Transport of proton in polymeric ionic exchange membranes in relation with the dissociated sorbed acid

Desufinution, 80 (1991) 193409 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

193

Transport of Proton in Polymeric Ionic Exchange Membranes in Relation with the Dissociated Sorbed Acid GERALD POURCELLY, MAGUELONNE BOUDET-DUMY, ARLElTE LINDHEIMER and CLAUDE GAVACH Labomtoty of Physical Chemistry of Polyphasic Systems, CNRS, URA 330, BP 5051, 34033 Montpellier Cede (Fmnce) (Received December 17,199O)

SUMMARY

The transport of proton in ion exchange membranes in contact with HCl and H2S04 solutions is studied. The membranes are the Nafione 117 cation exchange membrane and, on the other hand, the Selemion@ AAV and the Morgane ARA anion exchange membranes. Sorption and water content measurements combined with the radiotracer technique point out the low dissociation degree of the acid present in the membrane phase. This low dissociation leads to the excellent permselectivity towards proton of the Nafion membrane, and it is also the factor which decreases the proton leakage in the two studied anion exchange membranes.

INTRODUCTION

In water, proton has the highest electrical mobility. This property is at the basis of the good permselectivity of commercial cation exchange membranes in contact with acid solutions. On the other hand, the high mobility of proton is at the origin of the proton leakage shown by the standard commercial anion exchange membranes. Because of the poor permselectivity of anion exchange membranes in the presence of protons, until very recently high current efficiencies could not be obtained in the electrodialysis for the recovery and the reconcentration of acids [l-9]. The Nafionm 117 cation exchange membrane, which has been especially developed for proton transport [lO,ll], shows a very high permselectivity towards protons. ooll-9164/91/$03.50

0 1991 Elsevier Science Publishers B.V.

194

The Selemion AAV and the Morgane ARA anion exchange membranes have been designed for the recovery of acids by electrodialysis and electroelectrodialysis [U-14]. In these new commercial membranes the proton leakage is lower than in other anion exchange membranes. In contact with concentrated acid solutions the three membranes contain a certain amount of acid [ 11,151,but with the three membranes the transport numbers of the counter-ion have values which are higher than in the hypothetical case where the ratio of the mobilities of the proton and of the associated anion is the same as in water. The dissociation and the transport properties of the sorbed acid must be taken into consideration. The aim of this work is to quantify the contribution of the membrane material to the reduction of the loss of permselectivity of the studied membranes in contact with acid solutions. The permselectivity is determined by zero-current membrane potential measurements as well as by radiotracer flux measurements of the anion under an applied current. Special attention is paid towards the dissociation of the sorbed acid.

EXPERIMENTAL The main characteristics

of the studied membranes are shown in Table I. Before utilisation the membranes are soaked in water for more than 48 h, and before the experiments they are maintained in the studied solution for 24 h. Measurements are carried out at 25°C. TABLE I Main characteristics of the studied membmnes ARA

Nafion 117

Membrane Exchange capacity (m.eq./g) Thickness (pm) Water content (weight S) Transport number*

PM) DuPont de Nem. 0.890

PM) Asahi Glass 0.54

Morgane 0.60

180 25

110 8.6

160 8.7

(H+): = 1.0

(Cl-): = 0.92

(Cl-): = 0.94

*Measured from membrane potential (low2 - 10-l NaCl).

195

Dete~‘~tion

of the amount of sorbed acid

After having been equilibrated in a given acid solution, the membrane is then removed from the solution, its faces are swept using blotting paper and the membrane is again immersed into a given volume of water with a strong stirring. For the Nafion cation exchange membrane the amount of sorbed acid is determined by acid+basic titration. For the anion exchange membranes the amount of sorbed HCl or HzS04 has been determined from HCl labelled with 36Cl or from H$O4 labelled with 35S. Se&dimion

flux and transport number measurements

Radiotra~ers are used for measur~g the transmembrane unid~~tion~ fluxes of labelled ions when the membranes are maintained between two symmetrical acid solutions. By this technique the values of the self-diffusion fluxes as well as the transport number can be evaluated [16,173. Determination of the water content The water content is usually defined by the following relation:

where mw is the weight of the membrane sample equ~brated with HCl 0.1 M aqueous solution, and md is the weight of the dried membrane under the H ’ form for the cation exchange membr~e and under the Cl- form for the two anion exchange membranes. For a complete dehydration the membrane having been maintained under vacuum at 35°C until a constant weight has been obtained is sufficient for the two anion exchange membranes, while a more severe dehydration under the tetrabutylammonium form is required for the Nafion 117 membrane [ll]. It has been checked by IR spectroscopy that after this treatment the membrane contains no more water. Zero-current membrane potential After having been immersed in a 0.1 M solution of acid for 12 h, the membrane is placed inside a plexiglass cell between two aqueous solutions at different concentrations: 0.1 M on one side and variable concentrations up to 4 M on the other side. In each compa~ment the solutions are continuously renewed by means of two peristaltic pumps. The steady-state values of

1%

the membrane potential are measured using two saturated calomel reference electrodes and a high impedance milli-voltmeter [18].

RESULTS AND DISCUSSION The measurements of sorption, water content, zero-current exchange flux, unidirectional fluxes and membrane potential have been achieved on the Nafion 117 membrane and on the AAV and AM membranes. With H,SO, solutions the measurements of sorption, water content, zero-current exchange flux and unidirectional fluxes are achieved only on the Nafion 117 membrane.

Water and acid sorption equilibrium The permselectivity of an ion exchange membrane results from both by the amount of aqueous co-ion penetrating into the membrane phase and by the ratio between the co-ion and counter-ion mobilities in the membrane. The measurements of sorption and water content are shown in Tables IIV. Tables II and III refer to the Nafion 117 membrane, respectively, in HCl and H,SO, solutions. Tables IV and V refer, respectively, to the AAV and to the ARA membranes in HCl solutions. In these tables the following are reported acid concentration: l

l

l

as a function of the external

the water content expressed, respectively, by the number of water molecules per fixed exchange site n H20/ns or by the water content percentage w.c.; the amount of sorbed acid expressed, respectively, by the number of moles of sorbed acid HA per fixed site nHA/ns or per gram of dried membrane; the number of water molecules per mole of sorbed acid nHzO/nHA that gives a good insight into the hydration state of the sorbed species.

The results show that swelling is very important for the Nafion membrane which is not cross-linked, while the amount of sorbed acid remains very low. For the AAV anion exchange membrane the swelling and the sorption are more important than for the ARA membrane. These results may be compared to the theoretical values of sorption calculated from the classical Donnan relation.

197

TABLE II Sorption and water content for HCl in the Nafion 117 membrane vs. the external concentration C H?l (mol/l)

3i201ns0y

5%H,O W.C.

%rClkg

(wt. W

nH&

of

dried membrane

“H2O’“HCl

(mmolk) 0.25 0.50 1.0 2.0 3.0 4.0

0.022 0.034 0.08 0.15 0.26 0.36

28.2 27.5 26.3 24.8 24.6 22.4

23.9 23.0 22.4 20.7 20.5 18.1

0.021 0.032 0.07 0.14 0.24 0.33

1045 677 280 137 79 50

TABLE III Sorption and water content for H,SO, in the Nafion 117 membrane vs. the external concentration Cext

nH20’nSOj-

H2S04

(mol/l)

96 H,O

“H2S04h@-

W.C.

“H2SO4’g or aned

(wt. 96)

membrane

“H20’ nH2S04

(mmolk) 0.25 0.50 1.0 2.0 3.0 4.0

26.0 24.2 22.2 20.0 18.4 18.2

29.9 28.5 26.2 24.7 23.2 23.0

0.005 0.015 0.04 0.08 0.12 0.16

0.005 0.014 0.037 0.073 0.110 0.115

5200 1613 560 250 153 114

For the HCl solutions (1: 1 electrolyte): rn,l =

-x/2

+ [Y/4

+ “xc1 *WICI

1

(1)

where mHClis the modalityof the_sorbedacid in the membrane phase, mHCI its molality in the bulksolution,X the exchange capacity, i.e., the molality of the fixed sites, and y 2 HcI the mean activity coefficient in the bulk.

198 TABLE IV Sorption and water content for HCI ia the AAV anion exchange membraue

c

~H&R+

HZ (mol/l)

(mol/site)

96H,O w.c. W.

bdBR+

%c!4

of

~~~~~H~I

(mol/mol)

membme

%%)

(mmolk) 0.5 1.0 2.0 3.0 4.0 5.0 6.0

15.9 14.4 14.0 12.9 13.2 12.1 9.1

19.49 17.55 16.93 15.54 15.93 14.60 10.73

1.78 2.59 3.04 3.72 3.94 4.94 7.41

O.% 1.40 1.64 2.01 2.13 2.67 4.00

7.82 5.32 5.01 3.95 3.87 2.97 1.58

TABLE V Sorption and water content for HCl in the ARA anion exchange membrane C Hz (molll)

“H2o/~R+

96 Hz0

nHCl’nR+

%Ic14!

of

dried membrane

“H20’nHC1

(mmoI@ 0.5 1.0 2.0 3.0 4.0 5.0 -6.0

11.89 9.73 9.36 9.17 9.0 8.28 7.16

14.2 12.4 12.0 11.9 11.7 11.0 10.0

0.52 1.05 1.22 1.38 1.77 2.25 1.88

0.31 0.62 0.73 0.83 1.06 1.35 1.88

11.80 9.41 7.87 6.65 5.08 3.70 2.29

Eqn. (1) has been derived by simulating the membrane as an homogeneous gel phase where the ionic dissociation is complete and the electrolyte activity coefficient in the membrane is taken as unity. The experimental values of mHCl are calculated from: ;

HCI

I-.-‘-

loo0 18

fiHC1 n,

“, nHw

with “s” the fined change sites.

(2)

199

For HJS04 solutions (1:2 electrolyte) and for the Nafion membrane, if r55+ and m_ are. respectively the molalities of the counter-ion and of the coion within the membrane phase: (3)

Gi+= x + 2iii_

supposing a complete dissociation. The classical Donnan’s relation leads to the equation -(3! + Zn_)m_

= mi’*y*3

(4)

with I?i_ = mEIzso4 For the Nafion membrane equilibrated with HCl solutions (Fig. l), a from Dorman’s equation is observed from mHc1 greater than discrepan 1 mole.kg’“y, while, in the higher molality range, the rejection of co-ions is greater than that predicted. For the same membrane equilibrated with H$O, solutions (Fig. 2), the discrepancy from Donnan’s equation is observed in the whole range of external concentrations; the rejection of coions is about one order of magnitude less than the theoretical one.

Naf ion 117

Fig. 1. Experimental and calculated values of sorbed molalities of hydrochloric acid as a function of the external mokdity. N&on 117 membrane.

200

43

.a2

Fig. 2. Experimental and calculated values of sorbed molalities of hydrosulphuric function of the external molality. Nafion 117 membrane.

acid as a

For the AAV and AKA membranes the experimental values of the amount of sorbed species are relatively high. In Fig. 3 we can see that the experimental values of the sorption 6&+, are higher than the theoretical values m,, The behavior of these two membranes is quite the same with respect to HCl solutions. Keeping the same gel model for the membrane and taking into consideration the non-homogeneity in the fixed charge distribution, Glueckauf [19] obtained the following relation: r;; =

k,m@-@

(5)

where &is the mean molality of the sorbed electrolyte, k a constant and z a factor reflecting the non-homogeneity in the fixed charge distribution. Fig. 4 shows that forge = 1.7, there is good agreement between the experimental values and Eqn. (5) for the AAV and ARA anion exchange membranes.

201

Fig. 3. detents and calculated values of sorbed molalities of hydrochloric acid as a function of the external molality. AAV and ARA membranes.

~*.,_ . Fig. 4. elevation

.

km

of the exponent of the Glue&auf relation & =

202

Zero-current membrane potentials In Figs. 5 and 6 the experimental values of the diffusion membrane potential are plotted vs. the logarithm of the ratio of the HCl mean activity on both sides of the membrane. For the Nafion membrane, deviation from Nernst’s law starts from 2 M. For the AAV membrane it occurs from the same value of 2 M while for the ARA membrane it occurs from 1 M. The values of the so-called apparent transport numbers of ions t- in the membrane phase are commonly related to the membrane potential by means of the following equation:

with u1 and a2 the mean activities. They are reported in Table VI.

’ E/tv /’ . . 100

Naflon 117

,

.’

Fig. 5. Variation of the zero-current membrane potential of the cek HCl (0.1 M, al) 11 N&on 117 membrane 11HCl (C, az).

For the Nafion membrane the values of the transport number of H+ confirms the high permselectivity of the membrane in contact with protons. For the two anion exchange membranes the proton leakage becomes significant from a 2 M HCl concentration, the behavior of these two membranes seeming to be the same.

0.28

$4

1.0 1

1.7

2

3

lO@$

qAnol.1-’

Fig. 6. Variation of the zero-current membrane potential of the cek HCl(O.1 M a,) 11AAV or AFU membrane 11HCl (C, az). TABLE VI Experimental values of the transport numbers of counter-ion: (a) from zero current srb= El;rn: HCl (0.1 M) 1 1 membrane 1 1 HCl C(M); (b) from

Membrane

N&m

117

CHCI (M)

G+ (a)

AAV G,(a)

0.2 1.0 2.0 3.0 4.0

1.00 1.00 0.97 0.94 0.92

1.00 0.99 0.92 0.88 0.80

&IV %*tW

G(a)

I 0.97 0.94 0.74 0.54

1.00 0.97 0.90 0.84 0.77

I 0.87 0.82 0.66 0.55

Transport number measurements un&r a constant current With the AEM the values of fhe zefo-current exchange fluxes Jo and those of the u~~~tion~ fluxesJ and J of the counter-ion and the transport number under a constant current are reported in Tables VII-X. The transport number of a counter-ion ‘3”is calculated from the relation: ii = (1 - J')F/Z

(7)

204

with I the applied current density, J’ the unidirectional flux from the cathode to the anode and J the unidirectional flux in the opposite direction. TABLE VII HCl in the Nafion 117 membrane - Zero current exchange flux (Jo) and unidirectional flux%(J) ofCl- ions crossing the membrane between two identical HCl solutions. The flux J and J are, respectively, the unidirectional fluxes in the same and in the opposite direction as the electrical driving force

C

I=0

I = 18.25 mA/cm2

HCl (mol/l)

lo* Jo (molh. cm2)

108 J’ (mol/s.cm2)

lo8 ?-

0.5 1.0 2.0 3.0 4.0

0.60 1.66 4.21 8.97 10.70

0.60 1.91 5.30 8.97 10.70

0.60 1.95 4.50 8.15 10.70

Tables VII and VIII concern the Nafion 117 membrane equilibrated respectively with HCl and H,SO, solutions. The main feature is the ve low value of the co-ion transport number, Cl’, for HCl and HSO;i or SO,P for sulphuric acid, that confirms the Nafion 117 membrane as an excellent proton conductor. Within the experimental error estimated at 20.01, tclremains lower than 2% whatever the external concentration. For H,SO,, too, the transport number of the ion bearing a sulphur atom remains lower than 2%. The analysis of these results is quite difficult because the degree of dissociation of these acids within the membrane phase is unknown. The radiotracer technique does not allow for distinguishing the transport number of ion,s to that of the non-dissociated species. For example, the unidirectional flux J in the opposite direction as the electric driving force may be due either to the drift move of Cl’ anion due to the electroosmotic flux in the membrane or to non-dissociated HCI molecules forming the solvation shell of the proton with H,O. The problem is the same for H2S04.

205

TABLE VIII Hydrosulphuric acid& the+Na&m 117 membrane - Zero-current exchange flux Jo, unidirectional fluxes J and J and transportnumber TBof species bearing a sulphur atom as a function of the extemal acid concentration I=0

I = 18.25 mAlcm2

C (H2SO4) (mol/l)

lo* Jo mol/s.cm2

1087 (mol/s.cm2)

1087

r _ (S-W # Z

0.25

0.50 1.00 2.00 4.00

0.12 0.29 0.84 2.10 3.80

0.09 0.32 0.99 2.40 3.57

0.08 0.26 0.89 2.27 3.25

1 3 5 7 10

2.00

2.01

4.00

3.89

x x x x x

10-3 10-3 1o-3 10-3 1o-3

I = 186 mA/cm2 3.14

1.82

7 x 10-S I = 383 mA/cm2 5.47

3.53

5 x 10-3

Nevertheless, the analysis of the unidirectional fluxes ? and f compared with the zero-current exchange flux J, allows one to propose some explanations about the transport within the membrane. For HCl in the Nafion membrane the two unidirectional fluxes of the 36C1 labelled species increase with the external concentration, keeping values quite equal to the zero-current exchange fluxJu (Table VII). That is to say that the chloride species is not carried by the electrical current. On the basis of a_low a@d dissociation within the membrane phase, the equality between J,, J and J for all external acid concentrations signifies that the transport of chloride species in the same electric driving force is exactly balanced by the transport of the chloride ion in the opposite direction. This last transport may be due either to the migration of Cl- ions by the frictional interaction with the migrating water hydrating the proton or to the formation of aggregates such as [H,HZO,HCl] +. Nevertheless, the electroosmo$c fly decreases when the external concentration increases [20] while Jo, J and J are quite identical for all the external concentrations. Thus, the only possibility is a very low dissociation of sorbed HCl favouring the formation of aggregates.

206 TABLE IXA AAV membrane - Zero-current exchange flux (Jo) and unidirectional fluxes (7 and 7) of chloride ion crossing the AAV membmue vs. the external concentration of HCl

C (HCl) (mol/l) 0.5 1.0 2.0 3.0 4.0 5.0 6.0

I=0

I = 30 mA/cm2

I = 30 mA/cm2

I = 30 mA/cm2

lo* J,, (mol/s.cm2)

lo* J’ (mol/s.cm2)

1097 (molk. cm2)

X

2.1 1.4 2.2 1.2 3.5 1.4 1.5

28.0 26.8 22.5 19.6 16.4 14.6 12.8

4.3 7.9 8.2 12.2 9.1 9.6 8.7

0.015 0.029 0.036 0.062 0.055 0.065 0.068

TABLE IXB AR.4 membrane - Zero-current exchange flux (Jo) and unidirectional fluxes (7 and J’ of chloride ion crossing the ARA membrane vs. the external concentration of HCl

C (HCl) (mol/l) 0.5 1.0 2.0 3.0 4.0 5.0 6.0

I=0

I = 30 mAkm2

I = 30 mAlcm2

I = 30 mA/cm2

lo* J,, (mol/s.cm2)

1087 (mol/s.cm2)

1097 (mol/s.cm2)

X

6.1 5.0 6.4 7.3 6.7 4.6 4.8

28.6 26.8 25.6 21.3 17.3 14.6 12.5

3.2 2.9 17.2 28.6 26.2 22.6 51.5

0.011 0.011 0.067 0.134 0.151 0.154 0.410

For H$O, in the Nafion membrane the comparison between J,, J and J’ is quite the same as for HCI. In bulk aqueous solutions sulphuric acid is known to dissociate in two steps. The first reaction gives HSOi; its equilibrium constant is high, while that of the second dissociation to SO: is 0.01 mole.kg-’ [21,22]. By raman investigations Young et al. [23] established that in aqueous sulphuric acid solutions, the ratio [SO$l/[HSO;l remains lower than one-third within a concentration range from 0.1-4 M. In the membrane

207

phase under the H+ form, the presence of H ’ balancing the sulphonic sites tends to reduce the dissociation of sulphuric acid. For a given external acid concentration the values of the zzro-ctrrent exchange flux J,-, and those of the forward and backward fluxesJ andJ are lower than those of HCl. These results must be related to the lower penetration of H2S04 as compared to Lhose of HCl (Tables II and III). HoweverLthe forward unidirectional flux J is always greater than the backward flux J and in most cases greater than Jo (Table VIII). Thus, in the same type of membranes the species bearing a sulphur atom have the same behavior as the chloride species in the case of HCl. If the motion of HSOi species in the same direction as the electrical driving force may be slowed down by the electroosmotic flux, the solvation of proton by the H2S04 molecules seems to be more difficult than for HCl due to their size. TABLE!X Comparisonbetweenthe transportnumberof the counter-ionin the AAV and the ARA anion exchangemembranes C (HCl) (mol/l)

AAV membrane

ARA membrane

G

tc,-

0.5 1.0 2.0 3.0 4.0 5.0 6.0

0.92 0.97 0.74 0.62 0.54 0.46 0.41

0.94 0.87 0.82 0.66 0.55 0.48 0.41

For the AAV and the ARA membrane the transmembrane fluxes of chloride and the associated transport numbers are sho_wn in Tables IXA, IXB and X. The decrease of the unidirectional flux J with the external concentration points out the proton leakage. The values so thus obtained of the transport number of chloride species are lower than the values deducted from potential measurements. Therefore, the permselectivity which would be estimated from the membrane potential measurements is higher than the permselectivity calculated from radiotracer measurements. This discrepancy might be due to the calculations of the activity of the chloride ion involved in Eqn. (6). With the>e_svo membranes and more especially for the ARA membrane, the ratio J /J of unidirectional fluxes increaFs with the external electrolyte concentration; that is to say, the forward flux J decreases with the

208

external concentration and J’ increases, remaining lower both than.? and Jo This result means that chloride ions are associated with the motion of positively charged species. In this case too, this fact might be due to the formation of aggregates such as w,HzO,HCl] + resulting from the solvation of a proton by a water molecule .and an HCI molecule, an ion association inside the membrane overcoming the state of a neutral HCl. For these two anion exchange membranes one notes the very high amount of sorbed electrolyte in the membrane with respect to the amount of water (Tables IV and V). For example, in the membrane equilibrated with 4 M HCl solutions the number of sorbed HCl molecules over the umber of fixed sites nHCl/ n s is 3.94 for the AAV and 1.77 for the ARA membrane; the number of water molecules over the number of total chloride is 3.87 for the AAV and 5.08 for the ARA membrane. For 6 M HCl solutions, these last two values decrease to 1.58 for the AAV and to 2.29 for the ARA membrane. The low values of these data suggest an incomplete ionic dissociation inside the membrane phase, an important fraction of sorbed HCl molecules probably remaining in a neutral molecular form.

CONCLUSION

The main feature of the transport of HCl and H,SO, in the Nafion 117 cation exchange membrane and of HCl in the AAV and ARA anion exchange membranes is the very low dissociation of the sorbed acids pointed out by the unidirectional radiotracer measurements. Moreover, the excellent permselectivity of the Nafion membrane is due to a very low sorption of these mineral acids, the H2S04 sorption remaining two times lower than the HCl sorption. For the AAV and the ARA membranes the loss of proton leakage cannot be attributed to a Donnan’s exclusion but also to a weak dissociation of the sorbed HCl favoured by a high amount of sorbed acid with respect to the hydration state of the membrane.

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