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Studies on the synthetic inorganic ion exchangers—I
J. Inorg. Nucl. Chem., 1964, Vol. 26, pp. 2241 to 2253. Pergamon Press Ltd. Printed in Northern Ireland
STUDIES ON THE SYNTHETIC I N O R G A N I C ION EXCHANGERS--I S Y N T H E S I S OF S T A N N I C P H O S P H A T E A N D SOME OF ITS P R O P E R T I E S YASUSHI INOUE The Research Institute for Iron, Steel and Other Metals, Tohoku University, 75, Katahira-cho, Sendai, Japan (Received 26 July 1962; in revised form 18 March 1963)
Abstract--For the purpose of developing ion exchangers which are expected to have radiation stability, the author has studied the synthetic inorganic ion exchangers. First, stannic phosphate was prepared by various methods, and its properties, such as composition, exchange capacity, and chemical stability, were investigated in relation to the methods of synthesis. The content of phosphate increases with increase of the mixing ratio of PO~S-/Sn4+ up to a value 2 and then becomes constant at the maximum phosphate to stannic ratio of ~ of the stoichiometric value. Exchange capacity depends on the phosphate content of the exchanger, and it is assumed that fixed ion originates from phosphate group. The exchanger, which has maximum phosphate to stannic ratio, has maximum exchange capacity of 1.3-1.5 meq/g. This exchanger is unstable in concentrated hydrochloric acid and alkali even in dilute solution, but fairly stable in nitric acid as zirconium phosphate. Attempts were made on mutual separation of alkali metals for the examination of the selectivity and availability for the column operation of the exchanger. By using various concentration of ammonium chloride as an eluent, complete separation of sodium, potassium and cesium from each other was achieved with relatively small column. INTRODUCTION ION EXCHANGEprocesses are widely used in nuclear engineering, such as for purification of nuclear fuels, fuel reprocessing, waste disposal, enrichment and purification of useful radioactive isotopes because of the selectivity and completeness of the separation that may be obtained. In addition, the ease of operation combined with the application of remote-handling methods renders such procedures peculiarly suitable to operations in the nuclear energy field. Although the ion exchange resins commercially available at present have excellent ion exchange properties, there are two serious limitations for use in the atomic energy field. One is that ion exchange resins suffer changes of characteristics, such as capacity, selectivity and exchange rate, by the irradiation of the high dose of ionized radiation, m Another is that this material breaks down in water at high temperature. Because of this decomposition, the high-pressure cooling water of pressurized watercooled reactors, if to be purified by ion exchange, must be cooled to normal temperatures before purification and then released, which is wasteful both in power and space. Most inorganic adsorbents are expected to have radiation stability, and some of these m a y also possess high-temperature stability. It has been known for a long time that m a n y inorganic substances exhibit ion exchange properties, but the use of these [1}G. T.
fATHERS,
United Nations Conference on the Peaceful Uses of Atomic Energy 7, 490 (1955).
2241
2242
YASUSHIINOUE
substances is restricted because the exchange capacity is lower than that o f ion exchange resins and such substances are useful only in a limited range o f pH. The production o f a chemically stable inorganic ion exchanger would enable ion exchange process to be used in an intense radiation field. In addition, it is conceivable that in some cases a special peculiarity in comparison with the properties o f organic resins m a y lead to improvements even under normal operating conditions. Thus, m u c h attention has been given to the development o f synthetic, inorganic ion exchangers during the last decade. These materials are classified into two groups: first, acid salts having cation exchange properties such as zirconium, (z-13) titanium,(a) tin, (e) and antimony (2s) phosphate, zirconium tungstate/4) a m m o n i u m and thallium molybdophosphate, (~22) zirconium arsenate, c~7) a m m o n i u m phosphotungstate, (~9) tungsten oxide, (~) and zirconium molybdate; t27) and secondly, hydrated oxides having cation and anion exchange properties, such as zirconium, (2'4"~'2~'~7) thorium, c~z'27) titanium,(23) tin,t 4.~) chromium,(4) niobium,(4) tantalum, t4) vanadium, ~4) uranium ~4) and aluminium t~) oxide. O f these, zirconium phosphate and a m m o n i u m m o l y b d o phosphate, belonging to the first group, are the only ones that have been investigated in detail. I n order to develop a radiation-resistant ion exchanger, the a u t h o r plans to investigate the synthetic inorganic ion exchangers systematically. I n this paper, studies o f the preparation o f stannic phosphate in a granular f o r m adequate for the column operation are described. (2) C. B. AMPHLETr,Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, A/Conf. 15, 271 (1958). (8) I. J. GAL and O. S. GAL, ibid. 15, 468 (1958). (~) K. A. KRAOS, H. O. PHILUPS,T. A. CARLSONand J. S. JOHNSON,ibid. 15, 1832 (1958). (~) C. B. AMprmerr, L. A. McDoNALDand M. J. 1LEDMAN,J. lnorg. NucL Chem. 6, 220 (1958). (e) E. MeRz, Z. Electrochem. 63, 288 (1959). (7) E. M. LARSeNand D. R. VISSERS,J. Phys. Chem. 64, 1732 (1960). (a) L. BAeTSLI~and J. PELSMAEKERS,J. Inorg. Nucl. Chem. 21, 124 (1961). (g) L. BAETSLI~and D. HuYS, ibid. 21, 133 (1961). (10)K. A. KRAUSand H. O. PHILLIPS,dr. Amer. Chem. Soc. 78, 694 (1956). m) C. B. AMPI-ILETT,L. A. McDoNALD, J. S. BURGERSand J. C. MAYNARD,J. Inorg. NucL Chem. 10, 69 (1959). (t2) C. B. AMPHLeTT,L. A. MCDONALDand M. J. REDMAN,Chem. & Industr. 1314 (1956). (is) H. O. PHILLIPS,F. NELSONand K. A. KRAUS,AEC 14 ORNL, 2159, p. 37 (1956). (t~) O. MraER and W. D. TREADWELL,Heir. Chim. Acta 34, 155 (1951). ~ls) H. BUCHWALDand W. P. THISa'LEraWAITE,J. Inorg. Nucl. Chem. 5, 341 (1958). tie) j. VANSMIT,J. J. JACOBSand W. RoBs, ibid. 12, 95 (1959). (17)j. VANSMIT,W. ROaB and J. J. JAcoas, ibid. 12, 104 (1959). ~ls) j. VANSMIT,W. ROBS and J. J. JACOBS,Nucleonics 17, No. (9) 116 (1959). (ts) j. KRTILand V. KO~RIN,3". lnorg. NucL Chem. 12, 367 (1960). (20)j. VANSUIT, Nature Lond. 181, 1530 (1958). ~21)T. HARA,BuL Chem. Soc. Jap. 31, 635 (1958). (22)H. BUCHWALDand W. P. THISTLETHWAITE,J. Inorg. Nucl. Chem. 7, 292 (1958). tzs) C. B. AMPI-ILe'rr,L. A. McDONALDand M. J. REDMAN,ibid., 6, 236 (1958). (~4)K. A. KRAUS,T. A. CARLSONand J. S. JOHNSON,Nature Lond. 177, 1128 (1956). t25) K. A. KRAUSand H. O. PHILLIPS,J. Amer. Chem. Soc. 78, 249 (1956). t26)C. B. AMPHLETT,L. A. McDONALDand M. J. REDMAN,Chem. & Industr., p. 365 (1957). (27)T. A. CARLSON,D. J. COOMBE,J. S. JOHNSON,K. A. KRAUSand H. O. PHILLIPS,AEC 14, ORNL 2159, p. 40 (1956). (28)T. ITO and M. ABe, BuL Chem. Soc. Jap. 34, 1736 (1961).
Studies on the synthetic inorganic ion exchangers--I
2243
Stannic p h o s p h a t e was first p r e p a r e d b y MERZ, (6) a n d he confirmed t h a t this m a t e r i a l h a d a l m o s t the same i o n exchange p r o p e r t i e s as z i r c o n i u m p h o s p h a t e . H e was able to s e p a r a t e the rare earths, h a f n i u m a n d zirconium, a n d the l i t h i u m isotopes (lithium-6 a n d lithium-7), a n d to ascertain t h a t this m a t e r i a l was useful as a c a t i o n exchanger. Because the p r o d u c t by MERZ'S m e t h o d ( p r e c i p i t a t i o n o f the stannic p h o s p h a t e b y the dissolution o f metallic tin granules in ca. 40 p e r cent nitric acid c o n t a i n i n g desired a m o u n t o f p h o s p h a t e ion) a n d s t a n n o u s p h o s p h a t e are fine p o w d e r s , they are u n s u i t a b l e for c o l u m n o p e r a t i o n . T h e a u t h o r was able to p r e p a r e stannic p h o s p h a t e in g r a n u l a r f o r m a n d to investigate the c o m p o s i t i o n , exchange capacity, a n d chemical stability. EXPERIMENTS
1. Preparation of stannic phosphate The general procedures for the preparation of stannic phosphate are as follows, except for sample No. 24, which was prepared by MERZ'S method. Aqueous phosphate and stannic solutions were prepared by the dissolution of sodium phosphate and stannic chloride, respectively. Acidities of these solutions were controlled by the addition of sodium hydroxide. Concentrations were adjusted to obtain a desired mixing ratio of phosphate to stannic ion. Any turbidity in the solution, if present, was removed by filtration through fine filter paper before use. The conditions of preparation are summarized in Table 1. Into a definite volume of the phosphate solution, a definite volume of stannic solution was poured rapidly, with vigorous stirring, at a definite temperature (usually 25°C). The mixed solution, the volume of which is given in column 5 of Table 1, stood for several hours at constant temperature. The precipitate was washed several times with distilled water by decantation until the pH of the washings remained constant. The precipitate was filtered rapidly through filter paper under suction, washed once or twice with distilled water, removed from the funnel, dried in air at room temperature or at ll0°C until it shrank, and immersed in water to break down the gel. If the gel did not break down to the desired particle size, it was ground in a porcelain mortar and sieved to obtain fractions of uniform particle size. To convert the exchanger completely to the hydrogen form, the phosphate was immersed in about 1 N hydrochloric acid for 2 or 3 days with intermittent shaking, sometimes renewing the solution in the course of immersion, and dried at room temperature.
2. Method of the column operation The column was prepared as follows. To 2 g of dried exchanger in hydrogen form (100-200 mesh) was added sufficient distilled water to allow thorough mixing to expel adhering air bubbles. After mixing, the slurry was poured into glass column, 7 mm i.d., with glass wool as a column support. The column, thus prepared, had the height of 20-50 mm, dependent on the density of the exchanger. The flow rate of influent was kept nearly constant, 10-12 drops/min, corresponding to 0-5-0.6 ml]min.
3. Determination of the exchange capacity Generally, the total exchange capacity of a weakly acidic cation exchanger must be determined with an alkaline solution. The stannic phosphate exchanger, however, is unstable in alkaline solution. Therefore, a measure of exchange capacity was taken as the amount of hydrogen ion liberated by a neutral salt, such as potassium chloride, was used as an eluent. Exchange capacity also depends on the concentration of the eluent. To determine the most suitable concentration for use, the elution curves of the hydrogen ion were found for various concentrations of the eluent. The hydrogen ion eluted from the column was titrimetrically determined by using methyl orange as an indicator. The results, obtained with the use of 0-1 M and 1.0 M potassium chloride solution, are shown in Fig. 1. Apparently, the exchange proceeds rapidly in the beginning, but the elution continues for a long time. This phenomenon prevented an accurate determination of the exchange capacity, but the amount of hydrogen ion eluted after 100 ml of the eluent was so little that the exchange capacity, as an approximate value, was calculated from the amount of hydrogen ion liberated by 100 ml. The exchange
0"15
0"15
0"15
0"3
0"3
0-3
0'3
0"3
0"3
0"3
0"3
0"3
0-3
0-3
0"3
0"3
0"3
0"3
17
18
191
2O
23
22
4
5
2
3,
6
7
7'
0"6
0'6
0'3
0"3
0-3
0"3
0"15
0"15
0-15
0'15
1"2
1-2
I "5
1"2
1"2
0"9
0"6
1"11
1"0
0"91
0"6
0"6
0-45
0"15
(M)
(M)
16
NaOH
HAP04
Sn(tV) (M)
No.
Amount of reagent in mixed solution
600
600
600
600
600
600
1800
630
620
600
600
600
600
650
(ml)
Volume of mixed solution
boil
31
25
25
25
25
25
21
21
23
26
26
27
25
(°C)
Temperature at the time of precipitation
R* 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R
(°C)
Drying temperature
1'06
1"26
0"576
0-902
0'928
1 "03
0"916
0"55
0"55
0"55
0"58
0"56
0"65
0'60
po~3-/Sn4+
TABLE 1 . - - M E T H O D S OF PREPARATION, COMPOSITION AND EXCHANGE CAPACITY
0"75 0"68 0"65 0'69 0"70 0"70 0"74 0"70 0"78 0"78 0"77 0-77 0"78 0"78 0"98 0"96 1"17 1-11 1"18 1"19 1"21 1"21 0-98 1"03 1"45 1 '34 1"56
Hydrogen liberation capacity (meq/g)
Hydrogen
0"65 0"65 0'60 0'66 0"68 0"64 0'70 0"63 0"70 0"66 0"64 0"61 0"70 0"65 0'92 0"88 1"10 1"05 1"11 1'09 1"04 1"08 0"70 0-83 1"32 1"06 1'24
absorption capacity (me,q/g)
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~.
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2246
YASUSHI INOUE I-0
-i
0"8
,E
I
0-6 _1
I I + I -I'0-4 _1 I
g
i=
°'2~._r._ 0
20
40
Effluenf,
I
I
I
60
80
I00
mL
FiG. 1.--Elution curve of hydrogen ion Straight line: Eluent: 0.1 M KC1 Dotted line: Eluent: 1.0 M KC1 capacity obtained by this method is represented on Fig. 2 as a function of the concentration of the eluent. As a constant value was obtained for concentrations of potassium chloride solution greater than 1 M, the exchange capacity was determined by the use of 1 M potassium chloride solution. The capacity, in this meaning, is tabulated in Table 1, next-to-last column. The exchange capacity based on the amount of hydrogen ion taken up when about 0.1 N hydrochloric acid was passed through the potassium form of the exchanger, is also tabulated in the last column of Table 1.
4. Chemicalanalysis of the exchanger After the exchangers were dissolved in concentrated hydrochloric acid, stannic oxide was determined by the usual iodometric titration, and phosphorus pentoxide by ammonium phosphomolybdate precipitation, followed by the neutralization titration.
t.~ 5 EEl.4
if
0,,
"~ 1.3
o,,9 8,
1.2
1.0
I
I
I
2 Concentro'lion
I
5 of KCL,
I
4 M
FIG. 2.--Exchange capacity as a function of the concentration of eluent.
Studies on the synthetic inorganic ion exchangers--I
2247
5. Chemicalstability (a) Stability against water and nitric acid: After being washed with boiling water to avoid the influence of phosphate and stannic ion which might be occluded in the exchanger due to the incompleteness of the washing, the exchangers were tested by the following procedure. Five hundred milligrams of the exchanger (in hydrogen form, 100-200 mesh) were refluxed with 50 ml of 7 N nitric acid or distilled water for an hour. After cooling, the supernatant liquid was filtered through fine filter paper. If the filtrate was turbid, the filtration was repeated till the filtrate became clear. The phosphate and stannic ions in the filtrate were determined by the usual spectro-photometric methods, viz., molybdenum blue and phenylfinorone method, ~9~respectively. (b) Change of exchange capacity in the course of absorption-regeneration cycles" In conformity with the procedure for column operation mentioned above, the following cycles were repeated about ten times. In this experiment, 1 g of sample No. 7 was used. The column was regenerated by 50 ml of acid (1'2 N hydrochloric acid, 1"0 N nitric acid, or 7.0 N nitric acid). After the column was washed with 50 ml of water, hydrogen ion was eluted with 100 ml of 1.0 M or 0.5 M potassium chloride solution. Again the column was washed with 15 ml of water followed by acid regeneration and so on. RESULTS
1. Chemical composition of the exchangers The molar ratio of PO~-/Sn 4+ of the exchanger prepared by various methods is presented in Table 1. With few exceptions, the ratio is largely dependent upon the mixing ratio of the two components, that is, the ratio in the exchanger increases as the mixing ratio increases up to a value 2, and with further increase remains constant. For each mixing ratio, the acidity of the solution was varied over the range in which stannic phosphate could be precipitated. The ratio of phosphate to stannic ion is low in the limiting condition for precipitation, especially in the alkaline side, in which the yield is extremely low. The maximum ratio of PO4a-/Sn 4+ in the exchanger prepared by various methods is about 1.25, which corresponded to the stoichiometric value of PO43-/Sn4+ = ~. The external appearance also changed with the difference in the mixing ratio, namely, when the mixing ratio was less than 2, the product was white, opaque, and brittle like chalk. When the mixing ratio was above 2, the product was a white, semi-transparent or transparent solid, and hard as compared with the above opaque materials. The transparent solid breaks down easily to small particles with cracking and slight evolution of heat when immersed in water. X-ray or electron diffraction studies are being carried out to determine the structure of the product. Results will be reported in the near future.
2. Exchange capacity of the exchanger Two kinds of exchange capacity have been defined and experimental values are reported in Table 1. The "Hydrogen absorption capacity" is slightly lower than the "Hydrogen liberation capacity," but both capacities appear to be proportional, so that these can be used as a measure of exchange capacity. Exchange capacity is strikingly dependent upon the ratio of PO43-/Sn4+ in the product; namely, the greater the ratio, the higher the capacity. The same trend is shown between the mixing ratio of two components and the exchange capacity up to a value of 2 for the former quantity. ~zg~M. IsmsAsm, T. SHIOEMATSU,Y. YAMAMOTOand Y. INOUE,Japan Analyst 7, 473 (1958).
2248
YASUSHI INOUE
The exchange capacity of stannic phosphate dried at 110°C is slightly lower than that of stannic phosphate dried at room temperature, but the difference is not so large. For the purpose of estimating the reproducibility, the capacities of the four different batches of sample No. 7 were determined. As shown in Table 2, the capacities of three of the four batches were in fair agreement with one another, but the fourth had a higher capacity. Irregularities in behaviour are attributable to fluctuations. The capacity of sample No. 24 prepared by MERZ'S method was not determined because it could not be used in a column. TABLE 2.~REPRODU¢ImLITYOr TI-1ERESULTS Exchange capacity of exchanger No. 7 Batch No.
1 2 3 4
Hydrogen liberation capacity (rneq/.g)
Hydrogen absorption capacity (meq/g)
1.62 1"42 1"42 1"35
1.43 1"26 1'26 1"32
For the purpose of comparison, the capacity of a commercial exchanger of zirconium phosphate (Ionite C) and that of a weakly acidic ion exchange resin (Amberlite CG 50) were determined by the same method; they had capacities of 1.60 and 0.24, respectively. The measured capacity of stannic phosphate was slightly lower than that of Ionite C, but higher than that of Amberlite CG 50. Consequently, stannic phosphate possesses an exchange group which is more acidic than the carboxylic group.
3. Chemical stability Stannic phosphate is stable in dilute hydrochloric acid (less than 1 N), but it is solublein6 N hydrochloric acid due to the formation ofachlorocomplex. 13°~ Although various batches of the exchanger did not always behave identically, they are fairly unstable in basic solution; that is, they dissolve or peptize even in 0.1 N sodium hydroxide solution. However, the product is stable in nitric acid. The material can be used in this medium without suffering the great deterioration in the ion exchange properties or undergoing a breaking down of the particles. (a) Stability against water and nitric acid: In order to evaluate the stability against nitric acid and water in detail, the amount of phosphorus pentoxide and stannic oxide dissolved in these media was determined by means of the method described in section 3 of the experimental part. The tests, except for sample No. 7, were conducted only for the samples dried at room temperature. As shown in Table 3, the phosphate released to water varied between 2 and 7 mg, but the amount of stannic oxide was 30/~g or less. On the contrary, the phosphate ~30~M. IsmBASm,Y. YA~AMOTOand Y. INOUE,Bull. Inst. Chem. Res., Kyoto Univ. 37, 38 (1959).
Studies on the synthetic inorganic ion exchangers--I
2249
TABLE 3.--DISSOLUTIONOF PHOSPHORUSPENTOXIDEAND STANNICOXIDE INTO WATERAND 7 N NITRICACID Amount dissolved in H20 (mg)
Amount dissolved in 7 N HNO3 (mg)
Sample No. P20s 2 3 4 5 6 7 7* 7' 8 9 10 11 12 13 14 16 17 18 19 20 22 23 Ionite C
2.9 6"5 1-8 3-2 4.6 4.2 5"6 7.9 5"4 4"0 2.2 4'4 7'0 5"8 6-2 2'5 4'2 2'8 2'4 2"9 4"3 2.4 3.9
Sn or Zr 0.03 0.01 0.03
0-01
0.03
0.00 0.00
P20~ 3.4 3.4 1-7 1'8 2.9 3.4 2-5 2-1 4-0 2.0 3.0 2.7 3.1 2.4 2-6 2.3 1'5 1.4 3"3 0"4 2.5 1'9 4-7
Sn or Zr 1"5 1'3 1-7
2.5
1"8
1'3 0.02
* Samples dried at llO°C. dissolved in 7 N nitric acid to an extent slightly less than in water, but stannic oxide dissolved only to the extent of a few milligrams. F r o m the fact that even in nitric acid the molar ratio of dissolved phosphate to dissolved stannic ion is 2- or 6-fold higher than that of original material, it is apparent that phosphate ion selectively dissolves in these media. Although the amount of dissolved phosphate rises with increasing phosphate content, the distinct difference in chemical stability is n o t observed among exchangers prepared by various methods, so that by such a criterion the method of synthesis is not so important. Moreover, it is shown from the comparison of the results concerning samples No. 7 and No. 7* that the heat treatment a t 110°C does not stabilize the exchanger against these media. For the purpose of comparison, the chemical stability of Ionite C was tested in the same way. As may be seen from Table 3, phosphate ion in Ionite C has almost the same solubility as that in stannic phosphate, but the solubility of zirconium ion is less than that of stannic ion. Consequently, stannic phosphate is slightly unstable compared with Ionite C. (b) Change of exchange capacity in the course of absorption-regeneration cycles: The results obtained with sample No. 7 by the method described above, are presented in Table 4. The capacity gradually decreased by the repetition of the cycle regardless of the reagent used. 14
2250
YASUSHIINOUE
TABLE4. Tim CHANGEOF CAPACITYIN THE COURSEOF ABSORPTION-REGENERATIONCYCLES (HYDROGENLIBERATIONCAPACITY) 1
2
3
Cycle Regenerant: 1'2 N HCI eluent: 0.5 M KCI
Regenerant: 1.0 N HNO3 eluent: 1"0 M KC1
Regenerant: 7.0 N HNO3 eluent: 1'0 M KC1
1
1.45
1.48
1.50
2 3 4 5 6 7 8 9 10
1.38 1.39 1.38 1.34 1.33 1.31 1.30 1.25 1.22
1.44 1.44 1.40 1.41 1.34 1.34 1.32 1.30
1.45 1.45 1.38 1.41 1.34 1.34 1.33 1.28
4. Mutual separation of the alkali metals In order to ascertain that the stannic phosphate exchanger could be used as a typical ion exchanger and to investigate its properties of selectivity, an attempt was made to separate a mixture of alkali metals. Samples Nos. 2, 4 and 7 (dried at room temperature) were used; the same results were obtained from each. The results discussed are those obtained with No. 7. In the determination of elution curves, sodium-22, sodium-24, potassium-42, rubidium-86, and cesium-137 were used as tracers. The activity of the effluent was measured by means of a well-type scintillation counter, and the ~,-ray spectrum and decay curves were measured to ascertain the nuclides in each fraction. 0.01 meq. of each metallic ion was loaded for all experiments unless otherwise described. (a) Separation of carrier-free sodium-22 and cesium-137: As shown in Fig. 3, the separation of carrier-free cesium-137 and sodium-22 by the use of ammonium chloride as an eluent is very sharp except for a slight tailing of cesium fraction. This tailing is, perhaps, due to slow rate of exchange, corresponding to the prolonged elution of hydrogen ion shown in Fig. 1. (b) Separation of sodium, potassium and cesium: As shown in Fig. 4, the separation of each element was carried out by a stepwise elution technique in which various concentrations of ammonium chloride were used as eluents. Sodium, potassium and cesium are eluted with 0.05 M, 0.20 M and 3.0 M ammonium chloride respectively. Though the figure is not presented, the elution curves obtained with ammonium nitrate instead of ammonium chloride were the same as those of Fig. 4. When the load was scaled up to 0-1 meq., slight overlapping of each fraction occurred and separation was not complete. In this case, sodium was eluted even by washing with water, so that two sodium peaks appeared. The extremely low peak of potassium fraction is attributable to the low specific activity of potassium-42. (Fig. 5). (c) Separation of potassium and rubidium: Potassium from rubidium could not be separated by ammonium chloride because
Studies on the syntheticinorganicion exchangers--I
- ~ - 0 M NH4CL
0.I M NH4C'C
x .c
2251
Na zz
Cs137
'E (..3
I0
ZO
~0
Effluent,
FIG.
40
50
60
rnL
3.--Separation of carrier-free Na" and CsTM
of the overlapping of two peaks. To make this separation possible, the ammonium chloride solution containing organic solvent which is miscible with water, such as acetone or alcohol, was used as an eluent. As shown in Fig. 6, potassium is selectively eluted with 0"4 M ammonium chloride solution containing 12.5 per cent of alcohol, and then rubidium is quantitatively eluted with 3.0 M ammonium chloride. In the case of acetone, also, a similar result is obtained. (d) Separation of sodium, potassium, rubidium and cesium: Referring to above results, the mutual separation of the four alkali metals was made by a method similar to that illustrated by Fig. 4 except for the use of 0.2 M ammonium chloride. In its place, a 12"5~o alcohol-0.4 M ammonium chloride solution was used. But the fraction of potassium was contaminated with rubidium, presumably due to the rapid development of the latter during the elution of sodium. In addition, the cesium fraction piled up on the tail of rubidium fraction. Although the complete separation of the alkali metals is impossible by this method, their separation except for rubidium is easy, and it is concluded that this exchanger has an excellent selectivity.
2252
YASUSHI INOUE
0
i LV r~ L~ 20
40 Effluent,
60
80
mL
FIG. 4.--Separation of Na, K and Cs
0"2M NH4~L 3'0M NH4CL 1.0
Na
K
Cs
x c
•3
0.5
0
4O
20 Effluent,
6o
mk
FIG. 5.--Separation of Na, K and Cs (high load)
Studies on the synthetic inorganic ion exchangers--I
r
-L1
.c:
0
i
12.5%atcoho~ 0.4M NH4CL
2253
~
3.0 M NH4CL
Rb
I
I
20
40 mL
Effluent,
60
FIG. 6.--Separation of K and Rb
CONCLUSION As mentioned above, stannic phosphate was found to be a promising cation exchanger. Studies of other fundamental properties, such as structure, exchange rate, separation ability for various cations, and stability against radiation and heat treatment, will continue. These results will be reported in the near future.
Acknowledgment The author wishes to thank Dr. H. GOTOand Dr. S. SuzuKi of Tohoku University for their guidance, discussion and continuous encouragement.
STUDIES ON THE SYNTHETIC I N O R G A N I C ION EXCHANGERS--I S Y N T H E S I S OF S T A N N I C P H O S P H A T E A N D SOME OF ITS P R O P E R T I E S YASUSHI INOUE The Research Institute for Iron, Steel and Other Metals, Tohoku University, 75, Katahira-cho, Sendai, Japan (Received 26 July 1962; in revised form 18 March 1963)
Abstract--For the purpose of developing ion exchangers which are expected to have radiation stability, the author has studied the synthetic inorganic ion exchangers. First, stannic phosphate was prepared by various methods, and its properties, such as composition, exchange capacity, and chemical stability, were investigated in relation to the methods of synthesis. The content of phosphate increases with increase of the mixing ratio of PO~S-/Sn4+ up to a value 2 and then becomes constant at the maximum phosphate to stannic ratio of ~ of the stoichiometric value. Exchange capacity depends on the phosphate content of the exchanger, and it is assumed that fixed ion originates from phosphate group. The exchanger, which has maximum phosphate to stannic ratio, has maximum exchange capacity of 1.3-1.5 meq/g. This exchanger is unstable in concentrated hydrochloric acid and alkali even in dilute solution, but fairly stable in nitric acid as zirconium phosphate. Attempts were made on mutual separation of alkali metals for the examination of the selectivity and availability for the column operation of the exchanger. By using various concentration of ammonium chloride as an eluent, complete separation of sodium, potassium and cesium from each other was achieved with relatively small column. INTRODUCTION ION EXCHANGEprocesses are widely used in nuclear engineering, such as for purification of nuclear fuels, fuel reprocessing, waste disposal, enrichment and purification of useful radioactive isotopes because of the selectivity and completeness of the separation that may be obtained. In addition, the ease of operation combined with the application of remote-handling methods renders such procedures peculiarly suitable to operations in the nuclear energy field. Although the ion exchange resins commercially available at present have excellent ion exchange properties, there are two serious limitations for use in the atomic energy field. One is that ion exchange resins suffer changes of characteristics, such as capacity, selectivity and exchange rate, by the irradiation of the high dose of ionized radiation, m Another is that this material breaks down in water at high temperature. Because of this decomposition, the high-pressure cooling water of pressurized watercooled reactors, if to be purified by ion exchange, must be cooled to normal temperatures before purification and then released, which is wasteful both in power and space. Most inorganic adsorbents are expected to have radiation stability, and some of these m a y also possess high-temperature stability. It has been known for a long time that m a n y inorganic substances exhibit ion exchange properties, but the use of these [1}G. T.
fATHERS,
United Nations Conference on the Peaceful Uses of Atomic Energy 7, 490 (1955).
2241
2242
YASUSHIINOUE
substances is restricted because the exchange capacity is lower than that o f ion exchange resins and such substances are useful only in a limited range o f pH. The production o f a chemically stable inorganic ion exchanger would enable ion exchange process to be used in an intense radiation field. In addition, it is conceivable that in some cases a special peculiarity in comparison with the properties o f organic resins m a y lead to improvements even under normal operating conditions. Thus, m u c h attention has been given to the development o f synthetic, inorganic ion exchangers during the last decade. These materials are classified into two groups: first, acid salts having cation exchange properties such as zirconium, (z-13) titanium,(a) tin, (e) and antimony (2s) phosphate, zirconium tungstate/4) a m m o n i u m and thallium molybdophosphate, (~22) zirconium arsenate, c~7) a m m o n i u m phosphotungstate, (~9) tungsten oxide, (~) and zirconium molybdate; t27) and secondly, hydrated oxides having cation and anion exchange properties, such as zirconium, (2'4"~'2~'~7) thorium, c~z'27) titanium,(23) tin,t 4.~) chromium,(4) niobium,(4) tantalum, t4) vanadium, ~4) uranium ~4) and aluminium t~) oxide. O f these, zirconium phosphate and a m m o n i u m m o l y b d o phosphate, belonging to the first group, are the only ones that have been investigated in detail. I n order to develop a radiation-resistant ion exchanger, the a u t h o r plans to investigate the synthetic inorganic ion exchangers systematically. I n this paper, studies o f the preparation o f stannic phosphate in a granular f o r m adequate for the column operation are described. (2) C. B. AMPHLETr,Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, A/Conf. 15, 271 (1958). (8) I. J. GAL and O. S. GAL, ibid. 15, 468 (1958). (~) K. A. KRAOS, H. O. PHILUPS,T. A. CARLSONand J. S. JOHNSON,ibid. 15, 1832 (1958). (~) C. B. AMprmerr, L. A. McDoNALDand M. J. 1LEDMAN,J. lnorg. NucL Chem. 6, 220 (1958). (e) E. MeRz, Z. Electrochem. 63, 288 (1959). (7) E. M. LARSeNand D. R. VISSERS,J. Phys. Chem. 64, 1732 (1960). (a) L. BAeTSLI~and J. PELSMAEKERS,J. Inorg. Nucl. Chem. 21, 124 (1961). (g) L. BAETSLI~and D. HuYS, ibid. 21, 133 (1961). (10)K. A. KRAUSand H. O. PHILLIPS,dr. Amer. Chem. Soc. 78, 694 (1956). m) C. B. AMPI-ILETT,L. A. McDoNALD, J. S. BURGERSand J. C. MAYNARD,J. Inorg. NucL Chem. 10, 69 (1959). (t2) C. B. AMPHLeTT,L. A. MCDONALDand M. J. REDMAN,Chem. & Industr. 1314 (1956). (is) H. O. PHILLIPS,F. NELSONand K. A. KRAUS,AEC 14 ORNL, 2159, p. 37 (1956). (t~) O. MraER and W. D. TREADWELL,Heir. Chim. Acta 34, 155 (1951). ~ls) H. BUCHWALDand W. P. THISa'LEraWAITE,J. Inorg. Nucl. Chem. 5, 341 (1958). tie) j. VANSMIT,J. J. JACOBSand W. RoBs, ibid. 12, 95 (1959). (17)j. VANSMIT,W. ROaB and J. J. JAcoas, ibid. 12, 104 (1959). ~ls) j. VANSMIT,W. ROBS and J. J. JACOBS,Nucleonics 17, No. (9) 116 (1959). (ts) j. KRTILand V. KO~RIN,3". lnorg. NucL Chem. 12, 367 (1960). (20)j. VANSUIT, Nature Lond. 181, 1530 (1958). ~21)T. HARA,BuL Chem. Soc. Jap. 31, 635 (1958). (22)H. BUCHWALDand W. P. THISTLETHWAITE,J. Inorg. Nucl. Chem. 7, 292 (1958). tzs) C. B. AMPI-ILe'rr,L. A. McDONALDand M. J. REDMAN,ibid., 6, 236 (1958). (~4)K. A. KRAUS,T. A. CARLSONand J. S. JOHNSON,Nature Lond. 177, 1128 (1956). t25) K. A. KRAUSand H. O. PHILLIPS,J. Amer. Chem. Soc. 78, 249 (1956). t26)C. B. AMPHLETT,L. A. McDONALDand M. J. REDMAN,Chem. & Industr., p. 365 (1957). (27)T. A. CARLSON,D. J. COOMBE,J. S. JOHNSON,K. A. KRAUSand H. O. PHILLIPS,AEC 14, ORNL 2159, p. 40 (1956). (28)T. ITO and M. ABe, BuL Chem. Soc. Jap. 34, 1736 (1961).
Studies on the synthetic inorganic ion exchangers--I
2243
Stannic p h o s p h a t e was first p r e p a r e d b y MERZ, (6) a n d he confirmed t h a t this m a t e r i a l h a d a l m o s t the same i o n exchange p r o p e r t i e s as z i r c o n i u m p h o s p h a t e . H e was able to s e p a r a t e the rare earths, h a f n i u m a n d zirconium, a n d the l i t h i u m isotopes (lithium-6 a n d lithium-7), a n d to ascertain t h a t this m a t e r i a l was useful as a c a t i o n exchanger. Because the p r o d u c t by MERZ'S m e t h o d ( p r e c i p i t a t i o n o f the stannic p h o s p h a t e b y the dissolution o f metallic tin granules in ca. 40 p e r cent nitric acid c o n t a i n i n g desired a m o u n t o f p h o s p h a t e ion) a n d s t a n n o u s p h o s p h a t e are fine p o w d e r s , they are u n s u i t a b l e for c o l u m n o p e r a t i o n . T h e a u t h o r was able to p r e p a r e stannic p h o s p h a t e in g r a n u l a r f o r m a n d to investigate the c o m p o s i t i o n , exchange capacity, a n d chemical stability. EXPERIMENTS
1. Preparation of stannic phosphate The general procedures for the preparation of stannic phosphate are as follows, except for sample No. 24, which was prepared by MERZ'S method. Aqueous phosphate and stannic solutions were prepared by the dissolution of sodium phosphate and stannic chloride, respectively. Acidities of these solutions were controlled by the addition of sodium hydroxide. Concentrations were adjusted to obtain a desired mixing ratio of phosphate to stannic ion. Any turbidity in the solution, if present, was removed by filtration through fine filter paper before use. The conditions of preparation are summarized in Table 1. Into a definite volume of the phosphate solution, a definite volume of stannic solution was poured rapidly, with vigorous stirring, at a definite temperature (usually 25°C). The mixed solution, the volume of which is given in column 5 of Table 1, stood for several hours at constant temperature. The precipitate was washed several times with distilled water by decantation until the pH of the washings remained constant. The precipitate was filtered rapidly through filter paper under suction, washed once or twice with distilled water, removed from the funnel, dried in air at room temperature or at ll0°C until it shrank, and immersed in water to break down the gel. If the gel did not break down to the desired particle size, it was ground in a porcelain mortar and sieved to obtain fractions of uniform particle size. To convert the exchanger completely to the hydrogen form, the phosphate was immersed in about 1 N hydrochloric acid for 2 or 3 days with intermittent shaking, sometimes renewing the solution in the course of immersion, and dried at room temperature.
2. Method of the column operation The column was prepared as follows. To 2 g of dried exchanger in hydrogen form (100-200 mesh) was added sufficient distilled water to allow thorough mixing to expel adhering air bubbles. After mixing, the slurry was poured into glass column, 7 mm i.d., with glass wool as a column support. The column, thus prepared, had the height of 20-50 mm, dependent on the density of the exchanger. The flow rate of influent was kept nearly constant, 10-12 drops/min, corresponding to 0-5-0.6 ml]min.
3. Determination of the exchange capacity Generally, the total exchange capacity of a weakly acidic cation exchanger must be determined with an alkaline solution. The stannic phosphate exchanger, however, is unstable in alkaline solution. Therefore, a measure of exchange capacity was taken as the amount of hydrogen ion liberated by a neutral salt, such as potassium chloride, was used as an eluent. Exchange capacity also depends on the concentration of the eluent. To determine the most suitable concentration for use, the elution curves of the hydrogen ion were found for various concentrations of the eluent. The hydrogen ion eluted from the column was titrimetrically determined by using methyl orange as an indicator. The results, obtained with the use of 0-1 M and 1.0 M potassium chloride solution, are shown in Fig. 1. Apparently, the exchange proceeds rapidly in the beginning, but the elution continues for a long time. This phenomenon prevented an accurate determination of the exchange capacity, but the amount of hydrogen ion eluted after 100 ml of the eluent was so little that the exchange capacity, as an approximate value, was calculated from the amount of hydrogen ion liberated by 100 ml. The exchange
0"15
0"15
0"15
0"3
0"3
0-3
0'3
0"3
0"3
0"3
0"3
0"3
0-3
0-3
0"3
0"3
0"3
0"3
17
18
191
2O
23
22
4
5
2
3,
6
7
7'
0"6
0'6
0'3
0"3
0-3
0"3
0"15
0"15
0-15
0'15
1"2
1-2
I "5
1"2
1"2
0"9
0"6
1"11
1"0
0"91
0"6
0"6
0-45
0"15
(M)
(M)
16
NaOH
HAP04
Sn(tV) (M)
No.
Amount of reagent in mixed solution
600
600
600
600
600
600
1800
630
620
600
600
600
600
650
(ml)
Volume of mixed solution
boil
31
25
25
25
25
25
21
21
23
26
26
27
25
(°C)
Temperature at the time of precipitation
R* 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R 110 R
(°C)
Drying temperature
1'06
1"26
0"576
0-902
0'928
1 "03
0"916
0"55
0"55
0"55
0"58
0"56
0"65
0'60
po~3-/Sn4+
TABLE 1 . - - M E T H O D S OF PREPARATION, COMPOSITION AND EXCHANGE CAPACITY
0"75 0"68 0"65 0'69 0"70 0"70 0"74 0"70 0"78 0"78 0"77 0-77 0"78 0"78 0"98 0"96 1"17 1-11 1"18 1"19 1"21 1"21 0-98 1"03 1"45 1 '34 1"56
Hydrogen liberation capacity (meq/g)
Hydrogen
0"65 0"65 0'60 0'66 0"68 0"64 0'70 0"63 0"70 0"66 0"64 0"61 0"70 0"65 0'92 0"88 1"10 1"05 1"11 1'09 1"04 1"08 0"70 0-83 1"32 1"06 1'24
absorption capacity (me,q/g)
O
z
,.<
b~ .,~
~- o ,.t
t~N" ~
.o
.o
o
.o
o.
.o
.o
o
.~
&
&
&
&
&
&
&
~
•
.
,~
.
=
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8 0
g.
~Z
0
g.
-]
0" 0
N
g,
0
.o
o
~.
0
~.
0
~..o
0
~
0 c,Z,
glrZZ
I--SaO~ueqoxo uo! o!ue~aou! o!laqlu,(s aql uo sa!pn~s
2246
YASUSHI INOUE I-0
-i
0"8
,E
I
0-6 _1
I I + I -I'0-4 _1 I
g
i=
°'2~._r._ 0
20
40
Effluenf,
I
I
I
60
80
I00
mL
FiG. 1.--Elution curve of hydrogen ion Straight line: Eluent: 0.1 M KC1 Dotted line: Eluent: 1.0 M KC1 capacity obtained by this method is represented on Fig. 2 as a function of the concentration of the eluent. As a constant value was obtained for concentrations of potassium chloride solution greater than 1 M, the exchange capacity was determined by the use of 1 M potassium chloride solution. The capacity, in this meaning, is tabulated in Table 1, next-to-last column. The exchange capacity based on the amount of hydrogen ion taken up when about 0.1 N hydrochloric acid was passed through the potassium form of the exchanger, is also tabulated in the last column of Table 1.
4. Chemicalanalysis of the exchanger After the exchangers were dissolved in concentrated hydrochloric acid, stannic oxide was determined by the usual iodometric titration, and phosphorus pentoxide by ammonium phosphomolybdate precipitation, followed by the neutralization titration.
t.~ 5 EEl.4
if
0,,
"~ 1.3
o,,9 8,
1.2
1.0
I
I
I
2 Concentro'lion
I
5 of KCL,
I
4 M
FIG. 2.--Exchange capacity as a function of the concentration of eluent.
Studies on the synthetic inorganic ion exchangers--I
2247
5. Chemicalstability (a) Stability against water and nitric acid: After being washed with boiling water to avoid the influence of phosphate and stannic ion which might be occluded in the exchanger due to the incompleteness of the washing, the exchangers were tested by the following procedure. Five hundred milligrams of the exchanger (in hydrogen form, 100-200 mesh) were refluxed with 50 ml of 7 N nitric acid or distilled water for an hour. After cooling, the supernatant liquid was filtered through fine filter paper. If the filtrate was turbid, the filtration was repeated till the filtrate became clear. The phosphate and stannic ions in the filtrate were determined by the usual spectro-photometric methods, viz., molybdenum blue and phenylfinorone method, ~9~respectively. (b) Change of exchange capacity in the course of absorption-regeneration cycles" In conformity with the procedure for column operation mentioned above, the following cycles were repeated about ten times. In this experiment, 1 g of sample No. 7 was used. The column was regenerated by 50 ml of acid (1'2 N hydrochloric acid, 1"0 N nitric acid, or 7.0 N nitric acid). After the column was washed with 50 ml of water, hydrogen ion was eluted with 100 ml of 1.0 M or 0.5 M potassium chloride solution. Again the column was washed with 15 ml of water followed by acid regeneration and so on. RESULTS
1. Chemical composition of the exchangers The molar ratio of PO~-/Sn 4+ of the exchanger prepared by various methods is presented in Table 1. With few exceptions, the ratio is largely dependent upon the mixing ratio of the two components, that is, the ratio in the exchanger increases as the mixing ratio increases up to a value 2, and with further increase remains constant. For each mixing ratio, the acidity of the solution was varied over the range in which stannic phosphate could be precipitated. The ratio of phosphate to stannic ion is low in the limiting condition for precipitation, especially in the alkaline side, in which the yield is extremely low. The maximum ratio of PO4a-/Sn 4+ in the exchanger prepared by various methods is about 1.25, which corresponded to the stoichiometric value of PO43-/Sn4+ = ~. The external appearance also changed with the difference in the mixing ratio, namely, when the mixing ratio was less than 2, the product was white, opaque, and brittle like chalk. When the mixing ratio was above 2, the product was a white, semi-transparent or transparent solid, and hard as compared with the above opaque materials. The transparent solid breaks down easily to small particles with cracking and slight evolution of heat when immersed in water. X-ray or electron diffraction studies are being carried out to determine the structure of the product. Results will be reported in the near future.
2. Exchange capacity of the exchanger Two kinds of exchange capacity have been defined and experimental values are reported in Table 1. The "Hydrogen absorption capacity" is slightly lower than the "Hydrogen liberation capacity," but both capacities appear to be proportional, so that these can be used as a measure of exchange capacity. Exchange capacity is strikingly dependent upon the ratio of PO43-/Sn4+ in the product; namely, the greater the ratio, the higher the capacity. The same trend is shown between the mixing ratio of two components and the exchange capacity up to a value of 2 for the former quantity. ~zg~M. IsmsAsm, T. SHIOEMATSU,Y. YAMAMOTOand Y. INOUE,Japan Analyst 7, 473 (1958).
2248
YASUSHI INOUE
The exchange capacity of stannic phosphate dried at 110°C is slightly lower than that of stannic phosphate dried at room temperature, but the difference is not so large. For the purpose of estimating the reproducibility, the capacities of the four different batches of sample No. 7 were determined. As shown in Table 2, the capacities of three of the four batches were in fair agreement with one another, but the fourth had a higher capacity. Irregularities in behaviour are attributable to fluctuations. The capacity of sample No. 24 prepared by MERZ'S method was not determined because it could not be used in a column. TABLE 2.~REPRODU¢ImLITYOr TI-1ERESULTS Exchange capacity of exchanger No. 7 Batch No.
1 2 3 4
Hydrogen liberation capacity (rneq/.g)
Hydrogen absorption capacity (meq/g)
1.62 1"42 1"42 1"35
1.43 1"26 1'26 1"32
For the purpose of comparison, the capacity of a commercial exchanger of zirconium phosphate (Ionite C) and that of a weakly acidic ion exchange resin (Amberlite CG 50) were determined by the same method; they had capacities of 1.60 and 0.24, respectively. The measured capacity of stannic phosphate was slightly lower than that of Ionite C, but higher than that of Amberlite CG 50. Consequently, stannic phosphate possesses an exchange group which is more acidic than the carboxylic group.
3. Chemical stability Stannic phosphate is stable in dilute hydrochloric acid (less than 1 N), but it is solublein6 N hydrochloric acid due to the formation ofachlorocomplex. 13°~ Although various batches of the exchanger did not always behave identically, they are fairly unstable in basic solution; that is, they dissolve or peptize even in 0.1 N sodium hydroxide solution. However, the product is stable in nitric acid. The material can be used in this medium without suffering the great deterioration in the ion exchange properties or undergoing a breaking down of the particles. (a) Stability against water and nitric acid: In order to evaluate the stability against nitric acid and water in detail, the amount of phosphorus pentoxide and stannic oxide dissolved in these media was determined by means of the method described in section 3 of the experimental part. The tests, except for sample No. 7, were conducted only for the samples dried at room temperature. As shown in Table 3, the phosphate released to water varied between 2 and 7 mg, but the amount of stannic oxide was 30/~g or less. On the contrary, the phosphate ~30~M. IsmBASm,Y. YA~AMOTOand Y. INOUE,Bull. Inst. Chem. Res., Kyoto Univ. 37, 38 (1959).
Studies on the synthetic inorganic ion exchangers--I
2249
TABLE 3.--DISSOLUTIONOF PHOSPHORUSPENTOXIDEAND STANNICOXIDE INTO WATERAND 7 N NITRICACID Amount dissolved in H20 (mg)
Amount dissolved in 7 N HNO3 (mg)
Sample No. P20s 2 3 4 5 6 7 7* 7' 8 9 10 11 12 13 14 16 17 18 19 20 22 23 Ionite C
2.9 6"5 1-8 3-2 4.6 4.2 5"6 7.9 5"4 4"0 2.2 4'4 7'0 5"8 6-2 2'5 4'2 2'8 2'4 2"9 4"3 2.4 3.9
Sn or Zr 0.03 0.01 0.03
0-01
0.03
0.00 0.00
P20~ 3.4 3.4 1-7 1'8 2.9 3.4 2-5 2-1 4-0 2.0 3.0 2.7 3.1 2.4 2-6 2.3 1'5 1.4 3"3 0"4 2.5 1'9 4-7
Sn or Zr 1"5 1'3 1-7
2.5
1"8
1'3 0.02
* Samples dried at llO°C. dissolved in 7 N nitric acid to an extent slightly less than in water, but stannic oxide dissolved only to the extent of a few milligrams. F r o m the fact that even in nitric acid the molar ratio of dissolved phosphate to dissolved stannic ion is 2- or 6-fold higher than that of original material, it is apparent that phosphate ion selectively dissolves in these media. Although the amount of dissolved phosphate rises with increasing phosphate content, the distinct difference in chemical stability is n o t observed among exchangers prepared by various methods, so that by such a criterion the method of synthesis is not so important. Moreover, it is shown from the comparison of the results concerning samples No. 7 and No. 7* that the heat treatment a t 110°C does not stabilize the exchanger against these media. For the purpose of comparison, the chemical stability of Ionite C was tested in the same way. As may be seen from Table 3, phosphate ion in Ionite C has almost the same solubility as that in stannic phosphate, but the solubility of zirconium ion is less than that of stannic ion. Consequently, stannic phosphate is slightly unstable compared with Ionite C. (b) Change of exchange capacity in the course of absorption-regeneration cycles: The results obtained with sample No. 7 by the method described above, are presented in Table 4. The capacity gradually decreased by the repetition of the cycle regardless of the reagent used. 14
2250
YASUSHIINOUE
TABLE4. Tim CHANGEOF CAPACITYIN THE COURSEOF ABSORPTION-REGENERATIONCYCLES (HYDROGENLIBERATIONCAPACITY) 1
2
3
Cycle Regenerant: 1'2 N HCI eluent: 0.5 M KCI
Regenerant: 1.0 N HNO3 eluent: 1"0 M KC1
Regenerant: 7.0 N HNO3 eluent: 1'0 M KC1
1
1.45
1.48
1.50
2 3 4 5 6 7 8 9 10
1.38 1.39 1.38 1.34 1.33 1.31 1.30 1.25 1.22
1.44 1.44 1.40 1.41 1.34 1.34 1.32 1.30
1.45 1.45 1.38 1.41 1.34 1.34 1.33 1.28
4. Mutual separation of the alkali metals In order to ascertain that the stannic phosphate exchanger could be used as a typical ion exchanger and to investigate its properties of selectivity, an attempt was made to separate a mixture of alkali metals. Samples Nos. 2, 4 and 7 (dried at room temperature) were used; the same results were obtained from each. The results discussed are those obtained with No. 7. In the determination of elution curves, sodium-22, sodium-24, potassium-42, rubidium-86, and cesium-137 were used as tracers. The activity of the effluent was measured by means of a well-type scintillation counter, and the ~,-ray spectrum and decay curves were measured to ascertain the nuclides in each fraction. 0.01 meq. of each metallic ion was loaded for all experiments unless otherwise described. (a) Separation of carrier-free sodium-22 and cesium-137: As shown in Fig. 3, the separation of carrier-free cesium-137 and sodium-22 by the use of ammonium chloride as an eluent is very sharp except for a slight tailing of cesium fraction. This tailing is, perhaps, due to slow rate of exchange, corresponding to the prolonged elution of hydrogen ion shown in Fig. 1. (b) Separation of sodium, potassium and cesium: As shown in Fig. 4, the separation of each element was carried out by a stepwise elution technique in which various concentrations of ammonium chloride were used as eluents. Sodium, potassium and cesium are eluted with 0.05 M, 0.20 M and 3.0 M ammonium chloride respectively. Though the figure is not presented, the elution curves obtained with ammonium nitrate instead of ammonium chloride were the same as those of Fig. 4. When the load was scaled up to 0-1 meq., slight overlapping of each fraction occurred and separation was not complete. In this case, sodium was eluted even by washing with water, so that two sodium peaks appeared. The extremely low peak of potassium fraction is attributable to the low specific activity of potassium-42. (Fig. 5). (c) Separation of potassium and rubidium: Potassium from rubidium could not be separated by ammonium chloride because
Studies on the syntheticinorganicion exchangers--I
- ~ - 0 M NH4CL
0.I M NH4C'C
x .c
2251
Na zz
Cs137
'E (..3
I0
ZO
~0
Effluent,
FIG.
40
50
60
rnL
3.--Separation of carrier-free Na" and CsTM
of the overlapping of two peaks. To make this separation possible, the ammonium chloride solution containing organic solvent which is miscible with water, such as acetone or alcohol, was used as an eluent. As shown in Fig. 6, potassium is selectively eluted with 0"4 M ammonium chloride solution containing 12.5 per cent of alcohol, and then rubidium is quantitatively eluted with 3.0 M ammonium chloride. In the case of acetone, also, a similar result is obtained. (d) Separation of sodium, potassium, rubidium and cesium: Referring to above results, the mutual separation of the four alkali metals was made by a method similar to that illustrated by Fig. 4 except for the use of 0.2 M ammonium chloride. In its place, a 12"5~o alcohol-0.4 M ammonium chloride solution was used. But the fraction of potassium was contaminated with rubidium, presumably due to the rapid development of the latter during the elution of sodium. In addition, the cesium fraction piled up on the tail of rubidium fraction. Although the complete separation of the alkali metals is impossible by this method, their separation except for rubidium is easy, and it is concluded that this exchanger has an excellent selectivity.
2252
YASUSHI INOUE
0
i LV r~ L~ 20
40 Effluent,
60
80
mL
FIG. 4.--Separation of Na, K and Cs
0"2M NH4~L 3'0M NH4CL 1.0
Na
K
Cs
x c
•3
0.5
0
4O
20 Effluent,
6o
mk
FIG. 5.--Separation of Na, K and Cs (high load)
Studies on the synthetic inorganic ion exchangers--I
r
-L1
.c:
0
i
12.5%atcoho~ 0.4M NH4CL
2253
~
3.0 M NH4CL
Rb
I
I
20
40 mL
Effluent,
60
FIG. 6.--Separation of K and Rb
CONCLUSION As mentioned above, stannic phosphate was found to be a promising cation exchanger. Studies of other fundamental properties, such as structure, exchange rate, separation ability for various cations, and stability against radiation and heat treatment, will continue. These results will be reported in the near future.
Acknowledgment The author wishes to thank Dr. H. GOTOand Dr. S. SuzuKi of Tohoku University for their guidance, discussion and continuous encouragement.