Sorption Behavior of Trivalent Actinides and Rare Earths on Clay

12 Sorption Behavior of Trivalent Actinides and Rare Earths on Clay Minerals

1

G. W. BEALL, B. H. KETELLE, R. G. HAIRE, and G. D. O'KELLEY

Downloaded by UNIV OF PITTSBURGH on October 22, 2014 | http://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0100.ch012

Oak Ridge National Laboratory, Oak Ridge, TN 37830

Much interest in the past few years has been generated in connection with problems of radioactive waste isolation in a growing nuclear economy. Many studies have been initiated to find the most suitable sites for waste repositories, and the environmental impact i f breaches occur in such repositories. These studies must ultimately result in safe methods of storage for nuclear waste for the present and for times that are measured on a geologic scale. Salt mines have been suggested as possible sites for such repositories owing to their geologic stability and limited presence of water. The experiments which we wish to report in this paper were fashioned with this type of repository in mind. Clay minerals are potentially useful in several applications of exchange of radionuclides. They have been shown to be quite useful in removal of specific nuclides from waste streams (1). In connection with the Swedish waste isolation program they are being considered for secondary containment (2). A special case of secondary containment occurs i f waste material should escape from their casks and from the surrounding salt beds. They can be expected to be held by geologic formations surrounding such beds. Therefore, studies of rates of exchange and equilibrium constants are important. Experimental Distribution coefficients were measured employing batch methods. The solution (ml) to clay (mg) ratios were approximately as follows: 1:5 attapulgite; 1:60 montmorillonite; and 1:25 kaolinite. The solutions were brines (NaCl) buffered with pH 5 acetate solution. The original stock solution contained 3 M NaCl and 1 M Na acetate buffer. The lower [Na] solutions were made by Research sponsored by the Office of Basic Energy Sciences, Division of Nuclear Sciences, U.S. Department of Energy, under contract (W-7405-eng-26) with the Union Carbide Corp. 1

0-8412-0498-5/79/47-100-201$05.00/0 © 1978 American Chemical Society

In Radioactive Waste in Geologic Storage; Fried, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

RADIOACTIVE W A S T E IN GEOLOGIC

202

appropriate dilution of this stock solution. The clays employed were obtained from Source Clay Minerals Repository (3:). Attapulgite and kaolin were used as received. A 2% dispersion montmorillonite was prepared and centrifuged to remove approximately 80% of the sand. The remaining suspended clay was converted to the sod ium form by passing i t through a Dowex-50 cation exchange column (Na form) at 60°C. The tracers employed in the exchange studies were Am, ^ C m , C f , 5 3 - ^ E s , *UoLaj i 5 7 i69 i34 R b , *+K and S r . The actinides and most of the other isotopes were made at the High Flux Research Reactor at Oak Ridge National Laboratory. The Yb, C s , R b , and S r isotopes were ob tained from New England Nuclear. 2l+1

2

2 i + 9

2

2

S

86

2

m

j

Y

b

i

C

S

j

8 5

1 6 9


STORAGE

13l+

86

8 5

Results The three clays used in this study have different morpholo gies. The montmorillonite and kaolinite are typical sheet or plate-like clays, whereas the attapulgite exhibits a needle-like morphology (4j. The distribution coefficients (D) in units liters/kg have been measured as a function of concentration of sodium varying from .25 M to 4 M. They were studied as a function of [Na] be cause from the ion exchange equilibrium (5) the equation relating D to [Na] is given by: d log D = -M/N d log[Na] where: Ν = charge of cation on the exchanger M = charge of cation exchanging with cation of charge N. The distribution coefficients (D) in units liters/kg for the rare earths and actinides studied are listed in Tables 1, 2, and 3. Graphical representation of the data for typical rare earths and actinides, contained in Tables 1, 2, and 3, can be seen in Fig ures 1 and 2, where log D's vs. log [Na] are plotted. Under a given set of conditions there is very l i t t l e difference between the rare earths and actinides in any of the clays. All the clays exhibit slopes of -1.1 to -1.7, which is in better agreement with a monoacetate complex that would be expected to y i e l d slopes of -2 than the bare Am ion. There are no good activity coefficients available for NaCl, Na acetate and AmCl mixtures; therefore, no attempt was made to correct the experimental numbers. The D val ues for kaolinite and montmorillonite are in reasonable agreement when normalized by their respective capacities of 3.0 and 78 m.e./ 100 gm, indicating that the mechanism of exchange for the two clays are quite similar. In contrast, the D values for attapul gite are completely out of proportion with its respective 13 m.e./ 100 gm capacity. Another striking difference can be seen in the kinetics of exchange between the clays. Kaolinite and montmorillonite have very rapid exchange reactions that take less than fifteen minutes to come to equilibrium. Attapulgite in contrast takes as long as nine days to come to equilibrium (Figure 3). The D values for Am in a 0.5 M [Na] solution are given as a function of time in Figure 4. It can be seen that there is a very rapid i n i t i a l re+3

3

21+1


12.

BEALL ET AL.

Sorption

Behavior

on Chy

Minerah

203

Table 1


Distribution Coefficients U / k g ) for M(III)/Na(I) Exchange on Kaolinite at pH 5 Molarity [Na] Am Cm Cf Sm Es(pH 4) La 4 2 1 0.5 0.25

0.22 0.45 1.07 1.9 4.3

0.3 0.5 0.7 1.5 3.2

0.16 0.5 1.1 2.2 5.3

0.5 2.0 1.4 2.4 4.9

0.2 0.33 0.44 0.85 3.0

Yb

0.15

0.07 0.11 0.32 0.63 1.83

-

0.44

-

3.95

Table 2 Distribution Coefficients U / k g ) for M(III)/Na(I) Exchange on Montmorillonite at pH 5 Molarity [Na] Am Cm Yb Cf Es(pH 4) La Sm 4 2 1 0.5 0.25

2.2 2.9 6.4 11.5 33.9

2.4 3.6 6.6 14 37

1.6 3.8 7.7 18.0 43.5

3.1 3.5 9.1 25.0 54.0

1.7 2.2 4.0 12.2 38.5

1.7 2.5 5.4 13.7 39.2

1.5 3.1 5.5 18.2 48.7

Table 3 Distribution Coefficients U / k g ) for M(III)/Na(I) Exchange on Attapulgite at pH 5 Molarity [Na] Am Cm Sm Yb Cf La 4 2 1 0.5 0.25

163 550 1,700 6,600 20,400

251 600 1,600 4,000 13,800

310 778 2,300 9,500 24,600

98 201 306 680 1670

99 194 437 1000 1490

103 177 248 490 830



204

Π


10,000

STORAGE

I—I—I—F

Ο Sm • Yb

/ /

/ 1000

ATTAPULGITE

A

100

MONTMORILLONITE

/ /

, 8

10

/O

ν 6

•

0.1 '

iZl 4

2

KAOLIN

I 1

I L 0.5

0.25

[Να] Figure 1.

Flot of Sm and Yb exchange on kaolin, montmorillonite, and attapul gite: log D vs. log [Na].


12.

BEALL E T AL.

Sorption

Behavior

on Clay

Minerah

τ

O Am • Cf

/Ο

V

10,000 h —

/ο


/ο

1000

/ ATTAPULGITE /Ο

• / / /

ο'

100

/ο MONTMORILLONITE · '

/ο

10

V ,

/ D

/

/

KAOLIN

/ 0.1

Figure 2.

J 4

L 2

J

L

1 0.5 0.25 [Να]

Plot of Am and Cf exchange on kaolin, montmorillonite, and attapul gite: log D vs. log [Na].


205


STORAGE


206

Figure 3.

Log D vs. log [Na] for Am on attapulgite as a function of time of equilibrium


12.

BEALL ET AL.

Sorption

Behavior

on Clay

207

Minerals


100,000

10,000 h—

1000

100

60

80

100

120

140

160 180

TIME (hr) Figure 4.

Kinetics of exchange of Am on attapulgite



208


STORAGE

action requiring less than a day for equilibrium, with a second very slow reaction requiring days to come to equilibrium. The D values for Am, Cs, Rb, Κ and Sr on attapulgite are plotted in Figure 5. The D values increase from Κ to Rb and f i n ally Cs. The Cs behavior has been previously reported (7,8). The rates of exchange also follow this order. The Κ and Rb are quite rapid reactions while Cs has a two component reaction, one rapid requiring approximately thirty minutes with a second requir ing almost a day. A l l of the preceding data indicate some highly unusual and specific reactions of attapulgite for the rare earths, actinides, and Cs. These highly specific reactions cannot be ex plained satisfactorily by analogy to the comparable reactions of kaolin and montmorillonite. The most striking difference between these three clays has already been mentioned previously. The morphological differences deserve more scrutiny as to their structural origins. Structural representation of kaolin, montmorillonite, and attapulgite are given in Figures 6, 7 and 8, respectively. It can be seen that kaolin and montmorillonite do derive their morphologies largely from their structures. These two clays are basically layered stru ctures consisting of aluminum oxygen octahedra, and silicon oxygen tetrahedra, differing only in the order of stacking of the com ponents. Attapulgite likewise is built up of these basic units, but the major difference occurs in the arrangement of these units. The f i r s t two clays were extended layers of alternating planes of Al and S i . Attapulgite has this same layered structure on a micro scopic level. These microscopic units are then linked across a single oxygen shared between two Si tetrahedra. This linking bond exhibits prominent cleavage which results in the needle-like morph ology of attapulgite. This linking of the microscopic layered structures also result in a very open channeled structure which contains about 8 z e o l i t i c waters per unit c e l l . These channels could possibly be sources of the high specificity for the rare earths and actinides. It can be speculated that the size of the channels must also be the reason for the differences in both the kinetics and D values seen for Cs, Rb, and K. The two rates of reaction seen for Am and Cs could be explained by these channels also, since the needles would contain two types of channels. The f i r s t would be channels that are quite open resulting from the pro minent cleavage exhibited across the weak bond shown in Fig. 8. The second type of channel would be internal having only ends ex posed to exchange. The rapid reaction could result from interac tion with these highly exposed channels, with the second slower reaction limited by diffusion into the inner channels. The D values on the three clays studied indicate very l i t t l e difference between the rare earths and actinides. The D values also indicate that there is a small increase in D when going from low atomic number to higher atomic number in agreement with re sults of (9). The most unexpected results are that geometric or structural considerations can cause high specificity for some


BEALL ET AL.

Sorption

Behavior

on Clay


12.

209

Minerah

Sr ON ν Κ ATTA

0

,I

Figure 5.

I

I

4

2

I

I

I

I

1 0.5 0.25 [No]

Comparison of the exchange of Am, Cs, Rb, K, and Sr on attapulgite



210


Figure 6.

Structural representation of kaolin


STORAGE

BEALL ET AL.

Sorption

Behavior

on Clay

Minerals


12.

Figure 7.

Structural representation of montmorillonite


211


RADIOACTIVE W A S T E

IN GEOLOGIC

YJk&r^&o—o-o—Λ-Α-Ρ

οο

οο /S^t

Figure 8.

V-*»V

V-*-V

ν-Κ/

Ν#«*>ν

VÎSV

Structural representation of attapulgite


STORAGE

12.

BEALL ET AL.

Sorption

Behavior

on Chy

213

Minerals

ions. This high specificity for the actinides in particular has important implications for waste isolation,since * the long term these elements present the greatest hazards in nuclear waste. This behavior of attapulgite toward the actinides could also have possible applications in purification of waste streams or in cleanup of radioactive s p i l l s . Ί


Literature

η

Cited

1. Tamura, Τ and Jacobs, O. G . , Health Phys. 5, 149-154 (1961). 2. "Handling of Spent Nuclear Fuel and Final Storage of Vitri fied High Level Waste," Technical Reports by the Swedish Nuclear Fuel Safety Project (KBS),1978. 3. Source Clay Mineral Repository, University of Missouri, Rolla, Mo. 4. Grim, Ralph E., "Clay Mineralogy," 2nd E d . , McGraw-Hill, Ν. Y . , New York (1968). 5. Nelson, F., Murase, T., and Kraus, K . , J. Chromatog. 13, 503-535 (1963). 6. Choppin, Gregory R., and Schneider, Joan K . , J. Inorg. Nucl. Chem., 32, 3283-3288 (1970). 7. Chandra, Umesh, Master's Thesis, University of Bombay (1969). 8. Merriam, C.Neale, and Thomas, Henry C., J. Chem. Phys., 24, 993-995 (1956). 9. Adgaard, P . , Bull. Groupe franc. Argiles, 26, pp 193-199. RECEIVED January 16, 1979.


Sorption Behavior of Trivalent Actinides and Rare Earths on Clay

Recommend Documents