28 Near-Surface Alkali Diffusion into Glassy and Crystalline Silicates at 25°C to 100°C Art F. White and Andy Yee Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Alkali uptake by obsidian and feldspar was investigated in batch and recirculating column experiments and by XPS profiling using ion sputtering and variable take-off angles. No measurable alkali diffusion occurred in the feldspars. Diffusion coefficients were determined for Rb, Cs and Sr in glass using a one-dimensional model assuming interdiffusion with Na. Activation energies for Rb, Cs and Sr in glass were 50, 83, and 70kJ respectively which were significantly lower than that measured in previous high temperature experiments and indicate diffusion into a hydrated near-surface matrix. T h e i m m o b i l i z a t i o n o f dissolved c h e m i c a l species b y a d s o r p t i o n a n d i o n exchange o n t o m i n e r a l surfaces is a n i m p o r t a n t process affecting both natural a n d environmentally perturbed geochemical systems. However, s o r p t i o n o f e v e n c h e m i c a l l y s i m p l e a l k a l i e l e m e n t s s u c h as C s a n d S r o n t o c o m m o n r o c k s often does n o t achieve e q u i l i b r i u m n o r is e x p e r i m e n t a l l y reversible (1). P e n e t r a t i o n o r diffusion o f s o r b e d species i n t o t h e u n d e r l y ing m a t r i x has been p r o p o s e d as a c o n c u r r e n t n o n - e q u i l i b r a t i o n process (2). H o w e v e r , m a t r i x o r solid state diffusion is m o s t often considered e x t r e m e l y slow at ambient temperature based o n extrapolated d a t a from h i g h tem perature isotopic o r tracer diffusion experiments. Only limited attempts have been m a d e t o measure m a t r i x diffusion i n silicates a t lower t e m p e r a ture. T h i s paper reports results o f alkali diffusion experiments o n o b s i d i a n a n d f e l d s p a r s o v e r a t e m p e r a t u r e r a n g e o f 25° t o 100°C. T h e i m m e d i a t e significance o f this s t u d v is o n potential retardation o f radioactive alkali isotopes s u c h as Cs, S r a n d ^ R a during transport i n potential reposi t o r y h o s t r o c k s s u c h as b a s a l t a n d tuff w h i c h c o n t a i n glass a n d f e l d s p a r phases. T h e p a p e r h o p e f u l l y w i l l also s h e d light o n t h e m o r e general ques t i o n o f t h e i m p o r t a n c e o f l o w temperature diffusion i n geological e n v i r o n ments. 1 3 7
9 0
2 6
Experimental Results T h e three silicates obtained from W a r d s Scientific E s t a b l i s h m e n t a n d e m p l o y e d i n this s t u d y are obsidian from St. Helena, C a l i f o r n i a , microline from O n t a r i o , C a n a d a a n d albite from S o u t h D a k o t a . T h e bujk of t h e s a m p l e s w e r e c r u s h e d a n d s i e v e d t o b e t w e e n 2 1 1 a n d 4 2 3 μτη. F i n e s w e r e r e m o v e d b y r e p e a t e d u l t r a s o n i c c l e a n i n g a n d décantation. M e a s u r e d B E T 0097-6156/ 86/ 0323-0587$06.00/ 0 © 1986 American Chemical Society
588
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
surface areas were low; 0.07, 0.08 a n d 0.05 m for the K-feldspar, p l a gioclase feldspar a n d o b s i d i a n , respectively. I n d i v i d u a l larger cleavage and fracture fragments were preserved for surface studies using X - r a y p h o 2
T h e extent of allcali loss from aqueous solution as a result of s o r p t i o n a n d diffusion were determined by b o t h b a t c h a n d c o l u m n experiments. B o t h types of experiments u t i l i z e d l x l O m o l a r solutions of the a l k a l i chloride. B a t c h experiments consisted of reacting 2.0 L of air-saturated s o l u t i o n w i t h 200 gms of feldspar a n d glass at 25°C. B a t c h e s were s t i r r e d once a day over a one-year period. In the c o l u m n experiments, 650ml of s o l u t i o n stored i n a n air-saturated reservoir was recirculated b y a peristal tic p u m p t h r o u g h glass-jacketed columns c o n t a i n i n g 165 g m of o b s i d i a n . E x p e r i m e n t s were r u n at 25°, 50° a n d 75°C for periods up to t w o m o n t h s . T h e p H range d u r i n g b o t h b a t c h and c o l u m n experiments was buffered by atmospheric C 0 , resulting i n a p H range of between 6.5 a n d 7.5. Cleavage a n d fracture fragments of the m i n e r a l a n d glass phases were reacted separately at 25°, 50 , and 100°C. In order t o produce levels measurable b y X P S , l x l O m o l a r a l k a l i chloride solutions were required. A f t e r reaction, these samples were washed w i t h deionized water a n d stored i n a v a c u u m dessicator. - 4
2
- 1
Experimental Results
M a s s fluxes of a l k a l i elements transported across the solid-solution inter faces were calculated from measured decreases i n s o l u t i o n a n d f r o m k n o w n surface areas a n d mineral-to-solution weight-to-volume ratios. R e l a t i v e rates of C s u p t a k e by feldspar and o b s i d i a n i n the b a t c h experiments are i l l u s t r a t e d i n F i g u r e 1. A f t e r i n i t i a l u p t a k e due t o surface s o r p t i o n , little a d d i t i o n a l C s is removed from solution i n contact w i t h the feldspars. In contrast, parabolic u p t a k e of C s b y o b s i d i a n continues t h r o u g h o u t the reaction period i n d i c a t i n g a lack of s o r p t i o n e q u i l i b r i u m a n d the possibility of C s penetration i n t o the glass surface. T y p i c a l results for the shorter-time c o l u m n experiments show R b u p t a k e p l o t t e d against the square root of t i m e at several temperatures i n F i g u r e 2. N o measurable decreases i n L i , Κ or B a were observed for c o l u m n experiments conducted at 25°C. T h e d a t a of the type shown i n F i g u r e s 1 a n d 2 were fitted w i t h a linear regression program to the expression; M
=
x
M
0
+ kt / 1
2
(1)
where M is the t o t a l elemental uptake ( m o l e s - c m ) , M is the intercept at zero time ( m o l e s - c m ) a n d k is the parabolic rate constant (moles-cm" -s / ). T h e M t e r m can be used to approximate i n i t i a l s o r p t i o n or desorpt i o n o n the glass surface, a n d the k t ' t e r m the longer-term diffusion t r a n sport i n t o or out of the surface (3). A s shown i n F i g u r e 2, the s o r p t i o n t e r m decreases a n d the diffusion t e r m increases w i t h temperature for the o b s i d i a n experiments. T a b u l a t e d values for E q u a t i o n 1 are presented i n T a b l e 1 along w i t h the regression coefficient, r , for glass d a t a . T h e near-surface alkali-element concentrations i n ^ obsidian a n d feldspar were characterized b y their photoelectron peak intensities. A n example of the relative C s 3 d / peak intensities for o b s i d i a n a n d p l a gioclase p l o t t e d against the electron b i n d i n g energy is shown i n F i g u r e 3. T h e upper solid lines indicate slightly higher levels of C s i n the o b s i d i a n . A f t e r reaction for 10 minutes w i t h 0 . 1 N H C 1 , C s i n plagioclase is reduced t o a b a c k g r o u n d level i n d i c a t i n g reversible i o n exchange. In contrast, a -2
T
-2
2
-1
2
0
1
2
2
5
2
Q
WHITE A N D YEE
Near-Surface
Alkali
Diffusion
t'^SxIO
3
F i g u r e 1. R a t e of C s uptake from b a t c h solutions at 25°C. lines are least square regressions to E q u a t i o n 1.
S" x I 0 2
Straight
3
F i g u r e 2. R a t e of R b u p t a k e by o b s i d i a n from recirculating c o l u m n experiments at different temperatures. Straight lines are least square regressions t o E q u a t i o n 1.
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
590
T a b l e I. Diffusion rate parameters for E q u a t i o n 1. (units are defined i n text) Temp C°
M xlO"
-13
t xlO
k
0
1 0
1 0
r
2
6
Rb
25 50 75
0.69 0.46 0.01
0.61 1.03 2.66
2.95 2.95 0.94
.83 .89 .97
Cs
25 25* 75
1.47 1.97 0.23
0.28 0.15 1.76
2.09 2.09 0.89
.74 .92 .94
Sr
25 50
1.06 0.64
0.32 0.97
1.25 1.57
.94 .88
* b a t c h experiment s u b s t a n t i a l a m o u n t of C s remains after acid treatment of o b s i d i a n demons t r a t i n g non-reversibility a n d the potential for penetration below the glass surface. T w o methods employing X P S analysis, ion s p u t t e r i n g a n d variable take-off angles, were employed to produce elemental profiles w i t h d e p t h . T h e s p u t t e r i n g technique, used to create the profiles i n F i g u r e 4, involves sequential removal of surface layers b y b o m b a r d m e n t w i t h a positively charged A r beam followed b y m u l t i p l e x X P S analysis. T h e s p u t t e r time c a n be correlated w i t h penetration depth if rates have been calibrated for a given m a t r i x . S p u t t e r rates for o b s i d i a n a n d K - f e l d s p a r are not k n o w n b u t are estimated to be 10 Â per m i n u t e based o n amorphous S i 0 s t a n dards (4) and i n s t r u m e n t operating parameters. T h e elemental concentrations to the left of the zero time line i n F i g ure 4 are duplicate analyses p r i o r to the onset of s p u t t e r i n g . C s concent r a t i o n s i n o b s i d i a n shown i n F i g u r e 4 reach a m a x i m u m after s p u t t e r i n g for one m i n u t e (~ 10Â) and decrease to a stable b a c k g r o u n d after approxi m a t e l y 4 minutes (~ 40Â). In contrast, the C s profile for K - f e l d s p a r achieves a m a x i m u m at the surface and decreases more r a p i d l y w i t h d e p t h i n d i c a t i n g less penetration. T h e high C s b a c k g r o u n d intensities at greater depths relative to N a a n d Κ intensities are related to the greater a n a l y t i c a l s e n s i t i v i t y of C s (5). B o t h the N a a n d Κ intensities i n the K-feldspar profile of F i g u r e 4 are stable w i t h depth i n d i c a t i n g a previously documented l a c k of a l k a l i m o b i l i t y i n the surface layers of feldspars at low temperature (7). In contrast, Κ increases and N a decreases w i t h depth beneath the o b s i d i a n surface d e m o n s t r a t i n g s u b s t a n t i a l elemental m o b i l i t y . T h e Κ loss near the surface corresponds^to a concentration increase measured i n aqueous s o l u t i o n . S o d i u m profiles i n obsidian s h o u l d exhibit even greater near-surface losses relative to Κ based o n profiles measured b y H F leaching (3) a n d sputteri n d u c e d o p t i c a l emission studies (6). T h e anomalous N a decrease w i t h depth i n F i g u r e 4 illustrates a prob lem w i t h ion beam profiling i n that some light ions, i n c l u d i n g N a , are mobile under the beam a n d can be s t r i p p e d from the surface or embedded deeper i n the m a t r i x . Κ ions do not appear to be susceptible to this phenomenon a n d heavier elements such as R b , C s a n d Sr w o u l d be expected to be even less affected. Profiles of Si a n d A l , a l t h o u g h not s h o w n i n F i g u r e 4, were essentially constant w i t h d e p t h d o c u m e n t i n g the lack of secondary mineral f o r m a t i o n o n the surfaces of the o b s i d i a n a n d feldspar. 2
WHITE AND
Near-Surface
YEE
Alkali
Diffusion
591
5.0 4.0
>
3 0
LU
~Z.
2.0 1.0 "734
-730
-726
Binding Energy, eV
-790
-786
-782
Binding Energy, eV
F i g u r e 3. P e a k intensity versus electron b i n d i n g energies for the C s 3d / photoelectron peaks. U p p e r solid lines are after washing w i t h D.I. water, lower dashed lines are after washing w i t h 0 . 1 N H C 1 , lower solid lines are b a c k g r o u n d C s concentrations. 5/
2
10
Sputter Time, min.
Sputter Time,mm.
F i g u r e 4 . E l e m e n t a l peak intensities versus d u r a t i o n of A r ing i n o b s i d i a n .
+
sputter-
592
G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S
A second profiling m e t h o d , termed angle resolved X P S analysis, was used to confirm the penetration of R b , C s a n d Sr. Photoelectrons measured b y X P S can travel only a very l i m i t e d distance t h r o u g h the sample before i n t e r a c t i n g w i t h interior atoms a n d losing energy. T h e inelastic m e a n free p a t h (~ 30Â) is generally independent of the trajectory relative to the plane of the sample surface. B y p r o v i d i n g a n axis of r o t a t i o n i n the plane, the electron take-off angle changes. T h e m a x i m u m a n a l y t i c a l d e p t h is the product of the inelastic mean free p a t h a n d the sine of the take-off angle. A resulting take-off angle profile for C s i n o b s i d i a n is s h o w n i n F i g u r e 5. I n t h i s case, C s concentrations w i t h d e p t h are p l o t t e d as the mole ratio w i t h respect to S i . T h e use of elemental ratios overcomes the c o m m o n effect of carbon c o n t a m i n a t i o n o n the glass surfaces (8). T h e m a x i m u m C s / S i ratio for reaction at 25°C occurs at a depth of a p p r o x i m a t e l y 20Â a n d decreases up to depths of 50Â. T h e more l i m i t e d d a t a at 100°C exhib i t higher C s / S i ratios at greater d e p t h . T h e positions of the C s maxi m u m and d e p t h of penetration are comparable i n the take-off angle a n d s p u t t e r i n g techniques. C a l c u l a t i o n of Diffusion
Parameters
T h e diffusion coefficients for R b , C s a n d Sr i n o b s i d i a n c a n be calculated f r o m the aqueous rate d a t a i n T a b l e 1 as well as from the X P S d e p t h profiles. A simple single-component diffusion model (9) characterizes onedimensional transport i n t o a semi-infinite solid where the diffusion coefficient ( c m s ) is defined by; 2 ,
_ 1
D
-
