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The mechanism of transport of sulfuric acid through an anion exchange membrane is studied on the basis of the proton transport number, OH+ called proton ...
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DESALINATION ELSEVIER

Desalination 109 (1997) 231-239

Transport mechanism of sulfuric acid through an anion exchange membrane Yves Lorrain, G6rald Pourcelly*, Claude Gavach Laboratoire des MatOriaux et Proc~dOs Membranaires, UMR 5635 CNRS, BP 5051, 34293 Montpellier C~dex 5, France Tel. +33 (04) 67.61.33.98; Fax +33 (04) 67.04.28.20; E-mail: [email protected]

Received 11 October 1995; accepted 28 March 1997

Abstract

The mechanism of transport of sulfuric acid through an anion exchange membrane is studied on the basis of the proton transport number, OH+ called proton leakage. The comparison of OH+values between HC1 and H2SO4 solutions shows that this proton leakage is always greater for H2SO 4 than for HC1. Taking into account relatively close values of water content for the membrane in contact with these two acids, the difference of OH+ is explained by different mechanisms of transport for C1- ions and sulfate species. A specific mechanism of proton transport is presented, which takes into account the nature of the sulfate ion which behaves as a proton acceptor mediating the proton leakage.

Keywords:Anion exchange membrane; Electrolysis; Sulfuric acid; Proton leakage

1. Introduction

For all the separation processes involving ionexchange membranes (IEMs), the selectivity of ion transport has to be improved in order to obtain performance of the process which is compatible with the technical and economic conditions. Classical anion exchange membranes (AEMs) developed for the electrodialysis of *Corresponding author.

brackish waters, sea water or demineralization of whey are not suitable for the recovery of acids by electrodialysis because of their too high proton leakage. IEMs are swollen polymeric ion exchangers. When they are submitted to an electrical field, due to the presence of sorbed water, AEMs behave as proton conductors through two mechanisms: on the one hand the classical Grotthus mechanism in which the proton migrates from one water molecule to another, and the classical co-ion leakage associated with the

0011-9164/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PH S0011-9164(97)00069-6

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transport of the sorbed electrolyte [1]. This phenomenon of proton leakage associated with the transport of water are the two main problems encountered during the reconcentration of acids by electromembrane processes. AEMs having a reduced proton leakage have been specially designed for the recovery of acids by electromembrane processes, and their transport properties in acidic media have been widely investigated [2-6]. It was established that the best membranes suitable for the reconcentration of H2SO 4 by electrodialysis are those in which the sorption of acid and diffusion of protons are minimized [7]. By means of Raman spectroscopy and radiotracer measurements, the identification of the ionic species in the ARA membrane in contact with H2SO 4 solutions showed that the undissociated sulfuric acid was not present in the membrane and that, without any applied driving force, the only counter-ion equilibrating the charge of the ion exchanging sites was the bisulfate ion HSO 4 [8]. Nevertheless, these results were obtained with a membrane not under electrodialysis conditions but at the equilibrium state in the H2SO 4 solution. A previous study dealing with the proton leakage through the ARA membrane in contact with inorganic acid-alkaline solutions showed that the proton leakage was always greater for H2SO 4 solutions than for HC1 solutions [9]. The system studied was 200 mL of [xMHCI-(I -x) M MC1] or [xMH2SO 4-(1-x)MM2SO4] for the anodic compartment and 20 mL of 1 M HCI or 1 M HzSO 4 for the cathodic compartment, M being an alkaline metal. Fig. 1 shows the results obtained, indicating that, for symmetrical conditions of concentration, (x = 1), the transference number of protons, called the proton leakage, is more important for the H2SO 4 than for the HCI system (=0.50 instead of 0.2). In this case, the proton leakage was only due to a migration under the electrical field. Previous studies, achieved with symmetrical conditions of acid concentrations revealed that the selectivity of the ARA membrane towards anions was more

[] LiCI@ L2iSO 4 0,5 ~

-~7- NaCI ---~--- Na2SO4 -- ~-

- KCI

-- ~

~

0,4

- K2SO 4

/ " ," ~ ' "

/

0,3 0,2

.-

/

0,1

0,0

0,2

0,4

0,6

0,8

! ,0

x=acid/(acid+salt) Fig. 1. Transference number of the proton (proton leakage 0u+) through the ARA membrane (from [9]): [+HC1 x M-MC1 (1-x)M //ARA// HCI 1 M-] or [+ H2SO4 x M-M2SO4 (1-x)M//ARA//H2SO 4 1 M -]; j = 0.1 A.cm-2. important for 1 M HC1 solution than for 1M H2SO 4 solution [4,5]. The importance of the difference between the two values of for x = 1 should be justified either by a significant difference of water content of the ARA membrane in contact with the two acids or by a more important amount of protons in 1 M H2SO 4 than in 1 M HCI. Water content measurements in these acid solutions gave 8.9 wt% and 9.6 wt% for the membrane in contact with 1 M HC1 and 1 M H2SO 4, respectively. This difference was too weak to explain the difference in the proton leakage. For H2SOa solutions, the partition diagram of ionic species was investigated by Raman spectroscopy [10]. For 1 M H2SO a , a free proton concentration CH+--1.20M is reported in the literature. This value is relatively close to CH+ = 1 M for 1 M HCI. The proton leakage was, therefore, measured for the system [(+) (H2SO4), CH+ = I M [[ARA[I (H2SO4), CH+ = 1 M, (-)] where the initial concentration of H 2SO4 was 0.82 M corresponding to a free proton concentration of 1 M. We found OH+ = 0.46, which was a value close to that of 0.50 obtained for symmetrical conditions of 1 M H2SO 4. The two previous assumptions could not therefore justify the difference between the

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Y. Lorrain et al. ,/Desalination 109 (1997) 2 3 1 - 2 3 9

values of OH+ for the two acids. So, a specific contribution of the anion was revealed, and the only possible explanation should be different transport mechanisms of the proton through the ARA membrane in contact with these two acids. For HCI, the proton leakage is due to a Grotthus mechanism along the water molecules present both at the solution-membrane interface and within the membrane material. For H2SO4, this Grotthus mechanism occurs with the same intensity as for HC1 because the ARA membrane has almost the same water content when in contact with two acid solutions of similar concentrations. But the presence of two anions, HSO 4 and SO 2- complicates the transport mechanism because both of them could migrate through the ARA membrane. The aim of this work is to investigate the transport mechanism of HzSO 4 through the ARA membrane.

3. Experimental The membrane studied is the ARA membrane whose chemical composition is known. It consists of poly(4-vinylpyridinium) chains (in acidic solutions) grafted onto a FEP matrix 130#m thick [a copolymer of poly(tetrafluoroethylene) and poly(hexafiuoropropylene)] and cross-linked with divinylbenzene. The intrinsik pK of this membrane was determined by Raman spectroscopy and was 2.24 Ill]. Its exchange capacity was 0.6 mequiv./g of dried membrane [3]. Before use, the membrane samples (4cm diameter disks) were soaked in a large volume of deionized water for several days and, before the experiments, maintained in 1 M HCI or H2SO 4 solutions during 24 h. All the measurements were carried out at 25 °C. The proton leakage through the ARA membrane was characterized by electroelectrodialysis measurements performed in an asymmetrical Teflon cell, with platinizedtitanium electrodes (Fig. 2). The anodic compartment contained 200mL while the cathodic compartment was filled with only 20 mL in order

Anode

~th~e

®

®

Membrane

s~-~

- so~

,.~H +

(H2o

H+J

Fig. 2. Experimental cell for the proton leakage measurements. Electrodes: platinized titanium; volumes of compartments:anodic 200 mL, cathodic: 20 mL; anion exchange membrane: ARA; current density: j = 0.1 A. c m -2.

to generate more important variations of proton concentration. The systems studied were [+ [H+] x M IIARAI[ [H+] 1 M- ], where [H+] was the concentration of free protons coming either from HCI, in this case CHC1= [H+], or from H2SO 4. In this latter case the concentration of sulfuric acid Cn2so 4 corresponding to a given value of [H+] =x M was obtained from Raman spectroscopy data [10]. For the two acids, x ranged from 0.1 to 4 M. The working surface of the membrane was 7 cm 2 and the applied current density was 0.1 A.cm -2. The proton concentration was determined by acid-base titration. The water content is usually defined by the following equation: m

w.c.---w

-

m

md×100

(1)

w

where m w is the weight of the membrane sample equilibrated with a given acid solution and m d is the weight of the dried membrane under the Cl form. For complete dehydration, the membrane was maintained under vacuum at 35°C until a constant weight was obtained. The dialysis flux of acid, which can occur with a difference of acid concentration between

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Y Lorrain et al./Desalination 109 (1997) 231-239

the two compartments, was measured in the same two-compartment cell but without any applied current.

d

diffusion mechanism (nil÷). This latter case occurs under asymmetrical conditions of acid concentration. We have T

m

d

nil+ : nil+ + nil+

(3a)

3. Results

The values of the water content of the ARA membrane in contact with HC1 or H2SO 4 solutions are reported in Table 1. The results of acid dialysis are reported in Table 2 for different configurations of the dialysis cell. These fluxes are derived from the variation of acid concentration in the dilute compartment after 30min, which corresponds to the duration of the electrolysis process. d JHd = nil+

(2)

The results of acid-base titration gives the variation of the amount of protons in the cathodic compartment, AnH÷ given by

where C ~, Vi and C f Vf are respectively the concentration and the volume of the cathodic compartment at the initial and final time of electrolysis.

S.t

(3C)

/XnH+ = qF -nHr, where S is the membrane area and t the time expressed in seconds. Under the experimental conditions described previously, the value of the transport number of protons through the membrane OH+ is deduced from the variation of the acid concentration in the cathodic side. When qF Faraday of current crosses the membrane, assuming that the cathodic reaction has a current efficiency of 100%, qF mole of protons is reduced at the cathode, while nH[ mole of protons is transferred from the anodic compartment to the cathodic. This amount of protons is composed of protons transferred by a migration r?l mechanism (nr~,), and protons transferred by a

The transport number of proton, due to the migration process is therefore given by

0.7

0,6

0,5

E

a

0,4

0.0,3 I/) e."

0,2

Table 1 Values of the water content of the ARA membrane in contact with HCI or H2SO4

0,1

0,1

Acid concentration (M)

1

2

3

x (H+) mol/I

0.5

0.1

2.0

3.0

4.0

w.c. (wt%) HCI 9.5 w.c. (wt%) H2SO4 9.4

8.9 9.6

8.7 9,7

8.5 9.9

8.3 10.3

Fig. 3. Variation of the transport number OH+through the ARA membrane.j=0.1 A.cm-2. System: [(+) [H+]=x M IIARAII[H+] 1M(-)]. (a): [ H 2 S O 4 l[ H 2 9 0 4 ] ; (b) : [HCI 11H2SO4]; (c) : [H2SO4 IIHCI] ; (d) : [HC1 II HCI].

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E Lorrain et al./Desalination 109 (1997) 231-239

Table 2 Values of dialysis flux of protons (deduced from acid-base titration after a 30-rain dialysis). The accuracy is about 5% Concentration: free protons (xM) 0.1

1.0

2.0

3.0

4.0

1.7×10-8 3.3×10 -8 0.9×10 -8 0.7×10 -s

0 0 0 0

5.1×10-8 6.3×10 -8 1.2×10-8 1.l×10 -8

9.2×10 -8 1.1×10-7 3.9×10-s 3.8×10 -8

1.2×10-7 1.3)