GB2240342A

GB2240342A - Regeneration of electrode used in electrochemical ion exchange

Info

Publication number: GB2240342A
Application number: GB9100670A
Authority: GB
Inventors: Andrew Derek Turner; Nevill John Bridger
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1990-01-30
Filing date: 1991-01-11
Publication date: 1991-07-31
Anticipated expiration: 2011-01-11
Also published as: JPH04215885A; GB2240342B; US5766442A; GB9100670D0; JP3093291B2; DE4102539A1; FR2657600B1; GB9002079D0; DE4102539C2; FR2657600A1

Abstract

An electrode which includes a hydrolysable metal phosphate such as zirconium phosphate (Zr(HPO4)2) which is hydrolysed at least in part during the electrochemical ion exchange is regenerated by making the electrode an anode of an electrochemical cell in which the electrolyte is an aqueous solution containing one or more phosphate ions provided, for example, by a dissolved salt (eg Na3PO4, Na2HPO4 or NaH2PO4) or orthophosphoric acid, or both. Preferably, the electrolyte is acidic (eg pH2). Operation of the cell reverses the hydrolysis reaction and enhances the performance of the electrode.

Description

Electrode Regeneration This invention relates to the regeneration of

electrodes for use in electrochemical ion exchange.

The electrochemical removal of ions from aqueous solutions onto-ionexchange materials, which may be referred to as electrochenical ion exchange or electrochemical deionization, is known for example from UK patents GB 1 247 732, GB 2 150 598, and GB 2 187 761. It involves establishing an electrochenical cell comprising the aqueous solution as electrolyte, a working electrode and a counter electrode, where at least the working electrode incorporates an ion exchange material such as a resin, and applying a D.C. voltage between the electrodes. To remove cations from the solution the working electrode incorporates a cation responsive ion exchange material and is made the cathode. A localised change of pH occurs at the electrode due to generation of OH- ions which in turn produce active sites thereon for absorption of cations from the aqueous solution. Subseauent reversal of the voltage enables the absorbed ions to be eluted, so the workina electrode can be readily regenerated.

The working electrode may conprise a current feeder carrying an intimate mixture of an ion exchange material and a binder. The mixture may also include an electrically conducting material. GB-B-2 150 598 describes one form of such an electrode.

An example of a cation exchange material for use in such a working electrode is zirconium phosphate of the formula Zr(HP04)2, Zirconium phosphate is a very useful material in this respect but has the disadvantage of being gradually hydrolysed by hydroxl ions during continued operation in electrochemical ion exchange according to the following equation (l):

Zr (HP04) 2 + 4H 20 2H 3P04 + Zr (OH) 4 (1) Consequently, electrochemical ion exchange performance deteriorates and cell voltage increases.

A way of regenerating an electrode such as the above has now been devised. Thus, according to the present invention there is provided a method of regenerating an electrode for us6 in electrochemical ion exchanqe which electrode includes a hydrolysable metal phosphate as cation exchange material, which phosphate has been hydrolysed at least in part, which comprises conditioning the electrode by operating an electrochemical cell in which the electrode is an anode and the electrolyte is an aqueous solution containing one or more phosphate ions under conditions such that the hydrolysis reaction of the metal phosphate is reversed to reqenerate the metal phosphate.

It has been found that the original performance of the electrode in electrochemical ion exchange can be substantially restored or even enhanced by the present method. Where the cation exchange naterial is zirconium phosphate, it is believed that the present method-reverses equation (1) above.

The phosphate ions in the electrolyte may be one or more of P04 3- r HP04 2-, and H2P04-, provided for example by appropriate dissolved salts such as Na3P04, Na2HP04 or NaH2P04 respectively or by orthophosphoric acid (H3P04) or by both. Preferably, the electrolyte is acidic, for example, of pH less than 4 such as about pH 2.

The metal phosphate is preferably a transition metal i i phosphate such as a zirconium or titanium phosphate, the above-mentioned zirconium phosphate being preferred.

The invention will now be particularly described, by way of example only, as follows.

EXAMPLE

A zirconium phosphate electrochemical ion exchange electrode (54 g) had been used continuously for 3 months in a flow cell in an electrochemical ion exchange process. The process had been about 80% cathodic and 20% anodic. Some of the zirconium phosphate had, as a result, been hydrolysed to Zr(OH)4. A standard electrochemical ion exchange batch test on the resulting electrode showed that it would reduce 200 p.p.m. caesium (as Cs2CO3) to only 20 p.p.m., compared with the usual figure of 0.1 p.p.m. Also, the cell voltage required for a current of 3 mA/cm2 was increased to about SOV compared with the usual figure of about 30V, i.e. the cell had a very high resistance.

An electrochemical cell was established in which the anode was the electrode used as above, the electrolyte was an aqueous solution comprising 1M NaH2P04 and 0.2M H3P04, and the counter electrode was platinised titanium. The used electrode was anodised at 3 mA/cm2 for about 5 days following which it was tested in the above-mentioned standard test and shown to reduce 200 p.p.m. caesium (as Cs2C03) to 0.1 P. P.m. Moreover, the cell voltage was reduced to 10 V which was considerably lower than the initial voltage.

EXAMPLE

An electrochemical ion exchange electrode was made from zirconia (25g zirconia, area 50 CM2). To remove sorbed ions the electrode was held anodic for 2 ours in pure is water, to desorb cations, and then after thorough flushing it was held cathodic for 2 hours in pure water to desorb anions. It was then tested as an electrochemical ion exchange electrode to remove caesium ions from an aqueous solution; 200 p.p.n. caesium as Cs2C03 was reduced to 60 p.p.m. and the pH decreased.

Anions and cations were again desorbed into pure water. The electrode was then held at anodic potential with a current of 5 mA/cm2 in dilute phosphoric acid, for a period of about-5 hours. The electrolyte was 1 litre of phosphoric acid with initially about 1080 p.p.m. P04 ions. The phosphate ions were absorbed into the electrode, the concentration in the electrolyte falling to about 0.2 p.p.m., and the end point being indicated by a sigmoidal increase in both pH and cell voltage to respective plateaux. This procedure was repeated three tines in all. If the formula of the zirconia is assumed to be Zr(OH)4, then the amount of phosphate absorbed (3.22g) would lead to about 10% conversion to zirconium phosphate. If the zirconia is more highly hydrated then the extent of conversion is actually higher.

After desorption of any cations into pure water by holding the electrode anodic for 2 hours, the electrode was then tested as an electrochemical ion exchanqe electrode. Very little phosphate was eluted (final concentration less than 10 p.p.m.). After 3 hours absorption it had reduced the caesium. concentration to 14 p.p.m. This shows a considerable improvement over the performance of the initial zirconia electrode. It is thus apparent that hydrous zirconia can be converted into zirconiuri phosphate in an electrochemical ion exchange electrode, so improving its ability to absorb cations.

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Claims

1. A method of regenerating an electrode for use in electrochemical ion exchange which electrode includes a hydrolysable metal phosphate as cation exchange material, which phosphate has been hydrolysed at least in part, which comprises conditioning the electrode by operating an electrochemical cell in which the electrode is an anode and the electrolyte is an aqueous solution containing one or more phosphate ions under conditions such that the hydrolysis reaction of the metal phosphate is reversed to regenerate the metal phosphate.

2. A method a claimed in Claim 1 wherein the phosphate ions comprise one or more of P04 3-, HP04 2-, or H2P04_.

3. A method as claimed in Claim 1 or Claim 2 wherein the electrolyte is acidic.

4. A method as claimed in Claim 3 wherein the electrolyte is of pH less than 4.

5. A method of regenerating an electrode for use in electrochemical ion exchange substantialy as hereinbefore described.

Published 1991 at7be Patent Office, State House. 66/71 High Holborn. London WCIR47P. Further copies may be obtained from sake Branch. Unit

6. Nine Mile point, Cwnffelinfach. Cross Keys, NewporL NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent