GLASS SURFACES - 1982

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GLASS SURFACES - 1982

L. Hench

To cite this version:

L. Hench. GLASS SURFACES - 1982. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-625-C9-

636. �10.1051/jphyscol:19829125�. �jpa-00222431�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°12, Tome 43, decembre 1982 page C9-625

GLASS SURFACES - 1982

L . L . Hench

Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, U.S.A.

Résumé

Une nouvelle catégorie de surface des verres est décrite. Les surfaces du type IIIB sont constituées par des couches multiples d'oxydes, hydroxydes et des silicates hydrates, provenant des reactions de précipitation entre la surface du verre et la solution de lixiviation. Les très faibles vitesses de lixiviation ,ainsi que les faibles épaisseurs d'érosion pour certaines compositions des verres complexes de déchets nucléaires sont principalement dues a une surface du type IIIB. Les limites de solubilité qui établissent l'équilibre des concentrations ioniques pour les eaux d'immersion, établissent aussi les muliples barrières de couches du type IIIB qui protègent les verres de déchets nucléaires en contact avec ces eaux d'immersion.

Abstract

A new category of glass surface is described. Type IIIB surfaces are composed of multiple layers of oxides, hydroxides, and hydrated silicates resulting from a sequence of solution-precipitation reactions between the glass surface and leaching solutions. Very low leach rates and then ion depletion depths of certain compositions of complex nuclear waste glasses are due to Type IIIB surfaces. The solubility limits that establish the equilibrium ionic concentrations for burial ground waters also establish the multiple barrier Type IIIB films, to protect nuclear waste glasses in contact with those ground waters.

Introduction

Understanding and controlling the surface reactions of glass is a subject of major technical importance. It has long been recognized that glass performance, such as the durability of glass containers, the useful life of windows and optical glasses, pH electrode lifetime, and static fatigue is related to glass surface - environment interactions. During the last five years there has been a growing awareness that the reliability of glasses developed for immobilization of high level radioactive wastes is also dependent on glass surface chemistry. We now know that the mechanisms of nuclear waste glass leaching comprise an important new category of glass surface behavior.

Understanding the kinetics and mechanisms of the new type of glass- environment interaction characteristic of nuclear waste glasses is important for several reasons. First, the quantity of nuclear waste glass required for immobilization of defense and commercial radioactive waste is millions of kilograms and the estimated cost is billions of dollars. Secondly, there must be reasonable assurance that the glass surface will be stable in contact with

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19829125

C9-626 JOURNAL DE PHYSIQUE

ground w a t e r s d u r i n g g e o l o g i c b u r i a l f o r thousands t o hundreds of thousands of y e a r s . T h i r d l y , t h e mechanisms r e s p o n s i b l e f o r s t a b i l i z a t i o n of n u c l e a r waste g l a s s e s might be a p p l i e d i n t h e design of g l a s s e s f o r redox-sensing (Eh) e l e c t r o d e s o r g l a s s e s w i t h unique g r a d i e n t s i n o p t i c a l p r o p e r t i e s . L a s t l y , our a b i l i t y t o g e n e r a l i z e glass-environment i n t e r a c t i o n s i s expanded w i t h t h i s new c a t e g o r y of s u r f a c e behavior.

Thus t h e primary o b j e c t i v e s of t h i s paper a r e t o review t h e evidence f o r a new c l a s s of g l a s s s u r f a c e behavior, compare t h i s behavior with o t h e r c a t e g o r i e s of g l a s s s u r f a c e s , and r e l a t e t h e s i g n i f i c a n c e of t h e s u r f a c e s t r u c t u r e and r e a c t i o n k i n e t i c s of t h e s e g l a s s e s t o t h e geologic performance of n u c l e a r waste g l a s s e s .

Background

P r e v i o u s e f f o r t s t o g e n e r a l i z e t h e s u r f a c e behavior of s i l i c a t e g l a s s e s proposed f i v e t y p e s of g l a s s s u r f a c e s t o r e p r e s e n t a broad range of g l a s s - environment i n t e r a c t i o n s , F i g u r e 1. ⁽

'

^{r 2 ) I t} was suggested t h a t any s i l i c a t e g l a s s could be d e s c r i b e d i n terms of one of t h e f i v e s u r f a c e t y p e s a t any p a r t i c u l a r i n s t a n t i n i t s p r o c e s s i n g and environmental h i s t o r y . Recent s t u d i e s i n d i c a t e however t h a t a n o t h e r t y p e of s u r f a c e , I I I B , must be added t o t h i s scheme. F i g u r e 1 i n c o r p o r a t e s t h i s change.

The o r d i n a t e i n F i g u r e 1 r e p r e s e n t s t h e r e l a t i v e c o n c e n t r a t i o n of Si02 ( o r o t h e r o x i d e s i n Type I11 s u r f a c e s ) i n t h e g l a s s and t h e a b s c i s s a corresponds t o t h e depth i n t o t h e g l a s s s u r f a c e . If s p e c i e s a r e s e l e c t i v e l y d i s s o l v e d from t h e g l a s s s u r f a c e t h e r e l a t i v e S i 0 2 c o n c e n t r a t i o n w i l l i n c r e a s e producing a Si02-rich s u r f a c e l a y e r . I f a l l s p e c i e s i n t h e g l a s s a r e d i s s o l v e d simultaneously (congruent d i s s o l u t i o n ) t h e r e l a t i v e c o n c e n t r a t i o n of

Fig. 1. Five types of g l a s s s u r f a c e s and s i x s u r f a c e

c o n d i t i o n s r e s u l t i n g from g l a s s - environment i n t e r a c t i o n s . 3

'

1 Olms on gloss I

I

Soluble glasr

trlm on glass

D I S T A N C E - DISTANCE

-

Si02 w i l l remain t h e same a s i n t h e o r i g i n a l g l a s s . When combinations of s e l e c t i v e d i s s o l u t i o n , congruent d i s s o l u t i o n and p r e c i p i t a t i o n from s o l u t i o n occur t h e n any one of t h e 6 s u r f a c e c o n d i t i o n s shown i n F i g u r e 1 i s p o s s i b l e .

Type I g l a s s e s have undergone a s u r f a c e r e a c t i o n t h a t i s only a monolayer t h i c k and no compositional p r o f i l e i s measureable. High p u r i t y v i t r e o u s s i l i c a exposed t o n e u t r a l s o l u t i o n s i s an example of a Type I s u r f a c e . Exchange of a l k a l i and a l k a l i n e e a r t h i o n s w i t h hydrogen and/or hydronium i o n s

( i . e . , s e l e c t i v e d i s s o l u t i o n ) r e s u l t s i n Type I1 g l a s s s u r f a c e s i f t h e r e i s s u f f i c i e n t c o n c e n t r a t i o n of network formers i n t h e s u r f a c e f i l m t o s t a b i l i z e it. Type I11 s u r f a c e s w i l l be d e s c r i b e d l a t e r . I f network formers a r e n o t s u f f i c i e n t , o r i f t h e environment i s r i c h i n OH- o r o t h e r s p e c i e s which can break Si-0-Si network bonds, t h e s u r f a c e l a y e r i s u n s t a b l e and a Type I V s u r f a c e i s produced. A g l a s s t h a t i s undergoing t o t a l network a t t a c k ( a l s o r e f e r r e d t o a s congruent d i s s o l u t i o n ) is d e s c r i b e d a s having a Type V s u r f a c e . Often from t h e p e r s p e c t i v e of average s u r f a c e compostion t h e r e i s l i t t l e d i s t i n c t i o n between Type I and Type V s u r f a c e s . However, l a r g e q u a n t i t i e s of i o n s a r e being l o s t from a Type V s u r f a c e d u r i n g c o r r o s i o n and consequently e x t e n s i v e s u r f a c e p i t t i n g can r e s u l t due t o l o c a l i z e d heterogeneous a t t a c k . A d d i t i o n a l l y , l a r g e dimensional changes o f t e n accompany c o r r o s i o n of g l a s s e s w t i h Type V s u r f a c e s .

S u r f a c e Reaction K i n e t i c s

S t u d i e s of t h e k i n e t i c s of t h e above p r o c e s s e s show t h a t Type I1 s u r f a c e s a r e p r e s e n t when

where C, is t h e c o n c e n t r a t i o n of s p e c i e s i n s o l u t i o n , t i s r e a c t i o n time and k l is t h e r a t e c o n s t a n t f o r t h e d i f f u s i o n c o n t r o l l e d process. Type V s u r f a c e s correspond t o a regime of k i n e t i c s where

where C2 and t have t h e same meaning a s i n Equation 1 and k2 i s t h e r a t e c o n s t a n t f o r t h e i n t e r f a c e c o n t r o l l e d network d i s s o l u t i o n process. Type I V s u r f a c e s r e s u l t when both p r o c e s s e s occur t o g e t h e r ; e.g.

Continued exposure of a g l a s s t o many chemical environments l e a d s t o p r o g r e s s i v e changes i n both compositon and t h i c k n e s s of t h e s u r f a c e f i l m s . F i g u r e 2 d e s c r i b e s such a change under s t a t i c c o r r o s i o n o r v a r i o u s flow r a t e s where t h e i o n s from t h e g l a s s a r e p e r m i t t e d t o b u i l d up i n s o l u t i o n and i n c r e a s e t h e pH. S e l e c t i v e l e a c h i n g ( d i s s o l u t i o n ) behavior r e s u l t s i n formation of a s i l i c a - r i c h s u r f a c e f i l m and a Type I1 s u r f a c e

.

^With

i n c r e a s i n g time i n s t a t i c c o n d i t i o n s o r slow flow r a t e s , d i s s o l u t i o n of t h e s u r f a c e f i l m network o c c u r s due t o b u i l d u p of OH- i n t h e s o l u t i o n l e a d i n g t o a Type I V s u r f a c e . Eventually a t high pH's t h e s u r f a c e f i l m network i s t o t a l l y destroyed and t h e g l a s s i s c o r r o d i n g congruently ( i . e . , Type V b e h a v i o r ) . Congruent d i s s o l u t i o n proceeds u n t i l complete d i s s o l u t i o n of t h e g l a s s occurs o r u n t i l t h e s o l u t i o n i n c o n t a c t with t h e g l a s s becomes s a t u r a t e d with r e s p e c t t o t h e i n d i v i d u a l s p e c i e s . F a s t flow r a t e s p r e v e n t a pH e x c u r s i o n and l i m i t t h e o n s e t of congruent d i s s o l u t i o n .

JOUZNAL DE PIIY S IQUE

STATIC CORROSION

TYPE I

Gloss

I I

Orig~nol Surface

F i g . 2. Time dependent changes of

TYPE x Fast g l a s s s u r f a c e s w i t h s t a t i c l e a c h i n g

( b ) ^{a = o ,} A%. Flow

Gloss s e ~ e c t t s ~ i e k h v n g o r flow.

Leached Loyer

1

^so!n

^{B T}

^Glass o < a c l ; + - = - ^{TYPE 7P Slow} Flow Selective Leaching 8 Network Attack

I

^{I TYPE P very}

( d )

1

^Sol"

1 ^:y:s

Q = I , E i O SIO;

Network Attack Comruent D~ssolutlon

I ' I

DEPTH

A g l a s s d i s s o l u t i o n parameter ( a ) and a f i l m formation parameter ( E ) can be c a l c u l a t e d t o d e s c r i b e t h e s e p r o c e s s e s . ( 3 a 4 ) The r e l a t i o n of t h e s e parameters t o g l a s s s u r f a c e t y p e and r e a c t i o n sequence i s shown i n Figure 2.

moles S i 0 2 i n s o l u t i o n moles SiOl i n g l a s s

a =

moles a l k a l i i n s o l u t i o n

'

moles a l k a l i i n g l a s s when a + 0, incongruent d i s s o l u t i o n and f i l m formation;

a ^-t 1 , f i l m d e s t r u c t i o n ; a = 1 , congruent d i s s o l u t i o n ;

I - a

E = ppm SiO ( i n s o l u t i o n ) ₂

---

a ( 6 )

when =

+,

f i l m deformation

A t

when OE =

-,

f i l m d e s t r u c t i o n A t

when & = 0 , congruent d i s s o l u t i o n

The f i l m formation parameter, E , i s r e l a t e d t o t h e q u a n t i t y of excess s i l i c a a v a i l a b l e f o r formation of a s u r f a c e film. However, t h e d e n s i t y of t h e s i l i c a - r i c h f i l m can vary considerably. Consequently, t h e r e may be a c o n s i d e r a b l e q u a n t i t y of e x c e s s s i l i c a on t h e g l a s s s u r f a c e of a t h i c k n e s s of 10-100 u m , b u t because of p o r o s i t y and low d e n s i t y t h e f i l m i s n o t a b a r r i e r t o d i f f u s i o n (Type IV)

.

I n c o n t r a s t , a t h i n ( < l p ) f i l m of dense S i 0 2 remaining a f t e r s e l e c t i v e i o n exchange can s e r v e a s a h i g h l y p r o t e c t i v e s u r f a c e (Type 11).

A rimer of s t u d i e s of g l a s s e s of v a r i o u s compositions have shown depth compositional p r o f i l e s f o r Type 11, I V , and V s u r f a c e s . The t e c h n i q u e s used i n c l u d e Auger e l e c t r o n spectroscopy (AES)(5-8), secondar i o n mass spectroscopy ( SIMS) ('), r e s o n a n t n u c l e a r r e a c t i o n s ( RNR) ( l'), e l e c t r o n spectroscopy f o r chemical a n a l y s i s (ESCA) ( 'I l 2 )

,

secondary i o n photoemission spectroscopy (SIPS) ( 1 3 ~ 1 4 ) , i n f r a r e d r e f l e c t i o n spectrosc%py ( IRRS) ( 15-17).

Film depths i n t h e range of 0.01-1.0 Pm a r e g e n e r a l l y observed f o r Type I1 g l a s s e s . Type I V g l a s s e s t y p i c a l l y e x h i b i t f i l m s of 1.0-100 p depth. Adding network m o d i f i e r s of high e l e c t r i c f i e l d s t r e n g t h t o t h e g l a s s changes Type I V s u r f a c e s t o Type 11, g r e a t l y d e c r e a s e s t h e t h i c k n e s s of t h e s i l i c a - r i c h f i l m s and i n c r e a s e s t h e f i l m d e n s i t y .

Type 111 G l a s s S u r f a c e s

Previous s u r f a c e s t u d i e s of b i o l o i c a l l y a c t i v e i n v e r t s i l i c a t e g l a s s e s c o n t a i n i n g CaO and P205 ( b i o l a s s e s ) ( 19) and c e r t a i n compositional ranges of Li20-A1203-SiO, glasses(') showed t h a t dual f i l m s developed on t h e g l a s s . The secondary f i l m s were e i t h e r hydrated CaO-P205 o r A1203-Si02 l a y e r s . Chemical depth p r o f i l e s u s i n g AES showed t h a t t h e d u a l f i l m s formed on t o p of a s i l i c a r i c h f i l m which r e s u l t e d from r a p i d i o n exchange of a l k a l i f o r p r o t o n s ( o r hydronium i o n s ) . Other work showed t h a t such a dual f i l m could be formed by a d d i t i o n of e i t h e r phosphate(19) o r aluminum i o n s i n s o l u t i o n ( 2 ' ) a s w e l l a s through r e l e a s e of phosphate o r aluminum i o n s from t h e g l a s s .

Analysis of anion c o n c e n t r a t i o n s of t h e d u a l a p a t i t e f i l m formed on b i o g l a s s e s showed some exchange of co3*- i o n s f o r OH- i o n s s i m i l a r t o t h a t found f o r bone. ( 2 2 ) This s t u d y a l s o showed t h a t i n c o r p o r a t i o n of CaF2 i n t h e g l a s s r e s u l t s i n a f l u o r a p a t i t e f i l m ( 2 2 ) forming on t h e g l a s s . Previous d e n t a l implant s t u d i e s suggested a f a v o r a b l e t i s s u e response t o t h e f l u o r i d e c o n t a i n i n g g l a s s e s ( 2 3 ) and r e c e n t r e s u l t s show e x c e l l e n t b i o c o m p a t i b i l i t y of t h e f l u o r i d e anion doped s u r f a c e l a y e r s . ( 2 4 )

S t u d i e s a l s o show t h a t t h e e f f e c t i v e n e s s of a number of a l k a l i n e c o r r o s i o n

inhibitor^(*^',

such a s s o l u b l e calcium o r beryllium s a l t s , a r e due t o formation of dual f i l m s composed of i n s o l u b l e a l k a l i n e e a r t h - s i l i c a t e c ~ r n p o u n d s ( ~ ~ ~ ~ ~ )

.

This type of g l a s s s u r f a c e i s d e s i g n a t e d t y p e I11 A, d u a l p r o t e c t i v e f i l m , i n Fig. 1. The t h i c k n e s s of t h e secondary f i l m s can vary c o n s i d e r a b l y , from a s l i t t l e o r 0.01 pm f o r A1203-Si02 r i c h l a y e r s o r a s much a s 30

v m

f o r CaO-P205 r i c h l a y e r s .

S t u d i e s i n a number of l a b o r a t o r i e s have shown t h a t a l k a l i b o r o s i l i c a t e g l a s s e s designed t o immobilize h i g h l e v e l r a a d i o a c t i v e wastes have an even more complex s u r f a c e behavior t h a n Type I11 A g l a s s e s . There i s c o n s i d e r a b l e evidence t h a t such g l a s s e s develop m u l t i p l e l a y e r s of o x i d e s o r hydroxides on t h e i r s u r f a c e when exposed t o water. This behavior i s d e s i g n a t e d Type I11 B i n F i g u r e 1.

The t h e o r e t i c a l b a s i s f o r Type I11 B g l a s s e s i s t h e r e c e n t i n v e s t i g a t i o n of B. Grambow which p r e d i c t s t h e formation of a s e r i e s of i n s o l u b l e r e a c t i o n p r o d u c t s on g l a s s s u r f a c e s . ( 2 8 ) He shows t h a t t h e s t r o n g pH dependence of n u c l e a r waste g l a s s l e a c h i n g can be explained i n terms of t h e pH dependent s o l u b i l i t y l i m i t s of simple r e a c t i o n compounds of many c o n s t i t u e n t s i n t h e g l a s s . I n low and h i g h pH l e a c h a n t s g l a s s d i s s o l u t i o n i s c o n t r o l l e d by congruent d i s s o l u t i o n (eq. 2 ) b u t a t i n t e r m e d i a t e pH v a l u e s s o l u b i l i t y of r e a c t i o n s p e c i e s such a s Fe(OH)3, Zn(OHI2, Nd(OH)3 SrC03, CaC03, o r CaSi03 c o n t r o l t h e s u r f a c e r e a c t i o n s . Thus k3 i n eq. 4 i n v o l v e s a summation of t h e s o l u t i o n - p r e c i p i t a t i o n r e a c t i o n s of a l l of t h e above c o n s t i t u e n t s .

C9-630 JOURNAL DE PHYSIQUE

A mathematical model h a s been r e c e n t l y developed which a t t e m p t s t o d e s c r i b e some of t h e above p r o c e s s e s . (*') It is based on t h e e m p i r i c a l o b s e r v a t i o n t h a t t h e c o n c e n t r a t i o n r a t i o s of v a r i o u s elements i n f o u r d i f f e r e n t waste g l a s s systems were independent of time i n s t a t i c l e a c h t e s t s a f t e r 3 days; however, t h e elemental r a t i o s i n s o l u t i o n d i f f e r e d from t h e corresponding r a t i o s i n t h e g l a s s e s throughout t h e 28 day s t a t i c l e a c h p e r i o d . Thus, t h e model assumes t h a t t h e g l a s s corrodes congruently and t h e r a t e of c o r r o s i o n i s c o n t r o l l e d by t h e r a t e of r e a c t i o n of amorphous s i l i c a w i t h water a s w e l l a s t h e r a t e of d i f f u s i o n of s o l u b l e s i l i c a t e s through an i n s o l u b l e l a y e r between t h e g l a s s and bulk s o l u t i o n . This l e a d s t o t h e s t a b l e incongruency i n t h e l e a c h a n t s . The t h i c k n e s s of t h e i n s o l u b l e l a y e r is p r o p o r t i o n a l t o t h e amount of g l a s s t h a t has d i s s o l v e d from t h e s t a r t of t h e experiment with a s i l i c a t e c o n c e n t r a t i o n g r a d i e n t i n t h e l a y e r e q u a l t o t h e d i f f e r e n c e i n s i l i c a t e c o n c e n t r a t i o n a t t h e g l a s s l a y e r i n t e r f a c e and t h a t i n t h e l e a c h r a t e d i v i d e d by l a y e r t h i c k n e s s .

Wicks and Wallace c o n c l u d e , ( 2 9 ) "The model p r e d i c t s t h a t t h e s l o p e of l o g c o n c e n t r a t i o n vs. l o g time p l o t should be i n i t i a l l y 1 t h e n s h i f t t o 0.5 a t a l a t e r time and f i n a l l y approach 0 a s t h e l e a c h a t e becomes s a t u r a t e d with s i l i c a t e s . The p o i n t a t which t h e t r a n s i t i o n from a s l o p e of 1 ( m a t r i x d i s s o l u t i o n c o n t r o l l e d ) t o 0.5 ( d i f f u s i o n c o n t r o l l e d ) occurs depends p r i m a r i l y on t h e r a t i o of t h e r e a c t i o n r a t e c o n s t a n t t o t h e d i f f u s i o n c o e f f i c i e n t . The p o i n t a t which s a t u r a t i o n becomes important depends p r i m a r i l y on t h e r a t i o of s u r f a c e a r e a of g l a s s t o volume of l e a c h a n t . For l a r g e v a l u e s of t h i s r a t i o , s a t u r a t i o n may become important b e f o r e t h e s h i f t from m a t r i x d i s s o l u t i o n t o d i f f u s i o n c o n t r o l l e d k i n e t i c s occurs. I n open systems where t h e p r o d u c t s of l e a c h i n g a r e n o t allowed t o accumulate ( b u t s u r f a c e l a y e r s a r e formed) t h e s l o p e s should s h i f t from 1 t o 0.5 b u t not drop below t h a t value. A simple m a t r i x d i s s o l u t i o n model ( i n v o l v i n g no p r o t e c t i v e s u r f a c e l a y e r s ) p r e d i c t s a s l o p e of 1 which remains n e a r t h a t value u n t i l s a t u r a t i o n becomes important when it then drops toward zero.

Experimental v a l u e s of t h e s l o p e s i n f o u r s e t s of s t a t i c t e s t s v a r i e d between 0.5 and 0.25. The e s t i m a t e d f r a c t i o n a l s a t u r a t i o n of s i l i c a t e s i n t h e s e t e s t s was s u f f i c i e n t t o account f o r s l o p e s from 0.48 t o 0.41 f o r t h e s u r f a c e l a y e r model." Even lower v a l u e s approaching 0.05 a r e observed i n l o n g term t e s t s . t30r3')

: Waste Compositions

: F e 2 0 3 : 6.4 : 4.0 : 1.601: 2.8 : 2 . 9 6 : 2 . 9 6 : 2 . 9 6 :

: (Weight %).

% Sjnmlated Waste Products

The solubility limited-multiple layer rnodels imply that certain compositional ranges may exist where glasses can form a sequence of stable and continuous films capable of greatly inhibiting diffusion of other species through them or dissolution of the matrix underneath them. Results of several nuclear waste glass compositional studies show that there is a critical range of glass composition where leaching in minima1.(32-35) For example, one of the studies (33) comparing seven glasses (Table 1 ) for solidification of French nuclear wastes showed that glass surface attack and leach rates were strongly dependent on the concentration of network formers in the glass, Fig. 3. On the ordinate of Fig. 3 is plotted the number of days required for a glass surface to be attacked sufficiently for its IRRS spectrum at 950

6 '

(Si-0-alkali region) to be decreased by 15%. The abscissa is the sum of various combinations of glass network formers. The graph shows that a critical concentration of SiOZ

+

^A1203

+

^Fe203 > approximately 55% (by weight) or SiO, + A1,0,

.,

> ^{53% must be} present in the glass for the surface to be resistkt to leaching. Comparison of pH changes and ion solution concentrations after 28 days, 90°C yield the same conclusion.

Fig. 3. Time required for IRRS spectra of nuclear waste glasses to degrade 15% during static 90°C leaching.

E (NETWORK F O W R S I

Another study investigated the relative importance of ZnO and Fe203 in stabilizing the surface film on alkali-zinc-borosilicate glasses(341. For the composition studied, replacement of half of the 6% ZnO with Fe203 degraded the leach resistance by 3X. Both glasses appeared to be protected by dual films, one rich in SiOZ and a second vexy thin film rich in multivalent species. The second film containing the mixture of ZnO and Fe203 was less stable than ZnO alone, suggesting critical concentrations of film formers are necessary to produce a film sufficiently dense and continuous to serve as an effective diffusion barrier.

McVay and Buckwalter have shown that the role of Fe in leaching can be especially complicated when iron silicate colloids form in the leachant. ⁽³⁶ Formation of iron silicate precipitates effectively removes many elements from

C9-632 JOURNAL DE PHYSIQUE

s o l u t i o n and t h e r e f o r e e l i m i n a t e s many of t h e e f f e c t s of ground water chemistry. S i n c e t h e p r e c i p i t a t e s r e t a r d t h e s a t u r a t i o n e f f e c t s by removing s p e c i e s from s o l u t i o n , h i g h e r s u s t a i n e d g l a s s l e a c h r a t e s and g r e a t e r t o t a l elemental removal from t h e g l a s s is observed. Leaching i n t h e presence of s o l i d p i e c e s of d u c t i l e i r o n give r i s e t o t h i s e f f e c t and e x c e s s i v e c o n c e n t r a t i o n s of Fe i n t h e g l a s s may a l s o , s i n c e high waste g l a s s e s c o n t a i n i n g l a r e i o n i c Fe c o n c e n t r a t i o n s l e a c h e s more t h a n lox f a s t e r t h a n a high A 1 waste. F137)

I n o r d e r t o g e n e r a l i z e composition e f f e c t s of Type I11 B g l a s s e s a computerized comparison of t h e compositional de endence of 27 n u c l e a r waste g l a s s e s s t u d i e d worldwide has been conducted. (3g) F i g u r e 4 i l l u s t r a t e s t h e f i n d i n g s . A l l 27 g l a s s e s were leached i n D I water u s i n g MCC-1 s t a t i c l e a c h procedures a t 90°C f o r 28 days. The g l a s s compositions were d i v i d e d i n t o 3 groups; t h e weight percentage of oxides of S i , B, and Na a r e l o c a t e d a t t h e t o p and r i g h t c o r n e r s of t h e t e r n a r y p l o t . The o x i d e s of A l , Fe and a l l o t h e r c o n s t i t u e n t s , l a b e l e d a s WP ("waste p r o d u c t s " ) a r e added t o g e t h e r and comprise and 3 r d a x i s . A l l of t h e g l a s s e s i n t h e d a t a f i l e were enclosed w i t h i n t h e compositional space l a b e l l e d 45.0 i n F i g . 4. Thus a l l n u c l e a r waste g l a s s e s s t o r e d have a l e a c h r a t e f o r S i a t 28 days of l e s s t h a n 45 g/m2/day. A narrower f i e l d of compositions, b u t s t i l l e x t e n s i v e , p o s s e s s l e a c h r a t e s of 0.1 t o 0.2 g/m2/day. However, only a few compositions e x h i b i t t h e very low range of 0.02 g/m /day and t h e s e a l l c o n t a i n n e a r l y e q u i v a l e n t 51-53 w/o SiOZ 2 and +2% of t h e o t h e r c o n s t i t u e n t s .

NUCLEAR WASTE GLASS CONSTITUENT OXIDES

WPsSIMULATED WASTE PRODUCTS CONTAINING ALL ELEMENTS

NOT INDIVIDUALLY SHOWN

AlFeWP

w

B Na

Glass Composition (weight %I for Glasses with Leach Rates at 28 Days of not More Than:

45.0, 0.2, 0.1, 0.02 g/m2/day for Element SI at a Temperature of 90 OC and SA/V of 10 m"

Fig. 4. Compositional dependence of n u c l e a r waste g l a s s l e a c h i n g .

The r e s u l t s of F i g u r e 4 f o r S i i n s o l u t i o n a r e s i m i l a r t o t h e g e n e r a l f i n d i n g s of t h i s study; i . e . , t h e r e i s a narrow compositional range where n u c l e a r waste g l a s s e s achieve leach r a t e s a s low a s 0.02 g/m2/day i n a s t a t i c MCC-1, 90°C l e a c h t e s t . Howevera a much broader range of g l a s s compositions w i l l y i e l d l e a c h r a t e s of 0.1 g/m /day. The b e s t compositional range t e n d s t o be approximately 51-53% ( S i O Z ) , 24-28% (NaZO + B203), and 21-25% (A1 0

+

2. 3 Fe203 + WP). Analysis of t h e time dependent changes of t h e compositional space f o r lowest l e a c h r a t e s l e a d s t o t h e conclusion t h a t a c r i t i c a l c o n c e n t r a t i o n of A1203, Fe20?, and waste product c o n s t i t u e n t s a r e needed t o produce low l e a c h r a t e s . This i s c o n s i s t e n t with s u r f a c e a n a l y s e s t h a t show many of t h e s e s p e c i e s being i n c o r p o r a t e d i n s u r f a c e f i l m s which s e r v e a s d i f f u s i o n

barrier^(^'^^^-*^)

slowing down long term r e l e a s e r a t e s .

I t has a l s o been shown t h a t t h e t h i c k n e s s of t h e Type I I I B f i l m s d e c r e a s e s with i n c r e a s i n g percentage of waste c o n s t i t u e n t s and m u l t i p l e valence o x i d e s . ( 2 9 ) Leach r a t e s f o r a l l elements a r e much lower f o r t h e g l a s s e s with t h i n n e r f i l m s which i n d i c a t e s n o t only t h a t t h e f i l m s formed q u i c k l y but t h a t t h e y were s u f f i c i e n t l y continuous and dense t o p r e v e n t congruent a t t a c k of t h e g l a s s . I t i s e s p e c i a l l y important t h a t b u r i a l s t u d i e s of n u c l e a r waste g l a s s e s i n deep Swedish g r a n i t e w i t h 90°C c e n t e r l i n e h e a t e r s have shown t h a t Type I I I B s u r f a c e s form under c o n d i t i o n s expected i n l o n g term geologic r e p o ~ i t o r i e s . ( * ~ ' * ~ ) The presence of m e t a l l i c overpack i n t e r f a c e s such a s Pb, T i , o r Cu, a c c e l e r a t e formation of t h e m u l t i p l e l a y e r s a p p a r e n t l y due t o high s u r f a c e a r e a t o s o l u t i o n volume r a t i o s which l e a d t o r a p i d p r e c i p i t a t i o n of i n s o l u b l e m e t a l l i c compounds. ( 4 2 )

Summary

Type I I I B , m u l t i p l e b a r r i e r s u r f a c e s c o n s t i t u t e an important new c l a s s of g l a s s s u r f a c e r e a c t i o n s . A number of a l k a l i b o r o s i l i c a t e n u c l e a r waste g l a s s e s t h a t e x h i b i t T e

4'

I I I B s u r f a c e behavior have elemental l e a c h r a t e s a s low a s 0.02 t o 0.2 g/m /day with a time dependence of s t a t i c l e a c h i n g of to.=

t o t o e 2 o r l e s s a f t e r 28 days a t 900C. A d i s c u s s i o n of t h e v a r i a b l e s c o n t r o l l i n g formation of Type I I I B s u r f a c e s follows.

Although a s h o r t ( s e v e r a l days) p e r i o d of alkali-hydrogen ion exchange may occur f o r I I I B g l a s s e s , t h e dominant, long term mechanisms c o n t r o l l i n g c o r r o s i o n is a combination of matrix d i s s o l u t i o n followed by incongruent d i s s o l u t i o n and s o l u t i o n / p r e c i p i t a t i o n r e a c t i o n s . The e x t e n t of m a t r i x d i s s o l u t i o n and o n s e t of s u r f a c e p r e c i p i t a t i o n w i l l depend on t h e time r e q u i r e d f o r v a r i o u s s p e c i e s i n t h e g l a s s t o reach s a t u r a t i o n i n s o l u t i o n . S a t u r a t i o n of s p e c i e s ( i ) w i l l be a f u n c t i o n of t h e i n i t i a l s o l u t i o n pH, amount of a l k a l i i n t h e g l a s s and r a t e of a l k a l i r e l e a s e , temperature, i n i t i a l c o n c e n t r a t i o n of s p e c i e s ( i ) i n t h e s o l u t i o n , SA/V which i n f l u e n c e s s o l u t l o n c o n c e n t r a t i o n , o r flow r a t e which a l s o a f f e c t s s o l u t i o n c o n c e n t r a t l o n . U n t i l s a t u r a t i o n of some s p e c i e s i n s o l u t i o n i s reached, t h e g l a s s d i s s o l v e s congruently a t a r a t e p r o p o r t i o n a l t o k t ' .

When s o l u t i o n s a t u r a t i o n of s p e c i e s ( i ) i s reached t h e r e i s no longer any d r i v i n g f o r c e f o r t h a t s p e c i e s t o l e a v e t h e g l a s s s u r f a c e . Consequently s p e c i e s ( i ) w i l l accumulate a t t h e g l a s s - s o l u t i o n i n t e r f a c e a s t h e matrix d i s s o l v e s , l e a v i n g s p e c i e s ( i ) behind i n t h e g l a s s . I f t h e m a t r i x d i s s o l u t i o n r e l e a s e s a l k a l i i o n s , a s w i l l be t h e case f o r most g l a s s e s , t h e r e w i l l be a concomitant r i s e i n pH p r o p o r t i o n a l t o t h e flow r a t e o r SAD of t h e system.

An i n c r e a s e i n pH can have s e v e r a l simultaneous e f f e c t s on t h e g l a s s , t h e s o l u t i o n , and the g l a s s - s o l u t i o n i n t e r f a c e . A t t h e new pH, a second s p e c l e s ( j ) may reach s o l u t i o n s a t u r a t i o n and subsequently be r e t a i n e d i n t h e g l a s s s u r f a c e a l o n g with s p e c i e s ( i ) The e x t e n t of incongruent d i s s o l u t i o n of t h e g l a s s i s thereby i n c r e a s e d . I n a d d i t i o n , t h e pH can have e i t h e r one of

C9-634 JOURNAL DE PHYSIQUE

t h r e e e f f e c t s on s p e c i e s ( i ) , p r e v i o u s l y i n s a t u r a t i o n ; 1 ) it remains s a t u r a t e d b u t a t a h i g h e r c o n c e n t r a t i o n ; 2 ) it becomes s u p e r s a t u r a t e d and p r e c i p i t a t e s e i t h e r on t h e g l a s s o r o t h e r s u r f a c e o r a s a c o l l o i d ; o r 3 ) it becomes u n d e r s a t u r a t e d and s p e c i e s ( i ) i n t h e g l a s s s u r f a c e once a g a i n begins t o be r e l e a s e d . The sequence of e v e n t s t h a t occurs i s p r e d i c t a b l e based u on t h e s o l u b i l i t y l i m i t s of each s p e c i e s a t a given pH, a s shown by Grambow ^{( 3 8 )} F i g u r e 5, based upon Grambow's work, shows t h a t t h e Fe(OHI3 s o l u b i l i t y l i m i t s h o u l d be exeeded over a broad range of pH and t h e r e f o r e n u c l e a r waste g l a s s e s c o n t a i n i n g Fe o x i d e s s h o u l d c o n c e n t r a t e Fe w i t h i n s u r f a c e l a y e r s . Zinc, N d , S r , and Ca should be c o n c e n t r a t e d a s w e l l i n n e a r l y n e u t r a l o r s l i g h t l y a l k a l i n e s o l u t i o n s w i t h Na and B d e p l e t e d . F i g u r e s 6 and 7 o b t a i n e d by e l e c t r o n micro r o b e a n a l y s i s and AES p r o f i l i n g of corroded n u c l e a r waste g l a s s e s , (29838Q shows t h e c o n c e n t r a t i o n of Fe ( F i g . 6) and o t h e r heavy metal s p e c i e s (Fig. 7 ) i n t h e g l a s s s u r f a c e . Man? o t h e r s t u d i e s confirm t h e p r e d i c t i o n of s o l u b i l i t y l i m i t e d s u r f a c e l a y e r s . '29-3 36-42)

A consequence of t h e formation of t h e m u l t i p l e b a r r i e r , I I I B f i l m s i s low o v e r a l l l e a c h a b i l i t y of many n u c l e a r waste g l a s s e s over a pH range from 4.5 t o 9.5. Figure 5 superimposes a p l o t of S i l e a c h a b i l i t y from a Savannah River Lab composite waste g l a s s immersed i n a 5 day s a t i c 23OC s o l u t i o n b u f f e r e d t o v a r i o u s pH v a l u e s from 3.5 t o 10.7. Wicks (43f d a t a shows t h a t over t h e p H range expected f o r r e p o s i t o r y ground w a t e r s , ( 4 3 ) shown by arrows, g l a s s l e a c h a b i l i t y i s lowest. Thus, we can conclude t h a t t h e s o l u b i l i t y l i m i t s t h a t e s t a b l i s h t h e e q u i l i b r i u m i o n i c c o n c e n t r a t i o n s f o r t h e ground w a t e r s should a l s o e s t a b l i s h t h e m u l t i p l e b a r r i e r f i l m s t o p r o t e c t n u c l e a r waste g l a s s e s i n c o n t a c t with t h o s e ground w a t e r s .

Fig. 5. C o r r e l a t i o n between s o l u b i l i t y l i m i t s of n u c l e a r waste g l a s s s u r f a c e r e a c t i o n products and 23°C g l a s s l e a c h r a t e s i n d i f f e r e n t pH s o l u t i o n s .

20 w t % Waste 5 yrn H 30 wtX Waste 2 urn H L (mass loss) = 1.19 g/(m2-day) L (mass loss) = 0.51 g/(m2-day)

Cross sections of leached Iayers for 20% and 30% waste loaded glasses with x-ray line profiles of waste constituent Fe.

Fig. 6. Fe concentration in leached nuclear waste glass surface layers.

Fig. 7. Surface chemical composition profiles of a leached nuclear waste glass.

DEPTH (i)

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2 4 . WILSON, J . a n d HENCH, L. L . , p r i v a t e c o m m u n i c a t i o n s .

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2 9 . WICKS, G. C . a n d WALLACE, R. M., p r i v a t e c o m m u n i c a t i o n s .

3 0 . MENDEL, J . E. e t a1 , P a c i f i c N o r t h w e s t L a b R e p o r t 3 8 0 2 , A p r i l 1981 3 1 . PLODINEC, M. J., WICKS, G. C. a n d B I B L E R , N.E., R e p o r t D P - 1 6 2 9 , M a y 1 9 8 2 . 3 2 . HENCH, L . L. and URWONGSE, L . , SKBF/KBS R e p o r t , 8 - 8 2 ( 1 9 8 0 ) .

3 3 . NOGUES, J. L . , HENCH, L . L . a n d ZARZYCKI, J . , 5 t h I n t e r n a t i o n a l S y m p . o n t h e S c i e n t i f i c B a s i s f o r R a d i o a c t i v e W a s t e M a n a g e m e n t , B e r l i n , J u n e 1 9 8 2 . 34. NOGUES, J. L. and HENCH, L . L . , 5 t h I n t e r n a t i o n a l S y m p . o n t h e S c i e n t i f i c

B a s i s f o r R a d i o a c t i v e W a s t e M a n a g e m e n t , B e r l i n , J u n e 1 9 8 2 . 3 5 . HENCH, A. A. a n d HENCH. L . L . , p r i v a t e c o m m u n i c a t i o n s .

3 6 . McVAY, G. L . a n d BUCKWALTER, C. Q . , S u b m i t t e d t o J. Am. C e r a m . S o c . , M a y 1 9 8 2 .

3 7 . CLARK, D. E . , MAURER, C. A., JURGENSEN, A. R. a n d URWONGSE, L . , 5 t h I n t e r n a t i o n a l S y m p . o n t h e S c i e n t i f i c B a s i s f o r R a d i o a c t i v e W a s t e M a n a g e m e n t , B e r l i n , J u n e 1 9 8 2 .

3 8 . HENCH, L . L., CLARK, D . E . a n d LUE YEN-BOWER, E . , N u c l e a r a n d C h e m i c a l W a s t e M a n a g e m e n t ,

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4 0 . McVAY, G. L . a n d BUCKWALTER, C. Q., N u c l e a r Technologies, V o l . 5 1 , ( 1 9 8 0 ) . 4 1 . HENCH, L . L . , WERME, L a r s a n d LODDING, A l e x

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t h e S c i e n t i f i c B a s i s f o r R a d i o a c t i v e W a s t e M a n a g e m e n t , B e r l r n , J u n e 1 9 8 2 . 4 2 . WERME, L a r s , HENCH, L . L . a n d LODDING, A l e x , 5 t h I n t e r n a t i o n a l S y m p . on

t h e S c i e n t i f i c B a i s f o r R a d i o a c t i v e W a s t e M a n a g e m e n t , B e r l i n , J u n e 1 9 8 2 . 4 3 . WICKS, G . C . , P r o c e e d i n g s o f W a s t e M a n a g e m e n t 1 9 8 1 C o n f e r e n c e , T u c s o n ,

A r i z o n a , F e b . 2 3 - 2 7 , 1 9 8 1