Influence of thermal treatment on the rate of ion exchange of zirconium phosphate

J. Inorg. NucL Chem.. 1966, Vol. 28. pp. 225 to 231. PergamonPren Ltd. Primedin Northern Ireland

INFLUENCE OF THERMAL TREATMENT ON THE RATE OF ION EXCHANGE OF ZIRCONIUM PHOSPHATE G. ALBERTI, A. CONTE and E. TORRACCA Laboratorio di Chimica delle Radiazioni e Chimica Nucleate del C.N.E.N. and Istituto di Chimica Generale d'Inorganica dell'Universitii, Rome, Italy

(Received 17 May 1965) A l m m e t - - F o r thermally-treated samples of zirconium phosphate the variations of Ka for Cs + and titration curves as function of time were determined. It was found that condensation of phosphate groups to pyrophosphate groups starts at --~180°C. From determinations of water lost and of pyrophosphatz formed at 260°C it is possible to estimate the anhydrousz P content in given ZP samples dried at room temperature. Finally, the interrelation between pyrophosphate formed, lossintotal ionexchange capacity, and variation in selective properties of thermally-treated ZP samples is discussed.

E ~ L l ~ papers (Ls) have reported the effects of drying temperatures for different lengths of time on the ion-exchange properties of zirconium phosphate (ZP). The mass distribution coefficients, Kd, of various inorganic ions on ZP dried at 260°C for 1 hr, showed that such a moderate thermal treatment induced remarkable changes in the selectivity of ZP compared with the same material dried at 50°C for 24 hrs. However, a marked decrease in the rate of ion exchange was always observed. These results were related to increased cross-linking induced by condensation of phosphate groups to P-O-P groups. Variations in selectivity for certain ions on ZP dried at different temperatures were employed to improve certain separations of inorganic ions and in the extraction of 187Cs from fission products. (8) Further investigations on variations in the ion-exchange properties of ZP dried at high temperatures were carried out by VmEL'~ and PEK~K. (4) These authors confirmed by infra-red absorption spectra the condensation of phosphate groups to condensed P---O--P structures, as proposed by AMPHL~TT(5) and ourselves, but, contrary to our results, they observed a decrease of Kd for all the ions investigated on ZP dried at 260°C. Since such a disagreement could lead to confusion and uncertainty in the reported data (e) it seems proper to reconfirm and to extend the research on the ion-exchange properties of ZP dried at high temperatures, in order to clarify the outstanding problems on the mechanism of the observed phenomena. Owing to the differences between the properties of materials prepared in different laboratories we have used commercially available ZP (Bio Rad Laboratories 50-100 mesh). ~x) G. ALnERTZand A. CONT,, Rend. Acc. Naz. Llncei serie VIII 26, 782 (1959); 27, 224 (1959). ~*) G. ALnlmTI and A. CoN'IX, J. Chromatogr. S, 224 (1961). ~a~ G. ALnFa~TIand A. CONTE, Energia Nucleare 9, 43 (1962). ~4) V. VESEL~'and V. I~KI~nEK,J. lnorg. Nuci. Chem. 25, 697 (1963). cs~ C. B. AMPHLETT,L. A. MCDONALD and M. J. REDMAN,J. lnorg. Nucl. Chem. 6, 236 (1958). ~6~C. B. AMrHLETr, Inorganic Ion ExchanGers, p. 119. Elsevier (1964). 15

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G. ALBER'IX,A. CONTE and E. TORRACCA EXPERIMENTAL

Chemicals

Analytical grade chemicals (Carlo Erba R.P.) were used without further purification. Analytical procedures

a. Titration curves (without added salt). Several ZP samples (0-5 g) were weighed into dry flasks and transferred to a dry-box under nitrogen. To each flask re-distilled water was added and finally 0.01 M NaOH to give a constant ratio of 50 ml solution to 0.5 g ZP. The pH of these solutions was frequently determined (always in the dry-box) with a Beckman GS pH meter. b. Mass distribution coefficients. In order to compare Vesel~, and Pek~trek's results with our own, K~(Cs+) was determined under their conditions,c~ (0.1 N HNOa, 10-a N Cs+; 100 ml solution/g). c. Determination of P/Zr ratio. 0.15 g ZP was dissolved in 5 ml 1 M NH4HF2 at room temperature and diluted to 100 ml. Orthophosphate was determined colorimetrically on 2 ml of this solution according to the method of Bemhart and Wreath, employing a sulphuric acid solution of ammonium molybdate in an acetonewater medium. ~7~ Owing to the presence of fluoride, colorimetric determinations were performed with Perspex cells on a Cary 14 recording spectrophotometer. A calibration curve (orthophosphate content vs. absorbance) was obtained using standard phosphate solutions containing known amounts of zirconium and ammonium fluoride. Zirconium was determined gravimetrically as follows: 0.15 g ZP was dissolved in 5 ml NH4Fs and diluted to 100 ml with 10 ~o H2SO4. Owing to the fluoride ion interference, zirconium is determined with cupferron, following the procedure of ELVING and OLSON. is) d. Determination of pyrophosphate. The method described by NETHERTONet al. tS~ was suitably modified. 0.15 g ZP was dissolved in 5 ml NH,HFs 1 M and the orthophosphate content was determined as previously described. This determination is possible since we observed that pyrophosphate (as Na4P2OT) is not converted to orthophosphate by the ammonium molybdate---H2SO4 solution within 2 hr, which is easily sufficient to complete the analysis. 0.15 g ZP was dissolved in 5 ml of the ammonium fluoride solution; 7.5 ml of concentrated H~SO, were successively added. This mixture was gently heated for ,.~30 min to completely convert pyrophosphate to orthophosphate. The total orthophosphate content was then determined. The difference between the orthophosphate contents determined before and after the hydrolysis equals the orthophosphate produced by hydrolysis of pyrophosphate. The concentration of pyrophosphate was calculated as half the concentration of phosphate produced in the hydroylsis reaction. The results are expressed as mmol. of pyrophosphate per g of anhydrous ZP. RESULTS AND DISCUSSION F i g u r e 1 shows a p l o t o f K ~ ( C s +) vs. time o n Z P Bio R a d , ( P / Z r = 2), dried at 50°C for 24 h r ( Z P 50/24), 260°C f o r 1 h r ( Z P 260/1), 260°C f o r 24 h r ( Z P 260/24) a n d 300°C for 2 h r (ZP 300/2) respectively. D o t t e d curves refer to s h a k e n solutions. T h e following observations c a n be d r a w n : a. the rate o f exchange on Z P d r i e d at high t e m p e r a t u r e s increases r e m a r k a b l y on shaking, b u t shaking has little influence on the rate o f exchange on Z P which has n o t been t r e a t e d thermally. T o interpret these plots it m u s t be p o i n t e d o u t t h a t shaking induces a m a r k e d decrease in the Z P particle size. ~7~D. N. BERNHARTand A. R. WREATrI,Anal. Chem. 27, 440 (1955). is) p. I. ELVINGand E. C. OLSON,Anal. Chem. 27, 1817 (1955). ~*~L. E. NETHERTON,A. R. WREATHand D. N. Bemm-IART,Anal. Chem. 27, 860 (1955).

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b. The rate of ion exchange decreases considerably, on increasing the temperature and time of heating, confirming earlier results (t'~) c. At equilibrium, Kn(Cs+) on ZP dried at high temperatures is higher than Kd on unheated ZP, again confirming our previous observations. {2) The results also explain the disagreement between the results of VES~L'~ and PEr,X~K and our previous determinations. The former authors give Ka values determined on ZP dried at 260°C and at room temperature, after three days of equilibration with constant shaking. Since the time of heating is not reported, it seems 72' ¢)

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FIG. 1.--Variation of K~(Cs+) as function of time on ZP 50124 (a) with and (b) without shaking; ZP 260/1 (c) with and (d) without shaking; ZP 260124 (e) with and (f) without shaking; (g) ZP 300/2 without shaking.

proper to assume that ZP was dried to constant weight (at least 12-24 hr). By comparing the results of V~SeLq and PEKAS~Kwith the plots in Fig. 1 it is seen that three days are sufficient to reach equilibrium on ZP 50/24, but not for ZP samples heated at 260°C. At least 10-15 days are necessary to reach equilibrium on ZP 260/1 and still longer times for ZP 260/24, Figure 1 shows that after three days Ka(Cs +) onZP dried at high temperatures is smaller than K a obtained on ZP 50/24, as V~ELq and PEKAS~K have reported. On increasing the time of equilibration, Ka(Cs +) becomes greater than Ka on ZP 50/24. This inversion takes place after 18 days for ZP 260/24 with shaking and after 60 days if solutions are not shaken. Since the rate of ion exchange on ZP dried at high temperatures is very low, titration curves of ZP 50/24 and ZP 260/1 were studied as a function of time. The plots in Fig. 2 show that the titration curves of ZP 50/24 are only slightly modified with the passage of time whereas those of ZP 260/1 are continuously shifted and

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FIG. 2.--a. Variation of the titration curve (with 0.01 N NaOH) of ZP 50/24 as a function of time. b. Variation of the titration curve with0.01 NNaOH of ZP 260/1, as function of time. The integers onthe graphs indicate the number of weeks spent in equilibration. do not remain unchanged until after 60-70 days. Similar results were recently obtained by MAa~ROVA and Sr~mscI-mVSKY. tl°~ Fig. 3 shows titration curves of Bio Rad ZP 50/24, ZP 50/24 prepared by the method o f GAL and GAL tm and Bio Rad ZP 260/1 respectively. From Fig. 3 the following observations are drawn: a. up to pH 7 the titration curve o f Bio Rad ZP is similar to that of ZP prepared in our laboratories. At alkaline pH the titration curve o f the latter is slightly below that o f the former. b. Relative to ZP 50/24, ZP 260/1 shows a higher exchange capacity at acid pH, decreasing at alkaline pH. The curves coincide between pH 5 and 7. These results confirm earlier findings, t~J t~o~E. A. MATEROVAand P. A. SKABISCHEVSKY, Vestnrk Leningrad Univ. 10, 65 (1964). U.N. Int. Conf. Peaceful Uses of Atomic Energy, 2nd Geneva

~xl~ I. J. GAL and O. S. GAL, Proc. 24 (1958).

28,

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The ion exchange capacity as function of drying temperature is at present subject to discussion. MA~SltOVAand S~ISCaEvsKY reported that after heating at 300°C the capacity does not decrease markedly,(1°) whereas other authors found a weak (5) or a strong reduction(m of the exchange capacity of ZP dried at the same temperature. Since the rate of exchange on thermally treated ZP is very low, such a disagreement, in our opinion, is probably due to non-equilibrium conditions, i.e. in some cases the time was not sufficient to reach complete equilibrium. Furthermore, since ZP is a 14"

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Fie. 3.--Titration curves (without added salt) of (a) Bio Rad ZP 260[1 after ten weeks' equilibration, (b) Bio Rad ZP 50/24 and (c) ZP 50/24 prepared in our laboratories.

weak cation exchanger, the total ion exchange capacity should be determined at fairly high alkaline pH. Since it is not possible to achieve these conditions, owing to the large extent of hydrolysis of the material, various authors were forced to determine an apparent capacity under faintly alkaline conditions. Since thermal treatment modifies the selectivity of ZP the change in the total capacity might be different from the change in the apparent exchange capacity. In fact the decrease in the apparent capacity is strongly affected by the experimental conditions employed (nature of the ion and pH). To obtain data on the reduction in the total ion exchange capacity of thermally-treated ZP it seemed proper to determine quantitatively the pyrophosphate groups present in the material, produced by the condensation of --POH groups (2-P--OH ~ - - P - - O - - P - - ) . Each retool, of pyrophosphate determined must correspond to a decrease of 2 mmol. in the total capacity. Since the sensitivity of the infra-red method used by Vr:S~L? and PEKAaU~K(4) to determine pyrophosphate appears to be rather low, the method described in the experimental section was preferred. It should be pointed out that the determination (x~) j. PROSPERTand T. IOKrND^I, C.R. Acad. Sci. Paris 254, 860 (1962).

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G. ALBERTI, A. CONTEand E. TORRACCA

of water loss and pyrophosphate formed on heating at 260°C, also allows the standardization of ZP. Since for each millimole of pyrophosphate formed one millimole of HzO is irreversibly lost, the water content of a sample of ZP at room temperature can be calculated as mg of H20 lost at 260°C----(18 retool, of pyrophosphates formed at 260°C). * By subtracting this quantity from the weight of ZP sample, the weight of completely anhydrous ZP is obtained. Comparison may then be made between results for various ZP samples having different water contents at room temperature

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FIG. 4.--Variation of K~(Cs+) as a function of time, on ZP 260/1 samples, proviously soaked for 100 days in solutions at (a) pH = 7 Co) pH = 2.5 and (c) pH ----2.3. /G values were obtained without shaking. For comparison curve d of Fig. 1 is included. by reference to 1 g of anhydrous ZP. Pyrophosphate determinations on Bin Rad ZP samples heated from 110 ° to 260°C showed that condensation to form the P---O--P linkage starts at >180°C, and it is concluded that thermal treatment below 180°C produce only dehydration without decreasing the total capacity. Experimentally it was found that ZP 260/1 and ZP 260/24 contain 0.32 and 0.81 retool, of pyrophosphate per gram of anhydrous ZP respectively. Therefore the resultant loss in total capacity is 0-64 meq per g of anhydrous ZP for ZP 260/1 and 1.62 meq per g of anhydrous ZP for ZP 260/24. The titration curves in Fig. 3 confirm that at alkaline pH, where the apparent capacity is slightly less than the total capacity, the thermally-treated ZP exhibits a * It was found, as will be more fully described in a later publication, that the water lost by ZP in H+ form, at temperatures higher than 260°C, is only condensation water.

Thermal treatment on the rate of ion exchange of zirconium phosphate

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lower capacity than ZP 50/24. At pH 7 the apparent capacity of ZP 260/1 is equal to or higher than that of ZP 50/24. These results are to be expected if it is assumed that condensation of--P---OH groups increases the selectivity for the Na + ion, owing to the increased cross-linking. Furthermore, the increase in selectivity induces an increase in the apparent capacity at acid pH (where only a small fraction of the total capacity is active). The changes produced by thermal treatment remain after repeated ion-exchange cycles. Amphlett reported that it is possible to reconvert pyrophosphate groups (formed by condensation at high temperature) to phosphate groups by immersion in aqueous solutions at 300°C. c1~1 When, however, samples of ZP 260/1 were soaked in solutions of different acidities for 100 days at room temperature, quantitative determinations of pyrophosphate showed that, within experimental errors, such a long soaking does not modify the quantitiy of pyrophosphate formed. These results show that at room temperature conversion o f - - P - - O - - P - - groups to - - P - - O H groups is very slow. Further support for this view was obtained by determining the change in Ka(Cs+) as a function of time on ZP 260/1 soaked in solutions of different acidity for 100 days (Fig. 4.) At equilibrium, it can be seen that Ka(Cs+) for such samples is similar to the value on ZP which has not been previously soaked. The more rapid rate of attainment of equilibrium for soaked ZP must be partially connected with a decrease in the particle size during the processes of soaking, filtering, washing and drying. The initial rate of exchange may also be affected by the ability of the exchanger to regain hydration water which has been lost after thermal treatment, when soaked in aqueous solutions, tm CONCLUSIONS

On the basis of the above results it follows that: a. Since the condensation of - - P - - O H groups to - - P - - O P--- condensed structures starts at ,-~180°C (probably with a shift of ~: 10°C for ZP samples prepared in different ways), water lost up to this temperature is water of hydration. Therefore the dehydration of ZP below 180°C does not affect its ion exchange properties. b. At temperatures higher than 180°C partial condensation of--P---OH groups to --P---O--P-- groups decreases the total capacity of ZP and increases its degree of cross-linking. The properties of ZP are related to the extent of formation of--P--O--P---- groups, which depends on the drying temperature and the time of heating. c. By increasing the cross-linking, the rate of exchange decreases markedly and variations in selectivity are induced. High values of Ka(Cs+) must be related to presence o f - - P - - O - - P - - groups in the ion exchanger. d. In some cases, at acid pH, changes in selectivity may increase the apparent capacity. e. Changes in exchange properties induced by thermal treatment persist over many exchange cycles. ~xal C. B. AU~I-ILETT, Inorganic Ion Exchangers, p. 107. Elsevier (1964). (141 S. AI-IRLAND,J. ALBERTS~:)N,L. JOHAN~.gON,B. NlrtLOKRD and L. NIt.SSON, Acta Chem. Scand. 18, 1357 (1964).