO-TFC was more smoother than the unmodified membranes. Surface zeta potential decreased due to negatively charged functional groups of GO. Water contact angle was decreased. GO-TFC membrane showed higher water flux and the same level of salt rejection. Too much GO on other hand, decreased water flux. As GO content increased, the anti- fouling properties increased. Lastly, the GO-TFC membranes were more resilient to chlorine than TFC membranes.

    PA membrane is susceptible to facile degradation of amide bonding in oxidizing agents such as chlorine[21]. In practice, the PA membrane’s efficiency is lowered. In order to increase the efficiency, GO was highlighted for its properties. However, incorporating GO still faces many problems. Chae et el. studied GO-hydrogel nanocomposite membranes introduced as energy-efficient RO desalination method. The membrane is physically bound by the linked polymer networks and show remarkable water flux and salt rejection rates. It could be fabricated at industrial scale easily. GO was synthesized using a modified Hummer’s method. NIPAM, MBA, and APS were dissolved in GO solution. Then filtered through the PES substrate then the substrate was placed in convection oven for complete polymerization. For characterization, the weight ratio was determined by thermogravimetric analysis. FE-SEM was used for the morphology of the GO membrane. XRD was used to determine the crystalline structure of the membrane. Then salt rejection rate and water flux were measured. NaCl aqueous solution was used for measuring the water flux and salt rejection rate. The GO laminates were more dense closer to the membrane, forming asymmetric structure of the GO-polymer layer. In the SEM images, aligned and uniform GO-polymer layers were resulted and was more think than the precursor membrane. XRD spectrum showed GO-polymer layer showing a broad/sharp band around 8 degrees. Commercial membrane showed higher water flux, but it is a tradeoff of salt rejection rate and water flux as GO-polymer membranes showed better salt rejection rates. Although introducing other nanomaterials improved water flux and/or salt rejection rates, the GO-polymer membrane is better in large-scale fabrication. GO-polymer water flux increased as water pressure increased. Although being exposed to extreme 1000 ppm chlorine, GO-polymer membrane showed excellent chlorine resistance. GO-polymer had no change in salt rejection rate whether the pressure in RO was high or low.

    Shao et el. studied few-layered GO was incorporated to PA-TFC membrane to increase membrane chlorine resistance[22]. GO layers were deposited on the membrane surface through a spin-coating method. The GO powder was prepared by the improved Hummers method. PA-TFC membrane and the carbon membrane support were purchased. GO layers were coated onto selective surface of PA-TFC membrane through a spin-coating method. GO was dispersed in two different solutions, one with water and one with water mixed with ethanol. Then the solutions were spin coated onto the membrane at 60 degrees Celsius with 600 rpm. Then the membrane was naturally dried. The FTIR spectra of GO nanosheets showed peaks of C-O stretching, C-OH stretching, C=C stretching and carboxylic acid C=O stretching. An intense band indicates that GO has many free hydroxyl groups that can form hydrogen bonds and enlarge intermolecular forces of each GO nanosheet and membrane surface. FTIR, SEM, and AFM were used to characterize the GO membrane. The water contact angle decreased, and the surface was made smoother. At PH=7, the membrane performance was tested. The GO dispersed with ethanol/water solution showed better performance in permeability than the water dispersed GO. GO showed resistance to chlorine compared to the pristine membrane. As the number of GO layers increased, the water flux decreased but the salt rejection rates increased, which is typical trade off in desalination. pH level significantly affected the desalination process.

    While TFC membranes are widely used, membranes fabricated by interfacial polymerization (IP) of aromatic amines and acyl chlorides, the active layer is a highly crosslinked polymer which are difficult to control[23]. Furthermore, RO membranes are permeable to small, hydrophilic neutral solutes (SNSs). GO modification can impact the rejection of NDMA and water/NDMA permselectivity. GO was prepared by modified Hummers method. PA TFC RO membranes surface were functionalized with GO by Perreault et al. protocol. Nanosheets are tethered to PA via amine coupling chemistry. Carboxylic acid groups of PA layer are converted to amine-reactive esters by using EDC and NHS then ED solution is used to wash the membrane. Then GO solution is added to convert carboxylic acid groups that decorate GO to amine-reactive esters. Then sonicated and put into ultrapure water until use. GO nanosheets on PA surface were verified by Raman spectroscopy. FESEM was used to characterize membrane surface morphology. Water contact angle, zeta potential were measured. NDMA removal efficiency was also measured. Crumpled sheet of GO overlayed in GO modified membranes. SW30-GO showed smoother surface and higher water contact angles. Zeta potential was significantly increased. NDMA rejection was increased. ED linker also had a role in removing NDMA. However, the improved rejection was at cost of water permeability. The membrane functionalization with GO does not affect interfacial properties of PA.

    2.2. Nanofiltration

    Pure GO membranes and pure polymer for NF is very ineffective in durability and in performance in water purification technology[24]. Therefore, TFC PA membranes are used for NF processes. To increase performance if TFC PA in NF, GO was used by Bano et el. Graphite flakes for GO was purchased, and other chemicals for fabricating membranes and additives were used without purification. GO was prepared by the modified Hummers method and PA membranes by interfacial polymerization. The nanocomposites of GO were added onto membranes by dipping the membrane in the aqueous solution of GO and additives. Then after immersing in isoparaffin, heat was used for setting the GO into the PA membranes. The membrane was characterized by FT-IR for identifying GO on the membranes. The surface and cross-section morphologies was observed with SEM and TEM. Surface roughness and water contact angle were also measured. Membrane performance were tested on salt rejection rate and antifouling test concerning the permeate flux. The GO added PA membrane showed wider absorption band in FTIR spectra, meaning the higher hydrophilicity. The water contact angle decreased with increasing GO added to the PA, resulting in greater water flux. The surface was rougher and had more “ridge-and-valley” when GO was added to PA. Water flux was increased when GO was added, but only up to 0.2% and decreased afterwards. The increasing water flux with increasing GO may be due to several factors of GO. No significant change was observed in permeation properties of PA whether the change of GO was applied or not. Antifouling properties were significantly increased with the addition of GO in PA. In conclusion, the GO is a promising nanocomposite for additive in PA membranes, increasing performance and efficiency at low cost.

    Nanofiltration membrane is the mainstream of desalination process due to its high energy efficiency and salt rejection rate, compared to the reverse osmosis membrane[25]. There are two type of NF membrane, asymmetric and thin film composite. TFC NF membrane consists of PA layer and porous substrate layer. Each layer can be modified each for optimizing the performance. GO is an attractive choice for incorporating into membrane due to oxygen functionalities, improving water permeation, anti-fouling, anti-microbial filtration. Furthermore, GO has a unique characteristic of having high negative charge when applied to NF. Materials were used without modifications. Using the Hummer’s method, GO was synthesized. Graphite was oxidized and cooled in the ice bath, then stirred with slight heating up and adding mixtures. Then residues were washed with HCl aqueous solution and then RO water until reaching pH 5~6. Then ultrasonification, drying, dissolving, drying were used for obtaining pure GO. PSF-GO substrates were fabricated by adding GO and PVP together then using ultrasonication for removing air bubbles. Pouring PIP aqueous solution and PSf-GO on the substrate created thin PA layer on the membrane. GO was characterized by FTIR. The substrate layer’s contact angle was measured. Membrane surface roughness were also measured for each membrane. Also cross sectional and surface morphologies were visualized by FESEM. While all measurement, gold was used for coating, to prevent charging when characterizing membrane. The performance, mainly water permeability, was measured in steady pressure, to achieve steady condition for water flux. R = (1-Cp/Cf) × 100. XRD spectrum shows peaks when GO was incorporated into membrane. GO shows different spectrums in FTIR, showing various functional groups, a strong signal of GO nanosheets. The structures, as seen in other studies, show ridge and valleys. Also, thickness is increased in PA layer due to GO incorporation. GO loadings did not affect contact angle in the substrates. The angle was between 25~27, which is just the hydrophilic nature of PA layers. The incorporation increased water flux due to structures created by GO incorporation in membrane. The thickness of PA layer did not affect membrane permeability until reaching a certain point. The negative charge of GO also increased performance in desalination. However, due to Donnan exclusion effect, there is a difference in rejection rate in different type of salts.

    The researched membrane surface modification by using nanoparticles can hardly be put to practical use due the scalability and stability[26]. Therefore, the need for new membrane materials have appeared. GO is an attractive material for fabricating the NF membrane. GO membranes were prepared by filtrating GO nanosheets on the surface of PES UF membrane. GO modified with Hummers method were dispersed and used to prepare GO membranes. GO nanosheet and stacked GO nanosheets were tested on its antimicrobial performance and antifouling tests were done. GO nanosheets lose antimicrobial activity when they are stacked together. This is seen to the experiment done on the incubation of cells for 12 hours. GO membrane does not kill cells while GO nanosheets does. However, when stacked together, being no longer nanomaterials, they lost their property. Being complex in the feed waters, the practical use of GO membrane still faces a lot of difficulties. Conventional experiments on the GO membrane were done on short period, hence a longer period (200 hours) of test were done by Wang et el. GO membrane showed excellent reversible filtration resistance meaning it had good antifouling property. However, for complex contaminants including microorganisms, the fouling resistance is greatly reduced, meaning that surface modification of GO membrane is a necessity. Surprisingly, the GO membrane exhibited excellent mechanical stability for about 14 days which was contrary to many researchers’ original belief. Therefore, GO membrane has the potential especially due to the antifouling property.

    GO is a promising candidate for membranes for desalination and water purification due to superb anti- fouling, anti-bacterial, and chlorine resistance capacities[ 27]. One of the methods for fabricating GO based membranes, is layer-by-layer (LbL) assembly is a simple but efficient method for creating GO membranes. However, there are few deficits to the membranes in the conventional LbL assembly. Recently, external electric field (EF) was employed during LbL for better construction time and enhanced rate of film deposition. For the preparation of EF assisted LbL membranes, first the PAN substrate was prepared through phase inversion method. The PAN fiber and LiCl were dissolved into DMF for the casting solution. Then the LbL membrane was fabricated on the H-PAN (hydrolyzed PAN) substrate within an external EF. Magnetic stirring was used for concentration of the membrane. TEM was employed for analyzing the structural morphology of GO. X-ray diffraction, FTIR spectrum, and SEM was also used for characterization. GO layer showed large flat surface with nano-wrinkles and showed the particular FTIR spectrum showing the presence of OH groups, C=C bond, carbonyl in carboxyl, and epoxy groups, these lead to negative surface charge. Different from the conventional GO membrane, the EF assisted GO membranes showed a new peak at approximately 1667 cm-1. The thickness, zeta potential, water contact angle was also measured to ensure the formation of EF assisted LbL films. The EF assisted GO LbL membranes showed a better water permeability and better salt rejection. Furthermore, the stability was more desirable than that of conventional GO membranes.

    PECs are a high-performance material for NF membranes[ 28]. Easily modifiable, when fabricated in the NF membranes, it shows high water permeability and decontaminating performances. PEC composite membranes are more effective in separating organic molecules than salt ions. Therefore, GO has been incorporated in PEC composite membranes. Incorporation of GO increased stability and salt rejection rate. Using Hummer’s method, GO was created from graphite powder by ultrasonication. Then dried in oven. The non-woven/PVDF supporting layer was tried to keep homogeneity. PEC solution was dissolved and acidified at first then dried. The layer of PEC was pre-crosslinked for stability. For incorporation of GO, GO solution is was added into PEC solution when creating the layer. The GO was analyzed by FTIR for the functional groups. XPS was used for membranes’ chemical compositions. The thickness of the membranes was also measured. Microstructures and permeability (water contact angle) were analyzed. In the desalination process, 1000 ppm NaCl or Na2SO4 in D.I. water was used in low pressure of 1 MPa. GO incorporated nanosheets had higher oxygen content. And specific peak was observed in FTIR spectrums. The application of top separation PEC layer made the pores more dense and pore-free. When the composite is crosslinked, the surface morphology and cross-sectional view resembled the uncrosslinked sample. Only difference was in the water contact angle. PEC improved the desalination performance and in instability in desalination performance of unlinked PEC-GO layers.

    2.3. Forward osmosis

    GO membrane with modified active layer can enhance the filtering performance[29]. Hageb et el. adopted a novel modification using tannic acid to immobilize the GO nanosheets on the nanofiltration’s membrane. This modification method showed excellent anti-bacterial properties, which were 99.9% improved compared to the pristine membrane.

    Surface roughness were also measured for each membrane. Also, cross sectional and surface morphologies were visualized by FESEM. While all measurement, gold was used for coating, to prevent charging when characterizing membrane. The performance, mainly water permeability, was measured in steady pressure, to achieve steady condition for water flux. XRD spectrum shows peaks when GO was incorporated into membrane. GO shows different spectrums in FTIR, showing various functional groups, a strong signal of GO nanosheets. The structures, as seen in other studies, show ridge and valleys. Also, thickness is increased in PA layer due to GO incorporation. GO loadings did not affect contact angle in the substrates. The angle was between 25~27, which is due to the hydrophilic surface of PA. The thickness of PA layer did not affect membrane permeability until reaching a certain point. The negative charge of GO also increased performance in desalination. However, due to Donnan exclusion effect, there is a difference in rejection rate in different type of salts.

    A TFC-FO membrane is typically composed of thin active layer and porous structure as a mechanical support layer[30]. Manipulation of the support layer with incorporation of nanomaterials can enhance the performance of the FO membrane. Park et el. studied GO nanosheets that were used as fillers to modify the PSf support of TFC-FO membranes. GO nanosheets were prepared according to a modified Hummer’s method. Graphite was dispersed in H2SO4 and stirred and then NaNO3 was added for more stirring. KMnO4 was added and DI water addition to heat at 95 degrees Celsius. The dispersion was vacuum filtered to avoid mellitic acid and rinsed with HCl solution to remove residues. Then sonicated in DI. PSf/Go substrates were fabricated by conventional phase inversion technique. Different loadings of GO were added to PSf. On one side, dense active PA layer was formed through interfacial polymerization method.

    Oxygenous groups of GO nanosheets were analyzed by FTIR spectroscopy. Structure of GO nanosheets was examined by TEM. FE-SEM was used for morphologies of PSf, PSf/GO supports and PSf/GOT membranes. For the cross section, the samples were fractured in liquid N2 then observed. XPS and AFM images were taken. Water contact angle were measured for the hydrophilicity of the membranes. Membrane porosity and mechanical strengths were also measured. Water permeability and salt rejection were measured by crossflow RO filtration system. NaCl was used for the salt solution. FTIR showed the oxygenous groups in GO and it showed that the prepared GO was highly hydrophilic. GO showed brown compared to pure PSf membrane. Water contact angle was decreased with the GO loading increasement. GO increased the hydrophilicity up to only a certain point. Incorporation of GO nanosheets in PSf support enhanced the performance of TFC-FO membrane. However, only up to a certain amount of GO was optimum for enhancement of the performance.

    2.4 Pressure retarded osmosis

    Pressure-retarded osmosis process, PRO, is a process that can be beneficial to the energy aspects and it is widely used with an asymmetric membrane[31]. However, the asymmetric back side, which is rough, traps foulings which affect the efficiency of the membrane. When the foulants are trapped in the support layer, they are irreversibly fouled. Therefore, this study focuses on GO effects on the backside and the dense side of the membrane, prepared by layer-by-layer (LbL) assembly. GO was prepared by the modified Hummer’s method. GO membrane was synthesized by LbL assembly of GO and PAH. PAN membrane was hydrolyzed in NaOH then soaked in GO solutions and PAH solutions to create GO-PAH double layer film on both sides. PA membrane was prepared by interfacial polymerization. For the performance tests, water permeability and solute permeability were measured. For the membrane fouling, after performing the desalination process with foulant and NaCl feed solution, the membrane was washed with DI water and then desalinated again to compare the flux recovery. QCM-D was used to quantify the LbL assembly of GO membrane. For the measurement, the GO and PA membranes were coated then done the sensory run. Surface hydrophilicity were measured by measuring the water contact angle. Fouling occurrence were characterized by FTIR spectroscopy. GO membrane showed higher water flux and lower solute flux compared to PA membrane. The overall antifouling performance was better in GO membranes, even when desalination process was in FO mode. The GO membrane water flux level stayed the same after the cleaning, but only due to the fact that the structure of PAH and GO double-layers changed during the cleaning. The GO membrane showed more negatively charged surface then the PA membrane. The adsorption of the GO nanosheets did not affect the water flux of the membrane. The GO layer also acted as effective barriers in PRO mode. Thus, GO nanosheets on the back of the membrane prevents the irreversible foulings done in PRO mode.

    3. Modified Graphene Oxide

    3.1. Reverse osmosis

    The microorganisms attaching to the filtration membrane is the biggest problem since it creates microcolonies that produce extracellular polymeric substances (EPS) leading to biofilms on the surface[32] (Figs. 2 & 3). However, chlorine-based cleaning is not suitable due to chlorine degradation of PA polymers. Therefore, azide-functionalized GO was employed to modify membranes. AGO produce a highly reactive singlet nitrene intermediate that reacts with the abundant aromatic rings within PA membrane active layer. AGO is synthesized by route developed by Eigler et al. Sodium azide was added to GO aqueous solution then freeze-dried. While freeze-drying, the solid state azidation reaction takes place in which azide groups substitute for the sulfonate and epoxide groups. The final product was redispersed in water to make AGO aqueous solution. Then UV rays were irradiated on the AGO dispersed membrane for bonding. Water contact angle were measured to characterize the AGO membranes. The contact angle decreased indicating higher affinity between water and the membrane due to oxygen-containing functional groups within the AGO structure. XPS showed the modification took place which showed chemical composition similar to the AGO powder. AFM showed much smoother surface of the GO-RO membrane. The AGO membrane showed increased NaCL rejection rate and negligible change of permeability. The anti-fouling activity of the AGO-RO membrane significantly increased, and the rinsing was more effective compared to the commercial RO membrane. Therefore, AGO membrane showed more hydrophilicity, smoother surface, and antibacterial with resistance to protein fouling and biofouling.

    GO is widely used for its thermal stability, oxidation stability, biocidal properties, mechanical strength, high water permeability, high durability, and chlorine resistant properties[33]. Plant-induced natural polyphenols including tannic acid can be easily obtained from common plants at low costs. They show unique properties as modification materials due to good adhesion, coordination with metal ions, antimicrobial properties, broad chemical versatility, and radical scavenging ability. This study modified GO by TA and incorporated into the PA membrane. GO was prepared through the modified Hummers method. The surface of GO was coated with TA by self-polymerization of TA. TA was dissolved in GO buffer solution and stirred in room temperature. Then filtered and dried in vacuum oven. The membranes were prepared by the typical interfacial polymerization between MPD aqueous solution and TMC organic solution. Water flux and salt rejection rates were obtained by RO membrane test unit. Only a certain size of the membrane area were measured in brackish water reverse osmosis conditions. Antimicrobial property were evaluated by a hake flask method. Bacteria inhibition rate were calculated. Raman spectroscope was used to observe the structure of GO and GOT surfaces. For Raman spectroscopy, thin active layer of membrane was transferred to silicon wafer. Surface composition were analyzed by XPS. FTIR spectra were measured and compared. FESEM for the surface morphologies. The formation of TA coating on GO were confirmed by FTIR, Raman spectroscopy, XPS and TGA analysis. FTIR spectra were similar to that of TA powder. TA coating layer on GOT increased the number of phenolic groups. PA-T membrane showed little ridge-and-valley structures compared to the smooth surface of GO membranes. PA-GO and PA-GOT membranes showed better salt rejection rates than the pristine membranes. PA-GOT showed the best chlorine resistant properties. When TA was added to PA membranes, it showed effective antimicrobial properties.

    TiO2 and GO both can enhance the performance of RO membranes when incorporated[34]. TiO2 can be connected to PA membrane forming a bidentate coordination between Ti4+ and two oxygen atoms of -COOH group or by H-bond between surface hydroxyl group of TiO2 and -COOH group of the membrane surface. Shao et el. studied LbL self-assembly of TiO2 and GO nanoparticles onto PA membrane surface by H-bond and physical absorption is achieved. The flat PA composite membranes were fabricated by the interfacial polymerization on the PSf membrane surface. The TiO2 and GO nanoparticles were dispersed in water the sonicated for even dispersion. Then the PA membrane was dipped in TiO2 solution and naturally dried. Then GO solution was coated on the PA membrane. GO nanosheets were characterized through many different methods. The FTIR spectra showed characteristic peaks which indicated rich oxygen-containing groups. XPS showed oxidization degree of 56.2%. SEM and XPS analyzed the membrane morphology and surface properties. The water flux increased as the membrane is modified. The salt rejection rate declined negligibly. Ion adsorption and ion transmission in coating competes. The coated membrane showed better chlorine resistance according to C-N content dropping. The coated membrane showed better antifouling properties as water flux was higher than pristine membrane after anti-biofouling test. Furthermore, coated membrane showed better flux recovery after membrane rinsing.

    3.2. Nanofiltration

    The materials were purchased from organizations and the chemicals had no modification applied and. rGO@ TiO2@Ag nanocomposites were prepared by first creating mixture of TiO2 and AgNOy on same mass[35] (Figs. 4~7). GO is added to ethylene glycol. In the oil bath, add the nanocomposite mixture and stir for an hour at 50 degrees Celsius. Then microwave irradiation was use for drying the samples. In order to fabricate the membrane, first ceramic structure was created for fabrication of the PES ultrafilter. On the PES ultrafilter, IP process was done with organic solution and m-PDA aqueous solution. After drying and heating the film, PA layer is created on the PES ultrafilter. Then for the nanocomposites to incorporate in the membrane, ultrasonification was used for the IP process upon the membranes. Characterization of the membrane are done with; morphology and topography, where nanocomposites are shown in FE-SEM and shows a rough surface. X-ray diffractograms. FTIR spectra but with little difference only shown by a peak due to small number of nanocomposites. Water contact angle measurement, for the incorporation of GO improved the hydrophilicity. Different from the conventional single layer membranes, incorporating nanocomposite materials into the membrane gives better performance. By better performance, the experiment was done on permeability, desalination, dye retention, and antibacterial activity. The abundant oxygen functional groups in GO should reduce the permeability, however the nanocomposite did not reduce the hydrophilicity significant enough. In desalination, the rejection differed between the different salts, however rejection was increased significantly. Dye retention was increased due to the while the water flux was increased too. While Graphene based nanosheets are widely known for their antibacterial properties, the differently arranged nanoparticles enhanced the antibacterial properties.

    While GO membranes are a promising type of membrane for NF, it exhibits poor rejection rates for small ions[36]. This is due to the widening of layers when the membrane is wetted. Therefore, modification should be on the GO membranes to not widen when wet. This study aims for better performing membranes by affecting GO by positively charged chelates (EDTA-GO) and chemically reduced (EDTA-rGO). The surface of the membrane was treated by O2 plasma treatment. (hereafter, plasma treated EDTA-GO and EDTA-rGO will be called P-EDTA-GO and P-EDTA-rGO) The GO structures were characterized by SEM and EDS scan analyzation on structures. FTIR spectra was analyzed and water contact angles were measured. EDTA-GO and EDTA-rGO showed better salt rejection rate than the commercial NF membranes. EDTA-rGO showed better rejection rate than EDTA-GO due to the unwidening characteristic when wetted. The interlayer space is significantly smaller in EDTA-rGO. The water contact angle was in order GO (50) < EDTA-GO (60) < EDTA-rGO (74) due to combination of hydrophilicity and hydrophobic composites. Salt rejection rate differs in ion rejection rate, which behavior is differentiated due to the electrostatic repulsion between ions and GO. Size exclusion was dominant in EDTA-rGO. There is a tradeoff of rejection rate and permeability. The plasma treatment on EDTA-GO and EDTA-rGO affected the water permeability, which was increased. However, the NaCl ion rejection rate was not affected on both type of GO. The hydrophilicity change can be explained by changing water contact angle and oxygen functional groups analyzed by FTIR spectra and EDS spectra. Also, the pore density increased due to the plasma treatment. The plasma treatment increased the performance in anti- biofouling.

    Using piperazine (PIP), polyethylene imine (PEI), trimesoyl chloride (TMC) for preparing PA on a membrane created a better performance[37]. Further performance improvement could be achieved by incorporating GO into the membrane. However, incorporation of GO reduced the NaCl rejection rate 98% to 95%. Therefore, modifying of GO was needed for improvement. This study compares GO-membrane and R-GONH2- membrane. All materials are analytical reagents and used without further purification. GO-membrane was fabricated by interfacial polymerization. R-GO-NH2- membrane was fabricated by pumping R-GO-NH2 solution which was ultrasonic dispersed into the membrane. The chemical compositions were obtained by ATR-FTIR, an ATR method. Also, surface structures were analyzed by XPS. X-ray diffraction was used for characterization. The morphologies of the membranes were also analyzed by SEM and AFM. Water contact angle was also measured for hydrophilicity of the membranes. The membrane performance test was done on water permeability, salts rejection and PEG neutral solute rejection at room temperature and low pressure. FTIR spectra showed the typical peaks of NH2 groups in R-GO-NH2 membrane, leading to the increase of -OH content in membrane. The energy peaks analyzed from XPS showed the successful incorporation of R-GO-NH2. The morphology showed R-GO-NH2 created different structure formed in the membranes. Incorporating R-GO-NH2 created a smoother surface when viewed horizontally. Water contact angle decreased slightly. The pure water flux was increased in R-GO-NH2 membrane and showed better separation property. The modified membrane showed superior Na2SO4 rejection rate but when RGO- NH2 was added too much, the rejection rate would deteriorate suddenly. Due to pore size and higher electrostatic repulsion, modified membrane showed better salt rejection rate. Although showing difference in different type of salts, having different charges. The antifouling performance have decreased overtime for both membranes, but the modified membrane always had the higher antifouling performance rate. In conclusion, R-GO-NH2 incorporated membrane had better performance overall than the GO incorporated membrane.

    Sulfone containing polymers such as PES and PSF are commonly used in NF and ultrafiltration (UF) membranes due to hydrolytic, thermal, mechanical, and chemical stability[38]. Go based membranes also show attractive properties for separation. Addition to hydrophilic nanofillers, the NF membrane can be also modified with pore forming agents. PES asymmetric membranes with 3-aminopropyltriethoxysilane (APTS) functionalized GO nanofillers are studied. GO was fabricated by a modified Hummer’s method and functionalized with APTS. Characterizing by FTIR, XPS, same procedure described by Leaper et al. PES membranes were prepared by inversion technique. The cross sectional and top surface morphology were analyzed by SEM. Surface morphology were characterized by Fastscan atomic force microscope. The membranes showed asymmetric structures with a dense top layer and finger-like macrovoids and sponge-like mesoporous structures. In comparison to the pure PES membrane which had smaller finger like voids. When the finger like pores increase in size, the numbers decreased. This showed clear change in membrane morphology. When pore forming additives were present, porosity values and roughness increased. Also, the tensile strength is improved when pore forming agents and GO were both present. Water contact angle decreased with the modification. The FTIR does differ much unless in higher concentrations.

    Just the addition of pore forming agents did not show significant improvement of pure water permeability (PWP) or neither the filtration performance improved. However, the addition of APTS-GO fillers was added with pore forming agents did show improvements in filtration and rejection performance. For the antifouling characteristics, the incorporation of APTS-GO and APTS-GO/PVP improved the antifouling properties due to hydrophilic character, surface roughness and higher contact angle value. Most of the membranes in NF are PA and most of the research focuses on modifying the type of membranes. APTS-GO synthesized membranes showed increase in membrane hydrophilicity, mechanical stability, permeability, and rejection rates.

    There is the necessity of creating membrane that can increase water flux and separate large organics from wastewater[39] (Figs. 8~11). Loose NF membranes with high water permeability and separating organics are of increasing interest for desalination, organic separation, and reusability. Instead of creating dense membrane for NF, creating hydrophilic structure on the membrane surface are being studied increasingly. Zhu et el. studied copper nanoparticles - CuNP, for antibacterial membranes due to antifouling abilities. H2O2/CuSO4 triggered PDA codeposition with GO/rGOC nanomaterials are compared. GO was first synthesized, then GO suspension solution was prepared. CuNPs were grown onto the GO suspension by adding CuNPs, CuSO4, and EDTA⋅2Na⋅ 2H2O then mixed through sonication. Then NaOH solution and NaBH4 reducing agent were added and then stirred at room temperature. Then rinsed by DI water and vacuum dried for grinding to powder for use. The PAN membranes were modifided with rGOC and GO in different concentrations. Electron microscopy imaging, SEM were used to image the membranes. X-ray diffraction patterns were analyzed. The surface roughness, morphology was analyzed. Surface hydrophilicity were analyzed by water contact angle measurements and water uptake. Membrane separation performance were measured in pressurized conditions. Then antimicrobial properties were measured. Oxidated CuNPs appeared in the XRD analyses. rGOC modified membranes showed higher water uptake and water permeability. Salt permeation, dye retention, high water permeability is shown in rGOC modified membranes. Antibacterial properties were enhanced.

    3.3. Forward osmosis

    FO, although having less foulant layer and more easily cleanable than RO, fouling is detrimental due to cake-enhanced concentration polarization[40]. This decreases the osmotic force for permeation and requires frequent cleaning which drives the cost up. Therefore, the need for membranes with both antimicrobial and antifouling properties is present. This study evaluated GO as biofouling mitigation in FO. GO was produced by chemical oxidation of graphite by KMnO4 in a mixture of H2SO4 and H3PO4. GO was characterized by measurements done on dry GO powders. FTIR spectra, Raman spectroscopy, XPS, SEM were done to characterize GO. The antimicrobial performance of GO was verified by microbials put on a pure GO layer and then quantifying dead and live cells. GO was covalently bound to FO membranes by amide coupling reaction. The carboxyl groups of the PA layer are converted to amine-reactive esters by reacting with EDC and NHS. The amine-reactive esters are used to attach ethylenediamine to the membrane. Then GO is reacted with EDC and NHS to activate its carboxyl groups. Then put in contact with membrane for amide coupling. Raman spectra, SEM image, membrane hydrophilicity, surface roughness, water permeability, salt rejection rate was measured to characterize the membrane. GO showed high defect density as identified by FTIR and XPS spectroscopy. GO showed strong antimicrobial performance to different types of microbials, and it showed decrease of cell viability. The GO did not affect the transport properties of the membrane. The surface roughness did not change, however due to high density of oxygen functional groups in GO, hydrophilicity increased. GO incorporation showed antiadhesive and antimicrobial properties, which is viable for antifouling properties that can be added to FO membranes.

    4. Conclusions

    For one goal of enhancing the nanofiltration desalination membranes, various researchers are working on the different modification on different nanofiltration process. The variety of modification for the nanofiltration membranes were introduced in this article. In most common case of reverse osmosis nanofiltration method, the incorporation of graphene oxide significantly increased the flux of the membrane and increased the antifouling and antibacterial properties. Furthermore, the resistance to chlorine, the most common cleaning agent in nanofiltration process, increased by graphene oxide incorporation. Some cases of graphene oxide modification of the membrane significantly raised the antifouling performance of the membrane, however at cost of the water flux decreasing the efficiency. Other cases of modification apart from the graphene oxide also showed the enhancement of the antibacterial and antifouling properties. While the modifying of the nanofiltration membranes enhanced its properties and showed bright prospective for the desalination technology, there is still the left task of commercializing the nanofiltration membrane to a real- life scale outside of the lab. Another task to be solved is the longevity of the membranes. With enhanced antifouling and antibacterial properties, the thin bio film creation is accelerated, hindering the performance of nanofiltration. Although the performance of the membranes is increased, there are still a lot of room for possible improvements. Considering the importance of desalination process, the nanofiltration membrane should be researched continuously and relentlessly.

    Figures

    MEMBRANE_JOURNAL-31-1-16_F1.gif

    Schematic representation of thin film composite membrane.

    MEMBRANE_JOURNAL-31-1-16_F2.gif

    Synthesis of azide functionalized graphene oxide (AGO) and its attachment onto a polyamide RO membrane surface via UV activation of azide functional groups (Reproduced with permission from Huang et al., 32, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F3.gif

    (a) Long-term BSA fouling test on the control and modified membranes showing the differences in flux decline; (b) Fluorescence and SEM images showing the percentages and condition of Escherichia coli cells on membrane surfaces after contact for 24 h; (c) quantitative analysis of live (green) and dead (red) cell percentages on both membrane surfaces (Reproduced with permission from Huang et al., 32, Copyright 2016, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F4.gif

    Schematic illustration of the fabrication of rGO@TiO2@Ag nanocomposites by microwave irradiation (Reproduced with permission from Abadikhah et al., 35, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F5.gif

    Cross-sectional structure of the composite nanofilter membrane and nanocomposite incorporation in the PA layer (Reproduced with permission from Abadikhah et al., 35, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F6.gif

    Surface morphology and topography of the membranes: (A,a) TFC, (B,b) MTFN-1, (C,c) MTFN-2, and (D,d) MTFN-3 (Reproduced with permission from Abadikhah et al., 35, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F7.gif

    Antibacterial properties of the membranes evaluated by the (A) plate colony-forming count experiments, where (a) is TFC, (b) G-TFN, (c) MTNF-1, (d) MTNF-2, and (e) MTN-3, (B) the % bacterial viability, as a measure of the antimicrobial activity of the membranes, and (C) schematic illustration of antibacterial activities of MTFN membranes (Reproduced with permission from Abadikhah et al., 35, Copyright 2019, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F8.gif

    Schematic routes of (a) in situ growth of Cu NPs onto the surface of rGO nanosheets to make rGOC nanocomposites and (b) fast codeposition of PDA and rGOC nanocomposites triggered by CuSO4 and H2O2 (Reproduced with permission from Zhu et al., 39, Copyright 2017, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F9.gif

    Water permeability of hydrolyzed PAN membranes and PDA-modified membranes. Reported values represent the average water permeability over four varied pressures (HPAN 1~4 bar; other membranes 2~8 bar). LMH bar-1 is short for L m-2 h-1 bar-1. The operational condition was maintained at 30 L/h and 25 ± 2°C (Reproduced with permission from Zhu et al., 39, Copyright 2017, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F10.gif

    (a) Salt retention and (b) permeation flux of the membranes modified with PDA and GO/rGOC nanomaterials. Temperature was maintained throughout the filtrations at 25 ± 2°C. Influent salt concentration used was 1.0 g L-1. Pressures applied were 4 bar for all as-prepared membranes. (Reproduced with permission from Zhu et al., 39, Copyright 2017, American Chemical Society).

    MEMBRANE_JOURNAL-31-1-16_F11.gif

    (a) Demonstrated antibacterial properties of the membranes based on the plate counting method: (a’) control without membrane, (b’) HPAN membrane, (c) PDA membrane, (d) PDA-GO2 membrane, (e) PDA-rGOC2 membrane, and (f) PDA-rGOC3 membrane; (b) uantified antimicrobial ability of the HPAN, PDA-modified, and codeposition- modified membranes. Mussel-inspired architecture of high-flux loose nanofiltration membrane functionalized with antibacterial reduced graphene oxide-copper nanocomposites (Reproduced with permission from Zhu et al., 39, Copyright 2017, American Chemical Society).

    Tables

    Summary of Membrane Separation Process

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