Geochemical Behavior of Disposed Radioactive Waste - American

and l a b r a d o r i t e , C a A l 2 S i 2 0 8 as representative .... to ultrasonic treatment in acetone for periods of up to 1 hour. Transmission IR...
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13 Surface Studies of the Interaction of Cesium with Feldspars

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D. L. BROWN, R. I. HAINES, D. G. OWEN, F. W. STANCHELL, and D. G. WATSON Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba R0E 1L0 Canada

The interaction of cesium ions with feldspars at 150°C and 200°C has been studied i n d i s t i l l e d water, granite groundwater and saline solution. Pollucite, CsAlSi O , was identified by infrared spectroscopy, and was formed as a cubic c r y s t a l l i n e phase. Sur­ face analytical techniques (XPS, SAM, SIMS and SEM/ EDX) show Cs to be sorbed onto the mineral surfaces and alteration products. The mechanism of pollucite formation and i t s relevance to cesium transport/ retardation i n the near f i e l d of a nuclear waste­ -disposal vault i s discussed. 2

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In the Canadian Nuclear F u e l Waste Management program, d i s p o s a l o f r a d i o a c t i v e waste i s envisaged w i t h i n an engineered m u l t i - b a r r i e r v a u l t , deeply embedded i n a p l u t o n i c formation w i t h i n the Precambrian S h i e l d ( 1 ) . The g e o l o g i c formation i s the f i n a l b a r r i e r i n the scheme, and the i n t e r a c t i o n o f r a d i o n u c l i d e s w i t h rock-forming minerals i s the important element of t h a t b a r r i e r . T h i s paper d e s c r i b e s our r e s e a r c h i n t o the behaviour o f cesium i n contact with f e l d s p a r s . Feldspars c o n s t i t u t e a major p o r t i o n o f the igneous rock o f a p l u t o n . We have chosen m i c r o c l i n e , K A l S i 3 0 and l a b r a d o r i t e , C a A l S i 0 as r e p r e s e n t a t i v e f e l d s p a r s o f g r a n i t e and gabbro rocks, r e s p e c t i v e l y , f o r t h i s study. We have i n c l u d e d a l b i t e , N a A l S i 0 f o r comparison. T h i s f e l d s p a r occurs together with the former two minerals i n nature. Cesium has been chosen f o r study f o r s e v e r a l reasons. Although the amount o f cesium present i n the f i s s i o n products of used f u e l i s low (0.7%) ( 2 ) , i t ' s isotopes have r e l a t i v e l y long h a l f - l i v e s ( τ / ( C s ) 30 years, T # ( C S ) » 3 χ 10 y e a r s ) , they emit p e n e t r a t i n g r a d i a t i o n , and are deemed t o be r e l a t i v e l y mobile i n aqueous s o l u t i o n s , p e r c o l a t i n g through rocks and s o i l s . Hence, t o be able to p r e d i c t the movement of t h i s element i n the geosphere, i t i s necessary t o understand i t s p h y s i c a l and chemical i n t e r a c t i o n s with the g e o l o g i c m a t e r i a l s i t i s l i k e l y t o c o n t a c t . 8

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0097-6156/ 84/ 0246-0217S06.00/ 0 © 1984 American Chemical Society

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Two extreme scenarios are p l a u s i b l e . F i r s t l y , the waste v a u l t remains i n t a c t f o r a long p e r i o d , f o l l o w i n g which f i s s i o n products escape i n t o the immediate v i c i n i t y of the v a u l t . Here, i n t e r ­ a c t i o n s w i l l occur between the r a d i o n u c l i d e s and the geosphere, which has been hydrothermally a l t e r e d by the groundwater, heated by the thermal energy r e l e a s e d by r a d i o a c t i v e decay. At the other extreme, we may consider the case where the v a u l t i s breached e a r l y i n the thermal p e r i o d , r e l e a s i n g r a d i o n u c l i d e s i n t o a hydrothermal environment. I t i s t h i s second case which i s being considered here. We have t r e a t e d f e l d s p a r s with hydrothermal s o l u t i o n s of non-active cesium c h l o r i d e i n water, and s e l e c t e d groundwaters, and analyzed the r e s u l t i n g s o l u t i o n s and m i n e r a l s u r f a c e s , t o e l u c i d a t e the nature of the i n t e r a c t i o n of cesium with f e l d s p a r s u r f a c e s , and to determine whether t h i s n u c l i d e w i l l be m o b i l i z e d or retarded by the geosphere under these c o n d i t i o n s . Experimental M i c r o c l i n e , a l b i t e and l a b r a d o r i t e were obtained from Ward's Canada L i m i t e d . Cleaved c r y s t a l fragments and 60 mesh-sized samples were u l t r a s o n i c a l l y cleaned p r i o r to use. Powdered samples ( p a r t i c l e s i z e < 25 ym) were prepared by g r i n d i n g i n a tungsten c a r b i d e b a l l m i l l , wet-sieving and washing i n water. The water was doubly d i s t i l l e d and d e i o n i z e d before use. G r a n i t e groundwater (G.G.W.) and standard Canadian S h i e l d s a l i n e s o l u t i o n s (SCSSS) were prepared according to the method of Vandergraaf ( 3 ) . Experiments were performed i n T e f l o n - l i n e d t i t a n i u m auto­ c l a v e s , submerged i n a thermostated o i l bath. The supernatant l i q u o r s were analyzed using atomic absorption spectrometry. Solid r e s i d u e s were washed with water, d r i e d and examined by the f o l l o w i n g surface a n a l y t i c a l methods: 1.

SEM/EDX s t u d i e s were performed with an ISI DS 130 scanning e l e c t r o n microscope, u s i n g gold-sputtered samples. XPS measurements were made u s i n g a McPherson ESCA-36 spectrometer, with an operating pressure of 5 χ 10" Pa. Spectra were obtained u s i n g the A l ^ e x c i t i n g r a d i a t i o n . Energy d i s ­ p e r s i o n c a l i b r a t i o n s were made using the known energy d i f f e r e n c e of 1253.6 eV between the Mg (3p) and Mg(ls) l i n e s f o r evaporated magnesium metal. An i n t e r n a l carbon standard was used f o r measuring b i n d i n g e n e r g i e s . Auger s p e c t r a of the mineral surfaces were obtained using a P h y s i c a l E l e c t r o n i c s I n d u s t r i e s (PHI) Model 590A scanning Auger microprobe (SAM). Spectra were recorded i n the pulse count (N(E)*E) mode at 0.6% r e s o l u t i o n (ΔΕ/Ε) u s i n g a 3 kV, 3 nA primary e l e c t r o n beam. Background instrumental pressure was maintained at 1.33 χ 10" Pa during data a c q u i s i t i o n . Secondary i o n mass s p e c t r a (SIMS) were c o l l e c t e d using a PHI Model 3500 SIMS I I , attached to the SAM. A PHI Model 04-303 7

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d i f f e r e n t i a l l y pumped Ar i o n gun was used as the e x c i t a t i o n source- The secondary i o n s p e c t r a were e x c i t e d by a 4.5 kV, 400 nA A r beam, r a s t e r e d over a 1.5 mm-square area. F o u r i e r transform i n f r a r e d (FTIR) s p e c t r a were obtained using a N i c o l e t 10 MX FTIR spectrometer, i n the s p e c t r a l range 4000 cm" t o 270 cm" . Two hundred mg KBr d i s c s , c o n t a i n i n g about 1 mg sample were used, i n a nitrogen-purged system. D i s c s of r e a c t i o n product were prepared by u l t r a s o n i c a l l y removing the product from the sample surface under acetone, decanting, c e n t r i f u g i n g and d r y i n g the r e s u l t a n t product suspension, and mixing w i t h an appropriate amount of KBr. +

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Results Cesium-ion c o n c e n t r a t i o n s i n d i s t i l l e d water and s y n t h e t i c groundwaters were measured a f t e r contact w i t h the f e l d s p a r s f o r v a r i o u s periods of time, over the temperature range 150 C t o 200 C. I t was found that f o r short r e a c t i o n times (< 5 days), there was l i t t l e r e d u c t i o n i n the c o n c e n t r a t i o n of cesium i o n , i . e . l i t t l e s o r p t i o n of C s by the m i n e r a l s . Removal of C s from s o l u t i o n was enhanced by increased mineral surface area, r e a c t i o n temperature and time. I t was observed that i n the extreme case f o r powdered l a b r a d o r i t e , 98% of an i n i t i a l 10" mol dm" s o l u t i o n of C s was sorbed a f t e r 14 days at 200°C i n d i s t i l l e d water. The morphology, composition and chemical s t r u c t u r e of the mineral surfaces were i n v e s t i g a t e d by s e v e r a l a n a l y t i c a l methods, as described below. +

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Scanning E l e c t r o n Microscopy. Examination of the surface of cleavage fragments of m i c r o c l i n e and a l b i t e , a f t e r r e a c t i o n f o r 14 days at 200 C with 10~ mol dm"" aqueous cesium c h l o r i d e s o l u t i o n , revealed the formation of l a r g e (10 - 20 ym length) cubic c r y s t a l s (see F i g u r e 1 ( a ) ) . Sub-micron c r y s t a l s were observed covering the m i n e r a l surface surrounding the cubes, formed by hydrothermal a l t e r a t i o n of the m i n e r a l . EDX spot analyses showed the cubes to be composed, of Cs, A l and S i (elements l i g h t e r than F cannot be detected by t h i s method). Reaction times of £ 5 days produced no d e t e c t a b l e (by SEM) cesium-containing phases. However, a f t e r t r e a t i n g the m i n e r a l s with water at 200 C f o r 14 days i n the absence of cesium, then adding the a p p r o p r i a t e amount of C s C l , we observed cesium a l u m i n o s i l i c a t e c r y s t a l s w i t h i n 42 hours of f u r t h e r hydrothermal reaction. The cubic c r y s t a l s were not observed i n experiments i n v o l v i n g low (< 10" * mol dm" ) cesium c o n c e n t r a t i o n s . For runs i n v o l v i n g powdered and coarse-grained m i n e r a l s , spherular and 30-sided p o l y h e d r a l cesium a l u m i n o - s i l i c a t e c r y s t a l s were observed ( F i g u r e 1(b) and ( c ) ) . For powdered l a b r a d o r i t e , r o s e t t e formations were seen (Figure 1 ( f ) ) . Such morphologies are t y p i c a l of analciraetype m i n e r a l s , p a r t i c u l a r l y p o l l u c i t e , C s A l S i 0 ( 4 ) . EDX a n a l y s i s of the a l t e r a t i o n product surrounding the cesium alumino2

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s i l i c a t e showed that Cs was a l s o sorbed onto t h i s m a t e r i a l . S i m i l a r behaviour was observed f o r r e a c t i o n s at 150 C, although r e a c t i o n times were much longer (47 days), and c r y s t a l growth much l e s s (Figures 1(d) and ( e ) ) . A l b i t e behaved i n a s i m i l a r f a s h i o n to raicrocline under a l l c o n d i t i o n s . Experiments performed i n G.G.W. a l s o produced r e s u l t s analogous to those obtained using d i s t i l l e d water as the medium. T h i s observation i s not unexpected, s i n c e the GGW contains q u i t e low concentrations o f d i s s o l v e d i o n s , and contact of d i s t i l l e d water with f e l d s p a r s at high temperatures i s expected to produce s o l u t i o n s of s i m i l a r i o n i c strength (due to K or C a , N a and S i species) q u i t e r a p i d l y . For example, l a r g e cubes were produced by r e a c t i o n of cleaved m i c r o c l i n e with 10" mol dm" CsCl f o r 14 days i n G.G.W. (Figure 2 ( a ) ) . For experiments performed with SCSSS, no l a r g e cubes were detected. However, considerable a l t e r a t i o n product was observed, as 1 - 2 ym-sized c r y s t a l s of montmorillonite ( F i g u r e 2 ( b ) ) . EDX showed them to c o n s i s t of Mg, Ca, A l , S i (and 0 ) .

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FTIR Spectra. A f t e r r e a c t i o n , the mineral samples were subjected to u l t r a s o n i c treatment i n acetone f o r periods of up to 1 hour. Transmission IR s p e c t r a of the r e l e a s e d product f i n e s from r e a c t i o n s of 60-mesh l a b r a d o r i t e and m i c r o c l i n e with 10" mol dm" CsCl i n d i s t i l l e d water at 150 C f o r 47 days are presented i n F i g u r e 3. The spectrum obtained from of the l a b r a d o r i t e r e a c t i o n product (Figure 3(a)) i s i d e n t i c a l w i t h that of a sample of n a t u r a l p o l l u c i t e ( F i g u r e 3 ( c ) ) . The spectrum obtained from the m i c r o c l i n e r e a c t i o n product (Figure 3(b)) contains bands i n the 500 cm" to 800 cm" r e g i o n , due to unreacted m i c r o c l i n e , i n a d d i t i o n to the major bands of p o l l u c i t e . Heating the samples overnight at 105 C r e s u l t e d i n the disappearance of the water a b s o r p t i o n bands at about 3500 cm" and 1680 cm" .

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XPS, SAM, SIMS Analyses. For r e a c t i o n times < 5 days, or [ C s ] < 10""* mol dm" , XPS r e s u l t s showed about 10 atomic % Cs to be present on the surface of the mineral samples. Depth p r o f i l i n g using SIMS i n d i c a t e d that i n these cases, the sorbed Cs was present f o r only the f i r s t 200 nm. Table I l i s t s the binding energies f o r the major c o n s t i t u e n t elements present on the mineral s u r f a c e s , both before and a f t e r r e a c t i o n . These values are, i n general, i n good agreement w i t h l i t e r a t u r e values f o r a l u m i n o s i l i c a t e s ( 5 ) . The value f o r Cs corresponds to that f o r i o n i c Cs s a l t s . No l a r g e s h i f t s are expected i n these numbers, s i n c e there i s no o x i d a t i o n s t a t e d i f f e r e n c e between reactants and products. A d e t a i l e d XPS study of the i n t e r a c t i o n of cesium (and strontium) with f e l d s p a r surfaces i s i n progress, and w i l l be the subject of a f u t u r e paper. The 0/Cs r e g i o n (500 - 600 eV) of the n u m e r i c a l l y d i f f e r e n t i a t e d Auger spectrum of one of the cesium a l u m i n o s i l i c a t e c r y s t a l s , found on the face of m i c r o c l i n e exposed to 10" mol dm" 1

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Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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F i g u r e 1. SEM photomicrographs of m i c r o c l i n e ( a , c and d ) , a l b i t e (b) and l a b r a d o r i t e (e-g) a f t e r hydrothermal r e a c t i o n with CsCl (see t e x t ) . S c a l e : 4.5 mm - 19.1 urn (a) , 2.98 μπι (b) , 3.64 urn (c) , 1.98 urn ( d ) , 2.36 urn (e) , 2.42 ( f ) , and 2.34 p i (g) .

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F i g u r e 2. SEM photomicrograph of (a) cleaved m i c r o c l i n e surface a f t e r r e a c t i o n with 1 0 ~ mol dm" CsCl at 200°C f o r 14 days i n G.G.W; (b) as ( a ) , but i n SCSSS. S c a l e : 6 mm = 10.9 ym (a) and 5.62 um ( b ) . 2

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Table I . Elemental Binding Energies ( e V ) f o r Feldspars ( i ) Before and ( i i ) A f t e r Hydrothermal Reaction with Aqueous Cesium S o l u t i o n s Mineral (i)

Al(2p)

K(2p)

Na(ls)

Ca(2p)

102.7 103.0 102.50 102.80

293.65

1071.95 1071.85 1072.95

348.20

Cs(3d)

f r e s h minerals

Microcline Albite Labradorite Pollucite Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch013

Si(2p)

74.50 74.65 74.60 76.10

724.6 *~2 *"3 ο ( i i ) a f t e r r e a c t i o n with 1θ" mol dm" Cs at 200 C f o r 14 days Microcline 75.45 103.10 293.50 1071.75 724.95 Albite 75.70 103.20 725.05 L a b r a d o r i t e 74.9 102.65 1072.2 348.45 724.4 a

E r r o r i s + 0.25 eV.

CsCl a t 200°C f o r 14 days, i s shown i n F i g u r e 4. Before t h i s spectrum was c o l l e c t e d , the cube was sputtered with Ar ions to remove s u r f a c e contamination. Two Cs MNN t r a n s i t i o n s a r i s e a t 554 and 566 eV (6) and the l a r g e r 0 KLL peak i s a t 506 eV. Super­ imposed on F i g u r e 4 i s a spectrum of n a t u r a l p o l l u c i t e , recorded under the same instrument c o n d i t i o n s and c o r r e c t e d f o r a -4 eV p o s i t i o n a l d i f f e r e n c e observed i n each of the three peaks. The same charging s h i f t was observed i n the S i peaks (not shown i n F i g u r e 4 ) , which were found a t 76 eV (cube) and 72 eV ( n a t u r a l p o l l u c i t e ) . T r a n s i t i o n s from four elements, A l , S i , 0, and Cs, were observed i n both s p e c t r a . Potassium was not detected i n e i t h e r spectrum. The chemical composition of the cube was c a l c u l a t e d from the Auger s p e c t r a u s i n g elemental s e n s i t i v i t y c o e f f i c i e n t s d e r i v e d from the spectrum of n a t u r a l p o l l u c i t e , assuming the composition of the l a t t e r to be C s A l S i 0 . In t h i s manner, an e m p i r i c a l composition of Csi. Α1χ ι S i 0 was determined f o r the cube on the s u r f a c e of the m i c r o c l i n e . Several p o s s i b l e sources of e r r o r may c o n t r i b u t e to the observed d i f f e r e n c e i n composition. Other than the assumption that the n a t u r a l p o l l u c i t e was anhydrous, the most l i k e l y reason f o r the d i f f e r e n c e between the two i s the electron-beam-induced r e d u c t i o n of S i i n the matrix, r e s u l t i n g i n a s h i f t of the 76 eV peak ( S i 0 ) to 88 eV ( S i ) , and p a r t i a l des o r p t i o n of 0. Indeed, some r e d u c t i o n of S i 0 t o elemental S i was observed i n the spectrum of the cube. The apparent h i g h Cs con­ c e n t r a t i o n may be due to Cs adsorbed on the s u r f a c e that was i n ­ completely removed by s p u t t e r i n g . A s i m i l a r q u a n t i t a t i v e a n a l y s i s of a cubic c r y s t a l found on a l b i t e r e s u l t e d i n the e m p i r i c a l formula C s ^ A l j ^ j S i j ^ 0 . According to the r e l a t i v e i n t e n 2

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