Behavior of Ruthenium in Glass J. Mukerji Central Glass and Ceramic Research Institute, Jadavpur, Calcutta 32, I n d i a
The behavior of ruthenium in silicate and phosphate glasses is studied by spectrophotometric, solubility, and evaporation measurements. Two of the available oxidation states of ruthenium are in equilibrium in silicate glasses, whereas only one oxidation state is evident in phosphate glasses. Ru(IV) is present along with Ru(lll) and Ru(VI) in silicate glasses having below 25 mol % and above 2 5 mol % NazO, respectively. In soda phosphate glasses, Ru(VI) and Ru(VII) are the species expected to be above and below 50 mol % PzOj. Solubilities of ruthenium in soda-silica, sodium borosilicate, soda phosphate, sodium borophosphate, and lead phosphate glasses are reported. The influence of ruthenium solubility on its evaporation is investigated. Ruthenium volatilization may be minimized by rendering ruthenium insoluble in the glass.
Fixation of fission products in glass for ultimate disposal of high-level nuclear waste has been actively studied since the last decade. One of the problems one encounters in carrying out the fixation operation at the glass melting temperature (-1 100OC) is the volatilization of radioruthenium. lo6Ruof one-year half-life may be present in nitric acid waste solution as trivalent nitrosyl ruthenium complexes in which the ligands may be nitrato, nitro, hydroxo, and aquo groups (Fletcher et al., 1955). Evaporation loss of ruthenium takes place in two stages: during evaporation and concentration of waste solution as soon as the nitric acid concentration reaches 5M (Wilson, 1960; Elliot et al., 1963) and during processing of glass with the amount evaporated dependent on glass composition and time and temperature of processing. Ruthenium escapes from fixation in the glassy matrix by forming volatile oxides RuOl and RuOo. According to Wayne and Tagami (1963), a mixture of RuO4 and RuOp is present in the vapor phase, the former predominating at temperatures below 13OOOC and the latter above. The work reported here was undertaken to study the behavior of ruthenium in both phosphate and silicate glasses. The main points of interest are: the oxidation states in which ruthenium is likely to be present in both types of glasses of varying composition; the way in which i t goes into solution in glass and the extent of its solubility in different glasses, i.e., probable position of ruthenium in t h e glass structure; and the interrelation between evaporation of ruthenium, its oxidation states, and solubility in glass with methods chosen to prevent its volatilization. A combined discussion of separate studies (Dhargupta and Mukerji, 1968; Mukerji and Biswas, 1969) is presented here. Experimental methods and other details are dealt with briefly. Experimental
Spectrophotometric Studies. Neutral ruthenium has the electronic structure 4 8 b l after the completed Krypton shell. All the oxidation states of ruthenium show color in aqueous solution owing to ligand field splitting and charge transfer between the central cation and its ligands. Absorption spectra of glasses containing ruthenium were determined in the range 330-700 m p and were compared with absorption spectra of known ruthenium species in solution. 178 Ind. Eng. Chern. Prod. Res. Develop., Vol. 11, No. 2, 1972
Silicate glasses in which ruthenium was introduced as pure RUOZcrystals were melted in a platinum crucible until homogeneous and seed free. It was cast into 2-in. discs and optically polished for transmission measurement. Phosphate glasses in the simple system NaaO-PzOs were melted in a platinum dish at 1000°C until homogeneous and seed free and cast into discs and annealed. No polishing was done. Aluminophosphate glasses were melted in washed diaspore crucibles, cast, annealed, and polished. Spectra of the glasses were measured with a Uvispeck Hilger and Watts spectrophotometer. Ruthenium concentrations in glass samples were estimated colorimetrically with Rubeanic acid after dissolving t h e glass with a mixture of HC1 HF acids (Biswas and Mukerji, 1968). Solubility Measurements. The solubility of ruthenium was measured by using R u 0 2 in various glasses. The amount of Ruthenium going into solution a t a given temperature was estimated colorimetrically. Equilibrium was attained at each run (Biswas and Mukerji, 1968). Glasses in the simple systems NapO-SiO2, Na20-PzO6, NazO-Bz03-Si02, NazOB203-P203, and PbO-P206 were studied a t different experimental temperatures ranging from 900- 1100OC. Evaporation Study. T h e evaporation reaction may be written as
+
(RUO4)=
+
'/p
02
=
R u O ~t
+ 0'
(1)
The solubility of ruthenium is one among other factors, such as diffusion in glass and oxygen partial pressure, which will determine its evaporation from glass. Since solubility of ruthenium in glass could be adjusted by appropriate control of glass composition or by the use of reducing agents, i t was thought desirable to see if any relationship exists between solubility and evaporation. The evaporation set up consisted of a Kanthal AI wound tubular furnace through which a n impervious combustion tube was passed. Samples for the evaporation run were taken in platinum boats. A known amount of dry air was drawn into the furnace with a water pump connected to the exhaust end of the tube, A linear air velocity of 118 2 cm/Mt was used, and the surface subjected to evaporation was about 2.7 cm2. The time of each volatilization run was 4 hr. A thermocouple was placed in a vitreous silica sheath, the tip of which was placed just above the boat. The amount of
*
Table 1. Composition and Other Characteristics of Silicate and Phosphate Glasses Containing Ruthenium Glass composition, mol
No.
Si02
Nan0
1 2
20 30 40 28.5 28.14 22.5 54.4 44.0 36 17.1
3 4 5 6 7 8 9 10
80
...
70
... ... ... ... ...
60 66.5 65.66 66.5
...
... ...
...
70
PzO~
4 0 8
... ... 1.2
...
45.6 56.0 64 65.8
... 17.1
COO
... 5 5 11.o
.
I
.
...
Mp,"
O C
1500 1400 1400 1400 1400 1420 1000 1000 1000 1350
Color
Pink Orange Orange Yellow (greenish tinge) Pink Greenish yellow Yellow Orange Orange Light orange
Under laboratory atmosphere.
360
460
560
660
760
SbO
WAVE LENGTH IN MILLIMICRON
Figure 1. Absorption spectra of ruthenium in soda-silica glasses Curve I: Composition 1 Curve 11: Composition 2: thickness, 4.76 mm; Ru concn, Curve 111: Composition 3: thickness, 3.03 mm; Ru concn,
21 8 p p m 343 ppm
ruthenium soluble in t h e glass and t h e amount of undissolved ruthenium were estimated separately by the method detailed elsewhere (Dhargupta and llukerji, 1968). All the chemicals used in the experiments were of Guaranteed Pure quality. Pure RuOz crystals prepared in the laboratory by chemical vapor deposition from oxygen laden with RuOd were used as a source of R u 0 2 . Results and Discussion
Absorption spectra of ruthenium in silicate and phosphate glasses (Table I) in t h e region 350-700 mu (28,570-14,290 cm-l) are shown in Figures 1-3. Transmission has been plotted against wavelength. Although the total amount of ruthenium in t h e glass was known, exact calculation of the extinction coefficient was not possible as ruthenium was present (discussed later) in two oxidation states in the glasses, and the concentration of each oxidation state in t h e glass was not known. I n each figure total ruthenium and thickness of glass piece for transmission measurement are indicated.
WAVE LENGTH IN MILLIMICRON
Figure 2. Absorption spectra of ruthenium in soda-lime glasses Curve I: Composition 6 Curve II: Composition 4:thickness, 4.22 mm; Ru concn, Curve 111: Composition 5: thickness, 4.53 mm; Ru concn,
394 ppm 263 ppm
Absorption Bands. 680-90 J f p (14,700 cm-l). This band occurs in all the silicate glasses studied but does not occur in any of the phosphate glasses. Rlukerji and Biswas (1969) identified this band as owing to tetravalent ruthenium by comparison of its spectrum in solution (Figure 4,Curve 111). This band may be attributed to t h e one possible spin allowed transition 5r3-+ 5r5. 655 J f p (15,260 cm-1). This band was found only in phosphate glasses having below 50 mol % PzOS.The same glass shows a charge transfer band at 385 mp. Connick and Hurley (1952) reported t h a t the 385-mp band in solution may be owing to both octavalent and heptavalent ruthenium (Figure 5). With the argument t h a t octavalent ruthenium with its d subshell vacant should not show any ligand band, the absorption at 655 mp has been attributed to heptavalent ruthenium. Ru(VI1) in potassium perruthenate is tetrahedrally coordinated, and the transition may be attributed to the one possible transition for d1 ion, Le., *r3 +.T 5 . 600 J f p (20,000 cm-1). This band appears mostly as a shoulder in acidic silicate glasses and may be caused by Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No.
2, 1972 179
WAVE NUMBER
‘O0’ 25000
20000
Cm” 14290
16700
/
I
/
g60-
300
4 00
WAVE 350
400 5 00 600 WAVE LENGTH IN MILLIMICRON
-
I
I: Composition 10 II: Composition 7 111: Composition 9 IV: Composition 8
I
I
I
loo[
80
-
z
0 z60-
g a
c
i 4 0 t
300
400
500
600
I
I
600
700
LENGTH
IN
800
MILLIMICRON
7 0 0 750
Figure 3. Absorption spectra of ruthenium in phosphate glasses Curve Curve Curve Curve
I
500
700
800
WAVE L E N G T H IN MILLIMICRON
Figure 4. Absorption spectra of Ru(VI) in 2M KOH (Curve I ) and Ru(IV) in 6M KOH (Curve II)
trivalent ruthenium. The spectrum of octacoordinated trivalent ruthenium in 431 HC1 has a redox band a t 330 mp (30,300 cm-l) and a ligand band a t 510 mp (19,600 em-l) (Jorgensen, 1962). Ballhausen (1962) has assigned T2,+ 2A2, to this transition. 455-65 M p (11,000 cm-l). This band, which appears both in phosphate and in silicate glasses, is believed to be owing to tetracoordinated hexavalent ruthenium. The spectrum of Kz (Ru04) has been reported, among others, by Mukerji and Biswas (1969). It shows two bands, one a t 465 m p ( e = 1742) and the other a t 375 mp ( E = 809). The 465 mp band is probably a mixture of the ligand and charge transfer band and has been given the gross assignment y3 + y 5 . I n phosphate glasses this band appears a t 455 mp. 180 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1, No. 2, 1972
Figure 5. Absorption spectra of Ru(VII) in 2M KOH (Curve I) and Ru(VIII) in 1M perchloric acid (Curve II)
A close inspection of the absorption spectra reveals that two of the available oxidation states are in equilibrium in silicate glasses. Ru(1V) is in equilibrium with Ru(II1) and with Ru(V1) in acidic (25 mol % T\’a20), respectively. I n a silicate glass the percentage of Sa20 decides the valency states to be expected. The alkaline earth cation does not influence to any detectable degree the oxidation states of ruthenium (see Curve I of Figures 1 and 2). X small percentage of A1203favors trivalent ruthenium (see Curves I1 and I11 of Figure 2). An examination of the depth of the Ru(1V) absorption trough (680-90 nip) reveals that the amount of Ru(1V) in glass is less affected by a change in glass composition, whereas the amount of Ru(V1) increases sharply with the increase of X a z 0 above 25 mol %. Belolv 25 mol % K a 2 0 ,ruthenium is present mainly in octahedral coordination since Ru(1V) and Ru(II1) are both octahedrally coordinated. Above 25 mol % Xa20, octahedrally coordinated ruthenium gives place to tetrahedrally coordinated ruthenium, the amount gradually increasing with the increase in NazO content. I n the phosphate glasses studied, only one of the available oxidation states of ruthenium is present. Ru(V1) and Ru(VI1) are the valence species expected to be above and below 50 mol % P20a, respectively. According to the Van Wazer theory of phosphate glasses, a sharp structural change is expected a t this composition. Since Ru(V1) and Ru(VI1) are both tetracoordinated, there is no change in the oxygen coordination consequent to a valency change. Solubilities have been plotted in ppm R u against glass composition. The results are shown in Figures 6-8. Results of evaporation studies are shown in Figure 9. Solubility of ruthenium (Figures 6 and 7 ) increases sharply above 25 mol % S a 2 0 in soda-silica and above 50 mol % ’ P205 in soda-phosphate glasses. Ruthenium solubility below this composition changes slowly with N a 2 0 and PeOr concentration. A correlation of these findings with the discussion on the oxidation state indicates immediately that tetracoordinated hexavalentruthenium can be accommodated in the glasses studied in any appreciable amount. Other oxidation states of ruthenium with different oxygen coordinations can be accommodated only to a limited extent. This is further evidenced bj- the fact that the Ru(1V) absorption troughs in soda-silica glasses are close to each other (within the limits of variation of the thickness of glass pieces on
Mole Per c e n t
Nap
-.
IO
0 1
Mole
20
O/o
30
of B 2 0 3 Replacing G l a s s
-
40
Figure 6. Solubility of ruthenium in soda-silica glasses of varying composition at 1300°C (Curve I), 1200°C (Curve II), 1 100°C (Curve Ill),and 1000°C (Curve IV)
Figure 8. Solubility of ruthenium in borosilicate glasses. Base glass. 0:Na2O:SiOz = 1 :2; A: Na20:SiOS = 1:4
which absorption was measured), whereas t,he Ru(V1) absorpt'ioii increases rapidly with t.he S a z O content in the glass. Solubility of RuOp in the system PbO-P205 was studied for the composition range varying from 45-61 mol % P205. Similar to soda-phosphate glasses, a sharp rise in solubility was observed for composition having greater t'haii 50 mol yo P205.I t exhibited a maximum of about 1.01% at' 56 mol % PzOa. I n borosilicate glasses a gradual decrease in solubility occurs aft'er an initial increase (Figure 8). B203 was added in gradually increasing amounts to two base glasses of S a z O: Si02 of 1 : 4 and 1:2. I n bot,h glasses a n initial increase of solubility occurred to a maximum of 5 mol % &Oar irrespective of the glasses, followed by a decrease to a minimum a t 12 mol % and 23 mol % B203for the two types of glasses, respectively. Boron has a planar triangular oxygen coordination in
B2O3 glass and assumes tetrahedral coordination where
n ' 1 7'0
0
singly bonded oxygen atoms are available. Gradual disappearance of single-bonded oxygen atoms owing to formation of BOa tetrahedra leads to a decrease in solubility of ruthenium. A computation of the ratio of (Si B) atoms to oxygen atoms in the glasses a t the minimum solubility works out to 1 : 2.05 and 1: 2.02, respectively. This is the ratio of Si: 0 in SiO, which, according to the Zachariaseii concept of glass structure, has all its oxygen bonded to two silicon atoms. The initial increase of solubility up to 5 mol 7 0 may be explained as the contribution of B203over that of the glass probably because B 2 0 3 does not enter the glass structure below 5 niol yoB203. 1 similar decrease in solubility in borophosphate glasses is evident from Table 11. This may also be attributed to the formation of BP04 tetrahedron in glass. This indicates that
+
I
'
"
"
"
'
600
I
4'0
'30
A 4 0
5 0
60
7 0
Mole % of P2 Os-
0
Figure 7. Solubility of ruthenium in soda-phosphate glasses of varying composition at 1IOO"C (Curve I), 1OOO"C (Curve II), and 9 0 0 ° C (Curve 111)
2
4
6
8
IO
12
14
16
T i m e in (hr)
Figure 9. Variation of ruthenium solubility and ruthenium evaporation with time Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
181
not, however, remain constant throughout the volatilization run. The change in solubility with time and change in the volatilization rate with time for a soda-silica glass have been plotted in Figure 9. An equilibrium seems to have been established between the rate of solubility and volatilization. Results of the evaporation run are shown in Figure 10. The solubilities plotted here are initial solubilities. The best fits of experimental points are two straight lines of different slopes. The first part passes through the origin with a slope of 0.41 X 10+ mg/hr/ppm ruthenium. Below 61.6 mol yo Si02 in the Na20-Si02 system of glasses, a notable increase in the volatilization rate was observed a t a solubility value of 525 ppm. This was explained to be owing to the formation of RuOc in the bulk of the glass because of increased oxygen diffusion in glass (Ilhargupta and Mukerji, 1968). The activation energy for evaporation of ruthenium from glass was calculated from the Brrhenius plot for 62 mol % Si02 glass. This works out to 18.65kcal/niol. Prevention of Ruthenium Volatilization. From previous discussions i t is apparent that evaporation of ruthenium can be kept low by rendering ruthenium insoluble in the glass. This may be done in three ways:
207 t /I
R u Solubility i n P P M
Figure 10. Volatilization of ruthenium in milligrams plotted against ruthenium solubility at 1 1 OOOC
Table II. Solubility of Ruthenium in Borophosphate Glass at 1000°C
Base glass Na20:P20s= 1:2 B z 0 8 replacing base gloss, mol %
Solubility of Ru, W t %
Remarks
0 10 20
6.21 2.82 1.1
Glass Glass Crystallized
single-bonded oxygen in these glasses should be directly or indirectly responsible for taking ruthenium in solution in glass. The reaction by which ruthenium goes into solution may be written as:
+
RuIV02
1/2
02
+ 2 - 0-
-P
+
( R U ~ ' O ~ - ) ~ -0 2 - (2)
where, -0- represents singly bonded oxygen. Although a polymerization of the network, 0 2 ( R U O ~ ~ - )indicates ~should be considered as breaking the -Si-0-Sibond a t some other place. Heats of solution of RuOz in simple soda-silica glasses have been computed from the slope of the log Seatvs. 1/T plot for a given glass composition where Sa,% is the saturation solubility. For 20, 25, and 33.3 mol % S a 2 0 glasses, a straight line is obtained for experimental points above llOO°C, a curvature in the plot being apparent for points below 1100OC. The 40 mol % NazO glass yields a straight line for points above 1000°C. Computed heats of solution decrease with increases in Ea20 content and are 7.8, 7.3, 6.8, and 4.9 kcal/mol for 20, 25, 33.3, and 40 mol % Na20, respectively. Log SSatvs. 1/T plot for phosphate glasses produces curved lines showing perhaps a deviation from Henry's law. The approximate calculation of the heats of solution for 50 mol yo P205glass works out to 1.1 kcal/mol which is low compared to silicate glasses. Results of evaporation runs have been plotted as milligrams of ruthenium lost vs. solubility of ruthenium in glass. I n all these experiments sufficient ruthenium was added in the glass so that insoluble R u 0 2 remained in the melt and could go into solution when the ruthenium solubility fell below saturation owing to evaporation. The solubility did 182 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
Use of suitable glass composition Use of a n oxygen-free atmosphere for glass melting Use of reducing agents The effect of glass composition on ruthenium solubility has already been discussed. The range of glass composition for minimum evaporation should be evaluated in terms of the atomic ratio (Si H Al)/O (1:2) for silicate glasses and in ternis of percentage Pzos (2:1) indicates that these are no B-0-B linkages of the type found in fused B203. The Si-0-B bands predominate with B being tetrahedrally coordinated when sufficient NazO is present. “If B 2 0 3were added to a sodium silicate glass (e.g., NazO: Si02 equals 1 : 4 or 1: 2) up to about 5 niol %, there should be enough Ya20 to convert the €3 from tri- to tetrahedral coordination; therefore, the B would be a n integral part of the structure and for the most part would play a similar role to that of Si.” Acknowledgment
The author conveys his sincere thanks to K. D. Sharma, Director of the Institute, for his interest. He is also thankful to K. T. Thomas, Head, Waste Treatment Division, Bhabha Atomic Research Centre, Trombay, for stimulating discussions. Literature Cited
Ballhausen, C. J., Introduction t o Ligand Field Theory,” McGraw-Hill, New York, S Y , 1962, p 275. Biswas. S. R., Mukerii. J.. Cent. Glass Ceram. Res. Inst.. Bull.. “
I
I
15, 99-102 (1968).
Bonniaud, R., ORNL, AEC Official, 1966. Brezneva, N. E., Golovanov, Yu. S., Oziraner, S. N., Ermin, A. A,, Rozanova, V. N., “Treatment and Storage of High Level Radioactive Waste,” IAEA, Vienna, Austria, 1963, pp 441-64.
Clarke, W. E., Goodbee, H. W., ibid., pp 411-39. Connick, R. E., Hurley, R. C., J . Amer. Chem. Soc., 74, 5012-15 11952).
Dhargupta, K. K., Mukerji, J., Trans. Zndian Ceram. Soc., 27, 123-9 (1968).
Elf;lot, 31. N., Gayler, R., Grover, J. R., Hardwick, W. H., Treatment and Storage of High Level Radioactive Waste,” IAEA, Vienna, Austria, 1963, pp 489-506. Fletcher, J. &I., Jenkins, I. L., Lever, F. A I . , Martin, F. S., Powell, A. R., Todd, R., J . Znorg. Sucl. Chem., 1, 378-401 119.55). \ - -
- - I
Johnson, K. D. B., Grover, J. R., Hardwick, W. H., “Third United Kations Conference on Peaceful Use of Atomic Energy,” A/COSF, 28jP/188, 1964. Jorgensen, C. K., “Absorption Spectra and Chemical Bonding in Complexes,” Pergamon Press, Oxiord, England, 1962, p 287. Mukerii. J.. Biswas. S.R.. Indian J . Chem.. 7. 1239-42 11969). Wayne: ’E. Bell, Tagami ST.,J . Phys. Chem., 67,2432-6 (1963). Weyl, W. A., Canadian Clay Ceramics, 37, 19-24 (1968). Wilson, A. S.,J . Chem. Eng. Data, 5,521-4 (1960). RECEIVED for review December 8, 1970 ACCEPTEDJanuary 16, 1972
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
183
