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Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes
~D E S A L I N A T I O N
Desalination 122 (1999) 53-62
ELSEVIER
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Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes Mustapha Hichour, Frangoise Persin, Jean Mol6nat, Jacqueline Sandeaux*, Claude Gavach Laboratoire des Mat&iaux et Procddds Membranaires, UMR 5635, CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France Tel. +33 (4) 67 61 34 14; Fax +33 (4) 67 04 28 20; email [email protected]
Received 20 November 1998; accepted 18 February 1999
Abstract Too many or too few fluoride ions in drinking water are harmful to the consumer's health. The acceptable fluoride concentration is generally in the range of 0.5 to 1.5 mg.L -~. In the present study, Donnan dialysis (DD) with an anionexchange membrane (AEM) was applied for the defluoridation of diluted NaF solutions. The initial concentration of the feed solution was maintained at 10-3 mol.L -l, corresponding to a 19 mg.L -~ fluoride concentration. Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested. First, membrane properties were studied at equilibrium. The values of the exchange capacity of the membranes in CI- and F- form, water content, selectivity coefficient for the CI-/Fexchange, diffusion coefficient of CI- and F- ions in the membrane, were determined for each membrane. DD experiments, performed using a laboratory cell, showed that the DSV membrane is the most effective AEM, despite its electrolyte leakage. Subsequently, a pre-industrial pilot with a total membrane area of 1760 cm 2 was used to study the different physico-chemical and hydrodynamic parameters of the process. As the driving ion, the chloride ion is more efficient than the sulfate ion. At flow rates lower than 0.6 L.h -~, the fluoride concentration remains lower than the permitted values despite the presence of others anions generally present in ground water such as chloride, sulfate and bicarbonate ions. Keywords: Fluoride ion; Donnon dialysis; Anion-exchange membrane; Defluoridation; Diluted solution
1. Introduction Although a suitable low concentration o f fluoride in drinking water is beneficial for the *Corresponding author.
prevention o f dental caries, a high concentration can produce dental and bone fluorosis [1,2]. According to the OMS norms, the lower value is 0.5 mg.L -1 while the upper limits vary from 0.7 to 1.5 mg.L -1 at the average temperature [3].
0011-9164//99/$ - see front matter © 1999ElsevierScience B.V. All rights reserved PII S0011-9164(99)00027-2
54
M. Hichour et al. /Desalination 122 (1999) 53-62
However the concentration of fluoride ions in groundwater o f some places in the world exceeds the acceptable values. To solve this problem, various methods to remove fluoride ions from groundwater have been proposed and tried [4]: precipitation with calcium and aluminum salts [5-8], adsorption on active alumina, alum, charcoal, ash [9-14], ion exchange [15-17]. Membrane processes such as reverse osmosis [ 18], nanofiltration [19], electrodialysis [20] and Donnan dialysis [21 ] were recently investigated to reduce fluoride concentrations in water. Donnan dialysis (DD) is a potentially attractive membrane separation process for concentrating valuable materials in ionic form from diluted solutions or removing undesirable ionic species from solutions [22-25]. DD is a process used to exchange ions between two solutions, the feed (A) and receiver (R) solution, separated by an ion-exchange membrane. The difference in the electrochemical potential on both sides of the membrane acts as the driving force. Thus, fluxes of the two counter-ions moving through the membrane appear in opposite directions. In the present study, DD was applied for the fluoride removal from diluted NaF solutions. The initial concentration of the feed solution was maintained at 10 -3 mol.L -1, corresponding to a 19mg.L -1 fluoride concentration. Preliminary experiments were performed using a laboratory cell to select the most effective anion-exchange membrane (AEM). Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested and their properties were also studied at equilibrium. Subsequently, a pre-industrial pilot with a total membrane area 50 times larger was used to study the different physico-chemical and hydrodynamic parameters of the process. The nature and concentration of the receiver electrolyte were varied. The influence of other anions generally present in ground water such as chloride, sulfate and bicarbonate ions was studied under various flow rates.
2. Experimental The commercially available AEMs tested were Selemion-DSV (Asahi Glass), and Neosepta-AFX, AFN, AMX, ACS (Tokuyama Soda). The AFN membrane contains vinylpyridinium groups; it was resistant against organic fouling. The others contain quarternary ammonium groups making them strong anion exchangers. The AMX specification is its high chemical strength while the ACS membrane is mono-anion permselective. The two latter membranes are mainly used in electrodialysis and three others in dialysis. All the measurements were performed at 25 °C.
2.1. Exchange capacity
The membrane samples (3x3cm 2) were immersed for 24 h in 200 mL of 0.5 mol.L -1NaCI or NaF solution. The samples removed from the equilibrating solution were washed in deionized water, quickly blotted and immersed for 24 h in 200 mL of 0.5 mol.L -l Na2SO 4 solution. The latter operation was performed twice and the two solutions were mixed. The amount of Cl- and Fions was measured in Na2SO4 solution. The membrane samples were dried at 50°C until their weight reached a constant value. The exchange capacity was expressed in milliequivalent per gram of dry membrane.
2.2. Water content of the membrane
After the membrane samples were immersed for 24 h in the 10g.L -~ NaC1 solution, they were wept between two sheets of blotting paper, weighed and dried at 50°C until their weight reached a constant value. The water content (Pw) is expressed as the percentage of weight of water sorbed in the membrane over the weight of the dry membrane.
M. Hichour et al. / Desalination 122 (1999) 53-62
2.3. Ion exchange The membrane samples, being initially in CIform, were immersed for 24h in 10-2 moI.L -~ NaF solution. Once equilibrium was reached, determination of C1- and F- concentration in the equilibrating solution allowed the anion concentration in the membrane to be derived by difference. 2.4. Conductivity measurements The electrical conductivity measurements were performed for the NaCI or NaF solution ranging from 10 -2 to 8.10 -l mol.L -1 by using the clip-cell already used and described elsewhere [26].
55
Before the dialysis operations, the anion exchange membranes were equilibrated in CIform.
The electrolyte solutions flowed as a batch system in the laboratory cell and as a single pass system in opposite directions in the feed and receiver compartments of the pilot. A mixture o f 10 -3 moI.L- l NaF (19 rag.L- l F-) with various salts (NaCI, Na2SO4, NaHCO3) circulated in the feed compartment, while NaCI or Na2SO 4 solutions at various concentrations circulated in the receiver compartment. Various flow rates varying from 0.1 to 2L.h -~ were used. The concentration at the outlet of the feed and receiver compartments were determined by taking samples of solutions at regular time intervals.
2.5. Donnan dialysis The flow diagram of the DD set-up is shown in Fig. 1. The modules of plate-and-frame design as filter press utilize flat sheet membranes. The laboratory cell used to test the different AEMs was composed o f two compartments separated by an AEM. The working membrane area was 35 cm 2. The preindustrial pilot was composed of eleven compartments (five feed compartments and six receiver compartments) separated by ten DSV AEMs having a total area of 1760 cm 2.
NaF lo'3M + NaX
The fluoride concentration was measured with a specific ion electrode by use of a total ionic strength adjustment buffer (TAFIC) to maintain the ionic strength at 1.75 mol.L -1, pH at 5.3 and to eliminate the interference effects of complexing ions. The chloride concentration was measured by coulometric titration by means of a titrator (Yacussel TACL2). The hydrogenocarbonate concentration was measured by acido-basic titration by means of a titrator (Tacussel TT Processor 2). The sulfate concentration was measured by spectrophotometry (Hach, Drel/2000) at 450nm. Sulfaver reactive was added to precipitate the sulfate ions with barium.
~)
xM
(A)
Feed
NaY yM
2. 6. Anion concentration measurement
3. Results
(R)
Receiver
Fig. 1. Schematic flow diagram of Donnan dialysis system.
First, membrane properties were studied at equilibrium. The selectivity coefficients for the CI-/F- exchange and diffusion coefficients of C1and F- ions in the membrane were determined for different AEMs. In the DD operations, the initial concentration of fluoride ions was maintained at 10 -3 mol.L -~
56
M. Hichour et al. / Desalination 122 (1999) 53-62
(19mg.L -1 F-). The influence of the nature and concentration o f the driving ion, feed composition and flow rate on the process efficiency was analyzed. 3.1. Choice o f the anion exchange membrane 3.1.1. Characterization o f anion exchange membranes at equilibrium Five AEMs were tested: Selemion-DSV (Asahi Glass), and Neosepta-AFX, -AFN, -AMX, -ACS (Tokuyama Soda). The values of the exchange capacity in the CI- and F- form, water content, selectivity coefficient for the C1-/F- exchange, diffusion coefficient of Cl- and F- ions inside the membrane are given in Table 1. The exchange capacity and the water content values were directly obtained from experiments. The selectivity and diffusion coefficients were deduced from ion-exchange and conductivity measurements, respectively. When the membranes, equilibrated in CI- form, are immersed in a solution containing F- ions, an ion exchange reaction occurs as follows: F - +CI-,~ F - +C1where the bars refer to the membrane. Instead o f an equilibrium constant, a selectivity coefficient is often used for describing an ionexchange equilibrium [27]:
= C ×cc,-
(1)
C F- × Cci-
The diffusion coefficients were determined using the method proposed by Gnusin et al. [28] and developed by Zabolotsky et al. [29] in which the membrane may be considered as a combination of two microphases. The phase consisting o f the matrix polymer chains with the functional groups is called the gel phase. The second phase contains the equilibrating electroneutral solution filling the inter-gel spaces. The diffusion coefficient of counter-ions in the membrane was calculated from the magnitude of the membrane conductivity at the isoconductance point, Nso, through the following relation: RTKiso
(2)
F 2 Ce (gel)
where R is the gas constant, T the absolute temperature, F the Faraday constant, Ce(gel), the exchange capacity of the gel phase. This method was described in a previous paper [30] and applied to determine the diffusion coefficient of CI- and F- ions in the AFN, AMX and ACS membranes. In this work, it was used for the DSV and AFX membranes.
Table 1 Exchange capacity, water content, selectivity coefficient and diffusion coefficient of anions for AEMs
Ce (in CI- form), meq.g-l C, (in F- form), m e q . g -1 Pw, % K~d_ Dct_ x 101°, m2.s-1 DF_ x 101°, m2.s-1
DSV
AFX
AFN
AMX
ACS
2.0
1.3
1.9
1.3
1.6
2.0
1.3
1.7
1.3
--
40 0.12 2.3 2.5
32 0.16 1.7 1.7
32 0.18 0.50 2.4
20 0.08 0.46 0.45
21 -0.19 0.12
M. Hichour et al. / Desalination 122 (1999) 53-62
The results of Table 1 show that the exchange capacities on the one hand, and the diffusion coefficients on the other, are close whatever the nature of the counter-ion, except for the AFN membrane as already mentioned by the authors. As expected, the diffusion coefficients decreased with the water content inside the membrane. For all the tested membranes, /~l was lower than unity, showing that the membrane affinity was largely greater for the chloride than the fluoride anion more hydrated.
3.1.2. Characterization of anion exchange membranes in Donnan dialysis In order to select the most effective membrane, DD experiments were performed with a laboratory cell requiring a membrane with a small area. The solutions circulated as a batch system in each compartment, 10 -3 mol.L -~ NaF and 10 -1 mol.L -~ NaCI in the feed and receiver compartments, respectively. The fluoride concentration at the outlet of the feed compartment was plotted for the five membranes in Fig. 2. It decreased as a function of time down to reach a constant value, corresponding to a pseudo-equilibrium. The equilibrium concentration, called ~q, follows the sequence: AMX > ACS > AFN > AFX > DSV. Only the AFX and DSV membranes did the fluoride concentration become lower than the upper norm (1.5 mg.L -l - 0.8 mol.L-]). During the DD operation, an ion exchange takes place through the membrane between the two aqueous solutions, the fluoride ions in the feed solution being substituted by the chloride ions coming from the receiver solution. The feed solution is enriched by chloride ions and vice versa. Since these anions have the same electrochemical valence, the number of exchanged anions must be identical on both sides of the membrane. However, with some membranes, the chloride concentration in the feed solution was higher than that required by the fluoride loss. This
57
10-
8-
o E
g o
2-
...........
~~-----~!
~ II
i
i
i
i 5OO
Time (mini
Fig. 2. Time dependence of the feed concentration of the F- ions. A: 10 -3 moI.L-] NaF; R: 10-l mol.L -l NaCI; Q^= QR= 0.4 L.h -1.
.oi A
,oi
,~0
2;0
g,
,~
Time (min)
Fig. 3. Time dependence of the feed concentration of the Na+ ions. A: 10 -3 moI.L-1 NaF; R: 10-l mol.L -1 NaCI; QA= QR= 0.4 L.h -]. was due to a NaC1 diffusion taking place from the receiver to the feed solution. Changes in the Na concentration in the feed solution allowed the comparison between the membranes in terms o f electrolyte leakage. Fig. 3 shows that the electrolyte leakage is significantly higher with the DSV than AFX and AFN membranes, while it was not detected with the AMX and ACS membranes. These two latter membranes, which are electrodialysis membranes, are more selective than the dialysis membranes but they are less efficient for the defluoridation by Donnan dialysis.
M. Hichour et at/Desalination 122 (1999) 53-62
58
In order to study the selectivity of membranes towards anions, experiments were performed with chloride, sulfate and hydrogenocarbonate ions in the feed compartment. The extraction of anions through the DSV, AFX and AFN membranes was compared in terms of extraction ratio, expressed as the percentage, as follows: cO ceq i ---i Rex t -
(3)
x 100
C~°
where C~ and C7q are the initial and equilibrium concentration of the anion, respectively. The results in Table 2 show that the DSV membrane gave the highest extraction ratio of fluoride with the highest selectivity. The AFX membrane gave similar extraction ratios for the F-, SO42-, and HCO3 anions. With the AFN membrane the extraction was lower for the fluoride than other anions. Taking into account these results, the DSV membrane was chosen to be introduced into the preindustrial pilot stack. Consequently, in subsequent experiments, only the DSV membrane was used. The working area was 50 times greater than that o f the laboratory cell. Moreover, in the preindustrial pilot the solutions circulated as a single-pass system in each compartment.
Table 2 Extraction ratio of anions by Donnan dialysis
3.2. Influence o f the receiver composition 3.2.1. Influence o f the nature o f the receiver electrolyte DD operations were performed with either 10-1mol.L -1 NaC1 or 5.10 -2 mol.L -1 Na2SO 4 receiver solution. Fig. 4 shows that the equilibrium concentration of the outlet feed solution, C~F, was equal or lower than the norm with the sulfate and chloride ion, respectively. The lower efficiency o f the sulfate ion could be explained by its higher affinity for the ionized sites of the membrane. Some authors have studied the role of the receiver electrolyte in optimizing the DD of feed monovalent cations and anions [21-23 ]. They have demonstrated that the receiver electrolyte would be selected to minimize the association between the fixed sites and the driving ions. Recently Miyoshi [31] has determined the diffusion coefficients of ions through ion-exchange membranes in DD using cations of different valence in the feed and receiver solutions. He has shown that it is better to use monovalent driving ions to obtain a larger flux because monovalent ions can move more easily inside the membrane than bivalent ones which
8.
~S
R~, %
DSV AFX AFN
F-
S O 2-
HCO;
83 72 43
67 71.5 75
50 78 68
A:10-3moI.L-INaF+10 -2 mol.L- 1 NaCI+3.10- 3 mol.L 1 NaaSO4+3.10-3 mol.L -1 NaHCO3; R: 10-1 mol.L -1 NaCI; QA=Qn=0.4 L.h- 1.
Ill i
i 100
Time
7 150
(rain)
Fig. 4. Time dependence of the feed concentration of the F- ions. A: 10 -3 mol.L-l NaF; R: 1110-1 mol.L -~ NaCI; [] 5.10-2 mol.L-I Na2SO4; QA= 0.2 L.h-l; QR= 0.4 L.h -l. AEM:DSV.
M. Hichour et aL / Desalination 122 (1999) 53-62 interact more strongly with the ionized sites of the membrane. Moreover, the electrolyte leakage through the membrane from the receiver to the feed solution is 2.5 times higher with NaCI than Na2SO 4 receiver solution. The high affinity of the ionized sites of the membrane for the bivalent sulfate ions induces a partial neutralization of the membrane increasing the diffusion of salt from the more to the less concentrated solution. Consequently, NaCI was chosen as the electrolyte receiver.
3. 2.2. Influence o f the receiver concentration Experiments were performed with a feed solution containing fluoride, chloride and sulfate ions and a NaCI receiver solution at various concentrations from 0.02 to 0.25 mol.L -~. The results were analyzed in terms of ion flux, J,, at the pseudo-equilibrium, given by the following relation: in
J~ =
QA Ci
eq I=SA "-'i
o _ / D out t'-'
(4)
S where C~ and
C~/q a r e
59
concentration of the ion, respectively, Q~ and Q~Ut the flow rates at the inlet and outlet o f the feed compartment, respectively, and S the total membrane area. As shown in Figs. 5 and 6, the fluoride transfer was not affected by the increase in the receiver concentration between 0.02 to 0.25 mol.L -~, but the sulfate flux and water transport from the A to R compartment and the NaCI diffusion from the R to A compartment increased significantly. These results showed that the nature and concentration of the driving ions are important parameters which must be optimized for each case.
3.3. Influence o f the f l o w rate in the f e e d and receiver compartment The influence of the flow rate in the feed and receiver compartment was studied with a 10- i mol.L-l NaCI solution as receiver electrolyte, the feed solution being 10 -3 mol.L-1 NaF. Fig. 7 shows that the fluoride flux increased significantly with the feed flow rate in the concentration range studied from 0.1 to 2 L.h -1. That was due to both the increase in the fluoride
the initial and equilibrium .50. ._.,.-..------e
/a "~ 4 0 .J
0.
i._~ ... 30-
~5.
20"
.-=
...j~ 10.
0
0.00 20O0
I 0.O0
I 0.10
i 0.16
I 0.20
I 0.25
0.05
0.10
0.1,5
0.20
0.25
CN,Cl ( moI.L'l )
CN=Q (moI.L'l)
Fig. 5. Variation of the fluoride and sulfate fluxes from the A to R compartment with the receiver concentration. A: 10-3 mol.L-1NaF+10 -2 mol.L-1NaCI+3.10-3 mol.L-1 Na2SO4; QA= 0.2 L.h-l; QR= 0.4 L.h-1; AEM:DSV.
Fig. 6. Variation of the sodium fluxes from the R to A compartment and water transport from the A to R compartment with the receiver concentration. A: 10-3 mol.L-I NaF + 10-2 mol.L-1 NaCI + 3.10.3 mol.L-I Na2SO4. QA= 0.2 L.h-1; QR= 0.4 L.h-1; AEM:DSV
M. Hichour et al. / Desalination 122 (1999) 53-62
60
1.0-
30
1.6251.42O-
" ~ - - + - - +
+
+
1.21.0•
1 t5°
-:£
~ 0.8- _ _ 0.6. 0.4. 0,2. 0.0 0.0
105. 0
0.0
0:1
012
o:~
014
&
0,6
Q^ (L.hq)
QA (k'h'l)
Fig. 7. Variation of the fluoride and sodium fluxes with the feed flow rate. A: 10-3 mol.L -l NaF; R: 10-l mol.L 1 NaCI; QR= 0.4 L.h-]; AEM:DSV.
2.01.01,6. 1,4~" 1,2-
Fig. 8. Variation of the outlet feed concentration of the Fions at the pseudo-equilibrium with the feed flow rate. R: 10-t mol.L -1 NaCI; QR = 0.4 L.h-1; AEM:DSV. A: 1-10 -3 mol.L -1 NaF. A: 2-10 -3 mol.L -l NaF + 10-2 mol.L-1NaCI. A: 3-10-3 mol.L -] NaF+ 10-2 mol.L- l NaCI + 3.10 -3 mol.L -] Na2SO4. A: 4--10 -3 mol.L -1 NaF + 10 2 mol.L-1 NaCI + 3.10 -3 mol.L -1Na2SO 4 + 3.10 -3 mol.L -1 NaHCO3 × French norm at 25-30°C + French norm at 8-12°C.
1,0O~'L0,8. 0.6" 0.4" 0.20.0
1'0
1'~
2'0
Cso4 (meq.L-D
Fig. 9. Variation of the outlet feed concentration of the F ions at the pseudo-equilibrium with the sulfate feed concentration. A: 10 -3 mol.L -l NaF + 10-2 mol.L -l NaCI +x mol.L-1 NazSO4; R: 10-l mol.L-[ NaCI; QA=0.2 L.h J; QR= 0.4 L.h-]; AEM:DSV.
content in the feed c o m p a r t m e n t because the solution circulated as single pass system, and the decrease in the thickness o f the unstirred layers. It was generally observed that the mass transfer reaches a constant value corresponding to the limit value o f the unstirred layer thickness. The fluoride concentration at the outlet o f the feed c o m p a r t m e n t corresponding to these experiments is plotted in Fig. 8.
Fig. 7 also shows that the NaCI leakage was not affected by the flow rate in the feed compartment. A second set o f experiments was performed maintaining a constant feed flow rate at 0.2 L.h -a and varying the receiver flow rate from 0.1 to 2 L.h -1 N o changes were observed either in the fluoride flux or in the NaCI leakage. At the concentration used (0.1 mol.L-1), the polarization phenomenon in the receiver solution is a negligible factor in the transport across the AEM.
3.4. Influence o f the f e e d composition The influence o f the feed composition on the fluoride transfer was studied under various feed flow rates• Chloride, sulfate and hydrogenocarbonate ions were progressively added to the feed solution in order to better analyze the effect o f each one. The values o f C~F are plotted in Fig. 8 and compared to the upper permitted values. The C~q values increased with the amount o f anions
M. Hichour et aL / Desalination 122 (1999) 5 3 4 2
and became higher than the norms if the feed flow rate became too high. Among the anions tested, the sulfate ions had the least favorable influence on the fluoride transfer. Thus, changes in the C~q values were measured as a function of the sulfate concentration in the feed solution (Fig. 9). Values higher than the upper limit were obtained for a low flow rate (QA=0.2 L.h-l) when the sulfate concentration reached 17 meq.L- I.
4. Conclusions
The results show that DD with DSV and AFX membranes allows efficient defluoridation in which the permitted values are reached. However, with these membranes, electrolyte leakage occurs from the receiver to the feed compartment, increasing the ionic concentration of the feed solution. This salt diffusion is higher with the DSV than AFX membrane. Nevertheless, the selectivity of the DSV membrane is better for the fluoride ion compared to the sulfate or hydrogenocarbonate ions. This can be explained by the high diffusion coefficient of the fluoride ion which can move more rapidly inside the membrane having a high water content. Therefore, among the AEMs tested, the DSV membrane is the more suitable membrane for fluoride removal. At low flow rates, the fluoride concentration remains lower than the norms although competition in the ionic transfer between the fluoride and others anions takes place. Therefore, DD, even with its low transfer rate, seems a promising defluoridation technique. The next paper will deal with the removal of excess fluoride in drinking waters of Africa and France.
5.
Symbols
A C C~
- - Feed compartment - - Concentration - - Exchange capacity
c•q D
DD F K J Pw Q R R
Rext S T
61
- - Outlet feed concentration of the fluoride ions at the pseudo-equilibrium - - Diffusion coefficient -Donnan dialysis process -Faraday's constant - - Selectivity coefficient - - Flux -Water content - - Flow rate - - Gas constant - - Receiver compartment -Extraction ratio -Membrane area - - Absolute temperature
Greek
- - Conductivity
References
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Desalination 122 (1999) 53-62
ELSEVIER
www.elsevier.com/locate/desal
Fluoride removal from diluted solutions by Donnan dialysis with anion-exchange membranes Mustapha Hichour, Frangoise Persin, Jean Mol6nat, Jacqueline Sandeaux*, Claude Gavach Laboratoire des Mat&iaux et Procddds Membranaires, UMR 5635, CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France Tel. +33 (4) 67 61 34 14; Fax +33 (4) 67 04 28 20; email [email protected]
Received 20 November 1998; accepted 18 February 1999
Abstract Too many or too few fluoride ions in drinking water are harmful to the consumer's health. The acceptable fluoride concentration is generally in the range of 0.5 to 1.5 mg.L -~. In the present study, Donnan dialysis (DD) with an anionexchange membrane (AEM) was applied for the defluoridation of diluted NaF solutions. The initial concentration of the feed solution was maintained at 10-3 mol.L -l, corresponding to a 19 mg.L -~ fluoride concentration. Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested. First, membrane properties were studied at equilibrium. The values of the exchange capacity of the membranes in CI- and F- form, water content, selectivity coefficient for the CI-/Fexchange, diffusion coefficient of CI- and F- ions in the membrane, were determined for each membrane. DD experiments, performed using a laboratory cell, showed that the DSV membrane is the most effective AEM, despite its electrolyte leakage. Subsequently, a pre-industrial pilot with a total membrane area of 1760 cm 2 was used to study the different physico-chemical and hydrodynamic parameters of the process. As the driving ion, the chloride ion is more efficient than the sulfate ion. At flow rates lower than 0.6 L.h -~, the fluoride concentration remains lower than the permitted values despite the presence of others anions generally present in ground water such as chloride, sulfate and bicarbonate ions. Keywords: Fluoride ion; Donnon dialysis; Anion-exchange membrane; Defluoridation; Diluted solution
1. Introduction Although a suitable low concentration o f fluoride in drinking water is beneficial for the *Corresponding author.
prevention o f dental caries, a high concentration can produce dental and bone fluorosis [1,2]. According to the OMS norms, the lower value is 0.5 mg.L -1 while the upper limits vary from 0.7 to 1.5 mg.L -1 at the average temperature [3].
0011-9164//99/$ - see front matter © 1999ElsevierScience B.V. All rights reserved PII S0011-9164(99)00027-2
54
M. Hichour et al. /Desalination 122 (1999) 53-62
However the concentration of fluoride ions in groundwater o f some places in the world exceeds the acceptable values. To solve this problem, various methods to remove fluoride ions from groundwater have been proposed and tried [4]: precipitation with calcium and aluminum salts [5-8], adsorption on active alumina, alum, charcoal, ash [9-14], ion exchange [15-17]. Membrane processes such as reverse osmosis [ 18], nanofiltration [19], electrodialysis [20] and Donnan dialysis [21 ] were recently investigated to reduce fluoride concentrations in water. Donnan dialysis (DD) is a potentially attractive membrane separation process for concentrating valuable materials in ionic form from diluted solutions or removing undesirable ionic species from solutions [22-25]. DD is a process used to exchange ions between two solutions, the feed (A) and receiver (R) solution, separated by an ion-exchange membrane. The difference in the electrochemical potential on both sides of the membrane acts as the driving force. Thus, fluxes of the two counter-ions moving through the membrane appear in opposite directions. In the present study, DD was applied for the fluoride removal from diluted NaF solutions. The initial concentration of the feed solution was maintained at 10 -3 mol.L -1, corresponding to a 19mg.L -1 fluoride concentration. Preliminary experiments were performed using a laboratory cell to select the most effective anion-exchange membrane (AEM). Five kinds of AEMs (DSV, AFX, AFN, AMX, ACS) were tested and their properties were also studied at equilibrium. Subsequently, a pre-industrial pilot with a total membrane area 50 times larger was used to study the different physico-chemical and hydrodynamic parameters of the process. The nature and concentration of the receiver electrolyte were varied. The influence of other anions generally present in ground water such as chloride, sulfate and bicarbonate ions was studied under various flow rates.
2. Experimental The commercially available AEMs tested were Selemion-DSV (Asahi Glass), and Neosepta-AFX, AFN, AMX, ACS (Tokuyama Soda). The AFN membrane contains vinylpyridinium groups; it was resistant against organic fouling. The others contain quarternary ammonium groups making them strong anion exchangers. The AMX specification is its high chemical strength while the ACS membrane is mono-anion permselective. The two latter membranes are mainly used in electrodialysis and three others in dialysis. All the measurements were performed at 25 °C.
2.1. Exchange capacity
The membrane samples (3x3cm 2) were immersed for 24 h in 200 mL of 0.5 mol.L -1NaCI or NaF solution. The samples removed from the equilibrating solution were washed in deionized water, quickly blotted and immersed for 24 h in 200 mL of 0.5 mol.L -l Na2SO 4 solution. The latter operation was performed twice and the two solutions were mixed. The amount of Cl- and Fions was measured in Na2SO4 solution. The membrane samples were dried at 50°C until their weight reached a constant value. The exchange capacity was expressed in milliequivalent per gram of dry membrane.
2.2. Water content of the membrane
After the membrane samples were immersed for 24 h in the 10g.L -~ NaC1 solution, they were wept between two sheets of blotting paper, weighed and dried at 50°C until their weight reached a constant value. The water content (Pw) is expressed as the percentage of weight of water sorbed in the membrane over the weight of the dry membrane.
M. Hichour et al. / Desalination 122 (1999) 53-62
2.3. Ion exchange The membrane samples, being initially in CIform, were immersed for 24h in 10-2 moI.L -~ NaF solution. Once equilibrium was reached, determination of C1- and F- concentration in the equilibrating solution allowed the anion concentration in the membrane to be derived by difference. 2.4. Conductivity measurements The electrical conductivity measurements were performed for the NaCI or NaF solution ranging from 10 -2 to 8.10 -l mol.L -1 by using the clip-cell already used and described elsewhere [26].
55
Before the dialysis operations, the anion exchange membranes were equilibrated in CIform.
The electrolyte solutions flowed as a batch system in the laboratory cell and as a single pass system in opposite directions in the feed and receiver compartments of the pilot. A mixture o f 10 -3 moI.L- l NaF (19 rag.L- l F-) with various salts (NaCI, Na2SO4, NaHCO3) circulated in the feed compartment, while NaCI or Na2SO 4 solutions at various concentrations circulated in the receiver compartment. Various flow rates varying from 0.1 to 2L.h -~ were used. The concentration at the outlet of the feed and receiver compartments were determined by taking samples of solutions at regular time intervals.
2.5. Donnan dialysis The flow diagram of the DD set-up is shown in Fig. 1. The modules of plate-and-frame design as filter press utilize flat sheet membranes. The laboratory cell used to test the different AEMs was composed o f two compartments separated by an AEM. The working membrane area was 35 cm 2. The preindustrial pilot was composed of eleven compartments (five feed compartments and six receiver compartments) separated by ten DSV AEMs having a total area of 1760 cm 2.
NaF lo'3M + NaX
The fluoride concentration was measured with a specific ion electrode by use of a total ionic strength adjustment buffer (TAFIC) to maintain the ionic strength at 1.75 mol.L -1, pH at 5.3 and to eliminate the interference effects of complexing ions. The chloride concentration was measured by coulometric titration by means of a titrator (Yacussel TACL2). The hydrogenocarbonate concentration was measured by acido-basic titration by means of a titrator (Tacussel TT Processor 2). The sulfate concentration was measured by spectrophotometry (Hach, Drel/2000) at 450nm. Sulfaver reactive was added to precipitate the sulfate ions with barium.
~)
xM
(A)
Feed
NaY yM
2. 6. Anion concentration measurement
3. Results
(R)
Receiver
Fig. 1. Schematic flow diagram of Donnan dialysis system.
First, membrane properties were studied at equilibrium. The selectivity coefficients for the CI-/F- exchange and diffusion coefficients of C1and F- ions in the membrane were determined for different AEMs. In the DD operations, the initial concentration of fluoride ions was maintained at 10 -3 mol.L -~
56
M. Hichour et al. / Desalination 122 (1999) 53-62
(19mg.L -1 F-). The influence of the nature and concentration o f the driving ion, feed composition and flow rate on the process efficiency was analyzed. 3.1. Choice o f the anion exchange membrane 3.1.1. Characterization o f anion exchange membranes at equilibrium Five AEMs were tested: Selemion-DSV (Asahi Glass), and Neosepta-AFX, -AFN, -AMX, -ACS (Tokuyama Soda). The values of the exchange capacity in the CI- and F- form, water content, selectivity coefficient for the C1-/F- exchange, diffusion coefficient of Cl- and F- ions inside the membrane are given in Table 1. The exchange capacity and the water content values were directly obtained from experiments. The selectivity and diffusion coefficients were deduced from ion-exchange and conductivity measurements, respectively. When the membranes, equilibrated in CI- form, are immersed in a solution containing F- ions, an ion exchange reaction occurs as follows: F - +CI-,~ F - +C1where the bars refer to the membrane. Instead o f an equilibrium constant, a selectivity coefficient is often used for describing an ionexchange equilibrium [27]:
= C ×cc,-
(1)
C F- × Cci-
The diffusion coefficients were determined using the method proposed by Gnusin et al. [28] and developed by Zabolotsky et al. [29] in which the membrane may be considered as a combination of two microphases. The phase consisting o f the matrix polymer chains with the functional groups is called the gel phase. The second phase contains the equilibrating electroneutral solution filling the inter-gel spaces. The diffusion coefficient of counter-ions in the membrane was calculated from the magnitude of the membrane conductivity at the isoconductance point, Nso, through the following relation: RTKiso
(2)
F 2 Ce (gel)
where R is the gas constant, T the absolute temperature, F the Faraday constant, Ce(gel), the exchange capacity of the gel phase. This method was described in a previous paper [30] and applied to determine the diffusion coefficient of CI- and F- ions in the AFN, AMX and ACS membranes. In this work, it was used for the DSV and AFX membranes.
Table 1 Exchange capacity, water content, selectivity coefficient and diffusion coefficient of anions for AEMs
Ce (in CI- form), meq.g-l C, (in F- form), m e q . g -1 Pw, % K~d_ Dct_ x 101°, m2.s-1 DF_ x 101°, m2.s-1
DSV
AFX
AFN
AMX
ACS
2.0
1.3
1.9
1.3
1.6
2.0
1.3
1.7
1.3
--
40 0.12 2.3 2.5
32 0.16 1.7 1.7
32 0.18 0.50 2.4
20 0.08 0.46 0.45
21 -0.19 0.12
M. Hichour et al. / Desalination 122 (1999) 53-62
The results of Table 1 show that the exchange capacities on the one hand, and the diffusion coefficients on the other, are close whatever the nature of the counter-ion, except for the AFN membrane as already mentioned by the authors. As expected, the diffusion coefficients decreased with the water content inside the membrane. For all the tested membranes, /~l was lower than unity, showing that the membrane affinity was largely greater for the chloride than the fluoride anion more hydrated.
3.1.2. Characterization of anion exchange membranes in Donnan dialysis In order to select the most effective membrane, DD experiments were performed with a laboratory cell requiring a membrane with a small area. The solutions circulated as a batch system in each compartment, 10 -3 mol.L -~ NaF and 10 -1 mol.L -~ NaCI in the feed and receiver compartments, respectively. The fluoride concentration at the outlet of the feed compartment was plotted for the five membranes in Fig. 2. It decreased as a function of time down to reach a constant value, corresponding to a pseudo-equilibrium. The equilibrium concentration, called ~q, follows the sequence: AMX > ACS > AFN > AFX > DSV. Only the AFX and DSV membranes did the fluoride concentration become lower than the upper norm (1.5 mg.L -l - 0.8 mol.L-]). During the DD operation, an ion exchange takes place through the membrane between the two aqueous solutions, the fluoride ions in the feed solution being substituted by the chloride ions coming from the receiver solution. The feed solution is enriched by chloride ions and vice versa. Since these anions have the same electrochemical valence, the number of exchanged anions must be identical on both sides of the membrane. However, with some membranes, the chloride concentration in the feed solution was higher than that required by the fluoride loss. This
57
10-
8-
o E
g o
2-
...........
~~-----~!
~ II
i
i
i
i 5OO
Time (mini
Fig. 2. Time dependence of the feed concentration of the F- ions. A: 10 -3 moI.L-] NaF; R: 10-l mol.L -l NaCI; Q^= QR= 0.4 L.h -1.
.oi A
,oi
,~0
2;0
g,
,~
Time (min)
Fig. 3. Time dependence of the feed concentration of the Na+ ions. A: 10 -3 moI.L-1 NaF; R: 10-l mol.L -1 NaCI; QA= QR= 0.4 L.h -]. was due to a NaC1 diffusion taking place from the receiver to the feed solution. Changes in the Na concentration in the feed solution allowed the comparison between the membranes in terms o f electrolyte leakage. Fig. 3 shows that the electrolyte leakage is significantly higher with the DSV than AFX and AFN membranes, while it was not detected with the AMX and ACS membranes. These two latter membranes, which are electrodialysis membranes, are more selective than the dialysis membranes but they are less efficient for the defluoridation by Donnan dialysis.
M. Hichour et at/Desalination 122 (1999) 53-62
58
In order to study the selectivity of membranes towards anions, experiments were performed with chloride, sulfate and hydrogenocarbonate ions in the feed compartment. The extraction of anions through the DSV, AFX and AFN membranes was compared in terms of extraction ratio, expressed as the percentage, as follows: cO ceq i ---i Rex t -
(3)
x 100
C~°
where C~ and C7q are the initial and equilibrium concentration of the anion, respectively. The results in Table 2 show that the DSV membrane gave the highest extraction ratio of fluoride with the highest selectivity. The AFX membrane gave similar extraction ratios for the F-, SO42-, and HCO3 anions. With the AFN membrane the extraction was lower for the fluoride than other anions. Taking into account these results, the DSV membrane was chosen to be introduced into the preindustrial pilot stack. Consequently, in subsequent experiments, only the DSV membrane was used. The working area was 50 times greater than that o f the laboratory cell. Moreover, in the preindustrial pilot the solutions circulated as a single-pass system in each compartment.
Table 2 Extraction ratio of anions by Donnan dialysis
3.2. Influence o f the receiver composition 3.2.1. Influence o f the nature o f the receiver electrolyte DD operations were performed with either 10-1mol.L -1 NaC1 or 5.10 -2 mol.L -1 Na2SO 4 receiver solution. Fig. 4 shows that the equilibrium concentration of the outlet feed solution, C~F, was equal or lower than the norm with the sulfate and chloride ion, respectively. The lower efficiency o f the sulfate ion could be explained by its higher affinity for the ionized sites of the membrane. Some authors have studied the role of the receiver electrolyte in optimizing the DD of feed monovalent cations and anions [21-23 ]. They have demonstrated that the receiver electrolyte would be selected to minimize the association between the fixed sites and the driving ions. Recently Miyoshi [31] has determined the diffusion coefficients of ions through ion-exchange membranes in DD using cations of different valence in the feed and receiver solutions. He has shown that it is better to use monovalent driving ions to obtain a larger flux because monovalent ions can move more easily inside the membrane than bivalent ones which
8.
~S
R~, %
DSV AFX AFN
F-
S O 2-
HCO;
83 72 43
67 71.5 75
50 78 68
A:10-3moI.L-INaF+10 -2 mol.L- 1 NaCI+3.10- 3 mol.L 1 NaaSO4+3.10-3 mol.L -1 NaHCO3; R: 10-1 mol.L -1 NaCI; QA=Qn=0.4 L.h- 1.
Ill i
i 100
Time
7 150
(rain)
Fig. 4. Time dependence of the feed concentration of the F- ions. A: 10 -3 mol.L-l NaF; R: 1110-1 mol.L -~ NaCI; [] 5.10-2 mol.L-I Na2SO4; QA= 0.2 L.h-l; QR= 0.4 L.h -l. AEM:DSV.
M. Hichour et aL / Desalination 122 (1999) 53-62 interact more strongly with the ionized sites of the membrane. Moreover, the electrolyte leakage through the membrane from the receiver to the feed solution is 2.5 times higher with NaCI than Na2SO 4 receiver solution. The high affinity of the ionized sites of the membrane for the bivalent sulfate ions induces a partial neutralization of the membrane increasing the diffusion of salt from the more to the less concentrated solution. Consequently, NaCI was chosen as the electrolyte receiver.
3. 2.2. Influence o f the receiver concentration Experiments were performed with a feed solution containing fluoride, chloride and sulfate ions and a NaCI receiver solution at various concentrations from 0.02 to 0.25 mol.L -~. The results were analyzed in terms of ion flux, J,, at the pseudo-equilibrium, given by the following relation: in
J~ =
QA Ci
eq I=SA "-'i
o _ / D out t'-'
(4)
S where C~ and
C~/q a r e
59
concentration of the ion, respectively, Q~ and Q~Ut the flow rates at the inlet and outlet o f the feed compartment, respectively, and S the total membrane area. As shown in Figs. 5 and 6, the fluoride transfer was not affected by the increase in the receiver concentration between 0.02 to 0.25 mol.L -~, but the sulfate flux and water transport from the A to R compartment and the NaCI diffusion from the R to A compartment increased significantly. These results showed that the nature and concentration of the driving ions are important parameters which must be optimized for each case.
3.3. Influence o f the f l o w rate in the f e e d and receiver compartment The influence of the flow rate in the feed and receiver compartment was studied with a 10- i mol.L-l NaCI solution as receiver electrolyte, the feed solution being 10 -3 mol.L-1 NaF. Fig. 7 shows that the fluoride flux increased significantly with the feed flow rate in the concentration range studied from 0.1 to 2 L.h -1. That was due to both the increase in the fluoride
the initial and equilibrium .50. ._.,.-..------e
/a "~ 4 0 .J
0.
i._~ ... 30-
~5.
20"
.-=
...j~ 10.
0
0.00 20O0
I 0.O0
I 0.10
i 0.16
I 0.20
I 0.25
0.05
0.10
0.1,5
0.20
0.25
CN,Cl ( moI.L'l )
CN=Q (moI.L'l)
Fig. 5. Variation of the fluoride and sulfate fluxes from the A to R compartment with the receiver concentration. A: 10-3 mol.L-1NaF+10 -2 mol.L-1NaCI+3.10-3 mol.L-1 Na2SO4; QA= 0.2 L.h-l; QR= 0.4 L.h-1; AEM:DSV.
Fig. 6. Variation of the sodium fluxes from the R to A compartment and water transport from the A to R compartment with the receiver concentration. A: 10-3 mol.L-I NaF + 10-2 mol.L-1 NaCI + 3.10.3 mol.L-I Na2SO4. QA= 0.2 L.h-1; QR= 0.4 L.h-1; AEM:DSV
M. Hichour et al. / Desalination 122 (1999) 53-62
60
1.0-
30
1.6251.42O-
" ~ - - + - - +
+
+
1.21.0•
1 t5°
-:£
~ 0.8- _ _ 0.6. 0.4. 0,2. 0.0 0.0
105. 0
0.0
0:1
012
o:~
014
&
0,6
Q^ (L.hq)
QA (k'h'l)
Fig. 7. Variation of the fluoride and sodium fluxes with the feed flow rate. A: 10-3 mol.L -l NaF; R: 10-l mol.L 1 NaCI; QR= 0.4 L.h-]; AEM:DSV.
2.01.01,6. 1,4~" 1,2-
Fig. 8. Variation of the outlet feed concentration of the Fions at the pseudo-equilibrium with the feed flow rate. R: 10-t mol.L -1 NaCI; QR = 0.4 L.h-1; AEM:DSV. A: 1-10 -3 mol.L -1 NaF. A: 2-10 -3 mol.L -l NaF + 10-2 mol.L-1NaCI. A: 3-10-3 mol.L -] NaF+ 10-2 mol.L- l NaCI + 3.10 -3 mol.L -] Na2SO4. A: 4--10 -3 mol.L -1 NaF + 10 2 mol.L-1 NaCI + 3.10 -3 mol.L -1Na2SO 4 + 3.10 -3 mol.L -1 NaHCO3 × French norm at 25-30°C + French norm at 8-12°C.
1,0O~'L0,8. 0.6" 0.4" 0.20.0
1'0
1'~
2'0
Cso4 (meq.L-D
Fig. 9. Variation of the outlet feed concentration of the F ions at the pseudo-equilibrium with the sulfate feed concentration. A: 10 -3 mol.L -l NaF + 10-2 mol.L -l NaCI +x mol.L-1 NazSO4; R: 10-l mol.L-[ NaCI; QA=0.2 L.h J; QR= 0.4 L.h-]; AEM:DSV.
content in the feed c o m p a r t m e n t because the solution circulated as single pass system, and the decrease in the thickness o f the unstirred layers. It was generally observed that the mass transfer reaches a constant value corresponding to the limit value o f the unstirred layer thickness. The fluoride concentration at the outlet o f the feed c o m p a r t m e n t corresponding to these experiments is plotted in Fig. 8.
Fig. 7 also shows that the NaCI leakage was not affected by the flow rate in the feed compartment. A second set o f experiments was performed maintaining a constant feed flow rate at 0.2 L.h -a and varying the receiver flow rate from 0.1 to 2 L.h -1 N o changes were observed either in the fluoride flux or in the NaCI leakage. At the concentration used (0.1 mol.L-1), the polarization phenomenon in the receiver solution is a negligible factor in the transport across the AEM.
3.4. Influence o f the f e e d composition The influence o f the feed composition on the fluoride transfer was studied under various feed flow rates• Chloride, sulfate and hydrogenocarbonate ions were progressively added to the feed solution in order to better analyze the effect o f each one. The values o f C~F are plotted in Fig. 8 and compared to the upper permitted values. The C~q values increased with the amount o f anions
M. Hichour et aL / Desalination 122 (1999) 5 3 4 2
and became higher than the norms if the feed flow rate became too high. Among the anions tested, the sulfate ions had the least favorable influence on the fluoride transfer. Thus, changes in the C~q values were measured as a function of the sulfate concentration in the feed solution (Fig. 9). Values higher than the upper limit were obtained for a low flow rate (QA=0.2 L.h-l) when the sulfate concentration reached 17 meq.L- I.
4. Conclusions
The results show that DD with DSV and AFX membranes allows efficient defluoridation in which the permitted values are reached. However, with these membranes, electrolyte leakage occurs from the receiver to the feed compartment, increasing the ionic concentration of the feed solution. This salt diffusion is higher with the DSV than AFX membrane. Nevertheless, the selectivity of the DSV membrane is better for the fluoride ion compared to the sulfate or hydrogenocarbonate ions. This can be explained by the high diffusion coefficient of the fluoride ion which can move more rapidly inside the membrane having a high water content. Therefore, among the AEMs tested, the DSV membrane is the more suitable membrane for fluoride removal. At low flow rates, the fluoride concentration remains lower than the norms although competition in the ionic transfer between the fluoride and others anions takes place. Therefore, DD, even with its low transfer rate, seems a promising defluoridation technique. The next paper will deal with the removal of excess fluoride in drinking waters of Africa and France.
5.
Symbols
A C C~
- - Feed compartment - - Concentration - - Exchange capacity
c•q D
DD F K J Pw Q R R
Rext S T
61
- - Outlet feed concentration of the fluoride ions at the pseudo-equilibrium - - Diffusion coefficient -Donnan dialysis process -Faraday's constant - - Selectivity coefficient - - Flux -Water content - - Flow rate - - Gas constant - - Receiver compartment -Extraction ratio -Membrane area - - Absolute temperature
Greek
- - Conductivity
References
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[24] J. Macenauer, M. Handlirova and I. Machac, J. Membr. Sci., 60 (1991) 157. [25] T. Ktari, C. Latchet and B. Auclair, J. Membr. Sci., 84 (1993) 53. [26] S. Soulier, P. Sistat, E. Dejean, J. Sandeanx, R. Sandeaux and C. Gavach, J. Membr. Sci., 141 (1998) 111. [27] F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962. [28] N.P. Gnusin, V.I. Zaboiotsky, V.V. Nikonenko, and M.R. Urtenov, Electrokhimiya, 22 (1986) 298; Russian J. Electrochem, 22 (1986) 273. [29] V.I. Zabolotsky and V.V. Nikonenko, J. Membr. Sci., 79 (1993) 181. [30] A. Elattar, A. Elmidaoui, N. Pismenskaia, C. Gavach and G. Pourcelly, J. Membr. Sci., 143 (1998) 249. [31] H. Miyoshi, J. Membr. Sci., 141 (1998) 101.