Applied C/ay Science, 1 (1985) 115--124 115 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE EFFECT OF BENTONITE ON THE INTERACTION OF I- W I T H

P b O *

D.W. OSCARSON, R. TAYLOR, H.G. MILLER and S.C.H. CHEUNG

Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba ROE 1LO (Canada)

(Accepted for publication March 11, 1985)

ABSTRACT

Oscarson, D.W., Taylor, R., Miller, H.G. and Cheung, S.C.H., 1985. The effect of bentonite on the interaction of I- with PbO. Appl. Clay Sci., 1: 115--124.

In the long-term disposal of nuclear fuel waste, the radioactive fission product 129I requires special attention. This is because of its long half-life (1.7 • 107yr), and the fact that it exists in solution as an anion (I- or IOn) and does not intereact strongly with most geological materials such as clays and rock. Mixtures of bentonite and sand are being evaluated as candidate buffer materials to surround nuclear fuel waste containers in an underground disposal vault in Canada. Since bentonite and sand sorb only minor amounts of I-, research is being conducted to identify materials that could be mixed with the buffer material to selectively 'sorb' 129 I-, and consequently retard its movement through the buffer after the waste containers are breached. PbO has been proposed as a potential buffer additive, since it has been found to be very effective in removing I- from solution. However, when bentonite is present (~ 75 wt% of the total solids) in the PbO/I- s~stem, the amount of I- removed from a solution with an initial I- concentration of 10- tool/1 is significantly decreased. On the other hand, kaolinite has little effect. The same phenomenon occurs when bentonite is physically separated from the PbO by a semi- permeable membrane. In the PbO/I- system, X-ray diffraction analysis indicates that the Pb phases present are PbO, Pb3(CO3)2(OH)2, and 7PbO'PbI2"2H20; however, when bentonite is present, 7PbO • PbI2" 2H20 is not detectable. When bentonite is treated with NaOAc to remove carbonates and then added to the Pb/I- system, the amount of I- released into solution is much less than in the system containing untreated bentonite. Furthermore, the addition of CaCO3 or a solution of NaHCO3 to the Pb/I- system causes a release of I- into solution. The data indicate that bentonite affects the stability of 7PbO'PbI2"2H20, shifting the equilibrium between it and Pb3(CO3)2(OH)2 by increasing the HCO3 activity in the system (the bentonite contains 0.29% carbonate-C). The results indicate that PbO would not be an effective additive to.a buffer material in a nuclear fuel waste disposal vault for the selective removal of 129 I- from solution.

INTRODUCTION

Research in Canada on the permanent disposal of nuclear fuel waste is focussed on the concept of waste emplacement in corrosion-resistant

*This paper is issued as AECL-8528.

0169-1317/85/$03.30 © 1985 Elsevier Science Publishers B.V.

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containers and their subsequent disposal in a vault mined deep in plutonic rock in the Ontario port ion of the Canadian Shield (Cameron, 1982). Mixtures of bentonite and sand are being evaluated as potential components of a buffer material to surround the nuclear fuel waste containers in a vault. An important function of the buffer is to retard the movement of radio- nuclides from the waste containers to the surrounding rock mass after the containers are breached. Bentonite has a number of desirable properties that make it a potentially effective buffer component ; these include a high affinity for many radionuclides, a low hydraulic conductivity, and a low ionic diffusivity {Oscarson and Cheung, 1983).

In the long-term disposal of nuclear fuel waste, the radioactive fission product 1291 is potentially the greatest hazard to man and the environment. This is because of its long half-life (1.7 • 107 yr), and the fact that it exists in solution as an anion (I- or IOn) and does not interact strongly with geological materials such as clays, sand, and rock. Therefore, of the radio- nuclides present in nuclear fuel waste, 129I has the greatest potential of reaching the biosphere before decaying to insignificant levels. Consequently, research is being conducted to identify materials that could be mixed with the buffer material, preferably in small quantities ( ( 1 wt% of the buffer), to selectively sorb and consequently retard the movement of 129 I- through the buffer after the waste containers are breached.

Many metal oxides and sulfides (e.g., PbO, PbS, Cu 2 O, HgS, and CuFeS2) effectively remove I- from solution (Bird and Lopata, 1980; Strickert et al., 1980) and therefore are potential buffer additives. With these materials, coprecipitation and/or incorporation of I- into the crystal lattice is the mechanism involved; this is indicated by the effective removal of I- from solution by materials that contain metal ions that can form metal iodides of low solubility. Even though these oxides and sulfides effectively remove I- from solution in relatively simple systems, this is not necessarily the case when a clay, such as bentonite, is present. In this paper the effect of bentonite on the PbO/I- system is described.

MATERIALS AND METHODS

Materials

The bentonite used in this s tudy is from the Bearpaw Formation in southern Saskatchewan; the formation is sedimentary, but probably derived from volcanic ash. The clay is mined by Avonlea Mineral Industries Limited, Regina, Saskatchewan. The mineralogical composit ion of this clay has been estimated from X-ray diffraction analysis by Quigley (1983) and his data are summarized in Table I. The clay has a specific surface area of 63 hm 2/kg and a cation exchange capacity (CEC) of 82cmol{+) /kg; the amounts of Na ÷, Ca 2÷, Mg 2+, and K + on the exchange complex of this clay are 47, 40, 7, and 0.7 cmol(+) /kg, respectively (Quigley, 1983). The fact that the sum of the

117

"exchangeable" cations is greater than the measured C.E.C. indicates that some dissolution of soluble salts occurred during the determination of the exchange cations.

The bentoni te was passed through a 325-mesh sieve, and the < 44-gm size fracti6n was used; a sample of this clay was treated with NaOAc (pH 5) to remove carbonates (Jackson, 1975). The clay was stored in a desiccator over silica gel; under these conditions, the moisture content of the clay was < 1%.

TABLE I

Approximate mineralogical composition of the Avonlea bentonite

Component Percentage

Montmorillonite 79 Illite 9.5 Quartz 5 Plagioclase feldspar 3 Gypsum 2 Carbonate 1.5" 1 Organic matter 0.3

• 1 A carbonate-C content of 0.29% was determined in this study by the method given by Bach and Deane (1979).

For comparison with bentonite, the effect of kaolinite on the PbO/I- system was also examined; the kaolinite (KGa-1) was obtained from the Clay Minerals Society (CMS) Source Clays Reposi tory, and its properties are given by van Olphen and Fripiat (1979). The ~ 44-#m size fraction was used.

The carbonate content of the clays was determined by measuring the CO2 evolved upon igniting a sample in an inert atmosphere at 750°C using an open-tube resistance furnace (Bach and Deane, 1979).

Lead(II) oxide was obtained from Alfa Corp and was listed as 99.9% pure; X-ray diffraction analysis indicated that it was a mixture of the dimorphs massicot (~ 95%) and litharge.

A NaI solution was prepared in deionized-distilled water (DDW) and standardized with AgNO3, using an I--selective electrode to determine the endpoint. The AgNO3 solution was in turn standardized using KI as a primary standard. The NaI solution was spiked with Na 12s I to give an initial activity of 10 to 100 MBq/1. The standard I- solution was stored in a dark bottle, and during the experiments the centrifuge tubes containing the suspensions were either covered with black tape or placed in light-proof containers, to minimize the possible oxidation of I- by UV radiation.

Experimental

Suspensions (9 ml) of bentonite or kaolinite containing 5, 10, or 25 wt% PbO (total mass = 300 mg) were placed in 16-ml glass centrifuge tubes and

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1 ml of a NaI solution was added to give an initial I- concentrat ion of 10 -5 tool/1 in a total volume of 10 ml. The experiments where PbO was present at the 5% level with bentoni te were also performed with the clay contained in a semi-permeable membrane. The membrane was obtained from Spectropore Inc. and had a pore size of 4.8 nm. Before use, the membrane was thor- oughly washed with DDW to remove any contaminating impurities.

In another experiment, 15-mg samples of PbO were allowed to react for two days with 5 ml of a 2" 10 -s -mol/1 solution of I-. Then 5 ml of a suspen- sion containing 285 mg of untreated bentonite, NaOAc-treated bentonite, or kaolinite, or 15mg of 325-mesh CaCO 3 (calcite}, or 5ml of a 2" 10-3-mol/1 solution of NaHCO3 was added. The experiment with the untreated bentoni te was repeated with the clay contained in a semi-permeable membrane.

The tubes containing the suspensions were tightly capped and placed on a vertical disc having a diameter of 20cm, which was rotated at ~ 9 revolutions/min at room temperature. After various reaction periods, the suspensions were centrifuged and 0 .2ml of the supernatant solution was withdrawn and analyzed for 12sI using a Nuclear Chicago, Mark II, Model 6844 liquid scintillation counter equipped with a thermosta t ted sample conveyor. From reference samples it was determined that the glass tubes themselves do not sorb detectable amounts of I-. The pH of the samples that contained clay was 9.0 + 0.5; in some samples without clay, the pH was slightly higher and was adjusted to 9.0 + 0.5 with HC1. The experiments were done in triplicate.

In another study, 0 .5g PbO was suspended in 0.41 of a solution containing either 10 -s or 10 -3 mol/1 of I-. After a week, 10g of bentoni te , contained in a semi-permeable membrane, were added to the suspensions; this study was done in duplicate. The systems were allowed to react for three more weeks. During the reaction period, the samples were agitated manually once or twice a day. The samples were kept in closed glass containers and the containers were opened only to add the bentonite. After the reaction period, the bentoni te was removed, dialyzed against DDW for 24 h, dried at 50°C and the amounts of Pb and I sorbed on the bentoni te were determined by wavelength-dispersive X-ray fluorescence spect rometry (XRF) and a UV spect rophotometr ic method (Grimaldi and Schnepfe, 1971), respectively. The Pb phase was also removed and dried at 50°C, and examined by X-ray powder diffraction (XRD), using Cu-Ka radiation and a Ni filter, and by scanning electron microscopy (SEM), and the I content was determined by XRF. The amount of I sorbed on the bentoni te was below the detect ion limit of XRF; consequently, the more sensitive UV spect rophotometr ic method was used for the bentoni te / I samples.

RESULTS

As shown in Fig. 1, PbO removes all the I- f rom solution within two days. However, when bentoni te is present, the rate and the extent {in the systems

1 1 9

where PbO is present at the 5 and 10% levels) of I- removal is significantly decreased (Fig. 1), even though there is as much as five times more PbO in the systems when bentonite is present. When bentonite is contained in a semi-permeable membrane, the results are similar to those obtained when a membrane is not used (Fig. 1); this indicates that bentonite is chemically rather than physically affecting the PbO/I- system.

~1o

~ 8 = $--________|,__

6

g4

0 = 1 11=2

0 - 4

0 i l i i 0 5 I0 15 2 0 2 5 3 0

Time (d)

Fig. 1. I- concentration in solution in contact with PbO and various mixtures of PbO and bentonite vs time. 1 = 15 mg PbO; 2 = 75 mg PbO and 225 mg bentonite; 3 = 30 mg PbO and 270 mg bentonite; 4 = 15 mg PbO and 285 mg bentonite; 5 = 15 mg PbO and 285 mg bentonite contained in a semi-permeable membrane.

Kaolinite has a relatively minor effect on the PbO/I- system. The rate of I- removal is only slightly decreased when PbO is present at the 5% level with kaolinite. When PbO is present at the 10 and 25% levels with kaolinite, the results obtained are similar to those in the absence of clay (data not shown). The slight decrease in the rate of removal of I- with 5% PbO may be the result of an increase in the tortuosi ty of the diffusion path between I- and PbO when kaolinite is present. Neither bentonite nor kaolinite sorbs I- in amounts detectable by scintillation counting.

When bentonite is added to a PbO/I- system where nearly all of the I- has been removed from solution, there is an immediate release of I- into solution (see Fig. 2). Similar results are obtained when bentonite is contained in a semi-permeable membrane (data not shown). The addition of kaolinite does not result in a detectable release of I- into solution.

X-ray diffraction (XRD) analysis of the solids formed when 0.5 g PbO is suspended in 0.4 1 of a 10-a-mol/1 solution of I- in the absence of bentonite indicated the presence of a Pb-I phase, along with Pba (CO3)2 (OH)2 (hydro- cerussite) and PbO (massicot and litharge) (see Table II). From XRD and thermogravimetric analysis of the solids formed in a system similar to the one used in this study, Taylor et al. (1983) reported the presence of a solid that they tentatively identified as 7PbO 'PbI2"2H20 , and their X-ray diffractogram of 7PbO ' Pb I2"2H: O matched the diffractogram obtained in this study. No Pb-I phase was detected by XRD when the initial I- concen-

120

tration was 10 -s mol/1 {Table II); however, 7PbO" PbI2 • 2H2 O should form under these conditions {Taylor et al., 1983). Therefore, it is probable that this phase was present, but in amounts below the detection limit of XRD. When 10 g of bentonite was added to the above systems in a semi-permeable membrane one week after the PbO/I- suspension had been prepared, there was no evidence, according to XRD, of a Pb-I phase at the end of the four- week reaction period.

I0

v

6

. ~

= 4 o

m

z

' - 0 0

• J . & J

I I I

I I

I

I, i/

5 I0 15

Time (dl

m=:

l = : n

; : &

Q ~ 3

1

2O

I F r I , J

1 25

Fig. 2. I - c o n c e n t r a t i o n in so lu t ion in c o n t a c t w i th PbO plus b e n t o n i t e and CaCO3 sus- pens ions and NaHCO3 so lu t ion vs t ime. 1 - - 1 5 r a g PbO; 2 = 1 5 r a g P b O + 2 8 5 r a g NaOAc- t r ea t ed b e n t o n i t e ; 3 = 1 5 r a g P b O + 1 0 - 3 m o l / l NaHCO3; 4 = 1 5 r a g P b O + 15 mg CaCO3 ; 5 = 15 rag PbO + 285 rag u n t r e a t e d b e n t o n i t e . A r r o w ind ica tes t ime w h e n clay and CaCO3 suspens ions and NaHCO3 so lu t i on were added.

T A B L E II

A m o u n t s of I Associa ted wi th the Pb Phase and Pb and I Sorbed on B e n t o n i t e for the Var ious Sys tems S tud ied

Sys tem .1 I in Pb Pb so rbed o n I so rbed on Final pH Solid phases phase b e n t o n i t e b e n t o n i t e iden t i f i ed .2 (wt%) ( m m o l / k g ) (p rao l /kg)

PbO ~ 0.06 *3 8.6 1,2 PbO, 1 0 - s I - 0 .15 -+ 0 .06 TM 8.7 1,2 PbO, 1 0 - 3 I - 3.8 + 0.4 8.7 1,2,3

PbO, b ~ 0.06 1.45 -+ 0.48 19.7 + 3.9 9.1 1,2 PbO, 10-s I-, b 0 . 1 0 - + 0 . 0 2 1 7 . 7 - + 2 . 2 1 1 8 - + 8 9.4 1,2 PbO, 1 0 - 3 I -, b 1.3 + 0.3 11.6 -+ 0 116 -+ 16 9.2 1,2 b ~ 0 .50 *3 <: 8 *3 9.2

* l P b O = 0 . 5 g PbO; 1 0 - s I - and 1 0 - 3 I - = 10 -s and 1 0 - 3 m o i I-/1, r e s p e c t i v e l y ; b = 1 0 g b e n t o n i t e ; to ta l vo lume = 0.41. *2 1 = PbO; 2 = Pba(CO3)2 (OH)2 ; 3 = 7PbO" PbI2 " 2H2 O. *3 De tec t i on limit. *4 Average -+ one-ha l f the d i f fe rence b e t w e e n the dupl ica te de t e rmina t ions .

121

No observable difference in the morphology of the solid Pb phases in any of the systems studied was detected using scanning electron microscopy.

The amount of I associated with the Pb phase is less when bentonite is present in the system (Table II); this is in agreement with the results shown in Figs. 1 and 2, where the I- concentration in solution is greater when bentonite is present.

The amount of Pb sorbed on bentonite is significantly increased when I- is present in the system (Table II). There is also a small amount of I sorbed on the bentonite (Table II). The mechanism of I sorption in these systems is not known, but it is possible that it could be sorbed as a positively charged Pb complex, such as PbI ÷.

DISCUSSION

The mechanism by which bentonite affects the Pb/I- system will be examined by considering the equilibrium relationships between the solid Pb phases that are present, i.e., PbO, Pb3 (CO3)2 (OH)2 and 7PbO" PbI2 "2H2 O.

As long as PbO is present and accessible, the I- activity is controlled by the equilibrium between it and 7PbO'PbI2 "2H20. However, Pb3(CO3)2 (OH)2 is also present (Table II), therefore, it is this phase that controls the Pb 2 + activity. Although PbO is still present, it is likely passivated by a surface layer of Pb3(CO3)2 (OH)2 (the Pb 2+ activity in these systems was found to be less than that maintained by PbO, indicating that PbO is not the saturating Pb phase) and, hence, the I- activity is controlled by the equilibrium between Pb3 (CO3)2 (OH)2 and 7PbO" PbI2 "2H20. This equilibrium reaction can be written as:

3[7PbO'PbI2 "2H20] (s) + 10 H + + 16 HCO~

8[Pb3(CO3)2(OH)2 ] (s) + 6 I- + 11 H20 log K ° = 189.9 (1)

The thermodynamic data for Pba (COa)2 (OH)2 and 7PbO" PbI:" 2H2 O were obtained from Lindsay (1979) and Taylor et al. (1983}, respectively. From reaction 1, the relationship between the I- and HCO~ activities at pH 9 can be written as:

log [I-] = 16.7 + 8/3 log [HCO3]

This equation shows that when the equilibrium between Pba(COa)2(OH)2 and 7PbO'PbI2 "2H2 O controls the I- activity, a unit change in the HCO] activity will cause the I- activity to change by a factor of 8/3. Consequently, the stability of 7PbO'PbI2"2H20 is a sensitive function of the HCO~ activity in solution.

Since the bentonite initially contains 0.29% carbonate-C, when it is present in the Pb/I- system the HCO~ activity increases, driving reaction 1 to the right. This results in an increase in the I- activity and a decrease in the amount of the solid 7PbO" PbI2 • 2H2 O phase.

122

The experimental results support the proposed mechanism. When CaCO3 or a solution of NaHCO a is added to the Pb/I- system, there is an immediate release of I- into solution (Fig. 2). Furthermore, significantly less I- is released into solution when NaOAc-treated bentonite is added to the Pb/I- system than is released when untreated bentonite is used (Fig. 2). The carbonate-C content of the NaOAc-treated bentonite is 0.08% (~ 72% of the carbonate-C was removed by the NaOAc treatment); apparently this is enough carbonate to cause the small amount of release of I- into solution that is observed in this system (Fig. 2).

The following observations are also consistent with the proposed mechanism.

(1) Increasing the mass ratio of PbO to bentonite results in a decrease in the concentration of I- in solution (Fig. 1); the less bentonite in the system, the lower the carbonate content and the greater will be the amount of 7PbO" PbI 2 • 2H 20 formed, via reaction 1.

(2) Bentonite has the same effect whether or not it is contained in a semi- permeable membrane (Fig. 1); if the proposed mechanism is correct, the presence of a membrane should have no effect since HCO~ ions can freely pass through the membrane.

(3) The carbonate-C content of the kaolinite (< 0.01%) is much lower than that of bentonite; consequently, kaolinite should have less effect on the Pb/I- system, and this is what is found.

(4) A SWy-1 montmoril lonite (carbonate-C content = 0.24%) obtained from the CMS Clay Minerals Repository was also examined for its effect on the Pb/I- system, and the results were very similar to those obtained with the Avonlea bentonite. Thus, the phenomenon is not unique to the Avonlea clay.

Significantly more Pb is sorbed on bentonite when I- is present in the system (Table II). Therefore, the possibility that bentonite is promoting the dissolution of 7PbO" PbI2 • 2H2 O by sorbing Pb should also be considered as another possible mechanism.

The equilibrium reaction between 7PbO'PbI2 "2H:O and its ions in solution can be written as:

1 / 2 [ 7 P b O ' P b I 2 " 2 H 2 0 ] ( s ) + 7H ÷ = 4Pb 2 + + I - + 9 / 2 H 2 0

When bentonite is present, it could disturb the equilibrium by sorbing Pb 2 +, and/or its hydrolysis products, and removing it from solution; bentonite is known to have a high affinity for Pb (Farrah and Piekering, 1977). Conse- quently, the reaction proceeds to the right and the amount of I- in solution increased until a new equilibrium is established.

The importance of this reaction in these systems is unknown. However, given the significant increase in the amount of Pb sorbed on the bentonite in the presence of I-, this mechanism could be at least a partial explanation for the results observed in this study. The reason for the enhanced sorption of Pb in the presence of I- will require further study.

Torstenfelt and Allard (1984) found little difference in the rate of

123

transport of I- through Wyoming bentonite (MX-80) compacted to a density of 2.0 Mg/m 3, with or wi thout the addition of 1 wt% PbO. They used a synthetic groundwater with a HCO~ concentration of ~ 2"10-amol /1 to saturate the bentonite prior to the experiment. The results reported here help to explain the ineffectiveness of PbO in slowing the movement of I- through compacted bentonite saturated with a solution that contains HCO~.

It should also be noted that the HCO~ concentration of groundwaters in the Canadian Shield is generally in the range of 10 -a to 10-4mol/1 (Fritz and Frape, 1982). PbO in contact with these groundwaters will be converted to Pb3(CO3)2 (OH)2 (or another lead-carbonate phase), which cannot remove I- from solution when it is present at trace levels (< 10 -s mol/1) because of the stability relationships of the Pb phases discussed above.

SUMMARY AND CONCLUSION

Bentonite, containing 0.29% carbonate-C, significantly decreases the amount of I- removed from solution by PbO; kaolinite has little effect. Our data indicate that, by increasing the HCO~ activity in the Pb/I- system, bentonite decreases the stability of the 7PbO" PbI2 • 2H2 O phase by shifting the equilibrium towards Pb3 (CO3)2 (OH)2.

Thus, our results indicate that PbO would not be an effective additive to a buffer material in a nuclear fuel waste disposal vault for the removal of 129 I- from solution, particularly in the Canadian Shield where groundwaters typically contain HCO~ concentrations of 10 -3 to 10-4mol/1.

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Fritz, P. and Frape, S.K., 1982. Saline groundwaters in the Canadian S h i e l d - A first review. Chem. Geol., 36:179--187.

Grimaldi, F.S. and Schnepfe, M.M., 1971. Determination of iodine in the p.p.m, range in rocks. Anal. Chim. Acta, 53:181--184.

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Strickert, R., Friedman, A.M. and Fried, S., 1980. The sorption of technetium and iodine radioisotopes by various minerals. Nucl. Techno]., 49:253--266.

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Torstenfelt, B. and Atlard, B. 1984. The retention of redox sensitive waste elements in compacted bentonite. Mat. Res. Soc. Syrup. Proc., 26: 789--795.

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