Radionuclide chemical species determination in waste management studies

Journal of Radioanalytical and Nuclear Chemistry, Articles, [Iot. 110, No. 1 (1987} 91-100

RADIONUCLIDE CHEMICAL SPECIES DETERMINATION IN WASTE MANAGEMENT STUDIES

G. BIDOGLIO, G. TANET, A. DE PLANO, G. P. LAZZARI

Commission of the European Communities Joint Research Centre, Ispr~ Establishment,

Radiochemistry and Nuclear Chemistry Division, 21020 Ispra (Va] (Italy]

(Received January 3, 1987)

The bio-geochemical processes of actinides potentially released to the environment from nuclear waste repositories are mainly determined by their chemical form. Results of direct and indirect, determinations of soluble and colloidal species of radionuciides are reported. Experimental investigations on americium speciation in sea water are compared with model calculations. Americium pseudocoUoids smaller than 0.45/an were identified. The application of the Specific Interaction Theory to activity coefficient correction i s shown for the chloro- and sulphate complexes of americium, Analytical techniques for direct speciation studies are briefly reviewed.

Introduction

The safe disposal of radioactive wastes is one of the main themes of the nuclear debate in many countries. There has been an increasing concern about the potential introduction of radioactivity to the environment resulting from the management of ~-bearing wastes and Highly Active Wastes (HAW) arising from nuclear power programs. Disposal options umder investigation involve burial in suitable geological formations (clay, granite, salt and sub-seabed). The assessment of the consequences of a potential release of radionuclides from these repositories requires the description of the source, the transport behaviour in the environment and the absorption by receptor organisms. All these processes cannot be simply understood either by computing or measuring the total amount of nuclide. It fs known that different dis- tributions of species at identical total concentration of pollutants in natural waters result in different biological availabilities. For instance, Cr(VI) and As(III) have been reported to be more toxic than Cr (III) and As (V), respectively i.

The geoChemical inter&ctions are determined by the charge carried

by the radionuclide. Negatively charged mineral oxides and clays repel anionic species. Cationic species or strongly complexed nuclides or nuclides associated with colloidal particles move through the environment with different mechanisms leading to different retardation. Modelling of nuclide migration through geological formations requires, therefore, an understanding of the speciation of the radionllclide in order to determine its long-term environmental impact.

After a description of the methodology, this report shows how the problem of the determination of individual chemical forms of radionu~ clides has been tackled in our laboratories.

Elsevier Sequoia S. A., Leusa~ne A lcaddmiai Kiad6, Budapest

G. BIDOGLIO eta].: RADIONUCLIDE CHEMICAL SPECIES DETERMINATION

Methodological approach

Figure 1 summarizes the approach followed. It is based on a sort of feed-back loop between three different research areas: simulation experiments, speciation studies and barrier modelling. The laboratory simulation of events resulting in the release and migration of radionuclides is a necessary step to bring about first indications on the variables affecting the transport processes. This task involves leach tests of the matrix containing the radionuclide under investigation, batch K d measurements, continuous flow experiments with soil columns, measurements of diffusion rates. Because of the different time scales

DIRECT �9 CHEMICAL SPECIES

DETERMINATION IN COLUMN EFFLUENTS

= DEVELOPMENT OF ANALYTICAL TECHNIQUES

SPECIATION STUDIES

,MU T,ON EXPERIMENTS

INDIRECT " MODEL CALCULATIONS �9 EQUILIBRIUM CONSTANTS

BARRIER"~ MODELING I

DETERMINATION

Fig. 1. Schernadc representation of the methodological approach

and dimensions, laboratory data alone can hardly be extrapolated to

real geological situations, unless mechanisms of transfer are known. The goal of speciation studies is the quantification of phenomena observed with simulation experiments. They can be divided into direct and indirect. In the first case, a direct identification of species released from the waste matrix and present in the column effluent is carried out. This task entails the development of analytical techniques working at very low concentration levels~ The analytical information obtained requires further confirmation, investigating whether the proposed chemical reactions are thermodynamically and kinetically permitted. Basic chemistry studies are, therefore, needed (indirect approach) and complexation and colloidal aggregation of radionuclides are thoroughly investigated. All these research activities result in the generation of a model describing the migration behaviour of radionuclides in the investigated geological barrier. The model is then in- troduced in the suitable geochemical box of the code evaluating the dose comm( tment to the population.

G. BIDOGLIO et al.: RADIONUCLIDE CHEMICAL SPECIES DETERMINATIOI~

Experimental

The experimental methods used (solvent extraction, electrom~gration) were similar to those reported previously. The reader will find more details in the references mentioned in the text.

Ultrafiltration analyses of the glass leachates and the sea water were carried out using Nuclepore membranes. Ultracentrifugation tests were performed employinq a L8-55M Beckman Ultracentrifuge equipped with a fixed angle rotor. Adsorption of 24JAm on the filters and the wall of the tubes (Quick-Seal, Beckman) was found to be minimal in the time span of the tests.

Solutions of 24JAm were supplied by the Radiochemical Centre, Amersham. The neptunium isotope 239Np was produced by milking of the generator 243Am. All chemicals used were of analytical grade.

Results and discussion

Determination of stability constants

The matrix of competing equilibria describing the reaction pathways of a radionuclide M in an aquatic environment is depicted in Fig. 2, where L indicates a general ligand. The main processes con- trolling the species distribution are redox reactions, complexation, colloid formation and sorption. A direct investigation of these processes is complicated by the very low e~r~ntal concentrations of

M + nL e ~" MLn

*M" "L" "ML "

iii'iii i : iii i iii iiiGiii i iiiii ::: Fig. 2. Major reaction pathways of a radionuclide in the water flow. Distinction is made oetween geologic materials

exhibiting either a permanent (e,g. clay) or a pH dependent (e.g. mineral oxides) surface charge

93

G. BIDOGLIO et al.: RADIONUCLIDE CHEMICAL SPECIES DETERMINATION

actinides. Nevertheless, a picture of the situation may be obtained whether the equilibrium constants for all complex species and the concentrations of radionuclides and ligands are known.

In general, the stability constants of soluble complexes are determined from the dependence on the concentration of the ligand of physico-chemical propezties such as solubility, optical absorbance, half-wave potential, extent of distribution between two phases. To detect a cha/ige of some of these parameters, high concentrations of radionuclides are often needed. Extrapolation to environme/Ital conditions of data obtained at macroconcentrations may overestimate the contribution of certain species and neglect the formation of some others, e.g. mixed cclnplexes. In this respect, solvent extraction is particularly s~itable to investigate the very low nuclide concentrations. This separation technique was therefore used for the determination of Np(i r) speciation in aqEeous solutions containing bicarbonate and carbonate ions, main inorganic ligands in natural waters.

Extraction of Np(V) at neutral pH was attained by using a benzenic solution contaiDd/Ig two different extractants: TTA (thenoyltrifluoro- acetone) and TOMA (tri-octyl methyl ammonium chloride). A mixed ligand complex involving one molecule of TEAand 2.5 molecules of TOMA bound to each NpO~ ion, was identified as the extracted species, both in the presence and in the absence of carbonate ions 2 . This allowed a simplified treatment of the distribution data, based on the equation

2- [ (i) [co 3 l i=1

where D ~ and D are the distribution coefficients of Np(V) between the organic and the aqueous solutions in the absence and in the presence,

a 0 i.- z _m o_ U_ U_ W 0 0 z o b-

111

rr_ 1 I-- m 121

0

1.2 ),Ax,, %

log CARBONATE CONCENTRATION (mole I- ') Fig. 3. Distribution coefficients for neptunium between benzenic solutions of the extractant and aqueous phases at

increasing concentrations of carbonates

94

G. BIDOGLIO et al.: RADIONUCLIDE CHE~CAL SPECIES DETERMINATION

respectively, of carbonates; 8i is the stability constant of the i-th complex and f is a correction factor accounting for the hydrolysis of Np(V) and its complexation with TTA in the aqueous phase. The distribution coefficients D of neptunit~n presented in Fig. 3 as a function of carbonate concentration, plot on the same line, indicating a negligible interaction of Np(V) with bicarbonates. The average number i of CO 2- attached to NpO~ ions is related to the slope of the curve shown Oln Fig. 3. It appears that for the predominating complex species, i is not higher than two, suggesting the formation of NP02(C03)- and

(CO3)3-. Accordingly, the experimental data r were found to be des- NpO 2 cribed by a 81/82 model. The stability constants at ionic strength 0.2 M (NaC104) calculated with Eq.(1) are logSl 4.13 and log82 7.06.

Activity coefficient correction

The interpretation of the chemical propertie s of multimetals/multi- ligands systems is nowadays simplified by the use of large computer codes (see Ref. 3 for discussions). However, data are often inhomo- geneous and must be corrected for the medium dependence to allow their extrapolation to ionic strengths at which they have not been measured. Among the several approaches to this objective, the ion interaction methods provide an estimation of the activities of mixed electrolytes from the properties of pure components. The addition of a few terms to the Debye-Hfickel limiting law accounts for the short range forces between ions at high concentration. In the Pitzer model these terms are ionic strength dependent 4. Nevertheless, such a detailed model would not appear to be of practical application to the complex groundwater -~ repository system. The Guggenhelm-Scatchard S~ecific Interaction Theory (SIT) provides a simpler description of the short range forces using only one additional term s assumed to be constant with ionic strength and reflecting the pairwise interactions between ions of opposite charge sign5, 6. For the general chemical reaction

Bi M z+ + nL- ~ ML z-n (2)

n

the formation constant 8 i at a fixed ionic strength is related to the thermodynamic constant through the ,expression

log 8i(1) = log 8i(0) + Az2D + AeI (3)

where:

0.51 II/2 D=

I + 1.5 11/2

At -- § n%-A§ - A+ or x-) n

and, for trace concentrations of the radionuclide and the ligand, A+X - is the bulk electrolyte used in the constant-ionic-medium method. Hence, the SIT model predicts that log 8i(I) - Az2D should be a linear function of I. A curved relationship is expected when the basic assump-

95


I(M)

a 04 N

I

1.5

1.15 1.0

T

0.2 0.5 1.0 I I i

AmCI =+

k log ,8 (0) I I , I

I I I

3 4 8 0 ~ A80.196

3. \0l log,8 (0)

I I

AmS0 + I

0.5 1.0 2.0 I(M)

Fig. 4. Application of t]~% Specific Interaction Theory (SIT) to activity coefficient correction. The equilibrium C 2+ . + c o n s t a n t s ~ for Am l were taken from Reference 7, 8 and from Reference 8-12 for the species AmSO4

tion of the model is not fulfilled and the e coefficients change with ionic strength. In this case, the Pitzer equation would describe more accurately the medium dependence of the equilibrium constants. Fig. 4 shows the application of the SIT model to the available equilibrium constants of AmCI 2+ and AmSO~, as examples of Eq.(3). From the slope

of the lines (Ae) and using e(Am3%,C1 -) = e (La3+,C1 -) = 0.22,

e(Am3+,ClO~) = 0.47 for analogy with lanthanides5, 6, the following

interaction coefficients are obtained

+ e(AmCl2+,Cl -) = 0.03; e(AmS04,ClO 4) = 0.I

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For its simplicity, the Specific Interaction Theory appears to be very helpful, as shown in the example, in establishing a set of cri- tically compiled thermodynamic data for transuranics.

Colloid formation

The geochemistry of radionuclides in aquatic environments may involve changes from the ionic to the colloidaT state. Since these processes may substantially alter reaction pathways, an ,Inderstanding of the colloid chemistry of trace radionuclides is needed 13.

From the stability constants reported in the literature7-12,14-16 and considering the concentration of natural inorganic agents 17, it would appear that the solution chemistry of americium in sea and pore water is dominated by hydroxide and carbonate cemplexation, despite the much hiqher concentration of chlorides. This is shown in Fig. 5, reporting the percentage of soluble species of americium as a function of carbonate concentration at the constant pH of 7.8. These model calculations are consistent with direct speciation studies by electromigration of americium in a s!nlthetic sea water containing inorqanic ligands only. Fig. 6 qives the relative concentrations of americium species found in the anodic and the cathodic compar~ent of the electromigration cell. It can be observed that the predicted cross over point of the mole fraction of anionic and cationic species was obtained also experimentally. However, the chemistry of americium i~ a real sea water is likely to be affected by the presence of suspended particles which may adsorb radionuclides generating pseudocolloids. Preliminary results on the pseudocolloidal behaviour of americium in a North Atlantic sea water are given in. Table i. These data were obtained by ultrafiltration with Nuclepore membranes of sea water sam- ples traced with 241Am. The reported trend was confirmed with ultra-

100-

5O'

~ 10.

g

5 / /

, . , . , . ,

-5.2 -5.0 -4.8 -4.6 -4.4 4 .2 -4.0 -3.8

=og [co~-] Fig. 5. Calctflated influence of carbonates on Am(HI) speciation in sea water at pH 7.8. Stability constant r

taken from the literature were corrected to ionic strength 0.7M using the SIT model

7 97


100

5O

~D 10.

s I -5.2

' ii

-g.o -.'8

�9 �9 �9 �9 �9 �9

* ' , . o % 8 0 0 0 0

A N I O N S . . �9 �9 * , �9

~] o ~176176 ~

"1 ~176 F

-4.6 -4'.4 -4:2 -4'.(

log [co~-]

o o 0

b ~

I e

-3is

Fig. 6. Experimental measurement of the relative percentage of cationic and anionic soluble spec!es of Am(III) in a synthetic sea water. Electromigrafion results are compared with computer evaluations of Fig, 5 (dotted curves)

Table 1

Ultrafiltration of americium pseudoeolloids in a N o r t h A t l a n t i c sea water traced with '2 4 2Am (<10-gM). The effect of ageing on the amount of soluble americium ( u n f i l t e r e d ) is r e p o r t e d

filter porosity (nm)

% of soluble Am

no ageing 35 days ageing

450 90.4 85.7 200 88.1 75.8 100 86.1 79.0 50 77.6 69.[

centrifugation experiments. Data in the table show the presence of colloidal particles smaller tha~ 450 nm whose size distribution varies

with ageing of the solution.

The formation of pseudocolloids is likely to dominate the colloidal behaviour of actinides in natural waters. In a flowing syste~ and if sufficiently small to be not filtered, these radionuclide bearing particles may move at the velocity of the water vector with little retardation. As a matter of fact, no electrostatic attraction exists with the surrounding geomedia from which the particles have originated.

~-xperimental investigations on the behaviour of pseudocolloids are, therefore, necessary to determine conditions under which colloid

t~ansport is significant.

98


Analytical techniques for direct spec~ation studies

Computer modelling predictions for radionuclide speciation need an independent experimental proof because of the variations existing in the literature for the stability constant data. This problem is far from being sol~ed, because of the great sensitivity required and the inevitable alteration of equilibria between chemical species due to the measurement itself. Nevertheless, a number of techniques are now available in addition to the classical electroanalytical and spectros- copic methods whose main limitation is the relatively high detection limit. Some of these techniques, shortly described in the following, are currently used in our laboratories.

EXAFS (_Extended x--ray Absorption Fine Structure) and XANES (X_-ray Absorption N__ear Edge Structure) spectroscopy using a synchrotron radiation, can be very helpful in ascertaining the oxidation state and[ the chemical structure of radionuclides in the bulk of the waste form and in aqueous solutions 18. These techniques have been used to investigate the chemical state of technetium in borosilicate glasses doped with 99Tc both under oxic and anoxic conditions 19 .

Photon Correlation Spectroscopy measures the light scattering of a laser beam to determine the particle size distribution of colloidal suspensions. This technique is being applied for the measurement of americium particulates in natural waters.

Thermal Lensing Spectrometry is based upon the absorption by the species to be determined of an incident laser radiation. This absorption is the/l detected as a change in the refracti~re index of the liquid. The advantage over conventional spectrophotometry is the con- siderable enhancement of sensitivity. As spectrophotometry, TLS does not alter chemical equilibria and provides information on oxidation states and chemical forms20, 21. The same kind of information is obtained by Photoacoustic Spectroscopy 22, which eliminates the difficul- ties in collection and detection of optical radiation.

The electromigration experiments reported for americium in sea water were carried out using a cell described elsewhere 23. Electrical mobilities and diffusion coefficients of technetium species in ground water were also measured. Adsorption and electro-osmosis effects often encountered with conventional paper electrophoresis are minimized whe/l using this free-liquid electromigration cell.

Wet chemical techniques such as ion-exchange, solvent extraction, coprecipitation, coupled with NAA, have also to be mentioned. They have been extensively reviewed by other authors and they are not

discussed here 24. Their main disadvantage is the physical alteration of the sample and the disruption of chemical equilibria. A good se- lectivity can, however, be attained with the application of combined procedures25,26.

Conclusions

The determination of individual chemical species of radionuclides is still an open question. The following research areas are suggested for future studies:

- role of organic matter, which appears to contribute significantly or even dominate the radionuclide speciation in natural waters;

- association with suspended particulates, which is likely to affect drastically the migration pattern of radionuclides;

7* 99


- estimation of thermodynamic constants, to improve modelling of radionuclide speciation which suffers from a lack of accurate data;

- development of analytical methods responding to individual chemical forms, to substantiate model calculations.

Unless the physico-chemical speciation of radionuclides is known, predictions of their behaviour and migration in a give~ aquatic system can hardly be made. The problem can be solved only by using an inte- grated approach and encouraging collaboration between laboratory and

field researchers.

References

1. E. SABBIONI, J. EDEL, L. GOETZ, Nutrition Research, Proc. Int. Syrup. on the Health Effects and Interactions of Essential and Toxic Elements, M. ABDULLA, M. B. NAIR, R. K. CHANDRA (Eds), Pergamon Press, New York, 1985, p. 35.

2. G. BIDOGLIO, G. TANET, A. CHATT, Radiochim. Aeta, (1985). 3. T. W. BROYD, R. B. DEAN, G, D. HOBBS0 N. C. KNOWLES~ J. M. PUTNEY, J. WRIGLEY, A directory of

computer programs for assessment of radioactive waste disposal in geological formations, Report EUR 8669 EN, 1983 (updated 1985).

4. K. S. PITZER, in Activity Coefficients in Electrolyte Solutions, R. M. PYTKOWICZ (Ed.), Vol. 1, CRC Press, Inc., 1979, p. 157.

5. G. BIEDERMANN, J. BRUNO, D. FERRI, I. GRENTHE, F. SALVATORE, K. SPAHIU, in Scientific Basis for Nuclear Waste Management, Vol. 5, by W. LUTZE (Ed.), Elsevier Science Publ., New York, 1982, p. 791.

6. L. CIAVATTA, Ann. Chim. Rome, 70 (1980) 551. 7. M. WARD, G. A. WELCH, J. Inorg. Nucl. Chem., 2 (1956) 395. 8. P. K. KHOPKAR, J. N. MATHUR, J. Inorg. Nucl. Chem., 42 (1980) 109. 9. T. SEKINE, J. Inorg. Nucl. Chem., 26 (1964) 1463.

10. R. G. DE CARVALHO, G. R. CHOPPIN, J. Inorg. Nucl. Chem., 29 (1967) 725. I1. A. AZIZ, S. J. LYLE, S. J. NAOVI, J. Inorg. Nuci. Chem., 30 (1968) 1013. 12. G. M. NAIR, Radiochim. Acta, 10 (1968) 116. 13. A. AVOGADRO, G. DE MARSILY, in Scientific Basis for Nuclear Waste Management VII, G. L. MCVAY (Ed.),

Elsevier Science Publ., New York, 1984, p. 495. 14. M. CACECI, G, R. CHOPPIN, Radiochim. Acta, 33 (1983) 101, 15. D. RAI, R. G. STRICKERT, D. A. MOORE, J. L. RYAN, Radiochim. Acta, 33 (1983) 201. 16. P. ROBOUCH, P. VITORGE, Final Report Contract CEA/CEC Nr. WAS 83.3617 (Ss), in preparation (1985). 17. J. LYMAN, R. H. FLEMING, J. Mar. Res., 3 (1940) 134. 18. D, P. KARIM, P. GEORGOPOULOS, G. S. KNAPP, Nuci. Technol., 51 (1980) 162. 19. M. ANTONINI, A. E. MERLINI, R. F. THORNLEY, J. Non-Cryst. Solids, 71 (1985) 219. 20. J. V. BEITZ, J. P. HESSLER, Nucl. Technol., 51 (1980) 169. 2t. T. BERTHOUD, P. MAUCHIEN, N. OMENETTO, G. ROSSI, Anal. Chim. Acta, 153 (t983) 265. 22. W. SCHREPP, R. STUMPE, J. I. KIM, H, WALTHER, Appl. Phys., B32 (1983) 207. 23. A. CHATT, G. BIDOGLIO, A. DE PLANO, Anal, Chim. Acta, 151 (1983) 203. 24. P. BENES, V. MAJER, Trace Chemistry of Aqueous Solutions, Elsevier Science Publ., New York, 1980. 25. A. SAITO, G. R. CHOPPIN, Anal. Chem., 55 (1983) 2454. 26. T.M. FLORENCE, G. E. BATLEY, Talanta, 24 (1977) 151.

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Radionuclide chemical species determination in waste management studies