H+ exchange of α-zirconium phosphate

J i!l.rz ,~l.i.(hem, 1976.Vol 38.pp. I085-1089. PergamonPress. Printedin GrealBritain

ON THE MECHANISM OF ION EXCHANGE IN ZIRCONIUM P H O S P H A T E S ~ X I V THE EFFECT EXCHANGE

OF CRYSTALLINITY ON NH.+/H + OF c~-ZIRCONIUM PHOSPHATE?

ABRAHAM CLEARFIELD and REX A. ftUNTER* Clippinger Graduate Research Laboratories, Department of Chemistry, Ohio University, Athens, OH 45701, U.S.A. (Received 2 September 1975)

Abstract--The ion exchange behavior of zirconium phosphates possessing different degrees of crystallinity was examined for the NH,~/H + system. In the most crystalline exchangers ammonium ion is initially taken into solid-solution without phase change. This initial uptake is followed by conversion of the solid solution to a fully exchanged ammonium ion phase Zr(NH,PO,):.H:O. The composition range of the solid solution increases with decreasing crystallinity until, in the amorphous exchanger it covers the entire range of ammonium ion uptake. The exchange process is reversible for the amorphous exchanger with K = 7.4 x 10 ~. However, with the more crystalline exchangers a half-exchanged phase, Zr(NH~)H(PO,)~.0.33H20, is obtained when H + replaces NH~+. These results differ significantly from other published data. INTRODUCTION I~ iS now abundantly clear that the ion exchange properties of c~-zirconium phosphate (a-ZrP), Zr(HPO4)2'H20, depend upon the crystallinity of the particular preparation[l-6]. Variations in degree of crystallinity can be induced by refluxing dispersions of the amorphous solid for different lengths of time in H3PO4 of increasing concentration[I,4]. Ion exchange isotherms for the entire range of crystallinitties have been determined for Na+/H+[4], Cs+]H+[5], Li+/H~[7] and K-/H +[8]. The isotherms for each of these systems are sufficiently different that as yet no generalizations can be made. Therefore, it is necessary to examine the exchange behavior of additional ions in order to more fully understand the operative mechanisms. In this paper we report upon the NHn*/H' exchange system. The potential use of zirconium phosphate as an NH4* scavenger in renal dialysis units added a further incentive to this study[9]. Ammonium ion exchange on t~-ZrP has been examined by Hasegawa and Aoki[10] using an exchanger prepared by refluxing a zirconium phosphate gel in 6.7 M phosphoric acid for 120 hr. They reported an inflection point in the titration curve at an uptake of 4.4 meq of NH4 ÷ per g of exchanger and proposed that a solid phase, whose formula is Zr(NH,)v~3Ho.67(PO4)2'HzO, formed in the process. This solid was then converted to the fully exchanged phase by further uptake of ammonium ion. The reactions were not reversible since in the backwards titration a half exchanged solid phase rather than the 1-33 NH4 ~ phase was obtained. In a subsequent paper Hasegawa reported upon the thermal decomposition of the exchanged phases [11].

EXPERIMENTAL Reagents Baker analysed reagent was used to prepare a stock solution of ammonium chloride approx. 0.5 M in concentration. This solution was standardized by gravimetric determination of chloride as AgCI. An ammonium hydroxide stock solution, of approx. 0.2 M conc, was prepared by dilution of Fisher's reagent grade ammonia solution and filtered. The stock solution was standardized by titration against standard HC1to the bromcresol green end point Samples and equilibration technique The zirconium phosphate exchanger samples were prepared as described previously [4] and in most cases were the same samples. Analytical and X-ray data for these samples were presented in Ref. [4]. New samples prepared in this study were 0.1:48, 12:190 and 12:384 (see Ref. [4] for an explanation of this sample designation). Equilibration technique The titrations were carried out by the static method as described earlier [4]. However, the treatment of the equilibrium mixture was different. After recording the final pH the solid was filtered off and X-ray powder patterns of the wet and desiccator dried samples obtained. The dried exchanger was then analysed for its nitrogen and hydrogen content using an F and M Carbon, Hydrogen Nitrogen Analyser. Aliquots of the filtrate were analysed for chloride ion gravimetrically and for phosphate ion by the spectrophotometric method of Michelson[12]. The amount of hydroxide ion consumed in hydrolysis reactions was obtained from the known amount of phosphate solubilized [4]. To obtain values of the hydrogen ion replaced by the exchanged ammonium ion, [H]~o,, ~e made use of the following relationships. For pH values below 7, [}{ +lRel ~ [ O H

].....

-- [ O H ] . . . . . . .

• [ H ~] [ q

(1)

and at pH values above 7, +This work was supported in part by a grant from the National Science Foundation, GP-36002X, for which grateful acknowledgement is made. ~Portions of this paper were taken from the M.S. thesis of R.A.H. presented to the Department of Chemistry, December 1974. Present address, Armco Steel Corp., Middletown, OH 45042, U.S.A.

[tt~]R~, = [OH]. . . . .

--

[OH]...... - [OH 1~.,

(2)

where [OH],~d~od is the total meq. of hydroxyl ion added, [OH]Hyde,,, the amount consumed by hydrolysis, and [H+]~, and [OH ],, the rneq of these ions in solution at equilibrium for each measured point.

1085

1086

A. CLEARFIELDand R. A. HUNTER

Instrumental

X-ray diffraction patterns were obtained with a Norelco wide angle ~oniometer with nickel filtered copper radiation (A = 1.5418A). Weight losses at elevated temperatures were determined by theromogravimetricanalysis using a Tem-Pres TG-2A unit. The heating rate was 5.5°/rain. RESULTS Analytical data and X-ray patterns for most of the samples were given previously[4]. The X-ray pattern for 0.1:48 was intermediate between that of 0-5:48 and the gel picutred in Ref. [4]. It was necessary to know the exact water contents of the exchangers since the ion uptakes were reported as meq per gram of ZrP2Or. Therefore, these values were redetermined by ignition at 800°C after drying overnight at 40-50°C. The weight losses were: 0.1:48, 15.73%; 0•5:48, 16.64%; 2.5:48, 14•23%; 4.5:48, 11.89%; 9:48, 11.29%; 12:190, 11•20%; 12:384, 11•86%• The theoretical water content for Zr(HPO4)2.H20 is 11•96%• The less crystalline samples always contain considerably more than the theoretical water content unless thoroughly dried[4].

of the theoretical capacity of 7.54 meq/g. This reduced uptake has been observed for other ions on amorphous exchangers and adequately explained[4, 5]. It is certainly not due to hydrolysis which in this instance is quite small. The titration curves (forward direction only) for exchanger 2.5:48 are shown in Fig. 2. The initial uptake in the absence of added base was 0.55 meq NtL÷/g at a pH of 2.51. The maximum uptake was 7.30 meq/g. There is a distinct change in slope of the curve at 2.4 meq/g. X-ray patterns, taken of exchanger samples derived from the initial steeply sloping portion of the curve, revealed that the solid phase was a-ZrP. Thus, the ammonium ion must distribute itself statistically among the a-ZrP exchange sites. Just beyond a loading of 2 meq/g a second phase makes its appearance. This phase was identified as the fully exchanged ammonium ion form. From this point on the relative amount of a-ZrP decreases while that of the fully exchanged form increases until, at a loading of 4.5 meq/g, it is the only solid phase present. This loading coincides with a noticeable increase in the slope of the titration curve.

Titration curves

The results for the static potentiometric titration of the amorphous exchanger 0.1:4.8 are shown in Fig. 1. In this as in the other exchangers there was a one for one exchange so that within the limits of error these zirconium phosphates behaved as perfect exchangers. Furthermore the NH4+/H + exchange is reversible for 0.1:48. This is not evident from the raw titration data where the curves representing the forward and backward directions do not coincide. However, the actual exchange curves do coincide• Initial equilibration of 0.1 : 48 with 0.1 N NH4CI resulted in an uptake of 1.1 meq NIL + per g of ZrP2OT. Furthermore, the total loading of ~ 5 meq/g falls far short

@ 0 7

0 @0

6 0 0@0 •I-

o

5

o oO

o

0

o@ 4 c01

o~ 3

0

i 2

i I

co

8 i

7

~

// /

x

rnecl/g ZrP20 ?

/• /

O I~l I ~i I

6

Fig. 2. Titration curve, forward direction only, for sample 2.5:48. Open circles are meq of OH- added and the half filled circles the actual NIL+ uptake. In the center section the two sets of points nearly superimpose.

/"

//

o ~/ :r IX

s~

o on 0

5

d

/

o/

/

4 oI

/

o~

3

./

I

i I

2

3

4

5

6

7

8

9

meq NH.~ looded Ig ZrPzO ? meq H+ odcled lq ZRPzO7

Fig: l. Forward and reverse titration curves for exchanger 0"l : 48. Titrant: forward direction, 0.1 N NILOH+0.1 N NILCI; backwards titration HCI+NH,CI at /~=0-1. Dashed line gives amount of OH added and open circles the actual NIL÷ loaded. The dot-dashed line gives the H÷ added (bottom scale) and the triangles represent the NIL+ in the exchanger in the backwards titration. The solid line at the extreme left represents the phosphate ion in solution.

Figure 3 illustrates typical results for the highly crystalline exchangers 12:190 and 12:384. Along the initial sharply rising portion of the forward titration curve the solid consisted of only the a-ZrP phase• However, at about 1 meq/g of NH, ÷ uptake the ammonium ion phase, which had earlier been shown to be Zr(NtLPO4)2"H20[13], made its appearance. Along the entire flat portion of the curve both phases existed together in relative amounts governed by the loading• At about 7 meq NIL ÷ uptake the a-ZrP phase was no longer observed• Exchangers 9:48 and 4.5:48 exhibited quite similar results• However, the initial steeply rising portion of the curve was shifted slightly to the right and the flat plateau occurred at somewhat lower pH values. These effects were more pronounced the lesser the degree of crystallinity, For example, with 4-5:48 the fiat plateau occurred at ~ 0.5 of a pH unit lower than the corresponding values for 12:384 and the appearance of the NIL ÷ phase was observed at about 1.3 meq of NIL ÷ uptake. The

1087

On the mechanismof ion exchangein zirconiumphosphates--XIV

8

J

I

2 meq

4

5 NH4+

5

6

Ioodad/g

7

6

9

Z r P 2 0 - ,,

Fig. 3. Titrationcurves for highlycrystallinezirconiumphosphate. Forward direction: 12:190 (IS]),12:384 (©); Reverse direction: 12:384 (@).Titrant same as for Fig. 1. results for 9:48 were about midway between those of 4.5:48 and the most crystalline exchangers. The exchange reactions are not reversible as shown by the reverse titration curve (exchanging hydrogen ions for ammonium ions) in Fig. 3. A sharp break was observed at exactly half-exchange. Along the flat plateau preceding this break two solid phases were present, the fully exchanged phase and a half-exchanged phase. At the half-exchanged point and beyond the solid consisted only of half-exchanged phase with the additional hydrogen ion going into solid solution. Finally, at an uptake of approx. 5.5 meq H+/g of exchanger the solid was converted to 0-ZrP which is an eight hydrate[14]. In the present case, however, the solid retained considerable ammonium ion which, as seen in Fig. 3, was not removed even when as much as 8 meq of HC1 has been added.

Identi~cation and thermal analysis of the exchanged phases The fully exchanged ammonium ion phase of a-ZrP has been characterized previously and an indexed powder pattern is available[13]. Table 1 contains a summary of the TGA results for the ammonium ion exchanged phases (the end products of the forward direction titrations). All of the thermograms, except the one for sample 0.1:48, exhibited three distinct weight losses, designated as steps Table 1. Thermogravimetricanalysis of ammonium ion exchange reactionproducts S:u:~i

( c- }',,o ',

(%oc'-aSi ° ~

Step (5o, -TXJ' )

•

3:. '

!7.54

2 .99

l

:b

,:

h,oret i:

~

Step 2

Equilibrium constant for NH4+/ H +exchange with 0.1 : 48 Since the exchange reaction in this case is reversible, it is possible to calculate an equilibrium constant. The original gel should contain approx. 5 moles of water on the assumption that it behaves similarly to the gel sample 0-5:48[4]. At the end of the titration it was shown (in the Sample O0

t~

200

D

'~00

Wt • 0 3384g

i /

[

Temp

J 40

~l ~4

3o 8

~ 6oo ~ aoo

~

I0

~

g

Wt.

TIME

Fig. 4. Thermogravimetric weight loss curve for the halfexchanged ammonium ion phase of 12:384 prepared by the reverse titrationof the fullyexchangedphase, Zr(NH,PO4)z.H20.

: N

%% Weight Loss Step

1-3, in Table I. The first step corresponds to the loss of I mole of water and I mole of NHs. This is followed by the loss of the remaining NH3 and, finally, by condensation of the monohydrogen phosphate groups to form ZrP2OT. These results are in substantial agreement with previous dehydrration studies [l l, 20]. For the non-crystalline sample 0.1:48 only a single, continuous weight loss, extending from 50° to 650°C was observed. This exchanger contained 1.33 moles of NH3 per mole of ZrP207 so that the 33% weight loss corresponds to 5.05 moles of zeolitic water in addition to the ammonia. This is in accord with our earlier observation that the amorphous zirconium phosphates swell in water with uptake of roughly 5 moles of solvent. Thus, the formula, of the ammonia exchanged 0.1:48 sample closely approximates Zr(NH4) I.33H,~.67(PO4)2"5H20. The TGA curve exhibited by the half exchanged solid obtained from the backwards titration of sample 12:384 is shown in Fig. 4. The first weight loss (below 300°C) amounts of 2.5% and is equivalent to 0.33 moles of water. The second sharper weight loss amounts to 5.18% and results from the loss of ammonia. Finally, condensation of phosphate groups takes place with loss of 5.32% H20. The original solid contained 4.47% N. Thus, the formula, Zr(NH,PO,)(HPO,).0.33H20, which requires 4-33% N and stepwise weight losses of 1.85%, H20; 5.25%, NH~; 5.5% H20. The powder pattern for this half exchanged phase is given in Table 2 where it may be compared to that of the unhydrated half exchanged solid obtained by heating Zr(NH4PO,)2.H.~O.

Table 2. X-raypowder patterns of half exchangedammonium ion forms of a-ZrP [.U,5

1. :

i5.5%

20.6C

;'.25

i L. 1'

1 ( , ~1

78.3~

"% 94

l;i,.

15,

J~

o. ~tl,

6. ~2

I('.4 ~

i ~ 5~

g<,. )O

.7i6

Zr'(r~l]l

,'Ix)*]:"~.q{:,;

r

< ,~,'H'

RA',

i/'i.

', A

?. %r 7. ~jO

~,3 //O

7.

,~. i.

:q

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.6! .:<~

<

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i;<

. :[

. .C

": ]

1088

A. CLEARFIELDand R. A. HUNTER

preceding section) that the exchanger still contained five moles of water. Thus, the change in water content is negligible and the data may be treated by the method of Argersinger, Jr. et al. [15]. The reaction can be written m

RH + NH4 + ~ RNH4 + H +

where R'--H= the swollen gel 0.1 : 48 and RNH, = the NH4 ÷ exchanged form of 0.1:48. The equilibrium constant is then aN/44aH

o.]

K NH4/H

--

fi.aNm

(4)

where the quantities with bars are the activities of the ions in the solid phase. Let Kc

£""'

a.

XH

~NH4"

(5)

Kc is the corrected rational selectivity coefficient for which log

0"1

fl

KNH,/H= | log Kc dXNa,.

(6)

3o

The integral was evaluated from a plot of log Kc vs .~,a, as shown in Fig. 5. In calculating Kc values, the theoretical capacity (7.54 meq/g) was chosen rather than the experimentally observed one. This introduced a rather large extrapolation, indicated by the dashed line in Fig. 5, into the evaluation of the integral. However, values of - l o g Kc vs -~NH,+were found to fall on a straight line so that a least squares fit of the data yielded the equation ' - l o g Kc = 6"81-~NrW+ 0'74.

(7)

This equation was then used to make the extrapolation. 0-I From the area under the curve KN~H was found to be 7.4x 10-5. This may be compared to the approximate value as obtained from - l o g Kc at XN,, = 0"5 which is 7"1 x 10-5.

8 7' 1

6 /

/

/

I

i

i

/

/

5 o

o °°

o

z~

o

T 3

oo

2

,

i

0.1

0.2

i 03

i 0.4

, 05 XNH4

i

i

i

i

0.6

0.7

0.8

0.9

I0

+

Fig. 5. -log Kc vs equivalentfraction of ammonium ion in solid for exchanger 0.1:48. Circles represent data obtained in the forward direction and triangles data obtained in the reverse direction.

DISCUSSION The results reported in this study do not agree with the previous work of Hasegawa and Aoki[10]. Our titration curves for the more crystalline exchangers did not show a break at 66% of exchange nor did we obtain a phase of composition Zr(NH4)l.33Ho.67(PO4)2'H=O. Comparison of the X-ray pattern attributed to this phase[10] with ours indicates that it is most likely a mixture of a-ZrP and the diammonium phase. Furthermore, it has been observed that an inflection point may be obtained at roughly 66% of added NH4OH during a progressive titration carried out under non-equilibrium conditions[16]. This may account for the shape of the curve reported by Hasegawa and Aoki and may be artificial. This point is under investigation. The most interesting feature of the present study is the fact that a single plateau is obtained in the forward titration curve for the more crystalline exchangers. This is in sharp contrast to the results for sodium and potassium ion exchange[17-19] where two plateaus and two exchanged phases were obtained. Thus, in contrast, both protons are exhibiting the same pK value toward ammonium ion. In fact the very varied behavior exhibited by highly crystalline a-ZrP towards different ions indicates that it is the nature of the exchanged phase(s) rather than proton acidity which determines the shape of the titration curve. The appearance of plateaus in the titration curve has its explanation in phase rule considerations[17]. When two solid phases are present, a total of three phases must be considered (the vapor phase can be neglected since the vapor pressure is constant during a titration). If the solid phases have constant composition, then three components are necessary to characterize the system[17]. Since temperature and pressure are held constant, the system has no degrees of freedom. Thus the composition of the solution remains constant while one solid is converted to the other. In the case that the solid(s) have variable composition the system has one degree of freedom and the titration curves exhibit a non-zero slope. With the most crystalline exchanger, 12:384, initially the ammonium ion distributes itself statistically throughout the a-ZrP crystal lattice with consequent sharp rise in pH. As soon as the solubility limit is exceeded (-13%) of exchange) a second, ammonium ion rich, phase forms. This latter phase does not have a constant composition since it is the only phase present from 93 to 100% replacement of H +. However, its composition must be reasonably constant during titration as long as the a-ZrP is also present since the slope of the curve is close to zero. As the crystallinity of the exchanger decreases the composition ranges of the two solid phases increase. Thus, with 2.5:48 the a-ZrP phase exists from 0 to -30% ammonium ion uptake and the ammonium rich phase from -60% to 100%. Finally, in the non-crystalline exchanger a single solid solution is observed. This increase in composition range with decreasing crystallinity appears to be the rule for a-ZrP [4, 5, 7, 8]. Another surprising feature of the exchange is the appearance of a half exchanged phase when hydrogen ions replace ammonium ions from the diammonium phase. The non-appearance of such a phase in the forward direction may stem from the special hydrogen bonding properties of the ammonium ion which causes it to form a particularly stable arrangement of two NH4+ and one H~O within the a-ZrP cavity[13]. Finally, it may be remarked that there seems to be

On the mechanism of ion exchange in zirconium phosphates--XIV nothing unusual in the titration curve for sample 0.1:48 whicb would indicate a special sorptive power for NH~ ~. Apparently the utility of amorphous zirconium phosphate as a sorbant in renal dialysis units must stem from the particular buffer system used in the units[9]. This point is under investigation. REFERENCES

I. A. Clearfield and J. A. Stynes, J. Inorg. Nucl. Chem. 26, 117 (1964). 2. J. Albertsson, Acta. Chem. Scand. 20, 1689 (1966). ~. S. Ahrland, J. Albertsson, A. Oskarsson and A. Nicklasson, J. lnorg. Nucl. Chem. 32, 1409 (t970). ~. A. Clearfield, A. Oskarsson and C. Oskarsson, Ion Exchange and Membranes 1, 91 (1972). 5. A. Clearfield and A. Oskarsson, Ion Exchange and Membranes 1, 2115 (1974). 6. G. Alberti, S. Altulli, U. Constantino, M. A. Massucci and N. Tomassini, Ji Inorg. Nucl. Chem. 36, 653 (1974). 7. A. Clearfield and Dinko A. Yuhtar, submitted to J. Phys. Chem. ~..~. Clearfield. A. Ruwm~c and S. Milonjic, to be published.

1089

9. A. Gordon, M. Popovtzer, M. Greenbaum, L. Marantz, M. McArthur, J. R. DePalma and M. H. Maxwell, Proc. Europ. Dialysis Transplant Assoc. 5, 86 (1968). 10. Y. Hasegawa and H. Aoki, Bull. Chem. Soc. Jap. 46, 836 (1973). 11. Y. Hasegawa, Bull. Chem. soc. Jap. 46, 3296 (1973). 12. O. B. Michelson, Anal. Chem. 29, 60 (1957). 13. A. Clearfield and J. M. Troup, J. Phys. Chem. 77, 243 (1973). 14. A. Clearfield, G. H. Nancollas and R. H. Blessing, Ion Exchange and Solvent Extraction (Edited by J. A. Marinsky and Y. Marcus), Chap. I, p. 33. Marcel Dekker, New York (1973). 15. W. J. Argersinger, Jr., A. W. Davidson and O. B. Bonner, Trans. Kan, Acad. Sci. 53, 404 (1950). 16. A. Ruvara~, Boris Kidric Institute, Belgrade, private communication. t7. A. Clearfield and A. S. Medina, J. Phys. Chem, 75, 3750 (1971). 18. F. Mounie~ and L. Winand, Bull. Soc. Chim. Fr. 1829 (1968). 19. A. Clearfield, W. k. Duax, J. M. Garces and A. S. Medina, J. Inorg. Nucl. Chem. M, 329 (1972). 20. S. E, Horsley and D. V. Nowell, Therm. Anal., Proc. 3rd ICTA DAVOS 2, 611 (1971).