Review ArticlePreparation of Polymer Membranes by In SituInterfacial Polymerization

Michał Adamczak , Gabriela Kamińska, and Jolanta Bohdziewicz

Institute of Water and Wastewater Engineering, Silesian University of Technology, Konarskiego St. 18, Gliwice 44-100, Poland

Correspondence should be addressed to Michał Adamczak; michal.adamczak@polsl.pl

Received 18 October 2018; Revised 19 January 2019; Accepted 27 January 2019; Published 14 March 2019

Academic Editor: Miriam H. Rafailovich

Copyright © 2019 Michał Adamczak et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Membranes currently have a wide application in sewage treatment and water purification processes, in seawater desalination, and invarious technological processes where high product purity is required. Deposition of an ultrathin skin layer of TFC (thin-filmcomposite) and TFN (thin-film nanocomposite) onto the surface of membranes is discussed in this article. Their presenceimproves membrane properties such as retention of impurities and permeability. The aim of this paper is to present the currentstate of knowledge about the methods of preparing composite and nanocomposite membranes. The properties of the preparedTFC membranes can be modified by changing the type and concentration of the reacting monomers, and the physicalconditions in which the membrane preparation process itself is carried out are also significant. Additionally, the properties ofTFN membranes can be further modified with nanocomposites. The membranes are characterized by different properties notonly because they have nanoparticles in their structure but also because their concentration and the way they are blended intothe membrane structure were changed. This paper provides information on modifications of TFN membranes withnanoparticles, as well as modification by changes in polymerization reaction conditions and monomer concentration. Examplesof the use of TFN and TFC membranes are also presented.

1. Introduction

Membranes are produced on a commercial scale for a widerange of applications including sewage treatment and waterpurification, seawater desalination, and various technologicalprocesses where high product purity is required. Most mem-branes are produced by the simple method of phase inver-sion. In this process, solvent and polymer solution arepoured onto a flat surface to form a film of appropriate thick-ness, and then, a controlled process of exchanging solvent toa nonsolvent takes place and a solid, uniformmembrane witha skin layer is formed. Membranes for microfiltration (MF)and ultrafiltration (UF) are most commonly produced withthis phase inversion method. Both MF and UF processesare considered low-pressure processes since they operateat relatively low pressures that range from 0.1 to 1.0MPa.Macromolecular impurities, such as emulsions, colloids,bacteria, proteins, and viruses, are removed through MFand UF. Due to various modifications, UF membranes can

also be used to remove organic micropollutants, which areusually removed by the use of high-pressure processes likenanofiltration (NF) and reverse osmosis (RO) [1–3]. Theseprocesses and also low-pressure membrane processes areshown in Figure 1.

Apart from phase inversion, another equally importantmethod of membrane formation is in situ interfacial poly-merization (IPIS). This method is used to form membraneson an industrial scale, mainly for NF and RO processes. NFand RO are high-pressure processes involving appliedpressure ranges from 0.5 to 5.0MPa. Substances includingdyes, polyvalent ions, monovalent ions, and simple sugarsare removed in NF and RO [1]. The first known applicationof the interfacial polymerization process was published in1959 by Wittbecker and Morgan [4]. The process was called“interfacial polycondensation,” and it was conductedbetween diamine and acidic monomer-containing chlorineatoms by the Schotten-Bauman reaction and was basedon the work from 1946 [4]. For preparation of the first

HindawiInternational Journal of Polymer ScienceVolume 2019, Article ID 6217924, 13 pageshttps://doi.org/10.1155/2019/6217924

thin-film composite membranes, this reaction was used byJohn Cadotte in 1985 for RO membrane preparation. Thismembrane showed 99% rejection of NaCl [5]. The next stepfor TFC membrane development was the use of nanocom-posites for modification of these membranes. The first useof this technique was noted in the years 2005-2007 by Jeonget al. [6] and it used zeolite nanoparticles dispersed within apolyamide film.

Nowadays, a relatively new type of membrane processwith the possibility of implementing TFC and TFN mem-branes is forward osmosis (FO) which occurs under the influ-ence of the osmotic pressure of water. Over the last decade,this method has earned more attention. The first articleapplying this method was published in 2012. In FO, theosmotic pressure is the driving force that is created by the dif-ference in concentrations between the solutions on both sidesof the membrane. The process makes use of the so-calledextraction solution, which consists of special dissolved sub-stances. Water passes very slowly through the semipermeablemembrane from the feed of lower pressure to the draw solu-tion of higher pressure, which is gradually diluted [7–10].

Membranes formed by the IPIS method are referred to asTFC (thin-film composite) membranes, or in the case ofmembranes modified with nanoparticles, they are referredto as TFN or TFNC (thin-film nanocomposite) membranes.Additionally, most of the currently applied desalinationplants in the world use nanofiltration or osmotic TFC mem-branes as the most significant part of the entire desalinationprocess. They are also used for separation processes in food,pharmaceutical, and chemical industries [11, 12].

2. Thin-Film Composite Membranes

TFC membranes are essentially composed of two separatelayers: a support layer prepared exactly like a classic mem-brane by means of phase inversion and an ultrathin skinlayer. Each layer has a different structure and is composed

of different polymers. The porous support provides the mem-brane with mechanical stability against cracking and tearingwhen operating at high pressures, whereas the ultrathin sur-face layer is directly involved in the rejection process. Com-pared to membranes produced by simple-phase inversion,TFC membranes allow high retention of substances in thefeed while maintaining the highest possible flow rates [11].TFC membranes are important in commercial applicationsof nanofiltration and reverse osmosis. TFC membranes alsooffer stabile at high pH (i.e., pH 5-13) and temperatures upto 70°C. Their widespread use is due to a higher retentionof the feed components with a simultaneously increased per-meate flux. Interestingly, there is a possibility of optimizingboth the support and the skin layer independently of eachother, depending on the desired effects. These propertiesmake TFC membranes suitable for use in the removal ofheavy metals, hardness, and micropollutants such as pesti-cides, endocrine disruptors, and pharmaceuticals from sew-age water [9, 13].

2.1. Thin-Film Composite Membrane Manufacturing. Anultrathin layer of polymer is generally obtained by themethod of forming a polyamide (PA) by polycondensationof two monomers, diamine and polyacyl chloride, on thesurface of a porous support. These monomers should be dis-solved in two immiscible liquids. One of them, with dissolvedpolyamine, should pretreat the support, and the reactionitself takes place at the interface [11, 14]. This process is sche-matically illustrated in Figure 2.

The support for composite membranes can be made of avariety of polymers, i.e., polyamides, polyethersulfone, poly-urethanes, and even polyesters [15]. Poly(vinylidene fluo-ride) (PVDF), poly(vinyl alcohol) (PVA), polyacrylonitrile,or polytetrafluoroethylene (PTFE) can also be used [16]. Inaddition, the solvent that is used also affects membrane prop-erties. The most commonly applied solvents are N,N-di-methylformamide (DMF), dimethyl sulfoxide (DMSO), and

Microfiltration (MF) Ultrafiltration (UF) Nanofiltration (NF) Reversed osmosis (RO )

Fats

ColloidsSuspension

Viruses

ProteinsBacteria

Ion

Low-molecular weight compoundsMacromolecular compounds

Figure 1: Separation in pressure membrane techniques depending on the size of the components [1–3].

2 International Journal of Polymer Science

N-methylpyrrolidone (NMP) [17]. However, the composi-tion of the skin layer, i.e., the monomers that can be usedduring the in situ interfacial polymerization, is constantlybeing modified. The previously mentioned diamines aremost commonly used—they are aliphatic or aromatic com-pounds and are reactive enough to form a layer of polyam-ide. Piperazine (PIP), m-phenylenediamine (MPD), andp-phenylenediamine (PPD) are used, among others, andthey may react with acidic monomers containing chlorineatoms: polyacyl chloride (TMC), isophthaloyl chloride(IPC), and 5-isocyanatoisophthaloyl chloride (ICIC). Struc-tural formulas of these substances are shown in Figure 3.The most commonly used reaction for commercial purposesis MPD/TMC, also shown in Figure 3, as it yields goodresults in seawater desalination [18].

2.2. Factors and Preparation Conditions Influencing TFCMembrane Properties

2.2.1. Effect of Support. As it was mentioned in the previouschapter, the first step of TFC membrane fabrication is thepreparation of a support by mixing a polymer and organicsolvent and casting the prepared solution. The main polymertype with the widest application is polysulfone (PSf) due toits high thermal and chemical stability, glass transition tem-perature, and its good ability for forming membranes. Theonly limit of this material is its hydrophilicity [20]. Higherglass transition temperature, important for further curing ofmembrane, has also uncommon polymers like poly(phtala-zione ether sulfone ketone) and poly(phthalazinone etheramide) [21]. Most works include the use of polysulfone. PSfsupport was used in the study of Ding et al. [22] with concen-trations ranging from 11 to 19wt.%. Values of water fluxesfor support layers dropped significantly from 1750 to 0(L/m2∗h) with a decrease in PSf concentration. A less signif-icant tendency was found for complete TFC membranes, i.e.,from 40 to 10 (L/m2∗h). This can be explained by the lowporosity of the support structure with higher concentrationsof polymer. Contrariwise, salt rejection increased when poly-mer concentration increased from 11 to 16wt.% and a fur-ther increase in polymer concentration did not affect saltrejection. Zhang et al. [23] proved that the size and pore

density became more visible and distinct when polymer con-centration in the support decreased from 20wt.% to 12wt.%.Moreover, supports with low porosity had less visible surfacestructures and lower thickness and had weak bonding to theactive layer. Contrarily, membranes with higher porosity hadmore desirable properties.

Properties of thin-film membranes can be changed notonly by use of different polymers but also by differentorganic solvents. Lu et al. [24] used dimethylformamide(DMF) and N-methyl-2-pyrrolidnone (NMP) to preparecasting solutions. The DMF-prepared membrane had alarger pore size and higher surface area. This membranealso had a greater amount of area available for water per-meation at the polysulfone-polyamide interface [24].Importantly, the fouling tendency was higher for theDMF-prepared membranes due to the greater hydrophilic-ity of NMP-prepared membranes.

A dependence between the pore size of the supportand the retention of salts by TFC membranes was verywell presented by Huang and McCutcheon [25] for com-mercially used supports and by Sharabati [26] forlaboratory-prepared membranes. Both reported that anincrease in support pore size was highly connected with adecrease in salt rejection and an increase in permeation.The porosity of the membrane support can be also changedby addition of a porogenic substance like polyvinylpyrroli-done (PVP). This is a water-soluble polymer and has excel-lent wetting properties in solutions This additive caused anincrease in the hydrophilicity of membranes, rejection ofNaCl, and an overall increase in porosity by a having a largernumber of pores with a smaller size. This substance is widelyrecommended for modification of support. PVP is nontoxicand it can be used in processes for drinking water [22, 27].

2.2.2. Effect of the Monomer Type. For TFC membranes, animportant factor is the selection of proper monomers. Inthe research paper written by Li et al. [28], PES was used asa support. The membrane obtained was submerged in anaqueous solution of one of the amines—diethylenetriamine(DETA), triethylenetetramine (TETA), tetraethylenepenta-mine (TEPA), or PIP—at a concentration of 0.2% by weightand left for 10 minutes. Excess solution was removed with

(a) (b)

(c)(d)

Figure 2: The process of in situ interfacial polymerization [11, 14]: (a) creation of a support layer by phase inversion; (b) distribution ofdiamine aqueous solution on the membrane surface; (c) distribution and mixing of polyacyl chloride dissolved in an organic solvent onthe membrane surface; (d) polymerization reaction, formation of the polyamide skin layer.

3International Journal of Polymer Science

tissue paper. Then, a 0.2%-by-weight TMC solution in hex-ane was slowly poured onto the membrane for two minutesto fully cover its surface and it was left in place for 30 minutesto allow the hexane to evaporate and the skin layer to reachthe desired stability. The membrane was then stored indeionized water until it was used. The membrane thusobtained had osmotic properties. Depending on the polymerused, different membrane properties were obtained. First ofall, the contact angle values were significantly different.While the DETA/TMC membrane had almost the same con-tact angle as the unmodified pristine PES membrane (66o),the use of TEPA or PIP produced membranes that werehydrophilic. This positively affected permeate flux, whichwas directly dependent on the hydrophilicity of the mem-brane. The second parameter that was subjected to changewas the value of the zeta potential. The name zeta potentialrepresents the electrostatic charge of a membrane, which isassociated with the presence of functional groups on its sur-face and is dependent on the pH value of the feed. Its valuerefers to the membrane’s ability to repulse or attract posi-tively or negatively charged substances present in the feedsolution. In a study already mentioned [28], a strong negativecharge was caused by the deprotonation of carboxyl groupsfrom the hydrolysis of acyl chloride from TMC; however, itmay be compensated by the protonation of amine groupsfrom the same reaction. Although both hydrophilicity andthe zeta potential of the membrane played a significant role

in the separation of contaminants, each membrane hasachieved the level of selected salt removal at over 80% (oneionic and two ionic). Permeate flux recovery ranged from87% to 100%, depending on the selected monomer. TheTFC membrane with PIP had the best properties both interms of permeability and rejection [28, 29]. The highest fluxof this membrane was related to its contact angle. Generally,membranes with high contact angle have hydrophobic prop-erties that limit water permeation, and thus, a lower waterflux is observed. But along with reduction of the contactangle, the hydrophilicity of membranes increases and theirpermeability is raised. Details regarding the membranes dis-cussed above are given in Table 1.

Similar tests were also reported by Saha and Joshi [30]but with other types of monomers. In addition to piperazineand MPD, n-2-diaminethylpiperazine (AEPIP) and 3,5-dia-minobenzoic acid (BA) were used. These monomers reactwith TMC and isophthaloyl chloride IPC. This undertakingdid not yield very good results. However, interesting correla-tions were observed based on differences in the concentrationof monomers. Namely, it was observed that an increase inPIP concentration from 0.5 to 5% by weight in relation toTMC caused an increase in NaCl retention from less than10% to about 70% and MgSO4 from about 25% to almost100% retention with a slight decrease in the value of thepermeate flux. A similar relationship was found forMPD-TMC membranes, where both NaCl retention and

HN

HN

H2N

H2N

NH2

H2N NH2

NH2

Piperazine (PIP) M-Phenylenediamine (MPD) P-Phenylenediamine (PPD)

COCI COCI

COCICOCI CIOC COCICIOC

NCO

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COCI

Isophthaloylchloride (IPC)

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5-Isocyanato-isophthaloylchloride (ICIC)

O

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O O

OH

C

O

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O

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C C

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NH NH NH NH

HN

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+

Figure 3: The examples of substances commonly used in the in situ interfacial polymerization process. PIP, MPD, and PPD are dissolvedusually in deionised water and IPC, TMC, and ICIC in organic solvent like hexane. Beneth presented typical reaction between MPD andTMC [18, 19].

4 International Journal of Polymer Science

the flux increased with an increase in the MPD monomerconcentration from 0.5% to 2% by weight. Sodium chlorideretention was reported to increase from 75% to more than90%. On the basis of the presented experiments, it can beconcluded that TMC is characterized by more favourableproperties than IPC, as it increased the hydrophilic proper-ties of the skin layer. On the other hand, the use of AEPIPresulted in a significant decrease in the permeate flux [30].

2.2.3. Effect of Reaction Parameters. It has already been men-tioned that the properties of composite membranes are alsoinfluenced by the conditions in which the processing is con-ducted. In a research undertaken by Khorshidi et al. [11],TFC membranes were prepared using PES as a support andMPD and TMC reactions were used to make the skin layer.Four membranes were made at four different temperaturesof organic solvent: −20°C, 1°C, 25°C, and 50°C. At the tem-perature of 25°C, a membrane surface topography withclassic “hills and valleys” was obtained. At elevated organicsolvent temperatures, the skin layer of the membranereached a greater thickness, as opposed to the membranesformed at −20 and 1°C, which had the smoothest surfaceand the thinnest layer on the PES membrane support. Ahigher temperature of skin formation caused the MPD/TMCpolymerization reaction to be faster, whereas at lower tem-peratures, an opposite phenomenon was observed. Changesin the value of the contact angle were mainly related to differ-ent morphologies of these membranes. The membranes pre-pared at room temperature were characterized by the highesthydrophobicity, whereas the membranes formed at lowerand higher temperatures were much more hydrophilic. Thepermeate flux was also temperature dependent. For TFC-1and TFC-2, a lower temperature of acid chloride solutionmeant lower thickness of the membrane and thus higher per-meability. At a temperature higher than room temperature,the permeate flux increased due to a larger and moreextended surface which made the surface of contact betweenwater molecules and the membrane surface larger [11].Detailed parameters of these membranes have been demon-strated in Table 2.

2.2.4. Permeability and Selectivity.However, the most impor-tant preparation factors for permeability and selectivity ofTFC membranes, according to Mah et al. [31], are the con-centrations of both monomers, reaction time, presence, andtime of curing membrane. Interestingly, the pH of the aque-ous solution during the preparation process had no influence

on increasing the permeability of the membrane. However,a slight increase in membrane permeation was observedwhen the first monomer concentration was increased andthe reaction time in the interphase was reduced. From otherworks [32–34], an increase in the concentration of MPDand TMC in solutions caused an increase in selectivity witha decrease in permeability. High MPD concentrations canproduce a dense and thick PA layer by promoting the IPreaction. The effect of this is reduction of water permeationand promotion of high salt rejection. The influence ofhigher TMC concentration is also connected with a higheramount of TMC that participates in the polymerizationreaction [32]. Opposite results for TMC, achieved by Khor-shidi et al. [34], were explained by a complicated influenceof the polyamide thin-film thickness and density on theproperties of membranes.

The dependence between TMC concentration and per-meability and selectivity, appropriate for low concentrationsof monomers, is shown in the flow chart (Figure 4). Gener-ally, lower flux and higher selectivity of TFC membranesare directly connected with reduction of pore size and overallmembrane thickness. This is a result of more cross-linkingoccurring between monomers during formation of the thinfilm. The hydrophilicity of a membrane depends on the func-tional groups attached to its surface. The presence of aminogroups changes the hydrophilicity of membranes dependingon pH [31]. The most significant influence on membranepermeability was curing temperature and time. Tests con-ducted byMah et al. [35] showed that longer exposure to cur-ing caused the top layer to become denser which decreasedthe flux of pure water and promoted the rejection of salt.The minimum time of hexane evaporation from the TFCmembrane (during curing) was 5 minutes. Fifteen minutesof curing was needed to enhance the selectivity of a mem-brane [31]. Khorshidi et al. [34] found that increasing thecuring temperature improved flux and rejection but temper-atures higher than 75°C lowered the flux. A decrease incuring temperature resulted in thinner membranes andhigher permeation. This factor contributed 40.7% to theimpact on the water permeation. MPD and TMC concentra-tion exerted a lower effect, around 28% of contribution.Changes in reaction time between MPD and TMC had theleast influence on water permeation—less than 2% [34].However, all mentioned factors influenced the porosity ofmembranes, including the properties of support used. Adecrease in concentration of the first monomer caused anincrease in pore size. A similar effect was caused by areduction in the time of soaking and immersion in both

Table 1: Parameters of the membranes formed, depending on themonomer used [17].

Membranename

Contactangle (o)

Zeta potential(mV)

Permeability(dm 3/m2h∗MPa)

DETA/TMC 64 5 ± 0 5 −13 7 ± 0 4 33 5 ± 2TETA/TMC 47 −18 9 ± 0 6 43 5 ± 3TEPA/TMC 35 4 ± 2 2 −17 7 ± 0 5 51 ± 4 5PIP/TMC 39 6 ± 1 6 −24 4 ± 0 8 75 ± 6

Table 2: Parameters of the membranes formed, depending on theorganic solvent [5].

Membranesymbol

Heptanetemperature (°C)

Contactangle (o)

Flux(LMH)

NaClretention (%)

TFC-1 -20 53 3 ± 1 2 92.1 94.8

TFC-2 1 56 9 ± 1 1 51 97

TFC-3 25 81 2 ± 1 6 10.7 98.8

TFC-4 50 66 2 ± 1 27.9 99.1

5International Journal of Polymer Science

monomers. A decrease in TMC concentration may cause thepore size to decrease. The porosity of TFC membranesmostly depends on the porosity and pore size of the supports.Thin-film membrane properties are also affected by the addi-tion of nanocomposites.

3. Thin-Film Nanocomposite Membranes

For TFN membranes, the “nano” prefix is the key aspect. Itindicates the presence of a filler in the form of nanoparticles,nanotubes, or nanosheets. This addition is used to enhancethe properties of the membranes. The specific features ofthe TFN membrane area significantly increase permeate fluxcombined with better separation properties, and an energycost reduction is associated with the operation of thesemembranes. These are new types of membranes formed bymeans of the process of in situ interfacial polymerization.The first membrane of this kind appeared in 2007, whenthe first publications on the subject were published [10]. In2010, the first desalination plant to use such membranesappeared. As in the case of TFC membranes, most applica-tions are related to nanofiltration or reverse osmosis but theyare increasingly being used in low-pressure processes such asUF and MF [10].

The preparation of TFNmembranes is almost identical tothat of TFC membranes. The only change in the membrane

preparation process is the addition of a nanomodifier. TheTFN membrane is made up of a thin skin layer of up to500 nm thick which is placed on a much thicker supportmade with a method other than the skin layer, usually bymeans of phase inversion. Nanoparticles can be placedwithin the membrane structure in two ways: by addingthem to the support layer to change the membrane charac-teristics or by adding them to the selective skin layer toimprove roughness, selectivity and, above all, permeability[13]. Additionally, nanocomposites can also be added tothe thin selective layer also in two ways. Namely, the nano-particles can be placed on the TFN membrane eithertogether with an aqueous solution or with an organic sol-vent solution. It is much more common to place the nano-composite together with the TMC solution in hexane. Thisis mainly due to better results achieved from these typesof membranes [36]. Figure 5 demonstrates the basic typesof nanocomposite membranes.

3.1. Modification of TFN Membranes. Nanomaterials in TFNmembranes belong to one of four basic groups. They can beclassified as dimensionless hydrophilic metals or metaloxides, one-dimensional carbon materials such as nanotubes,two-dimensional materials such as graphene or grapheneoxides, or also three-dimensional porous materials suchas nanozeolites [37]. The types of the most common

TFCmembranepreparation

Soaking ofmembrane in 1.

monomeraqueus solution

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Yes No

(i)(ii)

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(iii)

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(ii)(iii)

(i)(ii)

(i)

(i)

(ii)

(iii)

Figure 4: Flow chart of preparation of TFC membranes with general influence of the most important factors: time of reaction/process,concentration of monomers, and curing temperature. First and second monomer concentrations were in ranges 1-4 wt.% and 0.1-0.2 wt.%,respectively [31–34].

6 International Journal of Polymer Science

nanoadditives are listed in Table 3. As mentioned before,these ingredients can be added together with an aqueousamine solution or in an organic solvent, together withTMC. Besides, they can also be placed in the support of themembrane [13].

The variety of nanoparticles provides numerous sizes andshapes. They occur in the form of crystals (cellulose), tubes(carbon nanotubes, halloysite nanotubes), ribbons, platelets,dots, and sheets (graphene). Their size is in the range of afew nanometers to dozens of micrometers Another impor-tant factor is the spontaneous and preferential aggregationof many of them which causes their useful size to be multi-plied. As was already mentioned, filler-polymer interfacegreatly affects the overall TFN properties. Therefore, boththe size and shape the nanoadditives play an important rolein their fabrication. For example, Alberto et al. [40] foundthat the size of GO flakes affects the porosity of the mem-brane structure. Micrometer-sized flakes of GO generatedlarge and nonselective voids between polymer and graphene,while smaller flakes were aligned with polymer blocks [40].Another aspect is the shape of nanofillers. Halloysite nano-tubes tended to egress from polymer matrices, leaving a hol-low space inside the composite [41]. Chong et al. comparedthe topography and thickness of two TFN membranes mod-ified with (1) titanium oxide and (2) titanate nanotube. Tita-nate nanotubes, due to their long length, preferred to layhorizontally on the surface, simultaneously creating a slightlylower membrane thickness than TFN with titanium oxide[42]. On the contrary, Karkooti et al. [43] demonstrated noeffect of different shapes (sheet, platelet, or ribbon) of gra-phene oxide on TFN cross-section morphology. As they

observed, all membranes had an asymmetric structure witha dense top layer and a porous finger-like support [43].

The most common types of carbon nanomaterials used inthe preparation of nanocomposite membranes include car-bon nanotubes (CNTs), graphene, graphene oxide (GO),MXenes (two-dimensional inorganic compounds made upof carbides but also nitrides or metal cyanides), and carbonmaterials obtained from carbides and fullerenes. Due to theirunique physical properties, such as high thermal, chemical,and mechanical durability, high conductivity, good opticalproperties and low density, these compounds are widely usedin material engineering and chemistry. However, the mostpopular nanoadditives are carbon nanotubes (CNTs), whichwe have mentioned before. Their wide application is due totheir exceptional mechanical, electrical, and thermal proper-ties. Their main advantage is wide chemical modifiability, aswell as their large area in relation to the dimensions of thenanotube itself [17, 44]. A membrane with the addition ofnanotubes can be as durable as a ceramic membrane, whileretaining the flexibility of the polymer. Their special featureis the ability to modify the physicochemical properties ofthe membrane itself. Modification of membranes withhydrophilic particles increases the affinity of their surface towater, increasing permeability. The inner nanotube poresact as selective nanopores, which makes the modified mem-brane much more permeable than the unmodified mem-brane. However, the main disadvantage of nanotube use istheir tendency to agglomerate. Besides, some carbon nano-materials, e.g., CNTs, GO, and graphene, also have specificproperties, and when incorporated into the membrane, theyinhibit the growth of bacteria that is in direct contact with

(a)

(b)

(c)

Figure 5: Types of membranes with nanoparticles added [13]: (a) typical nanocomposite membrane; (b) TFN membrane with nanoparticlesin the skin layer; (c) TFN membrane with nanoparticles in the support.

7International Journal of Polymer Science

the surface of the membrane. This may have a positive effecton the reduction of fouling. Moreover, graphene-modifiedmembranes have the same durability as those with CNTaddition but they are much more flexible, whereas theaddition of carbon nanotubes makes them bactericidal—according to research, 61% of bacteria were killed in onehour [45, 46].

The presence of nanoparticles in the membrane structureresults in the membrane having better properties, most nota-bly making it much more permeable. For example, in testsconducted byMahmood et al. [47], brackish water from a rivermouth was filtered with TFC and TFN membranes with theaddition of multiwalled carbon nanotubes at a concentrationof 0.4% by weight. These were placed in the support of themembrane, and the MCM-41 substance (made of syntheticaluminosilicates of a special structure) was placed in the skinlayer at the concentration of 0.05% by weight. The result ofthese studies [47] was to obtain an almost identical level ofretention of salts for both membranes (about 93.7%) with amuch higher permeate flux for the TFN membrane by about54.85% in comparison to the TFC membrane. This increasein permeability was obtained thanks to the formation ofmacrochannels in the support layer and the strong antifoulingproperties of the nanocomposite membranes [47].

Membrane porosity depends mostly on the support struc-ture and the kind of modifier on its surface. Adamczak et al.[48] prepared UF membranes with PES support and modifiedthem with single-walled carbon nanotubes. The presence ofnanotubes caused the decrease in porosity. In these tests,porosity of the pristine support was around 70-75%, whilemembranes containing nanotubes had porosity at around30-50%, dependent on nanotube location in TFNmembranes:in the support or the in thin film. In the Emadzadeh et al. [49]

study, the overall porosity of membranes increased as the TiO2loading in the support increased; however, the average poresize decreased. Importantly, the water flux increased withincreasing TiO2 concentration. The membrane prepared fromthe PSf substrate bound with 0.5wt.% TiO2 has shown the bestperformance for the water desalination process. However,according to Dalvi et al. [50], in tests with polyhedral oligo-meric silsesquioxane TiO2-SiO2, nanoparticles, regardless ofsize and shape, can cause formation of defects in the polyam-ide layer. The defects appeared more often with larger size andhigher concentrations of nanoparticles. In this test, they low-ered rejection of salts.

A membrane was formed with the use of graphene for thepurpose of the Yin et al. study [51]. It was made in the sameway as reported by Mahmood et al. [47] where graphene wasadded to the TMC solution. The presence of grapheneallowed the membrane to maintain a similar retention valueof Na2SO4 and NaCl salts at the level of 93-98% with a signif-icant increase of the permeate flux from about 39 LMH toover 59 LMH at the concentration of 0.015% of GO. Simi-larly, with an increase in the concentration of nanosheets,there were slight fluctuations in the increased value of themembrane’s contact angle, between 62o and 68o, indicatingan increase in hydrophilicity and a decrease in the averageroughness of the membrane from approximately 60 nm toapproximately 40 nm. The most important aspect of thisundertaking was to maintain retention with increased per-meability through the introduction of nanoparticles [51].

Such tests, related to the method of introducing nano-composites, were conducted in the work of Huang et al.[36]. Three membranes based on a PS membrane wereformed with a skin layer based on MPD and TMC reactionswith zeolite nanoparticles: one with NaA added in the

Table 3: Division of nanoparticles used in the preparation of nanocomposite membranes [18, 20, 38, 39].

Particle shapes Particle types Description and examples

Dimensionless Metals Metals and metal oxides: silver, titanium, zirconium, iron, gold, and others

One dimensional

Carbon nanotubes (CNTs)Single walled (SWCNTs), multiwalled (MWCNTs), and modified with

carboxyl (-COOH), amine, amide, and hydroxyl (-OH) residues

Carbon nanofibers (CNFs) Available in bundles and modifiable as CNTs

Cellulose nanofibers (CNF)Contains a lot of hydroxyl groups on the surface; possibility of modification

similar to CNTs

Cellulose nanocrystals (CNC) Similar properties as CNF

Two dimensional

Graphene In the form of hexagonal nanosheets

Graphene oxide (GO) Like graphene, with oxygen and hydrogen atoms attached to carbon atoms

MXenesA substance consisting of several layers of metal carbides or carbon nitrides;

they have different conductivity and hydrophilicity depending on theircomposition

Two dimensional andthree dimensional

Nanozeolites

A group of hydrated clay minerals whose composition includes silicon,aluminium, magnesium hydroxides, and metal ions; this group also includes

synthetic materials; natural: montmorillonite and halloysite; synthetic:MCM-41

Three dimensionalFullerenes

Spherical allotropic form of carbon consisting of graphene platelets; fullereneswere components of the first nanocomposite membranes

Metal organic frameworks (MOFs)Nanoporous materials composed of inorganic subunits of metal ions and

polytropic organic ligands

8 International Journal of Polymer Science

aqueous phase, a second with hexane solution added duringthe phase, and a third with no nanoparticles added. In thetest, a NaCl salt solution was filtered through membranes ata pressure of 1.6MPa. It was demonstrated that the permeateflux increased with an increase of nanoparticle concentration(for concentrations from 0% to 0.2% by weight), while thedegree of retention slightly decreased. This was explainedby the presence of defects in the membrane structure, whichallowed cations and anions to permeate and pass through themembrane. Salt retention on the NaA membrane in organicsolvent solution was higher than that on the NaA membranein water and was therefore considered to be more beneficialfor particle dispersion on TFN membranes [36].

Less commonly, aluminosilicates, which are also knownas halloysites, are used as nanocomposites. They are used inthe shape of nanotubes (HNTs) and are extracted andformed from natural deposits of this substance. This sub-stance has unique properties. The outer layer is negativelycharged due to the presence of siloxane groups (Si-O-Si),while the inside of the nanotubes is filled with positivelycharged hydroxyl groups (Al-OH). Moreover, unlike carbonnanotubes, they do not have a tendency to agglomerate.Nowadays, they are increasingly used in research works.Also, their production cost is lower than CNTs [52]. Themost recent aspect related to the addition of nanoparticlesis the use of a new type of compound, the so-called metalorganic frameworks (MOFs). These are new-class nanopor-ous materials that are composed of inorganic subunits ofmetal ions and polytropic organic ligands. The presence ofthe organic fragments makes the substance more compatiblewith the polymer that the membrane is composed of. Fur-thermore, these nanomaterials can be freely modified onaccount of the presence of free bonds that can be supple-mented with functional groups. The presence of the alkylchains reduces nanoparticle aggregation. A study conductedby Guo et al. [53], where nanoparticles were added to theTMC solution in hexane, confirmed the desired effect of thistype of nanomaterial. The material maintained almost 100%tetracycline retention, and nearly 30% higher membrane per-meability was achieved compared to the unmodified mem-brane [53].

A quite new aspect of membrane modification by nano-composite is the application of nanofibers or nanocrystalsof cellulose (CNFs or CNC). These fillers are reminiscent ofcarbon nanotubes and nanofibers, but cellulose materialsare hydrophilic, naturally occurring in the environment,and they are a renewable resource. Similar to CNTs, theycan be functionalized by adding functional groups, such ascarboxyl groups (carboxycellulose nanofibers), on their sur-face. Nanofibers or nanocrystals may be extracted from cellu-lose, but this requires large energy consumption, and theirprocessing uses a large amount of acid [38, 54]. Extractionof these nanofillers may be conducted from jute fibers, bam-boo cellulose, or grass [8]. The use of these products from cel-lulose in membrane preparation is gaining more and moreattention each year. For example, Asempour et al. [55] usedCNC in a TFN membrane for RO application. Embeddednanocrystals improved antifouling properties, lowered thecontact angle to 20o, and doubled water flux with only a slight

decrease in salt rejection. Bai et al. [56] used CNC on the PESsubstrate and improved the removal of cationic and anionicdyes from around 96-97% to 99.83-99.9%. In the same work,the addition of CNC on the membrane surface caused a slightdecrease in NaCl and MgSO4 retention and a significantincrease in water permeability. They also found that the con-tact angles of the pristine substrate and unmodified TFC were66.5o and 59.2o, respectively, and CNC addition resulted in adecrease of the contact angle to 37.5o. For TFN membraneswith cellulose nanofibers, Carpenter et al. [38] provide dataabout an improvement of the mechanical strength of mem-branes filled with CNF. Similar to CNC, membranes withCNF are characterized by an increase in water permeabilityand hydrophilicity with constant contaminant removal.

4. Advantages and Disadvantages ofTFC/TFN Membranes

The main advantages of thin-film membranes produced inthe form of TFC are related to their good separation perfor-mance and wide pH tolerance range. However, compositemembranes with polyamide films are not free from limits.Typical disadvantages of unmodified polyamide films arethe relatively low water flux, poor antifouling property, andlow resistance to exposure to oxidizing agents such as chlo-rine [57, 58]. There are several mechanisms that have aneffect on polyamide films that are described by Verbekeet al. [59], but the effects are inconsistent. This may also bea result of a tightening or swelling effect. In some cases, thePA layer may dissolve or separate from the support. In somecases, reaction of chlorine with the PA top layer enhances thehydrophobicity of the membrane simultaneously with waterflux reduction [59]. Another limiting aspect of TFC is thelow stability of PA films in acidic pH. In this environment(pH < 5), the carbonyl groups of amide linkages in the PAchains are attacked [60]. The degradation of PA chains withpartial hydrolyzation of amide groups by the H+ or OH-

causes a reduction in the degree of cross-linking. Thisincreases membrane hydrophilicity and at the same resultsin larger permeability and lower selectivity [61]. In Rezaniaet al. [62], filtration of solution of Na2SO4 conducted at pH3 caused a decrease in rejection from 91.1% in pH 7 to81.3%, and at pH 5, rejection decreased to 88.2%. To restrainthis effect and also degradation by chlorine, nanocompositesare placed on the surface of the membrane [59].

Thin-film nanocomposite membranes have many advan-tages in comparison to TFC membranes such as higher waterflux and better bacterial resistance without a significantreduction in salt rejection. This is due to the small particleschanging the membrane performance. The presence of nano-composites may improve surface properties of membranessuch as hydrophilicity/hydrophobicity, porosity, zeta poten-tial, and antibacterial properties [63, 64]. For example, Banoet al. [65] compared the influence of TFC and TFN propertieson water flux and salt retention. They proved that addition ofgraphene oxide improves hydrophilicity, zeta potential, androughness while simultaneously maintaining the same levelof salt retention.

9International Journal of Polymer Science

The major obstacles for the commercialization of theTFN membranes are high production cost, leaching outof toxic nanoparticles into the permeate and retentate,and low interaction between nanoparticles and polyamidethin films [55]. Leached nanoparticles may be toxic forthe user and the environment. Elimination of this threatmay be done in two ways: (1) by preventing the leachingout of the nanoparticles or (2) by using biocompatiblenanoparticles [55]. For example, silver nanoparticles, takenorally, are distributed to all the organs and attack primarilythe liver and intestinal tract [66]. Leaching of silver nano-particles (AgNPs) was firstly proved and then limited byYin et al. [67]. In a batch test, they measured that within14 days, 12% of the total amount of AgNPs was leachedfrom the membrane. In a flow test, leaching of silver wasreduced to 3.5% by using cysteamine as a bridging agent.In a work presented by Wan et al. [68], ferrum/platinumnanoparticles were integrated into a membrane by functio-nalization with poly(acrylic acid). Another way to bypass thedanger of nanoparticle release is by the use of nontoxic mate-rials. An example of such a substance is cellulose nanocrystalthat is obtained from acid hydrolysis of native cellulose [55].The only limitation is the application of TFN membranes inacidic conditions. Some nanoparticles like aluminium-richzeolites may be less structurally stable in low-pH conditions(pH of less than 2) particularly in the presence of phosphate(pH of less then 5) [69]. However, by overcoming theseobstacles, both nanocomposite-free and modified mem-branes are being used more and more frequently inmanufacturing and technological processes.

5. Application of TFC/TFN Membranes

TFC/TFN membranes have a great potential for watertreatment. Due to their high-permeability and selectivity,good durability, and antifouling properties, they dominateover conventional polymer membranes. These membranesare mainly used in high-pressure membrane processes (i.e.,nanofiltration and reverse osmosis for desalination of sea-water). However, some cases of their use for micropollu-tants and organic matter removal have been alsoreported. Karkooti et al. [43] proved that a TFN mem-brane modified with 0.1wt.% graphene nanoribbons hadexcellent properties in terms of both water flux andorganic matter removal [43]. In the Bojnourd [70] study,the concentration of cephalexin was reduced by 98.8% byusing a TFN membrane with 0.4wt.% of modified mont-morillonite [70].

While TFC membranes are widely employed in desali-nation plants and in the food industry, the use of TFNmembranes on an industrial scale is negligible. The highproduction cost and the potential threats connected withnanoparticle release are obstacles to wider use. On theother hand, many studies have shown that TFN mem-branes are more effective than TFC membranes, especiallyin terms of the removal of toxic pesticides, arsenic salts,proteins, hormones, and micropollutants from wastewater(e.g., in the pharmaceutical industry) [49, 69, 71]. More-over, in comparison to other sewage treatment technologies,

such as biological methods, TFN results in a better quality oftreated sewage at lower operating costs. Additionally, TFNmembranes are used in laboratories to obtain high purityproducts [71].

Currently, TFC membranes made of polyamide andcellulose acetate are most commonly used for water desalina-tion. They take part in both RO and FO processes. TFNmembranes are also commercially available. One example isthe RO TFN membrane modified with zeolite nanoparticles(100 nm in diameter) that is intended for seawater desalina-tion. This has been available since August 2010. Its efficiencyis approximately 10-20% higher than that of the classic ROmembrane with a similar reduction in energy consumption.This type of membrane has been successfully installed onthe island of Curacao for the purpose of seawater desalina-tion [72, 73]. A similar cost reduction related to energyexpenditure was observed in the processes of reverse osmosiswith a TFN membrane [74].

6. Conclusions

Despite having many unquestionable advantages for bothTFN and TFC membranes, there are still many challengesthat modern science must face. The main problem is apoor dispersion of nanocomposites in inorganic solventsand, consequently, their agglomeration in the membranestructure. Surface modification of nanoparticles partiallysolves this problem, but it cannot eliminate this phenome-non completely [10]. Moreover, a problem of secondarycontamination from nanoparticles leaching from mem-branes should be solved.

Compared to TFC membranes, TFN membranes have ahigher potential for further development both in terms ofeliminating fouling and increasing permeability while main-taining a high level of retention. Yet, another differencebetween these two membrane types is the higher productionprice of TFN membranes. However, the use of TFC/TFNmembranes increases productivity and thus results in loweroperating costs [74].

However, it should be remembered that no universalmembrane exists that would be able to work with any kindof feed material. Moreover, the membrane manufacturingprocess needs to be optimized, depending on the needs, bothon a laboratory scale and an industrial scale.

Conflicts of Interest

The authors declare that there is no conflict of interestregarding the publication of this paper.

Acknowledgments

The paper was created as a result of a research projectfinanced from the funds of the National Science Centretitled “Modelling the Transport and Retention of OrganicMicropollutants by New Generation of Ultrafiltration Mem-branes,” decision number DEC-2016/21/B/ST8/03128.

10 International Journal of Polymer Science

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