Materials and engineering design of interfacial polymerized thin film composite nanofiltration membrane for industrial applications

2.5 Conventional Applications of TFC Nanofiltration 582.5.2 Wastewater and Water Treatment 59 2.6 Functionalized TFC Nanofiltration and Its Applications 61 2.6.1 Positively Charged Thin

C H A P T E R

2

Materials and Engineering Design of Interfacial Polymerized

Thin Film Composite

Nanofiltration Membrane for Industrial Applications

B.S Ooi * , J.Y Sum * , J.J Beh * , Woei Jye Lau†,

*School of Chemical Engineering, Universiti Sains Malaysia,

Nibong Tebal, Malaysia

2.5 Conventional Applications of TFC Nanofiltration 58

2.5.2 Wastewater and Water Treatment 59

2.6 Functionalized TFC Nanofiltration and Its Applications 61

2.6.1 Positively Charged Thin Film Composite Membrane 61

2.6.2 Chemical Resistance Nanofiltration 64

2.6.3 Thin Film Nanocomposite Membrane (TFN) 65

2.7 Separation Principles and Solute Transportation 66

2.7.1 Driving Force of NF Process 67

2.7.2 Membrane Transport Model 68

NF to have specific applications in water treatment [1], wastewatertreatment[2–4], hardness removal[5, 6]and bio-separation[7, 8].There are many ways to produce polymeric nanofiltration, includingphase inversion technique for asymmetric membrane, interfacial polymer-ization, coating [9–12], and layer-by-layer assembly methods [13, 14].Among these methods, interfacial polymerization (IP) is known to be asimple method due to its flexibility in optimizing the support layer andthe thin film layer, separately Interfacial polymerization was initiallyproposed by Wittbecker and Morgan in 1959 whereby polymerizationoccurs in the interface of two immiscible phases whereby one phase iscommon water, and the other phase consists of a low dielectric constantsolvent such as n-hexane The thin film composite membrane could be opti-mized by tailoring the properties of both support and selective layerseparately The semipermeable skin layer is designed for higher water fluxand better solute rejection whereas the desired qualities such as highporosity and excellent mechanical strength were optimized for thesupport layer[15]

In selecting a nanofiltration membrane for a specific application, tion and water permeability are two critical parameters to evaluate theirperformance Its performance relies on the membrane materials, operatingconditions, and solution chemistry Membrane rejection is typically evalu-ated using mono and divalent ions such as NaCl, Na2SO4, MgSO4, andMgCl2 It is commonly accepted that NF can reject at least 95% MgSO4.Interfacial polymerized TFC NF membrane has been successfully commer-cialized and used in the water treatment industry partly to replace theenergy-intensive reverse osmosis system Before the year 2020, NF willdominate the membrane market with an expected growth of$445.1 million

rejec-or annual growth rate of 15.6%[16] The market is shared by the top-rankednanofiltration manufacturers such as Koch Membrane, Microdyn-Nadir,Hydranautics, GE Osmonics, Toray, and Dow-Film.Table 2.1summarizesthe performance of some commercially available NF membranes In aver-age, the flux of an NF membrane is within 2–14 L/h m2bar with >95%rejection of the multivalent ions

2.2 MEMBRANE CHARACTERISTICS AND ITS

PERFORMANCE

A membrane prepared via IP process has different morphologydepends on the process conditions namely the reactant concentrations,the ratio of reactants, the solubility of the reactants in the organic phase,diffusion rate and the kinetics of hydrolysis and crosslinking[17].The ideal thickness of the ultra-thin skin layer that governs the membraneflux is preferably under 0.10μm It is necessary to produce membranes withhigher productivities without severely affecting the membrane selectivity.Various studies revealed that the membrane performance is related to themolecular arrangement or chemical structure of the skin layer The desiredproperties of the thin film included defined pore size (<1 nm), narrow poresize distribution, defect-free, smooth morphology, and thin and robustpores Research works to enhance the membrane performance could be cat-egorized into two areas namely (i) preparation condition, e.g., reaction time,concentration, pH, temperature, and humidity (ii) material selection such asfunctionality of the reactants and its solubility or partitioning coefficient.Significant improvement has been achieved in fabricating the poly-meric and the nanocomposite membrane via interfacial polymerization.The IP process can be further based on the type of monomer and reaction(polycondensation or polyaddition) Polycondensation is commonly used

to produce the commercially available polyamide NF membrane whereaspolyaddition involves radical polymerization of alkene and initiators thatbeing immobilized on the solid substrate The success of IP in producing

49

2.2 MEMBRANE CHARACTERISTICS AND ITS PERFORMANCE

Membrane Material Manufacturer

Contact angle Retention

Water permeability (L/m 2 h bar) References NF40 Polypiperazinamide Dow-Film Tec 20 bar, 2000 ppm

NaCl ¼ 45%

20 bar, 2000 ppm

Na 2 SO 4 ¼ 95%

20 bar, 2000 ppm MgCl 2 ¼ 70%

13.5 (PWP) 3.7  10 11 m 3 /(m 2 s Pa)

5.7 (PWP) [18, 22]

a The membranes are not produced via interfacial polymerization.

PWP, pure water permeability.

defect-free ultrathin films lies on the “self-sealing” and “self-termination”mechanism as a result of the slow solute diffusion[24].

The other parameters involved in IP process include the porosity ofsupport layer, wettability, monomer concentration, type of monomer,type of organic solvent, reaction time, solution pH, and the presence ofadditives In general, during polymerization two competing phenomenaare occurring namely nucleation and crystallization as well as polymergrowth (crosslinking, hydrolysis, and film growth) Fast nucleation willinduce smaller nodule structure or tighter skin layer whereas excess filmgrowth reduces the membrane permeability A membrane with thinnerskin layer has higher permeability but may compensate its structurerobustness Depending on the applications of the NF membrane, the oper-ation that required low transmembrane pressure has the luxury of havingthinner film layer

2.3 MATERIAL SELECTION

Compared to an RO membrane, nanofiltration has higher flexibility inchoosing the type of monomer involved in the IP process Nonplanarmolecules with a functional group provide a broader choice for an NFmembrane compared to an RO membrane, which is mainly limited bym-phenylenediamine To date, many monomers are being employed tosynthesize the TFC NF membrane The standard material used for thepolymerization process of reverse osmosis and NF include polyamide andpolyester but are not limited to other polymers such as polyurea, polyure-thanes, polyesters, and polycarbonate

2.3.1 Polyamide

TFC membranes are typically made from a thin polyamide layerdeposited onto the porous layer A thin polyamide layer with the thick-ness< 200 nm are formed by reacting the diacyl/triacyl chloride withdiamine monomers such as semiaromatic polyamide (piperazine-isophthaloyl chloride) [17, 25], aliphatic or aromatic diamine [26–28],aromatic-cycloaliphatic (cyclohexane-1,3,5-tricarbonyl chloride andm-phenylenediamine-4-methyl)[29, 30], polysulfonamide[31], polyviny-lamine[32], cyclen[33], and dopamine[34] The wholly aromatic MPD-membranes normally give higher water and salt transport compared tothe nonplanar piperazine membranes Different monomer combinationproduces membrane ranging from RO to NF with specific applications

in low-pressure desalination or selective organic solute separations Asthe flux is increased, the selectivity of the membrane may increase to allow

the permeation of monovalent ions while rejecting the hardness solution(divalent ions) and bigger organic compounds.

The thinner skin layer of the NF compared to the RO membrane enables

it to operate at higher flux but much lower pressure (<7 bar) Although the

PA membrane is relatively hydrophilic with good chemical stability, the

PA membrane is considered to have poor chlorine tolerance (<0.1 ppm)compared to other polymers The membrane is not biodegradable as com-pared to a cellulose acetate membrane and can operate within a pH of 3–9

[35] The membrane with amide group exhibits a positive charge at a pHlower their isoelectric point (around pH 5), which means that polyamidemembrane is mildly negative charge in most of the water stream

It is commonly known that a polyamide membrane with its hydrophilicnature has enhanced antifouling properties A TFC polyamide membraneutilizing natural polymer, such as the Sericin-TMC membrane, possessed

a better antifouling property compared to the conventional polyamidebased nanofiltration (NF270)[36] Besides antifouling properties, polyam-ide with antibacterial properties also could be tailored made via interfacialpolymerization A new nanofiltration membrane prepared by interfacialpolymerization of polyhexamethylene guanidine hydrochloride(PHGH) and trimesoyl chloride (TMC) demonstrated the improvement

in term of antibiofouling performance[37] Similar to the nanofiltrationcontaining zwitterionic moieties, polyamide with N-aminoethyl pipera-zine propane sulfonate also showed excellent bacterial adsorption resis-tance [38] Fig 2.1 shows that a polyamide TFC membrane can betailor-made using different bi-amine monomers or oligomers

2.3.2 Polyester

Polyester membrane exhibits better chlorine resistance compared to thepolyamide membrane The incorporation of the ester linkage can signifi-cantly increase chlorine resistance and oxidation of the membrane Thepolyester TFC membrane can be synthesized by incorporating monomerssuch as triethanolamine[39], bisphenol A[40], tannic acid[41], resorcinol

[42], pentaerythritol[43]and hyperbranched polyester[44, 45] In general,the polyester membrane exhibits negatively charged surface with higherrejection toward divalent anions like the common NF membrane It wasexpected that by having higher negative charge, the membrane could

be more resistant toward organic fouling such as humic acid

Because amines react with acyl chloride groups much more readilythan alcohols, the synthesis of a TFC polyester membrane is not as easy

as that of a TFC polyamide membrane In view of this, polyesteramidethin-film-composite (TFC) membranes that combine the benefits of poly-amide and polyester can be produced by tailoring the ester/amide ratio

53

2.3 MATERIAL SELECTION

FIG 2.1 Interfacial polymerized thin film composite polyamide membrane using different monomers.

The polyesteramide membrane has a relatively good oxidative resistancecompared to the polyamide membrane[46] The fluxes of the polyestermembranes are lower (0.7–2.5 L atm1m2h1) compared to the polyam-ide membrane[42], and their rejection is not comparable to polyamidemembrane For example, the polyester TFC membrane synthesized fromtriethanolamine (TEOA) and trimesoyl chloride (TMC) were tested onsalts separation at 0.6 MPa The rejection of the mono and divalent saltsdecreased following the order of Na2SO4> MgSO4> NaCl > MgCl2

[39] The obtained results showed that the polyester NF membrane isrelatively loose compared to polyamide TFC membrane.Fig 2.2showsthat polyester membrane can be produced via interfacial polymerizationbased on different alcohol groups

2.3.3 Polyamine

Compared to polyamide NF membrane, which usually containspiperazine in the aqueous phase, polyamine NF membrane uses oxidantreacting compound, which protects the membrane from damage by chlo-rine Theoretically, it is deduced that membranes, which are derived frompolyethyleneimine and trimesoyl chloride, have better pH stability andresistance toward a nucleophilic attack as compared to polyamidemembranes[47] Lee et al proposed the synthesis of a TFC polyamine mem-brane with low pH stability by reacting cyanuric chloride and diethylenetriamine monomeric amines The membranes were found to have highersalt rejection and water permeability[48] A fluorinated polyamine mono-mer [CF3(CF2)6CONH(CH2CH2NH)2CH2CH2NH2] was also synthesizedand utilized to perform interfacial polymerization with trimesoyl chloride

FIG 2.2 Polyester produced via interfacial polymerization using various alcohol groups.

55

2.3 MATERIAL SELECTION

The surface free energy of the NF membrane is as low as 23.0 mJ/m2, whichcould resist the adhesion of foulants such as bovine serum albumin (BSA)and humic acid[49].

2.3.4 Polyurethane

A new type and uncommon TFC-NF membranes can be prepared viainterfacial polymerization of poly(bis-MPA) and methylene diphenyldiisocyanate[50] The membrane rejection toward MgSO4is lower than40%, indicating that it is a loose NF membrane However, the membraneclaimed to have more organic fouling resistance even though not muchinformation could be obtained for the PU membrane synthesized viainterfacial polymerization

2.4 CONTROL OF INTERFACIAL POLYMERIZATION

Over the years, research has been carried out to enhance the membraneperformance by varying the reaction conditions such as types of mono-mer, monomer concentrations, curing conditions, types of solvent, andpresence of additives in the solvent[15, 51–55] These parameters showhigh impact toward the membrane properties such as surface morphol-ogy, chemical and bonding property, mechanical property, permeability,and selectivity

2.4.1 Monomer

The properties of a thin film can be manipulated by changing the ical structure, size, solubility, shape, and reactivity of the monomer[28].Besides, Chen et al found that monomer concentrations, as well as wet-ting/swelling agents, could play an essential role in the membrane proper-ties too[56] For example, a thin-film-composite (TFC) nanofiltration (NF)membrane composed of aliphatic piperazine and aromaticm-phenylenediamine mixture was employed to separate oleic acid It was found thatthe monomer concentrations and drying times have great impacts towardthe membrane’s properties[53] Roh and co-workers demonstrated thatincreasing the diamine concentration during MPD/TMC reaction caused

chem-a drop in surfchem-ace hydrophilicity chem-and thus led to the declinchem-ation of flux

[52] The authors pointed out that membrane permeability is determined

by counterbalancing both the film thickness and surface hydrophilicity.One of the methods to increase the flux is by hydrophilizing the mem-brane material In membrane technology, concern was given to producing

a membrane with superior permeability and selectivity, which can beachieved via chemical approach For instance, the hydrophilic

hydroxyl-ended hyperbranched polyester (HPE) was employed as a mer in synthesizing NF membrane by in-situ IP process with trimesoylchloride (TMC)[57] The membrane showed enhanced permeability with-out scarifying its rejection capability Another TFC membrane synthesizedfrom hyperbranched polyethyleneimine (PEI) renders similar result byincorporating the free rotating amines group that could enhance the waterpermeation[53, 58].

mono-2.4.2 Reaction Conditions

Many studies have been carried out to relate the membrane synthesisconditions toward their performance Past study suggested that temp-erature and relative humidity were dominant factors in tuning the finalthickness, structure, and performance of the skin layer Mickols found thatthe controlling parameters for the membrane flux are the reaction time aswell as the amine solution temperature[59] The skin thickness could alterthe membrane flux, but the membrane selectivity was found to beindependent of film thickness[15] The results obtained from the attenu-ated total reflectance infrared (ATR-IR) spectroscopy show that reactiontime, relative humidity, and reaction temperature are determining factors

An exciting study showed that the immerse time in an organic solution, aswell as the ratio of m-phenylene diamine and m-Aminophenol, couldaffect the water permeability[60]

2.4.3 Support Layer

A TFC membrane consists of a skin layer residing on a support brane The support aims to provide mechanical strength for moderate tohigh-pressure operations The support layer itself must have optimumpore size to diminish the additional resistance for water transport

mem-A TFC membrane is considered to be more advantageous than its cessor asymmetric membrane as it allows separate optimization of theskin layer and support layer[17, 61–63] However, recent studies revealedthat these two layers are in fact closely interrelated as the polyamide filmgrows directly on top of the support surface The surface features ofsupport are essential in defining the structural integrity and uniformity

prede-of the skin layer[63–66]

Singh and co-workers reported on the effect of different pore size tribution of polysulfone to the resulting polyamide thin layer properties

dis-[67] The substrate with a smaller pore size (0.07μm) shows higher saltrejection efficiency in comparison with the substrate of higher pore size(0.15μm) The hydrophilicity of the membrane substrate will determinethe penetration of water into the pores, and thus the thickness of skin layerformed[63].Fig 2.3illustrates the impact of pore opening and its surface

57

2.4 CONTROL OF INTERFACIAL POLYMERIZATION

chemical property to the film formation Support with more significant,hydrophobic characteristics will produce a more permeable TFC mem-brane with a rougher and thinner skin layer because less polymer isformed within the pores.

Besides asymmetric support, fibrous materials are also employed as aTFC support layer A study was carried out to synthesize thin film nano-fibrous composite (TFNC) membranes using the self-support nanofibrouspolyethylene terephthalate Results showed that the TFNC-NF membranehad improved salt rejection and 4-fold water flux than the TFC NF mem-brane The improved flux in TFNC membranes is due to the low waterpermeation resistance of the nanofibrous support and the more prominentpore structure[68] In another study, the polyethersulfone (PES) nanofi-brous supporting layer was modified with dopamine which offers aninnovative way for the synthesis of composite NF membranes with highsalt rejection (99.4%) and high flux (63.0 L/m2h)[69] Other scaffoldssuch as ultra-fine cellulose[70]and polyacrylonitrile nanofibers[71]werealso reported with good performance

2.5 CONVENTIONAL APPLICATIONS OF TFC

NANOFILTRATION 2.5.1 Water Softening

Water softening is a process that removes calcium, magnesium, andother metal ions from the resources such as groundwater and seawater

A hardness of 300–500 mg/L as CaCO3caused scaling in heating vessels

FIG 2.3 Conceptual model illustrating the impact of support pore structure and philicity toward polyamide thin film formation during interfacial polymerization [63]

hydro-and pipes The conventional methods for removing water hardness rely

on ion-exchange, precipitation, and sequestration by chelating agents.Reverse osmosis and nanofiltration are alternatives for water softeningwith NF have a typical TDS reduction of 70%–80% From the economicalaspect, the NF process is approximately 25% more costly that an anionexchange on a 20-year present worth basis[72]mainly due to the highercapital cost of NF alternatives However, as plant capacities increase, thecosts difference between the two processes decrease[72] Compared to theconventional softening method, operation costs of NF are considerablylow as expensive regenerations using large amounts of salt can beavoided Another report showed that for a small-scale plant, NF is a morecost-effective method for color removal For such a system, the NF systemscould treat the water at a much lower price compared to lime soda, ozon-ation, and adsorption using granulated activated carbon [73] It isexpected that following the recent improvement in NF performanceand lower operating pressure requirements, both CAPEX and OPEX of

NF can be further reduced

The ultimate aim of membrane development is to develop membraneswith increasing flux and enhanced rejections which are practical for desa-lination[28] Desalination is one of the central sectors that contribute to a15% market growth forecast of nanofiltration (NF) [16] Nanofiltration(NF) is a suitable seawater softening method that provides excellent per-meates for oilfield-water injection whereby the concentration of multiva-lent ions such as Ca2+and Mg2+in permeate were kept at very low level

[74] The pilot-scale testing revealed that with the presence of antiscalant,total hardness with a 87.7%–93.5% removal rate can be achieved using NFsystem[75] This enables NF to be effectively used in seawater softeningand provide excellent feed for seawater reverse osmosis[76]

2.5.2 Wastewater and Water Treatment

Nanofiltration has been applied as a hybrid technology to treat the water from leachate [77], textile effluent [3], tannery industries [78],pharmaceutical industries[79, 80], and agriculture industries[81, 82] Thecommon solute to be retained includes the divalent ions as well as medium

waste-to high molecular weight organic molecule (>200 Da) NF is synergeticallycombined with other technologies such as coagulation-flocculation [83],adsorption[84], ozonation[4, 85], and photocatalysis[86]to polish the qual-ity of the permeate further

Membrane fouling is a significant problem for the applications of NFmembrane in wastewater treatment A rapid decline in rejection efficiencyand permeate flux was commonly attributed to the fast concentration polar-ization buildup and deposition of hydrophilic small-molecular-weight

59

2.5 CONVENTIONAL APPLICATIONS OF TFC NANOFILTRATION

carbohydrates such as fulvic-like substances[87] Pretreatment prior to NF

is the most practical way to reduce fouling, prolong the membrane lifespanand improve its performance[88] In a technical and economic feasibilitystudy of an olive oil mill effluent treatment, nanofiltration was coupledwith dissolved air flotation pretreatment The system demonstrated itself

to be able to achieve high volume reduction factors and the rejectionfor total suspended solids (>80%), total organic carbon (>60%), chemicaloxygen demand (50%–70%), and oil and grease (60%–80%)[89]

Effluents from the textile industries consist of various pollutants such

as an organic material with high chemical oxygen demand, high pended solids, color, and other soluble substances[90] Nanofiltration iswidely applied to treat the textile wastewater by removing the colorand reuse the water Textile wastewater was recovered and reused using

sus-a membrsus-ane pilot plsus-ant It wsus-as found thsus-at NF90 membrsus-ane could yield sus-aCOD reduction of 99% and the salt retention of 75%–95% Later, it wasscaled up for long duration experiments using a spiral-wound module

It was found that the fouling phenomena were not significant and thefoulants can be easily cleaned[91]

2.5.3 Food Processing

One of the promising applications of NF in the food industry is related

to beverage industries Fruit juices have been traditionally concentrated

by the thermal operation; however, the high operating temperatureresulted in color changes as well as loss of juice aroma NF membranewhich is an isothermal process that offers a better way to preserve the orig-inal taste of the clarified juice by simple sieving mechanism The NFsystem was used to concentrate fruit juice from 10° Brix to 45° Brix at muchlower energy requirement compared to evaporation[92]

Other common applications of NF in food industries include wastestream whey processing Acid whey contains 0.55%–0.75% (w/v) of pro-tein, 4.2%–4.9% (w/v) of lactose and up to 93.5% (w/v) of water[93] Thepresence of high concentrations of lactate exerts operational problems inthe dryer due to its stickiness[94] Whey demineralization by nanofiltra-tion allows concentration and demineralization of the whey to be carriedout in a one-step process before spray drying[95] Under uncharged con-ditions, lactose can be retained preferentially over lactic acid due to its big-ger molecular weight On the contrary, under acidic condition, NF can beused to retain lactose while allowing the permeation of lactic acid Thehigh selectivity of NF demineralization process results in a 30% reduction

in lactic acid content and a reduction of 46%–60% in monovalent ions saltcontent[95]

Many attempts to produce a positively charged membrane using a ferent route has been reported Most of the methods are through grafting,coating, or the bulk polymerization with limited literature being reportedvia IP method The membrane bears the dissociable primary and second-ary amine, and its charge depends on the dissociation constant of theamine as well as the number of amine groups Commonly, macromoleculecarrying the polyamine groups could be employed to produce positivelycharged membrane A monomer with high amine groups has a lower par-tition coefficient and suffers from diffusional resistance during interfacialpolymerization, which explains why IP is not a favorable process to pro-duce NF membrane Monomers or polymers such as poly(ethylene imine),poly(vinylamine), poly(amidoamine) and poly(dopamine) are commonlyused to prepare positively charged nanofiltration.

dif-2.6.1.1 Poly (Ethylene Imine)

Despite its technical difficulties in preparing positively charged NFmembranes via an IP route, it was reported that quaternized branchedpolyethyleneimine (BPEI) had been successfully introduced into theTFC NF membrane via an IP process The membrane showed a typical

NF rejection trend of MgCl2> MgSO4> Na2SO4> NaCl [96] Anothersuccessful case was demonstrated by the membrane that bears fixed qua-ternary ammonium moieties that had been synthesized by functionalizingthe branched polyethyleneimine via reaction with glycidyl trimethylammonium chloride (GTACl) The membrane showed a significantincrease of selectivity toward chloride- and sulfate-bearing solutes[97]

In one of the studies, the skin layer of the composite hollow fiber (HF)was formed through an IP of branched polyethyleneimine (BPEI) and tri-mesoyl chloride (TMC) The resulting membrane acquired a positivelycharged surface with pure water permeability (PWP) of about 17 l/m2h barand a molecular weight cut-off (MWCO) of around 500 Da, or equivalent to apore diameter of about 1.29 nm[18, 98]

61

2.6 FUNCTIONALIZED TFC NANOFILTRATION AND ITS APPLICATIONS

A positively charged NF membrane had been employed for hardnessremoval and waste recovery For example, composite nanofiltration mem-brane with the positively charged surface was synthesized by reactingBPEI and trimesoyl chloride (TMC) The membrane was used for efficientrecovery of lithium from mixed LiCl/MgCl2solution The mass ratio of

Mg2+to Li+in the permeate decreased after the filtration[99] Such a brane was also employed for phosphorus recovery in which the mem-brane effectively rejected the heavy metal ions such as Pb, Cu, Zn, and

mem-Ni while allowing the passage of phosphate to achieve phosphorus ery[100] Other laboratory demonstrations found that the rejection rates ofcationic dyes at neutral pH were >96% using positively charged NFmembrane[101]

2.6.1.3 Poly (amidoamine)

Another easy way to introduce the positively charged moieties is bygrafting poly (amidoamine) dendrimer (PAMAM) on the interfaciallypolymerized layer The resultant membrane recorded>99% rejection overthe heavy metal ions such as Pb2+, Cu2+, Ni2+, Cd2+, Zn2+, and As5+, with amoderate pure water permeability (PWP) of 3.6 L m2h1bar1at 10 bar

[104] An easy method was also reported by reacting the carboxylic acids

on the surface of a polyamide thin film composite with poly(amidoamine)

in the presence of 2-chloro-1-methylpyridinium iodide as an activatingagent The membrane showed an elevated isoelectric point to pH 9.9because of the high density of free protonated amino groups The mem-brane showed excellent rejections toward metal ions including Cu2+,

Ni2+, and Pb2+[105]

The research was carried out by embedding poly(dopamine) modifiedmultiwall carbon nanotubes (PDA-MWCNTs) in polyamide thin-filmcomposite membranes The membranes showed improved flux with salt

rejection decreased in the sequence of ZnCl2(93.0%)> MgCl2(91.5%)>CuCl2 (90.5%) CaCl2, indicated that it is suitable for water softening

[106] A new nanofiltration membrane was synthesized by reactingpoly(amidoamine) and trimesoyl chloride During acidic solution filtra-tion, the pH in the feed increased from 2 to 9, which showed that the mem-brane could enhance the proton permeation [107] The membrane wasfound to be more positively charged due to the profound amine groups;however, its MgSO4rejection is poor due to the pore opening phenome-non This problem was solved by one of the recent work that incorporatedPAMAM and piperazine as co-monomer to form the skin layer with betterdivalent rejection[108] The membrane has a lower isoelectric point and atthe same time allow better rejection of MgSO4

Higher generation PAMAM (G4 and G5) were also incorporated into theTFC membrane It was found that the pure water flux was improved for106% at similar separation performance[109] Researchers found that thesurface charge of the thin layers was changed by incorporating PAMAMwith different generations and concentrations The membrane showednegatively charge behavior with higher Na2SO4 rejection compared toMgCl2[110] This phenomenon might be due to the competitive hydrolysisrate of the carboxyl group to form a negative carboxylate group due to thepoor partitioning of PAMAM into the organic phase By increasing the con-centrations and generations of the PAMAM, the cationic surface charge ofthe thin layers was increased[111] Based on this observation, it can beconcluded that the charge of the membrane does not only depend on thematerial itself but to a great extent depends on the processing conditions

It is especially true for the membrane prepared via the IP route

2.6.1.4 Poly (dopamine)

Positively charged composite NF membranes were synthesized bydepositing the poly (dopamine) (PDA) followed by grafting the poly(ethylene imine) (PEI) on the polyethersulfone support The salts rejec-tion followed the sequence of MgCl2> CaCl2> MgSO4> Na2SO4, show-ing that the membranes surface carried positive charge[112] Compared

to poly (ethylene imine), poly (vinylamine), and poly (amidoamine),polydopamine membranes are seldom synthesized via interfacial poly-merization due to its complex nature of the monomer The membranewas commonly prepared via self-polymerization [34], coating [113],and deposition [114] methods Nonetheless, TFC-NF membranes withgood structural stability were prepared via IP process under the media-tion of polydopamine (PD) Under optimal conditions, the membraneexhibited flux as high as 22.8 L/(m2h) while the rejection of Na2SO4

reached 93.5% under 0.2 MPa[115]without showing the evidence of itively charged surface

pos-63

2.6 FUNCTIONALIZED TFC NANOFILTRATION AND ITS APPLICATIONS

2.6.2 Chemical Resistance Nanofiltration

Since the first commercial applications of organic solvent nanofiltration(OSN) or solvent resistance nanofiltration (SRNF) around 1990, the NFmembrane with more robust materials has been continuously developed

to work in harsh conditions such as extreme pH conditions and theprocesses with organic solvents [116–118] It was reported that energyrequirement for solvents recovery could be reduced by>70% using NFassisted evaporation process[119]

SRNF membrane provides an effective chemicals recovery method foroleochemical industry[119, 120]and pharmaceutical industry[121] Forexample, SelRO@ NF membranes commercialized by Koch Membraneclaimed to have excellent stability in organic solvents Their membranesare found suitable for extreme conditions in the separation of heavymetals under both acids and alkaline conditions Although the composi-tion of the membrane is not entirely open, it is unlikely that themembrane is produced via interfacial polymerization SRNF membranesare commonly produced by coating or crosslinking the chemicallystable polymer such as poly (vinylidene fluoride) (PVDF), polyetherke-tons (PEEK), polyetherimide (PEI), poly(phenelene sulfide)(PPS) andpolybenzimidazole (PBI)

High flux solvent-stable TFC-OSN membranes are crucial In one article,diaminopiperazine (DAP) and trimesoyl chloride (TMC) reacted with thepolythiosemicarbazide support to give the high fluxes filtration toward sol-vents like dimethylsulfoxide, tetrahydrofuran, and dimethylformamide

[122] Another example includes a TFC membrane with cross-linkedpolyimide substrate with the skin being subjected to the posttreatment ofglycerol/sodium dodecyl sulphate and dimethyl sulfoxide (DMSO).The membrane was very permeable to methanol (5.12 lm2h1bar1),dimethylformamide (3.92 lm2h1bar1) and DMSO (3.34 lm2h1bar1)but could retain tetracycline well[123] Jimenez Solomon et al.[124]reportedthat solvent activation of the TFC membranes after IP could improve theorganic solvent permeation without compromising their selectivity

An OSN membrane is also applied to filter the solvent containingdye solution Mixed matrix membranes (MMMs) consist of carboxyl-functionalized multiwalled carbon nanotubes (MWCNTs-COOH) in P84polyimide with the aid of 1,6-hexanediamine (HDA) as chemical crosslinkerwere employed for organic solvent nanofiltration (OSN) The cross-linkedMMM has a rejection of 85% to rose bengal (1017.65 Da) while ethanol per-meance is 9.6 LMHbar1at 5 bar[125] Tris(hydroxymethyl) aminomethane(Tris), a hydrophilic monoamine, was added to the dope to modify polyi-mide OSN membranes during phase inversion [126] The isopropanol(IPA) permeability of the crosslinked membranes was increased as much

as 270% with slightly drop of dyes rejections