A chlorine-free anode for electrodialysis1

Separation and Purification Technology 15 (1999) 147–152

A chlorine-free anode for electrodialysis1 J. Pretz, E. Korngold *, O. Kedem The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, Israel Received 7 April 1998; received in revised form 23 June 1998; accepted 28 June 1998

Abstract The production of free chlorine at the anode in electrodialysis processes causes deterioration of the membranes. The production of free chlorine could be reduced significantly by decreasing the transport number of the chloride ions in the anode chamber. This was achieved by introducing a solid polymer electrolyte (ion-exchange resin) in the anode. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Electrodialysis; Ion-exchange resin

1. Introduction In electrodialysis ( ED) processes, the formation of chlorine in the anode compartment by oxidation of chlorides can cause destruction of membranes near the anode. The main reaction is of course oxygen production, and, considering the redox potentials, one would not expect chloride to be oxidized, as indicated in Eqs. (1) and (2): 1 H O2H++ O +2e− 2 2 2

(E =1.23 V ) 0

(1)

2Cl−Cl +2e− (E =1.37 V ) (2) 2 0 The amount of free chlorine produced depends on the anode material and on the overvoltage [1]. For oxygen production the exchange current density should be high while the charge transfer overvoltage and activation overvoltage should be * Corresponding author. Fax: +972 7 6472969. 1 This paper was presented at the IDA Congress in Madrid, Spain, October 6–9, 1997.

minimal. At the current densities applied in ED, the overvoltage for oxygen evolution may raise the electrode potential into the region of chlorine production. The overvoltage depends on the composition of the anode [1]. For a common Pt anode and current densities of 1 to 10 mA cm−2, the overvoltage for oxygen in 1 N H SO is 0.3 to 2 4 0.6 V [1], whereas that for chlorine is 0.1 to 0.5 V. Thus some chlorine can be produced. At higher current densities the percentage of chlorine production is higher [2–4] . Given these reaction parameters, the best way to suppress the side reaction appeared to be to decrease the concentration of chloride ions in the solution in contact with the anode. Accordingly, we attempted to minimize the amount of chloride transferred by the current to the anode by introducing cation-exchange resin into the anode compartment. Cation-exchange resin can serve as solid electrolyte even without electrolyte solutions (solid polymer electrolytes are used in fuel cells at high current densities [5–7]). The applied potential field

1383-5866/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S1 3 8 3 -5 8 6 6 ( 9 8 ) 0 0 09 3 - 8

148

J. Pretz et al. / Separation and Purification Technology 15 (1999) 147–152

decomposes water near the positive electrode into O and H+. In our experiments, decomposition 2 occurred at the contact surface between the solid polymer electrolyte and the anode. The transport number of the H+ ions in strong cation-exchange resins approaches one, because the amount of chloride in such resin is very small and the mobility of the H+ ions is much higher than that of the other ions. The decrease of chloride transfer leads to decreased chlorine production.

package was treated as a membrane in calculating the transport numbers, which were calculated as follows: E 1 R + (3) 2E 2 0 where E is the measured potential with the resin, R E is the potential for an ideal membrane and t is 0 the transport number.

t=

2.2. ED experiments 2. Experimental 2.1. Transport number To determine the transport number of the chloride ions in a cation-exchange resin package, a simple three-compartment cell was used (Fig. 1). The cation-exchange resin (Amberlite IR-120, 15–45 mesh) was introduced into the central compartment, which was confined by two cellophane membranes. The adjacent compartments were filled with electrolyte solutions of different concentrations, namely 0.001/0.01, 0.01/0.1 and 0.1/1.0. The potential was measured with calomel electrodes. In a second step, an electrolyte solution of lower concentration was passed through the resin package. The potential between the compartments was also observed in the absence of resin between the cellophane membranes. The resin

The ED experiments were carried out in a unit with five compartments: two brine, one diluate, one cathode and one anode (Fig. 2). The catholyte and anolyte circulated separately. Conductivity, pH and temperature were monitored. The thickness of the diluate and brine compartments was 2 mm and that of the electrode chambers was 15 mm. The membrane surface area was 120 cm2. The experiments were carried out with and without electrolyte solution in the anode compartment. The accumulating gases of the anode reaction were passed through gas-washing bottles filled with distilled water or dilute caustic solution to absorb the free chlorine. 2.3. Measurement of free chlorine Chlorine determination was carried out by two different methods for different amounts of free chlorine in the sample. In low concentration ranges the free chlorine was determined by the visocolorA test-kits for chlorine determination supplied by Marcherey–Nagel. For 0.02–0.60 mg l−1 the Cl 2 was determined with visocolor HE and for 0.1–2.0 mg l−1 with visocolor. At concentrations above 2.0 mg l−1 the Cl determination was carried 2 out by titration with sodium thiosulfate following an ASTM method [8].

3. Results and discussion 3.1. Transport number Fig. 1. Experimental set-up for resin package potential measurement.

The experiments for determination of the transport number in packed resin were first carried out

J. Pretz et al. / Separation and Purification Technology 15 (1999) 147–152

149

Fig. 2. Experimental set-up for the ED.

with a circulating electrolyte solution in the resin compartment with different flow rates. The concentration of this electrolyte solution was equal to the lower concentration in the adjacent compartment of the cell. In the second series of experiments no electrolyte was passed through this compartment, which was filled only with the wet resin. Fig. 3

shows the transport numbers with and without electrolyte. The transport numbers are significantly higher in the absence of electrolyte and, of course, both sets of results are higher than in free KCl solution without resin (transport number ~0.5). These simple experiments confirm our assumption that one can easily modify the ion transport in the desirable direction. More especially, when resin is used fewer chloride ions are carried to the anode. 3.2. Influence of ion-exchange resin on electrode reaction

Fig. 3. Transport numbers for K at different KCl concentrations, with and without flow in the resin package: (A) resin package with electrolyte; (B) resin package without electrolyte.

3.2.1. ED with anolyte For each experiment, 1 l of salt solution was 90% desalted at constant current. The desalting was followed by conductance measurements; the stack potential, pH and temperature were monitored. The amount of free chlorine was determined at the end of the desalination run. The compartment adjacent to the anode chamber was always a brine cell. In the first series of experiments, the salt concentration in the circulating anolyte was identical with that of the brine.

150

J. Pretz et al. / Separation and Purification Technology 15 (1999) 147–152

Desalting was carried out with and without resin in the anode chamber. Fig. 4 shows the chlorine production as a function of chloride concentration in the anolyte. At concentrations up to 0.1 N NaCl the resin reduces free chlorine by one order of magnitude. At 1 N NaCl the resin becomes ineffective. This dependence is to be expected from the transport numbers: at low salt concentrations most of the current is carried by the resin. Since the protons produced by the electrode reaction replace the sodium ions, this contribution is further increased by the high proton mobility. At high salt concentrations, the chloride flux, both in solution and in the resin, becomes a significant fraction of the current. This is also shown by the slight dependence on the rate of circulation: at higher circulation the back exchange to the sodium form is more effective. The smaller amount of chlorine found in the anolyte could be due to chlorination of the resin in the reaction scheme shown in Fig. 5. To test this possibility, the resin was prechlorinated under much more severe conditions than those in the ED stack. Chlorine was bubbled through resin in water, at 2 l h−1, for 24 h, with vigorous stirring. Fig. 4 compares the performances of prechlorinated and untreated resins.

Fig. 4. Free chlorine (milligrams of free chlorine per ampe`re hour) as a function of salt concentration in the brine compartment: (A) without resin; (B) with resin; (C ) with prechlorinated resin.

Fig. 5. Reaction of the resin with free chlorine. Table 1 Exchange capacity Resin

Unused Used (3 months) Unused prechlorinated Used prechlorinated

Exchange capacity (meq g−1) dry

wet

4.35 4.26 4.45 4.30

1.88 1.84 1.92 1.86

Clearly, the reduction of chlorine concentration in the anolyte reflects a real reduction of the side reaction and not chlorination of the resin, as one can see from Fig. 4, lines (B) and (C ). The stability of the resin was checked by comparing the exchange capacity before and after use and following chlorination. Table 1 shows that, within experimental error, the capacity remained unchanged by use in the anode chamber and by prechlorination. Even after at least 3 months of use in ED experiments, there was no significant change in the exchange capacity of the Amberlite IR-120 resin. Moreover, as the chlorine concentration in the anolyte is very low, chemical stability of the resin is adequate. 3.2.2. ED without anolyte In a packed bed of resin there is substantial contact between the grains, especially if the degree of crosslinking is not very high and the resin is swollen and somewhat deformable. It is therefore possible to use the resin itself as solid electrolyte, as is customary in fuel cells [4–6 ], without circulating electrolyte solution. This eliminates direct contact between chloride ions in solution and the electrode. Desalting experiments were carried out with resin in the anode compartment, without electro-

J. Pretz et al. / Separation and Purification Technology 15 (1999) 147–152

Fig. 6. Free chlorine (milligrams of free chlorine per ampe`re hour) as a function of salt concentration in the brine compartment: (A) without resin; (B) with resin and anolyte; (C ) with resin and without anolyte.

Fig. 7. Free chlorine (milligrams of free chlorine per ampe`re hour) as a function of current density: (A) without resin; (B) with resin and anolyte; (C ) with prechlorinated resin and anolyte; (D) with resin and without anolyte.

lyte (Fig. 6). As was to be expected, the amount of free chlorine decreased and was independent of brine concentration. Free chlorine increased at high current density, as can be seen from Fig. 7, but absolute values remained very low. It is obvious that in this particular convenient configuration, chemical stability to chlorine is not required. Earlier we saw that the transport number is higher without electrolyte than with (Fig. 3). To

151

Fig. 8. Free chlorine (milligrams of free chlorine per ampe`re hour) as function of flow rate. Resin was not prechlorinated (c =c =c =0.01 mol l−1). (A) Without resin; (B) with resin. B D 0

Fig. 9. Voltage drop for the complete ED cell during desalination (c =c =c =0.1 mol l−1; i=10 mA cm−2): (A) without B D 0 resin; (B) with resin with anolyte; (C ) with resin without anolyte.

check the influence of the flow rate, we looked at the amount of free chlorine produced at different flow rates in the anode chamber. As Fig. 8 indicates, within experimental error there was no influence of the flow. The resin is kept wet by the permeation of water as liquid or vapor through the first membrane. The flux of water permeation is greater than the amount of water decomposed near the anode.

152

J. Pretz et al. / Separation and Purification Technology 15 (1999) 147–152

There appears to be some rise of temperature. This is indicated by increased conductance. Fig. 9 shows the stack voltage during desalting in the different conditions. Note that the voltage with resin alone is lower than that of resin+electrolyte. This is understandable if we consider that, as long as the concentration remains below that of the equiconductance point, the conductivity of the system with resin will be higher than that of a system comprising only electrolyte, as reported in Ref. [9]. Resin packed between membranes was introduced some years ago by different researchers [10–18], also with beneficial effect.

4. Conclusions Production of free at chlorine the anode in ED can be almost fully suppressed by the introduction of cation-exchange resin into the anode, optimum results being achieved in the absence of anolyte in the anode. This system offers the possibility of reducing the amount of free chlorine by approximately two orders of magnitude or even a little more, eliminating the need for circulation. The chlorine-free anode could represent a significant technical advance in ED. It is inexpensive to operate, the resin involved being a commercial product available at low cost. The resin can be introduced into existing ED stacks.

Acknowledgments J. Pretz is indebted to the MINERVAFoundation for making this work possible. The authors are grateful to the Wolfson Foundation who supported this work in part. We also thank Mrs Alice Sen for editing the manuscript.

References [1] K.J. Veetter, Electrochemical Kinetics, 1st edition, Academic, New York, Press, 1967. [2] C.H. Hamann, W. Vielstich, Elektrochemie I/II, 2., u¨berarbeitete Auflage, Verlag Chemie, Weinheim, 1981. [3] G. Bianchi, J. Appl. Electrochem. 1 (1971) 231–243. [4] G. Faita, G. Fiori, J.W. Augustynski, J. Electrochem. Soc. 116 (1969) 928–932. [5] W. Gajewski, Spektrum der Wissenschaft, July, 1995. [6 ] G.G. Schere, Ber. Bunsenges. Phys. Chem 94 (1990) 1008–1014. [7] R. Nolte, K. Ledjeff, M. Bauer, R. Mu¨lhaumpt, J. Membr. Sci. 83 (1993) 211–220. [8] ASTM Handbook. [9] D. Davous, M. Metayer, E. Korngold, J. Chim. Phys. 83 (1986) 137–147. [10] P. Kollsman, US Patent 2 815 320, 1957. [11] W.R. Walters, D.M. Weiser, L.Y. Marek, Ind. Eng. Chem. 47 (1955) 61. [12] E. Glueckauf, Br. Chem. Eng. 4 (1959) 646. [13] E. Korngold, Desalination 16 (1975) 225–233. [14] E. Selegeny, E. Korngold, US Patent 3 686 089, 1972. [15] E. Selegeny, E. Korngold, British patent 1 240 710, 1968. [16 ] E. Selegeny, E. Korngold, French Patent 150 621, 1968. [17] A.J. Guifrida, A.D. Iha, G.C. Ganzi, US Patent 4 632 745, 1986. [18] A.J. Guifrida, A.D. Iha, G.C. Ganzi, US Patent 4 925 541 (1990).