- Email: [email protected]
Streaming effect of single electrolyte mass transfer in nanofiltration: potential application for the selective defluorination of brackish drinking waters
DESALINATION ELSEVIER
Desalination 15 l(2002) 267-274
www.elsevier.com/locate/desal
Streaming effect of single electrolyte mass transfer in nanofiltration: potential application for the selective defluorination of brackish drinking waters M. PontiC”‘, C.K. Diawarab, M. Rumeauc “Laboratoire d’Electrochimie et Chimie Analytique, UMR CNRS 7575, Ecole Nationale Superieure de Chimie de Paris, 1I rue Pierre et Marie Curie, 75231 Paris cedex OS, France Tel. +33 (1) 55-42-63-90; Fax + 33 (I) 44-27-67-50; email: [email protected] bLaboratoire & chimie analytique et minerale, Universite Chekh Anta Diop, Dakar Farm, Senkgal ‘Hydrosciences, Place Eugene Bataillon, case MSE, Universite de Montpellier 2, F-34095 Montpellier cedex 5, France Received 2 1 December 200 1; accepted 28 June 2002
Abstract This work deals with electrical properties of nanosurfaces in contact with electrolyte solutions. Single halide ion solutions were studied by streaming potential (SP) measurements and observed retention &,J of the F-, Cl-, and Brions across nanofiltration (NF) membranes. The detailed understanding of an electrolyte solution mass transfer requires an intimate knowledge of the physicochemical interactions occurring between nanoporous materials and electrolyte solutions across the first-generation composite membranes called NF55, NF70 and NF90. These membranes are composed of a polysulfone mesoporous sublayer and a microporous skin layer in polyamide. In order to get a better understanding of these effects, it seems attractive to compare the mass transfer permeation of the monovalent ions F-, Cl-, and Bi with the electrokinetic characterizations deduced from a properly developed SP apparatus. SP measurements is a very simple method to show the intrinsinc charges on membrane pore walls. The membrane’s electrical properties are studied with SP design modeling pH, ionic strength and kind of electrolyte solutions. We have observed that the isoelectric point (IEP) of the membrane materials is both dependent on the ionic strength and on the kind of electrolyte solution. The IEP in the presence of KC1 is 4.4 at 0.0001 mol/L and 5.8 at 0.001 mol./L, showing an increasing adsorption of the cation K’ by increasing its solution concentration. For a fixed concentration, the effect of the electrolyte solution has shown that a higher adsorption of Ca” occurs in comparison to K’ and Na’. But the adsorption of these electrolyte solutions is essentially reversible as observed under dilution conditions. Furthermore SP measurements were used for the first time to characterize the transmembrane pressure ranges where a convective and/or a diffusional mass transfer occurs. Such an approach was developed to correlate the %b, of the halide ions F-, Cl- and Br- with the kind of mass transfer (diffusional and/or convective) occurring predominantly under transmembrane pressure variations. Thus the NF70 membrane shows at low pressure (under 3 bar) the order of R,,b,following the hydrated ionic radius: ~bs.(F-)>~.(Cll)>&JBr-). For a higher pressure (> 3 bar) an inversion occurs between Cland Br-, but F was not affected. These results open a new prospective area for selective defluorination of brackish drinking waters using NF membranes under low transmembrane pressure.
*Corresponding
author.
001 l-91 64/02/$- See front matter 6 2002 Elsevier Science B.V. All rights reserved PII:SOOll-9164(02)01019-6
268 Keywords:
hf. Pontie’ et al. /Desalination
151 (2002) 267-274
Nanofiltration; Convective and/or diffusional mass transfer; Streaming potential; Fluoride ions removal; Brackish drinking waters
1. Introduction
The aim of the present work is to study the mass transfer of single electrolytes across NF membranes and its potential application for fluoride ion removal in drinking waters. In several sites in the world groundwaters contain fluoride ions at concentrations higher than the acceptable level of 2 mg.L-’ [l] causing serious diseases such as dental and bone fluorosis [ 1,2]. In a preliminary work in Senegal [2-S], it was shown for the first time that undesirable solutes can be removed selectively from high fluorinated brackish waters using NP membranes: partial demineralization and total disinfection were obtained with an observed rejection of the fluoride ions greater than 90% percent [24]. These previous results demonstrated that NF could be a new process for fluoride ion rejection in brackish drinking waters. As previously detailed [4], the removal of fluoride ions depends mainly on the operating conditions (transmembrane pressure, ionic strength of the bulk), on the contents of the feed water (divalent salts, humics), and on the membrane materials (hydration properties and/or charge presence). Usually divalent ions have always been better rejected than monovalent ones in the firstgeneration NF membranes, but recently [6,7] a second generation of charged NF membranes has appeared which ensures transmission of sufftcient Ca” to meet water quality requirements. In this study we focus on the electrical properties of the first-generation NF membranes. Then it was possible to separate ions of the same valency due to their different hydration properties, as previously reported [4,8]. The detailed understanding of the electrolyte solutions mass transfer requires an intimate knowledge of the physicochemical interactions
occurring between nanoporous materials and electrolyte solutions. In order to get a better understanding of these effects, it seems attractive to compare the mass transfer permeation of the monovalent ions F-, Cl-, and Br- by a properly developed SP design. A full description of the charge origin of these first-generation NF membranes will be studied to better understand NF selectivity among the halide ions. Furthermore, SP measurements are used for the first time to distinguish convective to diffisional mass transfer vs. transmembrane pressure conditions. This approach will establish a correlation between SP results and the halide ions observed retention in order to detail the reasons why the fluoride ions are always better rejected than other monovalent ions in NF. Our results will demonstrate the transmembrane pressure effects on the selective defluorination of brackish drinking waters. 2. Experimental 2.1. Membranes and module
The membranes under study are thin-film composite membranes composed oftwo layers a thin polyamide film active surface and a large mesoporous polysulfone support layer [6]. All NF membranes were purchased from Filmtec (Dow, Denmark). These membranes are usually called NF55, NF70 and NF90. The pore radii of the NF membranes studied were obtained using Ferry’s equation, as previously detailed [3], and the results were 0.65 nm, 0.62 nm and 0.54 nm, respectively. An FT30 polyamide reverse osmosis (RO) membrane was purchased from Filmtec (Dow, USA). Before all experiments the membranes were cleaned with standard procedures to remove preservatives and rinsed with UP
M. Pontie’ et al. /Desalination
MilliQ water (Millipore, France) until the conductivity of the permeate remained below 1 @/cm. The effective surface membrane area was 138 cm*. 2.2. Streaming potentials measurements SP measurements were made using a very simple method [9-l 11, adjustable easily to all kinds of modules. A pair of saturated calomel electrodes (SCE) from Radiometer-Analytical (France) were used to measure the potential difference between both sides of the membrane as a function of transmembrane pressure. They were judiciously placed in the SP device as previously detailed [9]. The electric contact between the electrode and the retentate solution was established via a glass tank filled with electrolyte solution isolated from the bulk solution by a glass vycor’. The electrodes were connected to a high impedance voltmeter. Our SP measurements across NF membranes created some theoretical problems due to the evaluation of the zeta potential materials [3,12,13]. But our SP measurement design enables characterization of the active layer of the membrane due to the main decrease in transmembrane pressure occurring in the microporous layer. Furthermore, we are currently only interested in a qualification of the pore wall charges without any quantification. 2.3. Chemicals All the salts used (KCl, KF, KBr, NaOH, HCl) were of analytical grade from Aldrich (France). All solutions were prepared from a MilliQ water design (Millipore system, USA) with a high-purity water presenting a conductivity lower than 1 @/cm. 2.4. Nanojiltration runs Runs were performed on rigs (Osmonics,
269
I51 (2002) 267-274
USA) equipped with a gear pump and a planar membrane design with a tangential flow velocity around 1 m.s-’ and a constant conversion rate of 10% minimizing the concentration polarization effect. The applied transmembrane pressures were in the range O-9 MPa. Permeated solutions were recycled during the runs except for samples withdrawn for the calculation of observed retention (R,& according to: R,,.=
1 - C/,/C,
where C, and C,, are the concentrations in the permeate and bulk solutions, respectively. The ions were assayed with specific electrodes from Radiometer-Analytical (France) calibrated for fluoride, chloride, bromide and iodide ions. The sulphate solutions were analysed with a conductivity cell correlating total salinity with conductivity under maintained temperature (298.15 K). 3. Results and discussion 3.1. Theoretical development
Expression of the membrane potential in a reverse osmosis (RO) membrane - For a charged membrane and by using the TMS model [ 141, the membrane potential (A@)+, for a monovalent electrolyte solution such as KC1 is expressed by A@ =$(l-2t+)lnlfi a2
where t’ denotes the transport number of the cation (i.e., K’) across the membrane phase, a, KC1 activity in compartment 1, a, KC1 activity in compartment 2, R = 8.31 L.mol-’ K-l, T is the temperature in K and F the Faraday constant in C.mole-‘. For a very selective membrane (%b, near the value of 100%) submitted to concentration polarization, the ratio of the solute concentration
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M. Pontid et al. /Desalination
in the bulk near the surface of the membrane, C,,, to C,, the solute concentration in the bulk is
J”b -c2 =expDf C2m
where 6 is the thickness of the active layer, 0, the average electrolyte diffusion coefficient and J, is the water flux across the membrane. Now the (A$)+,, expression can be explained as
A@ = $(l-2f’)
[[m$[:)]
In RO, the water flux under the transmembrane pressure is expressed as J, = Lp (AP - AII) where Lp is the water mass transfer coefficient, AP the transmembrane pressure applied and AII the osmotic pressure beween each sides of the membrane. Finally (A@),, can be explained as
151 (2002) 267-2 74
predominantly occurs and a pure UF mesoporous membrane where convective mass transfer mainly occurs. In the latter, a simple measure of SP (supposing the material carrying a charge) under diluted KC1 concentration is sufficient to prove that a predominantly convective mass transfer occurs. Thus NF mass transfer is highly dependent on permeation conditions, and transmembrane pressure plays a major role as illustrated earlier. 3.2. SP measurements: a new tool to distinguish between d@kional and convective mass transfer in NF under transmembrane pressure Fig. 1 illustrates SP values vs. transmembrane pressure for the membranes studied. It shows that for the NF70 membrane, SP is measured only when the transmembrane pressure level reaches a critical pressure, denoted P,(NF70) of 3 bars, indicating that the transport mechanism is changing around this critical pressure. Thus, before P,(NF70) a diffusional mass transfer is predominant, conforming to Eq.(l), and above this critical pressure a convective transport of the solutes occurs predominantly. For NF90 it is API bar
C A4 = aLp (AP-AII) + b ln -?f Cl
(1)
with b = $(l-zt+)
a = $(l-2t+)$; f
For a KC1 electrolyte solution, the transport number of K+ is near 0.5 (presumably assumed to be true in the membrane); then (l -2t’) is equal to zero and (A4),, is vanishing, conforming to Eq. (1). This result clearly explains the main difference between a pure RO dense (nonporous) membrane where diffusional mass transfer
8 -10
Fig. 1. SP measurements vs. transmembrane pressure for Filmtec NF55, NF70 andNF90 membranes ([KCI] = 1Om4 mot/L, pH = 6.8).
M. Pontit! et al. /Desalination
necessary to apply a critical pressure, denoted PJNF90), higher than 6.5 bars to measure SP. In the case of NF55 SP values were measurable just starting the transmembrane pressure. Then a convective transport is clearly predominant for the NF55 membrane in the range of all transmembrane pressures studied. Using firstgeneration NF membranes, the critical pressure values can be a limit usable to differentiate diffusional to convective mass transfer of the solutes. Furthermore, a correlation between the critical pressures of NF membranes and their pore radii can be observed: Pc(NF55,0.65nm) < Pc(NF70, 0.62 nm) < Pc(NF90, 0.54 nm). This order illustrates that SP effects increase with membrane pore size. Fig. 2 shows the evolution of a sodium electrolyte solution rejection observed vs. transmembrane pressure for theNF70 membrane. We have observed an inversion in R,,bs for bromide and chloride ions after 3 bars. Then for a pressure lower than the P,(NF70) value, the order of the observed rejection follows the 100 95
I
-*NaBr
Fig. 2. Observed rejection ratio of the halide ions Fe, Cl‘ and Br- sodium salts vs. the transmembrane pressure (NF70 membrane [sels] = 0.001 mol& pH = 6.8, membrane area 138 cm*).
I51 (2002) 267-274
271
hydration energy order (similar to the hydrated ionic radius): F-Xl->Br-3 proving that a pure diffusional mass transfer occurs. After Pc(NF70), a new order is observed F->Br-Xl-, proving now that NF mass transfer has changed and that a convective mass transfer driving process occurs predominantly. It now appears clearly that these halide ions are transferred by two mechanisms, separate or combined: (1) convective mechanism: the ions are carried by the solvent stream as a function of the transfer coefficient. The larger ions are more retained; (2) solubilization-diffusion: the ions are transferred under their partition coefficient which is the ratio of the solubility of the ion in the membrane under the solubility in the bulk solution. The larger the ion, the less well retained it is. Fluoride ion observed retention is not greatly affected by pressure modifications because its permeability is essentially due to diffusion. Convection has virtually no effect since a significant increase in pressure caused no significant change in the observed retention. In contrast, bromide, chloride or iodide ions as reported elsewhere [4] observed retention were much more influenced by the pressure because the permeability of these ions are mainly due to physical, rather than chemical, forces. We have also confirmed that the observed retention is often due more to the dissolution properties in the bulk solution than in the membrane media. Since the fluoride ion is the smallest monovalent anion, it is the most soluble ion in water with a high hydrated energy compared to the other halide ions and hence passes through the membrane with difficulty. In contrast, chloride and bromide ions are only slightly soluble in water and are less well retained. But as illustrated before, the transmembrane pressure can consequently modify the hydrated layers of the membrane pores and/or the hydrated layers of the ions showing some inversions, as previously reported [15-l 71.
272
M PontiB et al. /Desalination loo .j99.8
J” i
99.6 ...I
49.4 -
I
99.2 -1
x._.
x--~“-.~-.~x”---..~___,.
/~>z--__~; .:=---Gi” -.
‘/ J/-
I
/
.._..u ,--
/
98.8 _’ 0
3( -
’
/ 10
&_
--Q-t_
/
99.0 - I---
I .._.. ..._I
---a--a_
2b
/ 1 30
I, .X.“, .,..l
=4
f_
2.
_._.._. --........_..”.+._._-___. -t 40 SO 60 AP
(brr)
Fig. 3. Observed retention vs. transmembrane pressure for fluoride, chloride, bromide, iodide and sulfate ions in reverseosmosis(polyamidemembraneFT30, electrolyte sodium salt concentration = 0.05 mol/L, pH = 6.6).
The observed retention of the ions shows a constant value in the higher pressure range and a decrease in the selectivity between the ions. At the same time, retention increases with pressure due to an increase in the water flux due to dilution in the permeate, showing higher observed rejection. This is because convection predominates over diffusion. The observed retention vs. transmembrane pressure across a RO membrane is observed for divalent and monovalent anions shown in Fig. 3. As clearly demonstrated in Fig. 3, the observed retention of all the ions follows the order of the Hofmeister series (identical to the hydration energy of the ions): I-
151 (2002) 267-2 74
model [ 17-191, membrane water is bound to the polymer with solubility properties other than the free water of the bulk [ 16,191. That is the main reason for the better exclusion of highly hydrated ions. Sometimes the membrane material and/or streaming conditions can influence the retention observed, and some inversions have been reported [l&16]. This study clearly illustrates the essential differences between RO and NF processes. Indeed, RO cannot be used for partial and/or selective demineralization [22]. NF is more suitable for directly producing drinking water without the need of remineralized water. Furthermore, the present study clearly shows that NF selectivity for monovalent ions is higher at low pressure under diffusion mass transfer and consequently has a lower cost. 3.3. Origin of the membrane charge in firstgeneration NF membranes The SP measurements also allow to determine the IEP of the membrane under study and the kind of ions determining the charge density of these membranes which were initially not charged. But when a solid is dipped in an electrolyte solution, some ions are adsorbed at the interface. This results in a modification of the surface charge of the material. When adsorbing ions are those of the solvent itself, as is frequently the case in UF membranes [ 10,13,2 1, 221, the surface charge of the membrane is dependent on the pH of the solution, and at the IEP, the surface charge and the SP vanished. In NF the electrochemical interactions between electrolyte solutions and the membrane surface play a greater role in membrane performance. For our microporous PA membranes, the IEP is dependent on the ionic strength and the kind of electrolyte solution in contact with organic membranes materials, as recently detailed [23]. As seen in Fig_4a, displacement of the IEP with electrolyte concentration occurred. This
M PontiP et al. /Desalination 151 (2002) 267-274
273
This chemical adsorption may be explain by the following equilibria:
(a)
1‘5 1
2
Na’+H,‘-H’+Na,’
9 O2 E
&
O
(1)
and
4.5
-1 -1.5
Cl- +OHswOH-
, 2
+Cl,
(2)
I
’
3.5
5
6.5
8
9.5
11
PH
(b)
(S = membrane surface.) However, we have also checked that this chemical adsorption is not irreversible because all the initial properties were recovered before dilution or’ rinsing in a supersonic wave bath. 4. Conclusion
-1.5
’ 3.0
1
, 4.0
5.0
60
7.0
8.0
9.0
10
PfJ
Fig.4. (a) Growth of SP measurements vs. pH and ionic strength (membrane=NF55, electrolyte solution=KCl). (b) growth of SP measurements vs. pH for KCl, NaCl, and CaCI,electrolyte solutions (membrane=NFS& electrolyte concentration = 10e4 mol/L).
result confirms the hypothesis that ion adsorption occurs on the surface of the membrane used. Moreover, for the electrolyte KCI, NaCl and CaCl,. all the cations in solution are in competition to be adsorb first in the membrane electrochemical double layer. As seen in Fig.4b, the selectivity is due to different ions affinity of the surface. For a fixed electrolyte concentration (0.0001 mol.L-‘), the IEP ofthese membranes are 4.4, 5.6 and 4.8 for electrolytes KCl, NaCl and CaCl, electrolytes, respectively. Consequently, more Na’ is adsorbed compared to K’. Then the neutralization of the excess Na’ needs higher pH and a higher IEP is obtained.
A detailed understanding of the mass transfer of an electrolyte solution requires an intimate knowledge of the physicochemical interactions occurring between nanoporous materials and electrolyte solutions. SP measurements are one of the best methods to show the intrinsic charges brought to NF membrane pore walls. The origins of the charge on the polymers studied are the ions of the electrolyte solutions in contact with the materials. The IEP in the presence of KC1 is 4.4 at 0.0001 mol/L and 5.8 at 0.001 mol/L, showing an increasing adsorption of the K’ cation by increasing its concentration in solution. For a fixed concentration the effect of the kind of electrolyte solutions showed that a preferential affinity for Ca++is observed compared to K’ and Na’. But the adsorption of these electrolyte solutions is essentially reversible as observed under dilution conditions. Furthermore, SP measurements have been used for the first time to differentiate convective to diffusional mass transfer vs. transmembrane pressure. Our approach has established a correlation between the Kbs of the F-, Cl- and Brhalide ions and the kind of mass transfer diffusional or convective. Then for the NF70
274
M. Pontit! et al. /Desalination
membrane at low pressure work (less than 3 bar), the order follows the Hofmeister series R,b,.(F~)>~r,,.(C1)>&r,~.(Br). For a higher pressure range an inversion is observed between Cland Br-. But F- appeared unaffected by transmembrane pressure. This study also provides a clearer understanding of how solutes are transferred in uncharged NF membranes, which could lead to a better prediction of the possibilities of applications and to identify the optimal streaming conditions. These results hold the prospect for selective defluorination of brackish drinking waters using NF membranes under low transmembrane pressure. Furthermore, very recently a secondgeneration NF charged membrane, calledNF200, was developed, which ensures passage of sufficient Ca” to meet water quality requirements and is under study for its potential treatment of Senegal’s brackish drinking water.
Acknowledgements The authors are indebted to Lyonnaise des eaux for financial support. Lots of thanks to S. Chevallier for her technical help and to L. Durand-Bourlier, D. Lemordant and Oumar SARR for fruitful discussions.
References 111 Y. Travi , Hydrogt!ochimie et hydrologie isotopique des aquif&es fluorur& du bassin du Senegal Origine et conditions de transport du fluor dans les eaux souterraines. These Univ. de Paris-Sud, France, 1988. M. Pontie, M. Rumeau and M. Ndiaye, Revue Sante, PI Montrouge, 6 (1996) 27. [31 M. Pontie, Phenomenes &ctrocinetiques et transferts ioniques dans les membranes poreuses 21faible seuil de coupure - Application au traitement des eaux saumatres, University of Tours, France, 1996.
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I41 A. Lhassani, M. Rumeau, D. Benjelloun and M. PontiC, Wat. Res., 35(13) (2001) 3260. L. Durand-Bourlier and J.M. Lain& Use of NF and PI EDR technology for specific ion removal: fluoride. Proc., Membrane Technology Conference, New Orleans, 1997. PI J.P. DeWitte, The development of a nanofiltration membrane for the potabilisation of surface water. International Workshop, Membranes in Drinking Water Production, AIDE, Paris, 1995. I71 C. Ventresque, G. Turner and G. Bablon, J. AWWA 89 (1997) 65. PI M. Ruweau, P. Lerellier, S. Sanack, L. Schrive, P. Mandin, M. Minier, C. Jolivalt, M. Pontit, V. Week, R. Audinos, M. Cassir, F. Badioui, J. Devynck, R. Clewent and J.P. Labbt, Les techniques separatives a membranes: theorie, applications et perspectives, CFE, Paris, 200 1. PI M. Pontie, X. Chasseray, D. Lemordant and J.M. Lain& J. Membr. Sci., 129 (1997) 125. [lOI M. Pontib, X. Chasseray, D. Lemordant and J.M. Lain& J. Chim. Phys., 94 (1997) 1741. Vll M. Pontie, L. Durand-Bourlier, D. Lemordant and J.M. Laine, Sep. Purif. Techn., 14 (1998)l. P21 M. Pontit, J. Membr. Sci., 154 (1999) 213. v31 C. Causserand, Etude des mecanismes de selectivite d’une membrane d’ultrafiltration, Thesis, University of Toulouse, 1992, p.2 10. 1141J.O’M Bockris and A.K.N. Reddy, Modem Electrochemistry I, 2nd ed., Plenum Press, New York, 1998, p. 489. [W J.D. Rapoport and C.F. Abramovitch, Russian J. Chem., 3 (1976) 545. [161 H.N. Chang, Desalination, 42 (1982) 63. [I71 C. Menjeaud, M. Pontie and M. Rumeau, Entropie, 179 (1993) 13. W.S. Ho and K.S. Kamalesh, Membrane Handbook, WI Chapman and Hall, New York, 1992. WI M. Sourirajan and S.M. Loeb, Chem. Sot. Symposium Ser.,153 (1981) 11. M. Pontie and D. Lemordant, J. Membr. Sci., 141 PO1 (1998) 13. WI W. Conlon, Desalination, 56 (1985) 203. WI J. Benavente and G. Jonsson, Sep. Sci. Technol., 32 (1997) 1699. 1231R. Zimmermann, S. Dukhlin and C. Werner, J. Phys. Chem. B, 105 (2001) 8544.
Desalination 15 l(2002) 267-274
www.elsevier.com/locate/desal
Streaming effect of single electrolyte mass transfer in nanofiltration: potential application for the selective defluorination of brackish drinking waters M. PontiC”‘, C.K. Diawarab, M. Rumeauc “Laboratoire d’Electrochimie et Chimie Analytique, UMR CNRS 7575, Ecole Nationale Superieure de Chimie de Paris, 1I rue Pierre et Marie Curie, 75231 Paris cedex OS, France Tel. +33 (1) 55-42-63-90; Fax + 33 (I) 44-27-67-50; email: [email protected] bLaboratoire & chimie analytique et minerale, Universite Chekh Anta Diop, Dakar Farm, Senkgal ‘Hydrosciences, Place Eugene Bataillon, case MSE, Universite de Montpellier 2, F-34095 Montpellier cedex 5, France Received 2 1 December 200 1; accepted 28 June 2002
Abstract This work deals with electrical properties of nanosurfaces in contact with electrolyte solutions. Single halide ion solutions were studied by streaming potential (SP) measurements and observed retention &,J of the F-, Cl-, and Brions across nanofiltration (NF) membranes. The detailed understanding of an electrolyte solution mass transfer requires an intimate knowledge of the physicochemical interactions occurring between nanoporous materials and electrolyte solutions across the first-generation composite membranes called NF55, NF70 and NF90. These membranes are composed of a polysulfone mesoporous sublayer and a microporous skin layer in polyamide. In order to get a better understanding of these effects, it seems attractive to compare the mass transfer permeation of the monovalent ions F-, Cl-, and Bi with the electrokinetic characterizations deduced from a properly developed SP apparatus. SP measurements is a very simple method to show the intrinsinc charges on membrane pore walls. The membrane’s electrical properties are studied with SP design modeling pH, ionic strength and kind of electrolyte solutions. We have observed that the isoelectric point (IEP) of the membrane materials is both dependent on the ionic strength and on the kind of electrolyte solution. The IEP in the presence of KC1 is 4.4 at 0.0001 mol/L and 5.8 at 0.001 mol./L, showing an increasing adsorption of the cation K’ by increasing its solution concentration. For a fixed concentration, the effect of the electrolyte solution has shown that a higher adsorption of Ca” occurs in comparison to K’ and Na’. But the adsorption of these electrolyte solutions is essentially reversible as observed under dilution conditions. Furthermore SP measurements were used for the first time to characterize the transmembrane pressure ranges where a convective and/or a diffusional mass transfer occurs. Such an approach was developed to correlate the %b, of the halide ions F-, Cl- and Br- with the kind of mass transfer (diffusional and/or convective) occurring predominantly under transmembrane pressure variations. Thus the NF70 membrane shows at low pressure (under 3 bar) the order of R,,b,following the hydrated ionic radius: ~bs.(F-)>~.(Cll)>&JBr-). For a higher pressure (> 3 bar) an inversion occurs between Cland Br-, but F was not affected. These results open a new prospective area for selective defluorination of brackish drinking waters using NF membranes under low transmembrane pressure.
*Corresponding
author.
001 l-91 64/02/$- See front matter 6 2002 Elsevier Science B.V. All rights reserved PII:SOOll-9164(02)01019-6
268 Keywords:
hf. Pontie’ et al. /Desalination
151 (2002) 267-274
Nanofiltration; Convective and/or diffusional mass transfer; Streaming potential; Fluoride ions removal; Brackish drinking waters
1. Introduction
The aim of the present work is to study the mass transfer of single electrolytes across NF membranes and its potential application for fluoride ion removal in drinking waters. In several sites in the world groundwaters contain fluoride ions at concentrations higher than the acceptable level of 2 mg.L-’ [l] causing serious diseases such as dental and bone fluorosis [ 1,2]. In a preliminary work in Senegal [2-S], it was shown for the first time that undesirable solutes can be removed selectively from high fluorinated brackish waters using NP membranes: partial demineralization and total disinfection were obtained with an observed rejection of the fluoride ions greater than 90% percent [24]. These previous results demonstrated that NF could be a new process for fluoride ion rejection in brackish drinking waters. As previously detailed [4], the removal of fluoride ions depends mainly on the operating conditions (transmembrane pressure, ionic strength of the bulk), on the contents of the feed water (divalent salts, humics), and on the membrane materials (hydration properties and/or charge presence). Usually divalent ions have always been better rejected than monovalent ones in the firstgeneration NF membranes, but recently [6,7] a second generation of charged NF membranes has appeared which ensures transmission of sufftcient Ca” to meet water quality requirements. In this study we focus on the electrical properties of the first-generation NF membranes. Then it was possible to separate ions of the same valency due to their different hydration properties, as previously reported [4,8]. The detailed understanding of the electrolyte solutions mass transfer requires an intimate knowledge of the physicochemical interactions
occurring between nanoporous materials and electrolyte solutions. In order to get a better understanding of these effects, it seems attractive to compare the mass transfer permeation of the monovalent ions F-, Cl-, and Br- by a properly developed SP design. A full description of the charge origin of these first-generation NF membranes will be studied to better understand NF selectivity among the halide ions. Furthermore, SP measurements are used for the first time to distinguish convective to diffisional mass transfer vs. transmembrane pressure conditions. This approach will establish a correlation between SP results and the halide ions observed retention in order to detail the reasons why the fluoride ions are always better rejected than other monovalent ions in NF. Our results will demonstrate the transmembrane pressure effects on the selective defluorination of brackish drinking waters. 2. Experimental 2.1. Membranes and module
The membranes under study are thin-film composite membranes composed oftwo layers a thin polyamide film active surface and a large mesoporous polysulfone support layer [6]. All NF membranes were purchased from Filmtec (Dow, Denmark). These membranes are usually called NF55, NF70 and NF90. The pore radii of the NF membranes studied were obtained using Ferry’s equation, as previously detailed [3], and the results were 0.65 nm, 0.62 nm and 0.54 nm, respectively. An FT30 polyamide reverse osmosis (RO) membrane was purchased from Filmtec (Dow, USA). Before all experiments the membranes were cleaned with standard procedures to remove preservatives and rinsed with UP
M. Pontie’ et al. /Desalination
MilliQ water (Millipore, France) until the conductivity of the permeate remained below 1 @/cm. The effective surface membrane area was 138 cm*. 2.2. Streaming potentials measurements SP measurements were made using a very simple method [9-l 11, adjustable easily to all kinds of modules. A pair of saturated calomel electrodes (SCE) from Radiometer-Analytical (France) were used to measure the potential difference between both sides of the membrane as a function of transmembrane pressure. They were judiciously placed in the SP device as previously detailed [9]. The electric contact between the electrode and the retentate solution was established via a glass tank filled with electrolyte solution isolated from the bulk solution by a glass vycor’. The electrodes were connected to a high impedance voltmeter. Our SP measurements across NF membranes created some theoretical problems due to the evaluation of the zeta potential materials [3,12,13]. But our SP measurement design enables characterization of the active layer of the membrane due to the main decrease in transmembrane pressure occurring in the microporous layer. Furthermore, we are currently only interested in a qualification of the pore wall charges without any quantification. 2.3. Chemicals All the salts used (KCl, KF, KBr, NaOH, HCl) were of analytical grade from Aldrich (France). All solutions were prepared from a MilliQ water design (Millipore system, USA) with a high-purity water presenting a conductivity lower than 1 @/cm. 2.4. Nanojiltration runs Runs were performed on rigs (Osmonics,
269
I51 (2002) 267-274
USA) equipped with a gear pump and a planar membrane design with a tangential flow velocity around 1 m.s-’ and a constant conversion rate of 10% minimizing the concentration polarization effect. The applied transmembrane pressures were in the range O-9 MPa. Permeated solutions were recycled during the runs except for samples withdrawn for the calculation of observed retention (R,& according to: R,,.=
1 - C/,/C,
where C, and C,, are the concentrations in the permeate and bulk solutions, respectively. The ions were assayed with specific electrodes from Radiometer-Analytical (France) calibrated for fluoride, chloride, bromide and iodide ions. The sulphate solutions were analysed with a conductivity cell correlating total salinity with conductivity under maintained temperature (298.15 K). 3. Results and discussion 3.1. Theoretical development
Expression of the membrane potential in a reverse osmosis (RO) membrane - For a charged membrane and by using the TMS model [ 141, the membrane potential (A@)+, for a monovalent electrolyte solution such as KC1 is expressed by A@ =$(l-2t+)lnlfi a2
where t’ denotes the transport number of the cation (i.e., K’) across the membrane phase, a, KC1 activity in compartment 1, a, KC1 activity in compartment 2, R = 8.31 L.mol-’ K-l, T is the temperature in K and F the Faraday constant in C.mole-‘. For a very selective membrane (%b, near the value of 100%) submitted to concentration polarization, the ratio of the solute concentration
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M. Pontid et al. /Desalination
in the bulk near the surface of the membrane, C,,, to C,, the solute concentration in the bulk is
J”b -c2 =expDf C2m
where 6 is the thickness of the active layer, 0, the average electrolyte diffusion coefficient and J, is the water flux across the membrane. Now the (A$)+,, expression can be explained as
A@ = $(l-2f’)
[[m$[:)]
In RO, the water flux under the transmembrane pressure is expressed as J, = Lp (AP - AII) where Lp is the water mass transfer coefficient, AP the transmembrane pressure applied and AII the osmotic pressure beween each sides of the membrane. Finally (A@),, can be explained as
151 (2002) 267-2 74
predominantly occurs and a pure UF mesoporous membrane where convective mass transfer mainly occurs. In the latter, a simple measure of SP (supposing the material carrying a charge) under diluted KC1 concentration is sufficient to prove that a predominantly convective mass transfer occurs. Thus NF mass transfer is highly dependent on permeation conditions, and transmembrane pressure plays a major role as illustrated earlier. 3.2. SP measurements: a new tool to distinguish between d@kional and convective mass transfer in NF under transmembrane pressure Fig. 1 illustrates SP values vs. transmembrane pressure for the membranes studied. It shows that for the NF70 membrane, SP is measured only when the transmembrane pressure level reaches a critical pressure, denoted P,(NF70) of 3 bars, indicating that the transport mechanism is changing around this critical pressure. Thus, before P,(NF70) a diffusional mass transfer is predominant, conforming to Eq.(l), and above this critical pressure a convective transport of the solutes occurs predominantly. For NF90 it is API bar
C A4 = aLp (AP-AII) + b ln -?f Cl
(1)
with b = $(l-zt+)
a = $(l-2t+)$; f
For a KC1 electrolyte solution, the transport number of K+ is near 0.5 (presumably assumed to be true in the membrane); then (l -2t’) is equal to zero and (A4),, is vanishing, conforming to Eq. (1). This result clearly explains the main difference between a pure RO dense (nonporous) membrane where diffusional mass transfer
8 -10
Fig. 1. SP measurements vs. transmembrane pressure for Filmtec NF55, NF70 andNF90 membranes ([KCI] = 1Om4 mot/L, pH = 6.8).
M. Pontit! et al. /Desalination
necessary to apply a critical pressure, denoted PJNF90), higher than 6.5 bars to measure SP. In the case of NF55 SP values were measurable just starting the transmembrane pressure. Then a convective transport is clearly predominant for the NF55 membrane in the range of all transmembrane pressures studied. Using firstgeneration NF membranes, the critical pressure values can be a limit usable to differentiate diffusional to convective mass transfer of the solutes. Furthermore, a correlation between the critical pressures of NF membranes and their pore radii can be observed: Pc(NF55,0.65nm) < Pc(NF70, 0.62 nm) < Pc(NF90, 0.54 nm). This order illustrates that SP effects increase with membrane pore size. Fig. 2 shows the evolution of a sodium electrolyte solution rejection observed vs. transmembrane pressure for theNF70 membrane. We have observed an inversion in R,,bs for bromide and chloride ions after 3 bars. Then for a pressure lower than the P,(NF70) value, the order of the observed rejection follows the 100 95
I
-*NaBr
Fig. 2. Observed rejection ratio of the halide ions Fe, Cl‘ and Br- sodium salts vs. the transmembrane pressure (NF70 membrane [sels] = 0.001 mol& pH = 6.8, membrane area 138 cm*).
I51 (2002) 267-274
271
hydration energy order (similar to the hydrated ionic radius): F-Xl->Br-3 proving that a pure diffusional mass transfer occurs. After Pc(NF70), a new order is observed F->Br-Xl-, proving now that NF mass transfer has changed and that a convective mass transfer driving process occurs predominantly. It now appears clearly that these halide ions are transferred by two mechanisms, separate or combined: (1) convective mechanism: the ions are carried by the solvent stream as a function of the transfer coefficient. The larger ions are more retained; (2) solubilization-diffusion: the ions are transferred under their partition coefficient which is the ratio of the solubility of the ion in the membrane under the solubility in the bulk solution. The larger the ion, the less well retained it is. Fluoride ion observed retention is not greatly affected by pressure modifications because its permeability is essentially due to diffusion. Convection has virtually no effect since a significant increase in pressure caused no significant change in the observed retention. In contrast, bromide, chloride or iodide ions as reported elsewhere [4] observed retention were much more influenced by the pressure because the permeability of these ions are mainly due to physical, rather than chemical, forces. We have also confirmed that the observed retention is often due more to the dissolution properties in the bulk solution than in the membrane media. Since the fluoride ion is the smallest monovalent anion, it is the most soluble ion in water with a high hydrated energy compared to the other halide ions and hence passes through the membrane with difficulty. In contrast, chloride and bromide ions are only slightly soluble in water and are less well retained. But as illustrated before, the transmembrane pressure can consequently modify the hydrated layers of the membrane pores and/or the hydrated layers of the ions showing some inversions, as previously reported [15-l 71.
272
M PontiB et al. /Desalination loo .j99.8
J” i
99.6 ...I
49.4 -
I
99.2 -1
x._.
x--~“-.~-.~x”---..~___,.
/~>z--__~; .:=---Gi” -.
‘/ J/-
I
/
.._..u ,--
/
98.8 _’ 0
3( -
’
/ 10
&_
--Q-t_
/
99.0 - I---
I .._.. ..._I
---a--a_
2b
/ 1 30
I, .X.“, .,..l
=4
f_
2.
_._.._. --........_..”.+._._-___. -t 40 SO 60 AP
(brr)
Fig. 3. Observed retention vs. transmembrane pressure for fluoride, chloride, bromide, iodide and sulfate ions in reverseosmosis(polyamidemembraneFT30, electrolyte sodium salt concentration = 0.05 mol/L, pH = 6.6).
The observed retention of the ions shows a constant value in the higher pressure range and a decrease in the selectivity between the ions. At the same time, retention increases with pressure due to an increase in the water flux due to dilution in the permeate, showing higher observed rejection. This is because convection predominates over diffusion. The observed retention vs. transmembrane pressure across a RO membrane is observed for divalent and monovalent anions shown in Fig. 3. As clearly demonstrated in Fig. 3, the observed retention of all the ions follows the order of the Hofmeister series (identical to the hydration energy of the ions): I-
151 (2002) 267-2 74
model [ 17-191, membrane water is bound to the polymer with solubility properties other than the free water of the bulk [ 16,191. That is the main reason for the better exclusion of highly hydrated ions. Sometimes the membrane material and/or streaming conditions can influence the retention observed, and some inversions have been reported [l&16]. This study clearly illustrates the essential differences between RO and NF processes. Indeed, RO cannot be used for partial and/or selective demineralization [22]. NF is more suitable for directly producing drinking water without the need of remineralized water. Furthermore, the present study clearly shows that NF selectivity for monovalent ions is higher at low pressure under diffusion mass transfer and consequently has a lower cost. 3.3. Origin of the membrane charge in firstgeneration NF membranes The SP measurements also allow to determine the IEP of the membrane under study and the kind of ions determining the charge density of these membranes which were initially not charged. But when a solid is dipped in an electrolyte solution, some ions are adsorbed at the interface. This results in a modification of the surface charge of the material. When adsorbing ions are those of the solvent itself, as is frequently the case in UF membranes [ 10,13,2 1, 221, the surface charge of the membrane is dependent on the pH of the solution, and at the IEP, the surface charge and the SP vanished. In NF the electrochemical interactions between electrolyte solutions and the membrane surface play a greater role in membrane performance. For our microporous PA membranes, the IEP is dependent on the ionic strength and the kind of electrolyte solution in contact with organic membranes materials, as recently detailed [23]. As seen in Fig_4a, displacement of the IEP with electrolyte concentration occurred. This
M PontiP et al. /Desalination 151 (2002) 267-274
273
This chemical adsorption may be explain by the following equilibria:
(a)
1‘5 1
2
Na’+H,‘-H’+Na,’
9 O2 E
&
O
(1)
and
4.5
-1 -1.5
Cl- +OHswOH-
, 2
+Cl,
(2)
I
’
3.5
5
6.5
8
9.5
11
PH
(b)
(S = membrane surface.) However, we have also checked that this chemical adsorption is not irreversible because all the initial properties were recovered before dilution or’ rinsing in a supersonic wave bath. 4. Conclusion
-1.5
’ 3.0
1
, 4.0
5.0
60
7.0
8.0
9.0
10
PfJ
Fig.4. (a) Growth of SP measurements vs. pH and ionic strength (membrane=NF55, electrolyte solution=KCl). (b) growth of SP measurements vs. pH for KCl, NaCl, and CaCI,electrolyte solutions (membrane=NFS& electrolyte concentration = 10e4 mol/L).
result confirms the hypothesis that ion adsorption occurs on the surface of the membrane used. Moreover, for the electrolyte KCI, NaCl and CaCl,. all the cations in solution are in competition to be adsorb first in the membrane electrochemical double layer. As seen in Fig.4b, the selectivity is due to different ions affinity of the surface. For a fixed electrolyte concentration (0.0001 mol.L-‘), the IEP ofthese membranes are 4.4, 5.6 and 4.8 for electrolytes KCl, NaCl and CaCl, electrolytes, respectively. Consequently, more Na’ is adsorbed compared to K’. Then the neutralization of the excess Na’ needs higher pH and a higher IEP is obtained.
A detailed understanding of the mass transfer of an electrolyte solution requires an intimate knowledge of the physicochemical interactions occurring between nanoporous materials and electrolyte solutions. SP measurements are one of the best methods to show the intrinsic charges brought to NF membrane pore walls. The origins of the charge on the polymers studied are the ions of the electrolyte solutions in contact with the materials. The IEP in the presence of KC1 is 4.4 at 0.0001 mol/L and 5.8 at 0.001 mol/L, showing an increasing adsorption of the K’ cation by increasing its concentration in solution. For a fixed concentration the effect of the kind of electrolyte solutions showed that a preferential affinity for Ca++is observed compared to K’ and Na’. But the adsorption of these electrolyte solutions is essentially reversible as observed under dilution conditions. Furthermore, SP measurements have been used for the first time to differentiate convective to diffusional mass transfer vs. transmembrane pressure. Our approach has established a correlation between the Kbs of the F-, Cl- and Brhalide ions and the kind of mass transfer diffusional or convective. Then for the NF70
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M. Pontit! et al. /Desalination
membrane at low pressure work (less than 3 bar), the order follows the Hofmeister series R,b,.(F~)>~r,,.(C1)>&r,~.(Br). For a higher pressure range an inversion is observed between Cland Br-. But F- appeared unaffected by transmembrane pressure. This study also provides a clearer understanding of how solutes are transferred in uncharged NF membranes, which could lead to a better prediction of the possibilities of applications and to identify the optimal streaming conditions. These results hold the prospect for selective defluorination of brackish drinking waters using NF membranes under low transmembrane pressure. Furthermore, very recently a secondgeneration NF charged membrane, calledNF200, was developed, which ensures passage of sufficient Ca” to meet water quality requirements and is under study for its potential treatment of Senegal’s brackish drinking water.
Acknowledgements The authors are indebted to Lyonnaise des eaux for financial support. Lots of thanks to S. Chevallier for her technical help and to L. Durand-Bourlier, D. Lemordant and Oumar SARR for fruitful discussions.
References 111 Y. Travi , Hydrogt!ochimie et hydrologie isotopique des aquif&es fluorur& du bassin du Senegal Origine et conditions de transport du fluor dans les eaux souterraines. These Univ. de Paris-Sud, France, 1988. M. Pontie, M. Rumeau and M. Ndiaye, Revue Sante, PI Montrouge, 6 (1996) 27. [31 M. Pontie, Phenomenes &ctrocinetiques et transferts ioniques dans les membranes poreuses 21faible seuil de coupure - Application au traitement des eaux saumatres, University of Tours, France, 1996.
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I41 A. Lhassani, M. Rumeau, D. Benjelloun and M. PontiC, Wat. Res., 35(13) (2001) 3260. L. Durand-Bourlier and J.M. Lain& Use of NF and PI EDR technology for specific ion removal: fluoride. Proc., Membrane Technology Conference, New Orleans, 1997. PI J.P. DeWitte, The development of a nanofiltration membrane for the potabilisation of surface water. International Workshop, Membranes in Drinking Water Production, AIDE, Paris, 1995. I71 C. Ventresque, G. Turner and G. Bablon, J. AWWA 89 (1997) 65. PI M. Ruweau, P. Lerellier, S. Sanack, L. Schrive, P. Mandin, M. Minier, C. Jolivalt, M. Pontit, V. Week, R. Audinos, M. Cassir, F. Badioui, J. Devynck, R. Clewent and J.P. Labbt, Les techniques separatives a membranes: theorie, applications et perspectives, CFE, Paris, 200 1. PI M. Pontie, X. Chasseray, D. Lemordant and J.M. Lain& J. Membr. Sci., 129 (1997) 125. [lOI M. Pontib, X. Chasseray, D. Lemordant and J.M. Lain& J. Chim. Phys., 94 (1997) 1741. Vll M. Pontie, L. Durand-Bourlier, D. Lemordant and J.M. Laine, Sep. Purif. Techn., 14 (1998)l. P21 M. Pontit, J. Membr. Sci., 154 (1999) 213. v31 C. Causserand, Etude des mecanismes de selectivite d’une membrane d’ultrafiltration, Thesis, University of Toulouse, 1992, p.2 10. 1141J.O’M Bockris and A.K.N. Reddy, Modem Electrochemistry I, 2nd ed., Plenum Press, New York, 1998, p. 489. [W J.D. Rapoport and C.F. Abramovitch, Russian J. Chem., 3 (1976) 545. [161 H.N. Chang, Desalination, 42 (1982) 63. [I71 C. Menjeaud, M. Pontie and M. Rumeau, Entropie, 179 (1993) 13. W.S. Ho and K.S. Kamalesh, Membrane Handbook, WI Chapman and Hall, New York, 1992. WI M. Sourirajan and S.M. Loeb, Chem. Sot. Symposium Ser.,153 (1981) 11. M. Pontie and D. Lemordant, J. Membr. Sci., 141 PO1 (1998) 13. WI W. Conlon, Desalination, 56 (1985) 203. WI J. Benavente and G. Jonsson, Sep. Sci. Technol., 32 (1997) 1699. 1231R. Zimmermann, S. Dukhlin and C. Werner, J. Phys. Chem. B, 105 (2001) 8544.