4.1 LEACH TESTING OF WASTE FORMS: INTERRELATIONSHIP OF IS0 AND MCC TYPE

TESTS -- V. M. Oversby, Lawrence Livermore Laboratory \ a)

Abstract

1) Leach t e s t i n g experiments were conducted on SYNROC-D mate r ia l t o examine

the parameters which a f f e c t leaching r e s u l t s and t o measure the a c t i v a t i o n

energy f o r leaching o f elements f rom SYNROC-D. Measured leach r a t e s were

8 .. found t o be c o n t r o l l e d by p r e c i p i t a t i o n o f i n s o l u b l e phases f o r those t e s t s

where the sample surface area t o volume o f leachant (SA/V) m u l t i p l i e d by

leaching t ime ( t ) exceeded 0.3 cm-'d f o r leach t e s t s a t 90'~. I n these

cases the apparent a c t i v a t i o n energy f o r leaching was approximately

10 kcal/mole based on Na and S i data. For leach t e s t s a t 9 0 ' ~ w i t h

S A ( T ) ( t ) l ess than 0.2 cm-'d, the a c t i v a t i o n energy f o r Na and S i

d i s s o l u t i o n was 18.5 kcal/mole f o r sample S29 and 14.5 kcal/mole f o r sample

LS04. These a c t i v a t i o n energies are i n agreement w i t h values repor ted by Tole

and Lasaga (1981) f o r nephel ine d i sso lu t i on .

The e f f e c t o f sample geometry was i nves t i ga ted by leaching a se r ies o f

crushed samples o f d i f f e r e n t g r a i n size. The r e s u l t s support t he view t h a t

geometric surface area should be used i n leach r a t e c a l c u l a t i o n s r a t h e r than

I gas adsorpt ion BET sur face area.

Comparison o f r e s u l t s on 529 leaching o f crushed samples and mono1 i t h s

show t h a t data from MCC-1 and IS0 type leach t e s t s may be d i r e c t l y compared

$ when the data are examined a t constant ( y ) ( t ) . S A

(a ) Work performed under the auspices o f the U.S. Department o f Energy by the

1 Lawrence Livermore Laboratory under con t rac t No. N-7405-ENG-48.

9 7

Introduction

Leach testing of radioactive waste forms is done to satisfy a number of

information needs in waste management programs. Manufacturers of waste forms

need simple tests which can be conducted under hot cell conditions to provide

quality control during fabrication. Standardized tests such as the MCC-1

leach test developed by the Materials Characterization Center of Battelle

Pacific Northwest Laboratories can be very useful for this type of quality

control application. The MCC-1 test is also useful for comparing different

waste forms under a fixed set of conditions and for providing means of

interlaboratory comparison.

Persons involved with the design of geologic repositories for high level

nuclear waste need a different type of information. In this case

extrapolation of data to long times is needed and data on variation of

durability with temperature, solution composition, geometry of the waste form

and water to solid volume are needed. The draft IS0 (1979) leach testing

procedure involves variation of temperature; duration of leaching is a1 tered

by changing leach solutions at specified intervals. Variation of solution to

solid volumes is allowed within the test framework. The IS0 test can provide

data which will yield activation energies for leaching if care is taken to

measure initial leach rates. Slight modification of the IS0 test procedure

will provide information relevant to leaching mechanisms.

This paper reports a study of the leaching behavior of SYNROC-D, a

titanate ceramic waste form developed for imnobilization of Savannah River

defense high level wastes. Details of preparation methods and sample

properties are given in Campbell et al., (1982). The chemical composition of

the sample used in the leach testing studies is given in Table 1.

98

There were f o u r main o b j e c t i v e s t o t h i s work.

(1) Determine the a c t i v a t i o n energy f o r leaching from, o r d i s s o l u t i o n of,

SY NROC-D.

(29 Determine the e f f e c t o f s o l u b i l i t y l i m i t a t i o n s on leaching o f

SY NROC-D.

( 3 ) Examine the r e l a t i o n s h i p between sample geometry and leaching

behavior (crushed sample vs mono1 i t h s ) .

( 4 ) Determine whether BET o r geometric sur face area should be used i n

leach r a t e ca l cu la t i ons .

Experimental Method

Leach t e s t i n g described i n t h i s paper was c a r r i e d ou t i n Parr a c i d

d iges t i on bombs, 23 ml size, which consis ted o f an i nne r TFE t e f l o n capsule

conta in ing the t e s t sample and leach s o l u t i o n and an outer s t a i n l e s s s t e e l

casing. New t e f l o n capsules were cleaned p r i o r t o use by r i n s i n g w i t h

deionized water t h ree times. The bombs were then f i l l e d w i t h 16 m l o f

deionized water and p laced i n an oven h e l d a t 150'~. A f t e r t h ree days, t h e

c lean ing water was discarded and new water was p laced i n the bombs. This

water was used t o determine whether any element o f i n t e r e s t i n our leach ing

work cou ld be detected a f t e r heat ing a t 1 5 0 ' ~ f o r one day. No element o f

i n t e r e s t could be detected by I C P analys is , so i t was concluded t h a t t h e

c lean ing procedure was adequate. Use o f t h i s c lean ing procedure e l im ina tes

the use of ac id i n t h e t e f l o n capsules. When a c i d i s used i n t h e capsules,

some a c i d d i f f u s e s i n t o the t e f l o n and i s no t complete ly removed u n t i l a f t e r

several stages o f heat ing w i t h pure water. Consequently, i t i s p o s s i b l e t o

ob ta in spurious leach r e s u l t s when using ac id cleaned t e f l o n capsules i f the

capsule wa l l s s t i l l con ta in some a c i d which d i f f u s e s i n t o t h e s o l u t i o n du r ing

t h e leach tes t . When leach t e s t i n g i s c a r r i e d ou t i n deionized water t h e

99

change i n pH can be l a r g e due t o t h i s ef fect , and leaching i s a c t u a l l y t a k i n g

p lace i n d i l u t e ac id so lu t i on .

Leach t e s t s were c a r r i e d out on crushed and s i zed samples o f SYNROC-D

using a mod i f ied vers ion of t he IS0 leaching procedure ( ISO , DIS/6961, 1979).

The mod i f i ca t i ons inc lude use o f crushed samples r a t h e r than mono l i ths and

v a r i a t i o n s i n t he schedule f o r changing leach so lu t ions . Most o f t h e t e s t i n g

used the g r a i n s i z e f r a c t i o n 177 t o 246 urn. Surface area was c a l c u l a t e d

assuming cubic geometry w i t h a l l p a r t i c l e s 212 ~m size. Sample weights and

du ra t i on o f leach ing were va r ied t o i n v e s t i g a t e the e f f e c t s o f s o l u t i o n

composit ion on leach ra te .

A t t he s t a r t o f each t e s t a p o r t i o n o f crushed and s ieved sample was

weighed i n t o a c lean t e f l o n capsule. Approximately 16 grams o f de ion ized

water was added, t h e capsule placed i n s i d e the s t a i n l e s s s t e e l cas ing and then

placed i n an oven maintained a t the des i red leaching temperature.

Temperatures were maintained t o w i t h i n 2 2 CO o f t he nominal temperature.

A f t e r t he des i red leaching t ime the bombs were removed f rom the oven, cooled

on a s t e e l p l a t e and opened as soon as they cou ld be handled w i t h bare hands.

So lu t ions were decanted w i thout f i l t r a t i o n i n t o p l a s t i c tubes and s to red a t

room temperature u n t i l analysed. I f leach ing was t o cont inue, f r e s h deionized

water was then added t o the sample and t h e bomb re turned t o the oven. Because

o f t he storage o f s o l u t i o n s be fore analys is , i t was n o t poss ib le t o ob ta in

v a l i d data on T i . A l l o ther elements were shown t o be s t a b l e i n s o l u t i o n a t

t he concentrat ions r e l e v a n t t o these experiments w i thout a c i d i f i c a t i o n over a

p e r i o d o f a t l e a s t t h ree weeks. The longest t h a t samples were normal ly s to red

before ana lys is was two weeks.

So lu t ions were analyzed f o r Na, Al, S i , Ca and Sr by i n d u c t i v e l y coupled

plasma spectrometry (ICP). The de tec t i on l i m i t s were se t a t 4 t imes t h e

standard d e v i a t i o n o f an a c i d i f i e d water blank. The de tec t i on l i m i t s v a r i e d

100 C '

with the time when analyses were performed, but were generally better than

Na = 0.008 ppm, A1 = 0.015 ppm, Si = 0.022 ppm, Ca = 0.003 ppm and Sr = 0.008

I ppm. In no case were measurable amounts of these elements found in control

blanks run in teflon capsules in parallel with the leaching experiments.

4 Results and Discussion

The first series of experiments conducted using S29 SYNROC-D material were

designed to measure the activation energy for leaching for Na, Al, Si, Ca and

gill - . Sr. Samples were leached in deionized water at 50°c, 90°c, 120'~ and

150'~. Solutions were decanted and fresh water added after leaching times

of 1 day, 3 days (At = 2 days), 6 days (At = 3 days) and 10 days (At = 4

8 ) days). The concentrations of Na, Al, Si, Ca and Sr in the solutions were used

to calculate normalized elemental leach rates for each element using the

normalized elemental grams x in solution )( weiqht of solid leach rate = grams x in solid surface area of solid leaching time,

Y Leach rates calculated by this formula are those which would apply to the bulk

sample if the whole sample dissolved at the same rate as element x. Leach

rates for crushed samples reported in this paper are based on surface areas

which are calculated assuming cubic geometry with the linear dimension being

the mean value of the sieve fraction used.

Results for the first set of experiments are labeled Nov. '81 and are

given in Tables 2, 4, 5 and 6. Data for the second leach period (Day 2-3) are

1 0 -1 plotted in Figure 1 as In (leach rate) vs - ( K ). If the leaching

T process were controlled by an activation energy, the leach rate would follow

the empirical Arrhenius law

rate = Aexp (-EA/RT)

2 where A = experimental constant, g/m d

EA = a c t i v a t i o n energy f o r leaching, cal/mole

R = gas constant, c a l h o l e OK

T = temperature i n OK

1 For a s i n g l e fo rward reac t ion , a p l o t of I n ( r a t e ) vs - w i l l g i v e a s t r a i g h t T'

l i n e whose s lope i s -EA/R.

Leach data f o r 50, 90 and 1 2 0 ' ~ shown i n F igure 1 do fo rm q u i t e good

l i n e a r arrays i n t he Arrhenius p l o t . However, data f o r 1 5 0 ~ ~ show r a t e s

which are the same o r l ess than those f o r 1 2 0 ~ ~ . Since assumption o f a

negat ive a c t i v a t i o n energy o r a d r a s t i c change i n leaching mechanism between

1 2 0 ' ~ and 1 5 0 ' ~ seems unreasonable, we have our f i r s t i n d i c a t i o n t h a t t he

experimental cond i t i ons do not s a t i s f y t he r e s t r i c t i o n s necessary t o o b t a i n

the a c t i v a t i o n energy f o r leaching. A second i n d i c a t i o n t h a t something o ther

than simple leach ing o r d i s s o l u t i o n i s occur r ing i s prov ided by t h e value o f

EA ca lcu la ted f rom the 50 t o 1 2 0 ' ~ data. The leaching o f Na and S i f rom

SYNROC-D should be c o n t r o l l e d by the d i s s o l u t i o n o f nepheline. Tole and

Lasaga (1981) repo r ted a c t i v a t i o n energies o f about 15 kcal/mole f o r Na and S i

d i s s o l u t i o n f rom nephel ine wafers a t near neu t ra l pH. Since t h e pH o f t h e

leach s o l u t i o n s repo r ted i n t h i s paper was usua l l y between 6 and 7, we should

expect an EA c lose t o 15 kcal/mole. The low value o f about 10 kcal/mole

suggests t h a t p r e c i p i t a t i o n reac t i ons are occurr ing, and t h a t t h e apparent

leach r a t e i s the r e s u l t of two competing processes.

To t e s t t h i s hypothesis a se r ies o f leach t e s t s were r u n a t 70, 90, 120

and 1 3 8 ' ~ us ing t h e same water volume as i n t he November t e s t s , b u t a

smal ler amount o f sample a t each temperature. Resul ts f rom these t e s t s a re

g iven i n Tables 3 through 6 and are labe led Dec. '81A. The data a re p l o t t e d

as I n ( leach r a t e ) vs (%-I) i n F igu re 2. Again, a t h r e e p o i n t l i n e a r

a r ray can be de f i ned w i t h an apparent a c t i v a t i o n energy o f about 10 kcal/mole,

102

but the data for 90°c plots above the line in all cases. Tie lines

connecting the 70°c and 90'~ data indicate a higher activation energy

which is similar to the Tole and Lasaga (1981) nepheline value, Comparison of

the leach' rates at 90 and 120'~ for the Nov. '81 and Dec. '81A data show

that for all elements except Al, the leach rates had increased roughly in

proportion to the decrease in sample size. This was interpreted as

confirmation of control of the apparent leach rate by a precipitation reaction.

If apparent leach rate is determined by precipitation of new phases rather

than by kinetic control of the dissolution process, we would expect to find

constant concentrations of elements in the leach solutions at a given

temperature regardless of the sample size or the length of the leaching

period. Exarninat ion of the solution concentration data for 90'~ leaches

(Table 7) shows that this is the case for all elements except A1 in the

Nov. '81 and Dec. '81A data sets. The concentrations of Si, Na, Ca and Sr in

these solutions should represent the solubility 1 imits of the new phases which

are formed by precipitation. To determine the activation energy for leaching

in the absence of precipitation, we must have final leach solutions whose

concentrations are lower than these solubility limits.

A third set of experiments were conducted at 70, 90, 120 and 1 5 0 ~ ~ where

the sample size was again reduced and the duration of leaching was shortened.

Examination of the solution concentration data labeled Dec. '81B in Table 7

shows that concentrations of all elements at 90'~ for the single day leach

samples were less than in the previous two leaching experiments. Thus, we

should expect that these samples are not saturated with respect to phases

which had previously been precipitating from solution. The final leach test

of this series was conducted for 3 days in an attempt to produce saturation.

Despite the three times longer leach time, the solution concentrations

Increased only by 20 to 50% indicating that the single day samples were close

to the solubi 1 ity 1 imits of phases which had previously precipitated from

solution.

The Dec. '81B data for days 1 through 3 are listed as normalized elemental

leach rates in Tables 3 through 6. The data for Day 3 is plotted in Figure 3

along with the 50'~ data for Day 2-3 for the November experiments. The

slope of the line defined by the 50, 70 and 90°c data now gives an

activation energy of 19.3 kcal/mole for Na dissolution from SYNROC-D. A

similar plot for the Day 2 data gives 17.7 kcal/mole. The average activation

energy for the two data sets is 18.5 kcal/mole for both Na and Si leaching.

Substitution of this activation energy and the average measured leach rate at

90°c into the Arrhenius equation gives values of AN, = 5.8 x 1011 g/m2d

and ASi = 7.6 x 10" g/m2d.

The data for leaching at 120'~ and 1 5 0 ~ ~ do not 1 ie on the line

defined by the 50 to 90°c data for the Dec. '81B experiments. This is

interpreted to be due to precipitation of some insoluble materials from the

higher temperature solutions.

Leaching data for A1 did not follow the pattern set by Na and Si. At

50°c, the concentration of A1 in solution was only slightly above the ICP

2 detection limits, and the calculated leach rate was about 3 x g/m d. 2 At 70'~ the average A1 leach rate was 6 x 10'~ g/m d which represents a

factor of 20 increase in leach rate for 20'~ increase in temperature. The

Na and Si leach rates increased by a factor of 10 over this temperature

2 range. At 90°c, the A1 leach rate was about 1 g/m d which represented

nearly a 20 fold increase again for 20°c temperature rise, while Na and Si

leach rates increased only by a factor of about 4. Nepheline is the only

crystalline phase which contains Na and Si in SYNROC-D. A1 occurs in .. nepheline which makes up 18% by weight of S29 and in spinel which constitutes

48% by weight o f the S29 sample (Campbell et al., 1982). Assuming an average

of 36% Alp03 in both phases, about 30% of the A1 is present in nepheline

and 70% is in the nearly insoluble spinel. The lower leach rate for A1 at

90'~ as compared to Na and Si can then be explained by assuming no A1 is

leached from the spinel phase. This is a reasonable assumption since Fe, a

major component in the spinels, is not found in 1each.solutions in measureable

amounts.

While the presence of A1 in spinel can explain the 90°C leaching data,

the low leach rates at 70 and 50°C must be due to another cause. Tole and

Lasaga (1981) found that A1 precipitated from solutions which were near

neutral in pH as Al(OH)3, bayerite. They identified this product in a

nepheline dissolution experiment run at pH = 5, T = 80'~. Precipitation of

this material would explain the low apparent leaching of A1 from SYNROC-D at

50 and 70'~.

A second fabricated sample of SYNROC-D was also tested for leaching

characteristics at 70, 90 and 1 2 0 ~ ~ . This sample, LS04, had the same

chemical composition as S29 but during early testing by F. Bazan showed lower

Ca and Sr leach rates (personal comnunication). Leach data for crushed

material (177-246 of LS04 are given in Tables 3, 4 and 5. Solution

concentration data at 90°C are given in Table 7. Solution concentrations at

all three temperatures were low enough that precipitation of insoluble phases

from the leach solutions should not be a problem. Leach rates for Al, Si and

Na are in good agreement for LS04 and S29 (Dec. '810 data set), while the Ca

and Sr leach rates are substantially lower for the LS04 sample. This suggests

that the perovskite phase in LS04 was better formed during the hot-pressing

fabrication step than was the case for S29.

1 The leach data for LS04 are plotted in Figure 4 as In (leach rate) vs - T

0 -1 ( K ) The activation energy for Na dissolution from this sample is 14.5

kcal/mole. This is slightly lower than the activation energy found for the

105

$29 sample. Tole and Lasaga (1981) reported activation energies of 14.5

kcal/mole for Na at pH = 5.0 and 16.0 kcal/mole for Na at pH = 7.0. The pH

value of leach solutions reported in this paper was approximately 5.5 at the

start of leaching and increased slightly as leaching progressed. For samples

leached at 90'~ or less the final pH was approximately 6. The highest pH

values were 7 for samples leached for several days at 150'~.

The solution data given by Tole and Lasaga (1981) can be recalculated to

normalized elemental leach rates by using the relationship

1 V leach rate = ( [x] )(-)(5K)

fxt

where [x] = concentration of element x in solution, g/cm 3

fx = fraction of x in the solid

t = leaching time, d

V = volume of leach solution, cm 3

SA = surface area of sol id, m 2

The nepheline sample used by Tole and Lasaga (1981) had fNa = 0.12 and

f ~ i = 0.205. Their (1981) data for dissolution of natural (Na.76K.19)

nepheline are compared with the SYNROC-D data from this paper in Table 8. The

values used for SYNROC-D are the average of several separate leaches from

solutions thought to be unaffected by precipitation reactions. The data from

Table 8 are plotted in Figure 5; data points at pH 5 and 7 were averaged to

provide a single value for the 60 and 80'~ temperatures. There is excellent

agreement between the leach rates for Na and Si from nepheline dissolution and

SYNROC-D leaching. It i s interesting to note that the Tole and Lasaga (1981)

samples were 1 mn thick wafers (i.e., monoliths) and the surface area used to

caluclate the leach rates was "geometricN for all data in Table 8.

Shade (1981) r e p o r t e d data on leaching o f nephel ine g lass and ceramics a t

90'~. He noted t h a t h i s ceramic nephel i n e conta ined a second c r y s t a l 1 i n e

phase i n a d d i t i o n t o nephel ine, as we1 1 as some r e s i d u a l amorphous ma te r ia l .

2 The samples used f o r leach ing were "monol i ths" w i t h approximately 1 cm o f

sur face area. The leach r a t e s he measured du r ing the e a r l y stages o f leach ing

were 11 g/m2d f o r Na and 7 cJ/m2d f o r S i a t 90'~. A f t e r 2 days o f

leaching t h e r e s u l t s showed lower leach r a t e s and appeared t o be a f f e c t e d b y

p r e c i p i t a t i o n o f i n s o l u b l e phases. The leach r a t e f o r S i i s i n excel l e n t

agreement w i t h t h e value repo r ted i n t h i s paper f o r the case where no

p r e c i p i t a t i o n occurs. The Shade (1981) Na leach r a t e i s about tw i ce t h a t

measured f o r SYNROC-D. The h igher Na leach r a t e may be due t o t h e second

c r y s t a l l i n e phase which Shade (1981) n o t i c e d i n h i s sample. It i s i n t e r e s t i n g

t o note t h a t t h e S i concent ra t ion a f t e r 1 day o f leaching nephel ine ceramic

was about 13 ppm ( c a l c u l a t e d from Shade (1981) F igure 3). This i s f a r h igher

than the concent ra t ion o f Si which can be mainta ined i n contact w i t h SYNROC-D

a t 9 0 ' ~ w i thout back r e a c t i o n o r p r e c i p i t a t i o n occurr ing.

There has been a g rea t deal o f d iscuss ion i n t h e l i t e r a t u r e concerning

what i s the app rop r ia te sur face area t o use i n c a l c u l a t i n g leach ra tes .

Measurement o f t h e sur face area o f crushed m a t e r i a l s can be done by t h e BET

gas adsorpt ion method us ing gases which do n o t chemical ly r e a c t w i t h the

sample being measured. Coles and Bazan (1982) r e p o r t BET sur face area data

determined us ing N2 and K r f o r 150-300 pm s i z e f r a c t i o n s o f SYNROC-C and

Cs-hol landi te. The surface areas measured us ing N2 were l a rge r than t h e

surface areas determined us ing Kr by about a f a c t o r o f 2. I n turn, t h e K r BET

sur face area was about 15 t imes l a r g e r than the geometric sur face area which

can be ca l cu la ted assuming cubic geometry f o r t he grains.

To i n v e s t i g a t e t h e e f f e c t of sur face area on leach ing o f crushed samples a

ser ies o f samples were prepared by c rush ing and s i e v i n g a specimen of S29

SYNROC-D i n t o several g r a i n s i z e f rac t i ons . Surface area o f these samples was

measured us ing K r BET. Geometric sur face area was c a l c u l a t e d assuming a l l

g ra ins were cub ic w i t h l i n e a r dimensions equal t o t h e midpo in t o f t h e s ieve

range. The data are g iven i n Table 9. The r a t i o o f BET t o geometric surface

area increased sys temat i ca l l y as the g r a i n s i z e o f t h e p a r t i c l e s increased.

Thus, i t seemed poss ib le t h a t a se r ies o f leach t e s t s on these samples might

answer the quest ion o f whether BET o r geometric sur face area was t h e

appropr ia te area t o use i n leach t e s t ca l cu la t i ons .

Four s i z e f r a c t i o n s were i n v e s t i g a t e d - 177-246 ~m (Dec. '818 data se t ) ,

246-417 urn, 417-840 pm and 840-1650 pm. The leach t e s t s were c a r r i e d out a t

90°c, and sample s i zes were chosen so t h a t p r e c i p i t a t i o n reac t i ons would be

e l im ina ted o r minimized. Data f o r concent ra t ions i n t h e leach so lu t i ons are

given i n Table 7 and c a l c u l a t e d normal ized elemental leach r a t e s are g iven i n

Table 10. Note t h a t t he data conta ined i n column 2 o f Table 10 i nc ludes

sample weights, c a l c u l a t e d geometric sur face area, BET surface area and t h e

average volume o f leach s o l u t i o n d i v i d e d by geometric sur face area o f t h e

samples. Leach r a t e s f o r Days 1, 2 and 3 are g iven based on geometric sur face

area. Leach r a t e s f o r a l l elements show a decrease which i s p r o p o r t i o n a l t o

the decrease i n leach ing volume/surface area. This suggests t h a t

p r e c i p i t a t i o n reac t i ons were t a k i n g p lace desp i te the attempts t o e l i m i n a t e

them.

The l a s t column o f Table 10 shows leach r a t e s f o r Day 3 c a l c u l a t e d us ing

the measured BET sur face areas. Use o f BET sur face area causes a f u r t h e r

spreading o f t he data. During the e a r l y stages o f leaching, sample

d i s s o l u t i o n should be independent o f g r a i n size. Since use o f BET sur face

area causes the leach r a t e s t o spread, BET sur face area does n o t appear t o be

measuring the sur face area which i s appropr ia te t o leaching.

The geometric sur face area used i n t h i s paper i s based on cubic geometry

where the cubes are a1 1 assumed t o be the same size, namely the middle o f t he

s ieve f r a c t i o n s ize. This i s obv ious ly a s i m p l i s t i c model, e s p e c i a l l y f o r

SYNROC-0 where t h e g r a i n shape i s more l i k e rec tangu lar pr isms than cubes.

For coarse g r a i n s i z e f r a c t i o n s t h e number o f cubes per gram i s about t w i c e

t h a t ca l cu la ted assuming cubic geometry. Thus, " r e a l " geometric s u r f ace areas

would be about 1 1/2 t imes g rea te r than t h a t which was ca lcu la ted .

Table 11 shows a comparison of r e s u l t s f o r 529 leach ing a t 9 0 ' ~ us ing

the MCC-1 t e s t on a mono l i t h sample a f t e r leaching f o r 3 days under s t a t i c

cond i t ions w i t h da ta f o r crushed m a t e r i a l (177-246 pm) where t h e leach

so lu t i ons were changed d a i l y . The surface area used f o r t h e MCC-1 t e s t i s t h e

measured geometric surface area and the leach volume t o sur face area o f sample

i s f i x e d a t 10 cm. From t h e experience gained i n t h i s study concerning

p r e c i p i t a t i o n f rom S29 leach so lu t i ons we would expect t h a t t h e MCC-1 t e s t

would be l i t t l e a f f e c t e d by p r e c i p i t a t i o n a t 3 days leaching, b u t would be

s e r i o u s l y a f fec ted the rea f te r .

The average leach r a t e over the f i r s t t h r e e days by the MCC-1 method i s i n

good agreement w i t h t h a t measured on crushed m a t e r i a l us ing t h e mod i f i ed IS0

t e s t and geometric sur face area based on cubic geometry. The agreement i s

improved i f our est imate o f " r e a l " geometry sur face area i s used. Leach r a t e s

f o r " r e a l " sur face area are obta ined by d i v i d i n g crushed sample 1 day leach

r a t e s by 1.5 since we est imated " r e a l " geometric sur face t o be 1.5 t imes cub ic

geometric surface.

F u l l data f o r MCC-1 leaching r e s u l t s on S29 are g iven i n Table 12 along

w i t h s o l u t i o n concent ra t ions c a l c u l a t e d f rom t h e leach data, The leach r a t e s

f o r a l l elements decreased s t e a d i l y w i t h t ime. Th is cou ld be due t o t h e

109

fo rmat ion o f a p r o t e c t i v e sur face l aye r on t h e sample d u r i n g leach ing and

c o n t r o l o f subsequent leach ing by d i f f u s i o n through t h e s u r f ace 1 ayer.

However, examinat ion o f leached SYNROC-D samples does n o t show any p r o t e c t i v e

l aye r fo rmat ion on nephel i n e (R. Ryerson, personal comnunicat ion) . An

a l t e r n a t i v e exp lanat ion f o r t he decrease i n leach rate.s i s t h e fo rma t ion o f

i n s o l u b l e phases which p r e c i p i t a t e from the so lu t i on . The f i n a l

concentrat ions a f t e r 28 days of MCC-1 leaching are ve ry s i m i l a r t o those found *

f o r crushed sample leaching over 2 t o 3 day per iods i n t h e Nov. ' 81

experiments (compare Tables 7 and 12). The sur face area/volume o f leachant i n

the Nov. '81 experiments was 0.6 cm" as opposed t o 0.1 cm-' f o r t h e MCC-1

t es t s . We would thus expect s a t u r a t i o n a f f e c t s 6 t imes e a r l i e r f o r t h e

crushed ma te r ia l .

The Dec. '81A samples which were a lso a f f e c t e d by p r e c i p i t a t i o n had

sur face t o volume r a t i o s o f 0.3 cm-l. For t he 1 day leach (Day 1) t h e

so lu t i ons were probably on l y s l i g h t l y a f fec ted by p r e c i p i t a t i o n (compare leach b

ra tes f o r Day 1, Dec. '81A w i t h those f o r s i n g l e day Dec. '818 leaches a t

90°c, Table 4). Assuming constant leach ra tes , we would expect

p r e c i p i t a t i o n t o a f f e c t leach r e s u l t s when sur face area/volume m u l t i p l i e d by

t ime reached 0.2 t o 0.3 cm-ld. Table 11 showed t h a t r e s u l t s f o r MCC-1 t e s t s

a t 0.3 cm-'d agreed w i t h leaching o f crushed m a t e r i a l a t 0.2 cm-ld. Data

f o r Days 2-3 o f Dec. '81A se t had ( S A / V ) ( t ) = 0.6 cm-'d, and gave Na leach

2 2 r a t e s o f 1.8 g/m d. This i s i n good agreement w i t h t h e value o f 1.5 g/m d

S A f o r MCC-1 a f t e r 7 days ( T ) ( t ) = 0.7 cm-'d . The l a r g e s t value o f

S A ( T ) ( t ) f o r t he crushed m a t e r i a l i s 2.4 cm-'d f o r Days 7-10 i n t h e Nov.

'81 data s e t i n Table 4. This data should compare w i t h t h e MCC-1 r e s u l t s a t

S A 28 days where (T) ( t ) = 2.8 cm-'d. Comparison o f these r e s u l t s i s g iven

i n Table 13. The agreement between the data se ts i s exce l l en t , s t r o n g l y

support ing the i n t e r p r e t a t i o n t h a t both data sets are c o n t r o l l e d b y

p r e c i p i t a t i o n o f i n s o l u b l e phases.

Conclusions

The leaching s tud ies conducted on SYNROC-D ($29) repo r ted i n t h i s paper t

a l low us t o draw severa l conclusions concerning leaching mechanisms and

t e s t i n g methods.

(1) The apparent a c t i v a t i o n energy f o r leaching from wasteforms w i l l vary 8 .

i f more than one process i s occurr ing.

( 2 ) Where p r e c i p i t a t i o n o f phases can occur from leach ing so lu t ions , t he

apparent leach r a t e s w i 11 depend on s o l u t i o n composit ion.

( 3 ) If care i s taken t o minimize s a t u r a t i o n e f f e c t s , t h e measured leach

r a t e s f o r d i f f e r e n t sample geometries w i l l be the same.

( 4 ) I f p r e c i p i t a t i o n reac t i ons occur, leach r a t e s f o r d i f f e r e n t sample

geometries o r leaching times are comparable a t equal values o f

(sur face area o f sample/volume o f leachant) m u l t i p l i e d by leaching

S A t ime, (8 ) ( t ) . This r e l a t i o n s h i p a1 lows comparison o f data f ra

IS0 type t e s t s w i t h MCC-1 tes ts .

(5) Leach r e s u l t s on d i f f e r e n t sample geometries show b e t t e r agreement

when geometr ic sur face area i s used i n c a l c u l a t i o n s r a t h e r than BET

s u r f ace areas.

(6) I n order t o ob ta in data on a c t i v a t i o n energies f o r leach ing processes

and t o s tudy leaching mechanisms, t h e leach t e s t i n g methods must

S A a l l ow v a r i a t i o n o f (B)(t) .

Acknowledgments

The s o l u t i o n data used i n t h i s r e p o r t were determined by I C P ana lys i s

performed by A. Langhorst. I am g ra te fu l f o r h i s c a r e f u l work and h i s

w i 11 ingness t o ad jus t schedules t o f i t the needs o f these experiments. Thanks

are due t o 3. Campbell f o r support and encouragement ,of t h i s work and t o J.

Campbell and T. Wolery f o r rev iew o f the manuscript. BET sur face area

measurements were made by C. S le t tevo ld .

References

J. Campbell, C. Hoenig, F. Bazan, F. Ryerson, M. Guinan, R. Van Konynenburg,

and R. Rozsa, Proper t ies o f SYNROC-D Nuclear Waste Form: A S ta te-o f - the-Ar t

Review, Lawrence Livermore Nat ional Laboratory Report UCRL-53240, January,

D. G. Coles and F. Bazan, Continuous-flow Leaching Studies o f Crushed and

Cored SYNROC, Journal o f Nuclear Technoloqy, i n press. A lso Lawrence

Livermore Nat iona l Laboratory Report UCRL-84679, 1980.

I n t e r n a t i o n a l Organ isa t ion f o r Standardisat ion ( ISO) , Long-term Leach 4)

Test ing o f Rad ioac t ive Waste S o l i d i f i c a t i o n Products, D r a f t I n t e r n a t i o n a l

Standard DIS/6961, Be r l i n , 1979.

J. W. Shade, Comparison o f Glass and Ceramic Leaching Behavior by Natura l

Analogs, Nuclear and Chemical Waste Management 2, 219-228, 1981.

M. Tole and A. C. Lasaga, D i s s o l u t i o n Mechanism and D i s s o l u t i o n K i n e t i c s o f

Nepheline, NaA1Si04, a Sodium Host Phase f o r Defense Waste Ceramics, The

Mater i a l s Research Laboratory, Pennsylvania S t a t e Un ive rs i t y , Report No.

PSU-022, September, 1981.

This document was prepared as an account of work sponsored by an agency of the

United States Government. Neither the United States Government nor the

University of California nor any of their employees,. makes any warranty,

express or implied, or assumes any legal liability or responsibility for the

accuracy, completeness, or usefulness of any information, apparatus, product,

or process disclosed, or represents that its use would not infringe privately 8 -

owned rights. Reference herein to any specific cmercial products, process,

or service by trade name, trademark, manufacturer, or otherwise, does not

necessarily constitute or imply its endorsement, recommendation, or favoring

by the United States Government or the University of California. The views

and opinions of authors expressed herein do not necessarily state or reflect

those of the United States Government thereof, and shall not be used for

advertising or product endorsement purposes.

Table 1. Composition o f SYNROC-D sample S29 i n weight %.

Compound

Fe203

*'2'3 MnO

"3 '8 C a0 NiO

Si02

NapO

N a2 SO4

Cs*O SrO

Table 2. S29 - HPlOA Leach r a t e s a t 5 0 ' ~ 2 Normalized Elemental Leach r a t e i n g/m d

w t = 0 , 1 6 6 g 1 7 7 - 2 4 6 ~ (Nov. '81)

Time, days

Table 3. S29 - HPlOA Leach Data, 7 0 ' ~ 2 Normalized Elemental Leach r a t e i n g/m d

w t = 0.07829, 177 - 246u (Dec. '81A)

A 1 Time, days - S i - Na - Sr - Ca -

wt = 0.0499, 177 - 246u (Dec. '81B)

Time, day - A 1 - S i - Na - Sr - C a

LS04 Leach Data, 7 0 ' ~ 2 Normalized Elemental Leach r a t e i n g/m d

wt = 0.0370, 177 - 2461~ (Jan. '82)

Time, day

Table 4 . S29-HP10A Leach Data 90°c 2 Normalized Elemental Leach r a t e i n g/m d

w t = 0.1409, 177-246~ (Nov. '81)

Time, days - A 1 - S i - N a

w t = 0.06449, 177-246~1 (Dec. '81A)

Time, days - A1 - S i Na

w t = 0.04469, 1 7 7 - 2 4 6 ~ (Dec. '816)

Time, days

LS04 Leach Data 9 0 ' ~

w t = 0.03209, 177-246P (Jan. '82)

Time, days - A1 S i S r C A - N a - -

Table 5. S29-HP10A Leach Data 120°c 2 Normalized Elemental Leach r a t e i n g/m d

Time, days - A 1 - S i - Na

w t = 0.05799, 1 7 7 - 2 4 6 ~ (Dec. '81A)

Time, days - A 1 - S i - N a

w t = 0.03389, 177-246~ (Dec. '810)

Time, days

LS04 Leach Data 1 2 0 ' ~

w t = 0.02239, 177-246p (Jan. '82)

Time, days

Table 6. S29-HP10A Leach Data 2 Normalized Elemental Leach rate i n g/m d

1

w t = 0.1349, 177-246u (Nov. '81), 1 5 0 ~ ~

Time, days - A 1 - S i - N a Sr -_ C a - *

1 1.2 5.4 4.3 1.8 0.75

wt = 0.04069, 177-246 (Dec. '81A), 1 3 8 ' ~

Time, days - A1 - Si - N a Sr -

wt = 0.0349, 177-246u (Dec. '818) 1 5 0 ~ ~

Time, days

E'Z

(10'0)

(20'0)

(10'0)

EO'O

P

E 2

'I: 28, "Jet? 'POST

O'E

S ' Z

60 '0

LO'O

60 '0

60'0

60 '0

P'P

6'2

8'E

P ' E

P'E

I-S

t

E

2

T:

E'I 1.1

8'0

P'E

6 ' 2

2'2

0 ' 9

Z ' S

9'E

12 1 11'0 O ' t 2 '9 O't 9- P

tE ' 1 &I *o E ' P L O 9 1.5 E-Z 96'0 60'0 O'E 5 .9 O'E 1

udd u! 3,06 l e s u o ! ~ n l o s q x a 1 u j s u o ~ l e ~ ~ u a ~ u o 3 a l q P l

Table 8. Comparison o f D i s s o l u t i o n Rate f o r

Nephe 1 i ne and SY NROC-D

Temperature

O c --

2 Normal ized e lementa l leach r a t e g/m d

N a S i

0.19 0.22

0.95 0.60

0.64 0.46

1.3 1.5

2.91 1.96

2.27 1.54

5.0 6.5

Data f o r 60 and 8 0 ' ~ f r om To le and Lasaga.

Data f o r 50, 70 and 90°c, t h i s paper.

Table 9. BET and Geometric Surface Area Data

f o r crushed SYMROC-D

Sieve f r a c t i o n K r BET sur face Geometric su r f ace BET

(P m) 2 area, cm 2 a r e a , c m / g Geometric

Table 10. Grain Size Data 90'~ S29

Normalized Elemental Leach rate, g/m2d

For specified day Day 3 using

A 1 - using geometric SA BET SA Size Fraction wt(g) - 1 - 2 - 3 . - 3BET

geom SA cm 2 1 - 2 - 3 - 3BET -

BET SA cm'

Averaqe Vol /SA(G) 2 - 2 - 3 - 3BET

cm - 177 - 24611 5.0 0.91 0.81 0.97 0.16

246 - 417 p 4.2 0.62 0.60 0.90 0.13

417 - 840~ 3.2 0.61 0.54 0.70 0.09

840 - 165OP 2.3 0.63 0.44 0.40 0.04

Table 11. S29 Monolith leach rates vs 177-2461~ powders 2 a t 90 '~ i n g/m d, Geometric S. A.

Mono1 i th , 3 day Crushed, 1 day I.

[XI ppm average 1 .r . [XI ppm 1.r. - l . r . / l .5

Monol i t h Data by MCC-1 procedure, Campbell e t al . , 1982 (Table 24).

Crushed data from Table 4, Dec. '81B se t - Day 2.

Table 12. MCC-1 90°C Monol i t h Leaching

Normal ized Elemental 2 Leach ra te , g/m d

Solution concentration

PP'" 3-day d a y 14-day 28-day 3-day 7-day 14-day 28-day

#I Data from Campbell et a l . , (19821, Table 24.

Table 13. Comparison of Monolith and Crushed sample

leach results at similar vaiues o f

(Surf ace Area/Volume) (time)

2 Normalized Elemental Leach Rates, g/m d

MCC-1, 2.8 cm-'d Crushed, 2.4 cm-'d

Days 2-3

EA (Na) = 9.7 kcal - mole

l o 1 F i g u r e 1 N a t u r a l l og o f leach r a t e p l o t t e d aga ins t - ( K- ) f o r $29 Nov. T '81 data set. Arrow a t 5 0 ' ~ p o i n t s t o A1 da ta p o i n t which i s o f f

sca le . A c t i v a t i o n energy (EA) based on Na da ta i s 9.7 kcal /mole.

Days 2-3

EA(Na) from 3pt. line =l0.5 kcal /mole

1 0 -1 Figure 2 Natural log of leach rate vs - ( K ) for S29 Dec. '81A data T

set. Activation energy determined using 70, 120, and 138'~ data

for Na i s 10.5 kcal/mole.

S 29 Day 3

1 0 -1 Figure 3 Natural log of leach rate vs - ( K ) for S29 Dec. '818 data T

set plus 5 0 ' ~ data from Nov. '81. Activation energy for Na between

50 and 90°c i s 19.3 kcal/mole.

L S 04 Day 4

1 0 -1 Figure 4 Natural log of leach rate vs - ( K ) for LS04 data set. T

Activation energy for Na between 70 and 120'~ is 14.5 kcal/rnole.

1 0 -1, Figure 5 Natura l l o g o f leach r a t e vs 7 ( K ) f o r Na and S i f rom

SNYROC-D and nepheline. Data from Table 8; 60 and BO'C da ta are

the average o f the l i s t e d pH = 5.0 and 7.0 values.