Reactions Between Technetium in Solution and Iron-Containing

2

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Reactions Between Technetium in Solution and Iron-Containing Minerals Under Oxic and Anoxic Conditions T. T. VANDERGRAAF, Κ. V. TICKNOR, and I. M. GEORGE Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment, Pinawa, Manitoba ROE ILO Canada

The behaviour of technetium in the geosphere is of particular importance in nuclear fuel waste management studies because this man-made element has a long half-life and, under ambient conditions in the laboratory, is not readily sorbed on geologic materials. Autoradiographic analyses of rock and mineral thin sections contacted with TcO -containing solutions, under oxic and anoxic conditions, have confirmed that virtually no sorption takes place in the presence of oxygen. However, under anoxic conditions (< 0.2 μg/g oxygen in the atmosphere), sorption of technetium was observed on iron-oxide inclusions in ferrous-iron-containing minerals (biotite, olivine, pyroxene, hornblende) and on iron-oxide coatings on microfractures in granite, but not on the ferrous-iron minerals within the granite themselves. Subsequent static sorption tests with crushed magnetite showed that sorption is a function of the composition of the solution and of the radionuclide concentration, and again occurred only in the absence of oxygen. This behaviour is in contrast with that observed with metallic iron, which sorbs technetium strongly, even in the presence of air. These results show that technetium can be contained by magnetite in the geosphere, provided reducing conditions can be maintained. This can be aided, for example, by the incorporation of iron or iron oxides in the buffer and backfill materials in the waste disposal vault. 95m

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4

0097-6156/ 84/0246-0025S06.00/0 © 1984 American Chemical Society

Barney et al.; Geochemical Behavior of Disposed Radioactive Waste ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


26

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

The f i s s i o n i n g of U and Pu i n a nuclear r e a c t o r produces a l a r g e number of r a d i o a c t i v e f i s s i o n products. Most of these decay to s t a b l e isotopes w i t h i n a few minutes to a few years a f t e r the f u e l has been discharged from the r e a c t o r and therefore pose no problem i n the management of nuclear f u e l wastes. There are, however, a number of longer l i v e d r a d i o n u c l i d e s that must be considered i n assessing the environmental impact of any nuclear f u e l waste d i s p o s a l v a u l t i n the geosphere. For example, the f i s s i o n products technetium and promethium are unique, i n that they do not have any s t a b l e isotopes and do not occur i n nature i n measureable amounts. While promethium has a number of chemical analogues i n the other r a r e - e a r t h elements, t h i s i s not the case f o r technetium, and i t i s thus d i f f i c u l t to p r e d i c t i t s behaviour i n the geosphere. The technetium isotope of i n t e r e s t f o r nuclear f u e l waste disposal i s Tc. I t i s a pure G-emitter (E - 0.293 MeV) with a h a l f - l i f e of 2.13xl0 years. I t s high f i s s i o n y i e l d of 6% accounts f o r the r e l a t i v e l y high concentration 0.02% by weight) (1) i n f u e l discharged from a CANDU (CANada Deuterium Uranium) r e a c t o r (burnup * 650 GJ/kg U). Technetium i s a Group VII Β element. I t s chemical behaviour i s not well-known, but i s expected to f a l l between that of manganese and rhenium, and i s summarized f o r an aqueous medium i n Figure 1. Under o x i d i z i n g c o n d i t i o n s , technetium e x i s t s i n s o l u t i o n as the a n i o n i c species TcO^,, i n the 7+ valence s t a t e , and shows l i t t l e s o r p t i o n by geologic m a t e r i a l s (2-5). For t h i s reason, i n previous s a f e t y and environmental assessments of geologic d i s p o s a l of nuclear f u e l wastes, technetium has been assumed to t r a v e l at the same r a t e as moving groundwater. Under reducing c o n d i t i o n s , or i n the absence of oxygen, s o r p t i o n of technetium has been noted i n some cases. B o n d i e t t i and F r a n c i s (6) reported the removal of technetium from a nitrogen-sparged s o l u t i o n by b a s a l t and g r a n i t e , and A l l a r d et a l . (3) reported values (defined as the r a t i o of sorbed to nonsorbed concentrations) w e l l above zero, i n d i c a t i n g s o r p t i o n on f e r r o u s - i r o n - c o n t a i n i n g minerals i n contact with deaerated s o l u t i o n s » and on g r a n i t e i n a deaerated s o l u t i o n c o n t a i n i n g Fe ions. Figure 2 i s a combined E^-pH diagram f o r technetium and i r o n and shows that, under c e r t a i n c o n d i t i o n s , technetium can be reduced by ferrous i r o n . To study the r o l e of ferrous i r o n i n the removal of technetium from s o l u t i o n , experiments were c a r r i e d out with c r y s t a l l i n e rock and ferrous minerals and TcO^-containing s o l u t i o n s , under both anoxic and oxic c o n d i t i o n s . D

Experimental G e o l o g i c a l M a t e r i a l s . Granite was obtained from a quarry l o c a t e d on the Lac du Bonnet b a t h o l i t h near the W h i t e s h e l l Nuclear Re search Establishment. An o l i v i n e gabbro sample was obtained


VANDERGRAAF ET AL.

Reactions Between Tc and Fe-Containing Minerals 27


2.



28


through the G e o l o g i c a l Survey of Canada (GSC) from the Rouyn-Noranda area of western Quebec. The GSC a l s o s u p p l i e d the b i o t i t e mica, hornblende, and pyroxene samples. The o l i v i n e , epidote, i r o n oxides and i r o n oxyhydroxides were obtained from Ward's N a t u r a l Science E s t . Inc. In a d d i t i o n , some d r i l l - c o r e m a t e r i a l from the Eye-Dashwa Lakes p l u t o n near Atikokan i n northwestern Ontario was used. The chemical and m i n e r a l compositions of the g r a n i t e and gabbro specimens are given i n Tables 1(a) and 1 ( b ) . The unconsolidated m a t e r i a l was prepared by crushing the g e o l o g i c m a t e r i a l with a jawcrusher f i t t e d with tungsten carbide t e e t h to avoid contamination with m e t a l l i c i r o n , and was wet-sieved before u s i n g . T h i n s e c t i o n s of some of the rocks and minerals were made f o r s o r p t i o n / a u t o r a d i o g r a p h i c studies.

Table 1 ( a ) . Chemical Composition of Lac du Bonnet G r a n i t e and of O l i v i n e Gabbro used i n S o r p t i o n Studies Concentration i n wt% Granite 80-150 mesh

Gabbro Bulk 80-150

mesh

Oxide

Bulk

Si0

73.1 14.2 0.81 0.76 1.43 0.46 4.23 4.88 0.22 0.03 n.d.* n.d ·

76.3 12.1 0.88 1.06 1.05 0.49 3.66 5.03 0.23 0.03 n.d. n.d.

49.7 17.8 5.74 2.34 11.7 8.80 2.29 0.27 0.55 0.13 0.38 0.14

50.0 19.2 5.71 1.90 11.7 8.46 2.62 0.23 0.54 0.13 n.d. n.d.

100.09

100.83

99.78

100.49

2

A1 6 0

Q

3

FeO Fe 0. Ca6 MgO Na 0 KO TIO MnO H0 2

3

0

αδ

2

Total * n.d. • not

determined.

S o l u t i o n s . The f o l l o w i n g s o l u t i o n s were used i n one the experiments. With the exception of the d i s t i l l e d r e f l e c t the composition of the s o l u t i o n s that may be be present i n and around a nuclear f u e l waste v a u l t . chemical compositions are given i n Table I I . (1) (2)

or more of water, they expected to Their

D i s t i l l e d d e i o n i z e d water (DDW) Standard g r a n i t e groundwater (GGW), with a composition based on reported analyses of n a t u r a l l y o c c u r r i n g groundwaters a s s o c i a t e d with g r a n i t i c formations (7_).


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VANDERGRAAF ET AL.

Table 1 ( b ) .

M i n e r a l Composition of Lac du Bonnet Granite and of O l i v i n e Gabbro used i n S o r p t i o n Studies (Modal wt%)

Mineral



Lac du Bonnet g r a n i t e

Quartz K-feldspar Plagioclase Clinopyroxene Olivine Biotite Muscovite Opaques Epidote Chlorite S c a p o l i t e (?) Total

Olivine

25.8 28.1 33.9

46.2

5.2

41.7 6.5 1.2

2.5 0.7 1.5 2.3

0.3

-

0.5

gabbro

3.6

100.0

100.0

Table I I . Chemical Composition of S o l u t i o n s used i n the Technetium S o r p t i o n Studies Concentration i n mg/L Ion

GGW

Na Κ Mg Ca Sr Fe Si HC0~ CI SO, N0^ F ΗΡ0 * Humic a c i d pH J

=

GGW WN-1 SCSSS HA NAP

*

WN-1

SCSSS

HA

NAP

8.3 3.5 3.9 13.

1910 14 61 2130 24 0. 56

5 050 50 200 15 000 20

15

46

58. 5.0 8.6 0.62 0.19

68 6460 1040 33

15 10 34 260 790 50 95

6.5±0.5

7.0±0.5

7.0±0.5

85 8.2

g r a n i t e groundwater s a l i n e groundwater based on WN-1 analyses standard Canadian S h i e l d s a l i n e s o l u t i o n humic a c i d (100 mg/L) Na HP0, s o l u t i o n (140 mg/L) 4 i n e q u i l i b r i u m with PO^ and ^PO, o


8.0


30 (3)

(4)

(5)


(6)

S a l i n e groundwater (WN1), based on groundwater obtained from the 455-m l e v e l of the WN-1 borehole i n the Lac du Bonnet batholith (8). Standard Canadian S h i e l d s a l i n e s o l u t i o n (SCSSS), with a composition approximately that of s a l i n e s o l u t i o n s obtained from various sources i n the Canadian S h i e l d . Humic a c i d (HA). A 100 mg/L s o l u t i o n of the sodium s a l t of humic a c i d , to simulate groundwater c o n t a i n i n g an organic complexing agent. Na HP0^ s o l u t i o n (NAP) (140 mg/L), to provide a s o l u t i o n c o n t a i n i n g an i n o r g a n i c complexing agent to form a n i o n i c species w i t h reduced technetium ( 9 ) . T h i s anion was used instead of carbonate, as the p u r i f i c a t i o n system of the anaerobic chamber removes CO^ from the atmosphere, and a bicarbonate-carbonate s o l u t i o n would not be s t a b l e . 2

Technetium Isotopes. NH.^^VcO, ^ h a l f - l i f e 61 days), obtained from New England Nuclear, and NH^ TcO^, from Amersham-Searle, were used i n t h i s study. The presence of reduced technetium was determined by e x t r a c t i n g the Tc(7+) i n t o chloroform/tetraphenylarsonium c h l o r i d e , and assaying the aqueous phase f o r technetium a c t i v i t y . Experimental D e t a i l s and R e s u l t s . A s e r i e s of experiments was c a r r i e d out to study the behaviour of TcO, i n v a r i o u s s o l u t i o n s i n contact with a number of rocks and m i n e r a l s , under both o x i c and anoxic c o n d i t i o n s , to determine the c o n d i t i o n s that lead to removal of technetium from s o l u t i o n and the r o l e played by the v a r i o u s minerals i n t h i s process. Experiments under anoxic c o n d i t i o n s were c a r r i e d out i n a Vacuum Atmospheres Inc. anaerobic chamber c o n t a i n i n g a n i t r o g e n atmosphere, with an oxygen c o n c e n t r a t i o n of * 0.2 yL/L (as determined by a Teledyne Model 317-X trace oxygen a n a l y z e r ) . S o r p t i o n on Crushed Whole Rock under Oxic C o n d i t i o n s To determine the behaviour of TcO^ under o x i c c o n d i t i o n s ( i . e . c o n d i t i o n s s i m i l a r to those expected i n a nuclear waste d i s p o s a l v a u l t p r i o r to removal of atmospheric oxygen by g e o l o g i c and b a c t e r i o l o g i c a l p r o c e s s e s ) , crushed and s i n t e r e d g r a n i t e and gabbro,were contacted with g r a n i t e groundwater c o n t a i n i n g NH^ TcO, f o r 150 days. The p a r t i c l e s i z e of the rock was i n the range of 100-180 ym (80-150 mesh), and the s o l i d - t o - l i q u i d r a t i o was 1 g of rock to 10 mL of s o l u t i o n . ^ Τ * ^ i n i t i a l _^ technetium concentrations ranged from 3x10 mol/L to 10 mol/L. As a standard, crushed and sieved quartz was used. T h i s was obtained from a s i n g l e c r y s t a l , and washed with 6 mol/L HC1 to remove any i r o n . Small amounts (< 20 mg) of iron-metal f i l i n g s were added to one h a l f of a l l samples to check the e f f e c t of i n a d v e r t e n t l y i n t r o d u c i n g t h i s impurity during the crushing


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VANDERGRAAF ET AL.

process. The s o l u t i o n s were sampled a f t e r 60 and 150 days. The s o r p t i o n c o e f f i c i e n t s , expressed i n mL/g and defined as the r a t i o between sorbed and s o l u t i o n concentrations are given i n Table I I I . In the samples c o n t a i n i n g m e t a l l i c i r o n , s o r p t i o n was too great to give meaningful k, v a l u e s . Hence, the percent s o r p t i o n i s tabulated f o r those samples.


Table I I I .

S o r p t i o n C o e f f i c i e n t s f o r Technetium on Quartz, G r a n i t e , and Gabbro under Oxic Conditions k ** No i r o n metal present 35d 150d (mL/g) (mL/g)

Sorptionf Iron metal present 35d (%)

d

Material*

Acid-washed quartz Granite Gabbro

0.2±0.2tt 0.5±0.6 4.3±1.4

0.3±0.8tt 0.8±1.4 4.6±1.6

*

p a r t i c l e s i z e 100-180 m

**

^ d

t ft

99.6±0.3tt 99.8±0.2 99.7*0.5

(80-150 mesh).

[Tc] sorbed (mol/g) [Tc] s o l u t i o n (mol/mL)

99+% removal of the technetium from s o l u t i o n corresponds "calculated of > 2000 mL/g. e r r o r at 2σ·

to a

S o r p t i o n on G r a n i t e and Gabbro Coupons under Anoxic C o n d i t i o n s . Machined g r a n i t e and gabbro coupons, 19x19x4 mm, wçje contacted with 10 mL of g r a n i t e groundwater c o n t a i n i n g 3x10 mol/L ^c as TcO, f o r 35 days i n the anaerobic chamber. The s o r p t i o n coeff i c i e n t s , t h i s time expressed as k , where 2 , , ν moles of technetium sorbed/cm , κ. ( cm ι — - - - - ι a moles of technetium remaining i n solution/mL a

are shown i n Table IV. To determine the p o s s i b l e cause of the anomalous behaviour of one of the gabbro samples, a l l s i x coupons were autoradiographed using Kodak spectrum a n a l y s i s g l a s s p l a t e s #1 (Kodak catalogue number 156 7387). Representative autoradiographs are shown i n Figure 3, and these w i l l be discussed i n S e c t i o n 3. S o r p t i o n on G r a n i t e and Ferrous-Iron-Containing Minerals under Anoxic Conditions




32

FIGURE 3.

Photograph (a) and autoradiograph (b) of technetium sorbed on a gabbro coupon.


2.


VANDERGRAAF ET AL.

Table IV.

S o r p t i o n C o e f f i c i e n t s f o r Technetium on Granite and Gabbro Coupons under Anoxic Conditions Material

Granite


Gabbro

*

k

k * (cm) a 1 2 3 1 2 2t 3

k ** (mL/g) α

0.0031 0.0008 0.022 0.018 0.10 1.4 - 0.022

0.27 0.07 1.9 0.77 8.6 120 - 1.9

[Tc] sorbed (mol/cm ) [ T c ] s o l u t i o n (mol/mL)

a

2 ** t

k, c a l c u l a t e d assuming s p e c i f i c surface area of 43 cm Considering only the edge of the coupon with sorbed technetium.

/g.

To study the e f f e c t of f e r r o u s - i r o n - c o n t a i n i n g minerals on TcO^ i n s o l u t i o n , g r a n i t e , b i o t i t e , hornblende, epidote, o l i v i n e and pyroxene t h i n s e c t i o n s were^çontacted with g r a n i t e groundwater c o n t a i n i n g * 3x10 mol/L ^ c , again as TcO, , f o r one week i n the anaerobic chamber, and subsequently autoradiographed using the procedure o u t l i n e d elsewhere (10). These p a r t i c u l a r g r a n i t e samples were obtained at depths of 72 and 1074 metres from the ATK-1 borehole i n the Eye-Dashwa Lakes pluton near Atikokan, northwestern O n t a r i o . T h i s pluton has been used i n geochemical and h y d r o g e o l o g i c a l s t u d i e s of l a r g e i n t r u s i v e rock formations i n the Canadian S h i e l d . The samples were chosen because they c o n t a i n iron-oxide i n f i l l i n g s i n minute f r a c t u r e s , due to hydrothermal a l t e r a t i o n of the primary minerals of the rock matrix. The minerals were s e l e c t e d because they c o n t a i n F e ( I I ) (see Table V ) . Some r e p r e s e n t a t i v e autoradiographs of t h i n s e c t i o n s contacted i n the anaerobic chamber are shown i n Figures 4 to 6.

Table V.

Ferrous-Iron-Containing M i n e r a l s used i n Technetium S o r p t i o n Studies

Mineral Biotite Olivine Pyroxene Hornblende Ilmenite

General Chemical

Formula

K ( M g , F e T ) . S i A10 (0H) (Mg,Fe^) SiO^ (Ca,Fe )SiO~ .. (Na Ca )(Mg,Fe ) ( A l , F e )(Si.A10 Fe T i 0 2

)

o


(OH)


34


FIGURE 4.

Photograph (a) and autoradiograph (b) of technetium sorbed on a b i o t i t e mica t h i n s e c t i o n .


VANDERGRAAF ET AL.



2.

FIGURE 5.

Photograph (a) and autoradiograph (b) of technetium sorbed on a hornblende t h i n s e c t i o n .


36



FIGURE 6.

Photograph (a) and autoradiograph (b) of technetium sorbed on a g r a n i t e t h i n s e c t i o n from the 1074~m l e v e l i n the Eye-Dashwa Lakes p l u t o n .


2.

VANDERGRAAF ET AL.


S o r p t i o n on I r o n Oxides and I r o n Oxyhydroxides under Oxic and Anoxic Conditions Since i r o n oxides and i r o n oxyhydroxides are commonly found as f r a c t u r e - i n f i l l i n g m a t e r i a l s , samples with a p a r t i c l e s i z e of 100 to îeij^nm were^çontacted with g r a n i t e groundwater c o n t a i n i n g * 5xl0" mol/L ^ T c , as TcO^, f o r 30 days i n a i r , and i n the anaerobic chamber, and the s o l u t i o n s sampled p e r i o d i c a l l y . In a d d i t i o n , s y n t h e t i c hematite was used, prepared by adding 2 mol/L ΝΗ,ΟΗ to 1 mol/L Fe(N0«)^, washing and d r y i n g the p r e c i p i t a t e at 105 C f o r one day, followed by heating at ^0Q°C f o r two days. The minerals were analyzed f o r t o t a l i r o n , Fe and s u l f u r , and t h e i r concentrations are l i s t e d i n Table VI. Figures 7(a) and (b) show the decrease i n technetium c o n c e n t r a t i o n i n s o l u t i o n as a f u n c t i o n of time, under both oxic and anoxic c o n d i t i o n s .


i Z

m

V

|

Table VT.

T o t a l Iron, F e ( I I ) and S u l f u r . Concentrations "Opaques" (Ilmenite, Iron Oxides, and Iron Oxyhydroxides used i n Technetium Sorption Studies Concentrations i n wt%

Mineral

Formula

Ilmenite Goethite Hematite Specular Hematite Liraonite Magnetite Synthetic Hematite**

Fe TiO~ FeOOH Fe 0 Z J

* **

in

S

Fe(II)

Fe 40.8±0.1 55.1±0.7 68.5±0.3

0.9±0.3* 0.062±0.009 0.33±0.3

0.55±0.01 < 0.01 0.01±0.01

2°3 Fe OOH Fe0»Fe 0o

49.1±0.5 21.8±0.1 65.3±0.4

0.45±0.28 0.027±0.004 20.3±0.1

< 0.01 0.02±0.01 < 0.01

F e

72.6±2.2

< 3x10

F e

o

2°3

sample d i s s o l u t i o n prepared at WNRE.

J

—

incomplete.

E f f e c t of Groundwater Composition on S o r p t i o n on Magnetite under Anoxic Conditions One-gram samples of crushed, sieved magnetite ( p a r t i c l e s i z e 100 to 180 ym) were contacted with the s i x s o l u t i o n s described e a r l i e r i n the anaerobic chamber f o r 50_jjlays. The i n i t i a l technetium concentrations ranged from 3x10 mol/L to 10 mol/L, and the s o l i d - t o - l i q u i d r a t i o was 1 g/10 mL. The r e s u l t s f o r the two extreme s t a r t i n g concentrations are s^jiown j.n Figures 8(a) and ( b ) . At the end of the experiment, the Fe /Fe ratios i n solution were measured to estimate ^jie E^. In a l l cases where t h i s r a t i o could be measured, Fe /Fe > 5, i n d i c a t i n g reducing c o n d i t i o n s . 1


38

GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

\

•

ο υ Η 0.6


1

Magnetite Goethite Ilmenite Limonite Spec. Hematite Nat. Hematite Synth. Hematite Iron Metal

\ ι—I

0.4

10

15

20

30

25

t (days)

0

5

10

15

20

25

30

t (days)

FIGURE

7.

Decrease

in

time

for

solutions

technetium

iron

minerals

under

concentration

i n contact (a)

oxic

as a f u n c t i o n

with iron and (b)

metal

anoxic

of

and

conditions.


2.


VANDERGRAAF ET AL.

i 2

Initial [ T c ] : 3 x l d m o l / L Distilled Oeionized Water Granite Groundwater WN-I Saline Solution Std. Can. Shield Sal. S o i n


2

HPO4-

mic Acid -

10

20

30

I40mg/L

lOOmg/L

50

40

t (days)

Initial C T c D : l O "

20

4

mol/L

30 (tjdays

F I G U R E 8.

Decrease i n technetium c o n c e n t r a t i o n as a f u n c t i o n time for s i x s o l u t i o n s i n contact w i t h magnetite (FeO.Fe2Û3). I n i t i a l technetium concentration 3 χ 1 0 " m o l / L ( a ) a n d l x 10" mol/L (b). 1 2


of


40


Discussion The r e s u l t s obtained f o r crushed g r a n i t e and gabbro under o x i c c o n d i t i o n s , compared to those obtained f o r the acid-washed quartz (see Table I I I ) , c l e a r l y show that no s i g n i f i c a n t s o r p t i o n takes place on g r a n i t e , but that some technetium can be removed from s o l u t i o n by gabbro. Since small amounts (< 20 mg) of m e t a l l i c i r o n are able to remove as much as 99+% of the technetium from s o l u t i o n , any s o r p t i o n observed f o r g r a n i t e over the time s c a l e of the experiment may be a t t r i b u t e d to the presence of m e t a l l i c i r o n , i n a d v e r t e n t l y introduced i n the crushing and g r i n d i n g processes. In the absence of oxygen, again no s i g n i f i c a n t s o r p t i o n was noted f o r g r a n i t e . However, i n one case, some s o r p t i o n was observed f o r gabbro. Pétrographie a n a l y s i s of the area of the gabbro coupon that showed s o r p t i o n , as revealed by an autoradiograph (Figure 3 ) , i n d i c a t e d the presence of a t h i n i r o n - o x j ^ e band. Thus, even though gabbro contains ^ 4 2 % pyroxene ((Ca,Fe )SiOg), the ferrous i r o n i n t h i s mineral i s not able to remove the technetium from s o l u t i o n as e f f e c t i v e l y as the smaller amount of "opaques" ( i r o n oxides and i l m e n i t e ) . The i r o n oxide i s not d i s t r i b u t e d homogeneously throughout the gabbro rock matrix, and t h i s accounts f o r the wide v a r i a t i o n i n technetium removal from one coupon to the next. Crushing the rock f r e e s these opaques and d i s t r i b u t e s them more uniformly, and i t i s most l i k e l y that t h i s m a t e r i a l i s r e s p o n s i b l e f o r the s i g n i f i c a n t k^ values obtained with crushed gabbro under oxic c o n d i t i o n s . The autoradiographs of the rock and mineral t h i n s e c t i o n s (Figures 4 to 6) a l s o con££rm the importance of i r o n oxides: although b i o t i t e (K(Mg,Fe ) ^ S i ^ A 1 0 . ( 0 H ) ) and hornblende ((Na,Ca )(Mg,Fe ) ( A l , F e ) I S i ^ A 1 0 ^ ) ( O H ) ) contain ferrous i r o n , s o r p t i o n appears to take place s o l e l y on the small opaque ( i r o n - o x i d e ) i n c l u s i o n s . In the case of b i o t i t e , these oxides are l o c a t e d between the basal planes, and are randomly d i s t r i b u t e d i n the hornblende. S i m i l a r d i s t r i b u t i o n s are observed f o r o l i v i n e , pyroxene, and epidote. The r e s u l t s f o r pyroxene f u r t h e r confirm the low s o r p t i o n r e s u l t s obtained with gabbro, where i t i s one of the major m i n e r a l s . The autoradiographs a l s o show some s o r p t i o n on the g r a n i t e obtained from the Eye-Dashwa Lakes pluton ( F i g u r e 6 ) . Even though the i r o n c o n c e n t r a t i o n i s low, hydrothermal a l t e r a t i o n of the g r a n i t e has r e s u l t e d i n the i n f i l l i n g of the minute f r a c t u r e s i n the m i c r o c l i n e f e l d s p a r with i r o n oxides, which show technetium s o r p t i o n , while there i s no s o r p t i o n on the b i o t i t e c r y s t a l s . Thus, while f r e s h , unaltered g r a n i t e matrix rock has l i t t l e or no i r o n i n the form of i r o n oxides, a l t e r a t i o n zones around f r a c t u r e s do, and technetium s o r p t i o n may occur there. I t should a l s o be noted that s o r p t i o n of technetium i s l i m i t e d to s p e c i f i c mineral s u r f a c e s . Thus, i t appears that the r e d u c t i o n of TcO, to a lower o x i d a t i o n s t a t e occurs at or near the surface of the Iron oxide and not i n the bulk of the s o l u t i o n , by d i s s o l v e d f e r r o u s i o n s . n

2

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2

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2.

VANDERGRAAF ET AL.

Reactions Between Tc and Fe-Containing Minerais 41

The experiment i n v o l v i n g a s u i t e of iron-oxide minerals showed t h a t , with the exception of the s y n t h e t i c hematite, some s o r p t i o n took place on a l l minerals under anoxic c o n d i t i o n s and a l s o , i n some cases, i n the presence of a i r ( F i g u r e s 7(a) and (b)). Subsequent chemical analyses i n d i c a t e d small amounts of F e ( I I ) i n a l l n a t u r a l minerals, although g o e t h i t e , l i m o n i t e , and hematite should not contain any. These minerals may have been formed by the o x i d a t i o n and hydration of magnetite (FeO Fe 0^) and therefore contain r e s i d u a l amounts of unoxidized F e ( I I ) . At any r a t e , t h i s points out the importance of using p u r i f i e d and/or chemically analyzed m a t e r i a l . I t i s again noteworthy that the r a t e of s o r p t i o n on i l m e n i t e i s lower than that on n a t u r a l hematite, even though the former contains more F e ( I I ) . This can again be explained i n terms of a v a i l a b i l i t y of the ferrous i r o n , as was shown to be the case with the t h i n s e c t i o n s . Sorption on magnetite as a f u n c t i o n of groundwater composition shows that, under anoxic c o n d i t i o n s , technetium removal from s o l u t i o n i s e s s e n t i a l l y complete a f t e r 50 days, with the exception of s o l u t i o n s containing phosphate i o n s . As pointed out e a r l i e r , phosphate was used instead of carbonate, as both are known to form a n i o n i c complexes with Tc(IV) ( 9 ) . In these s t u d i e s , the presence of humic a c i d d i d not a f f e c t i t s s o r p t i o n . Strong s a l i n e s o l u t i o n s (up t o 34 000 mg/L CI) do not have a marked e f f e c t on the rate of technetium removal from s o l u t i o n e i t h e r , as evident from Figure 8.


e

Conclusions The r e s u l t s reported i n t h i s paper show that technetium i s removed from s o l u t i o n by i r o n oxides, and not by minerals c o n t a i n i n g ferrous i r o n as an i n t e g r a l part of t h e i r c r y s t a l l a t t i c e , such as b i o t i t e , pyroxene, or hornblende. I t was shown that there are cases where the small amount of i r o n i n g r a n i t e has a greater e f f e c t i n removing technetium from s o l u t i o n than the l a r g e r amounts of i r o n i n gabbro. Reduction of technetium occurs c l o s e to the mineral surface, and not i n the bulk of the s o l u t i o n , by d i s s o l v e d f e r r o u s i o n s . Technetium i s removed from groundwaters having widely d i f f e r e n t l e v e l s of t o t a l d i s s o l v e d s o l i d s , and i t s removal i s only a f f e c t e d by ligand-forming anions with a strong a f f i n i t y f o r technetium, such as phosphate (and presumably a l s o carbonate)· The s i g n i f i c a n c e of t h i s study f o r nuclear f u e l waste d i s p o s a l i s that i r o n - o x i d e - c o n t a i n i n g f r a c t u r e s i n hydrothermally a l t e r e d g r a n i t e are capable of sorbing technetium. Technetium transport i n the f a r - f i e l d region of a waste d i s p o s a l v a u l t can thus be impeded by iron-oxide coatings on h y d r o l o g i c a l l y conducting f r a c t u r e s u r f a c e s . I f necessary, r e t e n t i o n of technetium i n the n e a r - f i e l d region can be improved by i n c o r p o r a t i n g rock containing l a r g e amounts of i r o n oxides i n the b a c k f i l l material.


GEOCHEMICAL BEHAVIOR O F RADIOACTIVE WASTE

42

The presence of s a l i n e s o l u t i o n s at depth i n plutons i n the Canadian S h i e l d (11) should not be d e t r i m e n t a l t o r e t a i n i n g technetium i n the v a l u t s i n c e the experiments showed that technetium removal occurs from h i g h l y s a l i n e s o l u t i o n s . Throughout t h i s paper, r e f e r e n c e has beem made to reduced technetium s p e c i e s . Although i t has been assumed that Tc(IV) i s formed, there i s no d i r e c t evidence f o r t h i s . Experiments are now underway to determine the nature of the sorbed species u s i n g F o u r i e r Transform I n f r a r e d Spectroscopy, Downloaded by NORTH CAROLINA STATE UNIV on December 31, 2017 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch002

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Acknowl e dgment s The authors would l i k e to acknowledge the a s s i s t a n c e of J . Paquette and N. G a r i s t o i n p r o v i d i n g the E -pH diagrams f o r technetium and i r o n and of D,C. Kamineni, who performed the pétrographie analyses. The minerals from the GSC were obtained through i t s c u r a t o r , H.R. Steacy. R.F. Hamon, B.L. S a n i p e l l i and K. Ross performed the v a r i o u s chemical analyses reported i n t h i s paper. n

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Clegg, L. J; Coady, J. R., "Radioactive Decay Properties of CANDU fuel Volume 1. The Natural Uranium Fuel Cycle," AECL-4436/1, Chalk River, Ontario, Canada, 1977. Palmer, D. Α.; Meyer, R. Ε., "Adsorption of Technetium on Selected Inorganic Ion-Exchange Materials and on a Range of Naturally Occurring Minerals under Oxic Conditions," J. Inorg. Nucl. Chem. 1981, 43, 2979. Allard, B; Kipatsi, H.; Torstenfelt, Β., Radiochim. Radioanalyt. Letters, 1979, 37, 233. Sheppard, M. I.; Vandergraaf, T. T.; Thibault, I. M.; Reid, J. Α. Κ., Accepted for publication in Health Physios. Landa, E. R.; Thorvig, L. M.; Gast, R. H., J. Environ. Qual. 1977, 6, 181. Bondietti, Ε. Α.; Francis, C. W., Science 1979, 203, 1337. White, D. E.; Hem, J. D.; Waring, G. Α., "Chemical Composition of Subsurface Waters," In: M. Fleischer (Editor), Data of Geochemistry, U.S. Geolog. Survey Prof. Paper, 440-F, 6th ed. 66 pp. 1963. Analytical Science Branch, "Chemical Analysis of the Initial Groundwater Samples from the Lac du Bonnet Batholith," Whiteshell Nuclear Research Establishment, Pinawa, Manitoba, Canada, 1979. Paquette, J . ; Reid, J. A. K.; Rosinger, E. L. J., "Review of Technetium Behavior in Relation to Nuclear Waste Disposal," TR-25, Whiteshell Nuclear Research Establishment, Pinawa, Monitoba, Canada, 1980.


2. VANDERGRAAF ET AL. Reactions Between Tc and Fe-Containing Minerals 43

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Reactions Between Technetium in Solution and Iron-Containing

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