Sorption/desorption of Cs on clay and soil fractions from various regions of Turkey

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

SORPTION/DL~ORPTION OF Cm ON ClAY AND SOIL FRACTIO~ FROM VARIOUS

B ~ I O N S OF TUR]~f*

H N ERTEN 1 , S AKSOYOGLU 2 AND H GOKTURK 2

I Faculty of Arts and Sciences, Bllkent University, Ankara, Turkey 2 Department of Chemistry, Middle East Technical University, Ankara,

Turkey

ABSTRACT

The sorptlon desorptlon behavlour of Cs ion in the concentra-

tion region of 10 -8 to 10 -4 meqml -I have been studied

using clay and soil fractions from various regions of Turkey. The

sorption curves for all the material studied show similar behaviour

indicating at least two different sorption processes. One with high

and the other with low distribution coefficients. The results of

desorption studies indicate that Cs cation is to a large extent attached to the solid material in a reversible manner. The adsorp- tion isotherms were found to be nonlinear in all cases. The increase of R D values with decreasing particle size in most cases, suggests that sorption and or exchange is primarily a surface phenomenon in the clay and soil fractions studied.

* Supported in part by the International Atomic Energy Agency-Vienna

and by the Turkish Atomic Energy Authorlty-Ankarra

1 . INTRODUCTION

Sorption studies of various cations on soil material are quite important w i t h respect to the effects of radioactive waste on the g r o u n d w a t e r environment. In recent years, radioactive materials have been produced and used in ever i n c r e a s i n g quantities. Some of these materials have been released to the environment mainly as a result of nuclear weapons testing and accidents occuring at nuclear power or r e p r o c e s s l n g plants. M o s t recently the accident at the nuclear power plant at Chernobyl is an example of the environmental contamination w i t h radioactive nuclides.

Water present in the environment contacts many substances

during its movement on and beneath the surface of the earth. The

most common and w i d e s p r e a d of the various oil components are the clay

minerals. The interaction between clay minerals and water plays an

important role in controlling the concentralons of radioactive

substances in w a t e r and in p r e v e n t i n g their dispersal into the environment. The main factors w h i c h affect the transport of radio- nuclides by the g r o u n d w a t e r include chemical composition of ground- water, the g r o u n d w a t e r flow rate, the exchange capacity of soil, and temperature.

The effects of these factors on the sorptlon and transport properties of radlonuclides has been the subject of many investig-

ations. The general objective of these studies was to establish a

geologic environments.

The aim of this work was to study the sorption characteristics of some clays and soll fractions from various regions of Turkey.

2 . EXPERIMENTAL

The first cation chosen for study was caesium. The behaviour

of caesium in the soil is of considerable interest since the radio-

nuclide 137Cs due to its long half-life (t½=30.17 y), is a

principle radiocontaminant. Furthermore the chemistry of caesium is

simple and 137 Cs is commercially available as a suitable

tracer. TABLE I C a e s l u m l o n c o n c e n t r a t i o n s u s e d I n a d s o r p t l o n / d e s o r p t l o n s t u d l e s . [Ce] ° ( m e q / m l ) 1.19x10 -8 1.02x10 -7 1.02xi0-6 1.01xl0 -5 1.01xlO -4

The sorption experiments were carried out at initial caesium

obtained from the Radiochemical Centre, Amersham. According to the supplier's information the total activity was 5 mci, the volume I ml and the Cs ion concentration 210 ~ g Cs/ml. 0.3 ml of this solution was evaporated over a water bath to dryness. The residue was dis- solved in I0 ml bidistilled water. To four 2.5 ml portions of this solution nothing was added to the first, 1 ml of solution containing 1.693 mg CsCl/ml was added to the second, 1 ml of solution containing 16.93 mg CsCl/ml was added to the third and I ml of solution con- taining 169.3 mg CsCl/ml was added to the fourth portion. All the samples were then filled to the I00 ml mark with bidistilled water.

The resulting stock solutions were kept in plastic bottles. The

count rates of the stock solutions diluted I:I00 were about 80 cps.

The sorption, desorption experiments were carried out using the batch method. Weighed amounts of clay samples were kept in contact

with known volumes of solution for certain times. After separation

of the two phases the change in the concentration of the adsorbate in

the solution was determined radiochemically. The phases were

separated by centrifugation.

The clay minerals were obtained from the four regions indicated on the map given in Fig I, where the distribution of some mineral occurences in Turkey is shown. The clay samples were the following:

2- 3-

Resadiye bentonite (Na-bentonite); Region I. Giresun white bentonite (Ca-bentonite); Region 2. M i h a l i ~ G i k AhirSz~ (Kaolinite); Region 3.

4- 5-

Sindirgi (Kaolinite-alunlte); Region 4. K8re clay; Region 2.

.( ~ . PbZn lib . U I.u "~. e A n k = r = ph Zn ~ " ; . 2n Zn ~. ~ ~ - ~ M ~ cu . n , . F --- co AS " " r " ' / ) # ~ ; > ~ > ' F . A , u ~ ~ " . Pb F , _ ,_ "'~ F, e ( / V " A K D E N I ' i F i g 1 l ) I s t r l b u t l o n o f S O l e m l n e r a l o c c u r e n e ~ s I n T u r k e y

The first two clay minerals are of montmorillonite type, the rest are of kaolinite type with the general formulas;

AI4(Si4OIo)2(OH) 4 and A ~ S ~ 0 1 0 ( O H ) 8 respectively. Kaolinite is a two layer clay type whose layers consist of one tetrahedral and one octahedral sheet, whereas montmorillonite is a three-layer type clay

which has an octahedral sheet between two tetrahedral sheets. In

general, kaolinite clays have layers bound more tightly together than other clay types a n d they permit less substitution of other ions for

AI and Si. These structural differences are reflected in low Ion-

exchange capacity for the kaolinite clays and low plasticity because of a low capacity to absorb water. In the montmorillonite type clays

the layers are bound less tightly, this leads to properties such as swelling in polar solvents and very large ion-exchange capacities.

In the experiments Resadiye bentonite and M i h a l i ~ I k kaolinite were used as representatives of montmorillonite and kaolinite type clays.

The soll fractions from SaraykSy about 30 kms northwest of Ankara where the Turkish Atomic Energy Authority laboratories will be situated; were separated into various size fractions by wet seivlng

followed by sedimentation. The particle size distribution of the

clay samples and soll fractions of < 2 0 ~m size were determined by

using an Andreasen Pipette. The samples were first dried at II0°C

for 24 hours. I0 grams of the dried samples were then shaken for 12 hours with bldistilled water and then introduced into the pipette. At various preset times I0 ml solutions were taken, dried and the amount of solid material determined. The size of the particles were calculated according to their rate of sedimentation using Stokes' Law.

Sorption experiments were carried out using groundwater from

the Middle East Technical University (METU) water system. In the

case of Sarayk~y soil fractions, groundwater from Sarayk~y was used. The water samples were filtered through 0.22 ~ m Seitz bacteriological filter before use.

The following experimental procedure was used in the adsorption

studies; tubes were cleaned, dried at 60°C overnight, cooled and

weighed. About I00 mg of soil or clay was added and weighed. I0 ml of groundwater was added into the tubes and they were shaken for four days. They were then centrifuged for 30 minutes at 6000 rpm and the

liquid phases were discarded. This pretreatment step was aimed at

equilibrating the clay samples with the groundwater prior to adsorp-

tion experiments. I0 ml of Cs solution was then added from the

desired stock solutions after I:I00 dilution with groundwater. They

were then shaken for the desired sorption time. The samples were

then centrifuged again and the liquid phase was decanted into a clean tube. 5 ml of this liquid was counted using a 35 cm 3 calibrated Ge(Li) detector connected to a multichannel analyser. The adsorption

distribution ratio, RD,ad , was calculated from the measured

activities before and after shaking using the following relations;

The distribution ratio is defined by

[CS]s~ad R = D,ad [Cs] ad . . . ( I ) where [ C S ] s , a d ffi c o n c e n t r a t i o n o f Cs i n t h e s o l i d p h a s e a f t e r , s o r p t i o n ( m e q / g ) [ C S ] a d = c o n c e n t r a t i o n o f Cs i n t h e s o l u t i o n a f t e r sorptlon (meq/ml)

Since at the beginning of sorption V ml of solution with an

initial caesium concentration [Cs] ° was added and at the end of

sorption step ( V + A W p t ) ml of solution with concentration [CS]ad was present, here A Wpt is the amount of liquid remaining in the tube after pretreatment and decantation, the concentration of Cs in the solid after sorption is given by

V • [Cs] ° - [CS]ad . ( V + A W p t ) [ c s ] = s,ad w 8 ... (2) and A l ~ a d [Cs] ° [ C S ] a d = A o ... ( 3 )

Substituting equations (2) and (3) into equation (I) leads to; V • A ° - Al,ad ( V + A w p t )

R =

D,ad A . w

l,ad s

. . . ( 4 )

w h e r e A ° ffi Initial count rate of 5 ml of solution added for sorption

Al,ad ffi count rate of 5 ml of solution after sorption

W s = weight of solid material (g).

For desorption studies the following experimental procedure has been used;

Following the adsorption step, I0 ml of groundwater was added to the sample tube, shaken for the desired time, centrifuged and decanted. 5 ml of the liquid phase was counted.

The d i s t r i b u t i o n ratio of desorption, RD,de, was calculated from the following relation;

where; R D,de V . A ° - A l t a d ( V + A w p t - A W a d ) - A l t d e ( V + A W a d ) ..(5) A . w l,de s

AWad ffi the amount of liquid remaining in the tube after adsorption and decantation

Al,de = count rate of 5 ml of solution after desorption The rest of the terms in equation (5) have been defined earlier.

3 . RESULTS AND DISCUSSION

The size and distribution of clay and soll fractions are shown in Fig 2. Per cent Finer Than (FT) distribution is plotted against diameter of particles. It is seen that most of the particles were finer than I0 ~ m in diameter.

Chemical analysis of water samples used in the experiments as well as the solutions after pretreatment with various clay and soil fractions are given in Tables II and III for cations and anions respectively. The low concentrations of anions and cations in M E T U groundwater is striking. M E T U gets its water supply from a nearby lake. The water is treated in a treatment plant before distribution.

The water used in the sorption studies was this tap water. Water

treatment is probably responsible for low concentrations of anions and cations. The water compositions after pretreatment steps and the SaraykSy groundwater compositions are all similar and reasonable.

9oi

7o LoJ/ /

6o~ z

/ ,f

I l I I l I

10

20

30

40

50

60

d (~m)

---

F i g 2 S i z e d i s t r i b u t i o n o f c l a y s a m p l e s . Z F i n e r Than (FT) d i s t r i b u t i o n a g a i n s t d l a m e t e r o f p a r t i c l e s . o Sarayk6y s o l l @ R e s a d l y e c l a y + H i h a l l ~ ¢ I k c l a y

The pH of M E T U groundwater was measured as 7.4 and the conduct- ance as 0.8 mho. After pretreatment the pH became 8.0.

Neutron activation analysis was used in the d e t e r m i n a t i o n of thirteen element concentrations in clays from the four regions of

Turkey shown in Fig i. The clay samples were irradiated in the

TABLE I I C h e m i c a l A - ~ l y s i s o f W a t e r Samples Used I n A d s o r p t i o a / D e s o r p Z i o n S t u d i e s . C a t i o n C o n c e n t r a t i o n s W a t e r S a m p l e s M E T U G r o u n d - w a t e r SaraykSy ground- w a t e r N o p r e t r e a t m e n t P r e t r e a t m e n t w i t h M i h a l 1 ¢ ¢ i k c l a y (Z I0 pm) P r e t r e a t m e n t w i t h M i h a l 1 ¢ ¢ i k c l a y (~ I0 ~m) P r e t r e a t m e n t w i t h R e s a d i y e Clay (Z I0 ~m) P r e t r e a t m e n t w i t h R e s a d i y e c l a y (~ I0 ~m) N o p r e t r e a t m e n t P r e t r e a t m e n t w i t h s o i l f r a c t i o n s f r o m 1-4 m e t e r s C a t i o n C o n c e n t r a t i o n s (mg/l) Na* K ÷ C a 2+ M g l+ S r I+ Li + I . I 0 1 I I 0.80 0.34 < 0 . 0 0 2 0.003 85.00 5 01 35.50 14.90 0.220 0.016 197.00 4 50 8.20 14.90 0.031 0.020 314.00 I0 I0 13.30 71.50 0.076 0.033 336.00 13 40 13.90 81.40 0.115 0.037 102.00 103.00 4 61 23.30 39.00 0.991 0.051 7.20 46.00 27.00 0.830 0.052

TABI~ III

Chem/cal Analysls o f Water Samples Osed I n

A d s o r p t i o n / l ) e s o r p t l o n S t u d i e s . Anion C o n c e n t r a t i o n s M E T U G r o u n d - w a t e r S a r a y k 6 y g r o u n d - w a t e r W a t e r S a m p l e s N o p r e t r e a t m e n t P r e t r e a t m e n t w i t h M i h a l l ¢ ¢ I k c l a y (Z 10 ~m) P r e t r e a t m e n t w i t h M i h a l l ¢ ¢ z k c l a y (~ i0 Hm) P r e t r e a t m e n t w i t h R e s a d i y e C l a y (Z I0 pm) P r e t r e a t m e n t w i t h R e s a d i y e c l a y (~ I0 ~m) N o p r e t r e a t m e n t P r e t r e a t m e n t w i t h s o i l f r a c t i o n s f r o m 1-4 m e t e r s A n i o n C o n c e n t r a t i o n s (mE/l) - 2 - . c o ; Cl S O 4 0 60 10 40 19 20 8 40 7 40 43 O0 < 0 . 3 0 0 . 6 0 3 5 . 0 0 5 2 . 8 0 1 . 3 0 5 6 . 9 0 0 . 5 0 90.00 9 4 . 8 0 2 2 7 . 0 0 3 9 3 . 0 0 4 1 2 . 0 0 4 3 4 . 0 0 7 3 . 7 0 3 5 . 0 0 3 1 7 . 0 0 4 1 . 7 0 7 6 . 1 0 3 4 . 5 0 1 3 7 . 0 0

1.5x10 -13 n.cm-2.s -I . The irradiation time was 2 hours and counting was started after a cooling period of about 7 days. The activities were measured on a 35 cm 3 Ge(Li) detector connected to a 4096 channel analyzer.

The results of elemental abundances in clays are given in Table

IV. Besides neutron activation analysis, x-ray diffraction and

fourier transform infrared (FTIR) spectrometry were used to study the

structure of the various clay samples. In Fig 3 FTIR-spectra of

standard kaolinite type clay and M i h a l i ~ i k clay are shown. It is seen that Mihali~qik clay is of kaolinite type. Fig 4 shows FTIR- spectra of standard montmorillonlte type clay and Resadiye clay. It is seen that Resadiye clay is clearly of montmorillonite type. The x-ray diffraction spectra of glycolated and air dried samples of Mihaliqqik and Resadiye clays are shown in Figs 5 and 6 respectively.

No shift in peak positions are observed between glycolated and

unglycolated samples of Mihaliqqik clay. Whereas Resadiye clay

samples showed shifts in peak positions. These observations confirm

the identification of these samples as kaolinite and montmorillonite type clays respectively.

The evolution of R D values with time for the experiments with

[Cs]°=l.19xl0 -8 meq/ml and 1.01xl0 -5 meq/ml and particle size

fractions <I0 ~m and >I0 ~ m is shown in Fig 7 for MihaliGGik clay s a m p l e s . I t i s o b s e r v e d t h a t i n a b o u t f o u r d a y s o f s h a k i n g t i m e s a t u r a t i o n i s r e a c h e d . F o r b o t h Cs i o n c o n c e n t r a t i o n s , t h e R D v a l u e s f o r t h e s m a l l e r s i z e f r a c t i o n s a r e much h i g h e r t h a n t h o s e o f

the larger sizes. These observations suggest that the adsorption is primarily a surface phenomenon.

ill

'

'

'

t ~

/600

3000

1800

1000

Wavenumber =_ 6OO- FiB 3 F o u r i e r T r a n s f o r a I n f r a - R e d S p e c t r a o f a ) S t a n d a r d M o n t m o r i l l o n l Z e t y p e c l a y b) ~ s a d l y e c l a y

TABLg IV Abundances ( i n ppm) o f some g l e ~ n t s i n Turlctsh Clay8 R e g i o n 1 R e g i o n 2 R e g i o n 3 R e g i o n 4 R e s a d i y e K O r e M i h a l l ~ l k S l n d l r g l E l e m e n t c l a y c l a y c l a y c l a y Na 1 6 3 2 0 2 4 7 0 3 8 - 2 6 7 0 6 Ba 4 2 1 6 4 5 2 5 - 3 3 4 9 Ce 174 125 4 8 . 1 0 1 3 6 . 0 0 Co 15 3 5 . 7 1 0 . 9 0 2 . 8 0 C r 35 1 4 . 5 1 3 . 0 0 2 0 . 3 0 C s 0 . 6 0 . 6 2 . 1 0 0 . 7 0 H f 3 . 4 2 . 6 4 . 7 0 2 . 9 0 L a 2 0 . 5 4 2 . 4 6 . 9 0 4 3 . 6 0 T a 0 . 5 1 . 8 0 . 2 0 0 . 8 0 S b 2 . 3 - 3 6 . 0 0 1 4 . 0 0 S c 5 . 9 1 0 . 4 1 2 . 7 0 7 . 7 0 Sm 4 . 1 7 . 3 0 3 . 6 0 4 . 8 0 F e 4 7 2 5 0 6 3 0 0 4 8 6 5 5 6 0 0

I J r o t ~ r~

E

e- v ~ m ling i Jr)

aoue~!uusueJJ. %

~ -

I | | I i I I I

90

80

70

60 50 40 30 20 10 ! i 900 800

Flg

5

l I I I I 600

500

400

300

2e

X-~sy

Diffraction

spectrum

of

~[~ll~Ik

clay

a)

Glycolated

clay

salples

b)

Alr

dried

clay

salples

I

2O0

C=I I : In te ns it y (% ) I I I I I I I Q I--LI~ I d' I'd ~l 0 ',4

§

I I I I I I I I I

t/

'

2000

,L~

A

1800

, ooL/"

i

b

1000

~/'~

800~

30 , o°"~'-'-° •

20

~,o""

o

10

-

Flg

7 I | I I Z~ I • 0 0 I I I I

10

15

20

25

Time

(d)

Sorptlon Kinetlcs. Change of R d wlth tlme for Kthall~Ik clay. • Partlcle slze < I0 ~a, [Cs]'=l.OlxlO -5 neq/al o Parelcle slze >I0 Pn, [Cs]*-l.Olxlx -5 neq/al • Partlcle size < I0 Urn, [Cs]'=l.lgxlO -8 meq/ml A Partlcle slze >I0 pa, [Cs]S=l.19xlO -8 neq/al

The adsorption as well as desorptlon results of distribution ratio, RD, measurements as a function of Cs-ion concentration in the solid phase for two different size fractions (<I0 B m and >I0 ~m) of M i h a l i G G i k clay are shown in Figs 8 and 9 respectively. It is observed that in both cases the adsorption-desorption phenomenon is

reversible. Furthermore the curves exhibit a characteristic inverse

S-shape. If the R D values are taken to be true equilibrium

constants, then they are not expected to show a variation with Cs-ion concentration. The results shown in Figs 8 and 9 suggest the exist- ence of at least two types of adsorption and/or exchange phenomena.

One taking place at low Cs-ion concentrations (till about

[CS]sf0.SxlO-5 meq/g) and the other type starting at about

[Cs]sffil.Oxl0-4 meq/g and continuing at higher concentrations.

A transition from one type into the other takes place between Cs ion

concentrations of 10 -5 meq/g to 10 -4 meq/g. At high Cs ion

concentrations one may probably better describe the phenomenon as an exchange rather than an adsorption.

The reversible behaviour may be attributed to the tightly bound layers in the case of kaolinite structure which does not permit the deep penetration of the solution into the clay structure. Thus, the adsorption process takes place primarily at the surface which leads to an easy and effective release of the ions in desorption.

The adsorption isotherm for M i h a l i ~ i k clay is shown in Fig I0. It is seen that the isotherm is not linear and it reflects the characteristic shapes shown in Figs 8 and 9.

' I ' I ~ I ~ I

tOO0

OI

I

100

t'Z _l I l I

i0

-6

10-

5

, I , I

1o

-4

1o-3

[cs]s

(

meq/g

)

. Fig

8

The changes of R D values with cesium Ion loadlng for Eihalt¢~ik c/ay. Particle size I0 ~n. • Adsorption o Desorptlon

1000 A r~ ns 100

1

'

I

'

I

'

I

o

~

o

~~~o O@

m

I

,

I

,

I

,

I

Io-6

io-s

~O-~

lO-3

[Cs] s (meq/g) Fig 9 The changes of R D values with cesium ion loadlng for N/hali~Lk c/ay. Partlcle size <I0 pm. • Adsorption o Desorptton

I I I 0 I z I

(6/b~uJ) S[s~]

] I e - u~ .PI ^ v

® , ~ . ~

. 0

o w

%

5000 A 400O E _v 3000 n,. q

I

2000

1000 f@m@ 0 fj'"' 0 0 0 O~ i i J I [ i I I I [ l ~ i i [ , L i i I l i I l [ I i I I 5 10 15 20 25 Time (d) Fig II Sorptlon K/netlcs. C~mnge of R D values with tlm~ for Resadlye c/my. [Cs ]'=I .OZxlO -8 mq/m.l • Partlcle size I, I0 ~• o Particle size E I0 P•

28000~

1

'

'

'

l

@ @

2/.000

"E

20001]

r~

16000

~

_a~

F

°

12000

8000

~000

5 I ' l , l I l , , , I ' ' ' i I ' ' ' ' ] ' ' i , & • 0 OD • • • il~ 0 O. I I I I I I i

10

15 20 25

Time

(d)

I I I I Flg 12 Sorptlou ~tnetics. Change of R D values with tlme for Sarayk6y soil frsetlons. [Cs ] "=1.02xi0 -8 nneq/=]. • Partlele slze <5 pm o Partlele slze = 5-10 Pm • Particle slze - I0~20 Pm Q Partlcle slze >20 ~a

10OOO

O~

E

_tY

1000

-o\

\\

\,

L I ~ l , I , L ,

10

-6

10-5

10-4

10-3

[Cs]

s

(meq/g)

Fig 13 The change of R D vaXues w/th cesium ion

loadtnK

for SaraykSy

sot1

fractions. • Adsorption, particle slze <5 ~s Desorptlon, partlcle size <5 Ua

•

Adsorption,

particle

size

<20

Us

o

Desorptton,

particle

size

~20

pm

' I ' I ' I I ' I ' I p 10 -3

10

--/,

g-

_E

lo-s

_¢,J

1

10"6

S

10-11

i I

_10-10

I I

_10-9

Fig

14

/

_o

I I 1 I I , I

10-8

10

-?

10

-6

10-5

[C~t

(meqlmt)

:

Sorption Isotherm of Sarayk~y soil fractions. • Particle size <5 ~• o Particle size • 5 ~m

The sorptlon behavlour of Cs cation with Resadiye clay was found to be similar to M i h a l i q q i k clay with somewhat higher R D values. As an illustration, the sorption kinetics are shown in Fig

11 for [Cs]°=l.02xl0 -8 meq/ml. The greater R D values for

larger particle size is not well understood.

The results of sorption studies using SaraykSy soil fractions are shown in Figs 12, 13 and 14. Fig 12 shows the change of R D

values with time for various particle size fractions. The large

values of R D as compared to those using pure clay fractions is

striking. This may be due to the presence of organic components,

particularly humic acid in the soll fractions. Increase in R D

values with decrease in particle size suggests primarily a surface phenomenon as in the case of M i h a l i g ~ i k clay. The change of R D values with Cs ion loading is shown in Fig 13. Similar character- istic curves as in the case of pure clay samples is observed. Fig 14 shows the adsorption isotherm. Again it is nonlinear.

It may be concluded that various clay and soil fractions adsorb or desorb caesium cation in a similar way. The differences observed are only in the magnitudes of R D values which may be a reflection of their different cation exchange capacities.