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Exchange properties of ammonium salts of 12-heteropolyacids—IV
J. Inorg. Nucl. Chem., 1962, Vol. 24, pp: 1139 to 1144, Pergamon Press Ltd. Printed in 17.n_wland.
EXCHANGE PROPERTIES OF A M M O N I U M SALTS OF 12-HETEROPOLYACIDS--IV Cs E X C H A N G E O N A M M O N I U M P H O S P H O T U N G S T A T E A N D PHOSPHOMOLYBDATE J. KRTIL Institute of Nuclear Research, Czechoslovak Academy of Sciences, l~e~ u Prahy, Czechoslovakia (Received 15 January 1962) Abstract--Functional relations have been established describing caesium sorption on ammonium phosphotungstate (NH4PW) and ammonium phosphomolybdate (NH4PMo) as a function of the concentrations of nitric acid, ammonium nitrate and caesium in the solution, and on the amount of sorbent. THE use of a m m o n i u m salts of heteropolyacids as selective ion exchangers for the isolation of 137 Cs from fission product solutions requires a knowledge of the behaviour of these substances under the conditions obtaining in the reprocessing of nuclear fuel, viz. the strongly acid medium (usually nitric acid) and the presence of high concentrations of sodium and a m m o n i u m salts. A study of Cs extraction under these conditions and of the mathematical representation of the sorption phenomena is therefore necessary. The aim of this work was to study the exchange of Cs in micro and macro-amounts on a m m o n i u m phosphotungstate and phosphomolybdate in the presence of nitric acid and a m m o n i u m nitrate, to find how these factors influence the exchange, and to establish the rules governing the exchange. There is little published information concerning the mechanism of sorption of alkali metals on a m m o n i u m salts of heteropolyacids. SMIT et al.tl), measured the distribution coefficients of alkali metals in 0.I NNH4NO3 solutions on a m m o n i u m salts of various heteropolyacids, and the validity of the mass-action law has been checked for the sorption of Rb on NH4PMo and of Cs on NH4PWt2,3). EXPERIMENTAL Experiments were carried out at room temperature under static conditions with acid ammonium salts of phosphotungstic and phosphomolybdic acids having the compositions (NH4)2.2H0.8[PW12040]'15 H20 and (NH4)2.38 H0.62[PM012040]-9-3 H20. Preparation and analysis of these salts are described elsewhere.t4) The ammonium salts (0.1 g) were weighed into 2.5 ml glass tubes and 2 ml of ammonium nitrate solution containing micro-amounts of 137Cs (10 -7 mole/L) was added. In other experiments nitric acid of varying concentrations, or a mixture of nitric acid and ammonium nitrate, containing a micro-amount of 137Cs as tracer was added to the weighed amount. After shaking until equilibrium was attained the solution was centrifuged and the distribution of Cs between the phases was determined radiometrically. The distribution coefficient for Cs, defined as activity of Cs in 0.1 g of NH4PW activity of Cs in 2 ml of solution at equilibrium = Ka, was then calculated. Analytical grade chemicals were used throughout. (1) j. VAN R. SMIT, J. J. JACOBSand W. ROBB, J. Inorg. Nud. Chem. 12, 95 (1959). (2) j. VAN R. SMIT, J. J. JACOBSand W. ROBB,J. Inorg. Nucl. Chem. 12, 104 (1959). (3) j. KRTIL, J. Inorg. Nucl. Chem. 19, 298 (1961). 1139
1140
J. KRTm RESULTS
AND
DISCUSSION
The percentage of Cs in the solid will be determined by the equilibrium concentrations of ammonium salts and of hydrogen ions present in the solution, since it has been shown ta,s) that the alkali metal (Cs) is exchanged isomorphously into the lattice partly for ammonium and partly also for the acid hydrogen of the sorbent. It has also been shown that in an acid medium the composition of the sorbentt 4) is changed. The sorption of Cs or another alkali metal on the ammonium salts of heteropolyacids is thus a complex process, and to express it mathematically it is necessary to simplify the system. This may be fulfilled by examining the sorption of micro-amounts of Cs under suitable conditions of ammonium salt concentration and/ or nitric acid concentration and for an amount of sorbent such that Cs is a microcomponent relative to ammonium or acid hydrogen; its distribution between the phases will then obey simple laws. The overall exchange may be regarded as a result of three separate phenomena, viz. exchange of Cs for NH4 and/or for acid hydrogen, and of acid hydrogen for NH4. We may express the individual equilibrium constants as follows
KCslNI_I4 _
[Cs] [NH4] s L
[Cs] [NH4] L
Kc~m
_
KH/ 4
S
[Cs] t'H] s I. [Cs] [H] L
(1)
(2)
S
[NH4] In] L S
--[NH4] [H] s I.
(3), L indicating the solution, and S the precipitate
The value of Knma, is not constant over the range of HNO3 concentration studied (0.1-10 M), varying by as much as one order of magnitudet4) this is due to the fact that only exchangeable ammonium and not total ammonium concentration in the solid phase should be substituted into Equation (3), since changes in the sorbent composition occur at lower acidities. In 3 M and higher concentration of nitric acid the composition of the acid salt is practically constant. An accurate determination of the amount of exchangeable ammonium is rather difficult, but it may be shown from the experiments which were carried out that this process will not be the determining one for obtaining sorption dependences. The exchange of Cs for NH4 at constant acidity was first investigated. Equation (1) may be modified to give log K d = log A -- log [NH4]L,
[Cs] where
s __. Ka
[Cs]
L
and Kc~mH, • [NH4] = A ts C4lj. KR'HLand V. Kotn~M,7. Inorg. Nucl. Chem. 12, 367 (1960). ts) H. BUCI-iWALDand W. P. TmSTLETHWAtTE,J. Inorg. NucL Chem. 5, 341 (1957).
Exchange properties of ammonium salts of 12-heteropolyacids--IV
1141
Thus if (1) is valid we will obtain a linear dependence of log K a on log [NH4]L, and this is confirmed by the results shown in Fig. 1. The slope of curve A equal 1, showing that one Cs ion replaces one ammonium ion. The amount of hydrogen replaced is constant and negligible within the range of NH4 concentrations investigated.
3.0
2.0
I.O
A
0
I -2.0
, -I.O
0 IOg
i t.O
CNH4NO B
Fro. l.--Dependence of the distribution coefficient for Cs on ammonium ion concentration in solution at equilibrium. A, 0.001 M HNO3 B, 5 M HN03 C, 10 M HNO3 0 NH4PMo • NH4PW The distribution coefficient for Cs is higher on NH4PMo than on NH4PW (for equal amounts of sorbent) over the whole range of ammonium salt concentrations investigated, but the corresponding equilibrium constants in 0.001M.HNO3 are 5.5 × 102 for NH4PW and 4.18 × 102 for NH4PMo concentration in moles 1-1) the latter having a lower value in conformity with the solubilities of the caesium and ammonium salts. The difference between the values of K a for NH4PW and NH4PMo disappears with increasing acidity of the solution. The constant KcaNm is, of course, valid only for a certain hydrogen ion concentration and decreases with increasing acidity, as may be seen from the other lines in Fig. 1. The value of A depends on acidity, and if we suppose that the ammonium concentration in the solid phase varies only slightly (if Cs is present in micro-amounts) we can plot log A ( = Kd. [NH4]L) against nitric acid concentration as in Fig. 2. The value of A varies only slightly at HNO3 concentrations less than 3 M, and for [HNOa] > 3 M we obtain a linear dependence of log A on log [HIE. Putting A = f ( H ) we obtain log K d. [NH4] L -- -- log [HIE + log (const). Kd. [NH4] L [H] L = 25 or
1/K,t =
0"04[H]L. [NH4]L
1142
J. KRTU,.
This relation holds well'both for NH4PW and NH4PMo. This empirical dependence may be explained on the basis of the sorbent acting as an ion exchanger. The acid ammonium salt of a heteropolyacid may be regarded as a strong acid ion exchanger, in agreement with the relationship A = f ( H ) . If we
20 ©
..a
zx* 2K~
0.1
I 0-5
] 1.0
I 3
I 6
I 10
CHNO 3
FIO. 2.--Relationship between K d [ N H 4 ] L and nitric acid concentration. treat the sorbent as a mixture of an ammonium salt and a free heteropolyacid, the ammonium salt being completely dissociated and the free heteropolyacid partly dissociated, then the dissociation may be written in the form N H 4 R ~ N H + 4 + R- (a) HR ~ H + + R(b) Assuming complete dissociation of the ammonium salt we may approximate as follows ['NH$] - [R_] For (b) the following expression is valid at equilibrium
[H +3 [R-] K'--
s
s
[HR]
(c)
S
Substituting (e) into (1) where, [NH4]s corresponds'to that fraction of ammonium ion which is exchangeable for acid hydrogen, and supposing Donnan's concept of equilibrium to hold, when the concentration of hydrogen ions in the ion exchanger is proportional to the concentration of H ions in the solution, i.e., [His = k [I-I]L,
[[--I][NH4] C' we obtain KctmH, = Kd
L
[HR]
L
'
S
where C ' - -
k
(4)
K' For low H + concentrations the ratio [H+]s/FHR]s will be constant, and therefore K# will also be constant. For a higher acidity the relation [HR] s = eonst, must be
Exchange properties of ammonium salts of 12-heteropolyacids--IV
1143
valid, as we have obtained the limiting acid salt and then the relationship (4) will be simplified into the form Const. = K d [NHn+]L. [H +] L in agreement with the results obtained. The results for Cs sorption as a function of HNOa concentrations at constant ammonium salt concentration agree with previous data (Fig. 3). A slow decrease in
B
\. ]
3,0
-2.0
\
I
i
I
-b0
0
I
log
CHN%
3.--Dependence of the distribution coefficient for Cs on nitric acid concentration. A, without NH4NO3 B, 0.1 M NH4NO3 C, 1.2 M NH4NO3 O NH4PMo • NI-I,PW Cs sorption with increasing HNO3 concentration changes abruptly near [HNO3] ~ 3 M, beyond which there is a linear variation with slope ~ 1. Thus at acidities > 3 M HNO3 the sorption properties of NHaPW and NH4PMo change, and the acid hydrogen and Cs may replace each other. The variation in sorption properties of NH4PW and NH4PMo could be due to variation in the sorbent structure. A study of the infra-red spectra of samples of NHaPW and NH4PMo which were obtained by shaking the salts with HNO3 in various concentrations to equilibrium showed no change as the concentration of acid was varied. This can be explained by assuming that there is no substantial change in either the heteropolyanion bonding or in the water of crystallization. As was previously shown analytically<4), exchange of NH4 for H occurs in an acid medium, the salt becoming more acid and at an acidity > 3 M HNO3 we obtain the limiting salt (NH4)2HPW. Under our conditions we may also suppose this acid salt to be formed and it should be further enriched with H + or Cs ÷. As our experiments show, Cs + and H + are equivalent. We may conclude from this that the acid hydrogen ions are not equivalent in their bond strength, and undergo substitution by Cs or by another alkali metal in stages. The equilibrium constant Kcs~rt reaches a constant value at > 3 M HNO3; below 3 M HNO3 it is not constant, due to the fact that the content of acid hydrogen in the NH4PW molecule changes considerably. FIG.
1144
J. KRTrL
It is necessary, of course, to know the range of Cs concentrations over which these relationships are valid. It may be seen from the dependence of the distribution
S
I-0
/ ~o 0
-I.0 I I
I 2
E 5 -10g
~ 4
I 5
I 6
I 7
COs
FIQ. 4,~Dependence of distribution coefficient for Cs on the initial Cs
A, 0.43 M NH4NO3
concentration. (sorbent: NH4PW) B, 0.5 M NH4NO3 -¢~- Previous results(3)
+ 5 M HNO3
coefficient Ka(Cs) on the initial concentration of Cs (Fig. 4) that they may be applied up to [Cs +] ---- 10-3 M. Above this concentration Ka falls sharply, since the amount of exchangeable hydrogen and ammonium ions is limited by the capacity of the exchanges.
EXCHANGE PROPERTIES OF A M M O N I U M SALTS OF 12-HETEROPOLYACIDS--IV Cs E X C H A N G E O N A M M O N I U M P H O S P H O T U N G S T A T E A N D PHOSPHOMOLYBDATE J. KRTIL Institute of Nuclear Research, Czechoslovak Academy of Sciences, l~e~ u Prahy, Czechoslovakia (Received 15 January 1962) Abstract--Functional relations have been established describing caesium sorption on ammonium phosphotungstate (NH4PW) and ammonium phosphomolybdate (NH4PMo) as a function of the concentrations of nitric acid, ammonium nitrate and caesium in the solution, and on the amount of sorbent. THE use of a m m o n i u m salts of heteropolyacids as selective ion exchangers for the isolation of 137 Cs from fission product solutions requires a knowledge of the behaviour of these substances under the conditions obtaining in the reprocessing of nuclear fuel, viz. the strongly acid medium (usually nitric acid) and the presence of high concentrations of sodium and a m m o n i u m salts. A study of Cs extraction under these conditions and of the mathematical representation of the sorption phenomena is therefore necessary. The aim of this work was to study the exchange of Cs in micro and macro-amounts on a m m o n i u m phosphotungstate and phosphomolybdate in the presence of nitric acid and a m m o n i u m nitrate, to find how these factors influence the exchange, and to establish the rules governing the exchange. There is little published information concerning the mechanism of sorption of alkali metals on a m m o n i u m salts of heteropolyacids. SMIT et al.tl), measured the distribution coefficients of alkali metals in 0.I NNH4NO3 solutions on a m m o n i u m salts of various heteropolyacids, and the validity of the mass-action law has been checked for the sorption of Rb on NH4PMo and of Cs on NH4PWt2,3). EXPERIMENTAL Experiments were carried out at room temperature under static conditions with acid ammonium salts of phosphotungstic and phosphomolybdic acids having the compositions (NH4)2.2H0.8[PW12040]'15 H20 and (NH4)2.38 H0.62[PM012040]-9-3 H20. Preparation and analysis of these salts are described elsewhere.t4) The ammonium salts (0.1 g) were weighed into 2.5 ml glass tubes and 2 ml of ammonium nitrate solution containing micro-amounts of 137Cs (10 -7 mole/L) was added. In other experiments nitric acid of varying concentrations, or a mixture of nitric acid and ammonium nitrate, containing a micro-amount of 137Cs as tracer was added to the weighed amount. After shaking until equilibrium was attained the solution was centrifuged and the distribution of Cs between the phases was determined radiometrically. The distribution coefficient for Cs, defined as activity of Cs in 0.1 g of NH4PW activity of Cs in 2 ml of solution at equilibrium = Ka, was then calculated. Analytical grade chemicals were used throughout. (1) j. VAN R. SMIT, J. J. JACOBSand W. ROBB, J. Inorg. Nud. Chem. 12, 95 (1959). (2) j. VAN R. SMIT, J. J. JACOBSand W. ROBB,J. Inorg. Nucl. Chem. 12, 104 (1959). (3) j. KRTIL, J. Inorg. Nucl. Chem. 19, 298 (1961). 1139
1140
J. KRTm RESULTS
AND
DISCUSSION
The percentage of Cs in the solid will be determined by the equilibrium concentrations of ammonium salts and of hydrogen ions present in the solution, since it has been shown ta,s) that the alkali metal (Cs) is exchanged isomorphously into the lattice partly for ammonium and partly also for the acid hydrogen of the sorbent. It has also been shown that in an acid medium the composition of the sorbentt 4) is changed. The sorption of Cs or another alkali metal on the ammonium salts of heteropolyacids is thus a complex process, and to express it mathematically it is necessary to simplify the system. This may be fulfilled by examining the sorption of micro-amounts of Cs under suitable conditions of ammonium salt concentration and/ or nitric acid concentration and for an amount of sorbent such that Cs is a microcomponent relative to ammonium or acid hydrogen; its distribution between the phases will then obey simple laws. The overall exchange may be regarded as a result of three separate phenomena, viz. exchange of Cs for NH4 and/or for acid hydrogen, and of acid hydrogen for NH4. We may express the individual equilibrium constants as follows
KCslNI_I4 _
[Cs] [NH4] s L
[Cs] [NH4] L
Kc~m
_
KH/ 4
S
[Cs] t'H] s I. [Cs] [H] L
(1)
(2)
S
[NH4] In] L S
--[NH4] [H] s I.
(3), L indicating the solution, and S the precipitate
The value of Knma, is not constant over the range of HNO3 concentration studied (0.1-10 M), varying by as much as one order of magnitudet4) this is due to the fact that only exchangeable ammonium and not total ammonium concentration in the solid phase should be substituted into Equation (3), since changes in the sorbent composition occur at lower acidities. In 3 M and higher concentration of nitric acid the composition of the acid salt is practically constant. An accurate determination of the amount of exchangeable ammonium is rather difficult, but it may be shown from the experiments which were carried out that this process will not be the determining one for obtaining sorption dependences. The exchange of Cs for NH4 at constant acidity was first investigated. Equation (1) may be modified to give log K d = log A -- log [NH4]L,
[Cs] where
s __. Ka
[Cs]
L
and Kc~mH, • [NH4] = A ts C4lj. KR'HLand V. Kotn~M,7. Inorg. Nucl. Chem. 12, 367 (1960). ts) H. BUCI-iWALDand W. P. TmSTLETHWAtTE,J. Inorg. NucL Chem. 5, 341 (1957).
Exchange properties of ammonium salts of 12-heteropolyacids--IV
1141
Thus if (1) is valid we will obtain a linear dependence of log K a on log [NH4]L, and this is confirmed by the results shown in Fig. 1. The slope of curve A equal 1, showing that one Cs ion replaces one ammonium ion. The amount of hydrogen replaced is constant and negligible within the range of NH4 concentrations investigated.
3.0
2.0
I.O
A
0
I -2.0
, -I.O
0 IOg
i t.O
CNH4NO B
Fro. l.--Dependence of the distribution coefficient for Cs on ammonium ion concentration in solution at equilibrium. A, 0.001 M HNO3 B, 5 M HN03 C, 10 M HNO3 0 NH4PMo • NH4PW The distribution coefficient for Cs is higher on NH4PMo than on NH4PW (for equal amounts of sorbent) over the whole range of ammonium salt concentrations investigated, but the corresponding equilibrium constants in 0.001M.HNO3 are 5.5 × 102 for NH4PW and 4.18 × 102 for NH4PMo concentration in moles 1-1) the latter having a lower value in conformity with the solubilities of the caesium and ammonium salts. The difference between the values of K a for NH4PW and NH4PMo disappears with increasing acidity of the solution. The constant KcaNm is, of course, valid only for a certain hydrogen ion concentration and decreases with increasing acidity, as may be seen from the other lines in Fig. 1. The value of A depends on acidity, and if we suppose that the ammonium concentration in the solid phase varies only slightly (if Cs is present in micro-amounts) we can plot log A ( = Kd. [NH4]L) against nitric acid concentration as in Fig. 2. The value of A varies only slightly at HNO3 concentrations less than 3 M, and for [HNOa] > 3 M we obtain a linear dependence of log A on log [HIE. Putting A = f ( H ) we obtain log K d. [NH4] L -- -- log [HIE + log (const). Kd. [NH4] L [H] L = 25 or
1/K,t =
0"04[H]L. [NH4]L
1142
J. KRTU,.
This relation holds well'both for NH4PW and NH4PMo. This empirical dependence may be explained on the basis of the sorbent acting as an ion exchanger. The acid ammonium salt of a heteropolyacid may be regarded as a strong acid ion exchanger, in agreement with the relationship A = f ( H ) . If we
20 ©
..a
zx* 2K~
0.1
I 0-5
] 1.0
I 3
I 6
I 10
CHNO 3
FIO. 2.--Relationship between K d [ N H 4 ] L and nitric acid concentration. treat the sorbent as a mixture of an ammonium salt and a free heteropolyacid, the ammonium salt being completely dissociated and the free heteropolyacid partly dissociated, then the dissociation may be written in the form N H 4 R ~ N H + 4 + R- (a) HR ~ H + + R(b) Assuming complete dissociation of the ammonium salt we may approximate as follows ['NH$] - [R_] For (b) the following expression is valid at equilibrium
[H +3 [R-] K'--
s
s
[HR]
(c)
S
Substituting (e) into (1) where, [NH4]s corresponds'to that fraction of ammonium ion which is exchangeable for acid hydrogen, and supposing Donnan's concept of equilibrium to hold, when the concentration of hydrogen ions in the ion exchanger is proportional to the concentration of H ions in the solution, i.e., [His = k [I-I]L,
[[--I][NH4] C' we obtain KctmH, = Kd
L
[HR]
L
'
S
where C ' - -
k
(4)
K' For low H + concentrations the ratio [H+]s/FHR]s will be constant, and therefore K# will also be constant. For a higher acidity the relation [HR] s = eonst, must be
Exchange properties of ammonium salts of 12-heteropolyacids--IV
1143
valid, as we have obtained the limiting acid salt and then the relationship (4) will be simplified into the form Const. = K d [NHn+]L. [H +] L in agreement with the results obtained. The results for Cs sorption as a function of HNOa concentrations at constant ammonium salt concentration agree with previous data (Fig. 3). A slow decrease in
B
\. ]
3,0
-2.0
\
I
i
I
-b0
0
I
log
CHN%
3.--Dependence of the distribution coefficient for Cs on nitric acid concentration. A, without NH4NO3 B, 0.1 M NH4NO3 C, 1.2 M NH4NO3 O NH4PMo • NI-I,PW Cs sorption with increasing HNO3 concentration changes abruptly near [HNO3] ~ 3 M, beyond which there is a linear variation with slope ~ 1. Thus at acidities > 3 M HNO3 the sorption properties of NHaPW and NH4PMo change, and the acid hydrogen and Cs may replace each other. The variation in sorption properties of NH4PW and NH4PMo could be due to variation in the sorbent structure. A study of the infra-red spectra of samples of NHaPW and NH4PMo which were obtained by shaking the salts with HNO3 in various concentrations to equilibrium showed no change as the concentration of acid was varied. This can be explained by assuming that there is no substantial change in either the heteropolyanion bonding or in the water of crystallization. As was previously shown analytically<4), exchange of NH4 for H occurs in an acid medium, the salt becoming more acid and at an acidity > 3 M HNO3 we obtain the limiting salt (NH4)2HPW. Under our conditions we may also suppose this acid salt to be formed and it should be further enriched with H + or Cs ÷. As our experiments show, Cs + and H + are equivalent. We may conclude from this that the acid hydrogen ions are not equivalent in their bond strength, and undergo substitution by Cs or by another alkali metal in stages. The equilibrium constant Kcs~rt reaches a constant value at > 3 M HNO3; below 3 M HNO3 it is not constant, due to the fact that the content of acid hydrogen in the NH4PW molecule changes considerably. FIG.
1144
J. KRTrL
It is necessary, of course, to know the range of Cs concentrations over which these relationships are valid. It may be seen from the dependence of the distribution
S
I-0
/ ~o 0
-I.0 I I
I 2
E 5 -10g
~ 4
I 5
I 6
I 7
COs
FIQ. 4,~Dependence of distribution coefficient for Cs on the initial Cs
A, 0.43 M NH4NO3
concentration. (sorbent: NH4PW) B, 0.5 M NH4NO3 -¢~- Previous results(3)
+ 5 M HNO3
coefficient Ka(Cs) on the initial concentration of Cs (Fig. 4) that they may be applied up to [Cs +] ---- 10-3 M. Above this concentration Ka falls sharply, since the amount of exchangeable hydrogen and ammonium ions is limited by the capacity of the exchanges.