Nanofiltration thin-film-composite polyesteramide membranes based on bulky diols

Desalination 16 1 (2004) 25-32

Nanofiltration thin-film-composite polyesteramide membranes based on bulky diols Uday Razdana*,S.S. Kulkarnib “RO Division, Central Salt h Marine Chemicals ResearchInstitute, Gijubhai Badheku Marg, Bhavnagar 364 002, India Fax +90 (278) 567562; email: t&y-razdan@[email protected] bC.E. Division, National Chemical Laboratoq Pune 41I 008, India

Received 29 July 2002; accepted 20 May 2003

Abstract Polyesteramide thin-film-composite (TFC) membranes have promise for diafiltration applications due to their relatively good oxidative resistance coupled with the ability to tailor the membrane rejection profile by varying the ester/amide ratio. The incorporation of ester linkages in interfacially prepared polyesteramide TFC membranes has been previously shown to increase the oxidation resistance of the membrane. It was also found that polyesteramide TFC membranes incorporating hydroquinone (HQ) or bisphenol-A (Bis-A) had high rejection for monovalent salts, i.e., their rejection profiles matched those of reverse osmosis rather than nanofiltration membranes. We report the properties of polyesteramide TFC membranes incorporating bulky diols such as phenolphthalein (Phe) and terabromobisphenol-A (TBrBis-A). The data were used to correlate the influence of different ester fknctionalities on membrane flu and rejection characteristics. Membranes incorporating TBrBis-A had relatively high rejections for monovalent saltscoupled with low water permeance. By contrast, membranes incorporating Phe showed 10 times higher flux and a rejection profile which appears to be of interest for diafiltration applications involving the separation of organics with molecular weight >400 gmol-’ corn low-molecular-weight organics and salts. The Phe-based membranes show rejection characteristics for monovalent and multivalent salts typical of negatively charged membranes. Keyworak

Nanofiltration; Phenolphthalein; Tetrabromobisphenol-A; separation; Flux; Donnan exclusion; Diafiltration

1. Introduction Nanofiltration (NF) processing is used for retaining organics (molecular weight 300-2000 daltons) and certain multivalent salts while *Corresponding author. Current address: Gharda Chemicals Limited, PlotNo. 3525-26-27, GIDC Estate,Panoli394116, Dist. Bharuch, Gujarat, India.

Interfacial; Reverse osmosis; Rejection/

allowing the permeation of monovalent salts [l-4]. Reject& in NF membranes is a function of concentration and valency of the feed electrolytes, the ion-exchange capability of the membrane material, feed pH, and size/polarity of the solute molecule [5]. NF membranes reject organic solutes based on size-sieving effects, whereas ionic rejection is based on both solute

00 l l-9164/04/$- See front matter 0 2004 Elsevier Science B.V. All rights reserved PII: SO01

l-9164(03)00694-5

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U Razdan, S.S.Kulkarni /Desalination 161 (2004) 25-32

size and valency. Thus, unlike reverseosmosis (RO) membranes,NF membraneshavethe ability to discriminate between ions basedon valency [3]. The rejection characteristics for neutral organic solutes (glucose, sucrose)and charged solutes comprising of single salt solutions of Na,SO,, CaCl, and NaCl have been correlated with NF membranecomposition [3,6]. Many RO andNF thin-film-composite (TFC) membranesare basedon interfacially polymerized polyamides and aregenerally susceptibleto oxidative/chlorine attack. Jayarani et al. [7-91 synthesizedseveralpoly(esteramide)basedTFC membranesand showedthat thesehad improved oxidative (chlorine) resistancecomparedto polyamidemembranes.However,previously prepared polyesteramidemembraneshavelow passagefor monovalent salts. For example, Kim et al. [lo] described polysulfone (PSF) supported TFC membranesconsistingof aromatic polyesteror a copolymer of an aromatic polyesterandaromatic polyamide. They reported that the interfacial reaction of 4,4’-dihydroxybiphenyl and mphenylenediamine(MPDA) in the aqueousphase with trimesoyl chloride (TMC) in the organic phase resulted in a TFC membranewith NaCl rejection of 96.5%. Similarly, Jayarani et al. [7-91 found that polyesteramide membranes synthesizedfrom MPDA andTMC incorporating either m-aminophenol (MAP), hydroquinone (HQ) or bisphenol-A (Bis-A) had RO-type rejection profiles with NaCl rejections in the rangeof -95-98%. Severalfactorsmay affect therejectionprofile of a TFC membrane:packing density in the film, hydrophilicity and electrical charge effects/ Donnan exclusion [S,11,121.Film thicknessand packing density can be affected by interfacial reactionkinetics andthermodynamiceffects [ 131 and alsopresumablyby the intrinsic free-volume of the polymer. In contrastto previouswork [7-l 01,this work involves synthesis of polyesteramide-based copolymers with two bulky diols: phenol-

phthalein (Phe) and tetrabromobisphenol-A (TBrBis-A). The membranes were prepared interfacially on nanoporouspolysulfonesupports by reactingone of thesediols along with MPDA in aqueousphasewith TMC in petroleum ether. It was expectedthat the high excludedvolume of the bulky substituentson thesebisphenolswould hinder polymer packing in the separatinglayer resulting in a more “open” membrane. The changein diol chemistry was also expectedto affectthe hydrophilic&y of the membraneaswell as its surfacechargeandpK, with corresponding effects on the membrane’s ability to separate monovalent salts from divalent salts containing co-ions. We also know [14] that electronwithdrawing substituentssuchasthe nitro group andhalogensreducethe basicity of diols, thereby slowing the forward rate constant of the polymerization reaction. Changes in kinetics and oligomer solubility will also affect membrane morphology and the resulting membrane permeability [ 131.This paperreportsempirical data correlating these various effects by comparing the performance of TFC membranesincorporating thesebulky dials with previously reported polyesteramideand polyamide membranes[9]. 2. Experimental 2.1. Materials

and methods

Phe(4,4’-isopropylidenediphenol)(LR grade) from Qualigens(India) was recrystallized from ethanolsolution beforesynthesis.TBrBis-A was synthesizedas reported previously [15]. Other chemicalsusedwereall of AR grade:MPDA and TMC from Aldrich (USA), tetrabutylammonium bromide from Fluka Chemie (Switzerland) and petroleumetherfrom SD Fine Chemicals (India). Asymmetric polyester/polyesteramide-based NF membranes were prepared by inter-facial polycondensation: hand-dipping a nanoporous polysulfone membrane (0.015 m’; 0.15 m x 0.10 m) in an aqueous solution (264 ml)

U Razdan. S.S. Kulkurni / Desalination 161 (2004) 25-32 Table 1 TFC membranes based on Phe or TBrBis-A Aqueous phase concentration used to prepare various membranes Membranes

Phe or TBrBis-A (mmole/ml)

MPDA (mmole/ml)

PheiTMC Phe/MPDA(7:3*)/TMC Phe/MPDA( 1: l”)/TMC TBrBis-ARMC TBrBisA/MPDA(7:3”)/TMC TBrBisA/MPDA( 1: l”)/TMC TBrBisA/MPDA( 1:S”)/TMC

0.0643 0.0561 0.0477 0.0376 0.0346

0.0241 0.0477 0.0148

0.0314

0.0314

0.0188

0.0943

“Molar ratio. Aqueous solution: Phe or TBrBis-A with/without MPDA (2% w/w) + 2.2:0.025 moles of NaOH:tetrabutylammonium bromide (TBAB) per mole of diol (Phe or TBrBis-A). Organic solution: Trimesoyl chloride (TMC) dissolved in petroleum ether [0.15% w/v)]. Conditions: PSF membrane (0.015 m* area) dipped in -260 ml aqueous phase for 3 min. Membranes were air dried for 7 min. Membrane dipped in -260 ml organic phase for 1 min. Cured in air-oven at 50°C for 5 min.

containing the diol and/or MPDA (2% w/w) in different proportions followed by hand-dipping the samein anorganicsolution of polyacyl halide (TMC) (260ml). The polysulfonemembranewas prepared on a continuous caster with a textile supportandwas characterizedas having a water permeance of 30-40 Lm-‘h-’ at 71 kP, and bovine serum albumin (BSA) rejection of 7080%. Phe and TBrBis-A basedmembraneswith adifferent proportionof MPDA weresynthesized using conditionsshown in Table 1.The detailsof the membrane preparation procedure are the sameas usedby Jayarani[7,9].

27

2.2. Characterizationof membranefor salt and sucroserejection Rectangularmembranecouponsweretestedat ambient temperature in flow-through cells (-25 cm2 exposedarea,2 mm channel height). An aqueoussolution of NaCl (2000 ppm) and sucrose(5%) in a -3L tank was pumpedthrough the systemat 1 Lmin-’ (cross-flow feed velocity -0.32 ms-‘) at a pressureof 2.76 MPa (400 psi). Sucroseconcentrationswere measuredwith an Abbe refractometerand salt concentrationsby a conductivity meter. Several coupons of a particular membranetypeweretestedat eachexperimental condition and the results were averaged. Membranes basedon Phe or TBrBis-A and/or MPDA mixed in differentproportionsin aqueous phaseandreactedwith TMC were testedfor salt and sucrose rejection (OAR,)and water permeability coefficient A, as defined by Eqs. (1) and (2): R (%) = [l-C/&]. J,=A(AP

- AII)

100

(1) (2)

whereCpis permeateconcentration,Cffeed concentration,J, is volumetric flux, AP is pressure difference, and AII is the osmotic pressure difference. The osmotic pressuresfor the feed and permeate streams were calculated from Sourirajan [ 161.These membrane performance parametersare listed in Tables 2 and 3. The valuesreportedarethe meanvalues from several replicate experiments. The standard deviation from replicate experiments are -16% of the reported water permeability mean values and -10% of the reportedsalt rejection mean values. The only exceptionis the water permeability for Phe/TMC in Table 2 where the standarddeviation was -30% of the mean value. For Phe-basedmembranes,the rejections of NaCl, CaCl,, andNa,SO, at 5000 ppm eachwere also measuredindividually. These results are shown in Table 4.

28

I/. Razdan, S.S.Kulkmni / Desalination I61 (2004) 25-32

Table 2 NaCl rejection and permeability, B [Eq. (3)], sucrose rejection and water permeability, A [Eq. (2)], of Phe-based membranes. Data obtained from measurements with solutions containing both NaCl (2000 ppm) and sucrose (5%) at 2.76 MPaIambient temperature Membranes

Permeability, A (x

PheiTMC Phe/MPDA(7:3)/TMC Phe/MPDA( 1: l)/TMC

lo-*’ m3m-*s-‘Pav1)

-0.56 0.33 0.28

Salt rejection (%)

Sucrose rejection (“Yo)

Salt permeability, B (x10-S m/s)

16 43 35

34 77 88

3.17 0.40 0.52

Table 3 NaCl rejection and permeability, B [Eq. (3)], sucrose rejection and water permeability, A [Eq. (2)], of TBrBis-A based membranes. Data obtained from measurements with solutions containing both NaCl(2000 ppm) and sucrose (5%) at 2.76 MPakmbient temperature Membranes

Permeability, A

TBrBis-A/TMC TBrBis-A/MPDA(7:3)/TMC TBrBis-A/MPDA( 1: l)/TMC TBrBis-A/MPDA( 1:S)/TMC

(XlO-‘2 m3 mm2se’pa-‘)

Salt rejection (%)

Sucrose rejection (%)

Salt permeability, B (x1o-s m/s)

0.32 0.32 1.29 0.89

12 49 84 94

40 84 93 98

0.24 0.032 0.025 0.0057

Table 4 NaCl, Na$O,, CaCl, rejection and water permeability, A, of Phe-based membranes. Data obtained from measurements with solutions containing NaCl, CaCl, and Na$O, (5000 ppm) measured individually at 2.76 MPakunbient temperature Membranes

Permeability, A NaCl (x lo-‘* ms“, Pa-‘) rejection (“W

Permeability, A NGQ (Xlo-” ms-’ Pa-‘) rejection W-1

Permeability, A CaCl, (x10-r’ ms-’ Pa-‘) rejection w

PhelTMC Phe/MPDA(7:3)/TMC Phe/MPDA( 1: l)/TMC

0.31 0.13 0.28

0.37 0.21 0.31

0.35 0.21 0.30

15 35 23

3. Results and discussion The

polyester/polyesteramide

TFC

mem-

branesdiscussedherewere preparedby reacting aqueous solution of Phe or TBrBis-A and/or MPDA mixed in different proportionswith TMC in the organic phase (Table 1). Previously reportedpolyesteramidemembranesincorporating hydroquinone(HQ) or bisphenol-A (Bis-A) [7,9]

52 58 60

12 40 28

were synthesizedunder similar conditions as reported above. Membranes incorporating HQ andBis-A hadshownrelatively high rejection for NaCl(95-98%) and sucrose(-100%). Phe was selected for this study with the expectationthat the bulky phthalido cardo group connectedto the bridge carbonwould renderthe polymer

chain more rigid and introduce

steric

U. Razdan, S.S. Kuhrni

/ Desalination 161 (2004) 25-32

hindrance for close packing. This change in packing density was expected to increase the passageof smaller species as well as overall permeability. In alkaline conditions,thephthalido group opens giving a trianion. Some phthalido groups may also have been left uncyclized leaving a residual surfacecharge. The results of incorporating TBrBis-A in comparison to the previously reported Bis-A resultsoffer a direct comparisonfor the influence of the bulky Br substituentson the aromatic ring, on flux and separation characteristics. Bulky, polar substituents will influence basic&y (pK, values) of the diols and excluded volume in the resulting polymer. The polymer molecularweight and resulting membrane morphology will be affected by the kinetics of the polymerization reaction,which in turn can be affectedby the diol substituents.For example, kinetic rate constants for reaction with terephthaloyl chloride are reported [ 14,171to increase in the following order:2,2-bis (4-hydroxy-3-nitrophenyl)propane < 3,3‘, 5,5‘-tetramethyl-, 3,3‘, 5,5‘-tetrachloro substituted derivatives < unsubstituted 2,2-bis (4-hydroxyphenyl) propane and 2,2-bis (4hydroxy-3-methylphenyl) propane. Figs. l-3 show the water permeability coefficient, A, andthe rejection of NaCl and sucrosein

29

Phe/MPDA+ TMC andTBrBis-A/MPDA+ TMC basedmembranes.These figures also show the comparativedata for previously reportedpolyesteramidemembranesbasedon HQ and Bis-A; as mentioned above,all theseTFC membranes were made under similar conditions. The TFC membranes incorporating Phe are the most “open” of thefive polyesteramideTFC structures reported. The water permeability (0.28-0.56x 10-l’ m3mV2s-‘Pa-‘)for the Phe-basedTFC series

0

0.25

0.5

MPDA:diol

1

0.75

mole

ratio

Fig. 2. NaCl rejection of Phe, TBrBis-A, HQ, and Bis-A based membranes (MPDA:Diol + TMC) having varying diol content, measured with 5% sucrose + 2000 ppm NaCl solution at 2.76 MPa and ambient temperature. 0.6 ‘; tT 7 E ?. ‘: 2 25

0.5

-a-&-

TErBisA Phe

0.4

4 -x-

Bis-A HQ

\

0.3

d -0-‘IIWir-A

4

&s-A

I

-X-IlQ I 0

0.25

0.5

MI’DA

0.75

I

: dial mole ratio

Fig. 1. Sucrose rejection of Phe, TBrBis-A, HQ and BisA based membranes (MPDA:Diol+TMC) having varying diol content, measured with 5% sucrose + 2000 ppm NaCl solution at 2.76 MPa and ambient temperature.

tz o a 3

0.2

5 a

O.’ 0 0

0.2

0.4

MPDA

0.6

0.8

1

: dial mole ratio

Fig. 3. Water permeability coefficient, A [Eq. (2)] for Phe, TBrBis-A, HQ and Bis-A based membranes (MPDA:Diol + TMC) having varying diol content, measured with 5% sucrose + 2000 ppm NaCl solution at 2.76 MPa and ambient temperature.

30

U Razdan, S.S.Kulkarni /Desalination 161 (2004) 25-32

is the highest, and the NaCl rejection (15-43%) is by far the lowest. Sucroserejections are also lower in the Phe-basedseriesbut the difference, relative to the other polyesteramidemembranes, is smaller. By contrast,the water permeability (0.3-l .3x lo-l2 m3m-2s-‘Pa-1)of TBrBis-A based membranesis one order of magnitude lower than the Phe-basedmembranes. While the flux is the lowest amongst all the polyesteramide membranes,the rejections for both sucroseandNaCl areonly moderatelyhigh. This poor performance in aqueousmedia may be related to the hydrophobic natureof the bromo substituent. Similar to the trend in previous dioliMPDA basedpolyesteramideTFC membranes,both the Phe and TBrBis-A seriesof membranesshow a trend towards a more “open” membraneas the diol content increasesrelative to MPDA. Fig. 1 shows the decreasingsucroserejection in both the Phe and TBrBis-A seriesas the diol content increases. The membrane flux increasedwith increasing phenol content in the caseof the Phe series.However, for the other diols, this trend is not seen or is weaker; the HQ and TBrBis-A basedmembraneseriesappearto showmaximum water permeability around 30% and 50% diol content, respectively (Fig. 3). This maximum may be a result of two opposingtendencies:the increasingly open structure as diol content increases coupled with the increasing hydrophobic characterof these bisphenolsrelative to MPDA. The NaCl rejection trend in TBrBis-A based membranesagreeswith the packingdensitytrend inferred from the sucrose rejection data (Table 3). Fig. 2 shows a similar continuous increaseinNaC1rejectionwith increasingMPDA content approaching the rejection value (9899%) for the MPDA/TMC polyamidemembrane at 20% TBrBis-A content. NaCl rejections (Table 2) in the Phe-based membrane series show some interesting differ-

encesand should be interpretedin combination with the data for other salts (NaCl, NqSO, and CaCl,) reportedin Table 4. It is known that in negatively charged NF membranes, bivalent anionsshowthehighestretention,bivalent cation shows the least retention whereas monovalent ion-pairs show intermediate retention [3,11]. Thus a typical retention sequencefor negatively chargedmembranessuchas ASP35 is NqSO, > NaCl > CaCl,. By contrast,a positively charged membrane such as MPF-21 shows a rejection sequenceof CaCl, > NaCl > Na,SO, [ 111.This ability of chargedNF membranesto differentiate betweenions is attributedto theDonnanpotential generatedat the water-polymer interface,which repels co-ions. Fig. 4 shows the NaCl, Na$O, and CaCl, rejections for Phe-basedmembranes.The values reportedin Table 4 areaveragescorrespondingto threereplicateswith eachindividual salt solution. Na,SO, rejection increaseswith decreasingPhe content,aswas the casewith the neutral sucrose. NaCl and CaCl, rejections are both similar and much lower in comparisonto Na$O, rejection. For the full polyester,Phe/TMC membrane,salt rejection is in the sequenceNa$O, >> NaCl > CaCl,; this is consistentwith the expectationsof negatively chargedmembranes.However, both the 7:3 and 1:1 Phe:MPDA-based membranes show salt rejections in the sequenceNa,SO, >> CaCl, > NaCl. Furthermore, while Na,SO, rejection correlates directly with increasing MPDA content,both NaCl and CaCl, rejections show a local maximum at 30% MPDA content. This small maximum in the NaCl and CaCl, rejections may be due to a combination of structural and morphological changesbalanced by chargeeffects. The hydrated ion radius is in the order:Ca2’> SO:- > Na’ > Cl- [ 181,while the diffusion coefficients vary in the order: NaCl > CaCl, > Na,SO, [l 11. Based on this, we may expect CaCl, rejection to increase faster than NaCl rejection asthe membranepacking density

U. Razdan. S.S. Kulkarni / Desalination 161 (2004) 25-32

31

.-v

90

80

i;-:*

70 5 ‘5

..i 50 d s

80

.$

50

!$

40

b

30

\

20 10

--__ \

--__

\ h

---_

---_

--•

0 0

0 0

0.2

0.1 MPDA

0.6

0.8

0.5

I

1

NaCl

1.5

2

2.5

3

3.5

permeability, B (xl O”ms“)

: diol mole ratio

Fig. 4. Rejection profile of single salt solutions of NaCI, Na.$O,, and CaCI, with Phe-based membranes having decreasing diol content measured with individual salt solutions (5000 ppm for each salt) at 2.76 MPa and ambient temperature.

Fig. 5. Cross-plot of NaCl and sucrose rejection against NaCl permeability coefftcient, B [Eq. (3)], for Phe and TBrBis-A based membranes (Diol/MPDA + TMC) measured with 5% sucrose + 2000 ppm NaCl solution at 2.76 MPa and ambient temperature.

increaseswith increasingMPDA content.However,a more negativemembranewould show an opposingbehavior;Nat hasa higher rejection in negatively charged membranesthan does Ca”. While the increasingMPDA content in the Phe seriesincreasespacking density, it may decrease the membrane negative charge,thus leading to the nonlinear variation in NaCl and CaCl, rejection with Phe content. The NaCl rejectionsfor Phe-basedmembranes at 2000 ppm feed concentration (Table 2) are higher than thosereportedin Table 4 (5000 ppm feed). Increase in salt concentration in the solution reducesthe effective negativechargeon the membrane.This leads to lower rejection of the salt in accordancewith Donnan exclusion [ 11,191 andthe extendedSpiegler-Kedem model [ 19-211. Several industrial diafiltration (dyes, antibiotics, biological molecules) processesrequire high rejection for specieswith MW > 400 daltons while allowing passageof smaller organicsand salts [22]. A useful membrane in such applications would needto combine a high rejection

with a high permeability

for organics such as sucrose (or larger) coupled

for monovalent

salts

such as NaCl. Fig. 5 shows the relative diafiltration capability of the Phe and TBrBis-A basedpolyesteramidemembranes,respectively, by plotting sucroseandNaCl rejectionsvs. NaCl passagerate. The salt permeability B (m/s) was calculatedfrom: J,=J,,C,=B(C’-C,)

(3)

where J, is the solute flux. TBrBis-A based membranes,like those based on HQ or Bis-A, show little ability to separateNaCl from sucrose solutions. However, the Phe/MPDA based membranes show promise for diafiltration applications. 4. Conclusions

PolyesteramideTFC membraneshavepromise for diafiltration applications due to their relatively goodoxidative resistancecoupledwith the ability to tailor the nanofiltration membrane rejection profile by varying the ester/amide ratio and the diol type. The use of bulky diols such as

32

U. Razdan, S.S. Kulkarni / Desalination 161 (2004) 25-32

Phe and TBrBis-A introduces steric hindrance preventingclose packing of the separatinglayer structure.TBrBis-A basedpolyesteramidemembraneshad moderatelylower rejectionsfor small molecules than the polyamide TFC, but the decreased membrane hydrophilic@ may be responsiblefor unattractively low water fluxes. By contrast, Phe-basedmembraneswere significantly more “open”. The observedrejection of sucrose and various mono- and divalent salts could be rationalized by postulating that increasingPhe content increasedthe membrane negative charge while decreasing its packing density. Phe-basedpolyesteramide membranes showpromisefor diatiltration applicationswhere organics with MW >400 need to be separated from salts.

Acknowledgement UR is gratefulto CSIR for financial assistance to UR and to Mr.V.J. Shah, CSMCRI for cooperation. The experimental work was done at NCL, Pune.

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