Uranium Mobility in the Natural Environment - ACS Symposium Series

17 Uranium Mobility in the Natural Environment Evidence from Sedimentary Roll-Front Deposits W. J. DEUTSCH and R. J. SERNE

Downloaded by SUFFOLK UNIV on January 21, 2018 | http://pubs.acs.org Publication Date: March 8, 1984 | doi: 10.1021/bk-1984-0246.ch017

Pacific Northwest Laboratory, Richland, WA 99352

Roll-front deposits consist of naturally occurring ore-grade uranium in selected sandstone aquifers throughout the world. The geochemical environment of these roll-front deposits is analogous to the environment of a radioactive waste repository containing redox-sensitive elements during its post-thermal period. The ore deposits are formed by a combination of dissolution, complexation, sorption/precipitation, and mineral formation processes. The uranium, leached from the soil by percolating rainwater, complexes with dissolved carbonate and moves in the oxidizing ground water at very low concentration (parts per billion--ppb) levels. The uranium is extracted from the leaching solution by the chemical processes, over long periods of time, at the interfaces between oxidized and reduced sediments. The Eh of the ground water associated with the reduced sediments (Eh = -100 mv to +100 mv) is higher than the Eh expected for most waste repository environments (Eh = -100 mv to -300 mv); this suggests that uranium solids will not be very soluble in the repositories. Data from in-situ leach mining and restoration of roll-front uranium deposits also provide information on the potential mobility of the waste if oxidizing ground water should enter the repository. Uranium solids probably will be initially very soluble in carbonate ground water; however, as reducing conditions are re-established through water/rock interactions, the uranium will reprecipitate and the amount of uranium in solution will again equilibrate with the reduced uranium minerals.

0097-6156/ 84/0246-0287S06.00/0 © 1984 American Chemical Society

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


288

GEOCHEMICAL BEHAVIOR OF RADIOACTIVE WASTE

During the past ten years, i n s i t u l e a c h mining of uranium deposits has become a commercially s u c c e s s f u l method of r e c o v e r i n g uranium from r o l l - f r o n t deposits that are e i t h e r too deep or of too low a grade to mine by conventional techniques. The r o l l - f r o n t deposits are formed through a secondary enrichment process, and the mechanism of r o l l - f r o n t emplacement provides us with i n f o r mation on the m o b i l i t y of uranium i n the n a t u r a l environment. At P a c i f i c Northwest Laboratory (PNL) we are c u r r e n t l y conducting research, sponsored by the Nuclear Regulatory Commission (NRC) i n t o methods of minimizing contamination from i n - s i t u l e a c h mining. As part of t h i s work we are studying the geochemistry of uranium i n an a q u i f e r environment, and the e f f e c t of leach mining on uranium m o b i l i t y . A l s o , we are studying the chemical i n t e r a c t i o n s between the l e a c h i n g s o l u t i o n and the sediments surroundi n g the leached ore zone. In t h i s paper we d i s c u s s the n a t u r a l occurrence of uranium i n an a q u i f e r environment and the e f f e c t of i n s i t u leach mining on uranium m o b i l i t y . We a l s o present the r e s u l t s of our l a b o r a t o r y s t u d i e s on the i n t e r a c t i o n of uraniumr i c h s o l u t i o n s and sediments c o n t a i n i n g reducing minerals. The behavior of uranium stored as nuclear waste i n a g e o l o g i c r e p o s i t o r y w i l l be p a r t i a l l y c o n t r o l l e d by the geochemical environment of the r e p o s i t o r y . The e f f e c t of ground-water l e a c h i n g on the waste can be simulated by e s t i m a t i n g a ground-water composition and then using an e q u i l i b r i u m thermodynamic computer model to simulate the i n t e r a c t i o n of the waste with the s o l u t i o n . We present the r e s u l t s of such a modeling study and d i s c u s s the e f f e c t s of s o l u t i o n pH, Eh, and temperature on the expected concentration of uranium i n s o l u t i o n . Uranium i n Ground Water A s s o c i a t e d with R o l l - F r o n t Deposits Uranium occurs as a trace element i n the major rock forming mine r a l s (quartz, f e l d s p a r , and mica) and tends to be concentrated i n accessory minerals, such as a l l a n i t e , a p a t i t e , monazite, sphene, and z i r c o n . The uranium c o n c e n t r a t i o n i n most rock types i s low. Rogers and Adams (1) have compiled a v a i l a b l e data on the abundance of uranium i n igneous and sedimentary r o c k s . They show that the average uranium c o n c e n t r a t i o n i n s i l i c i c rocks ( g r a n i t e s , r h y o l i t e s , and t u f f s ) i s approximately 5 parts per m i l l i o n (ppm), whereas i n more mafic igneous rocks the concentration i s l e s s than 1 ppm. Uranium concentrations d i f f e r with rock type because uranium i s segregated i n t o rock types c h a r a c t e r i s t i c of the l a t e r stages of p e t r o l o g i c e v o l u t i o n . In common sandstones, uranium averages about 1 ppm, whereas i n shales the concentration average i s on the order of 3 or 4 ppm. The uranium that i s found i n r o l l - f r o n t deposits i s g e n e r a l l y b e l i e v e d to be d e r i v e d from the d i s s o l u t i o n and l e a c h i n g of host minerals by s o i l water and ground water (2,3)· T y p i c a l source rocks f o r the uranium are g r a n i t e s , t u f f s , and tuffaceous sandstones that have r e l a t i v e l y high concentrations of uranium i n



17.

DEUTSCH AND SERNE

Uranium Mobility and Roll-Front Deposits

289

t h e i r minerals. Cowart and Osmond (4) s t a t e that the uranium found i n the south Texas uranium ore zones i n the O a k v i l l e Sandstone and the Catahoula Tuff came from the leaching of ash ( t u f f a c e o u s ) m a t e r i a l a s s o c i a t e d with the o r i g i n a l host sandstones. The source of uranium i n the r o l l - f r o n t deposits of Wyoming i s not known; however, tuffaceous and g r a n i t i c m a t e r i a l i s known to occur upgradient of the deposits i n most Wyoming a q u i f e r systems and may have been the host f o r the o r i g i n a l uranium. When s o i l water and ground water contact the uranium source m a t e r i a l , the minerals w i l l d i s s o l v e and leach, thereby r e l e a s i n g uranium i n t o s o l u t i o n . In the o x i d i z i n g , carbonate-bearing waters c h a r a c t e r i s t i c of ground waters near recharge zones, the uranium w i l l |>e mobile as U(VI) carbonate complexes [ I K ^ i C O ^ ^ " and U O 2 (CO^)^ ] . The uranium i s transported i n o x i d i z i n g ground water but i s removed from s o l u t i o n by chemical processes (e.g., o x i d a t i o n - r e d u c t i o n , a d s o r p t i o n , chemical p r e c i p i t a t i o n ) that take place at the i n t e r f a c e between o x i d i z i n g and reducing zones i n the a q u i f e r . Figure 1 shows the s p a t i a l r e l a t i o n between o x i d i z i n g and reducing zones i n an a q u i f e r and the presence of a r o l l - f r o n t deposit. T y p i c a l uranium concentrations i n ground water i n a q u i f e r s c o n t a i n i n g r o l l - f r o n t deposits are l i s t e d i n Table I . Although the concentrations of uranium found by i n v e s t i g a t o r s at d i f f e r e n t s i t e s diverge widely, as do the values f o r each s i t e s t u d i e d , we can frame some g e n e r a l i t i e s from the measurements reported i n Table I . The uranium concentration of ground water sampled from the ore zone i s t y p i c a l l y higher than the concentration i n waters sampled up and down the h y d r o l o g i e gradient from the uranium d e p o s i t . The low content of uranium i n the upgradient ground water r e f l e c t s the f a c t that uranium i s a trace c o n s t i t u e n t i n the source rocks, uranium i s removed from ground water at the redox i n t e r f a c e because of o x i d a t i o n - r e d u c t i o n processes that change U(VI) to U(IV) and because of mineral p r e c i p i t a t i o n a s s o c i a t e d with the low s o l u b i l i t y of U(IV) minerals, p r i n c i p a l l y u r a n i n i t e and c o f f i n i t e . The chemical system i s dynamic and the r o l l - f r o n t deposit can migrate as a r e s u l t of the ingress of ground water from the o x i d i z e d side of the redox i n t e r f a c e . Thus, uranium may be d i s s o l v e d on one side of the deposit and p r e c i p i t a t e d on the o t h e r . This e f f e c t would lead to l o c a l l y high concentrations of uranium i n the ground water at the ore zone, which i s what i s found at the s i t e s l i s t e d i n Table I . Downgradient of the r o l l f r o n t , reducing c o n d i t i o n s e x i s t and the uranium concentration i n these waters i s g e n e r a l l y the lowest of the three regions represented i n Table I . The concentration of uranium i n the sediments of r o l l - f r o n t deposits i s t y p i c a l l y i n the 1000 to 2000 ppm range ( 8 ) . The uranium occurs as coatings on grains and as i n t e r s t i t i a l m a t e r i a l ; the predominant uranium mineral i s u r a n i n i t e . C o f f i n i t e i s o f t e n


290


Table I .

Concentration of Uranium i n Parts per B i l l i o n i n Ground Water Associated with R o l l - F r o n t Uranium Deposits L o c a t i o n

G r o u n d W a t e r

P a w n e e ,

Oakville,

S o u t h

B r u n i ,

Z o n e

T e x a s

T e x a s

T e x a s

T e x a s

U p g r a d i e n t f r o m

0 . 1 t o

6 1

0 . 1 t o

3 5 0

1 0

R e d

D e s e r t ,

W y o m i n g

t o5 0

3 0 t o

2 5 0

2 0 t o

3 0 0

1 7 t o3

4 0 t o

7 6 0

5t o

2 0 0 0

t o2 5

5 3 t o

1 7 0

0 . 1 4 t o

Roll

F r o n t

O r e

Z o n e

D o w n g r a d i e n t f r o m

0 . 7 t o

6 . 8

0 . 0 2 t o

0 . 4

0 . 2 t o

< 1

3 2

3 7

3

1 . 9

Roll


F r o n t

R e f e r e n c e s

( 4 _ )

( 6 )

< 1 )

( 4 . )

reported as an accessory mineral i n r o l l - f r o n t d e p o s i t s . E q u i l i b rium c a l c u l a t i o n s o f t e n show a r e l a t i o n between the s a t u r a t i o n of the ground water with respect to u r a n i n i t e (and sometimes c o f f i n i t e ) and the presence of ore-grade m a t e r i a l ( 5 > 6 ) · Upgradient from the ore zone the o x i d i z i n g ground water i s undersaturated with respect to u r a n i n i t e , w i t h i n and downgradient of the ore zone the ground water computes to be at e q u i l i b r i u m or s l i g h t l y oversaturated. Whether ground water i s a s s o c i a t e d with source rocks c o n t a i n ing l e s s than 10 ppm uranium, as i s the case i n the recharge zones of the r o l l - f r o n t a q u i f e r s , or i f the ground water i s from an ore zone c o n t a i n i n g thousands of ppm of r e a d i l y a c c e s s i b l e uranium m a t e r i a l , the concentration of uranium d i s s o l v e d i n ground water r a r e l y exceeds 1 ppm and i s o f t e n at the low ppb l e v e l . Uranium i s g e n e r a l l y a trace c o n s t i t u e n t i n ground water, e i t h e r because of i t s low concentration i n the source m a t e r i a l or because i t s concentration i n the ground water i s l i m i t e d by r e l a t i v e l y i n s o l u b l e U(IV) m i n e r a l s . The s o l u b i l i t y of uranium minerals i s governed by the l o c a l environmental c o n d i t i o n of the a q u i f e r , and, as i s shown i n the f o l l o w i n g s e c t i o n s , a much higher concentration of uranium i n the ground water i s p o s s i b l e i f o x i d i z i n g conditions become e s t a b l i s h e d where U(IV) minerals are present. In S i t u Leach Mining of Uranium R o l l - f r o n t uranium deposits i n confined a q u i f e r systems are amenable to e x t r a c t i o n by i n s i t u leach techniques. This method of mining was f i r s t tested i n Wyoming approximately twenty years



17.

DEUTSCH AND SERNE


ago, and s i t e s i n Wyoming, Texas, New Mexico, and Colorado are now being mined commercially or are involved i n p i l o t - s c a l e t e s t i n g (9) . In s i t u leach mining of uranium involves l o c a t i n g the ore zone, d r i l l i n g a number of i n j e c t i o n and recovery w e l l s w i t h i n the ore zone, pumping l i x i v i a n t (leaching s o l u t i o n ) through the ore zone, and e x t r a c t i n g the d i s s o l v e d uranium from the pregnant l i x i v i a n t at a surface f a c i l i t y (Figure 2). The l i x i v i a n t used at most f a c i l i t i e s i s ground water that has been f o r t i f i e d with oxygen and carbon d i o x i d e . Oxygen i n the l i x i v i a n t o x i d i z e s U(IV) i n the ore-zone uranium minerals ( u r a n i n i t e and c o f f i n i t e ) to U(VI). Carbon dioxide i s added to the l i x i v i a n t because i t increases the carbonate concentration of the ground water and forms s t a b l e d i s solved carbonate complexes with U(VI). The carbonate complexation f u r t h e r increases the concentration of uranium that can e x i s t i n s o l u t i o n i n e q u i l i b r i u m with uranium minerals. Although p y r i t e i s the predominant mineral containing species with reduced valence s t a t e s i n the r o l l - f r o n t deposits, u r a n i n i t e appears to be p r e f e r e n t i a l l y d i s s o l v e d by the o x i d i z i n g l i x i v i a n t (10) . Depending on the amount of u r a n i n i t e present and ôn comp e t i t i o n by other reduced minerals f o r the a v a i l a b l e oxygen d i s solved i n the l i x i v i a n t , uranium can reach concentrations i n the range of 100 to 200 ppm i n the l i x i v i a n t . Although the amount of oxygen and carbonate i n the l i x i v i a n t used f o r i n s i t u mining i s w e l l above that of t y p i c a l confined a q u i f e r s , the p o t e n t i a l f o r m o b i l i z a t i o n of uranium by the n a t u r a l ingress of o x i d i z i n g , c a r bonate-bearing waters i s shown by the e f f e c t that the l i x i v i a n t has on the s t a b i l i t y of the reduced uranium minerals. Furthermore, the p r e c i p i t a t i o n of an o x i d i z e d uranium mineral (such as schoepite or c a r n o t i t e ) , i f i t does occur i n the leach mining system, does not s i g n i f i c a n t l y l i m i t the concentration of uranium i n s o l u t i o n . Consequently, the uranium present i n the r o l l - f r o n t deposit i s s t a b l e and r e l a t i v e l y immobile only i f reducing c o n d i t i o n s are maintained i n the a q u i f e r . I f conditions become o x i d i z ing the reduced uranium minerals can be expected to d i s s o l v e r a p i d l y and uranium w i l l be mobile. I n t e r a c t i o n of Uranium-Rich S o l u t i o n With Sediments Reduced Minerals

Containing

The l o c a l i z e d enrichment of uranium i n s o l u t i o n i n an i n s i t u leach f i e l d could be a source of contamination i n the a q u i f e r i f the pregnant l i x i v i a n t migrates from the mining zone. During mining a s e r i e s of monitoring w e l l s (shown i n Figure 2) are samp l e d to t e s t f o r unwanted movement of the l i x i v i a n t out of the leach f i e l d . A f t e r mining, the ore zone i s r e s t o r e d to a p r e determined chemical c o n d i t i o n i n accordance with r e g u l a t o r y guidel i n e s . The a q u i f e r i s r e s t o r e d through i n d u c e d - r e s t o r a t i o n t e c h niques and through the n a t u r a l solution/sediment i n t e r a c t i o n s . We


291



Figure 2. I n j e c t i o n , recovery, and monitoring w e l l p a t t e r n f o r an i n s i t u uranium mine.



17.

DEUTSCH AND SERNE


293

have studied the i n t e r a c t i o n between pregnant l i x i v i a n t and s e d i ments c o n t a i n i n g reduced minerals and have evaluated the m o b i l i t y of uranium i n such an environment. We obtained samples of pregnant l i x i v i a n t c o n t a i n i n g 52 ppm uranium from an operating i n s i t u leach f a c i l i t y i n south Texas. A l s o , the reduced sediments downgradient from the ore zone were sampled. The dominant mineral i n these sediments that contains components i n a reduced valence s t a t e i s p y r i t e , which makes up a small percentage of the sediment ( 7 ) . Marcasite i s a l s o present, but at a r e l a t i v e y low c o n c e n t r a t i o n . To simulate i n t e r a c t i o n of the l i x i v i a n t and the sediment i n the environment of an a q u i f e r , we b u i l t a flow-through column t e s t apparatus (Figure 3) that pumped l i x i v i a n t through the sediment. We monitored the pH and Eh of the column e f f l u e n t s by means of i n - l i n e sampling c e l l s equipped with glass and platinum e l e c trodes, r e s p e c t i v e l y . E f f l u e n t samples were c o l l e c t e d with a f r a c t i o n c o l l e c t o r . Three separate t e s t s were conducted w i t h columns of i d e n t i c a l diameter but d i f f e r e n t lengths: 11, 22, and 44 cm. The d i f f e r e n t column lengths allowed us to i n v e s t i g a t e the e f f e c t on s o l u t i o n chemistry of residence time i n the column and surface area of minerals contacted by the s o l u t i o n . At the flow r a t e s used i n our experiment, the residence times were a p p r o x i mately 1, 2, and 4 days, r e s p e c t i v e l y , f o r the three columns. The mineral surface area contacted by the s o l u t i o n s i s p r o p o r t i o n a l to the column lengths. Pregnant l i x i v i a n t was pumped through the 11- and 44-cm columns f o r a month and through the 22-cm column f o r 2 weeks. The uranium concentration i n the e f f l u e n t s from these columns was measured by l a s e r f l u o r i m e t r y and i s shown i n Figure 4. In t h i s f i g u r e we d i v i d e d the cumulative volume e l u t e d by the pore volume of the r e s p e c t i v e column to generate the x-coordinate of the p l o t . This normalizes the r e s u l t s on a pore volume b a s i s . Figure 4 shows that the concentration of uranium i n the e f f l u e n t s o l u t i o n d i d not reach the i n f l u e n t concentration (52 ppm) at any time during the experiment f o r any of the columns. Following an i n i t i a l peak i n uranium concentrations f o r the two shorter columns, the amount of uranium s t a b i l i z e d i n the 5 to 10 ppm range as the 2nd to 5th pore volumes were e l u t e d . A f t e r 5 pore volumes of e f f l u e n t were c o l l e c t e d from the s h o r t e s t column, the uranium c o n c e n t r a t i o n i n the e f f l u e n t dropped to the 10 to 50 ppb range. In the longest column the uranium concentration i n the e f f l u e n t r i s e s more slowly, but a l s o appears to s t a b i l i z e i n the 5 to 10 ppm range. We observed from the column data that uranium i n s o l u t i o n i s not very mobile when the s o l u t i o n contacts the sediments used i n the experiment. We expected that the o x i d i z e d uranium [U(VI)] i n the pregnant l i x i v i a n t would be reduced and immobilized by s o l u tion/sediment i n t e r a c t i o n s , and t h i s i s what happened i n the experiments a f t e r two to three pore volumes were e l u t e d . The a c t u a l removal of uranium from s o l u t i o n may occur by a d s o r p t i o n onto mineral s u r f a c e s , which produces l o c a l i z e d high concentra-



FEED FLASK

Eh pH ELECTRODES

I «

•

•

M

SEDIMENT


FLOW DIRECTION 1

ill

COLUMN 1ENT IMN

FRACTION COLLECTOR

SYRINGE PUMP

Figure 3. Test apparatus used i n column experiments of solution/sediment i n t e r a c t i o n s .

60 INFLUENT URANIUM CONCENTRATION

o ο ι

20 -

150 ML

ο

ρ.ν.

•

P.V. = 300 ML

=

Δ P.V. = 600 ML

ο

30 ο

i

ο

ο °ο

ο

5 0

• .

-

•

°ο

Uranium Mobility in the Natural Environment - ACS Symposium Series

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