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BEHAVIOUR OF PALLADIUM IN THE PUREX PROCESS
BEHAVIOUR OF PALLADIUM IN THE PUREX PROCESS
E. VIALARD and M. GERMAIN
Palladium is ah abundant fission product but a minor radioactive contaminant of nuclear fuels. Its peculiar chemical behaviour is troublesome because it reacts with most of the reagents used in partition and forms insoluble compounds or cruds. Fortunately palladium extraction by 30 % TBP-dodecane is moderate and high DF are expected as long as the diluent is not excessively degraded. However, y irradiated diluents display strong complexing power for palladium and retention of this element is observed.
INTRODUCTION A number of fission products are minor radioactive contaminants of irradiated nuclear fuels and have been neglected for a long time in designing reprocessing flowsheets. Today's increased fuel burn-up means a significant rise in the concentration of these elements which cannot longer be considered as traces ; any anomaly must be taken into account. A re-examination of fission product behaviour in the Purex process has therefore become necessary. This paper deals with an attempt to describe the metabolism of one of them, palladium, which appeared to exhibit chemical reactions with most of the usual process reagents. I. STATE OF PALLADIUM IN THE FUEL 1.1. Abundance Palladium is fairly abundant in the irradiated fuels. As we noted below its main isotopes are stable and long-lived, or very short-lived. Its precursors are very short-lived nuclides except 106-ruthenium. Thus palladium abundance in the fuel increases very slightly with cooling time in accordance with the extent of 106-ruthenium decay. It is estimated at 1.33 kg per metric ton of elemental primary uranium for a PWR fuel of a 33,000 MWd/mt burn-up after one year cooling [1], With FBR fuels burn-up is drastically increased and palladium abundance may exceed 5 kg per ton of material.
" Centre d Etudes Nucliaires de Fontenay-aux-Roses, BP n° 6, DGR-SEP-SCPR, 92260 - FONTENAY-AUX-ROSES (France) f
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I.CHEM.E. S Y M P O S I U M SERIES No. 88
1.2. Isotopic content After a reasonable cooling time, the isotopes of palladium from fission are 104, 105, 106, 107, 108 and 110. The only radioactive isotope is 107-palladium. It is a long-lived nuclide (half-life : 6.5 10^ years) with a low energy $ emission (30 keV) and without any significant y emission. Hence the radioactive contamination of the fuel by palladium is negligible and is never considered in the process. Palladium is only regarded as a chemical pollutant and no special decontamination performance is required. 1.3. Chemical state Palladium oxide, PdO, is not very stable compared to uranium dioxide or to other fission product oxides. It is therefore assumed to be mainly present as metallic species in the irradiated fuel [2]. It is known to form inclusions in the oxide phase, in combination with other fission products or with the fuel material. Several kinds of metallic alloy inclusions have been described [3]. The major compounds are UPd3 and PuPd3 and compounds of Tc, Ru, Rh, Pd and Mo. They are important because they are found in the residues from the dissolution of oxide fuel in nitric acid [4] and are therefore responsible for the low dissolution yield of palladium. II. SOLUTION CHEMISTRY OF PALLADIUM IN NITRIC ACID II.1. Oxidoreduction Palladium in the only platinum metal that is attacked by nitric acid. Its usual valency is Pd(II). The standard redox potential in non-complexing medium is assumed to be Eo « 0.99 V for the Pd(0)/Pd(II) system. As , complexation by nitrate ion remains weak, we estimated this potential to be not less than 0.98 V in 1M nitric acid. Like platinum and nickel, it has a second valency : Pd(IV). Though a nitrate salt has been described [5], it is of no importance in the Purex process. It is a strong oxidant (Eo - 1.6 V in non-complexing medium) and, in spite of a stronger tendency to complexation, the redox potential is not lowered much in nitric acid. Therefore we failed to observe it in synthetic Purex conditions. Thus the only valencies involved in the process are Pd(0) and Pd(II). As plutonium and palladium have close potential values, we may assume that plutonium reductants will be effective for the reduction of palladium, and that palladium will follow the oxidoreduction behaviour of plutonium. The major difference lies in the production of metallic palladium, when this reduction occurs ; hence it may cause unexpected precipitation or cruds formation in the extraction contactors. We therefore checked that palladium reduction is achieved when ferrous sulphate or uranous nitrate is used. With ferrous sulphate the reaction is fast and complete. It is slow with uranous nitrate in homogeneous aqueous phase but fast in 30 % TBP-dodecane phase or in emulsion. Hydroxylammonium nitrate (HAN) is also a palladium reductant. In warm IN nitric acid solution the reaction is slow (several minutes) and incomplete.
20
I.CHEM.E. SYMPOSIUM SERIES No. 88 II.2. Complexation Like most transition metals, palladium is involved in complex formation with many compounds. We therefore reviewed the reactions of divalent palladium with usual process reagents. 11.2.1. Complexation by nitrate In nitrate medium, divalent palladium exhibits a spectrum with a broad absorption band in the 350-500 nm region. This spectrum is slightly affected by a variation of the nitrate concentration showing the formation of a nitrato complex. This reaction has been extensively studied in different media with variable nitrate concentration up to 5M. A weak mono-nitrato complex is formed with a stability constant Kl - (1.2 ± 0.4) l.mole" [6]. The formation of higher complexes is likely but has not been demonstrated. 1
11.2.2. Hydrolysis The hydrolysis of divalent palladium is easily observed in weakly complexing media. It is a slow and practically irreversible process leading to the formation of dark brown species, polymerization and precipitation. It occurs above pH 0.5 with the formation of the mono-hydroxo complex Pd(0H) rather stable at pH 2. Precipitation of the hydroxide Pd(0H)2 then begins. Monomer species predominate at low palladium concentration. At higher concentration (> 3.10^ M ) , in the pH range 0.5 - 2.0, polymers are however formed. Nabivanets and Kalabina [7] have shown that these are mostly hexamers. These authors have measured the following hydrolysis constant values : +
[Pd(0H) ] [H ] +
+
= 5.0 10"" mole.r 3
[Pd ]
1
2+
[Pd(0H) ] [ H ] +
2
2
-5 2 -2 = 1.6 10 mole .1
[Pd ] 2+
Thus hydrolysis is an important reaction which must be taken into account especially when preparing and conditioning palladium solutions. 11.2.3. Complexation by nitrite Like nickel and platinum, divalent palladium is complexed by nitrite. Several compounds have been described or postulated (K Pd ( N 0 ) 4 , H Pd (N0 ) 4 ) but few data are available about the nature and stability of the species in aqueous solutions. 2
2
2
2
In 1M nitric or perchloric acid we observed a similar change in the palladium spectrum by the addition of sodium nitrite. The position of the band is not significantly shifted, but absorption is substantially enhanced. The molar extinction coefficient is increased by a factor of 10 and reaches 1200 l.mole !.cm *. Raman spectra of these solutions do not exhibit any characteristic band of a bonded NO or N0 but a band at 610 cm~l, encountered in other nitrite complexes. -
-
+
Spectrophotometric investigation of this reaction indicated complexation by at least one or two nitrite ions in IN HNO3. At lower acidity, the results are different and interpretation is difficult. Complexation by nitrite may be favoured by the resultant increase of the free nitrite concentration, but it 21
I.CHEM.E. SYMPOSIUM SERIES No. 88 also competes with hydrolysis. Titration of the solution by caustic soda indicated that nitrite prevents palladium hydroxide precipitation. However, titration curves exhibit different shapes according to the nitrite content, and changes in colour are observed. We therefore believe that partial hydrolysis may occur and that mixed complexes are formed. This complexation process is another difficulty in the preparation of nitric solutions of palladium. Possible nitrite complexes must always be destroyed by the elimination of nitrous acid. This is difficult to achieve by air bubbling and boiling the solution is preferable. 11.2.4. Complexation by sulphamic acid To avoid nitrite complex formation, the use of sulphamic acid, which is a good nitrous acid scavenger and has low reducing power, may be considered. However, sulphamic acid reacts on divalent palladium by complexation. The absorption spectra of palladium with various additions of sulphamic acid are presented in figure 1. Substantial enhancement of absorption recurs without any significant shift of the peak position. From figure 2, showing the variation of the absorption of the new complex versus reagent concentration, we may assume that one sulphamic acid molecule is involved in reaction with palladium. 11.2.5. Complexation by hydrazine Sulphamic acid is no longer used as a nitrite scavenger in reprocessing, and hydrazine is prefered. However, hydrazine has an effective reducing power, and we expected redox reactions with palladium. This actually occurs in basic medium, but in nitric acid solution, the addition of hydrazinium nitrate precipitates a yellowish salt. The formation of several hydrazine compounds of palladium, such as PdL2 ( ^ 5 ) 2 X2 and PdL2(N2H4) (N H )X where V - CI", Br" or N02~ and X = C I O 4 " , N O 3 " , Br" or CI", is reported in the literature [ 8 ] . Hence the precipitation of a hydrazine compound of divalent palladium is likely. We investigated this reaction by analysing the supernatant solution. Palladium concentration was measured by beta radioraetry using 103p
5
By mass balance we determined the hydrazine content of the salt. The hydrazine to palladium ratio was found equal to 2.05 t 0.25, confirming the assumption of a formula analogous to the salts already described. There the ligands L and X are assumed to be same anion NO^". To check this hypothesis, and to determine the protonation state of hydrazine in the complex, we investigated the influence of nitrate on the solubility of the complex, operating at constant acidity, 0.5 N nitric acid. Nitrate concentration was varied by the addition of sodium nitrate. Ionic strength was not buffered t o avoid the formation of other compounds. The value of the product [ P d ^ ] [ N H ] was measured in the supernatant solutions. The palladium concentrations were corrected for nitrate complexation assuming the formation of Pd NO3 with Kj = 1.2. We therefore plotted (figure 3) +
+
+
2
5
* [!f.2 fN0 -] [ N H ] v e r s u s log [ S O 3 - ] The slope was found equal to be -0.92, favouring a single nitrate complex. Hence another anion must be involved. The only anion available is OH" and the complex must be Pd(0H)(N03) ( N H ) . Another complex, [Pd(0H>2 ( N 2 H 4 ) ( N H + ) ] N03~ may be considered, but its formation is rather unlikely in our conditions of acidity. The solubility product may be written : l o
q
+
3
2
4
2
2
5
2
2
Ks - [ P d ] 2+
22
[0H"][N0 "][N H ] 3
2
4
2
5
I.CHEM.E. S Y M P O S I U M SERIES No. 88
Considering palladium complexation by nitrate and protonation of hydrazine the equation is : Ks » We obtained the following value : Ks - (2.8 _ 0.2) 1 0 ~
5 8
M~
5
11.2.6. Complexation by hydroxylamine The complexation of divalent nickel by hydroxylamine is reported [9] but we failed to detect it by spectrophotometry, only observing a redox process. 11.2.7. Complexation by carbonate It is very effective and prevents palladium hydrolysis in basic medium. III. PALLADIUM EXTRACTION BY 30 % TBP-DODECANE III.l. Extraction from nitric acid Moderate extraction of palladium from nitric acid has been reported [10, 11]. Our own results roughly confirm the data from the various sources (figure 4 ) . Extraction by 30 % dodecane is maximum from IN nitric acid and the partition coefficient reaches 0.35. At higher nitric acid concentration a drop in the coefficient is observed : from 3N it falls to 0.07. However, the data are considerably dispersed and we observed poor reproductibility in performing our own measurements. This is not caused by the state of purity of the solvent because we checked that dibutylphosphoric acid, its main pollutant, has no effect on palladium extraction. The preparation method of the stock solution appeared to be a more significant parameter. As we stated previously, palladium is easily hydrolysed or complexed by nitrite. So when metal is used as a palladium source, dissolution generates nitrous acid ; and if palladous nitrate is preferred hydrolysis products are obtained because of the strong hygroscopicity of the salt. Therefore, great care must be taken when conditioning the solution, and a boiling step in concentrated nitric acid is often desirable. In any case, spectrophotometric verification is necessary. We used a complicated process because ^ ^ P d in chloride from was added as a tracer. However, different conditioning methods were tested and we finally checked our results by a back-extraction procedure. The extraction mechanism has been investigated by several authors. There is general agreement on the TBP to palladium ratio in the organic complex, which has been estimated at 2. However, the formula of the complex remains debatable. Most of the authors believe that palladous nitrate is formed and solvated by TBP, but the dependence of the partition coefficient on nitric acid concentration cannot be explained easily. Lunichkina, Renard and Shevchenko operated at constant nitrate concentration and observed a drop in the partition coefficient with increasing acidity [11]. They assumed the formation of a less extractible protonated palladous nitrate at high acid concentration. This is unlikely because palladium complexation by nitrate is not effective enough. We are therefore inclined to favour the hypothesis of Ly and Poitrenaud [12] relative to the formation of a hydroxo-palladium nitrate : Pd(OH)(N0 )(TBP) 3
23
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I.CHEM.E. S Y M P O S I U M SERIES No. 88
III.2. Extraction in the presence of nitrous acid When nitrous acid is added to the system, complexation occurs in the aqueous phase as previously reported. However, we also observed complexation in the organic phase. Spectrophotometry evidence was obtained by mixing solvent samples respectively loaded with palladium and nitrous acid. The spectrum changes are similar to those previously observed in aqueous medium. Complexation is achieved by the addition of a slight excess of nitrous acid. We believe that two nitrites are involved in the process and that palladous nitrite is obtained, which is supported by infrared studies. Hence palladium extraction is assumed to be strongly affected by nitrous acid. We observed an enhancement of the partition coefficient at low nitric acid concentration (< 0.3 N ) , but at high acidity it is significantly decreased (figure 5 ) . On increasing the nitrous acid concentration, when large excess is used, extraction is lowered (figure 6 ) . This is explained by the salting-out effect of nitrous acid, because the extraction of nitrous acid is significant. Assuming that the partition coefficient of palladous nitrite is proportional to a certain power of the free TBP concentration : D
p d
* K [TBP ]
n
f
it may be expressed as a function of nitric and nitrous acid concentration because free TBP concentration is [ T B P
f
]
=
1 + k.[HN0j|' + 1
J aq
1
k [HN0 ] 9
9
I
L aq
if palladium extraction is ignored. Hence
jj '+ k J H N O j 1 J aq l l aq giving the following approximated equation : log D
p d
log K + n log
i +
l
k
fHNOJ
o aq
Such an equation explains the shape of the curves in figure 6 . Thus the extraction of palladium remains moderate in the presence of nitrous acid. III.3. Extraction by irradiated solvent In the Purex process the solvent is exposed to vigorous degradation conditions by radiolysis and hydrolysis. The main degradation product of the extractant is dibutylphosphoric acid and we already pointed out that it has no influence on palladium extraction. The presence of other degradation products (butanol, butyric acid) also has no marked effect [13]. However an enhancement of the partition coefficient with irradiated solvent has been reported by Soviet authors [13]. Therefore, the degradation products of the diluent must be responsible for the extraction and we limited our study to the diluent. We used a synthetic branched dodecane usually called HTP (hydrogenated tetrapropylene) from its preparation method. We first checked that fresh
24
I.CHEM.E. S Y M P O S I U M SERIES No. 88
diluent did not extract palladium because it has been established that sulphur containing hydrocarbons show a certain extraction power with respect to palladium [14]. Our diluent, having a low sulphur content, exhibits no significant extraction power. After y irradiation with a ^ C o source, some extraction of palladium was observed though the diluent was used alone. Partition coefficient was dependent on palladium concentration. We also found great enhancement of its value (by a factor of at least 100) when using a back-extraction procedure. Hence this is not an extraction process but we apparently observed strong complexation of palladium by degradation products of the diluent. To measure the extent of complexation we estimated the retention of palladium by a stripping test. It was found to increase with the irradiation dose (figure 7 ) . The maximum retention reached 5.10~5M in our experiments. The nitric acid concentration of the aqueous phase, used for the extraction measurements, has some influence (figure 8 ) as well as nitrous acid. But greater enhancement of retention was found when aging irradiated diluent samples : an increase by a factor of 5 was observed after aging for only 8 days. This complexing power was found to be unaffected by the usual solvent regeneration treatment (caustic soda, carbonate). Another property of irradiated diluents is their reducing power : precipitation of metallic palladium is often observed and causes cruds formation in the extraction contactors. We failed to characterise the complexing agent but there is strong evidence for refusing the assumption of hydroxamic acids.
CONCLUSION Palladium is an abundant fission product. It is a minor radioactive contaminant but its chemical behaviour is troublesome for the process. It has been shown to precipitate with most of the reagents used in the partition step (U(4), HAN, hydrazine). Fortunately its extraction by TBP is slight, and after scrubbing, a large decontamination factor is expected. However, while HDBP has no influence on its extraction, irradiated diluents exhibit some complexing power for palladium. This incurs the risk of palladium overspreading in the process, with the accumulation of solids at the partition step. On the other hand irradiated diluents may reduce palladium and form cruds in the extraction step. Therefore palladium displays acceptable behaviour in the Purex process as long as diluent degradation remains slight. REFERENCES [1]
(1970) ORNL - 4451
[2]
Bazin J.,Jouan J. Vignesoult N (1974) CEA, B.I.S.T., \96, 55
[3]
Kleykamp H. 7-11 aug. 1972 - Proceedings of a Panel, IAEA - PL - 46318, Vienna
[4]
Kleykamp H. 1983, Nukleare Entsorgung, Band 2, p. 151, Verlag Chemie, Weinheim.
[5]
Addison C.C., Ward B.G. (1966) Chem. Commune., 155
25
I.CHEM.E. S Y M P O S I U M SERIES No. 88
T6]
Jorgensen C.K., Parthasarathy V.
[7]
Nabivanets B.I., Kalabina L.V. (1970), Russ. J. Inorg. Chem. 15(6), 818
[8]
Klyuchnikov N.G. Para F.I. (1968) Russ. J. Inorg. Chem. 13(3), 416
[9]
Ivashkovich E.M., Skoblei M.I. (1974), Russ. J. Inorg. Chem., 19(3), 411
[10]
Gorskii B. Gorskii N.I.
[11]
Lunichkina K.P., Renard E.V., Shevchenko V.B. (1974) Russ. J. Inorg. Chem., 19(1), 110
[12]
Ly J., Poitrenaud C M . (Aug. 26 - sept. 2, 1983) ISEC, Denver, Colorado, USA
[13]
Zagorets, P.A., Smelov V.S., Ochkin A.V., Chubukov V.V., Tverdovskii A.N Kondratev B.A., Kirpikov S.V. (1982) Soviet. Radiochem., 24(1), 3&
[14]
Prihoda J. (1979), J. Radioanal. Chem.,
(1978) Acta Chem. Scand., A 3j2,957
c
(1979) Soviet. Radiochem., 21(2), 243
233.
26
I.CHEM.E. S Y M P O S I U M SERIES No. 88
log
|H NS0 H| 3
2
S •
/
/slopezzl.06
•3H log
Absorbance
Fig. 2 : Influence of sulphamic acid concentration on palladium absorbance.
mole!r
J
10-1
5
10- J 9
io-«J
NO3 —I
1—i
0.5
i
i
|
2
3
mole-1
1
Fig. 3 : Dependence of the hydrazino-palladium solubility product on nitrate. 27
I.CHEM.E. S Y M P O S I U M SERIES No. 88
Fig. 4 : Palladium extraction from nitric acid with 30 % TBP.
28
I.CHEM.E. S Y M P O S I U M SERIES No. 88
1.
D
P
(
J
o
no HNO2
added
•
|HN0 aq|=5.1
2
5 : Palladium extraction in the presence of nitrous acid.
0 1
0.05 1
To
0.1 '
20
|HN0 | . M 2
lHNQ |Pd
2
»gl
aq|
Fig. 6 : Influence of nitrous acid on palladium extraction.
29
I.CHEM.E. S Y M P O S I U M SERIES No. 8 8
dose — I — 50
W.h.l-
100
Fig. 7 : Influence of the y irradiation dose on palladium retention by diluent.
mg.!- | 1
|Pd
o r o
,
10J
I I
' " ' H 0.1
- 1 — I — I
I
I I
11
HNO 3
aq
-I—I
Fig. 8 : Influence of nitric acid on palladium retention by irradiated HTP (113 Wh.l'l). 30
I
I I
11 10
N
E. VIALARD and M. GERMAIN
Palladium is ah abundant fission product but a minor radioactive contaminant of nuclear fuels. Its peculiar chemical behaviour is troublesome because it reacts with most of the reagents used in partition and forms insoluble compounds or cruds. Fortunately palladium extraction by 30 % TBP-dodecane is moderate and high DF are expected as long as the diluent is not excessively degraded. However, y irradiated diluents display strong complexing power for palladium and retention of this element is observed.
INTRODUCTION A number of fission products are minor radioactive contaminants of irradiated nuclear fuels and have been neglected for a long time in designing reprocessing flowsheets. Today's increased fuel burn-up means a significant rise in the concentration of these elements which cannot longer be considered as traces ; any anomaly must be taken into account. A re-examination of fission product behaviour in the Purex process has therefore become necessary. This paper deals with an attempt to describe the metabolism of one of them, palladium, which appeared to exhibit chemical reactions with most of the usual process reagents. I. STATE OF PALLADIUM IN THE FUEL 1.1. Abundance Palladium is fairly abundant in the irradiated fuels. As we noted below its main isotopes are stable and long-lived, or very short-lived. Its precursors are very short-lived nuclides except 106-ruthenium. Thus palladium abundance in the fuel increases very slightly with cooling time in accordance with the extent of 106-ruthenium decay. It is estimated at 1.33 kg per metric ton of elemental primary uranium for a PWR fuel of a 33,000 MWd/mt burn-up after one year cooling [1], With FBR fuels burn-up is drastically increased and palladium abundance may exceed 5 kg per ton of material.
" Centre d Etudes Nucliaires de Fontenay-aux-Roses, BP n° 6, DGR-SEP-SCPR, 92260 - FONTENAY-AUX-ROSES (France) f
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I.CHEM.E. S Y M P O S I U M SERIES No. 88
1.2. Isotopic content After a reasonable cooling time, the isotopes of palladium from fission are 104, 105, 106, 107, 108 and 110. The only radioactive isotope is 107-palladium. It is a long-lived nuclide (half-life : 6.5 10^ years) with a low energy $ emission (30 keV) and without any significant y emission. Hence the radioactive contamination of the fuel by palladium is negligible and is never considered in the process. Palladium is only regarded as a chemical pollutant and no special decontamination performance is required. 1.3. Chemical state Palladium oxide, PdO, is not very stable compared to uranium dioxide or to other fission product oxides. It is therefore assumed to be mainly present as metallic species in the irradiated fuel [2]. It is known to form inclusions in the oxide phase, in combination with other fission products or with the fuel material. Several kinds of metallic alloy inclusions have been described [3]. The major compounds are UPd3 and PuPd3 and compounds of Tc, Ru, Rh, Pd and Mo. They are important because they are found in the residues from the dissolution of oxide fuel in nitric acid [4] and are therefore responsible for the low dissolution yield of palladium. II. SOLUTION CHEMISTRY OF PALLADIUM IN NITRIC ACID II.1. Oxidoreduction Palladium in the only platinum metal that is attacked by nitric acid. Its usual valency is Pd(II). The standard redox potential in non-complexing medium is assumed to be Eo « 0.99 V for the Pd(0)/Pd(II) system. As , complexation by nitrate ion remains weak, we estimated this potential to be not less than 0.98 V in 1M nitric acid. Like platinum and nickel, it has a second valency : Pd(IV). Though a nitrate salt has been described [5], it is of no importance in the Purex process. It is a strong oxidant (Eo - 1.6 V in non-complexing medium) and, in spite of a stronger tendency to complexation, the redox potential is not lowered much in nitric acid. Therefore we failed to observe it in synthetic Purex conditions. Thus the only valencies involved in the process are Pd(0) and Pd(II). As plutonium and palladium have close potential values, we may assume that plutonium reductants will be effective for the reduction of palladium, and that palladium will follow the oxidoreduction behaviour of plutonium. The major difference lies in the production of metallic palladium, when this reduction occurs ; hence it may cause unexpected precipitation or cruds formation in the extraction contactors. We therefore checked that palladium reduction is achieved when ferrous sulphate or uranous nitrate is used. With ferrous sulphate the reaction is fast and complete. It is slow with uranous nitrate in homogeneous aqueous phase but fast in 30 % TBP-dodecane phase or in emulsion. Hydroxylammonium nitrate (HAN) is also a palladium reductant. In warm IN nitric acid solution the reaction is slow (several minutes) and incomplete.
20
I.CHEM.E. SYMPOSIUM SERIES No. 88 II.2. Complexation Like most transition metals, palladium is involved in complex formation with many compounds. We therefore reviewed the reactions of divalent palladium with usual process reagents. 11.2.1. Complexation by nitrate In nitrate medium, divalent palladium exhibits a spectrum with a broad absorption band in the 350-500 nm region. This spectrum is slightly affected by a variation of the nitrate concentration showing the formation of a nitrato complex. This reaction has been extensively studied in different media with variable nitrate concentration up to 5M. A weak mono-nitrato complex is formed with a stability constant Kl - (1.2 ± 0.4) l.mole" [6]. The formation of higher complexes is likely but has not been demonstrated. 1
11.2.2. Hydrolysis The hydrolysis of divalent palladium is easily observed in weakly complexing media. It is a slow and practically irreversible process leading to the formation of dark brown species, polymerization and precipitation. It occurs above pH 0.5 with the formation of the mono-hydroxo complex Pd(0H) rather stable at pH 2. Precipitation of the hydroxide Pd(0H)2 then begins. Monomer species predominate at low palladium concentration. At higher concentration (> 3.10^ M ) , in the pH range 0.5 - 2.0, polymers are however formed. Nabivanets and Kalabina [7] have shown that these are mostly hexamers. These authors have measured the following hydrolysis constant values : +
[Pd(0H) ] [H ] +
+
= 5.0 10"" mole.r 3
[Pd ]
1
2+
[Pd(0H) ] [ H ] +
2
2
-5 2 -2 = 1.6 10 mole .1
[Pd ] 2+
Thus hydrolysis is an important reaction which must be taken into account especially when preparing and conditioning palladium solutions. 11.2.3. Complexation by nitrite Like nickel and platinum, divalent palladium is complexed by nitrite. Several compounds have been described or postulated (K Pd ( N 0 ) 4 , H Pd (N0 ) 4 ) but few data are available about the nature and stability of the species in aqueous solutions. 2
2
2
2
In 1M nitric or perchloric acid we observed a similar change in the palladium spectrum by the addition of sodium nitrite. The position of the band is not significantly shifted, but absorption is substantially enhanced. The molar extinction coefficient is increased by a factor of 10 and reaches 1200 l.mole !.cm *. Raman spectra of these solutions do not exhibit any characteristic band of a bonded NO or N0 but a band at 610 cm~l, encountered in other nitrite complexes. -
-
+
Spectrophotometric investigation of this reaction indicated complexation by at least one or two nitrite ions in IN HNO3. At lower acidity, the results are different and interpretation is difficult. Complexation by nitrite may be favoured by the resultant increase of the free nitrite concentration, but it 21
I.CHEM.E. SYMPOSIUM SERIES No. 88 also competes with hydrolysis. Titration of the solution by caustic soda indicated that nitrite prevents palladium hydroxide precipitation. However, titration curves exhibit different shapes according to the nitrite content, and changes in colour are observed. We therefore believe that partial hydrolysis may occur and that mixed complexes are formed. This complexation process is another difficulty in the preparation of nitric solutions of palladium. Possible nitrite complexes must always be destroyed by the elimination of nitrous acid. This is difficult to achieve by air bubbling and boiling the solution is preferable. 11.2.4. Complexation by sulphamic acid To avoid nitrite complex formation, the use of sulphamic acid, which is a good nitrous acid scavenger and has low reducing power, may be considered. However, sulphamic acid reacts on divalent palladium by complexation. The absorption spectra of palladium with various additions of sulphamic acid are presented in figure 1. Substantial enhancement of absorption recurs without any significant shift of the peak position. From figure 2, showing the variation of the absorption of the new complex versus reagent concentration, we may assume that one sulphamic acid molecule is involved in reaction with palladium. 11.2.5. Complexation by hydrazine Sulphamic acid is no longer used as a nitrite scavenger in reprocessing, and hydrazine is prefered. However, hydrazine has an effective reducing power, and we expected redox reactions with palladium. This actually occurs in basic medium, but in nitric acid solution, the addition of hydrazinium nitrate precipitates a yellowish salt. The formation of several hydrazine compounds of palladium, such as PdL2 ( ^ 5 ) 2 X2 and PdL2(N2H4) (N H )X where V - CI", Br" or N02~ and X = C I O 4 " , N O 3 " , Br" or CI", is reported in the literature [ 8 ] . Hence the precipitation of a hydrazine compound of divalent palladium is likely. We investigated this reaction by analysing the supernatant solution. Palladium concentration was measured by beta radioraetry using 103p
5
By mass balance we determined the hydrazine content of the salt. The hydrazine to palladium ratio was found equal to 2.05 t 0.25, confirming the assumption of a formula analogous to the salts already described. There the ligands L and X are assumed to be same anion NO^". To check this hypothesis, and to determine the protonation state of hydrazine in the complex, we investigated the influence of nitrate on the solubility of the complex, operating at constant acidity, 0.5 N nitric acid. Nitrate concentration was varied by the addition of sodium nitrate. Ionic strength was not buffered t o avoid the formation of other compounds. The value of the product [ P d ^ ] [ N H ] was measured in the supernatant solutions. The palladium concentrations were corrected for nitrate complexation assuming the formation of Pd NO3 with Kj = 1.2. We therefore plotted (figure 3) +
+
+
2
5
* [!f.2 fN0 -] [ N H ] v e r s u s log [ S O 3 - ] The slope was found equal to be -0.92, favouring a single nitrate complex. Hence another anion must be involved. The only anion available is OH" and the complex must be Pd(0H)(N03) ( N H ) . Another complex, [Pd(0H>2 ( N 2 H 4 ) ( N H + ) ] N03~ may be considered, but its formation is rather unlikely in our conditions of acidity. The solubility product may be written : l o
q
+
3
2
4
2
2
5
2
2
Ks - [ P d ] 2+
22
[0H"][N0 "][N H ] 3
2
4
2
5
I.CHEM.E. S Y M P O S I U M SERIES No. 88
Considering palladium complexation by nitrate and protonation of hydrazine the equation is : Ks » We obtained the following value : Ks - (2.8 _ 0.2) 1 0 ~
5 8
M~
5
11.2.6. Complexation by hydroxylamine The complexation of divalent nickel by hydroxylamine is reported [9] but we failed to detect it by spectrophotometry, only observing a redox process. 11.2.7. Complexation by carbonate It is very effective and prevents palladium hydrolysis in basic medium. III. PALLADIUM EXTRACTION BY 30 % TBP-DODECANE III.l. Extraction from nitric acid Moderate extraction of palladium from nitric acid has been reported [10, 11]. Our own results roughly confirm the data from the various sources (figure 4 ) . Extraction by 30 % dodecane is maximum from IN nitric acid and the partition coefficient reaches 0.35. At higher nitric acid concentration a drop in the coefficient is observed : from 3N it falls to 0.07. However, the data are considerably dispersed and we observed poor reproductibility in performing our own measurements. This is not caused by the state of purity of the solvent because we checked that dibutylphosphoric acid, its main pollutant, has no effect on palladium extraction. The preparation method of the stock solution appeared to be a more significant parameter. As we stated previously, palladium is easily hydrolysed or complexed by nitrite. So when metal is used as a palladium source, dissolution generates nitrous acid ; and if palladous nitrate is preferred hydrolysis products are obtained because of the strong hygroscopicity of the salt. Therefore, great care must be taken when conditioning the solution, and a boiling step in concentrated nitric acid is often desirable. In any case, spectrophotometric verification is necessary. We used a complicated process because ^ ^ P d in chloride from was added as a tracer. However, different conditioning methods were tested and we finally checked our results by a back-extraction procedure. The extraction mechanism has been investigated by several authors. There is general agreement on the TBP to palladium ratio in the organic complex, which has been estimated at 2. However, the formula of the complex remains debatable. Most of the authors believe that palladous nitrate is formed and solvated by TBP, but the dependence of the partition coefficient on nitric acid concentration cannot be explained easily. Lunichkina, Renard and Shevchenko operated at constant nitrate concentration and observed a drop in the partition coefficient with increasing acidity [11]. They assumed the formation of a less extractible protonated palladous nitrate at high acid concentration. This is unlikely because palladium complexation by nitrate is not effective enough. We are therefore inclined to favour the hypothesis of Ly and Poitrenaud [12] relative to the formation of a hydroxo-palladium nitrate : Pd(OH)(N0 )(TBP) 3
23
2
I.CHEM.E. S Y M P O S I U M SERIES No. 88
III.2. Extraction in the presence of nitrous acid When nitrous acid is added to the system, complexation occurs in the aqueous phase as previously reported. However, we also observed complexation in the organic phase. Spectrophotometry evidence was obtained by mixing solvent samples respectively loaded with palladium and nitrous acid. The spectrum changes are similar to those previously observed in aqueous medium. Complexation is achieved by the addition of a slight excess of nitrous acid. We believe that two nitrites are involved in the process and that palladous nitrite is obtained, which is supported by infrared studies. Hence palladium extraction is assumed to be strongly affected by nitrous acid. We observed an enhancement of the partition coefficient at low nitric acid concentration (< 0.3 N ) , but at high acidity it is significantly decreased (figure 5 ) . On increasing the nitrous acid concentration, when large excess is used, extraction is lowered (figure 6 ) . This is explained by the salting-out effect of nitrous acid, because the extraction of nitrous acid is significant. Assuming that the partition coefficient of palladous nitrite is proportional to a certain power of the free TBP concentration : D
p d
* K [TBP ]
n
f
it may be expressed as a function of nitric and nitrous acid concentration because free TBP concentration is [ T B P
f
]
=
1 + k.[HN0j|' + 1
J aq
1
k [HN0 ] 9
9
I
L aq
if palladium extraction is ignored. Hence
jj '+ k J H N O j 1 J aq l l aq giving the following approximated equation : log D
p d
log K + n log
i +
l
k
fHNOJ
o aq
Such an equation explains the shape of the curves in figure 6 . Thus the extraction of palladium remains moderate in the presence of nitrous acid. III.3. Extraction by irradiated solvent In the Purex process the solvent is exposed to vigorous degradation conditions by radiolysis and hydrolysis. The main degradation product of the extractant is dibutylphosphoric acid and we already pointed out that it has no influence on palladium extraction. The presence of other degradation products (butanol, butyric acid) also has no marked effect [13]. However an enhancement of the partition coefficient with irradiated solvent has been reported by Soviet authors [13]. Therefore, the degradation products of the diluent must be responsible for the extraction and we limited our study to the diluent. We used a synthetic branched dodecane usually called HTP (hydrogenated tetrapropylene) from its preparation method. We first checked that fresh
24
I.CHEM.E. S Y M P O S I U M SERIES No. 88
diluent did not extract palladium because it has been established that sulphur containing hydrocarbons show a certain extraction power with respect to palladium [14]. Our diluent, having a low sulphur content, exhibits no significant extraction power. After y irradiation with a ^ C o source, some extraction of palladium was observed though the diluent was used alone. Partition coefficient was dependent on palladium concentration. We also found great enhancement of its value (by a factor of at least 100) when using a back-extraction procedure. Hence this is not an extraction process but we apparently observed strong complexation of palladium by degradation products of the diluent. To measure the extent of complexation we estimated the retention of palladium by a stripping test. It was found to increase with the irradiation dose (figure 7 ) . The maximum retention reached 5.10~5M in our experiments. The nitric acid concentration of the aqueous phase, used for the extraction measurements, has some influence (figure 8 ) as well as nitrous acid. But greater enhancement of retention was found when aging irradiated diluent samples : an increase by a factor of 5 was observed after aging for only 8 days. This complexing power was found to be unaffected by the usual solvent regeneration treatment (caustic soda, carbonate). Another property of irradiated diluents is their reducing power : precipitation of metallic palladium is often observed and causes cruds formation in the extraction contactors. We failed to characterise the complexing agent but there is strong evidence for refusing the assumption of hydroxamic acids.
CONCLUSION Palladium is an abundant fission product. It is a minor radioactive contaminant but its chemical behaviour is troublesome for the process. It has been shown to precipitate with most of the reagents used in the partition step (U(4), HAN, hydrazine). Fortunately its extraction by TBP is slight, and after scrubbing, a large decontamination factor is expected. However, while HDBP has no influence on its extraction, irradiated diluents exhibit some complexing power for palladium. This incurs the risk of palladium overspreading in the process, with the accumulation of solids at the partition step. On the other hand irradiated diluents may reduce palladium and form cruds in the extraction step. Therefore palladium displays acceptable behaviour in the Purex process as long as diluent degradation remains slight. REFERENCES [1]
(1970) ORNL - 4451
[2]
Bazin J.,Jouan J. Vignesoult N (1974) CEA, B.I.S.T., \96, 55
[3]
Kleykamp H. 7-11 aug. 1972 - Proceedings of a Panel, IAEA - PL - 46318, Vienna
[4]
Kleykamp H. 1983, Nukleare Entsorgung, Band 2, p. 151, Verlag Chemie, Weinheim.
[5]
Addison C.C., Ward B.G. (1966) Chem. Commune., 155
25
I.CHEM.E. S Y M P O S I U M SERIES No. 88
T6]
Jorgensen C.K., Parthasarathy V.
[7]
Nabivanets B.I., Kalabina L.V. (1970), Russ. J. Inorg. Chem. 15(6), 818
[8]
Klyuchnikov N.G. Para F.I. (1968) Russ. J. Inorg. Chem. 13(3), 416
[9]
Ivashkovich E.M., Skoblei M.I. (1974), Russ. J. Inorg. Chem., 19(3), 411
[10]
Gorskii B. Gorskii N.I.
[11]
Lunichkina K.P., Renard E.V., Shevchenko V.B. (1974) Russ. J. Inorg. Chem., 19(1), 110
[12]
Ly J., Poitrenaud C M . (Aug. 26 - sept. 2, 1983) ISEC, Denver, Colorado, USA
[13]
Zagorets, P.A., Smelov V.S., Ochkin A.V., Chubukov V.V., Tverdovskii A.N Kondratev B.A., Kirpikov S.V. (1982) Soviet. Radiochem., 24(1), 3&
[14]
Prihoda J. (1979), J. Radioanal. Chem.,
(1978) Acta Chem. Scand., A 3j2,957
c
(1979) Soviet. Radiochem., 21(2), 243
233.
26
I.CHEM.E. S Y M P O S I U M SERIES No. 88
log
|H NS0 H| 3
2
S •
/
/slopezzl.06
•3H log
Absorbance
Fig. 2 : Influence of sulphamic acid concentration on palladium absorbance.
mole!r
J
10-1
5
10- J 9
io-«J
NO3 —I
1—i
0.5
i
i
|
2
3
mole-1
1
Fig. 3 : Dependence of the hydrazino-palladium solubility product on nitrate. 27
I.CHEM.E. S Y M P O S I U M SERIES No. 88
Fig. 4 : Palladium extraction from nitric acid with 30 % TBP.
28
I.CHEM.E. S Y M P O S I U M SERIES No. 88
1.
D
P
(
J
o
no HNO2
added
•
|HN0 aq|=5.1
2
5 : Palladium extraction in the presence of nitrous acid.
0 1
0.05 1
To
0.1 '
20
|HN0 | . M 2
lHNQ |Pd
2
»gl
aq|
Fig. 6 : Influence of nitrous acid on palladium extraction.
29
I.CHEM.E. S Y M P O S I U M SERIES No. 8 8
dose — I — 50
W.h.l-
100
Fig. 7 : Influence of the y irradiation dose on palladium retention by diluent.
mg.!- | 1
|Pd
o r o
,
10J
I I
' " ' H 0.1
- 1 — I — I
I
I I
11
HNO 3
aq
-I—I
Fig. 8 : Influence of nitric acid on palladium retention by irradiated HTP (113 Wh.l'l). 30
I
I I
11 10
N