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Migration and retention phenomena of radionuclides in clay-barrier systems
Applied Clay Science, 6 (1991) 195-214
195
Elsevier Science Publishers B.V., AmsterdAm
Migration and retention phenomena of radionuclides in clay-barrier systems U. Bartl a and K.A. Czurda b aLandratsamt Esslingen, Postfach 145, D-7000 Esslingen am NeCkar, Germany bDepartment of Applied Geology, University of Kalrsruhe, Kaiserstr. 12, D- 7500 Karlsruhe, Germany (Received September 27; accepted after revision May 28, 1991 )
ABSTRACT Bartl, U. and Czurda, K.A., 1991. Migration and retention phenomena of radionuclides in clay-barrier systems. Appl. Clay Sci., 6: 195-214. Within the scope of projecting a waste deposit for low- and medium radioactive materials we have investigated the migration behaviour of the nuclides cesium (Cs- 137 ), strontium (Sr-90), cobalt (Co60) and zinc (Zn-65) in the Badener Tegel, a Miocene clay of the Vienna Basin (Lower Austria). Two fundamental retention processes could be derived from batch tests for the nuclides mentioned above: Sorption processes, in general cation exchange processes on clay minerals (cesium and strontium) and precipitation reactions (zinc and cobalt) which are included by exceeding the solubility product in the changed equilibrium solution with the clay. The migration behaviour of the nuclides was investigated with the help of columns with undisturbed and orientated clay samples under advective conditions. The influences of the hydraulic gradient, fabric and solution composition on the migration behaviour of nuclides can be derived form breakthrough curves and retention coefficients. The grade of fixation of nuclides in the clay can be determined by desorption tests with water of different composition. A more realistic case of contaminant extension in low permeable clays is simulated by diffusion tests in undisturbed samples. For the Badener Tegel, diffusion coefficients and breakthrough times have been determined from which the diffusive nuclide extension can be derived. Again precipitation reactions (especially with zinc) lead to a drastic reduction of the nuclide amounts passing through.
1. I N T R O D U C T I O N
In isolating contaminants from the biosphere, clays serve as natural barriers on the one hand; on the other they function as an artificial technical barrier. Besides their low permeability, plasticity and the retention capacity for contaminants are further advantages of clay barriers. Therefore clays are in discussion besides salt and granite as host rocks for a waste deposit for lowand m e d i u m radioactive substances. The high safety required can be guaranteed by the multibarrier principle whereby the host rock and the technical barrier form a staggered security system. It is important to know the retention 0169-1317/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
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U. B A R T L AND K.A. CZURDA
capacity of clays and the migration behaviour of radionuclide in clay barriers under breakthrough conditions to evaluate its risks for the biosphere. The investigations described have been carried out by us at the Austrian Research Center Seibersdorf and the Department of Applied Geology, University of Karlsruhe. The scope of these investigations is Austria's search for an apt place for low- and medium radioactive waste. 2. CLAY ROCKS
The clay samples used come from the Badener Tegel, a marine clay of middle Miocene age of the Tertiary basin filling of the Vienna Basin and has been taken from its holotype locality in Baden-SooB/Lower Austria. Table 1 shows the mineral composition which has been determined semiquantitatively. The high amount of detritic components is indicated, besides the grain size distribution (Fig. 1 ), in the range of clay minerals where detritic minerals such as mica, illites and chlorites predominate. The amount of smectite comes from an illite-smectite mixed layer clay mineral of the mineral composition I + ( 1014M ) while kaolinite exists in traces only. The primary cation content of the Badener Tegel consists of Ca 2÷- and Mg2+-ions predominantly. Its cation exchange capacity (CEC) against NH~--ions amounts to 47.5 mval/100 g and is in the range of the CEC of comparable molasse clays (Czurda et al., 1987 ). grain size distribution clay
o
sand
silt middle
fine
fine
coarse
middle
coarse
r
lO ~20 ~-
soL Kottingbrunner Soh~chten ( Tonmergel ) Badener Tegel
¢D
100
1
2
6
20
50
200
600 2000 grain size (pro)
Fig. 1. Grain size distribution of tested Tertiary clays and marls: Kottingbrunn formation and Badener Tegel.
RADIONUCLIDESIN CLAYBARRIERS
197
TABLE l Parameters of tested clay rock: Badener Tegei Grain size fraction (/tm)
Mineral content (%) quartz
feldspar
calcite
dolomite
chlorite
kaolinite
illite
smectite
Total 125-60 60-30 30-10 < 10
31 61 45 29 14
5 5 5 5 5
13 20 12 12 21
7 4 6 6 5
18 18 17
2 2 2
16 10 32 18 19
8 7 17
CEC (meq/100 g) Moisture content (%) natural after convection
47.5 7.0 17.0
Heat loss (%) Effective porosity (%) Permeability ( m / s )
11.0 15.3 1 0 - 9 _ 1 0 - |o
TABLE 2 Hydrochemical analyses of ground water and concrete leachate used for desorption tests
Temperature ( ° C) pH-value Redox-potential (mV) Specific electric conductivity (/zS/cm) Acid capacity, pH 8.2, m-value (ml) Acid capacity, pH 4.3, p-value (ml) Water hardness ( ° dH) Carbonate hardness ( m e q . / l )
Ground water
Concrete leachate
21.0 7.2 + 61.0 499.0
21.0 10.5 + 57.0 1108.0
0.00 4.07 14.50 4.10
4.40 0.14 4.60 0.00
Cations (meq./1): sodium potassium ammonium calcium magnesium iron manganese
Na + K+ NH~ Ca 2÷ Mg 2÷ Fe 2÷ Mn 2÷
1.087 0.054 0.006 3.897 3.934 0.007 0.005
1.905 1.944 0.000 1.547 0.002 0.003 0.001
Anions (meq.1): hydrogen carbonate chloride nitrate sulfate phosphate hydroxide
HCO~CINO jSO 2HPO 2OH-
4.069 0.293 0.363 4.422 0.000 0.000
0.138 1.995 0.170 1.790 0.019 ca. 0.900
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U. BARTLAND K.A.CZURDA
3. R A D I O N U C L I D E S O L U T I O N S
The used radionuclides Cs-137, Sr-90, Co-60 and Zn-65 represent the main part of activity in low- and m e d i u m radioactive waste. Chloride solutions of the nuclide pairs cesium/strontium and cobalt/zinc have been prepared with a representative ground water from a gravel body nearby. Its hydrochemical analysis is shown in Table 2. The nuclide concentration in the inactive solution was 0.01M resp. 0.05M, an equivalent activity concentration of 1 resp. 5n C i / m l was standardized by addition of the respective radionuclides. The retention of the nuclides by the clay sample can be determined by the activity concentration which can be measured easily before and after the test. To simulate different conditions of the damage case, a slightly acid solution (pH 3.5 ) and an alkaline concrete leachate water (pH 10.5 ) was used besides the neutral alkaline-hydrogencarbonatic ground water. Table 2 shows the composition of the concrete water. The investigation of the contaminated concrete water-clay barrier interactions is interesting, as concrete is well suitable for conditioning low- and m e d i u m radioactive waste and is therefore increasingly used. 4. N U C L I D E E X T E N S I O N IN T H E CLAY
4. I. Nuclide transport If a solution passes through a porous medium, then its components are influenced in different ways, which leads to a smearing of the penetrating solution front and thereby to a mostly delayed and unprecise breakthrough of solution. In a mathematical model a transport equation can describe, e.g. the one-dimensional migration of a nuclide front through the m e d i u m in form of different terms: OC
OC
DfOS
Ot -Eff~x2 + l ~ x 2 - Vwox
02C
_02C
~ at
dispersive difJusive advective sorptive lerm
term
term
term
where C = concentration outside the sorbens, t =time, E = hydrodynamic dispersive coefficient, D = diffusive coefficient, x = length (flow path ), Vw = velocity of medium, pf = density of the fluid,
(1)
RADIONUCLIDES IN CLAY BARRIERS
E S
199
= porosity, = concentration in the sorbens.
The change of concentration in the eluate is expressed by a dispersive, a diffusive, an advective and a sorptive term. While advective transport which is caused by a hydraulic gradient works uniformly on the whole solution and would therefore effect a piston-like breakthrough of unchanged solution, hydrodynamic dispersion, diffusion and retention may effect either an acceleration or a retardation of solution components in different ways. The term retention signifies all interactions which allow a solid body to retain solution components. Under this term sorption processes are classed with, whereby rock components adsorb solution components, in this case certain cations, as well as chemical reactions like complex formation and precipitation reactions, whose products can be fitted mechanically by means of highly adsorptive and adhesive forces on hydrated particle surfaces. A precise distinction between those two retention processes cannot be carried out as complex compounds or colloids can be subjected to both adhesive and sorptive processes. Interactions of this kind have not been investigated thoroughly yet. Due to the different velocities of solution transport in pores of different size, hydrodynamic dispersion leads to a local running ahead of the displacing solution components in the biggest pores. Simultaneously the moment of reaching the initial concentration in the eluate is retarded whereby the Sshaped breakthrough curve flattens out at its right end. The hydrodynamic dispersion coefficient E contains properties of the pore system as well as the form and dimension of single passage ways. Concentration gradients of the solution components cause diffusive processes along the migration path of the solution. They appear in a percolating solution if the flow velocity is very low and the flow distance becomes very high. Diffusion processes lead to long-term flattening of the concentration ascent of the adsorption isotherms as they proceed very slowly. At the top of the velocity profile the diffusive movement runs towards the pore solution without nuclide content and adds therewith to an acceleration of the breakthrough of first nuclide parts. Under the conditions of high flow velocities the influence of slowly proceeding diffusion processes on the nuclide transport can be neglected (Nye, 1986). In any case, diffusion from the interlayer space of the clay minerals, especially from three-layer smectites, into the pore space has to be considered. The saturation of the Stern-layer with radionuclides and their inactive metal cations leads to a comparatively high concentrated double-layer space which finally releases cations and radionuclides out into the pore space by means of a diffusional migration. The released cations are restricted in passing the clay to differing degrees. The clay acts as a semipermeable membrane, allowing some components of
200
U. BARTLAND K.A. CZURDA
pore fluids such as water to pass through relatively freely, but restricting the various ionic species. Electroosmosis and streaming potential are the two main electrokinetic phenomena for clay-electrolyte systems and associated with them are hydraulic and electrical conductivities (Demir, 1987). The cation selectivity during membrane transport is increased at higher temperatures (Haydon and Graf, 1986 ). 4.2. Retention processes
The sorption processes on clay minerals always mean cation exchange. The cation exchange cap~tcity (CEC) of a clay is determined by the following parameters: content of sorbents, - size of accessible exchanger surface, kind and amount of surface charge, - primary ion occupancy of exchanger, ion strength of the solution, cation specific parameters. The cation exchange is of importance predominantly with highly sorptive, swellable three-layer clay minerals of the smectite- and vermiculite-group which show a high exchange capacity due to their large charge deficit and their accessible interlayer space. Many cations show only a restricted solubility under conditions which result from the contact of a solution with the clay. Different kinds of metal hydroxides, -oxides, -sulfates, -hydroxencarbonates and carbonates can be precipitated. As it is the case with the Badener Tegel and other carbonatic clays, this happens with the help of carbonate buffered solutions or the change of the solution pH-value by sorption resp. desorption of H ÷-ions on clay minerals. These clays are able to standardize and stabilize the pH-value of an equilibrium solution at different stages, if the solubility of a cation in the solution is exceeded the precipitation products are eliminated mostly in form of flakes or colloids. If sorption processes proceed between the solution components and the clay, then solution components are permanently adsorbed at the beginning of the contact of solution and sorbens. The amount adsorbed is at a maximum at the solution front and sorption tends towards an equilibrium state. But the amount retarded at the breakthrough front is lower than the exchange capacity of the sorbens, as not all sorptive surfaces had the possibility of coming into contact with the percolating solution. Before a complete sorption is possible, solution componentsr reach the eluate by means of flow passages. Many influences such as stream velocity, content of sorptive components along the flow passages and the kinetics of sorption processes control this dynamic process which is therefore a very complex quantity. In most transport equations the attempt is made to take into account its influence on migration by intro-
-
-
-
RADIONUCLIDES IN CLAY BARRIERS
201
ducing a separate sorption term, yet the necessary distribution coefficients, which are mostly deduced from adsorption isotherms assume the adherence to certain frame conditions which are often not fulfilled in rock/solution systems. 5. E X P E R I M E N T S
5.1. Batch tests The interaction of components of a clay with a nuclide solution can be comprehended by batch tests. In batch tests small amounts of rock powder ( 1 g) are shaken with a determined amount of nuclide solution (20 ml). The amount of nuclide adsorbed after the test can be expressed with the help of a distribution coefficient (Rd-value). It is defined as: nuclide concentration in the solide phase Rd --nuclide concentration in the liquid phase
ml g
--
(2)
The retention of the Badener Tegel has been investigated under different parameters. 5.1. I. Mineral composition and grain size distribution In Fig. 2 cesium and strontium show an infinitely small sorption with coarse grain fractions. Only when clay minerals occur in the fraction 50-10 ~tm the sorption of the two nuclides starts. Sorption increases clearly with cesium and drastically with strontium in the fraction < 10 ~tm where clay minerals are accumulated with an increased smectite amount with high sorption capacity. Hence it can be derived that the part of sorptive surface is a primary factor for the sorption of the two nuclides, whereby the sorption of bivalent strontium seems to depend especially on the increase of smectite content. Cobalt and zinc on the other hand show a certain retention even with coarse grain sizes which might be caused by the provision of carbonate especially with zinc, as both zinc and cobalt have only a small solubility under pH-conditions of about 7. Due to its complex chemism which allows sorption processes with the help of complex formation as well as precipitation reactions, cobalt shows a sorptive component which is expressed by the increase of retention by smallest grain sizes with their clearly higher content of sorptive components. Illite and chlorite which dominate in the clay mineral spectrum of the Badener Tegel are able to adsorb cations only on their outer surface due to their structure. Their strength of bonding is relatively small so that the adsorbed cesium and strontium ions can be mobilized again with desorption tests. IIlites show specific exchange areas and are able to adsorb cations predominantly along prism faces of their outer surface and along exposed basal planes. Monovalent cations like cesium are preferred. Contrarily, smectites adsorb
202
U. B A R T L AND K.A. CZURDA
botch test / sorption
>
100
2~ Z n 2~
0
m 50 .c_ Co 2. S
g~
0 (.3
125-60
60-30 30-10 grain size (/Jm)
OCs*
10
total clay
E]Sr 2÷ ACo 2. v Z n 2.
Fig. 2. Batch test results from Badener Tegel-marl with Zn, Co, Cs, St. Quantity of adsorbed cations in dependence on grain size distribution.
predominantly bivalent cations with an apt ion radius into their interlayer space due to their favourable charge compensation. Strontium is thereby preferred to the monovalent cesium. This fact is also shown by sorption tests made under identical conditions by Rashidchi (1987) with monomineralic clays where illites showed higher adsorption capacities for cesium and strontium. Of course, clays with high amounts of smectite show especially high adsorption rates for cesium and strontium. To evaluate the influences of the carbonate content on cobalt and zinc retention, batch tests have been carried out with decarbonated Badener Tegel as well as with pure calcium carbonate powder and calcium sulfate powder.
RADIONUCLIDES IN CLAY BARRIERS
203
Decarbonated clay samples show the expected decrease in retention, whereby the remaining retention at an equilibrium pH-value of 4-4.5 can be traced back to the precipitation of zinc and cobalt carbonates which causes a reduction of the zinc content of the nuclide solution of 91.9% and of the cobalt content of 20%. The lower amount of precipitation of cobalt can be explained by its slightly higher solubility and the competition of zinc for precipitating substances in the solution.
5.1.2. Contact time The contact times of the batch tests have been varied between 1 hour and 75 days. Prolonged contact time and a longer time of shaking causes a better dispersion of the clay particles and therewith an enlargement of the accessible sorptive surface, whereby mechanical spalling of mica and illites provides additional sorptive surfaces at their basal faces. On the other hand the standardization of an equilibrium between the cations adsorbed at the clay and the dissolved cation is a function of time due to slow diffusion processes. 5.2. Column tests Column tests allow undisturbed rock testing in the laboratory. The influence of rock composition and hingering time of solution in the rock on nuclide extension can be investigated by flow tests. The scheme of the test apparatus is shown in Fig. 3. By means of a nitrogen atmosphere a solution can be percolated through an undisturbed and oriented clay sample under different hydraulic gradients and in different orientations of the bedding planes. The nuclide solution was given by the continuous-feed method. The samples were saturated when the initial concentration in the eluate was gained. Desorption with the help of waters of different pH-values shows how much nuclides can be released from a contaminated clay. The resulting breakthrough curves suggest two different kinds of retention of the clays: ( 1 ) Sorption. Here the clay adsorbs cations from the solution until reaching its sorption capacity (relative concentration C/Co= 1 ). From this point on, solution passes the sample unchanged (Fig. 4, breakthrough curves). (2) Filter effect. The clay resp. the leaching solution (e.g. alkaline concrete leachate) causes certain cations to precipitate. The clay may gain lower kr values because of pore space fillings by precipitation products. In general, precipitation defers breakthrough effects and produces adsorption isotherms. The steep increase in the adsorption isotherms (Fig. 5 ) until 400 ml solution percolated through the clay denotes ion exchange reactions (adsorption) and the extremely slow ascent after the 400 ml turn precipitation of carbonates Permeability coefficients decrease during percolation as a function of time, of the ionic strength of the solution and of the reaction temperature. The tem-
204
U. B A R T L A N D K.A. C Z U R D A
2 J
"
J"
~-[-~
--7 f l
!
L
1 solutbon input 2 air pressure valve 3 steel lid of pressure column /, central part of pressure columr~
I
!
S clay sample 6 steel bottom of pressure column
i....... L.....
Fig. 3. Sketch of high pressure steel exchange column. Columns of this type were used for all tests discussed in this paper. Pressurefor solution input up to 20 bar. perature effect is different: at low ionic strength the permeability decreases with increasing temperature, and on the other hand at high ionic strength permeability increases when temperature also increases (Baudracco and Tardy, 1988 ). The flocculation-deflocculation behaviour of clays and therefore the extent o f dispersion is a function of the thickness variation of the double layer at the clay-water interface.
5.2. I. Nuclide retention The S-shaped breakthrough curve indicates the dispersion of cations during migration by hydrodynamic processes and sorption. Its ascent and shifting of the value C/Co = 0.5 towards higher percolation amounts is a measure for the sorption capacity o f the clay for the considered nuclide. Depending on the hingering time o f the nuclide solution in the sample a dynamic cation exchange occurs during which nuclides are adsorbed from the clay mineral specific surfaces and other cations are released for it. Kinetics of the exchange process play an important role. As it has been shown in the previous section,
205
RADIONUCLIDES IN CLAY BARRIERS J.ss C/Co
f'
.Lss
llss
II
ss
0.9 0.8
o Cs"
0.7
• Sr 2" kf= 4 .lO'l°m/s kf= 3 . l o - l ° m l s
Ilss: /ss:
pressure
0.6
0.2 MPo
0.5 0.4 0,3 0,2 0.1 i
200
400
600
i
800
i
1000
i
1200
i
1/,00
ml filtrote
Fig. 4. Breakthroughcurves from exchangecolumn test with Cs and Sr on Badener Tegel-marl. Breakthroughat concentration C/Co = 1 at different volumes of solution percolation according to convection direction and type of cation. diffusion processes in the interlayer space of the swellable three-layer clay minerals proceed relatively slowly, while adsorption along outer sorptive surfaces proceeds quickly. Caused by the relatively high flow velocity, adsorption into the interlayer space plays only an unimportant role, by which fact the sorption of strontium is disadvantages. Cesium is adsorbed predominantly on illites with outer sorptive surface and can be adsorbed quickly. If sorption processes occur, the resulting retention of the clay can be expressed by the retention coefficient Rf. It is defined as:
Vw Rf~vr
(3)
where Vw = flow velocity of water, Vr =migration velocity of nuclide. Table 3 shows the retention coefficients for the nuclide sorption of the Badener Tegel. Also in Table 3 a filter coefficient F is given for nuclides which are filtered under certain conditions by the clay sample. The filter coefficient F shows the efficiency of this process. It is defined as:
206
U . B A R T L A N D K.A. C Z U R D A
c'c°/
I AA A A
A
i
: I1 s s i
i L ss
1 i
i'll-'-i , ~--v
,/v~r
0,1
v - - v ~ * - -
v vl
"vv~
-V--
vv
'*
_vl II s s
i
,v
Z ~----iJ-ss •
kf = /. 10 '~rnfs
:7 Z n 2.
± ss k , = &.101Umls : Zn ~pressure 02 MPo
'
filtrate
Fig. 5. Breakthrough curves from exchange column tests with Co and Zn on Badener Tegel-marl. Steep part of isotherm denotes ion exchange reactions, slow ascent of curve denotes carbonate precipitation.
F=I---
C
G
(4)
where C G)
= nuclide concentration of eluate in equilibrium, =initial nuclide concentration.
Immediately after entering of the nuclide solution the sample of the Badener Tegel buffers the pH-value of the solution to 7-8. So the precipitation reactions described in the previous section can proceed in dependence on hingering time of the percolating solution and thus presuming the filter function of the clay sample. The assumption that the filter effect of the clay is an effective retention process is proved by te fact that, as it is shown with the Badener Tegel, no saturation of the clay sample could be reached for the nuclides cobalt and zinc even after percolating 1500 ml 0.05M solution (Fig. 5 ), while the adsorption capacity is quickly reached for cesium and strontium (Fig. 4). The filter effect depends highly on the sedimentological structure of the clay. Intercalated coarser layers form courses of higher permeability and higher pore diameters, so that the filter effect is diminished and the sample is satu-
RADIONUCLIDES IN CLAY BARRIERS
207
TABLE 3 R e t e n t i o n c o e f f i c i e n t s ( R r ) a n d f i l t e r c o e f f i c i e n t s ( F ) f o r t h e B a d e n e r Tcgel; p b p = p a r a l l e l b e d d i n g planes, vbp = vertical bedding planes; p = 0.2 M Pa Nuclide
Cs Cs Cs Sr Sr Sr Co Co Zn Zn
Solution pa
3.5 7.0 10.5 3.5 7.0 10.5 3.5 3.5 3.5 3.5
Concentration (M)
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.05 0.01 0.05
Rf
F
pbp
vbp
pbp
vbp
171.0 99.6 150.0 69.0 53.7 -
95.9 138.0 145.7 48.3 78.2 -
0.2 0.1 0.1 0.55 0.50
0.25 0.10 0.10 0.60 0.60
rated faster. Along those more permeable courses, solution is transported into the eluate, whereby dispersion of the nuclide front is increased (Bartl and Rashidchi, 1987). The Badener Tegel, which shows no defined microfabric direction, does not show a significant dependence on the fabric direction neither in the permeabilities measured nor in its retention behaviour. Another factor which is able to reduce retention capacity is the formation of complexes to which especially cobalt is inclined to form. By means of these processes cations with different valencies can be held in solution over wide pHranges nd in variable complexes. Different hydroxocomplexes of the cobalt ion are known with variable positive or negative charge (Baes and Mesmer, 1976). Those complexes can influence the sorption behaviour in different ways. Besides, redox reactions as they may occur in ferruginous clays can change the charge of cobalt and may thus decrease its solubility. Kinetics of precipitation reactions are decisive for the filtered amount of precipitative nuclide according to sorption processes which are based upon provision of precipitated substances. It could be proved that the amount of nuclide precipitated increases clearly after a 7-day contact time. This means a clear increase of cobalt and zinc retention for slow flow velocities.
5.2.2. Nuclide desorption Desorption tests were performed by means of ground water of different pHvalue. Obviously the equilibrium pH buffered by the clay controls the mobility of cations. Once zinc and cobalt are precipitated, they can hardly be remobilized. More than 90% of the zinc remained in the clay sample of the Badener Tegel with all desorption agents. Cobalt could be remobilized slightly by acid solutions. In total, the amount of cobalt remaining in the sorbens was
208
u . B A R T L A N D K.A. C Z U R D A
~1oo
~ 90
LI
c 80 "m (D
.- 70
6 ~D
c 60
N
50 L.O 30 20
"
Ilss
k , : 4 104°m/s
J. s s :
k,=4
lO~mls
I l s s k~=2 lO-'~/s ± s s : ki=L lO~'nfs
•
Cs I
pref, su,-~ 0 2 ;4~o
10
200
490
600
800
1000
1200
1&O0 ml desorDtion oQens
Fig. 6. Desorption isotherms for Co, Zn, Cs from exchange column test on Badener Tegel-marl. Desorption solution = natural ground water.
clearly smaller (70-55%) than that of zinc. Here again one can see the complicated chemistry of cobalt whose higher mobility may be traced back to its easy complexibility (Fig. 6). Only concrete leachate water effected 60% of the cesium and 85% of the strontium to remain in the sample. When neutral or acid ground water had been used, a slow but continuous release could be observed. After percolating 1500 ml 50% strontium and 30% cesium remained in the clay with neutral ground water. Acid ground water effected only 30% strontium and 20% cesium to remain in the sorbens at the same amount of percolate. Under these conditions the sorption of cations is largely a reversible process, while precipitation products can be remobilized only to certain extent in the dependence on their solubility and complexibility. Concrete leachate water showed the lowest desorption rates, yet in a more realistic scenario one has to deal with the percolation of contamined rock by neutral ground water. 5.3. Diffusion tests
If no advective solution transport occurs, then contaminants (in this case radionuclides) migrate only by diffusion processes. Diffusion processes are caused by thermal movements, temperature gradients or concentration gradients. In our case the translational diffusion is decisive, which is caused by
RADIONUCLIDESIN CLAYBARRIERS
209
the different nuclide concentrations in both sides of the barrier and which lead to a diffusive transport of the nuclides through the clay. Diffusion caused by temperature could be neglected due to the uniform temperature of the test system. The temporal change of nuclide concentration has been measured and the respective apparent diffusion coefficients have been calculated from the resulting breakthrough curves after equation (6). Mathematically, the diffusion process is described by the first Fick law (formula terms see equation ( 1 ) ): 0C
_02C
at_Z x
(5)
After the total transfer method at steady state an apparent diffusion coefficient can be determined from the measured data in analogy to an experiment carried out by Olsen et al. ( 1965 ): dV (C2 - C , ) Ds -ACo t2 - tl
(6)
where Ds = apparent diffusion coefficient (m2/s), C~ =concentration on the inactive side at a time t~, C2 =concentration on the inactive side at a time t2, t~,t2 = time in the linear range of the nuclide breakthrough (s), V = volume of the active solution added (m3), A = sectional area of the sample body (m 2 ), d = thickness of the sample column (m), Cmax = nuclide concentration maximally passed, tmax = time at which Cmax is reached, to = time of nuclide breakthrough (linear regression of the linear range of the breakthrough curve). With the help of the mathematical separation of the adsorbed amount of nuclide (Co= Cma~) a diffusion coefficient D* has been calculated which is independent on the nuclide adsorption of the rock. If compared with the diffusion coefficient D~ which is influenced by sorption and differs from the diffusion coefficient of the single nuclides in the range of some potency indexes, it can be shown that only slight differences occur at the diffusion coefficient D* of the single nuclides which is not influenced by sorption. This is caused by the predominant influence of retention processes on mobility of the nuclides which is especially expressed in the heavy metals cobalt and zinc, which are adsorbed by the clay sample to a high degree (91% resp. 99%) and are therefore hindered in their extension. If one regards values of the non-influenced diffusion coefficients D*, then
210
U. BARTL AND K.A. CZURDA
one can recognize an assimilation of the values of single nuclides in the range of 5-10- 10 -12 m2/s. The order of Z n > C s > C o > S r of the Ds* values may indicate the different diffusivities of the single nuclides at a presumable relatively high amount of free ion diffusion in the clay. Relatively long flow passages in the silty clay of the Badener Tegel may lead to continuous paths of water molecules which are necessary for the free diffusion of ions. The microfabric of the clay particles as well as intercalated ferro- and alumooxides and organic substances prevent free ion diffusion in the rock. Interactions with the solid (sorption/desorption) cause a combined ion flow over solution and solid. Diffusion of ions in heterogeneous materials like clays is a very complex process which depends besides other factors on the mobility of ions, concentration gradients, ions present in the solution, porosity of the rock, rock composition and size and structure of particles, i.e. the rock fabric. Very slow diffusion coefficients with dry soils and clays indicate the influence of water content on the diffusional behaviour of ions (Nye, 1979, Kahr et al., 1985 ). In our tests the clay samples had been watersaturated before test start. This corresponds to water contents of 15-20%. Then the sample has been covered by solutions of 10 mval of the nuclides used in the test, in pairs (cobalt + zinc, cesium + strontium ) as a chloride solution ( 1 m v a l / m l ) . From the measured nuclide breakthrough curves breakthrough times td and diffusion coefficients Ds of the single nuclides can be derived (Fig. 7 ). In comparison to the single breakthrough times td determined by linear regression of the linear range of the breakthrough curve and the nuclide part remaining in the clay sample at the time /'max of the maximal nuclide breakthrough, retention of the clay is expressed under diffusive conditions opposite to the single nuclides. As it could be expected from the previous column tests, zinc and cobalt had been maximally adsorbed with 99.9% resp. 91%; a fact which is also shown in the diffusion coefficient influenced by sorption of 5-10-10 -15 m2/s for zinc and 7- 10-13 m2/s for cobalt (Fig. 7). The reason for this strong obstruction may lie within the precipitation reactions mentioned in the previous sections, which allow only a very small a m o u n t of those elements to pass the clay sample. If compared to the non-influenced diffusion coefficients, then clearly smaller differences occur ( 7 - 1 0 . 1 0 -12 m2/s for zinc and 8- 10-12 m2/s for cobalt). These values signify the mobility of the remaining free amounts of zinc and cobalt. In opposition to the column tests, strontium is adsorbed clearly stronger under diffusive conditions and is even better adsorbed than cesium which is expressed by an influenced diffusion coefficient which exceeds the influenced coefficient by ten power. The higher strontium adsorption at longer contact time of the Badener Tegel might play an important role. Compared to the other nuclides investigated cesium shows relatively small differences in non-influenced (D*) and influenced (Ds) diffusion coefficients. This is caused by the high mobility of cesium which is predominantly adsorbed at surfaces. The value of the influenced diffusion coefficient Ds re-
RADIONUCLIDESIN CLAYBARRIERS 1
100
2
'
211
3
4
5
time t (weeks) 6 7
'
7
90 2t -~ 80 P J~
4O diffusion exoeriment
cloy: Bodener Tegel chloride II ss: x J.ss: •
213 / /
10 / /
cobalt Ilss: A .lss: •
Z ~
1
l 2
I 3
I
4 time t (sec.lO 6)
Fig. 7. B r e a k t h r o u g h i s o t h e r m s f r o m d i f f u s i o n t e s t s o n B a d e n e r T e g e l - m a r l . D i f f e r e n t b e h a v i o u r of Co + and CI-.
flects the obstructing influence of single retention processes on the extension of single elements. The amount of elements breaking through in a certain time is reflected in the order of C1- < Cs ÷ < Sr ÷ < Co 2+ < Zn ÷. Precipitation reactions with zinc and, at parts, cobalt cause a strong destruction, while sorption processes of strontium which can be better adsorbed into the interlayer space of swellable clay mineral components due to longer contact times and of cesium, which is predominantly adsorbed along surfaces of illites, cause a smaller obstruction of mobility. The non-influenced diffusion coefficients D* of the single nuclides (C1- = Cs ÷/Sr 2÷ < Co 2÷ < Zn 2÷ ) show relatively small differences which characterize the specific diffusion of ions. A clear order of mobility is also given by the breakthrough time td listed in Table 4. So the CI ion part breaks through the clay samples after 2-3 days (Fig. 7 ), while cesium and strontium pass after 11-12 day sand cobalt and zinc only after 13 resp. 16-17 days. The breakthrough time can be looked upon as a measure for the migration of the elements which is influenced in different ways. An anisotropic effect of the fabric is not reflected in the mea-
212
U. BARTL AND K.A. CZURDA
TABLE 4
Diffusion coefficients and amount of nuclide retention. No distinct differences in nuclide retention with respect to fabric orientation and diffusion direction.
Cs + Sr 2+ Co 2+ Zn 2+ C1-
pbp vbp pbp vbp pbp vbp pbp vbp pbp vbp
td (s) • 10 -6
/max (S) • 10 -6
D~* ( m 2 / s ) • l0 -12
Ds ( m 2 / s )
% nuclide in clay sample
1.00 0.95 1.00 1.08
3.98 5.11 4.59 5.11 3.98 3.98 3.98 2.69 3.98 3.72
10.4 6.3 6.1 4.4 8.2 7.0 7.0 9.8 6.4 8.3
2.6" 10- I: 1.4"10 I'~ 9.9.10 13 7.2"10 -t:~ 7.3-10 -13 6.7-10 ~-~ 5.2- 10-i~ 14.2.10 t5 3.1-10 -i~ 3.9" 10- l:
75.3 78.1 83.9 83.7 91.0 91.5 99.9 99.8 51.5 52.9
1.11 1.20 1.35 1.50 0.22 0.21
pbp vbp
diffusion direction parallel to bedding planes diffusion directions normal to bedding planes to elapsed time until first solution breakthrough tmax moment of maximum nuclide breakthrough D* diffusion coefficient without retention influence D~ apparent diffusion coefficient (calculated from temporal changes of nuclide concentrations).
sured diffusion coefficients due to the strong bioturbate mixing of the rock in analogy to the results of the column tests• 6. C O N C L U S I O N
In the course of our investigations two principles of retention in clays became apparent: First are sorption phenomena, which in clays are based upon cation exchange processes of clay mineral external and interlayer surfaces. They are to a large extent reversible and depend strongly on the mineral composition of the clay and on the ion strength of the solution. If cations are adsorbed on the sorptive surfaces as e.g. with illites, adsorption occurs quickly and its mobility remains relatively high. Diffusion processes in the interlayer space of the swellable clay mineral phases occur relatively slowly and presume a very low flow velocity. Yet swellable three-layer clay minerals effect a clear increase in sorptive surface and thus cause a remarkable increase in cation exchange capacity. A higher amount of cation specific adsorbing surfaces at certain clay minerals leads to a selective adsorption of certain cations. In the case of the Badener Tegel the amount of marginal adsorbing places is relatively high due to the high amount of mica and illite, which leads to a preference of monovalent cations such as cesium. Yet sorption processes occur as long as the sorption capacity is reached of the clay sample for the single nuclide.
RADIONUCLIDES IN CLAY BARRIERS
213
Second, precipitation reactions occur if the solubility of the nuclide concentration existing in the equilibrium solution with the clay is exceeded dearly. Different precipitation products form according to the offer of precipitating substances. Clays containing carbonates buffer percolating solutions to pHvalues of 7-8, whereby conditions are created in the contact with the clay where cations are insolvable or solvable only to a small extent. Kinetics of the precipitation reactions resp. the efficiency of such complex retention processes depend strongly on the nuclide specific properties and on the composition of the equilibrium solution. Precipitates can be filtered out according to the particle size distribution and pore diameters of the clay. The mechanisms which cause this filter effect are not known exactly; yet the filter effect depends on the hydraulic gradient besides the rock composition, which can influence the efficiency of the precipitation process over the hingering time of the percolation solution. According to the flow velocity a certain dynamic equilibrium condition is reached between filtered nuclide and nuclide content of the eluate, which can be shifted clearly to a higher nuclide content in the eluate by means of the formation of complex compounds. Filter processes cannot effect a total removal of nuclides from the percolating solution, yet the quantities retained are much higher than with sorption processes. Filters do not have a restricted retention capacity. They function as long as precipitation reactions occur. Filtered nuclide cations can be remobilized only under a drastic change of solution pH or with increased flow velocity of the percolating desorbent. The buffer which forms in contact with the carbonate containing Badener Tegel is rather resistant so that the equilibrium pH remains stable to a large extent. Complex compounds can become a problem, as they form easily with some nuclides and may lead to an increased mobility of these nuclides. The microfabric of a clay is important for its nuclide retention. Clay flakes may be preferred oriented, e.g. parallel to bedding planes. This depends strongly on the primary sedimentary environment and the extent of the normal stress caused by the overlying sediments. The microfabric effect is responsible for longer or shorter solution contact with the clay. Simultaneously, the filter effect is strongly reduced due to the bigger pore diameters in preferred directions. In case of the Badener Tegel no significant dependence of nuclide percolation on the microfabric could be observed which is due to strong bioturbation. Diffusion tests with undisturbed clay samples are a realistic case for the contaminant retention behaviour in the rock. The diffusion coefficient derived from it can be used for the mathematical modelling of nuclide propagation. Carbonatic clays such as the Badener Tegel proved to be a sufficient barrier by means of the different retention effects for certain nuclides. Due to its mineralogical composition with a high content of iUite and chlorite, but a low content of swellable smectites the Badener Tegel offers predominantly outer
214
U. BARTLANDK.A.CZURDA
particle surfaces for adsorption. They show a relatively good cesium exchange capacity, yet can adsorb the nuclides only weakly. To deduct the migration behaviour of nuclides in the rock from retention properties determined in the laboratory one has to take into consideration factors such as faults, fabric, sedimentological structure, thickness of sediments and hydraulics of the percolating solution. Often influences rising from these factors can hardly be estimated, and results from the laboratory can only with restrictions be transferred to field conditions. ACKNOWLEDGEMENTS
We thank Dr. P. Krejsa and Mr. Petschnik (Austrian Research Center, Seibersdorf ) for their friendly support. Mrs. Dipl.-Geol. H. Bradl translated the German original into English.
REFERENCES Baes, Ch.F. and Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley, New York. Bartl, U. and Rashidchi, A., 1987. Sorptionsprozesse von Radionukliden an tonigen Barrieregesteinen. In: 6. Nat. Ing.-Geol., Aachen, 1987, pp. 143-159. Baudracco, J. and Tardy, Y., 1988. Dispersion and flocculation of clays in unconsolidated sandstone reservoirs subjected to percolation with NaCI and CaCI2 solutions at different temperatures. Appl. Clay Sci., 3: 347-360. Czurda, K.A., Rashidchi, A. and Wagner, J.-F., 1987. Migration of radionuclides (Sr-90, Cs137 ) in clays from the Austrian Molasse. Appl. Clay Sci., 2:129-145. Demir, 1., 1987. Studies of smectite membrane behavior: electrokinetic, osmotic, and isotopic fractionation processes at elevated pressures. Geochim. Cosmochim. Acta, 52: 727-737. Haydon, P.R. and Graf, D.L., 1986. Studies of smectite membrane behavior: temperature dependence, 20-180 ° C. Geochim. Cosmochim. Acta, 50:115-121. Kahr, K. et al., 1985. Ionendiffusion in hochverdichtetem Bentonit. Nagra Techn. Ber. 85-23, Wiirenlingen, Schweiz. Nye, P.H., 1986. Diffusion of ions and uncharged solutes in soils and soil clays. Adv. Agron., 31 : 225-272. Olsen, S.R. et al., 1965. Self diffusion coefficients of phosphorus in soil measured by transient and steady-state methods. Soil Sci. Soc. Am. Proc., 29:154-158. Rashidchi, A., 1987. Zur Geologie des Ostlichen Hausruck mit der besonderen Berficksightigung der Radionuklid-Sorption in marinen und limnischen Molassetonen. UnverSff. Diss., Univ. Innsbruck.
195
Elsevier Science Publishers B.V., AmsterdAm
Migration and retention phenomena of radionuclides in clay-barrier systems U. Bartl a and K.A. Czurda b aLandratsamt Esslingen, Postfach 145, D-7000 Esslingen am NeCkar, Germany bDepartment of Applied Geology, University of Kalrsruhe, Kaiserstr. 12, D- 7500 Karlsruhe, Germany (Received September 27; accepted after revision May 28, 1991 )
ABSTRACT Bartl, U. and Czurda, K.A., 1991. Migration and retention phenomena of radionuclides in clay-barrier systems. Appl. Clay Sci., 6: 195-214. Within the scope of projecting a waste deposit for low- and medium radioactive materials we have investigated the migration behaviour of the nuclides cesium (Cs- 137 ), strontium (Sr-90), cobalt (Co60) and zinc (Zn-65) in the Badener Tegel, a Miocene clay of the Vienna Basin (Lower Austria). Two fundamental retention processes could be derived from batch tests for the nuclides mentioned above: Sorption processes, in general cation exchange processes on clay minerals (cesium and strontium) and precipitation reactions (zinc and cobalt) which are included by exceeding the solubility product in the changed equilibrium solution with the clay. The migration behaviour of the nuclides was investigated with the help of columns with undisturbed and orientated clay samples under advective conditions. The influences of the hydraulic gradient, fabric and solution composition on the migration behaviour of nuclides can be derived form breakthrough curves and retention coefficients. The grade of fixation of nuclides in the clay can be determined by desorption tests with water of different composition. A more realistic case of contaminant extension in low permeable clays is simulated by diffusion tests in undisturbed samples. For the Badener Tegel, diffusion coefficients and breakthrough times have been determined from which the diffusive nuclide extension can be derived. Again precipitation reactions (especially with zinc) lead to a drastic reduction of the nuclide amounts passing through.
1. I N T R O D U C T I O N
In isolating contaminants from the biosphere, clays serve as natural barriers on the one hand; on the other they function as an artificial technical barrier. Besides their low permeability, plasticity and the retention capacity for contaminants are further advantages of clay barriers. Therefore clays are in discussion besides salt and granite as host rocks for a waste deposit for lowand m e d i u m radioactive substances. The high safety required can be guaranteed by the multibarrier principle whereby the host rock and the technical barrier form a staggered security system. It is important to know the retention 0169-1317/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
196
U. B A R T L AND K.A. CZURDA
capacity of clays and the migration behaviour of radionuclide in clay barriers under breakthrough conditions to evaluate its risks for the biosphere. The investigations described have been carried out by us at the Austrian Research Center Seibersdorf and the Department of Applied Geology, University of Karlsruhe. The scope of these investigations is Austria's search for an apt place for low- and medium radioactive waste. 2. CLAY ROCKS
The clay samples used come from the Badener Tegel, a marine clay of middle Miocene age of the Tertiary basin filling of the Vienna Basin and has been taken from its holotype locality in Baden-SooB/Lower Austria. Table 1 shows the mineral composition which has been determined semiquantitatively. The high amount of detritic components is indicated, besides the grain size distribution (Fig. 1 ), in the range of clay minerals where detritic minerals such as mica, illites and chlorites predominate. The amount of smectite comes from an illite-smectite mixed layer clay mineral of the mineral composition I + ( 1014M ) while kaolinite exists in traces only. The primary cation content of the Badener Tegel consists of Ca 2÷- and Mg2+-ions predominantly. Its cation exchange capacity (CEC) against NH~--ions amounts to 47.5 mval/100 g and is in the range of the CEC of comparable molasse clays (Czurda et al., 1987 ). grain size distribution clay
o
sand
silt middle
fine
fine
coarse
middle
coarse
r
lO ~20 ~-
soL Kottingbrunner Soh~chten ( Tonmergel ) Badener Tegel
¢D
100
1
2
6
20
50
200
600 2000 grain size (pro)
Fig. 1. Grain size distribution of tested Tertiary clays and marls: Kottingbrunn formation and Badener Tegel.
RADIONUCLIDESIN CLAYBARRIERS
197
TABLE l Parameters of tested clay rock: Badener Tegei Grain size fraction (/tm)
Mineral content (%) quartz
feldspar
calcite
dolomite
chlorite
kaolinite
illite
smectite
Total 125-60 60-30 30-10 < 10
31 61 45 29 14
5 5 5 5 5
13 20 12 12 21
7 4 6 6 5
18 18 17
2 2 2
16 10 32 18 19
8 7 17
CEC (meq/100 g) Moisture content (%) natural after convection
47.5 7.0 17.0
Heat loss (%) Effective porosity (%) Permeability ( m / s )
11.0 15.3 1 0 - 9 _ 1 0 - |o
TABLE 2 Hydrochemical analyses of ground water and concrete leachate used for desorption tests
Temperature ( ° C) pH-value Redox-potential (mV) Specific electric conductivity (/zS/cm) Acid capacity, pH 8.2, m-value (ml) Acid capacity, pH 4.3, p-value (ml) Water hardness ( ° dH) Carbonate hardness ( m e q . / l )
Ground water
Concrete leachate
21.0 7.2 + 61.0 499.0
21.0 10.5 + 57.0 1108.0
0.00 4.07 14.50 4.10
4.40 0.14 4.60 0.00
Cations (meq./1): sodium potassium ammonium calcium magnesium iron manganese
Na + K+ NH~ Ca 2÷ Mg 2÷ Fe 2÷ Mn 2÷
1.087 0.054 0.006 3.897 3.934 0.007 0.005
1.905 1.944 0.000 1.547 0.002 0.003 0.001
Anions (meq.1): hydrogen carbonate chloride nitrate sulfate phosphate hydroxide
HCO~CINO jSO 2HPO 2OH-
4.069 0.293 0.363 4.422 0.000 0.000
0.138 1.995 0.170 1.790 0.019 ca. 0.900
198
U. BARTLAND K.A.CZURDA
3. R A D I O N U C L I D E S O L U T I O N S
The used radionuclides Cs-137, Sr-90, Co-60 and Zn-65 represent the main part of activity in low- and m e d i u m radioactive waste. Chloride solutions of the nuclide pairs cesium/strontium and cobalt/zinc have been prepared with a representative ground water from a gravel body nearby. Its hydrochemical analysis is shown in Table 2. The nuclide concentration in the inactive solution was 0.01M resp. 0.05M, an equivalent activity concentration of 1 resp. 5n C i / m l was standardized by addition of the respective radionuclides. The retention of the nuclides by the clay sample can be determined by the activity concentration which can be measured easily before and after the test. To simulate different conditions of the damage case, a slightly acid solution (pH 3.5 ) and an alkaline concrete leachate water (pH 10.5 ) was used besides the neutral alkaline-hydrogencarbonatic ground water. Table 2 shows the composition of the concrete water. The investigation of the contaminated concrete water-clay barrier interactions is interesting, as concrete is well suitable for conditioning low- and m e d i u m radioactive waste and is therefore increasingly used. 4. N U C L I D E E X T E N S I O N IN T H E CLAY
4. I. Nuclide transport If a solution passes through a porous medium, then its components are influenced in different ways, which leads to a smearing of the penetrating solution front and thereby to a mostly delayed and unprecise breakthrough of solution. In a mathematical model a transport equation can describe, e.g. the one-dimensional migration of a nuclide front through the m e d i u m in form of different terms: OC
OC
DfOS
Ot -Eff~x2 + l ~ x 2 - Vwox
02C
_02C
~ at
dispersive difJusive advective sorptive lerm
term
term
term
where C = concentration outside the sorbens, t =time, E = hydrodynamic dispersive coefficient, D = diffusive coefficient, x = length (flow path ), Vw = velocity of medium, pf = density of the fluid,
(1)
RADIONUCLIDES IN CLAY BARRIERS
E S
199
= porosity, = concentration in the sorbens.
The change of concentration in the eluate is expressed by a dispersive, a diffusive, an advective and a sorptive term. While advective transport which is caused by a hydraulic gradient works uniformly on the whole solution and would therefore effect a piston-like breakthrough of unchanged solution, hydrodynamic dispersion, diffusion and retention may effect either an acceleration or a retardation of solution components in different ways. The term retention signifies all interactions which allow a solid body to retain solution components. Under this term sorption processes are classed with, whereby rock components adsorb solution components, in this case certain cations, as well as chemical reactions like complex formation and precipitation reactions, whose products can be fitted mechanically by means of highly adsorptive and adhesive forces on hydrated particle surfaces. A precise distinction between those two retention processes cannot be carried out as complex compounds or colloids can be subjected to both adhesive and sorptive processes. Interactions of this kind have not been investigated thoroughly yet. Due to the different velocities of solution transport in pores of different size, hydrodynamic dispersion leads to a local running ahead of the displacing solution components in the biggest pores. Simultaneously the moment of reaching the initial concentration in the eluate is retarded whereby the Sshaped breakthrough curve flattens out at its right end. The hydrodynamic dispersion coefficient E contains properties of the pore system as well as the form and dimension of single passage ways. Concentration gradients of the solution components cause diffusive processes along the migration path of the solution. They appear in a percolating solution if the flow velocity is very low and the flow distance becomes very high. Diffusion processes lead to long-term flattening of the concentration ascent of the adsorption isotherms as they proceed very slowly. At the top of the velocity profile the diffusive movement runs towards the pore solution without nuclide content and adds therewith to an acceleration of the breakthrough of first nuclide parts. Under the conditions of high flow velocities the influence of slowly proceeding diffusion processes on the nuclide transport can be neglected (Nye, 1986). In any case, diffusion from the interlayer space of the clay minerals, especially from three-layer smectites, into the pore space has to be considered. The saturation of the Stern-layer with radionuclides and their inactive metal cations leads to a comparatively high concentrated double-layer space which finally releases cations and radionuclides out into the pore space by means of a diffusional migration. The released cations are restricted in passing the clay to differing degrees. The clay acts as a semipermeable membrane, allowing some components of
200
U. BARTLAND K.A. CZURDA
pore fluids such as water to pass through relatively freely, but restricting the various ionic species. Electroosmosis and streaming potential are the two main electrokinetic phenomena for clay-electrolyte systems and associated with them are hydraulic and electrical conductivities (Demir, 1987). The cation selectivity during membrane transport is increased at higher temperatures (Haydon and Graf, 1986 ). 4.2. Retention processes
The sorption processes on clay minerals always mean cation exchange. The cation exchange cap~tcity (CEC) of a clay is determined by the following parameters: content of sorbents, - size of accessible exchanger surface, kind and amount of surface charge, - primary ion occupancy of exchanger, ion strength of the solution, cation specific parameters. The cation exchange is of importance predominantly with highly sorptive, swellable three-layer clay minerals of the smectite- and vermiculite-group which show a high exchange capacity due to their large charge deficit and their accessible interlayer space. Many cations show only a restricted solubility under conditions which result from the contact of a solution with the clay. Different kinds of metal hydroxides, -oxides, -sulfates, -hydroxencarbonates and carbonates can be precipitated. As it is the case with the Badener Tegel and other carbonatic clays, this happens with the help of carbonate buffered solutions or the change of the solution pH-value by sorption resp. desorption of H ÷-ions on clay minerals. These clays are able to standardize and stabilize the pH-value of an equilibrium solution at different stages, if the solubility of a cation in the solution is exceeded the precipitation products are eliminated mostly in form of flakes or colloids. If sorption processes proceed between the solution components and the clay, then solution components are permanently adsorbed at the beginning of the contact of solution and sorbens. The amount adsorbed is at a maximum at the solution front and sorption tends towards an equilibrium state. But the amount retarded at the breakthrough front is lower than the exchange capacity of the sorbens, as not all sorptive surfaces had the possibility of coming into contact with the percolating solution. Before a complete sorption is possible, solution componentsr reach the eluate by means of flow passages. Many influences such as stream velocity, content of sorptive components along the flow passages and the kinetics of sorption processes control this dynamic process which is therefore a very complex quantity. In most transport equations the attempt is made to take into account its influence on migration by intro-
-
-
-
RADIONUCLIDES IN CLAY BARRIERS
201
ducing a separate sorption term, yet the necessary distribution coefficients, which are mostly deduced from adsorption isotherms assume the adherence to certain frame conditions which are often not fulfilled in rock/solution systems. 5. E X P E R I M E N T S
5.1. Batch tests The interaction of components of a clay with a nuclide solution can be comprehended by batch tests. In batch tests small amounts of rock powder ( 1 g) are shaken with a determined amount of nuclide solution (20 ml). The amount of nuclide adsorbed after the test can be expressed with the help of a distribution coefficient (Rd-value). It is defined as: nuclide concentration in the solide phase Rd --nuclide concentration in the liquid phase
ml g
--
(2)
The retention of the Badener Tegel has been investigated under different parameters. 5.1. I. Mineral composition and grain size distribution In Fig. 2 cesium and strontium show an infinitely small sorption with coarse grain fractions. Only when clay minerals occur in the fraction 50-10 ~tm the sorption of the two nuclides starts. Sorption increases clearly with cesium and drastically with strontium in the fraction < 10 ~tm where clay minerals are accumulated with an increased smectite amount with high sorption capacity. Hence it can be derived that the part of sorptive surface is a primary factor for the sorption of the two nuclides, whereby the sorption of bivalent strontium seems to depend especially on the increase of smectite content. Cobalt and zinc on the other hand show a certain retention even with coarse grain sizes which might be caused by the provision of carbonate especially with zinc, as both zinc and cobalt have only a small solubility under pH-conditions of about 7. Due to its complex chemism which allows sorption processes with the help of complex formation as well as precipitation reactions, cobalt shows a sorptive component which is expressed by the increase of retention by smallest grain sizes with their clearly higher content of sorptive components. Illite and chlorite which dominate in the clay mineral spectrum of the Badener Tegel are able to adsorb cations only on their outer surface due to their structure. Their strength of bonding is relatively small so that the adsorbed cesium and strontium ions can be mobilized again with desorption tests. IIlites show specific exchange areas and are able to adsorb cations predominantly along prism faces of their outer surface and along exposed basal planes. Monovalent cations like cesium are preferred. Contrarily, smectites adsorb
202
U. B A R T L AND K.A. CZURDA
botch test / sorption
>
100
2~ Z n 2~
0
m 50 .c_ Co 2. S
g~
0 (.3
125-60
60-30 30-10 grain size (/Jm)
OCs*
10
total clay
E]Sr 2÷ ACo 2. v Z n 2.
Fig. 2. Batch test results from Badener Tegel-marl with Zn, Co, Cs, St. Quantity of adsorbed cations in dependence on grain size distribution.
predominantly bivalent cations with an apt ion radius into their interlayer space due to their favourable charge compensation. Strontium is thereby preferred to the monovalent cesium. This fact is also shown by sorption tests made under identical conditions by Rashidchi (1987) with monomineralic clays where illites showed higher adsorption capacities for cesium and strontium. Of course, clays with high amounts of smectite show especially high adsorption rates for cesium and strontium. To evaluate the influences of the carbonate content on cobalt and zinc retention, batch tests have been carried out with decarbonated Badener Tegel as well as with pure calcium carbonate powder and calcium sulfate powder.
RADIONUCLIDES IN CLAY BARRIERS
203
Decarbonated clay samples show the expected decrease in retention, whereby the remaining retention at an equilibrium pH-value of 4-4.5 can be traced back to the precipitation of zinc and cobalt carbonates which causes a reduction of the zinc content of the nuclide solution of 91.9% and of the cobalt content of 20%. The lower amount of precipitation of cobalt can be explained by its slightly higher solubility and the competition of zinc for precipitating substances in the solution.
5.1.2. Contact time The contact times of the batch tests have been varied between 1 hour and 75 days. Prolonged contact time and a longer time of shaking causes a better dispersion of the clay particles and therewith an enlargement of the accessible sorptive surface, whereby mechanical spalling of mica and illites provides additional sorptive surfaces at their basal faces. On the other hand the standardization of an equilibrium between the cations adsorbed at the clay and the dissolved cation is a function of time due to slow diffusion processes. 5.2. Column tests Column tests allow undisturbed rock testing in the laboratory. The influence of rock composition and hingering time of solution in the rock on nuclide extension can be investigated by flow tests. The scheme of the test apparatus is shown in Fig. 3. By means of a nitrogen atmosphere a solution can be percolated through an undisturbed and oriented clay sample under different hydraulic gradients and in different orientations of the bedding planes. The nuclide solution was given by the continuous-feed method. The samples were saturated when the initial concentration in the eluate was gained. Desorption with the help of waters of different pH-values shows how much nuclides can be released from a contaminated clay. The resulting breakthrough curves suggest two different kinds of retention of the clays: ( 1 ) Sorption. Here the clay adsorbs cations from the solution until reaching its sorption capacity (relative concentration C/Co= 1 ). From this point on, solution passes the sample unchanged (Fig. 4, breakthrough curves). (2) Filter effect. The clay resp. the leaching solution (e.g. alkaline concrete leachate) causes certain cations to precipitate. The clay may gain lower kr values because of pore space fillings by precipitation products. In general, precipitation defers breakthrough effects and produces adsorption isotherms. The steep increase in the adsorption isotherms (Fig. 5 ) until 400 ml solution percolated through the clay denotes ion exchange reactions (adsorption) and the extremely slow ascent after the 400 ml turn precipitation of carbonates Permeability coefficients decrease during percolation as a function of time, of the ionic strength of the solution and of the reaction temperature. The tem-
204
U. B A R T L A N D K.A. C Z U R D A
2 J
"
J"
~-[-~
--7 f l
!
L
1 solutbon input 2 air pressure valve 3 steel lid of pressure column /, central part of pressure columr~
I
!
S clay sample 6 steel bottom of pressure column
i....... L.....
Fig. 3. Sketch of high pressure steel exchange column. Columns of this type were used for all tests discussed in this paper. Pressurefor solution input up to 20 bar. perature effect is different: at low ionic strength the permeability decreases with increasing temperature, and on the other hand at high ionic strength permeability increases when temperature also increases (Baudracco and Tardy, 1988 ). The flocculation-deflocculation behaviour of clays and therefore the extent o f dispersion is a function of the thickness variation of the double layer at the clay-water interface.
5.2. I. Nuclide retention The S-shaped breakthrough curve indicates the dispersion of cations during migration by hydrodynamic processes and sorption. Its ascent and shifting of the value C/Co = 0.5 towards higher percolation amounts is a measure for the sorption capacity o f the clay for the considered nuclide. Depending on the hingering time o f the nuclide solution in the sample a dynamic cation exchange occurs during which nuclides are adsorbed from the clay mineral specific surfaces and other cations are released for it. Kinetics of the exchange process play an important role. As it has been shown in the previous section,
205
RADIONUCLIDES IN CLAY BARRIERS J.ss C/Co
f'
.Lss
llss
II
ss
0.9 0.8
o Cs"
0.7
• Sr 2" kf= 4 .lO'l°m/s kf= 3 . l o - l ° m l s
Ilss: /ss:
pressure
0.6
0.2 MPo
0.5 0.4 0,3 0,2 0.1 i
200
400
600
i
800
i
1000
i
1200
i
1/,00
ml filtrote
Fig. 4. Breakthroughcurves from exchangecolumn test with Cs and Sr on Badener Tegel-marl. Breakthroughat concentration C/Co = 1 at different volumes of solution percolation according to convection direction and type of cation. diffusion processes in the interlayer space of the swellable three-layer clay minerals proceed relatively slowly, while adsorption along outer sorptive surfaces proceeds quickly. Caused by the relatively high flow velocity, adsorption into the interlayer space plays only an unimportant role, by which fact the sorption of strontium is disadvantages. Cesium is adsorbed predominantly on illites with outer sorptive surface and can be adsorbed quickly. If sorption processes occur, the resulting retention of the clay can be expressed by the retention coefficient Rf. It is defined as:
Vw Rf~vr
(3)
where Vw = flow velocity of water, Vr =migration velocity of nuclide. Table 3 shows the retention coefficients for the nuclide sorption of the Badener Tegel. Also in Table 3 a filter coefficient F is given for nuclides which are filtered under certain conditions by the clay sample. The filter coefficient F shows the efficiency of this process. It is defined as:
206
U . B A R T L A N D K.A. C Z U R D A
c'c°/
I AA A A
A
i
: I1 s s i
i L ss
1 i
i'll-'-i , ~--v
,/v~r
0,1
v - - v ~ * - -
v vl
"vv~
-V--
vv
'*
_vl II s s
i
,v
Z ~----iJ-ss •
kf = /. 10 '~rnfs
:7 Z n 2.
± ss k , = &.101Umls : Zn ~pressure 02 MPo
'
filtrate
Fig. 5. Breakthrough curves from exchange column tests with Co and Zn on Badener Tegel-marl. Steep part of isotherm denotes ion exchange reactions, slow ascent of curve denotes carbonate precipitation.
F=I---
C
G
(4)
where C G)
= nuclide concentration of eluate in equilibrium, =initial nuclide concentration.
Immediately after entering of the nuclide solution the sample of the Badener Tegel buffers the pH-value of the solution to 7-8. So the precipitation reactions described in the previous section can proceed in dependence on hingering time of the percolating solution and thus presuming the filter function of the clay sample. The assumption that the filter effect of the clay is an effective retention process is proved by te fact that, as it is shown with the Badener Tegel, no saturation of the clay sample could be reached for the nuclides cobalt and zinc even after percolating 1500 ml 0.05M solution (Fig. 5 ), while the adsorption capacity is quickly reached for cesium and strontium (Fig. 4). The filter effect depends highly on the sedimentological structure of the clay. Intercalated coarser layers form courses of higher permeability and higher pore diameters, so that the filter effect is diminished and the sample is satu-
RADIONUCLIDES IN CLAY BARRIERS
207
TABLE 3 R e t e n t i o n c o e f f i c i e n t s ( R r ) a n d f i l t e r c o e f f i c i e n t s ( F ) f o r t h e B a d e n e r Tcgel; p b p = p a r a l l e l b e d d i n g planes, vbp = vertical bedding planes; p = 0.2 M Pa Nuclide
Cs Cs Cs Sr Sr Sr Co Co Zn Zn
Solution pa
3.5 7.0 10.5 3.5 7.0 10.5 3.5 3.5 3.5 3.5
Concentration (M)
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.05 0.01 0.05
Rf
F
pbp
vbp
pbp
vbp
171.0 99.6 150.0 69.0 53.7 -
95.9 138.0 145.7 48.3 78.2 -
0.2 0.1 0.1 0.55 0.50
0.25 0.10 0.10 0.60 0.60
rated faster. Along those more permeable courses, solution is transported into the eluate, whereby dispersion of the nuclide front is increased (Bartl and Rashidchi, 1987). The Badener Tegel, which shows no defined microfabric direction, does not show a significant dependence on the fabric direction neither in the permeabilities measured nor in its retention behaviour. Another factor which is able to reduce retention capacity is the formation of complexes to which especially cobalt is inclined to form. By means of these processes cations with different valencies can be held in solution over wide pHranges nd in variable complexes. Different hydroxocomplexes of the cobalt ion are known with variable positive or negative charge (Baes and Mesmer, 1976). Those complexes can influence the sorption behaviour in different ways. Besides, redox reactions as they may occur in ferruginous clays can change the charge of cobalt and may thus decrease its solubility. Kinetics of precipitation reactions are decisive for the filtered amount of precipitative nuclide according to sorption processes which are based upon provision of precipitated substances. It could be proved that the amount of nuclide precipitated increases clearly after a 7-day contact time. This means a clear increase of cobalt and zinc retention for slow flow velocities.
5.2.2. Nuclide desorption Desorption tests were performed by means of ground water of different pHvalue. Obviously the equilibrium pH buffered by the clay controls the mobility of cations. Once zinc and cobalt are precipitated, they can hardly be remobilized. More than 90% of the zinc remained in the clay sample of the Badener Tegel with all desorption agents. Cobalt could be remobilized slightly by acid solutions. In total, the amount of cobalt remaining in the sorbens was
208
u . B A R T L A N D K.A. C Z U R D A
~1oo
~ 90
LI
c 80 "m (D
.- 70
6 ~D
c 60
N
50 L.O 30 20
"
Ilss
k , : 4 104°m/s
J. s s :
k,=4
lO~mls
I l s s k~=2 lO-'~/s ± s s : ki=L lO~'nfs
•
Cs I
pref, su,-~ 0 2 ;4~o
10
200
490
600
800
1000
1200
1&O0 ml desorDtion oQens
Fig. 6. Desorption isotherms for Co, Zn, Cs from exchange column test on Badener Tegel-marl. Desorption solution = natural ground water.
clearly smaller (70-55%) than that of zinc. Here again one can see the complicated chemistry of cobalt whose higher mobility may be traced back to its easy complexibility (Fig. 6). Only concrete leachate water effected 60% of the cesium and 85% of the strontium to remain in the sample. When neutral or acid ground water had been used, a slow but continuous release could be observed. After percolating 1500 ml 50% strontium and 30% cesium remained in the clay with neutral ground water. Acid ground water effected only 30% strontium and 20% cesium to remain in the sorbens at the same amount of percolate. Under these conditions the sorption of cations is largely a reversible process, while precipitation products can be remobilized only to certain extent in the dependence on their solubility and complexibility. Concrete leachate water showed the lowest desorption rates, yet in a more realistic scenario one has to deal with the percolation of contamined rock by neutral ground water. 5.3. Diffusion tests
If no advective solution transport occurs, then contaminants (in this case radionuclides) migrate only by diffusion processes. Diffusion processes are caused by thermal movements, temperature gradients or concentration gradients. In our case the translational diffusion is decisive, which is caused by
RADIONUCLIDESIN CLAYBARRIERS
209
the different nuclide concentrations in both sides of the barrier and which lead to a diffusive transport of the nuclides through the clay. Diffusion caused by temperature could be neglected due to the uniform temperature of the test system. The temporal change of nuclide concentration has been measured and the respective apparent diffusion coefficients have been calculated from the resulting breakthrough curves after equation (6). Mathematically, the diffusion process is described by the first Fick law (formula terms see equation ( 1 ) ): 0C
_02C
at_Z x
(5)
After the total transfer method at steady state an apparent diffusion coefficient can be determined from the measured data in analogy to an experiment carried out by Olsen et al. ( 1965 ): dV (C2 - C , ) Ds -ACo t2 - tl
(6)
where Ds = apparent diffusion coefficient (m2/s), C~ =concentration on the inactive side at a time t~, C2 =concentration on the inactive side at a time t2, t~,t2 = time in the linear range of the nuclide breakthrough (s), V = volume of the active solution added (m3), A = sectional area of the sample body (m 2 ), d = thickness of the sample column (m), Cmax = nuclide concentration maximally passed, tmax = time at which Cmax is reached, to = time of nuclide breakthrough (linear regression of the linear range of the breakthrough curve). With the help of the mathematical separation of the adsorbed amount of nuclide (Co= Cma~) a diffusion coefficient D* has been calculated which is independent on the nuclide adsorption of the rock. If compared with the diffusion coefficient D~ which is influenced by sorption and differs from the diffusion coefficient of the single nuclides in the range of some potency indexes, it can be shown that only slight differences occur at the diffusion coefficient D* of the single nuclides which is not influenced by sorption. This is caused by the predominant influence of retention processes on mobility of the nuclides which is especially expressed in the heavy metals cobalt and zinc, which are adsorbed by the clay sample to a high degree (91% resp. 99%) and are therefore hindered in their extension. If one regards values of the non-influenced diffusion coefficients D*, then
210
U. BARTL AND K.A. CZURDA
one can recognize an assimilation of the values of single nuclides in the range of 5-10- 10 -12 m2/s. The order of Z n > C s > C o > S r of the Ds* values may indicate the different diffusivities of the single nuclides at a presumable relatively high amount of free ion diffusion in the clay. Relatively long flow passages in the silty clay of the Badener Tegel may lead to continuous paths of water molecules which are necessary for the free diffusion of ions. The microfabric of the clay particles as well as intercalated ferro- and alumooxides and organic substances prevent free ion diffusion in the rock. Interactions with the solid (sorption/desorption) cause a combined ion flow over solution and solid. Diffusion of ions in heterogeneous materials like clays is a very complex process which depends besides other factors on the mobility of ions, concentration gradients, ions present in the solution, porosity of the rock, rock composition and size and structure of particles, i.e. the rock fabric. Very slow diffusion coefficients with dry soils and clays indicate the influence of water content on the diffusional behaviour of ions (Nye, 1979, Kahr et al., 1985 ). In our tests the clay samples had been watersaturated before test start. This corresponds to water contents of 15-20%. Then the sample has been covered by solutions of 10 mval of the nuclides used in the test, in pairs (cobalt + zinc, cesium + strontium ) as a chloride solution ( 1 m v a l / m l ) . From the measured nuclide breakthrough curves breakthrough times td and diffusion coefficients Ds of the single nuclides can be derived (Fig. 7 ). In comparison to the single breakthrough times td determined by linear regression of the linear range of the breakthrough curve and the nuclide part remaining in the clay sample at the time /'max of the maximal nuclide breakthrough, retention of the clay is expressed under diffusive conditions opposite to the single nuclides. As it could be expected from the previous column tests, zinc and cobalt had been maximally adsorbed with 99.9% resp. 91%; a fact which is also shown in the diffusion coefficient influenced by sorption of 5-10-10 -15 m2/s for zinc and 7- 10-13 m2/s for cobalt (Fig. 7). The reason for this strong obstruction may lie within the precipitation reactions mentioned in the previous sections, which allow only a very small a m o u n t of those elements to pass the clay sample. If compared to the non-influenced diffusion coefficients, then clearly smaller differences occur ( 7 - 1 0 . 1 0 -12 m2/s for zinc and 8- 10-12 m2/s for cobalt). These values signify the mobility of the remaining free amounts of zinc and cobalt. In opposition to the column tests, strontium is adsorbed clearly stronger under diffusive conditions and is even better adsorbed than cesium which is expressed by an influenced diffusion coefficient which exceeds the influenced coefficient by ten power. The higher strontium adsorption at longer contact time of the Badener Tegel might play an important role. Compared to the other nuclides investigated cesium shows relatively small differences in non-influenced (D*) and influenced (Ds) diffusion coefficients. This is caused by the high mobility of cesium which is predominantly adsorbed at surfaces. The value of the influenced diffusion coefficient Ds re-
RADIONUCLIDESIN CLAYBARRIERS 1
100
2
'
211
3
4
5
time t (weeks) 6 7
'
7
90 2t -~ 80 P J~
4O diffusion exoeriment
cloy: Bodener Tegel chloride II ss: x J.ss: •
213 / /
10 / /
cobalt Ilss: A .lss: •
Z ~
1
l 2
I 3
I
4 time t (sec.lO 6)
Fig. 7. B r e a k t h r o u g h i s o t h e r m s f r o m d i f f u s i o n t e s t s o n B a d e n e r T e g e l - m a r l . D i f f e r e n t b e h a v i o u r of Co + and CI-.
flects the obstructing influence of single retention processes on the extension of single elements. The amount of elements breaking through in a certain time is reflected in the order of C1- < Cs ÷ < Sr ÷ < Co 2+ < Zn ÷. Precipitation reactions with zinc and, at parts, cobalt cause a strong destruction, while sorption processes of strontium which can be better adsorbed into the interlayer space of swellable clay mineral components due to longer contact times and of cesium, which is predominantly adsorbed along surfaces of illites, cause a smaller obstruction of mobility. The non-influenced diffusion coefficients D* of the single nuclides (C1- = Cs ÷/Sr 2÷ < Co 2÷ < Zn 2÷ ) show relatively small differences which characterize the specific diffusion of ions. A clear order of mobility is also given by the breakthrough time td listed in Table 4. So the CI ion part breaks through the clay samples after 2-3 days (Fig. 7 ), while cesium and strontium pass after 11-12 day sand cobalt and zinc only after 13 resp. 16-17 days. The breakthrough time can be looked upon as a measure for the migration of the elements which is influenced in different ways. An anisotropic effect of the fabric is not reflected in the mea-
212
U. BARTL AND K.A. CZURDA
TABLE 4
Diffusion coefficients and amount of nuclide retention. No distinct differences in nuclide retention with respect to fabric orientation and diffusion direction.
Cs + Sr 2+ Co 2+ Zn 2+ C1-
pbp vbp pbp vbp pbp vbp pbp vbp pbp vbp
td (s) • 10 -6
/max (S) • 10 -6
D~* ( m 2 / s ) • l0 -12
Ds ( m 2 / s )
% nuclide in clay sample
1.00 0.95 1.00 1.08
3.98 5.11 4.59 5.11 3.98 3.98 3.98 2.69 3.98 3.72
10.4 6.3 6.1 4.4 8.2 7.0 7.0 9.8 6.4 8.3
2.6" 10- I: 1.4"10 I'~ 9.9.10 13 7.2"10 -t:~ 7.3-10 -13 6.7-10 ~-~ 5.2- 10-i~ 14.2.10 t5 3.1-10 -i~ 3.9" 10- l:
75.3 78.1 83.9 83.7 91.0 91.5 99.9 99.8 51.5 52.9
1.11 1.20 1.35 1.50 0.22 0.21
pbp vbp
diffusion direction parallel to bedding planes diffusion directions normal to bedding planes to elapsed time until first solution breakthrough tmax moment of maximum nuclide breakthrough D* diffusion coefficient without retention influence D~ apparent diffusion coefficient (calculated from temporal changes of nuclide concentrations).
sured diffusion coefficients due to the strong bioturbate mixing of the rock in analogy to the results of the column tests• 6. C O N C L U S I O N
In the course of our investigations two principles of retention in clays became apparent: First are sorption phenomena, which in clays are based upon cation exchange processes of clay mineral external and interlayer surfaces. They are to a large extent reversible and depend strongly on the mineral composition of the clay and on the ion strength of the solution. If cations are adsorbed on the sorptive surfaces as e.g. with illites, adsorption occurs quickly and its mobility remains relatively high. Diffusion processes in the interlayer space of the swellable clay mineral phases occur relatively slowly and presume a very low flow velocity. Yet swellable three-layer clay minerals effect a clear increase in sorptive surface and thus cause a remarkable increase in cation exchange capacity. A higher amount of cation specific adsorbing surfaces at certain clay minerals leads to a selective adsorption of certain cations. In the case of the Badener Tegel the amount of marginal adsorbing places is relatively high due to the high amount of mica and illite, which leads to a preference of monovalent cations such as cesium. Yet sorption processes occur as long as the sorption capacity is reached of the clay sample for the single nuclide.
RADIONUCLIDES IN CLAY BARRIERS
213
Second, precipitation reactions occur if the solubility of the nuclide concentration existing in the equilibrium solution with the clay is exceeded dearly. Different precipitation products form according to the offer of precipitating substances. Clays containing carbonates buffer percolating solutions to pHvalues of 7-8, whereby conditions are created in the contact with the clay where cations are insolvable or solvable only to a small extent. Kinetics of the precipitation reactions resp. the efficiency of such complex retention processes depend strongly on the nuclide specific properties and on the composition of the equilibrium solution. Precipitates can be filtered out according to the particle size distribution and pore diameters of the clay. The mechanisms which cause this filter effect are not known exactly; yet the filter effect depends on the hydraulic gradient besides the rock composition, which can influence the efficiency of the precipitation process over the hingering time of the percolation solution. According to the flow velocity a certain dynamic equilibrium condition is reached between filtered nuclide and nuclide content of the eluate, which can be shifted clearly to a higher nuclide content in the eluate by means of the formation of complex compounds. Filter processes cannot effect a total removal of nuclides from the percolating solution, yet the quantities retained are much higher than with sorption processes. Filters do not have a restricted retention capacity. They function as long as precipitation reactions occur. Filtered nuclide cations can be remobilized only under a drastic change of solution pH or with increased flow velocity of the percolating desorbent. The buffer which forms in contact with the carbonate containing Badener Tegel is rather resistant so that the equilibrium pH remains stable to a large extent. Complex compounds can become a problem, as they form easily with some nuclides and may lead to an increased mobility of these nuclides. The microfabric of a clay is important for its nuclide retention. Clay flakes may be preferred oriented, e.g. parallel to bedding planes. This depends strongly on the primary sedimentary environment and the extent of the normal stress caused by the overlying sediments. The microfabric effect is responsible for longer or shorter solution contact with the clay. Simultaneously, the filter effect is strongly reduced due to the bigger pore diameters in preferred directions. In case of the Badener Tegel no significant dependence of nuclide percolation on the microfabric could be observed which is due to strong bioturbation. Diffusion tests with undisturbed clay samples are a realistic case for the contaminant retention behaviour in the rock. The diffusion coefficient derived from it can be used for the mathematical modelling of nuclide propagation. Carbonatic clays such as the Badener Tegel proved to be a sufficient barrier by means of the different retention effects for certain nuclides. Due to its mineralogical composition with a high content of iUite and chlorite, but a low content of swellable smectites the Badener Tegel offers predominantly outer
214
U. BARTLANDK.A.CZURDA
particle surfaces for adsorption. They show a relatively good cesium exchange capacity, yet can adsorb the nuclides only weakly. To deduct the migration behaviour of nuclides in the rock from retention properties determined in the laboratory one has to take into consideration factors such as faults, fabric, sedimentological structure, thickness of sediments and hydraulics of the percolating solution. Often influences rising from these factors can hardly be estimated, and results from the laboratory can only with restrictions be transferred to field conditions. ACKNOWLEDGEMENTS
We thank Dr. P. Krejsa and Mr. Petschnik (Austrian Research Center, Seibersdorf ) for their friendly support. Mrs. Dipl.-Geol. H. Bradl translated the German original into English.
REFERENCES Baes, Ch.F. and Mesmer, R.E., 1976. The Hydrolysis of Cations. Wiley, New York. Bartl, U. and Rashidchi, A., 1987. Sorptionsprozesse von Radionukliden an tonigen Barrieregesteinen. In: 6. Nat. Ing.-Geol., Aachen, 1987, pp. 143-159. Baudracco, J. and Tardy, Y., 1988. Dispersion and flocculation of clays in unconsolidated sandstone reservoirs subjected to percolation with NaCI and CaCI2 solutions at different temperatures. Appl. Clay Sci., 3: 347-360. Czurda, K.A., Rashidchi, A. and Wagner, J.-F., 1987. Migration of radionuclides (Sr-90, Cs137 ) in clays from the Austrian Molasse. Appl. Clay Sci., 2:129-145. Demir, 1., 1987. Studies of smectite membrane behavior: electrokinetic, osmotic, and isotopic fractionation processes at elevated pressures. Geochim. Cosmochim. Acta, 52: 727-737. Haydon, P.R. and Graf, D.L., 1986. Studies of smectite membrane behavior: temperature dependence, 20-180 ° C. Geochim. Cosmochim. Acta, 50:115-121. Kahr, K. et al., 1985. Ionendiffusion in hochverdichtetem Bentonit. Nagra Techn. Ber. 85-23, Wiirenlingen, Schweiz. Nye, P.H., 1986. Diffusion of ions and uncharged solutes in soils and soil clays. Adv. Agron., 31 : 225-272. Olsen, S.R. et al., 1965. Self diffusion coefficients of phosphorus in soil measured by transient and steady-state methods. Soil Sci. Soc. Am. Proc., 29:154-158. Rashidchi, A., 1987. Zur Geologie des Ostlichen Hausruck mit der besonderen Berficksightigung der Radionuklid-Sorption in marinen und limnischen Molassetonen. UnverSff. Diss., Univ. Innsbruck.