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

DOI: https://doi.org/10.14579/MEMBRANE_JOURNAL.2021.31.1.16

1)

†

Corresponding author(e-mail: [email protected], http://orcid.org/0000-0002-3820-141X)

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

이 대 원ㆍ라즈쿠마 파텔^†

연세대학교 언더우드국제대학 융합과학공학부 에너지환경과학공학 (2021년 1월 13일 접수, 2021년 1월 17일 수정, 2021년 1월 21일 채택)

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

Daewon Lee and 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

(Received 13 January, 2021, Revised 17 January, 2021, Accepted 21 January, 2021)

요 약: 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.

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.

Keywords: nanofiltration, pressure retarded osmosis, desalination, antifouling

1. Introduction

Water resources are limited compared to the growing global demand due to exponentially growing population and increasing industrial activities. In recent years, an- tifouling and desalination techniques of wastewater and seawater are the most important methods of fresh wa- ter supply in water scarce regions. Currently, many dif- ferent methods of water desalination techniques are be- ing researched[1-19]. Among the water desalination techniques, using RO, NF, and FO are simple, econom- ical, and energy effective. Currently, number of tech- nologies are being researched such as distillation and advanced oxidization processes. However, the tech- nologies mentioned suffer from the complex equipment needed with the high energy consumption and high op- eration 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 oper- ation, chemical free, easy scale-up, low space require- ment, and low energy consumption. RO, NF, and FO techniques are also environmentally friendly as they re- move foulants and bacteria effectively. Various tech- nologies are being combined for enhanced properties for the membrane. However, the membrane fouling is the setback for the efficiency restrictions reducing wa- ter flux rate, deteriorating water quality, and increasing energy consumption. The membrane fouling can take place by pore blocking, bio film formation, organic ad- sorption, inorganic precipitation and cake formation.

There are different types of methods to improve the membrane fouling. The pretreatment of raw water, op- timizing 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 in- corporated into the membrane structure. Graphene oxide is one of the most promising material that can be in- corporated to the RO, NF, and FO membranes. Graphene oxide with its unique properties, shows excellent anti-

fouling 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-foul- ing, 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 neutral-

ized 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 aver-

age roughness were measured. Anti-biofouling proper-

ties 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

TFC layer Separation

method Advntage Modification Water

permeability Salt rejection rate Reference

PA layer RO Chlorine resistance GO embedded 18 LMH after

chlorination

48,000 ppm/h

after chlorination [20]

PA layer RO Non-swelling GO suspension by covalent bonding 33.5 L/m²h 98.5% NaCl [21]

PA layer RO Chlorine resistance GO spin coating onto surface - 75% after 16 h chlorine exposure [22]

PA layer RO NDMA GO functionalization on surface - - [23]

PA layer RO Antifouling toward

BSA and HA GO embedded 22 L/m²h Above 80% [24]

PSf layer RO Salt rejection GO incorporation - Na2SO4 95.2% [25]

PA layer RO Antimicrobial GO nanosheets - Na2SO4 65.23% [26]

PSf layer FO Higher stability Electric field assisted

layer-by-layer GO assembly 65.6 L/m²h MgCl2 80.9% [27]

PA layer RO Water permeability GO nano additive into

polyelectrolyte complex 89 kg/m²hMPa Na2SO4 62.1% [28]

PA layer FO Antibiofouling Immobilized GO nanosheet

by tannic acid - - [29]

PSf layer FO Water permeability GO (0.25wt%) incorporation - - [30]

Back side of asymmetric

membrane

PRO Fouling control GO barrier layer - - [31]

PA layer RO Antifouling Surface grafting GO - Nearly 90% [32]

PA layer RO Chlorine tolerant,

antimicrobial GO coated by tannic acid - 94% after CL

exposure [33]

PA layer RO Membrane

durability LbL assembly TiO2 and GO

17.9 L/m² after microbial incubation

75% after microbial incubation

[34]

PA layer RO Ion rejection EDTA functionalized GO 150 LMH bar^-1 NaCl 80% [36]

PA layer Low pressure

Antifouling,

water flux Reduced GO-NH2 38.57 L/m²h Na2SO4 98.21% [37]

PSf layer Low

pressure Water Permeability GO funtionalized with

3-aminopropyltriethoxysilane 9.9 LMHbar BSA 95% [38]

PA layer FO Biofouling

mitigation GO functionalization - - [40]

Fig. 1. Schematic representation of thin film composite membrane.

Table 1. Summary of Membrane Separation Process

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 lev- el of salt rejection. Too much GO on other hand, de- creased water flux. As GO content increased, the an- ti-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 nano- composite membranes introduced as energy-efficient RO desalination method. The membrane is physically bound by the linked polymer networks and show re- markable water flux and salt rejection rates. It could be fabricated at industrial scale easily. GO was synthe- sized 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 wa- ter flux and salt rejection rate. The GO laminates were more dense closer to the membrane, forming asym- metric 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 mem- brane. XRD spectrum showed GO-polymer layer show- ing a broad/sharp band around 8 degrees. Commercial membrane showed higher water flux, but it is a trade- off 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-pol- ymer 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 mem- brane 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 mem- brane 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 nano- sheet and membrane surface. FTIR, SEM, and AFM were used to characterize the GO membrane. The wa- ter 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 de- salination 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 per- mselectivity. GO was prepared by modified Hummers method. PA TFC RO membranes surface were func- tionalized with GO by Perreault et al. protocol. Nano- sheets 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 sol- ution 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 sig- nificantly 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 af- fect 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 mem- branes are used for NF processes. To increase perform- ance if TFC PA in NF, GO was used by Bano et el.

Graphite flakes for GO was purchased, and other chem- icals 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 aque- ous solution of GO and additives. Then after immers- ing in isoparaffin, heat was used for setting the GO in- to 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 perform- ance 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 proper- ties were significantly increased with the addition of GO in PA. In conclusion, the GO is a promising nano- composite for additive in PA membranes, increasing performance and efficiency at low cost.

Nanofiltration membrane is the mainstream of de- salination 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 mem- brane consists of PA layer and porous substrate layer.

Each layer can be modified each for optimizing the performance. GO is an attractive choice for incorporat- ing into membrane due to oxygen functionalities, im- proving 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 remov-

ing 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 sub- strate layer’s contact angle was measured. Membrane surface roughness were also measured for each mem- brane. 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 wa- ter 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 in- corporated into membrane. GO shows different spec- trums 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 hy- drophilic nature of PA layers. The incorporation in- creased water flux due to structures created by GO in- corporation in membrane. The thickness of PA layer did not affect membrane permeability until reaching a certain point. The negative charge of GO also in- creased performance in desalination. However, due to Donnan exclusion effect, there is a difference in re- jection 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 nano- sheets on the surface of PES UF membrane. GO modi- fied 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 to- gether, being no longer nanomaterials, they lost their property. Being complex in the feed waters, the prac-

tical 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 con- taminants including microorganisms, the fouling resist- ance is greatly reduced, meaning that surface mod- ification of GO membrane is a necessity. Surprisingly, the GO membrane exhibited excellent mechanical sta- bility 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 de- salination and water purification due to superb an- ti-fouling, anti-bacterial, and chlorine resistance capaci- ties[27]. One of the methods for fabricating GO based membranes, is layer-by-layer (LbL) assembly is a sim- ple but efficient method for creating GO membranes.

However, there are few deficits to the membranes in the conventional LbL assembly. Recently, external elec- tric 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 in- version method. The PAN fiber and LiCl were dis- solved into DMF for the casting solution. Then the LbL membrane was fabricated on the H-PAN (hydrolyzed PAN) substrate within an external EF. Magnetic stir- ring was used for concentration of the membrane.

TEM was employed for analyzing the structural mor- phology 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 ep- oxy groups, these lead to negative surface charge.

Different from the conventional GO membrane, the EF

assisted GO membranes showed a new peak at approx-

imately 1667 cm

. 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 bet- ter salt rejection. Furthermore, the stability was more desirable than that of conventional GO membranes.

PECs are a high-performance material for NF mem- branes[28]. Easily modifiable, when fabricated in the NF membranes, it shows high water permeability and decontaminating performances. PEC composite mem- branes are more effective in separating organic mole- cules than salt ions. Therefore, GO has been incorpo- rated 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-cross- linked for stability. For incorporation of GO, GO sol- ution is was added into PEC solution when creating the layer. The GO was analyzed by FTIR for the func- tional groups. XPS was used for membranes’ chemical compositions. The thickness of the membranes was al- so measured. Microstructures and permeability (water contact angle) were analyzed. In the desalination proc- ess, 1000 ppm NaCl or Na

SO

in D.I. water was used in low pressure of 1 MPa. GO incorporated nanosheets had higher oxygen content. And specific peak was ob- served in FTIR spectrums. The application of top sepa- ration PEC layer made the pores more dense and pore-free. When the composite is crosslinked, the sur- face morphology and cross-sectional view resembled the uncrosslinked sample. Only difference was in the water contact angle. PEC improved the desalination perform- ance 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 mem- brane. 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 wa- ter 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 perme- ability until reaching a certain point. The negative charge of GO also increased performance in desalina- tion. 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 sup- port layer[30]. Manipulation of the support layer with incorporation of nanomaterials can enhance the perfor- mance 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 H

SO

and stirred and then NaNO

was added for more stirring. KMnO

was add- ed 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 poly- merization 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 N

then observed. XPS and AFM images were taken. Water contact angle were measured for the hy- drophilicity of the membranes. Membrane porosity and mechanical strengths were also measured. Water per- meability and salt rejection were measured by cross- flow 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 hydro- philicity up to only a certain point. Incorporation of GO nanosheets in PSf support enhanced the perform- ance 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 foul- ings which affect the efficiency of the membrane.

When the foulants are trapped in the support layer, they are irreversibly fouled. Therefore, this study fo- cuses 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 per- meability and solute permeability were measured. For the membrane fouling, after performing the desalination process with foulant and NaCl feed solution, the mem- brane 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 coat- ed then done the sensory run. Surface hydrophilicity were measured by measuring the water contact angle.

Fouling occurrence were characterized by FTIR spect- roscopy. GO membrane showed higher water flux and lower solute flux compared to PA membrane. The over- all 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 mem- brane is the biggest problem since it creates micro- colonies 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 mem-

branes. 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 com- position 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 ac-

tivity 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 Fig. 2. 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).

Fig. 3. (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 (Re-

produced with permission from Huang et al., 32, Copyright 2016, American Chemical Society).

stability, biocidal properties, mechanical strength, high water permeability, high durability, and chlorine re- sistant properties[33]. Plant-induced natural polyphenols including tannic acid can be easily obtained from com- mon plants at low costs. They show unique properties as modification materials due to good adhesion, coordi- nation 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 mem- branes were prepared by the typical interfacial poly- merization 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 prop- erty 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 mem- branes. PA-GO and PA-GOT membranes showed bet- ter salt rejection rates than the pristine membranes.

PA-GOT showed the best chlorine resistant properties.

When TA was added to PA membranes, it showed ef- fective antimicrobial properties.

TiO

and GO both can enhance the performance of RO membranes when incorporated[34]. TiO

can be connected to PA membrane forming a bidentate coordi- nation between Ti

and two oxygen atoms of -COOH

group or by H-bond between surface hydroxyl group of TiO

and -COOH group of the membrane surface.

Shao et el. studied LbL self-assembly of TiO

and GO nanoparticles onto PA membrane surface by H-bond and physical absorption is achieved. The flat PA com- posite membranes were fabricated by the interfacial polymerization on the PSf membrane surface. The TiO

and GO nanoparticles were dispersed in water the so- nicated for even dispersion. Then the PA membrane was dipped in TiO

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 mem- brane 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@

TiO

@Ag nanocomposites were prepared by first creat- ing mixture of TiO

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 ultra-

filter, 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 nanocompo- sites are shown in FE-SEM and shows a rough surface.

X-ray diffractograms. FTIR spectra but with little dif- ference 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 mem- brane 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 per- meability, however the nanocomposite did not reduce the hydrophilicity significant enough. In desalination, the rejection differed between the different salts, how- ever rejection was increased significantly. Dye retention was increased due to the while the water flux was in- creased too. While Graphene based nanosheets are widely known for their antibacterial properties, the dif- ferently arranged nanoparticles enhanced the anti- bacterial properties.

Fig. 4. Schematic illustration of the fabrication of rGO@TiO

@Ag nanocomposites by microwave irradiation (Reproduced with permission from Abadikhah et al., 35, Copyright 2019, American Chemical Society).

Fig. 5. 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).

Fig. 6. 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).

Fig. 7. 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).

While GO membranes are a promising type of mem- brane 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 affect- ing GO by positively charged chelates (EDTA-GO) and chemically reduced (EDTA-rGO). The surface of the membrane was treated by O

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 ana- lyzed 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 an- ti-biofouling.

Using piperazine (PIP), polyethylene imine (PEI), tri- mesoyl chloride (TMC) for preparing PA on a mem- brane created a better performance[37]. Further per- formance improvement could be achieved by incorporat- ing GO into the membrane. However, incorporation of GO reduced the NaCl rejection rate 98% to 95%.

Therefore, modifying of GO was needed for improve-

ment. This study compares GO-membrane and R-GO- NH

-membrane. All materials are analytical reagents and used without further purification. GO-membrane was fabricated by interfacial polymerization. R-GO-NH

- membrane was fabricated by pumping R-GO-NH

sol- ution 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 ana- lyzed by SEM and AFM. Water contact angle was also measured for hydrophilicity of the membranes. The membrane performance test was done on water perme- ability, salts rejection and PEG neutral solute rejection at room temperature and low pressure. FTIR spectra showed the typical peaks of NH

groups in R-GO-NH

membrane, leading to the increase of -OH content in membrane. The energy peaks analyzed from XPS showed the successful incorporation of R-GO-NH

. The mor- phology showed R-GO-NH

created different structure formed in the membranes. Incorporating R-GO-NH

created a smoother surface when viewed horizontally.

Water contact angle decreased slightly. The pure water flux was increased in R-GO-NH

membrane and showed better separation property. The modified membrane showed superior Na

SO

rejection rate but when R- GO-NH

was added too much, the rejection rate would deteriorate suddenly. Due to pore size and higher elec- trostatic repulsion, modified membrane showed better salt rejection rate. Although showing difference in dif- ferent type of salts, having different charges. The anti- fouling performance have decreased overtime for both membranes, but the modified membrane always had the higher antifouling performance rate. In conclusion, R-GO-NH

incorporated membrane had better perform- ance 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 hydro-

philic nanofillers, the NF membrane can be also modi-

fied with pore forming agents. PES asymmetric mem- branes with 3-aminopropyltriethoxysilane (APTS) func- tionalized GO nanofillers are studied. GO was fab- ricated by a modified Hummer’s method and function- alized with APTS. Characterizing by FTIR, XPS, same procedure described by Leaper et al. PES membranes were prepared by inversion technique. The cross sec- tional 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 modi- fication. 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, mechan- ical 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 sur- face are being studied increasingly. Zhu et el. studied copper nanoparticles - CuNP, for antibacterial membranes due to antifouling abilities. H

O

/CuSO

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 suspen- sion by adding CuNPs, CuSO

, and EDTA⋅2Na⋅

2H

O then mixed through sonication. Then NaOH sol-

ution and NaBH

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

Fig. 8. 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 CuSO

and H

O

(Reproduced with permission from

Zhu et al., 39, Copyright 2017, American Chemical Society).

Fig. 9. 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

is short for L m

h

bar

. The operational condition was maintained at 30 L/h and 25 ± 2°C (Reproduced with per- mission from Zhu et al., 39, Copyright 2017, American Chemical Society).

Fig. 10. (a) Salt retention and (b) permeation flux of the membranes modified with PDA and GO/rGOC nanomate- rials. Temperature was maintained throughout the filtrations at 25 ± 2°C. Influent salt concentration used was 1.0 g L

. Pressures applied were 4 bar for all as-prepared membranes.

(Reproduced with permission from Zhu et al., 39, Copy- right 2017, American Chemical Society).

Fig. 11. (a) Demonstrated antibacterial properties of the membranes based on the plate counting method: (a’) con- trol without membrane, (b’) HPAN membrane, (c) PDA membrane, (d) PDA-GO2 membrane, (e) PDA-rGOC2 mem- brane, and (f) PDA-rGOC3 membrane; (b) uantified anti- microbial ability of the HPAN, PDA-modified, and code- position-modified membranes. Mussel-inspired architecture of high-flux loose nanofiltration membrane functionalized with antibacterial reduced graphene oxide-copper nano- composites (Reproduced with permission from Zhu et al., 39, Copyright 2017, American Chemical Society).

PAN membranes were modifided with rGOC and GO

in different concentrations. Electron microscopy imag-

ing, SEM were used to image the membranes. X-ray

diffraction patterns were analyzed. The surface rough-

ness, morphology was analyzed. Surface hydrophilicity

were analyzed by water contact angle measurements

and water uptake. Membrane separation performance

were measured in pressurized conditions. Then anti-

microbial properties were measured. Oxidated CuNPs

appeared in the XRD analyses. rGOC modified mem-

branes showed higher water uptake and water perme-

ability. Salt permeation, dye retention, high water per-

meability 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 de- creases 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 KMnO

in a mix- ture of H

SO

and H

PO

. GO was characterized by measurements done on dry GO powders. FTIR spectra, Raman spectroscopy, XPS, SEM were done to charac- terize 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 ethylenedi- amine 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 perform- ance 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 rough- ness 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 de-

salination membranes, various researchers are working on the different modification on different nanofiltration process. The variety of modification for the nano- filtration membranes were introduced in this article. In most common case of reverse osmosis nanofiltration method, the incorporation of graphene oxide sig- nificantly increased the flux of the membrane and in- creased the antifouling and antibacterial properties.

Furthermore, the resistance to chlorine, the most com- mon cleaning agent in nanofiltration process, increased by graphene oxide incorporation. Some cases of gra- phene 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 mod- ifying of the nanofiltration membranes enhanced its properties and showed bright prospective for the de- salination technology, there is still the left task of commercializing the nanofiltration membrane to a re- al-life scale outside of the lab. Another task to be solved is the longevity of the membranes. With en- hanced antifouling and antibacterial properties, the thin bio film creation is accelerated, hindering the perform- ance 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.

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