Zirconium arsenates and their ion exchange behaviour

1.inorg,nucl.Chem..1968,Vol.30, pp. 277 to 285. PergamonPressLtd.Printedin Great Britain

ZIRCONIUM

ARSENATES

EXCHANGE

AND

THEIR

ION

BEHAVIOUR*

A. C L E A R F I E L D , G. D. S M I T H t and B. H A M M O N D Department of Chemistry, Ohio University, Athens, Ohio

(Received 12 April 1967) Abstract-When aqueous solutions of Zr(IV) compounds are allowed to react with arsenate ion, amorphous gels of variable composition are obtained. However, a crystalline zirconium monohydrogen arsenate is obtained on refluxing the gels in a strongly acid (HNO3) solution of arsenic acid. The crystalline arsenate is isomorphous with crystalline zirconium monohydrogen phosphate and exhibits quite similar ion exchange behaviour. Some comments on the mechanism of exchange are presented based on structural considerations. INTRODUCTION

ZmCONIUMarsenate preparations have recently been discovered to behave as ion exchangers[l, 2]. These arsenates were prepared by mixing a soluble Zr(IV) salt with arsenic acid solutions. This method of preparation yielded amorphous gels of variable composition. The same type of behaviour was observed on mixing phosphoric acid with Zr(IV) solutions[3]. However, Clearfield and Stynes[4] have shown that the gel converts to a stoichiometric, crystalline phase on refluxing in strong phosphoric acid. The ion exchange behaviour of the crystals is considerably different than that of the original gel. It was therefore of interest to determine whether the same type of conversion would occur with the arsenates. A number of zirconium arsenate phases have been reported in the literature. Paykull[5] added Na~HAsO4 to an aqueous solution of zirconium sulphate and obtained a white powder which, on drying at 100 °, had the composition 4ZrO2-2As2Os'5H20. Kulka[6] mixed aqueous solutions of zirconium sulphate and arsenic pentoxide in the cold to obtain a white, gelatinous product of composition 2ZrO2"As20~.30H20. When dried to constant wt. at 100-110 °, the compound lost 29 moles of water. Moser and Lessnig[7] added sodium monohydrogen arsenate to a nitric acid solution of a zirconium salt. They also obtained a white solid which had a Zr-As ratio of one and assigned the formula ZrOH AsO4 to their preparation. *This work was supported by the National Science Foundation under foundation grant G P-2946. tPortions of this paper were abstracted from the Ph.D. Thesis of G. David Smith to be presented to the Department of Chemistry, Ohio University, September 1967. 1. K. A. Kraus, H. O. Phillips, T. A. Carlson and J. S. Johnson, Proceedings of the Second International Conference on Peaceful Uses of A tomic Energy, United Nations, Geneva, 28, 3 (1958). 2. C. B. Amphlett, Proceedings of the Second International Conference on Peaceful Uses c~f Atomic Energy, United Nations, Geneva, 28, 17 (1958). 3. C. B. Amphlett, Inorganic Ion Exchangers, p. 93. Elsevier, New York (1964). 4. A. Clearfield and J. A. Stynes,J. inorg, nucl. Chem. 26, 117 (1964). 5. S. R. Paykull, Bull. Soc. chim. Pans 20, 65 (1873). 6. O. Kulka, Dissertation, Bern., p. 4, 1902. 7. L. Moser and R. Lessnig, Monatsh. 45,323 (1925). 277

278

A. C L E A R F I E L D , G. D. S M I T H and B. H A M M O N D

Several workers[8-10] reportedly obtained a solid of composition 3ZRO2.2 As2Os-5HzO, or Zr~(AsO4)4.5H20, on adding either sodium or ammonium monohydrogen arsenate to a hydrochloric acid solution of zirconyl chloride. Wedekind and H. Wilke[11] washed hydrous zirconium oxide gel with arsenic acid solutions. They found that adsorption complexes of variable composition were formed. On longer contact or boiling with arsenic acid these complexes formed a definite compound to which they assigned the formula Zr(HAsO4)2. Blumenthal[12] preferred to represent this compound as a zirconyl salt, ZrO(HzAsO4)2. Peyronel[13] heated zirconyl nitrate and a strong (30-40 per cent) solution of arsenic acid together in an autoclave at 180-190 ° for 8-10 hr at 10 atm pressure. He obtained a product of the same composition as Wedekind and Wilke but represented it as a pyroarsenate, ZrO(HzAs20¢). Gump and Sherwood[14] claimed to have prepared a crystalline zirconium arsenate by homogeneous precipitation. Sodium arsenite was oxidized to arsenate by boiling with nitric acid in the presence of zirconyl chloride. No analysis or X-ray diffraction pattern of the product was given. Jean[15] determined the effect of varying the mole ratio of reactants and the acid concentration upon the composition of the precipitates obtained when Na2HAsOa was added to zirconyl chloride. He found that the compositions depended markedly upon these factors and only by chance did they approximate compositions reported by other workers. Since none of the phases discussed above have been characterized with certainty, and in view of Jean's results, we repeated the preparations in an attempt to establish the validity of previous claims. EXPERIMENTAL Starting materials Zirconyl chloride (Tizon Chem. Co.) was recrystallized from HCI until spectrographically pure. Zirconium sulphate (Tizon Chem. Co.) was purified by the method of Clabaugh and Gilchrist[16]. Solutions were prepared just prior to use to prevent precipitation of basic sulphates. All other chemicals were reagent grade and used without further purification.

Analytical method Approximately 0.35 g of zirconium arsenate (dried to constant wt. over P~Os) was dissolved in a minimum of hot concentrated H~SO4 ( - 2 ml). Then, 50 ml of 15 per cent HCI and 50 ml of hot 16 per cent mandelic acid solutions were added to the dissolved sample. The mixture was heated until a precipitate formed and then allowed to stand overnight to insure complete precipitation. The mandelate was then filtered and washed with a hot solution containing 5 per cent mandelic acid and 20 per cent HCI. The precipitate, after ashing the filter paper, was calcined at 1000°C in a muffle furnace for 1 hr and weighed as ZrO2. The filtrate and washings were combined and made alkaline with ammonia (methyl red indicator) and then slightly acid with HCI. To this solution was added 50 ml of magnesia 8. 9. 10. 11. 12. 13. 14. 15. 16.

M. Weibull, Acta Univ. lund, I118, 1 (1881). S. R. Paykttll, Ofvers. Sv.Akad. Forh. 30, 21 (1873). W. C. Schumb and E. J. Nolan, Ind. Engng Chem. anal. Edn. 9, 371 (1937). E. Wedekind and H. Wilke, Kolloidzeitschrift 34, 83 (1924). W. B. Blumenthal, The Chemical Behaviour of Zirconium, p. 307. Van Nostrand, Princeton, New York (1958). G. Peyronel, Gazz. chim. ital. 72, 89 (1942); 93 (1942). J. R. Gump and G. R. Sherwood, A nalyt. Chem. 22, 496 (1950). M. Jean, Analytica chim.Acta 3, 96 (1949). W. S. Clabaugh and R. Gilchrist, J.Am. chem. Soc. 74, 2105 (1952).

Zirconium arsenates and their ion exchange behaviour

279

mixture and the solution allowed to stand overnight. Then, the mixture was neutralized with ammonia and an additional 10 ml of ammonia added for each 100 ml of solution. The precipitate was then filtered, washed with 5 per cent ammonia solution and redissolved in excess 4 M HCI. To the acid arsenate solution was added l g of NaHCO3 in five 0.2-g portions with stirring to expel oxygen. Then 1.5 g of K1 was added and after allowing the mixture to stand for 5 rain, the liberated iodine was titrated with 0.1 N thiosulphate solution.

Preparation of arsenates All the syntheses were carried out in 3-neck round bottom flasks fitted with a mechanical tru-bore stirrer, a reflux condenser and an addition funnel. The analytical and X-ray results are summarized in Table 1. No. I (Kulka). To 50 ml of a 0-5 M arsenic pentoxide solution was added dropwise 100 ml of 0.5 M zirconium sulphate solution. The mixture was stirred overnight, filtered and washed free of sulphate ion with a l per cent arsenic acid solution. The precipitate was given two final washes with distilled water, dried overnight at 50°C and then dried to constant weight over P205. No. l 1. Repeated No. 1 except that 250 ml of the 0.5 M arsenic pentoxide solution was used. No. IIl. To 100 ml of hot 1.0 M zirconium sulphate solution was added 100 ml of 0-5 M arsenic pentoxide solution and boiling continued for 1 hr. The precipitate was then treated as in No. I. No. IV (Paykull). A 100 ml of 0.25 M sodium monohydrogen arsenate solution was added dropwise to 100 ml of a boiling 0.25 M Zr(SO4)~ solution. Stirring was continued for 1 hr without heating and the precipitate then treated as in No. I. No. V (Schurnb and Nolan). To 50 ml of 0.0388 M zirconyl chloride was added 620 ml of 3.75 M HNO3 and 100 ml of a 1 per cent (NH4)zHAsO4 solution. This mixture was brought to a boil and then 30 ml of a 10 per cent ammonium monohydrogen arsenate solution added and boiling continued for 20 min after the addition. The mixture was then cooled to 70°C, filtered and treated as in No. I. No. VI (Wedekind and Wilke). Hydrous zirconia was prepared by adding 6 M ammonia to a solution of 20 g of zirconyl chloride in 100 rnl of water. The hydrous oxide was filtered, washed free of CI- and reslurried with 100 ml of water. To the stirred slurry was added 200 ml of 1 M arsenic acid. The mixture was stirred for 24 hr and then treated as in No. 1. No. V l l (Peyronel). 1 gramme of hydrous zirconia and 10 ml of a 30 per cent arsenic acid solution were sealed in a pyrex tube and heated for 25 hr at 160-170°C. No. VII1 (Gurnp and Sherwood). To a 11. beaker was added 23.5g of ZrOCI2.8H20, 60ml concentrated HC1 and 40 ml of 6 M HNO:~. Then, 30 ml of concentrated H2SO 4 diluted with 200 ml of water was added. To this mixture was added a sodium arsenite solution containing 29.8 g NaAsO2 in 200 ml of water. The mixture was then brought to a boil and gently boiled until the volume was reduced to 500 ml. The mixture was then allowed to cool to room temperature, filtered and the solid washed with 21. of distilled water. No. IX. To 0.2 mole of As205 dissolved in 3{)0 ml of conc. HN 0:5 was added 0-1 mole of zirconyl nitrate in 85 mr of conc. HNO3 slowly and with stirring. The mixture was refluxed 24 hr, cooled and treated as in No. 1. The potentiometric titrations were carried out as described previously [4}. RESULTS

Table 1 summarizes the analytical data for all the preparations. The arseniczirconium ratios do not bear out the claims of earlier workers of the existence of compounds with 1 : 1 and 4 : 3 ratios. Rather gels of indefinite composition were obtained. The variable nature of these gels is best illustrated by reference to sample numbers I, II and III. In all three of these preparations zirconium sulphate was allowed to react with arsenic acid but the reaction conditions were different. The compositions of the three precipitates reflected these differences and tend to substantiate Jean's work. The preparations described by Gump and Sherwood and Peyronel yield a crystalline zirconium arsenate. However, the products obtained by these methods contained less than the expected 2 moles of arsenic per mole of zirconium. We

280

A. CLEARFIELD, G. D. SMITH and B. HAMMOND

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Zirconium arsenates and their ion exchange behaviour

281

attributed this deficiency of arsenic to hydrolysis and therefore refluxed mixtures of arsenic acid and zirconyl nitrate in concentrated nitric acid to obtain a stoichiometric product (Sample No. X). The composition ZrO2.As2Os.2HzO requires 31.85 per cent ZrO~, 58.92 per cent As2Os, and 9.24 per cent H20. Our values (Sample X) are slightly lower than this due to the presence of a small excess of water in the crystals as shown by dehydration studies. The crystals lost 4-72 per cent water at 110 ° without destroying the X-ray diffraction pattern of the original arsenate. However, at 500°C a total of 9.5 per cent water was lost and a new X-ray diffraction pattern obtained. This is similar to the behaviour of zirconium monohydrogenphosphate where 1 mole of zeolitic water is split out at 110°C and the second mole splits out at 450°C by condensation of the phosphate groups[4]. This information coupled with the ion exchange and X-ray data given below leads us to formulate the crystals as a monohydrogen arsenate, Zr(HAsO4)2. H20. Potentiometric titrations The ion exchange capacity of our crystalline sample No. X and gel sample No. IIl was determined by potentiometric titrations. The titration curves are shown in Figs. 1 and 2. The gel sample behaved in a manner similar to that of zirconium phosphate gels[4, 17]. No endpoints were distinguishable and the exchangeable hydrogens display a range of acidities. Hydrolysis set in at an early stage of the titration and at the end of the titration (pH = l l.0) 29 per cent of the arsenate ion was solubilized. Thus, it is impossible to determine the exchange capacity from the titration curve. However, it is certainly less than 5 m-equiv./g. The titration curves for the crystals are also similar to those obtained with ,21

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6

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2

I

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m-equiv. OH-/g

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Fig. 1. Potentiometric titration curves of amorphous zirconium arsenate. T i t r a n t 0.4 N N a C I + 0.0976 N N a O H . 17. C. B. Amphlett, L. A. McDonald, M, J. Redman, J. inorg, nucl. Chem. 6, 220 (1958).

282

A. C L E A R F I E L D , G. D. S M I T H and B. H A M M O N D 12-

m-equiv. OH-/g Fig. 2. Potentiometric titration curves of crystalline zirconium arsenate. T i t r a n t s C) 0.4N N a C l + 0 . 0 9 7 6 N N a O H ; • 0-5N LiCI+0-1010N LiOH; ~ 0.5N C s C l + 0-110 N CsOH.

crystalline zirconium monohydrogen phosphate[4]. The sodium and lithium ion titration curves exhibit two endpoints corresponding to the replacement of the two hydrogen atoms from the monohydrogen arsenate groups. The first endpoint occurred at 2.59 m-equiv, of base per g of exchanger. This is exactly the value required for the replacement of one hydrogen atom from Zr(HAsO4)2.H20. The second endpoint occurred at 5-21 m-equiv./g for the sodium ion titration and 5-38 m-equiv./g for the lithium ion titration. These differences are attributed to hydrolysis. Samples of titration liquor were removed just before and after the second endpoint and analysed for dissolved arsenic. Less than 1 per cent of the arsenate groups were hydrolysed before the endpoint of the sodium titration and 6 per cent at the completion of the titration (pH -- 11.6). For the lithium ion titration the corresponding values were 1.9 and 12 per cent. Cesium ion was not appreciably exchanged in acid solution. Some exchange occurred at a pH in the neighbourhood of 7 but this was accompanied by extensive hydrolysis. At the end of the titration 35 per cent of the arsenate groups were in solution and the X-ray pattern of the exchanger was destroyed. One mole of Rb + was exchanged in acid solution. The pH then rose into the alkaline region where further exchange was accompanied by extensive hydrolysis. This is different than the behaviour of zirconium monohydrogen phosphate which does not exchange rubidium ion in acid solution.

X-ray diffraction data The zirconium arsenate crystals obtained by refluxing were of the order of a few tenths of a micron in size. The powder pattern, taken with Pb(NO3)2 as an internal standard to obtain accurate d-spacings, is given in Table 2. Larger crystals were obtained by heating the microcrystals with a mixture of HNO3 and HaAsO4 in sealed tubes at 180°C for several weeks. Weissenberg and precession photographs showed that the crystals are monoclinic with a = 9.25/~,

Zirconium arsenates and their ion exchange behaviour

283

b = 5-34 A, c -----46.7/k and/3 = 90-4 °. The powder pattern was indexed on the basis of this unit cell and only those reflections which appeared on the single crystal photographs are included in the indexing. The arsenate is isomorphous with zirconium monohydrogen phosphate. During the course of exchange the X-ray powder pattern of zirconium arsenate underwent changes similar to those described previously for zirconium monohydrogen phosphate[4]. Table 2. Powder pattern of zirconium monohydrogen arsenate I

7"82 4-53 4.51 4.30

40 20 16 13

4.15 3.88 3-625 3.604 3.350 3.283

1 2 100 60 4 3

3.135

3

3.073 2.981 2.700 2.683 2-665

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2.631

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2.581 2-527

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2-457 2-366

14 3

2.148

10

2-075

2

2-024

I

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006 "112 202 204 114 1t 4 1-0.10 0-0.12 H8 208 2-14 2.0-10 1-1.10 0.1.12 1.1.11 300 218 1.1-14 2-0.14 020 310 7t3 313 0-0-18 3.1.6 026 316 1.1.16 319 029 319 408 228 2.0.20 ~-2.10 4.0-10 3-1-15 0-2.15 3.1.15 320

284

A. C L E A R F I E L D , G. D. SMITH and B. H A M M O N D Table 2. (Contd.) dobs. (A.)

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1.942 1.921

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1.900

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DISCUSSION

The analytical, ion exchange and X-ray data lead us to formulate the crystalline zirconium arsenate as Zr(HAsO4)2.H20. The arguments in favour of this formula are quite similar to those already presented in support of the formula Zr(HPO4)2"HzO for zirconium phosphate[4]. Crystals large enough for single crystal X-ray structure determinations of zirconium, hafnium, and titanium monohydrogen phosphate and zirconium arsenate have been grown. The structures of the two zirconium compounds are sufficiently advanced to permit some pertinent conclusions at this time. The structure is a layered one. Each layer consists of sheets of zirconium atoms held together by phosphate (or arsenate) bridges. The phosphate groups are situated above and below the sheets of metal atoms. Three phosphate oxygens are bonded to 3 different zirconium atoms but the fourth points towards the adjacent layer. This oxygen presumably bears the hydrogen atom and forms interlayer hydrogen bonds. The layers are oriented perpendicular to the C axis and there are six such layers in the unit cell. Thus, the interlayer distance is ~ C or 7.6 fl~ for zirconium phosphate and 7.7 fl~ for the arsenate. The arrangement of zirconium and HMO42- groups also produces zeolitic cavities within the crystal. When a univalent cation is exchanged for hydrogen, we believe the cation first enters the cavities since the (006) reflection in the powder pattern (which represents the interlayer distance) remains relatively undisturbed. There is exactly 1 mole of cavities per formula wt. of exchanger. Thus, when half the exchanged capacity for univalent cations is attained all the cavities are occupied. The cavities are too small to accommodate more than 1

Zirconium arsenates and their ion exchange behaviour

285

cation. The second mole of cation inserts itself between the layers. This is shown by the movement of the (006) reflection to higher d-spacings. At saturation the final interlayer distances for zirconium phosphate are: Li ÷, 8.9 A; N a ÷, 10-0 .A and K ÷, 10.8 ,~; similar results are obtained with zirconium arsenate. These distances indicate that the unhydrated cations are exchanged since the increase in interlayer distance is only slightly larger than the diameter of the inserted cation. The sieving properties exhibited by the crystals are also readily interpreted on the basis of the crystal structure. The entrance ways to the zeolitic cavities in zirconium monohydrogen phosphate are about 2.8 ,~ across. Thus, potassium ion (unhydrated) is just small enough to enter the cavities whereas rubidium is too large. In zirconium monohydrogen arsenate this distance must be somewhat larger to permit Rb + to exchange. Evidence supporting the conclusions given here will be presented upon completion of the refinement of the crystal structures,