Ion-exchange characteristics of a chromium tripolyphosphate glass

J. inorg,nucl.Chem.,1969,Vol.3I, pp. 1507to 1514. PergamonPress. Printedin Great Britain

ION-EXCHANGE CHARACTERISTICS CHROMIUM TRIPOLYPHOSPHATE

OF A GLASS

D. B E T T E R I D G E and G. N. S T R A D L I N G Chemistry Department, University College, Swansea

(Received 11 September 1968) Abstract-The results of batch and column ion-exchange experiments with a chromium tripolyphosphate glass and alkali metal ions are reported. The uptake, exchange capacity and log KD vary linearly with pH over a wide range. The glass is very selective towards alkali metal ions, which are adsorbed in the order Cs + > Rb + > K + > Na +. The glass is suitable for column chromatography; the alkali metal ions can be separated after sorption by elution with 1 M hydrochloric acid.

THE PREPARATION and analysis of a chromium tripolyphosphate glass* with useful ion-exchange properties has been described [ 1]. This paper reports the results of distribution studies made with the glass and on its performance as an ionexchange column. EXPERIMENTAL

Glass The glass was prepared according to the procedure described earlier[l]. Samples from different batches were used, but no detectable change in the ion-exchange properties was observed. The glass was ground and dry sieved to give particles of a predetermined size. The particles were then washed free of fines by decantation with distilled water, dried at room temperature and sieved again.

Solutions Solutions of caesium, rubidium, potassium and sodium were prepared by dissolving a known amount of the dried, analytical reagent grade chloride of the metal. These solutions were taken as standard and were kept in polythene bottles. They were labelled with a suitable radioactive isotope for some experiments. Solutions of other metal ions used were standardised by conventional procedures. Below pH 2 the acidity was controlled with hydrochloric or perchloric acid. Mixtures of nitric acid and pyridine or triethylamine were used to control the pH of the solutions between pH 2-6.5 and 6-5-10 respectively, whilst triethylamine was used for solutions of pH > 10.

Isotopic tracers C a e s i u m - 1 3 7 , rubidium-86, potassium-42, and s o d i u m - 2 2 were obtained from The Radiochemical Centre, Amersham, and used as tracers.

Procedures for batch equilibria Distribution ratios, exchange capacities and uptake of alkali metal ions were determined by standard procedures [2], by both sorption and desorption. The alkali metal ion was determined in the aqueous phase by flame photometry or by radiochemical techniques. All results are quoted relative to the dry weight of the glass.

Procedures for column equilibria The column consisted of 2.00 g of chromium tripolyphosphate glass of 100-140 mesh size packed so that it was 3-8 cm long and had a cross-sectional area of 0-55 cm 2. The flow-rate was determined *We shall use this name as a matter of convenience until the exact nature of the glass is known. 1. D. Betteridge and G. N. Stradling, J. inorg, nucl. Chem. 29, 2652 (1967). 2. C. B. Amphlett, L. A. McDonald and M. J. Redman, J. inorg, nucl. Chem. 6, 220 (1958). ! 507

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D. BETTERIDGE and G. N. STRADLING

by the column packing, taps and the small head of pressure above the column. The effluent was monitored continuously by a T-ray scintillation head connected to a recording ratemeter. Before use the column was treated with 25 ml of 4 M-hydrochloric acid and then washed with 50 ml of distilled water. After use it was washed with distilled water. Calculations Where appropriate the results are expressed in the form of a straight line, the calculations having been performed on an IBM 1600 computor using a standard least-squares fit program. RESULTS E x c h a n g e capacities T h e exchange capacity of the glass increased linearly with p H , the equation having the f o r m exchange capacity (meq/g) = a + b p H (Table 1). A t a given p H it was identical for potassium, rubidium and caesium within the limits of experimental error. It was independent of particle size o v e r the range invesitgated ( 4 0 - 2 0 0 mesh). Table 1. Exchange capacity of chromium tripolyphosphate glass as a function of pH. Exchange capacity, meq/g = a + b pH

Cation K(I) Rb(I) Cs(I) combined

No. of results 12 12 12 36

a

b

--0.377 -0.364 -0.408 -0.383

0-409s 0.4090 0.4132 0.4107

Standard error of b estimate 0.0035 0.0025 0.0031 0.0017

0.040 0.027 0.035 0-033

pH range 2.00-11.90; r, the correlation coefficient, is in all systems ~>0.9996. Uptake T h e uptake of each alkali metal ion also varied linearly with p H o v e r the range 2-80-11-90 and was also d e p e n d e n t upon the total concentration of alkali metal ion in solution. T h e uptake closely a p p r o a c h e d the exchange capacity when the concentration of the alkali metal ion solution was 0.10 M. T h e results are given in T a b l e 2. Distribution coefficients T h e distribution coefficients of several univalent ions were m e a s u r e d as a function of p H and in the p r e s e n c e of a large excess of another univalent ion, N ÷ (Table 3). In all s y s t e m s the slope of log KDMvS. --log [ N ÷] was unity. T h e separation factors predicted f r o m these results are given in T a b l e 4. Stability and reversibility o f glass in an ion-exchange column A column p r e p a r e d as described a b o v e was subjected to 64 sorption-desorption cycles with caesium. T h e sorption step consisted of passing 10 ml of 1-00 × 10 -3 M caesium chloride solution, labelled with c a e s i u m - 1 3 7 , through the column at a flow rate varying b e t w e e n 0.36 and 0.72 ml/min. T h e column was w a s h e d with 50 ml of distilled water. H y d r o c h l o r i c acid was used instead of nitric or sulphuric acid b e c a u s e it causes less interference in the flame-photometric determination

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Chromium tripolyphosphate glass Table 2. Uptake of alkali metal ions by chromium tripolyphosphate glass as a function o f p H . Uptake, m e q / b = a + b p H

Ion Cs(l) Cs(I) Cs(I) Cs(1) Rb(1) Rb(I) Rb(I) Rb(I) K(I) K(I) K(I) K(I)

[MI, M 1"00 x 2-00 x 5.00 x 1'00 × 1"00 × 1'00 × 5-00 x 1-00× 1"00 × 1"00 × 5.00 × 1.00×

10-1 10-~ 10-a 10-3 10-1 10-2 10-3 10-3 10-1 10-2 10-3 10-3

a

b

-0"427 -0"602 -0"577 -0"289 --0"419 -0"612 --0"563 --0"298 -0-415 -0-643 -0"571 -0"318

0'419o 0"401, 0"303~ 0' 133r 0"419o 0"4012 0'301o 0"1349 0"4165 0"4065 0"3032 0"1385

Standard error of b estimate 0.0058 0.0050 0.0034 0"0029 0.0059 0.0061 0.0027 0"0031 0-0064 0-0053 0.0039 0.0029

0.058 0-050 0"035 0.029 0.060 0.062 0-027 0"032 0-065 0.054 0-040 0"029

pH range 2.80-11.90. 11 determinations for each concentration, r, the correlation coefficient, is for all systems ~ 0.9976; t test shows all correlations highly significant.

Table 3. Distribution coefficients of univalent ion, M ÷, as a function of excess concentration of other univalent ions, N ÷. Log KDM= a + b pN

M+ Cs Cs Cs CS Cs K Na Rb Na K Rb

N+ Rb* K* Na* NH4 t H¢ H** H** H** NH4 ~ NH4 * NH4 *

No. determined

a

b

12 12 14 16 16 12 10 16 12 14 14

--0"1958 --0"159o 1"087 0"682 1 "220 0'1962 --0.7250 0-5461 - 1'247 -0.318 0'0623

1"0004 0"9994 0"9960 1"047~ 0"999r 1"0118 1.007~ 1.006 r 1"0105 1'020z 1.0003

Standard error of b estimate 0"0026 0"0047 0.0077 0"071 0"0050 0"0167 0.0062 0-0043 0.0120 0.028 0.0049

0"0012 0"0021 0.0033 0'0318 0"0017 0"0073 0.0041 0.0015 0.0063 0.0094 0.0017

r, the correlation coefficient for all systems is >I 0.9936; t test shows all correlations are highly significant; range of pN: *, 0.30-2.30; t, 0.30-1.40; ~, -0-40-2.00; **, 0.00-2.00 [M ÷] = 5 × 10 4 M; IN ÷] varied within the limits indicated, total volume of solution 10 or 20 ml, 0.300 g glass. T e m p 23 +_ 1°C. Time of equilibration, 8 hr.

of the alkali metals. The total effluent was collected and its radioactivity determined. The desorption step consisted of passing 20 ml of 2 M hydrochloric acid, or alternatively 2 M ammonium chloride, through the column followed by 50 ml of distilled water, the flow rate being 0.54-0-58 ml/min. The effluent was monitored as it left the column and its total radioactivity determined after collection. The column was then washed with 20 ml of 4 M hydrochloric acid and the washings

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D. B E T F E R I D G E and G. N. S T R A D L I N G

Table 4. Separation factors for some univalent ions on chromium tripolyphosphate glass MI+/M]

Na

Na K Rb Cs

0.12 0.053 0.011

K

Rb

Cs

8-4__+0.3

18-7__.0.3 2.2_+0.1

88_+3 10.5_+0.3 4.75 _+0.10

0.45 0.095

0.21

Separation factor - KD~/KD". Values taken from results with HC1 solutions, but those from ammoniacal solutions in agreement.

were collected and their radioactivity measured. The results remained constant for the 64 cycles (32 with HCI and 32 with NH4C1): less than 0.01% of the caesium leaked during the sorption step, and at least 99.9% of the caesium was removed during the desorption step. The maximum of the caesium desorption peak occurs at 0.4-0.5 ml and ends at 3.5-3.8 ml. The dead space of the column was 0 . 9 2 _ 0.02 ml. initially and 0.90 ___0.02 ml after the experiments. There was no obvious sign of material breakdown, but prolonged washing with > 1 M acid does bring about some dissolution.

Separation of alkali metal ions from other metal ions and from each other on a chromium tripolyphosphate glass column 450 ml of 1.00 × 10 -5 M caesium, sodium, or rubidium chloride was passed through the column at 0-6 ml/min and the column was then washed with 20 ml distilled water. The ion was desorbed with 20 ml of 2 M hydrochloric acid and washed with 50 ml of distilled water, the flow rate being 0.6 ml/min throughout. The effluent was continuously monitored and kept for total determination of alkali ion. The elution curves are shown in Fig. 1. At least 99.9% of the alkali

I0

>,4

*1 h\o . o, volume

I

2

Number of column volumes 2 MHCL

Fig. 1. Elution of alkali metal ions with 1 M hydrochloric acid; flow rate 0.55 ml/min. a, Na+; b, Rb+; c, Cs+; 2.00 g glass in hydrogen form, 100-140 mesh. Bed volume 2.10, void fraction 0.44.

Chromium tripolyphosphate glass

1511

metal was sorbed and desorbed. It is obvious that the flow rate is too great to allow separation of the alkali metals from each other, although it may allow separation of alkali metal ions from a large volume of solution. A solution containing 10 ml of 10 -a M caesium, rubidium or sodium chloride labelled with lsTCs, ~eRb and 2~Na respectively, and 10 ml each of 0.05 M solutions of the chlorides of Ni, Sr, Ba, In, Fe(III), Cr(III) and the nitrates of Cu(II), Zn, Co(II), Mn(II), Mg and Th(IV) were passed through the column in the acid form at a flow rate o f - 0-1 ml/min. The effluent was continuously monitored. The alkali metal ion was desorbed with 20 ml of 2 M hydrochloric acid solution at 0.55 ml/ min followed by 50 ml of distilled water; this fraction contained > 99.9% of the added activity. The sorption step necessitated a slower flow rate than with the alkali metal ions alone, but the elution curves were identical with those obtained in the absence of di-, tri- or quadrivalent ions. The alkali metal ions can be separated from each other after adsorption on the column by elution with 1 M hydrochloric acid (Fig. 2). Separation is achieved with

~e.

i:5"

0

i t 0

I

3

4

5

6

i

I

I

,

7 8 9 I0 II 12 13 14 15 16 17 Numbm" e( column ¥olumes IMHCL

18 19 20 21 22 23

Fig. 2. Elution of tracer amounts of alkali metal ions with 1 M hydrochloric acid; flow rate 0'02ml/min. 2.00g glass in hydrogen form, 100-140 mesh. Bed volume 2.10ml, void fraction 0-44.

a flow rate of 0.05 ml/min., but one of 0.02 ml/min is preferable. The results (Table 5) agree well with those calculated from the following standard equations: ~=X(Ka,,+~)

and

Kav=Kd.p

where ~ is the volume needed to elute a solute to its maximum concentration in the effluent, X is the total bed volume, Kay is the volume distribution coefficient, E is the void fraction and p = (gram of dry resin)/(ml of bed volume). The height equivalent to a theoretical plate was calculated both from simple plate theory[3, 4] and from Glueckauf's [5] modification of it, for elution at a flow rate of 0.02 ml/min. Both methods gave a value of 0.033 cm, for elution of both caesium and rubidium, which is approximately twice the mean diameter of the glass particles used (100-140 mesh). The agreement between the methods is evidence for the symmetry of the elution curves.

D. B E T T E R I D G E

1512

and G. N. S T R A D L I N G

Table 5. Separation o f alkali metal ions on c o l u m n

M+ Cs Rb K Na

Breakthrough volume (ml) .4 B 24.4 5.8 3-0 1.0

12.4 3.6 -1.0

V o l u m e to elute (ml) ,4 B

v, A

B

C

33-4 7.7 4.2 1-3

17-4 4-8 -1.3

34.6 8.0 4.3 1.3

44-6 10.1 5.4 1-7

34.8 9-8 -1.7

Kd A

B

C

16.3 3-4 1.6 0.2

8.2 1.9 -0.2

16.9 3.5 1.7 0.2

Separation factor A B C 4-7 2.1 8.5 --

4.2 1.6 6-0 --

4.7 2-3 8.4

A = 0-02 ml/min; B = 0,05 ml/min; C = value predicted from batch experiments; B e d v o l u m e = 2-10 m l ; p = 0.95 g/ml; void fraction = 0.44

Preliminary experiments on the rate of exchange under batch conditions suggest that 0.02 ml/min is probably the highest flow rate for equilibrium to be reached on a theoretical plate. Better separation could undoubtedly be achieved by gradient elution, but this was not attempted.

DISCUSSION

The glass appears to be a good compromise inorganic ion exchanger. It is not so selective towards caesium as ammonium phosphomolybdate nor does it have such a high capacity in concentrated acid, but it has better mechanical properties and is stable over a wider pH range [6]. It has similar mechanical properties to zirconium phosphate, but it is more selective, has a slightly higher capacity in acid solution and a slightly lower one in alkaline solution. It is more selective and has far better mechanical properties than the ferrocyanides which have been proposed as ion-exchangers[7]. Furthermore, the desorption step is easily accomplished and the preparation is simple. The chief difficulty in working with such a glass is in knowing what is its composition and what is the mechanism of exchange. The very properties which make it desirable as an ion exchanger make it difficult to establish its exact structure or composition. It is likely that the preparative reaction is a polymerisation reaction and that the glass contains many kinds of phosphate groups. To isolate and determine these groups by standard chromatographic procedures requires the glass to be dissolved. Dissolution in concentrated acid or alkali results in a conversion of phosphate groups to orthophosphate[8], and attempts to achieve solution in neutral or near-neutral solution with common oxidising, reducing or complexing reagents, including chromium(II) acetate, have failed. It is not volatile enough to give a meaningful mass spectrum on the MS9 and the i.r. spectrum between 1900 and 625 cm -1 only shows broad bands centred on 1625 3. 4. 5. 6. 7. 8.

H. F. Walton, Chromatography, (Edited by E. H e f t m a n ) Reinhold, N e w York (1961). M a y e r and Tompkins,J.Am. chem. Soc. 69, 2866 (1947). E. Glueckauf, Trans. Faraday Soc. 51, 34 (1955). C. B. A m p h l e t t , Inorganic Ion exchangers Elsevier, A m s t e r d a m (1965). G. N. Stradling, Proc. Soc. Anal. Chem. 3, 9 (1966). J. R. V a n Wazer, Phosphorus andits compounds, Vol. I Interscience, N e w York (1958).

Chromium tripolyphosphate glass

1513

and 1075 cm -1. The characteristic tripolyphosphate spectrum reported by Corbddge and Lowe [9], and confirmed by us on crystalline tripolyphosphates, is completely altered, all definition being lost. The titration curve is completely linear, in agreement with the uptake studies. As noted earlier, physical tests show that the glass is amorphous. It would be strange if such a material were obtained uniquely from the reactants. Over a period of a year we have obtained a reproducible product from a large number of preparations. However, other workers have produced glasses that are very similar, but nevertheless distinct [ 10-12]. The chief differences lie in the solubility and selectivity. The dissolution process is complex and is still being studied; after a variable induction period the kinetics resemble a second-order autocatalytic reaction. Whatever the details of the mechanism, it is clear that a prolonged column life depends upon washing with water and avoiding long periods of washing with 1 M hydrochloric or nitric acids. We have found the examples of dissolution time quoted earlier to be broadly correct; Wilson and Naylor[10] have noted that some of their preparations were initially much more soluble than ours, but after two months' storage they show comparable solubility. The differences in selectivity follow the same pattern but to different degrees. The exchange behaviour for caesium is that described above, but there is also an uptake of di- tri- and quadrivalent ions to about the same extent as that of sodium. This uptake has been noted under column conditions and under the batch conditions described earlier. A detailed study of this exchange has not yet been made, but it is possible to speculate on the cause of the difference. One possibility is that the polymerisation process is not exactly reproduced, and that phosphate groups suitable for chelating with multiply-charged cations have been introduced to a far greater degree than in our preparation. The other possibility is that there are orthophosphate groups which have not been completely removed by washing, and that uptake is occurring through the precipitation carder mechanism described by Vesrly and Pekarek [13]. The reaction is being studied in as much detail as possible to see if the reasons for these differences can be found, and whether it is possible to prepare an even better glass or series of glasses. The crystalline tripolyphosphates of Ba, Mn(I I), Zn, Pb, Co(I1), Cu(l l), N i and Cd have been prepared by the procedure used for the chromium tripolyphosphate glass, followed by recrystallisation from sulphurous acid[14]. The same preparation also yielded a cadmium and nickel glass. The uptake under comparable batch conditions for chromium, nickel and cadmium glasses and the crystalline materials was in the following ratios 7.5:4.5:3.6: l, the crystalline materials behaving almost identically. Amphlett has commented that this is analogous to the zirconium phosphate system, where a relationship between uptake and crystallinity has been established (the most crystalline having the lowest uptake) which 9. 10. 11. 12. 13. 14.

D. E. C. Corbridge and E. J. Lowe,J. chem. Soc. 493 (1954). P. D. Wilson and A. Naylor. Private communications. J. F. Cannell, B, D. H. Chemicals Limited. D. Betteridge and F. Snape. Unpublished studies. V. Ves~ly and V. Pek~rek, J. inorg, nucl. Chem. 27, 1419, (1966). A. Schwarzenberg, Annln. Chem. 65, 133 (1848).

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D. BETTERIDGE and G. N. STRADLING

merits further investigation [15, 16]. It certainly demonstrates that there may be a whose series of glasses hitherto undescribed, which may have very valuable ionexchange properties. The analogy with zirconium phosphate probably goes further; the centres of exchange are probably the acidic hydrogenions of phosphate groups, the linearity being due to a "smudging" effect of numerous phosphate groups having contiguous pK~ values over the pH range examined. However, at this stage the possibility of acidic protons of water molecules coordinated with chromium(III) playing a role cannot be ruled out. Work is in progress on the points discussed above and on the kinetics of the exchange process. Acknowledgements-We gratefully acknowledge a maintenance grant to one of us (G.N.S.) by the Science Research Council and the loan of equipment by AERE, Harwell. 15. C. B. Amphlett. Private communication. 16. (3. H. Nancollas and V. Pek~rek,J. inorg, nucl. Chem. 27, 1409 (1967).