POSIVA OY

POSIVA .. 97 .. 02

Model for diffusion and porewater chemistry in compacted bentonite Experimental arrangements and preliminary results

of the porewater chemistry studies

Arto Muurinen Jarmo Lehikoinen

VTT Chemical Technology

January 1997

Annankatu 42 D, FIN-00100 HELSINKI, FINLAND

Phone (09) 2280 30 (nat.), (+358-9-) 2280 30 (int.)

POSIVA OY

POSIVA-97-02

Model for diffusion and porewater chemistry in compacted bentonite Experimental arrangements and preliminary results

of the porewater chemistry studies

Arto Muurinen .Jarmo Lehikoinen

VTT Chemical Technology

January 1997

Annankatu 42 D. FIN-001 00 HELSINKI, F!NLAND

Phone (09) 2280 30 (nat.). (+358-9-) 2280 30 (int.)

Fax (09) 2280 3719 (nat.), (+358-9-) 2280 3719 (int.)

ISBN 951-652-027-8 ISSN 1239-3096

The conclusions and viewpoints presented in the report are

those of author(s) and do not necessarily coincide

with those of Posiva.

Tekija(t) - Author(s)

Arto Muurinen,

Posiva-raportti - Posiva report

PosivaOy Annankatu 42 D, FIN-00100 HELSINKI, FINLAND Puh. (09) 2280 30- lnt. Tel. +358 9 2280 30

T oimeksiantaja(t) - Commissioned by

J armo Lehikoinen Posiva Oy VTI Chemical Technology

Nimeke- Title

Raportin tunnus - Report code

POSIV A-97 -02

Julkaisuaika- Date

January 1997

MODEL FOR DIFFUSION AND POREW ATER CHEMISTRY IN COMPACTED BENTONITE- EXPERIMENTAL ARRANGEMENTS AND PRELIMINARY RESULTS OF THE POREWATER CHEMISTRY STUDIES

Tiivistelma - Abstract

This report describes the progress of the experimental research on the porewater chemistry in bentonite. The research is part of the project "Microstructural and Chemical Parameters of Bentonite as Determinants of Waste Isolation Efficiency" within the Nuclear Fission Safety Program organized by The Commission of the European Communities.

The study was started by a literature overview on the properties of bentonite, porewater-sampling methods and obtained results. On the basis of the literature study, pore water extraction by squeezing seemed the most promising method for further development. The apparatus developed in this study consists of a pressing apparatus, which is used to create the necessary long-term compression, and of the compaction cell where porewater is separated from bentonite and collected in a syringe. The constant log-term force is maintained by a strong spring.

An experimental study of solution-bentonite interactions was initiated. The parameters varied are the bentonite density, bentonite-water ratio, composition of the solutions, and the composition of bentonite. The report presents the experimental arrangements, the preliminary results for studying the evolution of water chemistry and the results of pre-modelling.

ISBN ISSN ISBN 951-652-027-8 ISSN 1239-3096

Sivumaara - Number of pages Kieli - Language 31 English

Tekija(t) - Author(s)

Arto Muurinen,

Posiva-raportti - Posiva report

PosivaOy Annankatu 42 D, FIN-001 00 HELSINKI, FINLAND Puh. (09) 2280 30- lnt. Tel. +358 9 2280 30

T oimeksiantaja(t) - Commissioned by

J armo Lehikoinen VTI Kemiantekniikka

Posiva Oy

Nimeke - Title

Raportin tunnus - Report code

POSIV A-97 -02

Julkaisuaika- Date

Tammikuu 1997

MALL! DIFFUUSIOLLE JA HUOKOSVESIKEMIALLE KOMPAKTOIDUSSA BENTONIITISSA- HUOKOSVESIKEMIAN TUTKIMUKSEN KOEJARJESTEL YT JA ALUST A V AT TULOKSET

Tiivistelma - Abstract

Tassa raportissa esitetaan kuvaus bentoniitin huokosvesikemian kokeellisen tutkimuksen edisty­misesUi. Tutkimus on osa projektia "Microstructural and Chemical Parameters of Bentonite as Determinants of Waste Isolation Efficiency", joka kuuluu Euroopan komission organisoimaan Nuclear Fission Safety ohjelmaan.

Huokosvesikemian tutkimus aloitettiin kirjallisuustutkimuksella bentoniitin ominaisuuksista ja huokosveden naytteenottomenetelmista seka noissa tutkimuksissa saaduista tuloksista. Kirjallisuus­tutkimuksen pohjalta huokosvesinaytteen ottaminen puristamalla naytti lupaavimmalta menetelmalta jatkokehityksen kannalta. Tassa tutkimuksessa kehitetty laitteisto koostuu puristuslaitteesta, jolla luodaan tarvittava pitkaaikainen puristus, ja puristussylinterista, jossa huokosvesi erotetaan bento­niitista ja kerataan ruiskuun. Pitkaaikainen vakiovoima yllapidetaan vahvalla jousella.

Kokeet veden ja bentoniitin valisista vuorovaikutuksista on aloitettu. Vaihdeltavat parametrit ovat bentoniitin tiheys, bentoniitti-vesisuhde, liuoksen ionivakevyys ja bentoniitin koostumus. Rapor­tissa esitetaan koejarjestelyt, alustavat tulokset vesikemian kehittymisesta ja esimallinnuksen tulok­set.

ISBN ISSN ISBN 951-652-027-8 ISSN 1239-3096

Sivumaara - Number of pages Kieli - Language 31 Englanti

FOREWORD

Within the Nucleal Fission Safety Program organized by The Commission of the European Communities, the research project "Microstructural and Chemical Parameters of Bentonite as Determinants of Waste Isolation Efficiency" has jointly been started by Clay Technology AB in Sweden, VTT Chemical Technology in Finland, Universitat Hannover in Germany and Kungliga Tekniska Hogskolan in Sweden. Professor Roland Pusch from Clay Technology AB is the co-ordinator of the project. The work in Finland is funded by The Commission of the European Communities, Posiva Oy and VTT Chemical Technology. The contact person for Posiva Oy is Jukka-Pekka Salo. The work in Finland is concentrated on the development of a model for diffusion and porewater chemistry in compacted bentonite.

ABSTRACT

TIIVISTELMA

FOREWORD

CONTENTS

CONTENTS

1 INTRODUCTION ..................................................................................... 1

2 PROPERTIES OF BENTONITE ............................................................... 2

3 LITERATURE REVIEW ON POREWATER CHEMISTRY STUDIES..... 5

4 EXPERIMENTAL STUDY ON WATER BENTONITE INTERACTION.. 10

4.1 EXPERIMENTAL ARRANGEMENTS AND CONDITIONS............. 10

4.2 PRELIMINARY EXPERIMENTAL RESULTS................................. 13

4.3 PRE-MODELLING OF THE EXPERIMENTS................................... 16

4.4 SQUEEZING APPARATUS FORPOREWATEREXTRACTION .... 25

5 SUMMARY............................................................................................... 27

6 REFERENCES........................................................................................... 29

1

1 INTRODUCTION

The understanding of the diffusion processes and porewater chemistry in bentonite are essential for the prediction of the release of radionuclides from the repository of nuclear waste. This study is part of a project where a simulation code for diffusion and porewater chemistry in bentonite will be developed. The model will be based on the micro- and macro-scale material properties of bentonite and surrounding conditions. The work consists of model development work and supporting experimental studies on porewater chemistry.

The evolution of porewater chemistry is determined by the dissolving components initially present in bentonite together with the ions coming with water from the surroundings. The most important easily dissolving components in natural bentonites are calcite, gypsum and pyrite. Ion-exchange processes occur between the exchangeable cations of montmorillonite, the main component of bentonite, and the cations in the porewater. In compacted bentonite, where the charged surfaces are close to each other, exclusion effects may be important, too. The small amount of free water in compacted bentonite may also affect the dissolution of sparingly soluble components.

The evolution of porewater chemistry will be studied in solution-bentonite interaction experiments. At the end of the experiment the equilibrating solution, the porewaters to be squeezed out of the bentonite samples, and bentonites themselves will be analysed to give information for the interpretation and modelling of the interaction.

2

2 PROPERTIES OF BENTONITE

The composition of the commercial MX-80 bentonite, selected as a reference buffer material in the Finnish disposal concept, is given in Table 1. The major component of MX-80 bentonite, montmorillonite, is an expanding 2:1 clay mineral. The observed negative charge on the clay particles mainly originates from the isomorphic replacement of constituent metal cations by lower-valency cations within the lattice. This charge deficiency is neutralized by exchangeable cations being readily replaceable by other cationic species from the solution in contact with the solids. The cation-exchange capacity is about 0.8 meq/g of bentonite. Part of the accessory minerals in bentonite, like carbonates and sulphates, are easily soluble and can produce dissolved ions in the porewater.

The small plate-like clay particles are composed of individual platelets of about 1 nm thickness stacked face to face. The lateral dimensions of a particle range from a few hundredths of a micrometer to several micrometers. The average number of platelets in a particle can be estimated from the ratio of the theoretical surface area to the BET area, which is 1 - 30 depending upon, e.g. the pretreatment of the sample. The measurement on compacted samples of a dry density 1.5 Mg/m3 suggested that each aggregate would be formed on an average of 30 montmorillonite layers (Muurinen et al., 1995).

Table 1. Composition of MX-80 bentonite (compiled from MiUler-Vonmoos and Kahr (1983), Pedersen and Karlsson (1995), and Wieland et al. (1994))

Mineral Montmorillonite Quartz Mica Feldspar Kaolinite Carbonate Sulphate Phosphate Fluoride Sulphide Organic matter

%by weight 65- 80 15 <1 5-8 < 1 1.4 0.3- 0.8 0.1 0.1 0.1- 0.3 0.3-0.4

3

The specific density of montmorillonite is about 2.7 - 2.8 Mg/m3, on the basis of which

the total amount of water in bentonite as a function of the dry density of bentonite has been calculated in Fig. 1. There are two major types of water in bentonite: that inside the granules and that between them. Various types of microstructural investigations have led to the relationship between internal and total water shown in Fig. 2.

0.9

0.8

c;) 0.7 E -m

0.6 ~ "E

0.5 Cl)

"E 8

0.4 ... .! as :: 0.3

~ ~~ ~ ~

i'-...

~ ""'-

"' ~ ..... 0.2

0.1

0

0 0.5 1 1.5 2 Dry density (Mg/m3)

Fig. 1. The total amount of water in saturated bentonite at different dry densities.

80

tR .... 60 Q)

0 3:

0 E 40 ~ .£

Sodium Calcium

20

04T~~~~rrnn~Trrn~TTrnrlh~~~rn~~~~rMnnn

0.00 0.40 0.80 1.20 1.60 2.00 Dry Density. g/ccm

Fig. 2. Theoretical relationship between dry bulk density and a content of "internal" water expressed as a per cent of the total pore volume (Pusch et al., 1990).

4

When water is added to dry bentonite, bentonite binds the water and swells. Fig. 3 presents the swelling pressures of bentonite as a function of bulk (wet) density and salinity in the solution used to saturate the samples (Pusch and Karnland, 1986). The results demonstrate that porewater chemistry does not affect the swelling pressure at bulk densities higher than about 2 Mg/m3

• The probable explanation is the small amount of external water, the swelling pressure at high densities being caused by the hydration of the exchangeable cations. At lower densities a double layer is formed on the external surfaces of the bentonite stacks. The double layers are sensitive to the salinity, thus affecting swelling pressure.

The swelling pressure has to be exceeded if porewater is squeezed out of the bentonite. A gradual expulsion of the interplanar water at constant temperature takes place at different pressures. According to Rieke and Chilingarian (1974), based on the data by van Olphen (1954) and Warner (1964), the first and second layers of water in the sodium montmorillonite surfaces are removed at pressures of 340 and 90 MPa, respectively.

The hydraulic conductivity of bentonite also depends on the clay density and the salinity of the solution used to saturate the bentonite. Squeezing of porewater from a sample of low density, or from a sample saturated with saline water, is easier than squeezing from a sample with high density or saturated with non-saline solution.

P. t/m3

2.2

2.1

2.0

1.9

1.8

1.7

1.6 ,.

1.5 0.1

/ /

/

/ / /

/0 /

r· /

I / /

/ ' /

~ / / / i

/; /~~ / )1'/1 ' i / , ' !

y / [7

/.c /; / /I i " ~~-.- . - I -~tr/ I

... ~--! I I

I

I

0.5

... ,..

+ /

v//, /c r / /

J / / / L:> / /

~i I ! I/~// I / / e! !

/ ... I / / I 0 / I

/

I i I

I I i I

l I !

i i

I I I

I

5 10 50

Fig. 3. The swelling pressure of MX -80 bentonite as a function of bulk (wet) density of the clay. Notes: + artificial groundwater, • distilled water, A 0.6 M NaCl solution, D 0.3 M CaCl solution (Pusch andKarnland, 1986).

5

3 LITERATURE REVIEW ON POREW ATER CHEMISTRY STUDIES

A wealth of attempts to study the interstitial waters of soils and clays has been made both by using samples from deep boreholes and by using controlled laboratory tests. A summary of the work up to 1974 is presented by Rieke and Chilingarian (1974).

There are three major methods for extracting interstitial fluids from sediments involving centrifuging, squeezing out and washing out. The first two methods are based on the same principle of compressing the sediment together and thereby forcing the fluid out of the pore space. The third method consists of determining the volume of the interstitial fluid by drying one sample, and the extraction of the soluble salts from the second sample by washing them out with distilled water. After analysing the solution, the composition of the interstitial water is calculated. In the squeezing methods the chemical composition of the expelled fluid often differs from that of the portion remaining in the pores. In the washing method, also solids in the sample may dissolve, and the components found in the washing solution do not only represent the soluble components coming from the porewater.

Rieke and Chilingarian (1974) give a comprehensive overview of the compaction and porewater-sampling equipment used for soils and clays since the beginning of the century. There are four basic requirements mentioned in Rieke and Chilingarian (1974) that should met by the equipment used in investigating the chemical and mechanical properties of sediments or clays under high pressures. These are, ( 1) application of a uniform stress on the contained sample, (2) measurement of the resulting sample deformation, (3) channelling and collection of the expelled fluid, and (4) utilization of a sample of sufficient size so that respective information can be obtained. In studying the chemistry of the interstitial fluids of hydrated clays and marine sediments, the problem of extracting such fluids for chemical analysis has been solved by various investigators utilizing a consolidometer, the principle of which is shown in Fig. 4.

LOAD APPLIED

l ,......DEFORMATION DIAL

STAND PIPE

POROUS DISKS

BASE

Fig. 4. Schematic diagram of an one-dimensional consolidometer (Rieke and Chilingarian, 1974 ).

6

Another method attempted has been centrifuging samples at high speed. It was shown, however, that centrifuging cannot achieve efficient separation of the fluid from the clay because of low differential pressures. The third method, used by Kryukov (1961), is based on a miscible-liquid displacement process. The principal concern is to maintain a "piston" type of displacement of the fluids by the alcohol.

The squeezing apparatus used by von Engelhard and Gaida ( 1963) consisted of a cylinder made from a nickel-iron-tungsten alloy, resistant to concentrated chloride solutions. The sieve plate through which the solution was pressed was made of the same material and covered with a membrane and paper filters. By using a hydraulic press, the pressure could be held constant during compaction over longer periods with a range of about 3 %. Pure montmorillonite and kaolinite clays saturated with different concentrations of NaCl and CaCl solutions were compacted at pressures between 3 and 320 MPa. For a given clay, the equilibrium porosity reached at a distinct pressure did not depend on electrolyte concentration. The compaction rate and clay mineral texture, on the other hand, were strongly influenced by salt concentration. The compaction rate was slow and texture good if the salt concentration was low, and vice versa. This behaviour was explained by an aggregate structure of salt-containing clays.

For pressures between 3 and 80 MPa with montmorillonite clay, the concentration of electrolyte in the pore solution diminished with increasing compaction.This was explained by the electrochemical properties of base exchanging clays. If the pore fluids contain an electrolyte, the liquid immediately surrounding the clay particle will contain less electrolyte than fluids farther away from the double layer. During compression the electrolyte-rich solution is removed, and the fluid of the double layers poor in electrolyte content is left behind. At higher pressures up to 320 MPa, the salt concentration within the remaining pore solution increased, which may be caused by the inclusion of small droplets of solution in the highly compressed clay acting as a barrier to ions. Fig. 5 gives the concentration vs. porosity curves for NaCl and CaCl.

The experimental results obtained by Rieke et al. (1964) and Chilingar and Rieke (1968) show that the salinity of solution squeezed out from argillaceous sediments progressively changes with increasing pressure. According to Warner (1964), the double-layer theory predicts that the electrolyte content of expelled interstitial waters should decrease with increasing compaction in the case of highly colloidal clays, when the interaction between the diffuse layers begins to occur.

The literature review by Chilingarian et al. (1973) indicated that the maJonty of investigators agree that above a certain pressure, depending on the type of clay, the mineralization of expelled solution decreases with increasing pressure. In the experimental study by Chilingarian et al. (1973) a sample of montmorillonite clay was saturated in seawater, the volume of water being in excess of that of clay. Then the supernatant liquid, which was assumed to have the same composition as the free interstitial water, was removed and analysed. The remaining saturated sample was placed in the triaxial compaction apparatus, described in detail by Sawabini et al. ( 1971) and the successive portions of the expelled solutions were analysed. The final remaining moisture was 62 %, which corresponds to 3.5 MPa pressure.The experimental results obtained show that the concentration of expelled solutions during the initial stages of compaction appear to be higher than that of the interstitial solution initially present in the montmorillonite clay sample saturated with seawater. The results also show that the

7

-Porosity 0.6 0,7 0.1

• 8. l5

J~r----,~----~~~~----~~--~

j nNICl

' ]

J o--~~~~~~~--~~--~ u 2 u 3 ~ 4 u 5

void ratio-

-Porosity 1.5 0 Q2 ll4 os 0.6 Q.7 0.75 Q.8JJ

void ratio-

Fig. 5. Concentration change of NaCl and CaCl2 solutions in compressing montmorillonite clay over pressure range 3 - 80 MPa (von Engelhard and Gaida, 1963).

concentration of expelled solution goes through a maximum, or at least remains constant, before starting to decrease with increasing pressure.

Chilingar and Rieke ( 197 5) conducted compaction experiments on montmorillonite plus illite mixture (50/50) saturated in seawater. Most of the salts were squeezed out during the initial stages of compaction, and the concentration of solutions expelled during these initial stages was higher than that of the initial interstitial solution. Subsequently, with increasing compaction pressures, the concentration of squeezed-out solution decreased. For chloride, the decreasing factor was about ten and for total dissolved solids about five. Water of higher salinity appears to be present in the central portions of the capillaries, whereas the adsorbed and/or oriented water next to the walls of the capillaries is fresher. The concentrations of Ca2

+ and Mg2+ ions, however, increase when the

oriented fresher water is being expelled. The final porosity of the sample tested, at a compaction pressure of 276 MPa, was equal to 14.8 %.

8

Rosenbaum ( 197 6) describes an apparatus for consolidating fine-grained materials under controlled conditions of total stress, pore fluid pressure and temperature. The apparatus enabled the samples to be consolidated over a range of stress from 0 to 70 MPa at temperatures of 0- 200 QC. The specimen of soil is contained in a stainless steel cylindrical cup of 10.16 cm internal diameter, which is then forced over a rigid cylindrical piston by hydraulic pressure, to permit consolidation. Porous sintered stainless steel discs at each end of the specimen enable uniform drainage of expelled pore fluid from the sample. The pore fluid expelled during consolidation is collected by displacement of a syringe inserted into the drainage plate at the base of the specimen. The material used in the experiment was Fuller" s Earth consisting of calcium montmorillonite with trace quantities of quartz and accessory heavy minerals. This material was mixed in distilled water, allowed to settle in a cup, which was then moved to the pressing apparatus. The concentrations of all the analysed ions decreased rapidly with increasing stress during the initial loading and thereafter the rate of decrease declined markedly. The decrease in concentration was exponential but showed a break in curvature indicating two distinct phases of pore fluid expulsion Fig. 6 gives the dependence of the concentrations of different ions as the function of effective axial pressure.

Morgenstem and Balasubramonian (1980) and Brightman et al. (1985) both found that there was a decrease in sodium and potassium, and an increase in calcium and magnesium concentrations with increased porewater extraction. They suggested that the changes were due to the double-layer effects.

Entwisle and Reeder (1993) have reviewed the older studies and report on a new apparatus for pore fluid extraction from mudrocks, together with the obtained results. The squeezing apparatus uses a commecially available hydraulic pump which has a maximum output stress of 70 MPa. The test cell and the metal filter are manufactured from Type 316 stainless steel. The diameter of the sample cell is 7 5 mm and the length 100 mm. The stress is added incrementally, the rate of increase being dependent upon the physical properties of the specimen and the test undertaken. The test may take from 1 h to 2 weeks. About 170 squeezing tests were reported. In general it was found that there is an increase in calcium, magnesium and silicon, and a decrease in sodium and potassium concentrations with decreasing pore size (progressive porewater extracts). Changes in the concentration of ions with the reduction of porewater content, and pore size, of the test specimen were suggested to be due to the interaction of "free" water and double­layer water.

' Cl' E

' "' E

0 z

.......

"' E

u 0. ~

9

• •

•

A A

• A

Pot<JS$iumv Sodium A Colc~um •

100 1000

lOQ effective oxia I pressure. kolcm 2

Magnesium o Chlorid~ • Sulphate 0

B 0 0 0

0

0 0

0 0 0

0 0 CO coo

0

0 0

• 0

• • • 0 • • oe • • • 0 •

• • eo!

lO<J effective axial pressure,

Fig. 6. Variation of the ion concentrations in expelled pore fluid with effective axial pressure (Rosenbaum, 1976).

10

4 EXPERIMENTAL STUDY ON WATER BENTONITE INTERACTION

4.1 EXPERIMENTAL ARRANGEMENTS AND CONDITIONS

The evolution of porewater chemistry is studied in solution-bentonite interaction experiments with the experimental apparatus shown in Fig. 7. The size of the bentonite sample is 20 mm in diameter and 9.5 mm in height. The size of the cell for the solutions is varied in view of different bentonite-water ratios. Bentonite is in contact with the external solution through a steel sinter of pore size 5 micrometers. The experiments will be continued until equilibrium between the solution and bentonite is reached. At the end of the experiment the equilibrating solution, the porewaters to be squeezed out of the bentonite samples, and bentonites themselves will be analysed to give information for interpretation and modelling of the interaction.

The parameters varied in the interaction experiments are the bentonite density, bentonite­water ratio, ionic strength of the solution, and the composition of bentonite. The experimental conditions have been presented in Table 2 and the solutions to be used in the experiments in Table 3.

Solution vessel

Sinter

Bentonite sample

Perforated plate

Sample plate

Bottom plate

Fig. 7. Schematic diagram of apparatus for water-bentonite interaction experiment.

11

Table 2. Conditions of the interaction experiments.

MX-80 bentonite Solution Density b/w-ratio

(Mg/m3) (g/ml) Allard w. 0.6 0.5 0.015 Allard w. 1.2 1.5 0.015 Allard w. 1.5 1.5 0.5 0.1 0.015 Allard w. 1.8 0.015 Saline w. 0.6 0.5 0.015 Saline w. 1.2 1.5 0.015 Saline w. 1.5 1.5 0.5 0.1 0.015 Saline w. 1.8 0.015 Di. w. 1.2 2.2 100% RH Di. w. 1.5 3.4 100% RH Di. w. 1.8 5.4 100%RH

Na-monUnorillonite + CaC03 Solution Density CaC03 b/w-ratio

(Mg/m3) (%) (g/ml) Di. w. loose 3.0 0.1 Di. w. loose 1.0 0.1 Di. w. loose 0.4 0.1 Di. w. loose 0.2 0.1 Di. w. loose 0 0.1 Di. w. 1.5 3.0 1.5 Di. w. 1.5 1.0 1.5 0.5 0.1 0.015 Di. w. 1.5 0.4 1.5 0.5 0.1 0.015 Di. w. 1.5 0.2 1.5 Di. w. 1.5 1.0 3.4 100% RH

Na-monUnorillonite + CaS04 Solution Density CaS04 b/w-ratio

(Mg/m3) (%) (g/ml) Di. w .. loose 8.1 0.1 Di. w. loose 2.4 0.1 Di. w. loose 1.0 0.1 Di. w. loose 0.4 0.1 Di. w. 1.5 3.0 1.5 Di. w. 1.5 1.0 1.5 0.5 0.1 0.015 Di. w. 1.5 0.4 1.5 0.5 0.1 0.015 Di. w. 1.5 0.2 1.5 Di. w. 1.5 1.0 3.4 100% RH

Na-monUnorilloniitti +CaC03 + CaS04 Solution Density CaC03 CaS04 b/w-ratio

(Mg/m3) (%) (%) (g/ml) Di. w. loose 1.0 2.4 0.1 Di. w. loose 1.0 1.0 0.1 Di. w. loose 1.0 0.4 0.1 Di. w. 1.5 1.0 1.0 1.5 0.5 0.1 0.015 Di. w. 1.5 1.0 1.0 3.4 100%RH

0 b . HDPY 100 rgano entomte Solution Density b/w-ratio

(Mg/m3) (g/ml) Di. w. 1.2 2.1 100% RH Di.w. 1.5 3.4 100% RH Di. w. 1.8 5.4 100% RH

Di.w. = deionized water; 100% RH= in 100% relative humidity

12

The artificial bentonites were prepared by mixing CaC03 or CaS 04 or both of them with Na-montmorillonite purified from MX-80. The purification procedure was modified from that by Sposito et al.(1981) and included the following steps:

• 450 g of air dry MX -80 bentonite was mechanically mixed with 9 liters of deionized water. The fraction < 2 j..Lm was separated by centrifuging.

• The suspension was flocculated by adding 9 liters of 1 M NaCl solution at pH 3. Bentonite was separated by centrifuging. The washing was repeated 13 times until the pH value was about three.

• Washing was continued with 0.1 M NaCl solution until a pH value of 5.8 was reached. Seven washings with 9 liters of solution was needed.

• Separated bentonite was then washed twice with 9 liters of 0.01 M NaCl solution at pH 5.8.

• Bentonite separated by centrifuging was moved to dialysis bags and dialyzed six times with 25 liters of deionized water until the obtained electrical conductivity of water during 20 hours changed from only 1.5 j..LS/cm to 4.5 j..LS/cm. The pH value of the dialysis solution at the end of the process was 6.5.

• Bentonite suspension was vacume freeze-dried.

The total recovery of the purification process was about 7 5 % .

Table 3. The compositions of the experimental solutions.

Component

Na+ K+ Ca2+

Mg2+ Sr2+

Si02 HC03-a-Br-F r so/­B pH

Allard water Saline water Concentration (mmol/L) Concentration (mmol/L)

2.26 0.10 0.464 0.190

1.80 1.48

0.10

9.1

209 0.54 99.8 2.3 0.4 0.055 0.035 417 1.3 0.063 0.007 0.044 0.85 8.2

13

4.2 PRELIMINARY EXPERIMENTAL RESULTS

Solution samples are taken from the experiments where b/w ratios are 0.01 g/ml, and the volume of the solution is large enough that sampling does not disturb the experiment too much. The results of the experiments with MX -80 bentonite for four months are presented in Tables 4 and 5. Typical behaviour of the concentrations for Allard water and saline water are seen in Fig 8.

Table 4. Concentrations in the equilibrating solution as a function of time. MX-80 bentonite, Allard water, b!w ratio 0.01 g!ml.

Density Time pH Na K Ca Mg Cl so4 HC03

g/cm3 day meq/L meq/L meq/L meq/L meq/L meq/L meq/L

--- 0 9.10 2.26 0.10 0.46 0.38 1.48 0.20 1.80

0.6 29 9.35 2.84 -- 0.38 0.37 1.73 0.59 1.87

71 9.00 3.97 -- 0.34 0.37 1.72 1.50

141 8.59 4.47 -- 0.34 0.41 1.71 1.53 1.85

1.2 29 9.89 3.63 -- 0.35 0.35 1.77 1.46

71 9.30 4.12 -- 0.33 0.43 1.70 1.40

141 8.79 4.44 -- 0.24 0.29 1.68 1.43 1.93

1.5 22 9.52 3.17 -- 0.30 0.37 1.71 0.78

64 9.20 3.74 -- 0.30 0.41 1.70 1.15 1.84

134 8.38 4.01 -- 0.25 0.30 1.65 1.30 1.62

1.8 22 9.58 2.73 -- 0.30 0.47 1.70 0.42

64 9.01 3.08 -- 0.28 0.32 1.71 0.60 1.92

134 8.64 3.39 -- 0.22 0.28 1.67 0.81 1.64

Table 5. Concentrations in the equilibrating solution as a function of time. MX-80 bentonite, saline water, b!w ratio 0.01 g!ml.

Density Time pH Na K Ca Mg Cl so4 HC03 Br

g/cm3 day meq/L meq/L meq/L meq/L meq/L meq/L meq/L meq/L

--- 0 8.2 209 0.54 100 4.60 417 0.09 0.03 1.30

0.6 22 7.83 203 0.43 89 3.91 425 0.80 -- 1.21

64 7.57 201 0.48 89 4.45 423 1.07 0.05 1.15

134 7.36 202 0.49 71 4.86 432 118 0.05 1.14

1.2 15 7.77 195 0.38 90 3.81 419 0.35 0.12 1.12

57 7.37 196 0.39 89 3.96 423 0.75 0.09 1.15

127 7.23 194 0.45 87 4.25 422 1.14 -- 1.12

1.5 15 7.80 200 0.38 92 3.78 422 -- 0.09 1.18

57 7.52 199 0.39 83 3.91 425 0.47 0.06 1.13

127 7.20 198 0.44 84 4.40 432 0.70 -- 1.11

1.8 15 8.04 199 0.36 94 3.80 422 0.23 0.10 1.19

57 7.46 195 0.36 83 4.13 417 0.32 0.07 1.11

127 7.21 199 0.41 92 4.55 423 0.40 -- 1.11

10

9

-~ 8 Q)

§. 7 c .2

6 '5 0 In 5 .5 c 0 4 :; .... -c 3 Q) () c 0 2 (.)

0

450

400

-~ 350 Q)

§. 300 c

.2 '5 250 0 In

.5 c 200 .2 ~ 150 E Q) () c

100 0 (.)

50

0

14

Allard water, dry density of bentonite 1.5 Mg/m3

~-1---- r--- -- - - --

.... ~- ---o--

/

•Y ~

, ... -= -

~~ == ~= - - - - F==::t I X X

0 20 40 60 80 100 120 140 160 Time (day)

Saline water, dry density of bentonite 1.5 Mg/m3

'e---X * X

l,t- - .... - ... ~

·&-- - - t---~ r-- - - --1-- - ~ ~ -- - - t--- r-- - - --I-- - -' .A. a. -A - -0 20 40 60 80 100 120 140

Time (day)

--c-Na

-x-K -6-Ca

-o-Mg

---cL ---------- 804 ~HC03

---pH

--c-Na

-x-K

-6-Ca

-o-Mg

~HC03

-x-CL _._804

-·-10x pH

Fig. 8. The concentrations of the experimental solution as a function of time. MX-80 bentonite with fresh (upper) and saline(lower) water.

15

The major changes in the Allard water samples are the diffusion of sodium and sulphate from bentonite to solution. The pH value first increases but then turns downward. The changes are small due to low bentonite to solution ratio. The experiments have not yet reached the equiliblium.

In the saline solutions, the pH values decrease and sulphate diffuses from bentonite to the solution as with the fresh water. Other changes are so minor that they cannot be found due to the high initial concentrations and low b/w ratio.

The weights of all the samples are monitored in order to control the loss of solution by evaporation during the long experiment. The weight decreases of the samples where the added solution volume is smallest has been in four months about 0.3 g, meaning 10 % of the added solution volume. The lost volume will be replaced by deionized water. In the case where the solution volume is large, no replacement is needed.

The weights of the samples to be saturated in 100 % relative humidity are also monitored in order to see when the samples have been saturated and can be opened. Fig. 9 presents the results for MX-80 bentonite together with the theoretical saturated value. As can be seen, the samples of the densities 1.8 and 1.5 Mg/m3 begin to be saturated while the sample of the lowest density is still far from saturation.

MX- 80 in 100 %RH

14~--------~--------~--------~--------~~----~------------------~

~-- r----- ---12 -- --....,.

- - ---= = -11

- -f--- ~c-: - - - - - - -

10 IY -A - - - - - - - - - - - ..,

_,6------~ 8~----+-----r-----r-----r---~----~----_, ·; --<>--1.8 Mg/m3, meas. Q) - -o- 1.5 Mg/m3, meas. "2 6 +------+-----r-----r-----r---~1----~----_, 0 - -IJ. - 1.2 Mg/m3, meas. -c: Q) --1.8 Mg/m3, theor m

4+------+-----r-----r-----r---~1----~----_, - - 1.5 Mg/m3, theor.

- - - - 1.2 Mg/m3, theor.

2+------+-----r-----r-----r---~1----~----_,

0+---------+---------~--------~--------~----~~----~--------~

0 10 20 30 40 50 60 70 Time (day)

Fig. 9. Bentonite weigts as a function of time in 100% relative humidity saturation.

16

4.3 PRE-MODELLING OF THE EXPERIMENTS

The effects of different parameters on the solution bentonite-interaction were studied by modelling for the artificial bentonites prepared from Na-montmorillonite, CaC03 and CaS04. The calculations were based on the method by Wieland et al. (1994). The modelling was performed with the HYDRAQL code (Papelis et al., 1988). The following assumptions ofWieland et al. (1994) were also followed in our modelling:

• highly compacted montmorillonite of Wyoming type is kept in contact with deionized water in equilibrium with 2 ppm C02 concentration in the atmosphere

• the whole water volume is available for the chemical species irrespective of compaction and b/w ratio

• the bentonite data are as given in Table 6

• the surface chemical equilibria comprises the reactions in Table 7

• calcite equilibria are reached according to a closed system

• CaS04 and CaC03 impurities are in equilibrium with the solution, i.e., dissolve according to their solubility products

• no dissolution of the clay fraction

• cation exchange modelled according to the Vanselow convention

• the equilibrium constants in the last column of Table 7 remain constant irrespective of compaction

• the total number of surface hydroxyl (SiOH, AlOH) and siloxane (X) sites are proportional to the solid content.

Table 6. Data for montmorillonite.

Parameter

Cation-Exchange Capacity, CEC

Silanol edge sites, SiOH

Aluminol edge sites, AlOH

Capacitance, C

Exchangeable Na

Specific density

Value

0.764 meq/g

6.39 ~mol/m2

5.32 ~mol/m2

1.06 F/m2

100%

2.70 Mg/m3

Reference

Miiller-Vonmoos and Kahr, 1983

White and Zelazny, 1988

White and Zelazny, 1988

Goldberg, 1993

17

Table 7. Suiface chemical reactions and equilibrium constants.

Reaction log K Reference

Si OH= SiO- + H+ -6.53 Schindler and Sposito, 1991

AlOH+ H + = AlOH; 7.2 Schindler and Sposito, 1991

AlOH = AlO- + H+ -9.5 Schindler and Sposito, 1991

x-+Na+ =XNa 20.00 By definition

x-+H+ =XH 20.10 Fletcher and Sposito, 1989

2X- + Ca2+ = X2Ca 40.17 Fletcher and Sposito, 1989

The results of the case where only CaC03 is added to Na-montmorillonite are presented in Figs. 10 and 11, which show the concentrations of the dissolved and non-dissolved compounds in the solution as a function of added CaC03. The b/w ratio in the calculation was 0.1 g/ml. Fig. 10 shows that calcium carbonate dissolves up to the content of about 0.5 %. At that point the solubility limit is reached and no more carbonate dissolves. In Fig. 11 it can be seen that sodium in montmorillonite (NaX) is partly replaced by calcium and calcium montmorillonite (CaX2) is formed.

In the case where only CaS04 is added (Figs. 12 and 13) no solubility limit is reached within the used CaS04 concentrations up to 5 % with the b/w ratio of 0.1 g/ml. The dissolved calcium replaces sodium in the montmorillonite forming Ca-montmorillonite and the sodium and suplhate concentrations in the solution increase.

The case where both CaC03 and CaS04 are added to montmorillonite is a combination of the two separate cases. Figs. 14 and 15 show the results for the case where the CaC03 concentration in the montmorillonite is 1 %, the CaS04 content varies from 0 to 3 % and the b/w ratio is 0.1 g/ml. CaS04 dissolves totally within the observed concentration range from 0 - 3 %. The dissolution of CaS04 diminishes the dissolution of CaC03, which can be seen as an increasing concentration of non-dissolved CaC03 in Fig.15.

18

Figs. 16 and 17 present the effect of the b/w ratio on the solubility behaviour in bentonite. The calculations have been made for two b/w ratios, 0.1 g/ml and 3.4 g/ml. The former is soft clay gel while the latter is water-saturated compacted bentonite with a dry density of 1.5 Mg/m3. The CaC03 content in the bentonite was kept at 0.1 % and the CaS04 content varied from 0 to 5 %. The effect of the b/w ratio clearly indicates the difference between a saturated solution (b/w = 3.4 g/ml) and a non-saturated solution (b/w = 0.1 g/ml).

With the b/w ratio of 3.4 g/ml, CaC03 and CaS04 are saturated right from the lowest values of 0.1 % and 1 %respectively. The concentrations of all the dissolved species are rather high and independent of CaS04 content. The pH value is 7.18. Due to the low solubility of calcium, the Na+/Ca 2+ ratio increases and montmorillonite is mostly in Na­form.

With the b/w ratio of 0.1 g/ml, all the CaS04 dissolves and CaC03 becomes solubility limited when the CaS04 content increases over 2 - 3 %. The Na+/Ca2+ concentration ratio decreases and montmorillonite turns from sodium form to calcium form. The pH value starts to decrease from 8.3 when CaC03 starts to precipitate and reaches 7.85 when the CaS04 content is 5 %.

The pH as a function of CaC03 and CaS04 content in bentonite is presented in Fig. 20. The b/w ratio in the calculations was 0.1 g/ml. The dissolution of CaC03 increases pH over ten until the solubility limit is reached at about CaC03 content of 0.5 %. The adding of CaS04 diminishes the dissolution of CaC03 and pH decreases.

19

Artificial bentonite, no CaS04, b/w 0.1 g/ml

1.00E-02 r----::;;:~=~====F====~====F===~ 1r I/~ = lr-: = :-: : r-_ -_ -=-- -_ ir = :-: : -_ -_ -=-- -_ -1,

-- ./• I 1.00E-03 +il -~).(:;;----+----+-----+-----t-------i

c .2 ; 0 .~ 1.00E-04 +-L----+----+-----+------+------1

c 0

~ E g 1.00E-05 X'-------+-----+-----+-----+-------1 0 0 r -1k--- - - - olk- - - - - - ...... ~

1.00E-06 +--.tj~--+----+-----+-----+--------4 0 2 3 4 5

CaC03 in bentonite (%)

--+--Na+

-A-Ca++

- -X- · C03---c- ·HC03-

Fig. 10. Model calculation for interaction of artificial bentonite with deionized water. Concentration of dissolved species in solution as a function of CaC03 concentration in bentonite.

Artificial bentonite, no CaS04, b/w 0.1 g/ml

0.08

·~ 0.07 ---I r--- ---1 ------- -- -·~ --- 0.06 0

.§. ...

0.05 s la 3: .~ 0.04 1: 0

:;o la 0.03 ... "E Cl) 0 1: 0.02 0 0

0.01

/~

/ /

/ V

/ /

·Y ~---- ------11 ~------ ------... ~ 0

--+-CaC03

-•-NaX

- -•- -CaX2

0 2 3 4 5 CaC03 in bentonite (%)

Fig. 11. Model calculation for interaction of artificial bentonite with deionized water. Concentration of non-dissolved components in solution as a function of CaC03 concentration in bentonite.

20

Artificial bentonite, no CaC03, b/w 0.1 g/ml

G.OOE-02

- 5.00E-02 ...._ 0 .§, s::: 4.00E-02 .2 5 0 m

3.00E-02 .5 s::: 0 :;: f!

2.00E-02 E 8 s:::

~ ~

/ --+-Na+ •-Ca++

/ _ __. -o- -804--

/ ---- -- -- - - -X- -CaS04 - ---/ .......... -+- -NaS04-

-.......... -0 (.)

1.00E-02 - --.,.... -y-""

O.OOE+OO

- --::--1~ -- - - .:;j ~:=-~=

-= ;:____;:: - - ----0 2 3 4 5

CaS04 in bentonite (%)

Fig. 12. Model calculation for interaction of artificial bentonite with deionized water. Concentration of dissolved species in solution as a function of CaS04 concentration in bentonite.

Artificial bentonite, no CaC03 , b/w 0.1 g/ml

?.OOE-02

G.OOE-02

...._ 0 .§. 5.00E-02

s::: .2 '5 4.00E-02 0 et)

.E c 3.00E-02 0

~ 'E Q) 2.00E-02 CJ c

~

~ ~

~ ~

--~ ------0 ---0 1.00E-02 ------

[""""'""

O.OOE+OO

0 2 3 4 5 CaS04 in bentonite (%)

Fig. 13. Model calculation for interaction of artificial bentonite with deionized water. Concentration of non-dissolved components in solution as a function of CaS04 concentration in bentonite.

21

Artificial bentonite, 1 °k CaC03, b/w 0.1 g/ml

1.00E+00 ........-----r------,..----r-------r------r-------,

1.00E-01 +----+----+---+-----+-----+-----i ---0 .s ~ ------c 1.00E-02 -=--=-=---t-----+c.--------=---+--------"=-t----t-----1

-(

.2 '5 0 w .5 c 0

lt-- - - - - - - n-1.00E-03 +------=-...,--t-----+---------+---------+------=--=t----:> .... -=111-.;

~- -­--~ --~ 'E 1.00E-04 +----+----+---_....=+--'....-""-----=-----+----t-----t

-. G) () c 0 ()

1.00E-05 +------;?""'-+----+---+-----+-----+-----i

1.00E-06 +----+-----+---+-----+----+-----! 0 0.5 1.5 2 2.5 3

CaS04 in bentonite (%)

~Na+

-£-Ca++

- -X- -C03--

-o- -S04--

-o- -HC03-

Fig. 14. Model calculation for interaction of artificial bentonite with deionized water. Concentration of dissolved species in solution as a function of CaS04 concentration in bentonite.

Artificial bentonite, 1 % CaC03, b/w 0.1 g/ml

?.OOE-02 . ._____

--- ---6.00E-02 --- ..... '--1 l-.....

........... --- --........._

0 5.00E-02 .s ...........

--...... --........._

c ...........

.2 'S 4.00E-02 0 w .5

--........._

........... ..........

--........._

---. -+-CaC03 c 3.00E-02 0

-11-NaX

~ - -•- -CaX2 c G)

2.00E-02 () c 0 ()

-- - -

- -- -- ------1.00E-02 - -- - 1

-=--- -----=-,__ -h

O.OOE+OO

0 0.5 1.5 2 2.5 3 CaS04 in bentonite (%)

Fig. 15. Model calculation for interaction of artificial bentonite with deionized water. Concentration of non-dissolved components in solution as a function of CaS04 concentration in bentonite.

22

Artificial bentonite, CaC03 0.1 %, b/w 0.1 g/ml

--- 1.00E-01 0 .s r::: -~ "5 -Na+ 0

_._Ca++ tl)

1.00E-02 ·= ~S04--r::: 0 ~CaS04

~ ~NaS04-E CD ~HC03-0 r::: 1.00E-03 0 u

0 2 3 4 5 CaS04 in bentonite (%)

Fig. 16. Model calculation for interaction of artificial bentonite with deionized water. Concentration of dissolved species in solution as a function of CaS04 concentration in bentonite.

Artificial bentonite, CaC03 0.1 %, b/w 0.1 g/ml

100

90

80

70

-~ 60 ~

~ ~ ~ ~ -...., ~ ~.

I: .g 50 -NaX a.. 0 -6-CaX2 a. 0 40 a.. c.

30

20

10

0

~~

~ ~ / ~ ~ /

~ ~ V

-o- CaS04 (non-diss.)

-<>-- CaC03 (non-diss.)

0 2 3 4 5 CaS04 in bentonite (%)

Fig. 17. Model calculation for interaction of artificial bentonite with deionized water. Proportion of montmorillonite in Na and Ca-form and proportion of non-dissolved CaS04 and CaC03 as a function of initial CaS04 in bentonite.

23

Artificial bentonite, CaC03 0.1 %, b/w 3.4 g/ml

1.00E+00 -r-----.,....----r-----,......------,r------,

-- 1.00E-01 0 .§. c .2 "5 -Na+ 0

-.-ea++ rtl 1.00E-02 .5 ~S04--c

0 -a-caS04 ~ ~NaS04-'E Cl) --+- HC03-0 c 1.00E-03 0 0

1.00E-04 +-----+-----+-----+-----f-------1 0 2 3 4 5

CaS04 in bentonite (%)

Fig. 18. M ode/ calculation for interaction of artificial bentonite with deionized water. Concentration of dissolved species in solution as a function of CaS04 concentration in bentonite.

100

90

80

70

-~ 0 - 60

1: 0

50 •t: 0 c. 0 40 0::

30

20

10

0

0

Artificial bentonite, CaC03 0.1 %, b/w 3.4 g/ml

) ~

/ V

/ /

<'f

2 3 Ca504 in bentonite (%)

----

4

~)

5

-NaX

.....-caX2

~ CaS04 (non-diss.)

~CaC03 (non-diss.)

Fig. 19. Model calculation for interaction of artificial bentonite with deionized water. Proportion of montmorillonite in Na and Ca-form and proportion of non-dissolved CaS04 and CaC03 as a function of initial CaS04 in bentonite.

24

Artificial bentonite, b/w 0.1 g/ml

12~------,-------~------~------~-------,

10 nO

~-~~ ---------- -<>-- ---- -----

1-I::Jtl:sl:J: - - - -4 - - - - - - - - - - - - - - - - - - - 1)1' - - - - - - - - - - - - - - - - - - -4

8 r -~- . . . . . . . ---- - - -r- • • - i

~~ a 6+-------;--------r-------r-------+------~ --o-- CaS04 = 0 %

- -o - CaS04 = 1 %

- - -tr - · CaS04 = 3 % 4+-------;--------r-------r-------+------~ - ·>E- • CaS04 = 5 %

2+-------,_-------r-------r-------+------~

0+-------,_-------r-------r-------+------~

0 2 3 4 5

CaC03 in bentonite (%)

Fig. 20. Model calculation for interaction of artificial bentonite with deionized water. The obtained pH as afunction ofCaS04 and CaC03 in bentonite.

25

4.4 SQUEEZING APPARATUS FOR POREW ATER EXTRACTION

The requirements for the squeezing apparatus are based on the planned water-bentonite interaction experiments. The equipment should cover the bentonite dry densities from 0.6 to 1.8 g/cm3.The size of the equipment is limited by the glove box where the experiments will be performed. Several samples should be pressed simultaneously. Constant pressure should be maintained for 1 - 2 weeks. The thickness of the bentonite sample is compromised by the need to obtain the equilibrium in the interaction experiment within reasonable time and by the need to obtain enough porewater for the analyses.

On the basis of the literature study, pore water extraction by squeezing seemed the most promising method. The apparatus developed for squeezing porewater from bentonite samples is seen in Fig. 21. It consists of a pressing apparatus which is used to create the necessary long-term compression, and of the compaction cell where the porewater is separated from the bentonite and collected in a syringe.

The constant long-term force is maintained with a strong spring by adjusting with nuts the length of the lower part of the pressing apparatus. The upper part of the pressing apparatus is intended to press the lower part with a hydraulic cylinder to the desired length prior to fixing the nuts to keep the selected length. After adjusting the length, the hydraulic cylinder can be moved to the next pressing apparatus.

Hydraulic cylinder

Fig. 21 Pressing apparatus (left) and compaction cell (right).

Piston

Cylinder

Bentonite

Sinter

Syringe

26

The compaction cell consists of the base part, syringe, sample cylinder, sinter and piston. The sinter is on the bottom of the cylinder and the bentonite is pressed with the piston against the sinter. The pore water flows through the sinter and the hole in the base part to a disposable syringe. The inner diameter of the cylinder is 20 mm. The planned height of the bentonite sample is about 10 mm, which allows for porewater samples of about 0.5 ml. The characteristic pressure vs. compression curve for the spring, 68 mm in length and 70 mm in diameter, is presented in Fig. 22. Higher or lower pressure scales are possible by changing the spring. Squeezing was successfully tested with bentonite samples at dry densities from 1.2 g/cm3 to 1.8 g/cm3

• The preliminary results for those samples show clear exclusion of almost all ions (Lehikoinen et al., 1996). Exclusion is strongest in the case of the divalent sulphate anion, as predicted by the double-layer theory. The pH values of the equilibrating solution and porewater seem in most cases to be rather equal. Only in saline solution under anaerobic conditions was there a clear discrepancy between the pH of porewater and outside solution.

160

140

120

Ci' 100 a.

~ Cl) 80 ... :::J 0 0 Cl) 60 ... a.

40

20

0

/ /

/ I-'"

/ /

/ /

/ /

/ 0 2 4 6 8 10 12

Compression length (mm)

Fig. 22. The characteristic pressure vs. compression curve for the pressing apparatus with a spring of 68 mm in length and 70 mm in diameter.

27

5 SUMMARY

This study is part of a project where a simulation code for diffusion and porewater chemistry in bentonite will be developed. The model will be based on the micro- and macro-scale material properties of bentonite and surrounding conditions. The work consists of model development work and supporting experimental studies on porewater chemistry.

The study on the porewater chemistry was started by a short literature overview on the properties of bentonite, porewater-sampling methods and obtained results. The total amount of water in saturated bentonite is a function of the dry density of the clay. There are two major types of water in bentonite: that inside the granules and that between them. The porewater chemistry is mostly determined by the easily soluble accessory minerals, like carbonates and sulphates, which can produce dissolved ions in the pore water, by the species from the solution in contact with the solids and by the exchangeable cations of bentonite being readily replaceable by the cations in the porewater.

There are three major methods used for extracting interstitial fluids from sediments, involving centrifuging, squeezing out and washing out. The most important factors affecting the sampling of porewater from bentonite are the swelling pressure and the hydraulic conductivity, both of which depend on the density of the clay and salinity of the solution used to saturate the sample. The swelling pressure has to be exceeded if porewater is to be squeezed out of bentonite. The hydraulic conductivity affects the rate by which the porewater can be squeezed out. The majority of investigators agree that the concentration of the dissolved solids in the expelled solution decreases with increasing compaction. The concentrations of sodium and potassium usually decrease, while calcium and magnesium increase with increasing density.

On the basis of the literature study, porewater extraction by squeezing seemed the most promising method for further development. The apparatus developed in this study consists of a pressing apparatus, which is used to create the necessary long-term compression, and of the compaction cell where porewater is separated from the bentonite and collected in a syringe. The constant long-term force is maintained by a strong spring. The maximum pressure obtained with the pressing apparatus is about 150 MPa with the selected cylinder diameter.

The evolution of porewater chemistry is studied in solution-bentonite interaction experiments. The parameters varied are the bentonite density, bentonite-water ratio, ionic strength of the solution, and the composition of bentonite. The bentonite types used in the experiments are commercial MX-80 and artificial bentonite prepared from purified MX -80 in sodium form, CaC03 and CaS04. Bentonite is in contact with the external solution through a steel sinter of pore size five micrometers. The experiments will be continued until equilibrium between the solution and bentonite is reached. At the end of the experiment the equilibrating solution, the porewaters to be squeezed out of the

28

bentonite samples, and bentonites themselves will be analysed to give information for interpretation and modelling of the interaction.

Solution samples are taken from the experiments where the volume of the solution is large enough such that sampling does not disturb the experiment too much. The samples of the experiments with MX -80 bentonite for four months have been analysed. The major changes in the Allard water samples are diffusion of sodium and sulphate from bentonite to solution. The pH value first increases but then turns downward. The changes are small due to the low bentonite to solution ratio. The experiment has not yet reached the equiliblium.

In the saline solutions the pH values decrease and sulphate diffuses from bentonite to the solution as with the fresh water. Other changes are so minor that they cannot be found due to the high initial concentrations and low b/w ratio.

The effects of different parameters on the interaction of deionized water with artificial bentonites were studied by modelling. The calculations were based on the method by Wieland et al. (1994) and modelling was performed with the HYDRAQL code (Papelis et al., 1988). The results show a complex dependence of dissolution, ion-exchange and pH on the content of the soluble components in the bentonite and bentonite/water ratio.

29

6 REFERENCES

Brightman, M. A., Bath, A., Cave, M. R., and Darling, W. G. 1985. Pore fluids from the argillaceous rocks of the Harwell region. Keyworth, Nottinham: British Geological Survey, Report FLPU 85-6.

Chilingar, G. V., and Rieke, H. H. Ill, 1968. Data on consolidation of fme-grained sediments. J. Sedim. Petrol., vol. 38, pp. 811- 816.

Chilingarian, G. V., Sawabini, C. T., and Rieke, H. H. 1973. Effect of compaction on chemistry of solutions expelled from montmorillonite clay saturated in sea water. Sedimentology, vol. 20, pp. 391 - 398.

Chilingar, G. V., and Rieke, H. 1975. Chemistry of interstitial solutions in undercompacted (overpressured) versus well-compacted chales. In: Proceedings of the International Clay Conference. Wilmette, Illinois, USA: Applied Publishing. Pp. 673 -678.

Entwisle, D. C., and Reeder, S. 1993. New apparatus for pore fluid extraction from mudrocks for geochemical analyses. In: Manning, D. A. C., Hall, P. L. and Hughes, C. R. ( eds. ). Geochemistry of Clay-Pore Auid Interactions. London: Chapman & Hall. Pp. 365- 388. ISBN 0 412 48980 5

Aetcher, P., and Sposito, G. 1989. The chemical modelling of clay/electrolyte interactions for montmorillonite. Clay Minerals, vol. 24, 375-391.

Goldberg, S. 1993. Constant capacitance model - Chemical surface complexation model for describing adsorption of toxic trace elements on soil minerals. In: Tedder, D. W., and Pohland, F. G. (eds.) Emerging Technologies in Hazardous Waste Management Ill, Am. Chem. Soc. Symp. Ser. 518. Pp. 278-307.

Kryukov, P.A. 1961. Toward procedures of squeezing out of solutions from sedimentary rocks. Gidrokim. Inst. Novocherkask., Gidrokhim Mater., vol33, pp. 191-197. (Adopted from Rieke and Chilingarian, 1973).

Lehikoinen, J.,Muurinen, A.,. Melamed, A. and Pitkanen P. (1996). Determination of porewater chemistry in compacted bentonite. MRS Meeting on Scientific Basis for Nuclear Waste Management XX, Boston, December 2- 6, 1996.

Morgenstern, N. R., and Balasubramonian, B. I. 1980. Effects of pore fluid on the swelling of clay shale. In: Proceedings of the Fourth International Conference on Expansive Soils, Denver, vol.1, pp. 190- 205.

30

Muurinen, A., Aalto, H., Carlsson, T., Lehikoinen, J., Melamed, A., Olin, M., and Salonen, P. 1995. Interaction of fresh and saline waters with compacted bentonite. Espoo: Technical Research Centre of Finland. 29 p. + app. 14 p. (VTT Research Notes 1714). ISBN 951-38-4869-8

Miiller-Vonmoos, M., and Kahr, G. 1983. Mineralogische Untersuchungen von Wyoming Bentonit MX-80 und Montigel. NTB 83-12, NAGRA, Baden, Switzerland. 15 p. + app. 13 p.

Papelis, C., Hayes, K.F. and Leclcie, J.O., Dept. Civil Engrg., Stanford Univ., CA, Technical Report No. 306 (1988), p. 130.

Pedersen, K., and Karlsson, F. 1995. Investigations of subterranean microorganisms. Their importance for performance assessment of radioactive waste disposal. Stockholm: Swedish Nuclear Fuel and Waste Management Co. 222 p. (Report SKB-95-10). ISSN 0284-3757

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LIST OF REPORTS 1 (1)

LIST OF POSIV A REPORTS PUBLISHED IN 1997

POSIVA-97-01

POSIV A -97-02

Model for diffusion and porewater chemistry in compacted bentonite Theoretical basis and the solution methodology for the transport model J armo Lehikoinen VTT Chemical Technology January 1997 ISBN 951-652-026-X

Model for diffusion and porewater chemistry in compacted bentonite Experimental arrangements and preliminary results of the porewater chemistry studies Arto Muurinen, Jarmo Lehikoinen VTT Chemical Technology January 1997 ISBN 951-652-027-8