Surface Studies of the Interaction of Cesium with Feldspars - ACS

13 Surface Studies of the Interaction of Cesium with Feldspars

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

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

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

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GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

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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BROWN ET A L .

Cesium-Feldspars Interaction

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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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Cesium-Feldspars Interaction


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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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Surface Studies of the Interaction of Cesium with Feldspars - ACS

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