Hydrolysis of uranium and thorium in surface-modified bentonite under hydrothermal conditions

ELSEVIER

Journal of Alloys and Compounds 249 (1997) 142-145

Hydrolysis of uranium and thorium in surface-modified bentonite under hydrothermal conditions D.M. Giaquinta’, Chemistry

Division,

Argonne

L. Soderholm, S.E. Yuchs, S.R. Wasserman

Natioml

Lnboratov,

9700 South Cuss Avenue,

Argonne,

IL 604394831,

(ISA

Abstract The speciation of (UO,)“and Th4’ -exchanged bentonite clay samples was probed by X-ray absorption spectroscopy (XAS). Surface modification reactions using hydrophobic silanes were performed in order to ascertain the effects of excluding the free exchange of external waterfrom theclay gallery. Hydrothermalconditionswereusedto mimicpotentialgeologicconditionsin a nuclearwastestorage facility. Both the (UO$+ andTh4’ samples showed evidence of polymeric hydrolysis products similar to the fluorite structure known for both UO, and ThO,.

1. Introduction Clays have been proposed

as storage media

for hazard-

ous waste, including radionuclides. Becauseof the ubiquitous presenceof clays in the environment, and their ability to incorporate cationic species,there has been considerable recent work detailing the interactions of radioactive cations with clay. For example, the exchange and sorption properties of bentonite, a naturally occurring smectite clay, have been studied. Further experiments have examined the swelling capacitiesand structural integrity of these materials as they relate to issuesinvolving the disposalof nuclear waste El- I 11.In this paper, the speciation of uranium- and thorium-exchanged smectite clays is examined as part of a continuing effort to develop improved media for hazardous waste storage. The effect of surface modification and hydrothermal conditioning of the clay, both alone and in tandem, on the speciation and interactions of uranium and thorium in exchanged bentonite are studied. Hydrothermal treatmentsare usedto simulateboth potential environmental conditions and potential waste-storagescenarios [121. Smectite-clay minerals consist of sheetsof aluminosilicates that are separatedby an interlayer that contains both cations, such as calcium and potassium,and water molecules. The inorganic layer structures are 2:l sheets of octahedrally coordinated cations, usually A13+, that share oxygen atoms with two tetrahedrally coordinated sheetsof Si4” [13]. Substitutions within the lattice, typically Mg2+ for A13’, result in a net negative charge that is balancedby the interlayer cations. These interlayer cations are hydrated and readily exchangeable.It is this cation-exchange capa“Corresponding

2. Experimental section The U- and Th-exchanged sampleswere preparedfrom the Ca2+ form of bentonite, [Bentolite L, Southern Clay

author.

0925~8388/97/$17.00 Pit SO925-8388(96)02823-X

bility that has made these materials of such widespread interest for waste storage. Clays are naturally hydrophillic. This affinity for water could potentially facilitate the leaching of any hazardous speciesstored within the mineral. In order to isolate the cationic specieswithin the clay interlayer, we have modified the surfaces of U- and Th-exchanged bentonite clays using alkylsilanes of the form C,,H$iX,, X=Cl, OCH, [ 141.These organosilanesbond covalently to the surface of the clay mineral. The resulting hydrophobic tail coats the external surface of the clay and inhibits the exchange of external water into and out of the interlayer. The exclusion of water reducesthe leaching of exchanged ions from the interlayer back into the environment [ 15,161. The conversion of the clay from a hydrophillic to a hydrophobic medium may have pronounced effects on the hydration of the cation and on the hydrolysis reactions involving the interlayer species. Both processesdepend strongly on the ratio of cation to water within the clay. It has been suggested that hydrolysis processesmay exacerbate leaching [17-191. Therefore the hydrolysis of metal cations within clays, particularly radionuclides, must be fully understoodto adequately resolve problems associated with their disposal.We have used X-ray absorption spectroscopy (XAS) to examine how hydrothermal treatments and surface modification, separately and together, affect the local environments of uranium and thorium within clays. Preliminary results are reported herein.

0

1997 Elsevier

Science

S.A. All rights

reserved

D.M.

Giaquinta

et al. I Journal

qf Alloys

Products], as described previously [20]. Portions of the exchanged samples were coated with octadecyl trimethoxysilane, CH,(CH,),,Si(OCH,), (denoted C,,Si(OMe),) [15]. Samples were hydrothermally treated for 20 h at 200 “C. Powder X-ray diffraction experiments were performed using CuKa radiation, h= 1.54184 A, on a Scintag PAD V diffractometer operating at 1.2 kW. All observed diffraction peaks could be accounted for by the clay structure. For the X-ray absorption experiments, uranium- and thorium-exchanged clay samples were finely ground and placed into 0.25 mm. thick aluminum sample holders and sealed with KaptonB tape. Uranium L,-edge (17166 eV) and thorium L,-edge (16300 eV) [21] X-ray absorption spectra, obtained using fluorescence detection, were collected at the National Synchrotron Light Source (NSLS) on beamline X23A2, and at the Stanford Synchrotron Radiation Laboratory (SSRL) on wiggler beamline 4-3 [20]. Energy calibration was maintained by simultaneously measuring the transmission spectrum of sodium uranyl acetate or thorium nitrate, respectively. The energy at the maximum of the first derivative of the inflection point was assigned a value of 17166 for U and 16300 eV for Th. Standard procedures were used for analyses [22,23]. Each data set represents the average of at least 2 scans. The averaged EXAFS spectra were fit using XAMath [24] from initial parameters of similar U-O or Th-0 systems [25,26].

3. Results Uranium L,-edge EXAFS obtained for the untreated (U02)‘+-loaded clay, shown in Fig. l(a) and (b), are indistinguishable from the data obtained from the same sample after hydrothermal treatment, or after silane-coating the sample. Three coordination shells are sufficient to fit the data, as summarized in Table 1. The coordination is typical of the uranyl ion except for the presence of an additional oxygen shell at 3.45 A. After hydrothermal treatment, the sample coated with C,,Si(OCH,), presents an EXAFS spectrum that is significantly different from the non-coated material, as shown in Fig. l(c) and (d). The typical axial and equatorial oxygen contacts of the uranyl cation have been replaced by a coordination environment very similar to UO, [27]. The first oxygen coordination shell, at 2.33 A, and the uranium-uranium contact, at 3.92 A, are consistent with UO,. The oxygen shell at 3.36 A, however, cannot be accounted for by the fluorite structure. High 2 metal backscattering is evident from .th& high intensity of the EXAFS oscillations above 10 A . This high intensity is not observed in the original (U02)” samples while such a high 2 contribution is expected from the UO, model. The EXAFS spectra obtained from the Th-exchanged clay also can be divided into two different types: those obtained from non-hydrothermally treated samples, and

and Compounds

249 (1997)

142-145

143

6 4

3

4

5

6

7 k (A-1)

8

9

10

11 I- (A)

3

5

2

4 8 .s;’

5 1 X0 s -1 -2

22 E 1

-3 3

4

5

6

7 8 k (A-1)

9

10

11

0 012345678 f (A)

Fig. 1. (a) &weighted EXAFS, k”~(k), of untreated (UO?)‘+-loaded clay. (b) Radial distribution of a; &=2-l 1 A-‘. *indicates peaks potentially due to uranium-clay interactions. Radial distances are not corrected for phase. (c) @,y(k) of hydrothermally treated, C,,Si(OCH,), coated, (UO>)‘+-loaded clay. (d) Radial distribution of b; Ak=2-1 IA-‘. *indicates peaks potentially due to uranium-clay interactions, Radial distances are not corrected for phase.

those from hydrothermally treated samples. Representative spectra are shown in Fig. 2(a)-(d). The thorium-loaded clays that were not treated hydrothermally display behavior indicative of a single hydration sphere with an average distance of 2.49 A. A similar distance and Debye-Waller factor were determined from the spectrum of the reference material, Th(N0,),.4H,O. An additional oxygen shell is seen at 3.66 A in the clay samples. In contrast, spectra from the thorium-loaded clays that were hydrothermally treated are similar to those obtained from the dioxide, ThO, (not shown). The first oxygen shell shifts to a slightly shorter distance upon hydrothermal treatment, and there are two tdditional peaks present at 3.54 A and 3.93 A. The 3.93 A peak is best represented as a thoriumthorium contacb.- ,High Z metal-metal backscattering is seen above 10 A in the EXAFS, previously discussed for the uranium-clay samples.

4. Discussion The coordination environments of uranium in the uncoated samples, before and after hydrothermal treatment, are consistent with individual uranyl units exchanged into the clay interlayer space [4,10,18,28,29]. Both a silane coating and hydrothermal treatment are necessary to produce any observable changes in the uranium-clay system. The C , ,Si( OCH,) ,-coated, hydrothermally treated

D.M.

144

Table 1 EXAFS parameters

for (U02)21-

Giaquintn

and Th’*-loaded

Shell 1 CN r (A,

sig AE Shell 2 CN

clays before

ht+C,,Si(OMc),

2.2

0.0255

0.0096

3.5

7.4

5.0

2.5 3.36 0.0154

2.1 3.66

3.8 3.54

0.0115

0.0229

3.45

sig R-space limits

0.0 175 1-3.6 with EXAFS

6.1 3.92

parameters

are 20% for coordination

Th-0

12

4 10 ‘22

*

0 012345678

1’ (A)

k (A-1)

2

-2 .$ 9

8 *0 B

$6 E -2

3

-4

0 k

12

0.0006

I-4.9

14

10

6.2 3.93

0.0085

sample has an EXAFS pattern that is unique among the uranium samples measured here. The two short axial oxygen atoms characteristic of (U0,)2’ are clearly absent, being replaced by six oxygen atoms at 2.33 A. The result is a spectrum very similar to that of the fluorite structure obtained from UO, [27,30]. Although the bond lengths are similar to those of UO,, the intensity ratio between the U-O shell and the U-U shell, as seen in the radial structure function from the k3-weighted EXAFS, k3.,y(k),

(I-1)

ht Th”+

6.8

2.6

6

ThJ+

6.9 2.39 0.0063 4.0

r 6,

4

(ht)”

(IJO,)‘+

10.7 2.49

0.0085

2

142-145

6.3 2.33

sig Shell 3 CN

associated

treatment

249 (1997)

1.77 0.0016

2.43

errors

and after hydrothermal

(uo,)2+

r (A,

“Estimated

of Alloys and Compounds

et al. I Journal

14

012345678 r' (.J4

Fig. 2. (a) k’-weighted EXAFS, !?~(k), of non-hydrothermally treated, thorium-loaded clay. (b) Radial distribution of a; Ak=2-11 A-‘. * indicates peaks potentially due to thorium-clay interactions. Radial distances are not corrected for phase. (c) k3,&) of hydrothermally treated, thorium-loaded clay. (d) Radial distribution of b; &=2-14 A. *indicates peaks potentially due to thorium-clay interactions. Radial distances are not corrected for phase.

l-3.9 numbers

l-4.2

and 2% for distances.

are not the same. This observation indicates a reduced coordination number in the coated, hydrothermally-treated clay. No indication of UOz(OH)2 was seen although uranyl hydroxide is prepared under similar hydrothermal conditions [31-331. The oxygen contact present at 3:36 A and the peak seen in the uncoated clay at 3.45 A are thought to originate from the clay lattice. Whereas we cam-rot distinguish between Al, Si or oxygen as the backscattering ion in this case, the dimensionsand structure of the interlayer environment suggestthat this feature is due to a clay lattice oxygen. The non-hydrothennally treated Th samplesdisplay the coordination expected for a highly hydrated thorium atom. The coordination number of approximately 11 corresponds to the expected value for the aqueous thorium ion. Thorium-oxygenw”r’ distances(2.5 A) are typically :horter than those found for thorium-oxygen”“‘n’c (2.6 A) in the solid [26,34]. The larger hydration sphere of thorium versus uranyl results in a slightly longer contact between thorium and the siloxane surface of the clay gallery. The hydrothermal treatment of thorium-exchanged bentonite results in the formation of the ThO,-like hydrous polymer that has been reported previously [35]. The EXAFS spectrum measured for the thorium-exchanged clay is similar to that calculated for ThO, using FEFFG [36,37]. The observed intensity of the Th-Th shell is smaller than expected from the calculations. This result is expected if one assumessmall clustersthat would give rise to lessthan the ideal coordination numbersin ThO,. The limited space of the clay interlayer would also be expected to constrain the size of the polymer network. Unlike the uranyl systems,the presenceof the silane coating does not seem to effect the resulting thorium compound. Changes in the speciation of (U0,)2* and Th4’ ions have been observed in exchanged clays as a function of hydrothermal conditioning andl or surface modification. There are indications of aggregation of both these ions within the interlayer spacing of the clay. The uranium

D.M.

Giaquinta

et al. 1 Journal

of Alloys

EXAFS of the hydrothermally treated, trimethoxysilanecoated clay mirrors that of UO,. The possible reduction of U(VI) to U(IV) is discussed elsewhere [20]. Thorium EXAFS of hydrothermally treated clays, both coated and uncoated indicate the presence the ThO,-like hydrous polymer. Further work to characterize these and related samples is currently underway.

Acknowledgments The authors thank Dr. J. Woicik and Dr. Mark R. Antonio for technical assistance. This research was supported by the U.S. DOE-Basic Energy Sciences, Chemical Sciences, under contract W-31-109-ENG-38 and the Strategic Environmental Research and Development Program, U.S. DOD.

References [l] [2] [3] [4] [5] [6] [7] [8] [9] [IO] [Ill [12]

[13] [14]

L.L. Ames, J.E. McGarrah, B.A. Walker and P.F. Salter, Chern. Geol., 35 (1982) 205. L.L. Ames, J.E. McGarrah and B.A. Walker, Clays Clay Minerals, 31 (1983) 321. 2. Borovec, Chejn. Geol., 32 (1981) 45. S.E. Miller, G.R. Heath and R.D. Gonzalez, Clays Clay Minerals, 30 (1982) 111. A. Tsunashima, G.W. Brindley and M. Bastovanov, Clays Cla) Minerals, 29 (1981) 10. C. Chisholm-Brause, S.D. Conradson, C.T. Buscher, P.G. Eller and D.E. Morris, Geochim. Cosmochim. Acta, 58 (1994) 3625. J.M. Oades, Clays Clay MBrerals, 32 (1984) 49. R.B. Heimann, Clays Clay Minerals, 41 (1993) 718. H. Yamada and H. Nakazawa, Clays Clay Minerals, 41 (1993) 726. D.E. Morris, C.J. Chisholm-Brause, M.E. Barr, S.D. Conradson and P.G. Eller, Geochim. Cosmochim. Acta. 58 (1994) 3613. A.J. Dent, J.D. Ramsay and S.W. Swanton, J. Colloid lnteq! Sci., 150 (1992) 45. R.W. Andrews, T.F. Dale and J.A. McNeish, Total System Performance Assessment, 1993: An evaluation of the potential Yucca Mountain Repository, DOE/RW/00134-T12; INTERA, Inc, 1994. R.E. Grim, Clay Mineralogy, McGraw Hill, New York, 1968. In this paper the behavior of the trimethoxysilane surface modi-

and Compounds

249 (1997)

142-145

145

lication is described. For a thorough description of the trichlorosilane surface modification and the surface modification reaction, the reader is urged to consult Ref. 20. K.B. Anderson, K. Song, S.E. Yuchs and C.L. [I51 S.R. Wasserman, Marshall, Method for Encapsulatin g and Isolating Hazardous Cations, Medium for Encapsulating and Isolating Hazardous Cations, U.S. Patent, Applied For. El61 S.R. Wasserman, K. Song, S.E. Yuchs and E.D. Carlson, (unpublished). J.M. Zachara, S.C. Smith and G.D. Turner, Clays Cl71 J.P. McKinley, Clay Minerals, 43 (1995) 586. [ISI J.M. Zachara and J.P. McKinley, Aquatic Sci., 55 (1993) 250. H. Diamond, L. Soderholm, E.P. Horwitz, L.M. [I91 P. Thiyagarajan, Toth and L.K. Felker, Inorg. Chem., 29 (1990) 1902. 1201D.M. Giaquinta, L. Soderholm, S.E. Yuchs and S.R. Wasserman, Envimn. Sci. Technol., in press (1996). Pll J.A. Bearden and A.F. Burr, Rev. Mod. Phys., 39 (1967) 125. I221 Boon K. Teo, EXAFS: Basic Principles and Data Analysis, Springer, Berlin, 1986. and R. Prins [231 D.E. Sayers and B.A. Bunder, in D. C. Koningsberger (eds.), X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley, New York, 1988, Chap. 6. (1995) XAMath is available on the World Wide ~241 S.R. Wasserman, Web at http:/lxafsdb.iit.edu:80. GE. Brown and G.A. Parks, Physica B, 209 ~251 H.A. Thompson, (1995) 167. 20 (1966) WI T. Ueki, A. Zalkin and D. Templeton, Acta Crystaliogr., 836. [271 F. Weigel, in J.J. Katz, G.T. Seaborg and L.R. Morss (eds.), The Chew&Q) of the Actinide Elements, Vol. 1, Chapman & Hall, New York, 1986, p. 260. 1281R. Calvet and R. Prost, Clays Clay Minerals, 19 (1971) 175. 12% M.D. Newsham, E.P. Giannelis, T.J. Pinnavaia and D.G. Nocera, J. Am. Chem. Sot., 110 (1988) 3885. E301 N.T. Barrett, G.N. Greaves, B.T.M. Willis, G.M. Antonini and F.R. Thornley, J. Phys. C, 21 (1988) L791. B, 27 (1971) 2018. [311 J.C. Taylor and H.J. Hurst, Acta Cystallogr. and J.C. Taylor, Acta Crystallogr. B, 26 (1970) [321 M.J. Bannister 1775. and E. Gebert, Acta Crystaflogr. B, 28 [331 S. Siegel, H.R. Hoekstra (1972) 3469. t341 G. Johansson, Acta Chem. &and., 22 (1968) 389. of Cations, Wiley, New [351 C.E Baes and R.F. Mesmer, The Hydrolysis York, 1976. and R.C. Albers, Phys. [361 J. Mustre de Leon, J.J. Rehr, S.I. Zabinsky Rev. B, 44 (1991) 4146. and R.C. Albers, J. Am. [371 J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky Chem. Sot., 113 (1991) 5135.