Journal of Radioanalytical and Nuclear Chemistry; Articles, Vol. 163, No. 2 (1992) 205-211

CESIUM SORPTION UNDER NONEQUILIBRIUM CONDITIONS

N. FILIPOVI~VINCEKOVI~ D. SKRTIC

Ruder Bo~kovi~ Institute, 41000 Zagreb, Bijeni~ka 54 (Croatia)

(Received December 13, 1991)

Sorption of cesium on montmorillonite has been studied under nonequilibrium conditions. Depending on the initial pH of the solutions, initial fast sorption was followed by slow additional sorption (acidic region) or desorption (basic region). Dissolution of main montmorillonite constituents in acidic solutions results in progressive changes in both surface structure and charge distribution. Additional sorption may be explained by an increase of charge density at the monlmorillonite/electrolyte interface. Suspensions aged in basic region revealed partial cesium desorption. The diminishing of sorptive properties is explained by the formation of secondary precipitate which may effectively coat the montmorillonite/electrolyte interface and/or readsorption of released clay constituents.

Introduction

In recent years the nonequilibrium behavior of many geochemical processes is becoming increasingly apparent. Departure from equilibrium and evidence of metastability of water/rock interactions emphasize a problem of using proper sorption value in modeling of pollutant transport in the environment. Long-term weathering reactions can change or remove the primary mineral phase and/or cause precipitation of secondary phase. Thus, the distribution of pollutant between the solid and the liquid

phase in natural systems may be affected. It is known that cations are adsorbed onto clays to balance the negative charge in the

aluminosilicate structure. The mechanism of low charge cations sorption appears to be ion exchange (usually a very fast process). In our earlier work we have shown that after the initial fast sorption, an additional long-term sorption of cesium on montmorillonite occurs. 1-2 This is due to the phenomena driven by the nonequilibrium state of the system) This paper deals with the identification of nonequilibrium processes which influence the sorption behavior of montmorillonite. The heterogeneous exchange of Cs ions between the solid and liquid phase is used as a tool to study the mechanisms of

solid phase equilibration and the way of ion incorporation into the solid phase. ~ Results reveal that nonequilibrium processes, dissolution of main clay constituents, precipitation of secondary phases, surface charge distribution change, etc., could significantly affect the distribution of cesium ion between montmorillonite and the solution phase during long periods of time.

Elsevier Sequoia S. A., Lausanne Akad~r Kiad6, Budapest

N. FILIPOVIC-VINCEKOVI~ D. ~KRTI~- CESIUM SORPTION

Experimental

The montmorillonite sample was obtained from the location near Ivanid Grad, Croatia. It was subjected to the least possible pretreatment (dried at 105 ~ and ground to pass a 165 mesh sieve). The characterization of sample was previously described, i

Systems were prepared by dispersing a dry montmoriUonite sample in electrolyte

solutions, and aged for different time intervals at 298 K. Initial pH of electrolyte solutions varied from 2.0 to 9.0 and was adjusted by addition of HC1 or NaOH.

The pH of montmorillonite suspensions was measured by using a combined

electrode connected to an Ionalyzer Model 901 (Orion Research). The distribution of radionuclide between the solid and the liquid phase was

measured by a simple batch method. 100 ml of labeled CsC1 solution (134Cs as CsC1

solution supplied by Amersham, England) was allowed to react with 1 g of montmorillonite in a polyethylene vial. At determined time intervals (after centrffugation) the liquid phase was analyzed for radioactivity using an LKB-WaUac "/-counter (1282 CompuGamma).

Results are presented as the changes of a (molar ratio of cesium ions in the solid phase (n o and in the liquid phase (nl)) with time. Definition of a is given by the equation:

a = n,/n i = (A o - At)/A t (1),

where A 0 and A t denote radioactivity of the solution at zero time and radioactivity of the liquid phase at time t, respectively.

In heterogeneous exchange experiments the suspensions were prepared with 0.01 mol. dm -3 CsCI solution and after a determined time interval labeled with t34CsC1.

The time interval from the moment of suspension preparation to the moment of labeling is called the aging time, tA, while the time interval after labeling is called the exchange

time, t E. The degree of exchange, F, was used for quantitative interpretation of heterogeneous

exchange experiments:

F = (A o - At)/(A o - A~) (2)

In Eq. (2) A~ denotes the equilibrium radioactivity of the liquid phase. The relation between A 0 and A~ is given by:

,% = AdO + ~) 0 )

206

n 0 v r

N. FILIPOVIC-VINCEKOVIC, D. SKRTIC: CESIUM SORPTION

Results and discussion

Kinetic investigations of cesium sorption on montmorillonite show that the ion-exchange process is usually finished within a few minutes. 1 The ion-exchange

equilibrium between the univalent cations from the solution and cations from the

montmorillonite can be described by the following equation

(basic cations)-montmoriUonite + H § + Cs+--,(H, Cs)-montmorillonite + + basic cations.

(4)

When the montmorillonite is kept in the solutions of different initial pH and over a long period of time a change of cesium ion distribution between the solid and liquid phase with time is observed (Fig. 1). Two different steps in the sorption of cesium ions on the montmorillonite can be distinguished, an instantaneous sorption due to ion exchanged and slow sorption during montmoriUonite aging in solution. Suspensions

prepared with initial pH 2.0 and 2.5 show additional sorption, while suspensions

A

g i.z5 C

1.00

n ~_._....~ ~ "%''~

0.75 I l I I = 2 3 4 5

log ( tA lon)

Fig. 1. Molar ratio of cesium in the solid, Us, and the liquid phase, n 1, as a function of time. The initial solution pH values are; O pH 2.0, �9 pH 2.5, • pH 6.0, Q pH 8.0 and A pH 9.0

prepared at higher initial pH (6.0, 8.0 and 9.0) show desorption. Extrapolated sorbed concentrations for the fast and slow processes are given in Fig. 2. It can be seen that Cs sorption on montmorillonite depends on the init.ial pH and contact time with aqueous

solution. The concentration of cesium sorbed during the ion-exchange process increases with increasing pH due to the less significant competition with H + ions and increase of net negative charge of the particles, (at pH > 8) caused by the reaction of hydroxide ions with positive edges of the clay platelets. 9

The changes of pH with time for the suspensions with different initial pH values are given in Fig. 3. If the initial solution pH was 2.0 and 2.5, an immediate increase in pH

207

N. FILIPOVIC-VINCEKOVI~ D. SKRTIC: CESIUM SORPTION

A I - '* ' ' ' ' ~

tA/rain 1.00j �9 lO

o 40000

I I I I I 1 I 0.75 2 3 4 5 6 7 8 9

pH

Fig. 2. Molar ratio of cesium ions in the solid, ns, and the liquid phase, nl, as a function of the initial pH of solution. Aging time, tA, as indicated

A

8 ~ A A

4 - 02.0 e2.5 x6.0 a8.0 a9.0

2 I ! 1 I I F 2 3 4 5

tog (tA/rain)

Fig. 3. pH of montmorillonite suspension as a function of time. The initial solution pH values are indicated

of suspensions occurred due to the release of the exchangeable basic cations and hydrogen ion consumption. At longer time periods a gradual increase with time (pH 2.0) or a practically constant value (pH 2.5) are seen. At higher initial pH values (6.0, 8.0 and 9.0) a rapid pH shift to -10.4 followed by a gradual decrease in time was observed. It can be concluded that the change of suspension pH depends on the initial pH of the solution and cation exchange capacity of montmorillonite. 3

In order to better understand the slow sorption/desorption processes, heterogeneous exchange experiments were performed. The heterogeneous exchange of ions between the solid and the liquid phase may be controlled by different processes, such as diffusion inside the liquid phase, surface reaction (sorption/desorption, diffusion through interface layer, chemical reaction), recrystallization and self-diffusion inside the solid phase (usually being the slowest process). Plots of the degree of exchange, F, vs. exchange time, tE, are usually used in identification of the mechanisms involved in heterogeneous

208

N. FILIPOVIC-VINCEKOVIC., D. ~KRTIC" CESIUM SORFTION

exchange processes. 4~ Processes controlled by surface reactions give typical plots with high F-values for short exchange time.5 Aging of precipitate in electrolyte solution often introduces recrystallization .inside the particles or some of the lattice layers and recrystallization in the sense of the Gibbs-Kelvin rule. Much lower exchange rates for aged systems compared to the "fresh" ones indicate that recrystallization has taken place. 4,7 In the systems when precipitates are aged for very long time periods, particles are in equilibrium with solution and the exchange of ions is often determined by diffusion through the solid phase. 4,6 The changes of the degree of exchange, F, [defined in experimental section, Eq. (2)], for differently aged suspensions (t A = 1500 and 20000 min) are shown in Figs 4 and 5. At chosen time intervals we suppose that sorption due to the inn-exchange process is completed. 1 Very high F-values attained after relatively short time intervals (10 minutes) in all systems indicate surface-controlled reactions. Such reactions are very sensitive to changes in solution composition and the electrochemical nature of the solid surface. Since the composition of the liquid phase (cesium in solution exists as Cs § over a wide range of pH and concentrations) remains constant during the aging time, changes of F-values are considered to be the consequence of changes in surface properties of montmofillonite. Such change of F-values obtained at the predetermined exchange th~ne is a "measure" of the changes in surface properties which influence the distribution of cations in the interfacial layer. Montmorillonite has a well-developed Stern layer in which most of the exchangeable cations reside. 1~ The net space charge in the Stem layer per unit area of the surface is equal in magnitude but opposite in sign to the difference between o 0 (charge density at the clay water/interface) and c~6 (charge density at the outer Helmholtz plane). The outer Helmholtz plane coincides with the plane of the shear and, according to MILLER and LOW, 1~ the fraction of exchangeable cations in the Stem layer is

I - % / % = nJno (5)

where n o denotes the total moles of cesium ions in system.

n o = n~ + n~ (6)

CombiningEqs~(t), (2) and (5) one can write:

F = [(A o - At)/Ao]oo/(O o - a~) (7)

A significant diffcrcncc was observed in the behavior of the suspensions with initial pH 2.0 and2.5 (Fig. 4) and that prepared with initial pH 6.0 or 9.0 (Fig. 5). While in acidicsystems (Fig. 4) an increase of F values was found, in the basic pH region a small

209

N. FILIPOVI~-VlNCEKOVI~ D. SKRTI& CESIUM SORPTION

pHo 2.5

13 r o ~

0 CsCl/mol .dm -3 =O01 tA/min �9 1500 o ooo

09[- 9 ; i

[, I 08 2 pH o 2.0

I l I t , 4 5

log (tE/min)

Fig. 4. Fraction of exchange, F, for the systems of initial solution pH 2.0 (lower part) and 2.5 (upper part) as a function of exchange time, t E. Aging times, tA, are indicated

i~ pHo 9.0 10 ~. ~, 0 0 C - -

0 ~"

CsC[ Imob drri 3 = 0.01 tAImin

�9 1500 A / 200O0 O

1;J I- pHo 6.0 1.0[- ~-- " -" "

- n

! I I I I ~ 09 2 3 4 5 -

log (t E/min)

Fig. 5. Fraction of exchange, F, as a function of exchange time, t E for the systems of initial solution pH 6.0 (lower part) and pH 9.0 (upper part). Aging time, tA, as indicated

decrease in F values was observed. Later it was less pronounced at higher initial pH value.

It seems that aging of montmoriUonite in acidic suspensions enhances surface charge (increase of F value in Fig. 4). The observed surface charge increase with time can be explained by the dissolution of main clay constitutents. 3 Incongruent dissolution with time results in progressive changes in both surface structure and charge distribution. In the systems aged under basic conditions several processes contribute to the decrease of F. Although an incongruent dissolution may cause the F-value to increase, such influence is compensated by at least two processes which have opposite effect on

210

N. FILIPOVR~-VINCEKOVI~ D. SKRTI~ CESIUM SORPTION

F-values: (a) precipitat ion o f secondary phase which may form a coating at the interface

be tween the solid and the l iquid phase 3 and (b) readsorption o f released main clay

constituents (especial ly a luminium), which compete for sites available for sorption. 13,14

The authors wish to thank Mrs. N. NEKIC for technical assistance. The financial support of the Ministry of Science, Technology and Informatics of Croatia (Grant 1-07-189) and the International Atomic Energy Agency (Research Contract 5243/RB) is gratefully acknowledged.

References

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(1990) 239. 3. N. FILIPOVIC-VINCEKOVI(~et al., Radionuclide migration mechanism in geosphere, Final Report, Res.

Contract No. 5243/RB, IAEA, Vienna. 4. K. E. ZIEMENS, Ark. Kemi Mineral. Geolog., 20 A, (1945) No. 18, 1. 5. M. J. HERAK, M. MIRNIK, Kolloid-Zeitschfift, 168 (1960) 139. 6. R. H. H. WOLF, M. MIRNIK, B. TE~AK, Kolloid Z. Z. Polymere, 205 (1965) 111. 7. N. FILIPOVIC.-VlNCEKOVI~ R. DESPOTOVI~ Radiochim. Aeta, 27 (1980) 213. 8. T. SUGIMOTO, G. YAMAGUCHI, J. Phys; Chem., 80 (1976) 1579. 9. S. L SWARTZEN-ALLEN, E. MATIJEVIC, J. Colloid Interface Sci., 30 (1975) 143.

t0. S. E. MILLER, P. F. LOW, Langmuir, 6 (1990) 572. 11. Y. HORIKAWA, R. S. MURRAY, J. P. QUIRK, Colloids Surfaces, 32 (1988) 181. 12. D. Y. C. CHAN, R. M. PASHLEY, J. P. QUIRK, Clays Clay Miner., 32 (1984) 131. 13. A. S. BUCHANAN, R. C. OPPENHEIM, Aust. J. Chem., 25 (1972) 1857. 14. H. S. HANNA, P. SOMASUNDARAN, J. Colloid Interface Sci., 70 (1979) 181.

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