Journal of Membrane Science 511 (2016) 65–75

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Journal of Membrane Science

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Novel methodology for facile fabrication of nanofiltration membranesbased on nucleophilic nature of polydopamine

Tunyu Wang a, Hazim Qiblawey a,n, Easan Sivaniah b, Abdolmajid Mohammadian c

a Department of Chemical Engineering, College of Engineering, Qatar University, Qatarb Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Japanc Department of Civil Engineering, University of Ottawa, CBY A114, 161 Louis Pasteur, Ottawa, ON K1N 6N5, Canada

a r t i c l e i n f o

Article history:Received 16 December 2015Received in revised form19 March 2016Accepted 21 March 2016Available online 26 March 2016

Keywords:PolydopamineNanofiltration membraneGraftingTrimesoyl chloridePoly(ethyleneimine)

x.doi.org/10.1016/j.memsci.2016.03.04388/& 2016 Elsevier B.V. All rights reserved.

esponding author.ail address: hazim@qu.edu.qa (H. Qiblawey).

a b s t r a c t

Novel methodology for facilely fabricating nanofiltration (NF) composite membrane has been success-fully developed by employing nucleophilic nature of polydopamine (PDA) chemistry. The self-poly-merized PDA coating over polysulfone (PSf) substrate was utilized as a key intermediate layer for tri-mesoyl chloride (TMC) grafting followed by poly(ethyleneimine) (PEI) deposition to construct the hier-archically structured separation layer of NF membrane. In contrast to the electrophilic quinone moietiesof PDA layer usually involved in the Michael addition and Schiff base reactions with polymeric amines formembrane preparation in previous reports, the phenolic hydroxyl groups of catechol moieties as well asamine groups at the PDA layer possess nucleophilic nature, which are capable of quickly coupling withthe highly reactive acyl chlorides to form ester and amide bonds in the step of TMC grafting, resulting inTMC moieties covalently anchored at the PDA layer with free acyl chloride groups. Such created acylchlorides are further coupled with the amine groups of branched PEI polymer in the PEI depositionprocedure to form the stable amide bonds linking the PDA base layer and PEI upper layer in the resultinghierarchical separation layer. The properties of membranes prepared at different stages were char-acterized with respect to surface chemistry, pore properties, and separation performances to understanddeeply the newly developed methodology for membrane preparation. Further studies focusing on NFproperties and stability of the developed NF membrane revealed that such membrane shows high effi-ciencies in retention of divalent cations, small organic molecules as well as heavy metal ions, and exhibitsdesirable thermal stability and long-term performance stability.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Over the past few decades, nanofiltration (NF) membrane withproperties between ultrafiltration (UF) and reverse osmosis (RO)has attracted considerable research interests because of its dis-tinguishing characteristics such as low rejection of monovalentions, high rejection of multivalent ions and small organic species,as well as low energy consumption [1]. In general, most of NFmembranes have a charged selective layer with negative or posi-tive characteristic due to the ionization of surface functionalgroups. During the NF process, rejection of uncharged solutes ismainly governed by the size exclusion mechanism; while theelectrostatic interaction based Donnan mechanism plays a key rolein determining the separation behaviors of the charged solutes [2].To date, most commercial NF membranes are thin film composite(TFC) membranes comprising polyamide separation layers

fabricated by interfacial polymerization (IP) technique. However,great deals of research efforts have been sustainably made fordeveloping novel materials and methodologies to fabricate the NFmembranes with enhanced separation properties over the decades[1]. It is believed that the deep understanding towards the novelmethodologies for membrane fabrication is definitely necessary toenable the rational design of advanced membranes for practicalapplications [3,4].

In 2007 Messersmith and co-workers reported breakthroughresearch in the chemistry of surface modifications inspired by thecomposition of adhesive proteins in mussels. Since then, dopa-mine, one kind of such catecholamines, has received a significantinterest in the field of material science [5–7]. It has been demon-strated that dopamine could undergo self-polymerization in al-kaline environment to form a thin and hydrophilic polymer coat-ing on a wide variety of materials with great adhesive strength.Moreover, the formed multifunctional polydopamine (PDA) filmcould serve as an extremely versatile platform for secondary sur-face-mediated reactions to build up additional layers with specificfunctionalities [5,8]. To date, the detailed polymerization and

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–7566

binding mechanisms of dopamine are not well understood. It isbelieved that dopamine undergoes a complicated process of self-polymerization based on both covalent and non-covalent inter-actions, accordingly introducing plenty of functional groups in-cluding free amine groups, phenolic hydroxyl groups of catecholmoieties, as well as carbonyl groups of o-quinone moieties at theresulting PDA layer surface [9–11]. Owing to such unique proper-ties of PDA, numerous studies have focused on the application ofdopamine chemistry into surface modifications and fabrications ofmembranes for water treatment [7,12]. On one hand, dopaminecan be utilized as hydrophilic surface modifier by simple coatingapproach. The self-polymerized PDA layers formed on the surfacesof various membranes such as microfiltration [13], UF [14–16] andRO [17,18] endow the resultant modified membranes with en-hanced surface hydrophilicity and fouling resistance. In addition,the modification of the support layer with PDA film prior to theconventional interfacial polymerization has also been proved as aneffective approach to improve the separation properties of theresulting TFC membranes for forward osmosis (FO) [19], pressureretarded osmosis (PRO) [20], pervaporation [21], as well as NFapplications [22]. On the other hand, PDA can be utilized as afunctional intermediate layer for further construction of hier-archically structured separation layers of membrane with ad-vanced filtration properties [7]. A great deal of research activitieshas been focusing on the development of functionalized desali-nation membranes by applying the chemistry of Michael additionand/or Schiff base reaction between thiol or amine groups offunctional polymers and o-quinone moieties of the mediating PDAlayer. For example, thermo responsive UF membrane, high fluxand anti-fouling NF membrane, as well as positively charged NFmembrane have been successfully fabricated by grafting the ami-no-terminated poly(N-isopropylacrylamide) [23], fluorinatedpolyamine [24], and poly(ethylene imine) at the PDA coated pre-cursor layers [25,26], respectively. Such chemistry of Michael ad-dition and Schiff base reactions is based on the electrophilicity ofPDA due to the electrophilic nature of quinone carbonyl groups.

Scheme 1. Schematic illustration of the pro

However, to ensure such reactions proceed to completion, longreaction times, high temperatures and alkaline conditions are re-quired in the membrane fabrication procedures. Nevertheless, incontrast to the quinone carbonyl groups, the phenolic hydroxyland amine groups of the PDA have nucleophilic nature, whichcould facilely react with active electrophiles such as acyl-chloridegroups to form covalent ester and amide bonds [27–29]. Comparedwith the reactant system of quinone moieties and amines involvedin Michael addition and Schiff base reactions, the polar phenolichydroxyl and amine groups possess higher reactivity to couplewith acyl-chloride groups, enabling the reaction to take place atroom temperature and accomplish within a short time. However,up to now, there are very few reported works focusing on theutilization of such nucleophilic nature of PDA chemistry to facilelyconstruct the membrane separation layers [29].

In order to further extend the variability of PDA layer as aneffective platform for constructing functional separation layers ofdesalination membranes, in this work we herein report the suc-cessful usage of self-polymerized PDA film as a key intermediatelayer for fabricating novel NF membrane based on the nucleophilicnature of PDA chemistry. As illustrated in Scheme 1, the fabrica-tion process started with coating PDA layer onto the polysulfone(PSf) substrate, followed by employing trimesoyl chloride (TMC) asactive electrophilic modifier to be covalently grafted at the surfaceof the PDA layer via the coupling of acyl chlorides with phenolichydroxyl and amine groups. Finally, branched poly(ethyleneimine)(PEI) polymer comprising plenty of free primary and secondaryamine groups was anchored on the surface of TMC grafted PDAlayer through the reaction of amino groups with acyl chlorides toproduce the defect free separation layer with hierarchical struc-ture. The membranes prepared at different stages were char-acterized in terms of surface chemistry, pore properties, andseparation properties to better understand the feasibility andmechanism of the newly developed methodology for membranepreparation. Additionally, separation efficiencies of the as-pre-pared NF membrane for retention of dyes and heavy metal ions,

cedure for NF membrane preparation.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–75 67

and membrane stability in terms of thermal stability and long-term stability were studied in detail to evaluate the functionalityand stability of the PDA based NF membrane. Results obtained inthis study provide new insight into the preparation of advancedmembrane materials based on the bio-inspired polydopaminechemistry.

2. Experimental

2.1. Materials

Polysulfone (PSf) substrate (PS 20) was purchased from Andemembrane Separation Technology & Engineering (Beijing) Co. Ltd.The chemicals including dopamine hydrochloride, trimesoylchloride (TMC, 98%), poly(ethyleneimine) solution (PEI, 50% (w/v)in water, 750 kDa), and tris-(hydroxymethyl)aminomethane (Tris)were all obtained from Sigma-Aldrich and used for mem-brane preparation. The organic dyes orange II sodium salt(C16H11N2O4S� Naþ , 350.32 Da) and safranin O (C20H19N4

þ Cl� ,350.84 Da) used for dyes separation test were also received fromSigma-Aldrich. A series of polyethylene glycol (PEG) polymersused for pore size analysis were purchased from Alfa Aesar. Thenatural organic solutes raffinose pentahydrate and alpha-cyclo-dextrin were purchased from AppliChem. The inorganic salts in-cluding NaCl, MgCl2, MgSO4, Na2SO4, Pb(NO3)2 and NiCl2 �6H2Owere all obtained from BDH Chmeical Ltd. Deionized (DI) waterwas produced from a Milli-Q ultrapure water purification system.

2.2. Membrane fabrication

All operations were performed in a clean assembly room. PSfsubstrate membranes were first immersed in DI water for 24 h,then rinsed thoroughly with DI water and air dried before thedopamine coating procedure. The dopamine solution for surfacecoating was prepared by dissolving 2 g dopamine hydrochloride in1 L Tris–HCl buffer solution with pH of 8.8. The coating was con-ducted at ambient temperature in air for 20 h, and only the topsurface of the PSf substrate was exposed to dopamine solution.After the PDA coating, the resulting membrane labeled as M-PDAwas thoroughly washed with DI water for 0.5 h to remove the un-reacted chemicals. Subsequently, after being dried in the air, thetop surface of M-PDA was covered with 0.2% (w/v) TMC n-dode-cane solution for 3 min. Afterwards, the excess TMC solution wasdrained from the soaked surface. The membrane was rinsed twicewith n-hexane to remove excessive TMC and n-dodecane solvent.Next, the surface of the as-prepared TMC grafted membrane wasimmediately covered with an aqueous solution of PEI with con-centration of 1% (w/v) for 5 min at ambient temperature, and thenthe membrane was thoroughly rinsed with DI water. The resultingPEI deposited membrane was labeled as M-PEI. Separately, in or-der to obtain the aforementioned TMC grafted membrane withstable physicochemical properties for further characterizations,the as-prepared TMC grafted composite membrane was immersedin warm DI water for around 24 h to hydrolyze the surface acylchloride groups to carboxyl groups. The resulting membrane islabeled as M-TMC.

2.3. Separation experiments

All of the separation experiments including pure water per-meation, retention of inorganic salts, heavy metal salts, neutralsolutes as well as dyes were all carried out in a commercial cross-flow cell apparatus (Sterlitech Corp.) consisting of two testing cells(model: CF042) which are placed in series. The cross-flow systemis equipped with temperature control system. Because of the

variable permeation characteristics of the membranes prepared atdifferent stages, variable preconditioning time and operationpressures were applied in the filtration tests. For case of PSf sub-strate and M-PDA membrane, the preconditioning time and op-eration pressures were fixed as 0.5 h/1 bar, and 2 h/2 bar, respec-tively. For both M-TMC and M-PEI membranes, the precondition-ing time and applied pressure were fixed as 2 h and 4 bar, re-spectively. The feed temperature in all the filtration runs wasmaintained at 2570.5 °C with constant feed flow rate of2.5 L min�1 and cross-flow velocity of 47 cm s�1.

The pure water permeability (PWP, L m�2 h�1 bar�1) was cal-culated using the equation:

=Δ ⋅ ( )

QP A

PWP 1

where Q is the water permeation volumetric flow rate (L h�1), A isthe effective filtration area (m2), and ΔP is the trans-membranepressure drop (bar).

In order to characterize the membrane separation properties,electrolyte solutions prepared by separately dissolving inorganicsalts including NaCl, MgCl2, MgSO4 and Na2SO4 in DI water withconcentration of 0.5 g L�1 (500 ppm) were used as the feed solu-tions. The feed pH was around 6.170.2. The solute rejection (R)was calculated using the following equation:

( ) = – ×( )

⎛⎝⎜

⎞⎠⎟R

cc

% 1 1002

p

f

where Cf and Cp are the solute concentrations in the feed andpermeate, respectively. The concentration of salt solution wasdetermined by an electrical conductivity meter (HACH, HQ440dmulti).

The pore size, pore size distribution and molecular weightcutoff (MWCO) of the membranes were characterized according tothe neutral solute transport method [30–35]. The solute rejectionfor membranes can be expressed by a log-normal probabilityfunction of solute size, as described in the following equation[31,33]:

∫ μσ

= ( ) =π

=−

( )−∞−( )R erf y u y

r12

e d , whereln ln

ln 3u

T

y/2 s s

g

2

where RT is the solute rejection, rs the solute radius, μs the geo-metric mean radius of solute at RT¼50% and sg is the geometricstandard deviation about μs, defined as the ratio of rs at RT¼84.13%to that at RT¼50%.

When the solute rejection of a membrane is plotted againstsolute radius on lognormal probability coordinates, a straight linecan be obtained:

( )( ) = + ( )F rR A B ln 4T s

By ignoring the influences of steric and hydrodynamic inter-actions between the solute and the pores on the solute rejection,the mean effective pore radius (μp) and the geometric standarddeviation (sp) can be assumed to be the same as μs and sg, re-spectively. Therefore, the PSD of a membrane can be expressed asthe following probability density function on basis of μp and sp:

( )( )σ π

μ

σ

( )= −

−

( )

⎡

⎣⎢⎢

⎤

⎦⎥⎥

R rr r

rdd

1ln 2

expln ln

2 ln 5

T p

p p p

p p2

p2

where rp is the effective pore radius of the membrane. The valuesof μp and sp determine the position and sharpness of the PSDcurves, respectively.

A series of neutral organic solutes (200 ppm) were used tomeasure the solute rejection (RT) of the membranes (Table S1,

Table 1Surface elemental composition of the membranes.

Sample C 1s (%) N 1s (%) O 1s (%) S 2p (%) N/C O/N

PSf 82.93 3.22 12.25 1.61 0.039 3.804M-PDA 77.13 5.24 16.61 1.02 0.068 3.170M-TMC 76.86 4.33 18.10 0.71 0.056 4.180M-PEI 67.57 12.78 19.66 – 0.189 1.538

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–7568

Supporting information). Between runs of different solutes, themembranes were thoroughly flushed with DI water. The MWCO,which is defined as the molecular weight that above 90% of thesolute in the feed solution is retained by the membrane, was ob-tained by the following equation [31]:

= − + ( )rlog 1.5575 0.4911 log MW 6s

The sp values of the membranes calculated according to theeffective rejection curves are presented in Table S2 (Supportinginformation). The concentrations of the feed and permeate solu-tions were measured using a total organic carbon analyzer (Shi-madzu TOC-LCPH/CPN). The solute rejection was calculated accord-ing to the Eq. (2).

To evaluate the retention properties of M-PEI NF membranetowards small organic solutes, three aqueous feed solutions se-parately containing 50 ppm safranin O (C20H19N4

þCl� , 350.84 Da),50 ppm orange II sodium salt (C16H11N2O4S�Naþ , 350.32 Da), and50 ppm saccharose (C12H22O11, 342.30 Da) were used as the feedsolutions with pH around 6.170.2 in the cross-flow NF experi-ments [34,35]. The dye concentrations in feed and permeate weremeasured using spectrophotometer (HACH DR-5000). The max-imum absorption wavelength of the dye safranin O and orange IIsodium salt are 520 nm and 485 nm, respectively. The concentra-tions of saccharose in feed and permeate were measured using atotal organic carbon analyzer (Shimadzu TOC-LCPH/CPN). The so-lute rejections were calculated according to the Eq. (2).

To examine the separation performances of membrane M-PEIfor heavy metal removals, two heavy metal salts Pb(NO3)2 andNiCl2 were chosen as feed solutes to prepare the respective feedsolutions at concentration of 500 ppm without pH adjustment, aswell with pH of 3.0 adjusted by hydrochloric acid. The con-centrations of heavy metal ions in feed and permeate were mea-sured using inductively coupled plasma-optical emission spectro-metry (ICP-OES, Thermo iCAP 6500). The heavy metal rejectionrates were calculated according to the Eq. (2).

Stability of the M-PEI membrane in terms of thermal stabilityand long-term stability was evaluated in the cross-flow filtrationexperiment using 500 ppm MgSO4 solution as model feed. For thethermal stability assessment, the testing feed temperatures wereranged from 25 °C to 70 °C. The dropping rate (dr) of the MgSO4

rejection with the elevated temperatures was calculated throughthe following equation defined by Liang et al. [36]:

= − ( − ) ( )d R R T T/ 2 1 7T Tr 2 1

where RT1 and RT2 are the MgSO4 rejection of M-PEI at the feedtemperature of T1 and T2, respectively. For the long-term stabilityassessment, continuous cross-flow filtration test was conductedfor 72 h at the operation pressure of 4 bar and feed temperature of2570.5 °C.

2.4. Surface characterization

The membrane samples used for surface characterizations werethoroughly rinsed with DI water, followed by drying under va-cuum at 45 °C for 24 h.

The surface chemical structures of membranes were studied onthe basis of X-ray photoelectron spectroscopy (XPS) measure-ments through the Kratos AXIS UltraDLD spectrometer (KratosAnalytical Ltd.) with a mono Al Kα X-ray source. The surveyspectra were recorded over the range of 0–1200 eV, and high re-solution spectra of O 1s was also recorded.

The membrane hydrophilicity was characterized in term ofcontact angle (CA) by using a video-based optical CA measuringinstrument (Dataphysics OCA15Pro, Germany). DI water dropletwith volume of 2 μL was carefully dropped onto membrane

surface at room temperature. The CA data for each membrane wascalculated by averaging the CA values obtained at 15 different lo-cations of sample surface.

Scanning electron microscopy (SEM) was performed withNova™ NanoSEM 50 Series from the FEI Company, and magnifi-cations of up to 20,000 were obtained. Atomic force microscopy(AFM) was carried out using Atomic Force Microscope with Nanoindenter (AFM-MFP-3D, Asylum Research).

3. Results and discussion

3.1. Surface properties

The chemical structures of the membrane surfaces were stu-died on the basis of X-ray photoelectron spectroscopy (XPS) ana-lysis. The quantitative information for elemental composition ofPSf substrate, M-PDA, M-TMC and M-PEI membranes are sum-marized in Table 1. The survey spectra of membranes are includedin Fig. S1 (Supporting information). Compared with the pristinePSf substrate, M-PDA shows dramatically increased nitrogen con-tent of 5.24%, as well as higher N/C molar ratio of 0.068, whilesulfur content dropped to 1.02% from 1.61% of PSf. This resultclearly demonstrates the successful formation of PDA layer overthe PSf substrate. It is noteworthy that the appearance of sulfursignal on the M-PDA surface indicates that thickness of the PDAlayer is less than the 10 nm of XPS detection depth limit. Thisphenomenon was also reported by other previous works[19,24,25]. Moreover, after TMC grafting and subsequent hydro-lysis treatment, the surface of resulting M-TMC membrane is ob-served to possess obviously higher oxygen content compared withthe previous PDA layer. The atomic ratio of O/N increased from 3.17of M-PDA to 4.18 of M-TMC as shown in Table 1. Such considerablyincreased oxygen content at the M-TMC surface is due to the in-troduction of carboxylic groups formed from the hydrolysis ofunreacted acyl chlorides within the grafted TMC moieties [37].However, the sulfur 2p signal as shown in Fig. S1(c) can be stilldetected on the M-TMC surface, while the content further de-creases to 0.71% from 1.02% of M-PDA. This result indicates thatthe TMC grafting marginally contributed to the increase in layerthickness, as a result the thickness of the entire layer formed overthe PSf substrate after TMC grafting process is still within the10 nm range of XPS detection depth. Moreover, after the PEI de-position, it could be found that the nitrogen 1s signal of M-PEIshown in Fig. S1(d) is significantly enhanced, meanwhile the ni-trogen content as well as N/C ratio notably increased from 4.33% ofM-TMC to 12.78%, and 0.056 of M-TMC to 0.189, respectively. Thehigh nitrogen content indicates the presence of abundant aminegroups at M-PEI surface, thus confirming the successful formationof PEI layer over the TMC grafting layer.

In order to get more detailed information of the membranesurface chemistry, the high-resolution oxygen 1s XPS spectra ofPSf substrate, M-PDA, M-TMC and M-PEI membranes were de-convoluted and fitted. As shown in Fig. 1a, there are two fittedpeaks at biding energy of 530.4 eV, and 532.1 eV in O 1s spectra ofPSf, which are assigned to the oxygen in sulfonyl group (S¼O2*),

Fig. 1. High-resolution and deconvolution of O 1s spectra of the membranes: (a) PSf substrate, (b) M-PDA, (c) M-TMC, and (d) M-PEI.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–75 69

and ester bond (C–O*–C) of polysulfone, respectively [19]. Thedeconvolution of the O 1s spectra of M-PDA results in threecomponent peaks as shown in Fig. 1b. The new emerging signal of530.2 eV is attributed to the carbonyl groups (C¼O*) in quinonemoieties of PDA layer, which clearly further confirms the forma-tion of PDA coating on the PSf substrate [15,38]. The small peak at530.9 eV is due to the S¼O2* of PSf substrate. The oxygen in car-bon-oxygen single bond (C–O*), including C–O*–C of PSf, and thenewly formed phenolic hydroxyl groups (¼C–O*H) in catecholmoieties of PDA contributes to the component peak at 532.0 eV.Additionally, as can be seen in Fig. 1a and b, the peak area ratio ofC–O*/S¼O2* in M-PDA is obviously higher than that in PSf, owingto the presence of phenolic hydroxyl groups at M-PDA surface.Furthermore, the O 1s high resolution signal of M-TMC shown inFig. 1c consists of three component peaks. The major peak at530.2 eV could be assigned to the oxygen in carbon-oxygen doublebond (C¼O*) in quinone moieties of PDA base layer, the newlyformed carboxy groups (HO–C¼O*) and ester groups (–O–C¼O*)of the grafted TMC moieties. The peak at 530.8 eV is due to theS¼O2* from PSf substrate, whose notably weaken signal is con-sistent with the decreased sulfur content of the M-TMC surface asverified by the aforementioned element content analysis. In ad-dition, four different types of C–O* contribute to the componentpeak at 531.9 eV, which are C–O*–C of PSf, ¼C–O*H in catecholmoieties of PDA, as well as newly formed –O*–C¼O and H*O–C¼Oin TMC moieties. Furthermore, for the membrane M-PEI, there areonly two component peaks fitted as shown in Fig. 1d. The widepeak at 531.3 eV is assigned to the oxygen in amide groups (N–C¼O*), which demonstrates the formation of covalent amide

bonds from the reaction of acyl chlorides with amines of PEIpolymer. The peak at 529.7 eV is likely due to the carboxylic acidgroups (–COOH) resulted from the hydrolysis of acyl chloridegroups at the TMC grafted sub-layer of M-PEI. However, thecounter ion OH� of ammonium salt (–Nþ) formed from protona-tion of the free amines of deposited PEI polymer may also con-tribute to such peak at 529.7 eV [23,32,39].

The surface morphology of PSf substrate, M-PDA, M-TMC, andM-PEI membranes were characterized by SEM and AFM analysis.Figs. 2 and 3 present the representative SEM surface images andthree-dimensional AFM images over 5�5 μm scan areas, respec-tively. The surface roughness information obtained on the basis ofAFM analysis are expressed in term of root means square rough-ness (RMS) and average roughness (Ra), which are given in Table 2.It is found that the unmodified PSf substrate has a smooth surfacewith the mean RMS and Ra values both less than 3 nm. After beingmodified with PDA, some unevenly distributed nodules could beobserved on the PDA layer surface as shown in Figs. 2b and 3b. Therelated RMS and Ra are increased to 4.61 nm and 3.14 nm, re-spectively. Such nodule structures are the aggregates of PDA par-ticles formed during the self-polymerization process, which couldbe clearly observed in Fig. 3b. Several previous works have alsoreported the formation of aggregated particles at the PDA layersurface [22–25,40]. Furthermore, the M-TMC surface shown inFigs. 2c and 3c is observed to still consist of some nodules, butwith smaller number and size compared to its precursor PDA layer.The values of RMS and Ra dropped to 3.78 nm and 2.76 nm, re-spectively. Such observations indicate that the grafting of TMConto the PDA layer could lower the roughness of PDA surface in a

Fig. 2. SEM surface images of the membranes: (a) PSf substrate, (b) M-PDA, (c) M-TMC, and (d) M-PEI.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–7570

small extent. Furthermore, the deposited PEI layer of M-PEI dis-plays much smoother surface as shown in Figs. 2d and 3d. Themean values of RMS and Ra are less than those of the original PSfsubstrate as seen in Table 2. The considerable decline in surfaceroughness of M-PEI suggests that the coupling reaction based PEIdeposition could result in a smooth and uniform coverage of PEIpolymer matrix as upper layer.

The surface hydrophilicity of the membranes was characterizedby measuring the static contact angle (CA) between the membranesurface and the air-water interface. As seen in Table 2, the mem-brane M-PDA has visibly lower contact angle compared to thepristine PSf substrate, suggesting the improved membrane hy-drophilicity after being modified with PDA coating. Such ob-servation agrees well with the reported results, which can be ex-plained by the introduction of hydrophilic phenolic hydroxyl andamine groups at the PDA layer [22–25]. After the TMC grafting andhydrolysis treatment, the average contact angle of M-TMC mem-brane increased to 61.5° from 53.3° of M-PDA, suggesting a rela-tively more hydrophobic surface compared with its PDA precursorlayer. Since the carboxylic groups at the M-TMC surface is bene-ficial to improving the membrane hydrophilicity, the observedhigher contact angle of M-TMC may be resulted from the existenceof hydrophobic benzene rings of grated TMC moieties [26].Moreover, the surface of composite M-PEI membrane shows themost hydrophilic feature with the mean contact angle value of49.8°, which can be ascribed to the presence of hydrophilic aminegroups at the PEI layer surface.

3.2. Pore properties

The pore properties in terms of mean effective pore radius(rp), molecular weight cutoff (MWCO) and pore size distrib-ution (PSD) of the PSf substrate, M-PDA, M-TMC, and M-PEI

membranes were characterized by means of organic solutetransport method, which has been widely used in several reports[30–35]. The data for rp and MWCO are summarized in Table 3.The values of pure water permeability (PWP) calculated accord-ing to Eq. (1) are also given in Table 3. The PSD curves of themembranes are displayed in Fig. 4. It could be observed that theoriginal PSf substrate membrane possesses mean pore radius of2.01 nm and MWCO of 38690 Da. After the PDA coating, theaverage PWP of M-PDA membrane sharply dropped to76.3 L m�2 h�1 bar�1 with smaller pore size of 1.3 nm and lowerMWCO of 26388 Da. It is believed that during the coating processthe self-polymerization of dopamine occurs not only onto the topsurface of PSf substrate but also within the surface pores of thesubstrate, which reduces water permeance and pore size[13,18,19]. Moreover, the membrane M-TMC is found to havefurther decreased pore radius, MWCO and PWP. Additionally, thePSD curve of M-TMC became remarkably narrower compared tothat of PSf substrate and M-PDA as shown in Fig. 4. During theTMC grafting process, TMC may act not only as the grafter mo-lecules to be covalently anchored at the PDA layer surface, butalso plays the role of cross-linker to couple with the phenolichydroxyl and amine groups of PDA chains at the layer surface aswell as inside the surface pores, endowing the resulting TMCmodified PDA layer with much tighter pore structures. Thesmaller pore size and lower water permeability of PDA coatedmembrane after TMC grafting were also observed by Chung et al.[19], suggesting that the electrophilic TMC molecules with highreactivity towards phenolic hydroxyl and amine groups coulddramatically alter the pore properties of PDA layer by covalentlyattaching to the surface structures during the short grafting time.Furthermore, as seen in Table 3, the membrane M-PEI displaysthe lowest values for pore size, MWCO and PWP, while its leftshifted PSD curve becomes narrowest among the curves of the

Fig. 3. AFM images of the membranes: (a) PSf substrate, (b) M-PDA, (c) M-TMC, and (d) M-PEI.

Table 2Data for surface roughness and contact angle of the membranes.

Membrane RMS (nm) Ra (nm) CA (deg)

PSf 2.7570.90 2.2070.76 64.1771.46M-PDA 4.6170.27 3.1470.06 53.3070.70M-TMC 3.7870.63 2.7670.25 61.5373.90M-PEI 1.9170.28 1.4370.16 49.8273.23

Table 3Mean effective pore size (rp), molecular weight cutoff (MWCO), and pure waterpermeability (PWP) of the membranes.

Membrane rp (nm) MWCO (Da) PWP (L m�2 h�1 bar�1)

PSf 2.01 38690 257.1710.7M-PDA 1.30 26388 76.379.5M-TMC 0.52 3030 11.472.3M-PEI 0.23 292 3.571.1

Fig. 4. Pore size distribution of the membranes.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–75 71

membranes. The mean pore radius of 0.23 nm and MWCO of292 Da demonstrate the NF characteristic of the newly developedM-PEI membrane. It is believed that the quick reaction of freeprimary and secondary amine groups of PEI with acyl chloridesresults in the defect free coverage of branched PEI polymer ma-trix covalently anchored on TMC grafting layer, consequentlygiving rise to the formed hierarchical separation layer with smallmean pore size [41].

3.3. Separation properties

The salt rejection characteristics of membranes at differentpreparation stages were studied by cross-flow filtration experi-ments using aqueous solutions of MgSO4, NaCl, Na2SO4 and MgCl2

Fig. 5. Salt rejection rates of the composite membranes.

Table 5Rejections of M-PEI NF membrane towards heavy metal ions.

Heavy metal ion Ni2þ[NiCl2] Pb2þ[Pb(NO3)2]

Hydrated radius (nm) 0.404 0.401Feed pH 5.7 3.0 4.9 3.0Rejection (%) 94.670.2 96.570.2 91.170.3 90.170.4

Testing conditions: 4 bar, 2570.5 °C, 500 ppm feed solution.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–7572

as model feed, respectively. As shown in Fig. 5, the general saltrejections of M-PDA membrane are in a low level, indicating thePDA modified PSf membrane is still too loose to enable the ef-fective retention of metal ions. After the TMC grafting and sub-sequent hydrolysis treatment, the salt rejections of resultingM-TMC membrane are significantly enhanced. The average rejec-tion of Na2SO4 and NaCl increased from 20.5% and 1.2% of M-PDAto 72.3% and 18.3%, respectively. This phenomenon is mainly as-cribed to the tighter pore structure of M-TMC as proved in Section3.2. In addition, it is found that the rejection rates of M-TMC followthe order of Na2SO44MgSO44NaCl4MgCl2, suggesting themembrane can highly reject the divalent anion SO4

2� while theability for retention of divalent cation Mg2þ is relatively poor.According to Donnan theory, this phenomenon clearly demon-strated the negatively charged feature of the M-TMC membrane atthe testing feed pH of 6.170.2 [2]. The negative charges at theM-TMC membrane surface are believed to stem from the ioniza-tion of hydrolyzed carboxylic acid groups [37]. Moreover, after PEIdeposition, the resulting M-PEI NF membrane exhibits furtherimproved general rejections compared with M-TMC, owing to itsmuch denser separation layer after the PEI deposition. Interest-ingly, in contrast to M-TMC, the membrane M-PEI could highlyreject the divalent cations Mg2þ with the average rejection of93.7%, and the salt rejections decrease in the order ofMgCl24MgSO44NaCl4Na2SO4. Such result is in line with theDonnan exclusion effect, which indicates that the M-PEI mem-brane is positively charged at the testing feed pH of 6.170.2[2,33]. The positively charged surface property is owing to theprotonation of amine groups of the deposited PEI polymer chains.Additionally, since the mean pore radius of the M-PEI membrane(0.23 nm) is smaller than the radius of the hydrated Mg2þ ion(0.43 nm) [33], the size exclusion mechanism is also responsiblefor the superior rejection of M-PEI membrane towards MgCl2 andMgSO4.

Table 4Rejections of M-PEI NF membrane towards small organic solutes.

Solute Safranin O Orange II sodium salt Saccharose

Molecular formula C20H19N4þCl� C16H11N2O4S�Naþ C12H22O11

MW (Da) 350.84 350.32 342.30Charge þ – neutralRejection (%) 99.470.1 96.870.2 94.470.4

Testing conditions: 4 bar, 2570.5 °C, 50 ppm feed solution.

3.4. NF functionalities of M-pei

Owing to the particular NF characteristics of membrane M-PEIsuch as small pore size, narrow pore size distribution, as well aspositively charged feature with high rejection towards divalentcation (Mg2þ), it is of interest to further examine the capacity ofthe M-PEI NF membrane for retention of organic dyes and heavymetal ions to further evaluate the NF functionalities of M-PEImembrane.

Two type of dye solutes, safranin O (C20H19N4þCl� , 350.84 Da)

and orange II sodium salt (C16H11N2O4S�Naþ , 350.32 Da), whichhave opposite charge feature but similar molecule weight, werechosen as model feed solutes in the cross-flow filtration experi-ments. In order to better understand the effect of solute charge onthe dye separation of M-PEI membrane, the neutral saccharose,whose molecule weight of 342.30 Da is nearly similar to that ofsafranin O and orange II sodium salt, is also employed as feedsolute for comparison [34,35]. The rejection data are listed in Ta-ble 4. Surprisingly, it could be found that M-PEI exhibits high re-jection efficiencies towards all the three solutes. For the neutralsolute retention in NF process, the size exclusion mechanismprincipally governs the solute transport property. Thereby theexcellent rejection to saccharose can be attributed to the smallmean pore size of M-PEI. Interestingly, the mean rejection rate of96.8% for orange II sodium salt is slightly higher than that of 94.4%of for saccharose as seen in Table 4. It is believed that the posi-tively charged M-PEI is supposed to reject natural saccharose moreeffectively rather than the negatively charged orange II sodiumsalt, because compared to the neutral saccharose, the dissociatedanion C16H11N2O4S� of orange II sodium salt is much more proneto pass across the positively charged separation layer. Hence thesuperior retention efficiency towards orange II sodium salt mightbe ascribed the existence of surface negative charges at the TMCgrafted sub-layer of M-PEI membrane. It is likely that the acylchloride groups which did not react with amines of PEI during thePEI deposition process were finally hydrolyzed into the carboxylicacid groups, hence enabling the TMC grafted layer with negativelycharged feature to serve as the secondary barrier to rejectC16H11N2O4S� . Similar phenomenon has been also observed in theresearch of Sun et al. [34]. Furthermore, as shown in Table 4, M-PEIexhibits highest average rejection rate of 99.4% towards the posi-tively charged safranin O. The combination of Donnan effect andsize exclusion effect are believed to contribute to the excellentretention towards safranin O.

The separation efficiency of NF membrane M-PEI for heavymetal removal was then evaluated. Table 5 presents the membranerejections of Pb(NO3)2 and NiCl2 in respective 500 ppm feed so-lutions without pH adjustment, as well with pH of 3.0 adjusted byhydrochloric acid. It is observed that M-PEI shows excellent re-jection of 94.6% to Ni2þ and acceptable rejection of 91.1% to Pb2þ

when the feed solutions without pH adjustment were used, whichare believed to be resulted from the combined determination byDonnan effect and steric size exclusion [42,43]. According to thepositively charged nature of M-PEI under feed pH of 6.170.2 asproved in the aforementioned ions rejection experiments in Sec-tion 3.3, it can be concluded that M-PEI has the increased positive

Fig. 6. MgSO4 rejection and permeate flux of M-PEI NF membrane as a function ofoperation temperature tested with 500 ppm MgSO4 at 4 bar.

Fig. 7. Long-term stability of M-PEI NF membrane tested with 500 ppm MgSO4 at4 bar and feed temperature of 2570.5 °C.

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–75 73

charge under the feed pH ranged from 5 to 6 due to the proto-nation of surface amine groups [44,45]. On the other hand, asshown in Table 5, the mean effective pore size of M-PEI (0.23 nm)is smaller than that of hydrated Pb2þ (0.401 nm) and Ni2þ

(0.404 nm) [42], which could facilitate the ions rejection of NFmembrane through the size exclusion effect. Furthermore, as ob-served in Table 5, there are not obvious differences between therejections of Pb2þ and Ni2þ in case of acidic feed at pH 3.0 andthose at feed pH without pH adjustment, indicating the acidic feedcondition has marginal effect on the rejections of M-PEI towardsheavy metal ions Pb2þ and Ni2þ .

3.5. Stability of M-PEI

Membrane stability is a very important factor which significantlydetermines the applicability of the membrane. It has been reportedthat the composite NF membranes comprising PDA mediating layersexhibit excellent structure stability under alcohol treatment, owing tothe strong adhesion between separation layers and PDA modifiedsubstrates [22,24]. In this work, the stability in terms of thermal re-sistance for PDA based NF membrane M-PEI was examined with feedtemperatures elevated from 25 °C to 70 °C. The filtration time at eachoperation temperature is constant for 2 h. Fig. 6 presents the MgSO4

rejection and corresponding permeate flux data. It could be observedthat the permeate flux increased linearly with the increase of feedtemperature, which could be ascribed to the increase in locomotion ofwater molecules and the reduction of water viscosity, as well as theincreased flexibility of polymer chains within the separation layer athigh feed temperature [46,47]. Moreover, as seen from Fig. 6, theaverage rejection of M-PEI dropped very slightly with the increase offeed temperature. The rejection rate decreased to its minimum valueof 86.4% when the feed temperature reached to 70 °C, suggesting only3.8% of average rejection rate lost comparing with the original value of90.2% at 25 °C. Furthermore, the calculated dropping rate of �0.084%at the end testing temperature according to Eq. (7) is comparable tothe value of �0.035% for the thermally stable polyamide NF mem-brane with annealed PSf supporting layer as reported by Liang et al.[36]. In addition, it is worth mentioning that the separation perfor-mance of M-PEI is nearly constant during additional 6 h of continuousfiltration under the feed temperature of 70 °C. Moreover, the separa-tion performance of M-PEI was estimated again after cooling down thefiltration system back to 25 °C. It was found that M-PEI showedaverage rejection of 90.5% and flux of 10.8 L m�2 h�1, which arenearly similar to the original values at the feed temperature of 25 °C.This phenomenon indicates that the change in rejection and flux ofM-PEI with the rising testing temperature is reversible [48], suggesting

the slight decrease in rejection of M-PEI is presumably due to theincreased flexibility of polymer chains within the separation layer atthe elevated feed temperature. Overall, these results clearly demon-strate that M-PEI is thermally stable in hot separation environmentwith temperature ranged from 25 °C to 70 °C. The strong adhesion ofPDA base layer on the PSf substrate, as well as the existence of che-mical bonds linking TMC grafting layer with both PDA base layer andupper PEI layers, which endow the entire separation layer of M-PEIwith high stiffness and integrity, may all contribute to the good ther-mal resistance of M-PEI NF membrane.

The long-term stability of M-PEI NF membrane was furtherevaluated with 500 ppm MgSO4 feed solution at the operationpressure of 4 bar and feed temperature of 2570.5 °C. Fig. 7 il-lustrates the changes of permeate flux and rejection rate of M-PEIduring 72 h of continuous filtration. It can be seen that there arevery slight variations in the flux and rejection, indicating goodperformance stability of membrane M-PEI in long-time running.

4. Conclusions

In this study we have presented a novel methodology for thepreparation of nanofiltration membrane based on the bio-inspiredpolydopamine chemistry. Compared with the previously reportedapproach focusing on the electrophilic nature of PDA, the hier-archically structured separation layer of developed NF membranewith excellent separation properties could be more facilely con-structed on basis of the nucleophilic nature of PDA. The nucleophilicphenolic hydroxyl as well as amine groups at the surface of PDA layercoated over PSf substrate are proved to be able to quickly couple withthe highly reactive acyl chloride groups of TMC at ambient tem-perature to form the TMC moieties covalently anchored at the PDAlayer by ester and amide bonds, accordingly endowing the TMCmodified PDA layer with visibly enhanced salt rejections and de-creased pore size. In the subsequent PEI deposition procedure,amine-rich polymer PEI was covalently anchored at the surface ofTMC grafting layer through the reaction of amine groups with thefree acyl chlorides of the grafted TMC moieties, forming the defectfree separation layer with hierarchical structure. The resulting PEIdeposited membrane M-PEI shows NF characteristic with high re-jection efficiency towards divalent cation, small organic solutes, andheavy metal ions because of its positively charged surface feature andsmall pore size. Furthermore, M-PEI has very smooth and hydrophiliclayer surface, and exhibits good thermal stability as well as long-termperformance stability. Further study focusing on the functionalitiessuch as chlorine resistance and anti-fouling of the desalination

T. Wang et al. / Journal of Membrane Science 511 (2016) 65–7574

membrane fabricated according to this newly developed methodol-ogy is ongoing in our lab.

Acknowledgment

This work is supported by Grant from the Qatar National Re-search Fund (QNRF) under its National Priorities Research Program(NPRP) award number NPRP 4-935-2-354. The statements madeherein are solely the responsibility of the authors. We acknowl-edge Gas Processing Center (GPC, Qatar University) for XPS mea-surements and Center of Advanced Materials (CAM, Qatar Uni-versity) for AFM measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.memsci.2016.03.043.

Nomenclature

PDA polydopamineNF nanofiltrationRO reverse osmosisUF ultrafiltrationPRO pressure retarded osmosisFO forward osmosisTFC thin-film compositeTMC trimesoyl chloridePEI poly(ethyleneimine)Tris tris-(hydroxymethyl)aminomethanePSf polysulfoneQ water permeation volumetric flow rate (L h�1)A effective filtration area (m2)ΔP trans-membrane pressure drop (bar)MWCO molecular weight cut off (Da)PWP pure water permeability (L m�2 h�1 bar�1)R solute rejectionCp solute concentration in permeateCf solute concentration in feedrp mean effective pore radius (nm)PSD pore size distributionRMS root means square roughness (nm)Ra average roughness (nm)dr dropping rate of rejectionCA contact angle (deg)ICP-OES inductively coupled plasma-optical emission

spectrometryXPS X-ray photoelectron spectroscopySEM scanning electron microscopyAFM atomic force microscopy

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