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Selenate-selenium sorption on a Columbia river basalt (Umtanum basalt, Washington, U.S.A.)
C h e m i c a l G e o l o g y , 43 (1984) 287--302
287
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SELENATE-SELENIUM SORPTION ON A COLUMBIA RIVER BASALT (UMTANUM BASALT, WASHINGTON, U.S.A.)*
L.L. AMES', P.F. S A L T E R 2, J.E. M c G A R R A H '
and B.A. W A L K E R '
'Pacific Northwest Laboratories, Richland, W A 99352 (U.S.A.) 2Rockwell Hanford Operations, Richland, W A 99352 (U.S.A.) (Received March 7, 1983; revised and accepted September 23, 1983)
ABSTRACT Ames, L.L., Salter, P.F., McGarrah, J.E. and Walker, B.A., 1984. Selenate-selenium sorption on a Columbia River basalt (Umtanum basalt, Washington, U.S.A.). Chem. Geol., 43: 287--302. Sodium selenate and Na 27sSeO4 were used with a synthetic groundwater and hydrazine to determine selenate-selenium sorption characteristics of crushed Umtanum basalt between 40 ° and 60°C. Selenium sorption kinetics from both oxidizing and reducing solutions followed an equation of the type: CI-
C = Kt b
where CI is the initial selenium concentration in molarity; C is the selenium concentration in molarity at time t in hours; and K and b are constants. Relatively constant b-values of a wide CI range allowed K determination over the whole initial concentration range investigated (CI ffi ~ 1 0 - ' - - 1 0 -'2 M). Selenium desorption kinetics in 60°C oxidizing systems were described by:
c =Ad where A and d are constants and other quantities defined above. Equilibrium results conformed to a Freundlich sorption isotherm. The enthalpy change was independent of temperature within the range studied and yielded AH-values of from +27.23 kcal. m o l - ' at 1.266 • 10 -'4 tool Se/g basalt to +28.47 kcal. tool-' at 2 . 0 6 1 . 1 0 -9 mol Se/g basalt. Chemisorption was indicated by the magnitude of the enthalpy change. Desorption kinetics at 60°C were only modestly less rapid than sorption kinetics, suggesting a relatively small sorption activation energy. Reduced (Eh ffi --150 to --200 mV) selenium did not desorb.
INTRODUCTION High-level nuclear reactor wastes contain a variety of radionuclides of varying toxicities, concentrations and transport hazards. Efforts have been *Prepared for Rockwell Hanford Operations, Prime Contractor to U.S. Department o f Energy under Contract DE-AC06-77RL01030. 0009-2541/84/$03.00
© 1984 Elsevier Science Publishers B.V.
288 made to obtain an overall ranking o f radionuclides that include the above variables (Barney and Wood, 1980). Selenium can usually be found in the top ten of such key radionuclide rankings. Dissolved selenium exhibits the three oxidation states of selenate (6+), selenite (4+) and selenide ( 2 - ) as a function of oxygen partial pressure. However, kinetic effects often result in the persistence of thermodynamically unstable species. Recent work by Cutter (1982) in sampling Saanich Inlet waters by Vancouver Island, British Columbia, Canada, showed a steady decrease of the Se(VI) to Se(IV) ratio with increasing depth. Though the Se(IV) oxidation to Se(VI) may be a relatively sluggish transition, it does occur in natural environments that range from oxidizing to reducing conditions. Thus selenate-selenium may be generated during the transient oxidizing environment present in the repository following closure. The present work was confined to selenate-selenium sorption reactions with U m t a n u m flow basalt (Washington, U.S.A.) and was undertaken as a step in assessing the migration potential of selenate-selenium through the basalt geohydrologic system. However, the results may be of interest to agronomists (Davies and Watkinson, 1966; Cary and Allaway, 1969; Gissel-Nielsen, 1973), who have applied various forms of selenium-bearing to selenium-deficient softs sometimes containing basalt particles, and to geochemists involved in exploration for various ores including uranium (Riese et al., 1979). MATERIALS USED A synthetic groundwater composition, based on chemical analyses of natural groundwaters found just above the U m t a n u m basalt, was used in the sorption work. The analysis of this synthetic groundwater (GR-3) is given in Table I. Reagents used in makeup of GR-3 included deionized water, Na2CO3, Na2SiO3.9H~O, NaOH, KC1, Na2SO4, NaF, HC1, CaC12 and MgC12. The synthetic GR-3 was made up as required and n o t stored. Radioactive ~SSe with a 120-day half-life was used as a selenium tracer. It was present as sodium selenate. All counting data older than two days were TABLEI Synthetic groundwater composition l- 1)
Constituent
(mg
Na ÷ K÷ Ca:÷ Mg2+
358 3.43 2.78 0.032
SiO 2
Constituent
(mg 1-1)
FC1SO~Alkalinity (as HCO~)
33.4 312 173 1.82 meq 1-1
76.2
pH -- 9.77 at 25°C; pH = 9.50 at 45°C; pH = 9.20 at 65°C. Ionic strength -- 0.03425.
289
decay-corrected due to the relatively short half-life of 7SSe. Nonradioactive sodium selenate was used with the selenium tracer to augment initial solution selenium concentrations above that of the 7SSe tracer (~5 • 10 -12 M 7SSe). Hydrazine (N2H4) was added to the GR-3 synthetic groundwater to create a reducing environment. Hydrazine reacts with oxygen to form nitrogen and water, N2H4 + O2 -~ N2 + 2H20, and is used extensively as a corrosion control agent in boilers and hot-water systems. The reaction is relatively slow at room temperature. Dibasic hydrazine is a slightly weaker base than ammonium, forming salts with inorganic acids. Hydrazine and its salts are good reducing agents as indicated by their standard redox potentials: N2H4 + 4OH- -~ N2 + 4H20 + 4e-,
E ° = +1.16 V
and N2Hs ÷ -+ N2 + 5H ÷ + 4e-,
E ° = +0.23 V
although hydrazine is clearly a better reducing agent in alkaline solutions. Hence, hydrazine is able to reduce Se(VI) and Se(IV) to Se metal. One of the disadvantages of using hydrazine for potential control is that it dissociates in basic aqueous solutions producing hydrazinium ions as follows (Hudson, 1967): [N2H~] [OH-] N=H4 ~ N2H; + OH-,
N2H~ -* N2H~÷ + OH-,
[N2H4] [N2H~÷] [OH-]
= 8.6 • I0 -~
= ~ 9 . 4 • 10 -is
[N2H:] Hence, if an ion-exchange reaction is under study, the hydrazinium ions m a y themselves compete with the lower radionuclide concentrations for cation-exchange sites on the basalt (Hayes et al., 1982). Measurements of the basalt--GR-3 groundwater system in a low-oxygen atmosphere chamber (< 0.1 ppm O2) with a Pt electrode consistently yielded potentials of from - 3 0 0 to - 4 0 0 mV at pH 9.0 and 25°C. Quinhydrone was used as a standard. The oxygenated solutions, on the other hand, contained from an average of 5.30 mg dissolved O2/1 at 60°C to 8.25 mg dissolved O2/1 at 25°C as measured with a calibrated oxygen diffusion membrane, and changed little over a 31-day period. Thus the potential at 25°C and pH 9.0 ranged from +550 to +650 mV. U m t a n u m basalt, a flow from the Columbia River Group (Ledgerwood et al., 1978), was collected at a field outcrop, crushed and screened to 0.85-0.30 m m in size (Hodge and Grutzeck, 1978). This fraction was washed, contacted three times with nonradioactive GR-3, dried at room temperature and stored in a low-oxygen (< 0.1 ppm 02) environment for later use. The properties o f the U m t a n u m basalt are summarized in Table II. For further in-
290
formation, consult Deju et al. (1978). The surface area was derived by nitrogen sorption at four relative nitrogen pressures and extrapolation to zero relative pressure (Lowell, 1979). The m e t h o d proposed by Heilman et al. (1965) was used to obtain an ethyleneglycolmonoethyl ether surface area on the same material. The t w o specific surface areas apparently do not measure the same surfaces because the BET measurement is usually considerably lower than the ethylene glycol m o n o e t h y l ether measurement. The Cs ÷ cation-exchange capacity is relatively low in comparison to smectites (60--90 meq/100 g) and other layer-silicates and zeolites. The groundmass was defined as the residual system remaining after the basalt cooled. It consists mainly of crystaUites of the more sodic plagioclases embedded in a glassy matrix composed of silica (50--75 wt.%), alumina (12--15 wt.%), potassium (2--8 wt.% K20) and other minor constituents (Ames, 1980). TABLE II Characterization of the Umtanum basalt used in the sorption work Size range (mm)
Mineralogy (vo1.%)
0.85--0.30
groundmass 38 plagioclase 26 pyroxenes 21 secondary minerals 8 metallic oxides 7
Specific surface (m ~ g-l)
Cesium cationexchange capacity, pH 7.0, 25°C (meq/100 g)
2.67
1.83 ± 0.02 ( l a )
Ethylene glycol monoethyl ether specificsurface
17.7 + 3.8 (lo)
Typical chemical analysis Oxide
(wt.%)
Oxide
SiO~ A1203 F%O 3 FeO MnO CaO MgO
55.64 13.62 2.00 10.68 0.20 7.17 3.33
Na20 K~O TiO 2 P205
3.30 1.62 2.05 0.39
Total
100.00
METHODS
(wt.%)
OF INVESTIGATION
To determine selenium sorption kinetics, 10 g of 0.85- to 0.30-mm Umtanum basalt were placed in a 125-ml Wheaton ® borosilicate glass serum bottle containing 100 ml of GR~q synthetic groundwater plus the 75Se and nonradioactive NacSeO4, b o t h at the selected system temperature. The Wheaton ® serum bottle was sealed with a Teflon ® -faced silicone septum and
291
secured with a crimped aluminum seal. Reducing solutions also contained ~ 0 . 0 5 M N2H4 and were made up and sealed in an anoxic (<0.1 ppm O~) chamber. Timed samples were taken after basalt--solution contact at temperature with a stainless-stee ! needle and syringe through the septum that resealed upon withdrawal of the needle, thus minimizing atmospheric contact. All samples were immediately filtered through a 0.22-#m filter unit that fit onto the end of the syringe. New Brunswick ® incubator--rotators were used for agitation and temperature control between samples. Sampling of solution for radionuclide removal rates was carried out at timed intervals. Upon approach to equilibrium, a last solution sample was taken to establish the radionuclide load on the basalt. The remaining solution was then removed and discarded, the basalt rinsed with methanol, 100 ml of GR-3 groundwater containing no radionuclide added at system temperature and the Wheaton ® bottle resealed. This operation was carried out in the anoxic chamber for the reducing systems. A sample for comparative counting was removed from the solutions before they were contacted with the basalt. All 75Se samples were gamma-counted on a sodium iodide crystal whose geometry was known. From the specific activity, a concentration of selenium could be obtained, assuming that the sorption behavior of the tracing 7SSeO~- and nonradioactive SeO~- was identical. The various solution containers were checked for selenium sorption with KOH. Container sorption was generally less than 2% of the contacting solution in the presence of basalt. Because all samples for counting were filtered, filtered and unfiltered samples of 40°C reduced selenium experiments at 1200 hr. of solution--basalt contact were taken and counted with the results shown in Table III. Unfiltered samples averaged ~3.72% higher than the 0.22-pm filtered samples. Hence, very little of the Se ° existed in solution as a colloid greater than 0.22 g m in size, and the selenium, filterable or unfilterable, was n o t in solution. T A B L E III C o m p a r i s o n o f filtered ( 0 . 2 2 u m ) a n d u n f i l t e r e d s o l u t i o n s a m p l e c o u n t i n g d i f f e r e n c e s for s e l e n i u m s o r p t i o n o n 20- t o 5 0 - m e s h U m t a n u m b a s a l t f r o m r e d u c i n g GR-3 g r o u n d w a t e r at 40°C at 1 2 0 0 hr. o f c o n t a c t C!
(100)(C, unfiltered)-
(C, filtered)] % d i f f e r e n c e
(M) 1.000
• 10-'
1.000 • 1 0 -8 1.440 o 10 -1' 4 . 4 3 7 • 10 - ' : M e a n -+ l o
+5.51 +3.51
+1.10 +4.75 + 3 . 7 2 -+ 1.93
RESULTS AND DISCUSSION
Selenate kinetic sorption isotherms on 0.85- to 0.30-mm U m t a n u m basalt were determined at 40 ° and 60°C for four selenate ion concentrations under
292 oxidizing and reducing conditions. The sorption kinetics for oxidizing and reducing conditions at 60°C are given in Figs. 1 and 2, respectively. The oxidizing kinetic sorption data show that equilibrium was approached and that the percentages of selenium CI sorbed with time are approximately the same over nearly a seven-fold selenate ion concentration range. -IO
'
r
'
I
'
I
'
u
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'
)
'
l
=
]
m .__.._._m ..------m ..._._.m _ _ m
-14
/
' •
•
•
•
m m--
_ mm_.__m_..._ m_._.- m CI= 1.000 x 10-5M Se
=~i - 1 8 A ~
5~
c -22
" .----- A
A- - ' - ' A ' - - A - ' - - j
~
A
~
A
A~
A_.._...~ & -
CI= 1.000 x 10-VM Se
CI= 4.393 x 10-12M Se CI= 1 . 4 3 2 x 10-11M Se \ _.._ x ~ x
-26
X .....=..X ...~... X..........-X
-30
"rO"d" -2 -1
i
T 0
l
....= ~
l 1
=
xX
~
~
x,_~...,x~ x , . . ~ o------- o
x x • •
• •~O
O ~
I 2
,
l 3
'
l 4
,
I 5
=
l 6
,
7
In t, hr Fig, 1. Sorption kinetics of selenium on Umtanum basalt in oxidizing GR-3 groundwater
at 60°C. -11
'
I
'
I
'
I
'
I
'
]
' I • n o , ~
' I ' I. u ''-'-" n" u - ' -
m~
-15
• l
m ~
m ~ m "
(b)
CI= 1 . 0 0 0 x 10-SM Se
(a)
-19
AT"--
A'A~CI _= - 2 3
m'
A'A~
A--A
1.000 X 10"SM Se
& ~ & ~ A Cl= 1.418 x 10-11M Se
x ~ X " - - - ~ . x _ _ x -, x e: •
-
_x~
x~x~e~
e'~
-31 -2
-1
0
1
CI= 4,215 x 10-12M Se i , h, 2 3 In t, hr
. 4
, ,
, 5
6
7
Fig. 2. Sorption kinetics of selenium on Umtanum basalt in reducing GR-3 groundwater at 60°C.
293
The reducing selenium kinetic sorption results indicate that 100% selenium removal was probable even at 40°C, presumably as selenium metal at the potential of the system. After ~ 4 6 days, the U m t a n u m basalt used for sorption was desorbed after removing all of the radioactive equilibrium solution and adding a 60°C, nonradioactive solution of the same composition. The results are given in Fig. 3. I
'
I
'
I
'
I
'
I
'
__._ n.___ n - - n-----'_=
-15
INITIAL (x/m) = 3 . 9 7 2 x 10-8M Se/g B A S A L T -19
INITIAL (x/m) = 4 . 2 6 5 x 10-11M Se/g B A S A L T
=~1 d -23 _c
INITIAL (x/m) = 6 . 4 4 1 x 10-14M Se/g B A S A L T -27 INITIAL (x/m) = 1.791 x 1 0 1 4 M Se/g B A S A L T ,...,....=..\~ X~
~ x ~ X -31
-...~m-
1
I
2
J
~
~k
__0~0-I i
l
3
4
I
x~X-----
^ ===~ O~
• I 5
J
1 6
0 =--~l
L 7
In t, hr
Fig. 3. Desorption of selenium from Umtanum basalt in oxidizing GR-3 groundwater at 60°C.
A comparison of sorption and desorption regression line slopes is shown in Fig. 4. Similar to sorption kinetics, the percentage of selenium desorbed with time remained nearly the same over a wide range in U m t a n u m basalt selenium loading. Attempts at desorbing the reduced selenium on Umtanum basalt were unsuccessful, suggesting once again t h a t the selenium was reduced to the metal and remained as metal as long as the low potential conditions prevailed. Treatment of the above kinetic data indicated that selenium sorption followed an equation of the type: Cz-
C = Kt b
where (CI -- C) is the selenium removed from solution onto the basalt in mol 1-1 (M); t is the solution--basalt contact time in hours; and K and b are constants. The above expression is the kinetic equivalent of the Freundlich sorp-
294
-25
I
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I
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' •
-
26
/ o J SORPTION SLOPE = 0.4426
:;t
•
°
• •
-27 J • J • "
e-
-29
-
DESORPTION SLOPE = 0.3250
28
/&
-
30
I
-2
I
-1
J
1
0
L
I
1
~l
I
~
L
2
3 In t,
~
I
4
i
I
5
i
I
6
L
7
hr
Fig. 4. Comparison of s o r p t i o n and desorption slopes for the system with C! ffi 1.432 • 10 -11 M Se.
tion isotherm. Thus, if ln(C~ - C) is plotted vs. In t, a straight line should result with K as the y-axis intercept when x -- 0 and b the regression line slope. There were two distinct slopes to the 60°C reducing regression line. The constants for the oxidizing and reducing 40°C results are listed in Table IV while the 60°C oxidizing and reducing results are given in Table V. Unlike the 40°C b-values, the b-values of the 60°C results are relatively constant for both the oxidizing and reducing results. This condition allows extrapolation of K-values over the initial selenium concentration (CI) range investigated (1.0.10 -s --4.2 • 10 -~2 M Se) in the 60°C U m t a n u m basalt--GR~3 solution system with the relationship: K = aCi d
where K is the y-axis intercept at x = 0; a and d are constants; and C~ is the initial selenium concentration in mol 1-~ (M). The constants for the above expression are given in Table VI. In similar fashion, the oxidizing selenium desorption kinetics at 60°C were found to follow an equation o f the type: C = At b
where C is the selenium concentration appearing in solution in mol 1-1 (M); A and b are constants; and t is the time in hours. The constants are listed in Table VII for the 60°C oxidizing desorption kinetic data. The b-values here also are approximately the same, enabling determination of an A-value over the range of selenium basalt loading x / m of the experiment (~1.8 • 10 -~4 to
295 TABLE IV C o n s t a n t s f o r s e l e n i u m s o r p t i o n at 4 0 ° C f r o m G R - 3 g r o u n d w a t e r o n 20- t o 5 0 - m e s h U m t a n u m b a s a l t - - t h e d e s c r i p t i v e e q u a t i o n is (CI -- C) = Kt b C I (M)
K
b
r
Oxidizing: • 10 -s 1 . 0 0 0 • 10 -8 1 . 4 5 0 o 10 -11 4 . 5 7 4 • 10 -12 1.000
7.1424 6.3443 2.5841 5.0443
• • • •
10 10 10 10
-9 -11 -13 -l'
0.8955 0.4603 0.2873 0.4298
+0.9920 +0.9946 +0.9953 +0.9982
3.86473.1478 • 2.7249 • 1.0143.
10 10 10 10
-9 -12 -14 -'4
1.0673 1.1311 0.8277 0.8150
+0.9965 +0.9951 +0.9949 +0.9936
Reducing: 1.000. 1.000 • 1.4404.473 •
10 10 10 10
-s -8 -11 -12
T h e r is a c o r r e l a t i o n c o e f f i c i e n t t h a t is a m e a s u r e o f h o w well t h e e x p e r i m e n t a l p o i n t s fit t h e r e g r e s s i o n line. A p e r f e c t fit is 1 . 0 0 0 . TABLE V C o n s t a n t s f o r s o r p t i o n k i n e t i c s o f s e l e n i u m o n U m t a n u m b a s a l t at 6 0 ° C f r o m G R - 3 g r o u n d w a t e r - - t h e d e s c r i p t i v e e q u a t i o n is ( C I - - C) = K t b C I (M)
K
b
r
Umtanum basalt, oxidizing, whole loading curve: 1.000 • 1.000 • 1.432. 4.393.
10 10 10 10
-5 -8 -11 -12
4.9888 5.2839 7.5568 2.1569
• • • •
10 10 10 10
-7 -1° -13 -13
Mean
0.3802 0.3819 0.4075 0.3785
+0.9975 +0.9980 +0.9962 +0.9922
0.3870
Umtanum basalt, reducing, lower loading curve (a): 1.000.
10 10 1 . 4 1 8 • 10 4 . 2 1 5 . 10 1.000
•
-s -8 -11 -12
3.0229 2.9124 4.7391 1.5635
• o • •
10 10 10 10
-7 -1° -13 -13
Mean
0.7563 0.7680 0.7449 0.7649
+0.9976 +0.9931 +0.9957 +0.9939
0.7585
Umtanum basalt, reducing, upper loading curve ( b ) : 1.000. 1.000 o 1.418 • 4.215. Mean
10 10 10 10
-s -8 -11 -12
1.2330. 10-' 1 . 2 3 7 1 • 10 `9 1 . 7 5 6 8 • 10 -12 7 . 1 2 1 3 • 10 -13
0.3057 0.3172 0.3219 0.2713 0.3040
+0.9727 +0.9810 +0.9704 +0.9633
296 TABLE VI Constants for the relationship between selenium C! and K in the equation K = aCi d for selenium sorption on Umtanum basalt in GR-3 groundwater at 60°C a
d
r
Oxidizing, whole loading curve:
0.050657
0.9995
+0.9999
Reducing, lower loading curve (a):
0.024960
0.9873
+0.9999
Reducing, upper loading curve (b):
0.101314
0.9863
+0.9998
TABLE VII Constants for desorption of selenium from Umtanum basalt in oxidizing GR-3 groundwater at 60°C Initial x / m (mol Se/g)
A
b
r
2.3521 • 10 -'4 7.3904 • 10 -14 5.3069 • 10 -11 5.0854 • 10 -g
0.3090 0.3223 0.3485 0.3065
+0.9936 +0.9919 +0.9961 +0.9964
C =Atb:
1.791.
I 0 -'4
6.441"
1 0 -'4
4.265- 10 -1' 3.972" 10-' Mean
0.3216
A = a(x/m) b :
1.31917
1.00225
+0.9999
4.0 - 10 -8 tool Se/g basalt). H e n c e , given an x / m - v a l u e w i t h i n t h e a b o v e range, the Table V I I c o n s t a n t s can be used t o d e t e r m i n e an a p p r o p r i a t e Avalues a n d t h e n t o c o n s t r u c t the kinetic selenium d e s o r p t i o n curve. I n an e f f o r t t o learn m o r e a b o u t h o w t h e selenium was held o n the basalt, T a m m ' s s o l u t i o n ( 0 . 1 7 5 M a m m o n i u m o x a l a t e - - 0 . 1 0 0 M oxalic acid, p H 3.3) was used in darkness at 60°C t o d e t e r m i n e the relationship b e t w e e n a m o r p h o u s m e t a l oxides p r e s e n t o n t h e w e a t h e r e d basalt surfaces and conc o m i t a n t selenium d e s o r p t i o n (Warnant et al., 1 9 8 1 ; J o h n et al., 1976). T h e r e was little c o r r e l a t i o n b e t w e e n a m o r p h o u s F e r e m o v a l a n d c o n c o m i t a n t Se removal. A certain a m o u n t o f A1 a n d Ti were r e m o v e d f r o m t h e basalt at the same t i m e as t h e a m o r p h o u s Fe, also with n o a p p a r e n t relationship bet w e e n c o n c o m i t a n t Se and A1 o r Ti r e m o v a l w i t h time. T h e Fe and Se re-
297
moval relationship is shown graphically in Fig. 5. Amorphous Fe removal from basalt was reasonably linear when plotted as a natural log function of In t. Little, if any, o f the Se was sorbed on the amorphous Fe. As mentioned previously, reduced selenium desorption did not occur. In one case, a reduced system was allowed to become partially oxidized and again reduced. U p o n oxidation, a portion o f the selenium was solubilized, but removed from solution again when reducing conditions were reestablished.
'
I
'
I
'
1
'
4 •
_
-
-
¢/1 z
~2
-- ~
AN
--
t-
/ ~df
I
0
L 2
'J 3
I
'J 4
i
1 5
, 6
In t, min Fig. 5. R e l a t i o n s h i p b e t w e e n a m o r p h o u s F e a n d Se r e m o v a l b y T a m m ' s s o l u t i o n at 60°C w i t h time. x / m at t i m e zero was 5 . 6 4 2 . 1 0 -14 M Se/g of U m t a n u m basalt.
The slopes b o f the 60°C sorption and desorption regression lines can be used to compare relative selenium loading and unloading rates onto U m t a n u m basalt as shown in Fig. 4. Selenium loading is only slightly more rapid than unloading. Selenium removal from solution at 60°C under reducing conditions is initially about twice as fast as from oxidizing solutions, b u t they eventually become about equal in reaction rate. The lower selenium concentration values of the 40°C experiments also show removal rates that are approximately twice as rapid under reducing conditions. The upper 40°C
298
selenium concentrations approach linear selenium removal rates. The 40°C kinetic sorption regression lines are n o t doubly sloped as the 60°C lines are. It should be noted that nearly all of the regression coefficients (r) for these data sets are 0.99 or higher, statistically significant at a very high probability level. As noted previously, the 40 ° and 60°C oxidizing selenate kinetic sorption results had approached equilibrium. By taking a mean of the last three or four C and x/m kinetic values (Table VIII), equilibrium isotherms can be constructed as shown in Fig. 6. These are Freundlich isotherms with the constants given in Fig. 6. The three isotherms allow derivation of a sorption reaction enthalpy change (~H). To determine AH, a derivative of the Gibbs-Helmholtz equation was used, because within the temperature range studied AH was independent of temperature (Fig. 7). The equation is:
log(KdJKd,) = [,~H(T2 -- TI)]/(2.303RTIT2) where Kd2 is the 60°C selenium equilibrium distribution coefficient at a given x/m-value; Kd, is the 40°C selenium equilibrium distribution coefficient at the same given x/m-value; R is the gas constant (1.987 cal. mol -~ K-~); 2.303 is a conversion factor for natural to base 10 logarithmic units; and T1 and T2 are in kelvins (Farrington and Daniels, 1979). The Kd is de fined as (x/m)/O.O01C, with units ml g-~, and was determined utilizing the applicable Freundlich constants. TABLE
VIII
Equilibrium groundwater
sorption of selenium on 20- to 50-mesh a t 4 0 °, 4 5 ° a n d 6 0 ° C
CI
C
x/m
(M)
(M)
(mol g ' )
Temperature
Umtanum
40°C:
1.000 1.000 1.450
• 1 0 -2 • 10 8 • 10 -'i
9.249 9.294 1.324
• I0-' • 1 0 -9 • 1 0 -11
7.510 7.066 1.253
• 1 0 -9 • 1 0 -I~ • 1 0 -14
4.574
• 10 -l:
4.115
• 1 0 -I~
4.588
• 10 -Is
• 10-' • 1 0 -9 • 1 0 -1~
8.272. 1 0 -9 8 . 3 3 8 • 1 0 -l~ 1 . 5 2 0 • 1 0 -15
Temperature 1.000. 1 0 -s 1 . 0 0 0 • 1 0 -8 1 . 4 5 8 • 10112
Temperature 1.000. 1.000. 1.432. 4.393 •
10 10 10 10
-5 -~ -11 -12
45°C: 9.173 9.166 1.306 60°C: 4.681 • 10-' 4 . 1 3 4 • 1 0 -9 5.507. 1 0 -1~ 2 . 0 3 3 • 1 0 -1~
5.185 5.688 8.551 2.301
• • • •
10-' 1 0 -11 10 -1' 10 -'4
basalt from oxidized
GR-3
299
1
{
I
-18
60oc
-22
45oc
40oc 0
E
- 26
x C
- 30
(x/m)= KC N TEMPERATURE, °C
K
N
r
40
6.0051 x 10 .4
0.9802
+0.999
45
7.1510 x 104
0.9833
+0,999
60
1.0388 x 102
0.9901
+0,999
- 34
-28
-24
{
I
I
-20
-16
-12
-8
In C , M
Fig. 6. Equilibrium isotherms for sorption of selenium from oxidized GR-3 groundwater.
It was assumed that the basalt surface coverage remained constant for the same x/m-values. The change of AH with x/m is shown in Fig. 8. No change in the selenium species sorbed over the investigated selenium concentration range was indicated although the species sorbed and the specific sorption mechanism were not determined. The best single criterion for distinguishing between chemisorption and physical adsorption is the heat of sorption (Trapnell, 1955). The rather high AH of oxidized selenium sorption indicates that chemisorption has occurred, with selenium chemically bonded rather than the physical bonding of sorption reactions. These conclusions were supported by the results of selenium sorption on Umtanum basalt at 25°C which was minimal. Because initial desorption apparently takes place with little difficulty (Fig. 3), it may also be concluded that the activation energy involved in the sorption reaction is probably relatively small in magnitude (Tompkins, 1978). Desorption would only occur with difficulty at the same temper-
300
2,0
E _= 1.0
3.00
3.05
3.10
3.15
3.20
1 / T x 103, K
Fig. 7. In K d p l o t t e d against reciprocal a b s o l u t e t e m p e r a t u r e at x / m = 1 . 2 6 6 . 1 0 -14 M Se/ g U m t a n u m basalt. 29.0
I
l
I
I
I
1
1
I
I
I
l
I
I
e
I
I
.J
O
E "-28.5 == ¢.1
",r-
Z
28.0
'I-
,.I
•.r 2 7 . 5
ee
j ~
=
.
+ 0.1035
[In(x/m)]
I--
z
I&.l
27.0
I'~"1
- 34
1 - 32
I - 30
I
1 - 28
I
I - 26
I
I - 24
I
I - 22
In(x/m), moles/g
Fig. 8. T h e c h a n g e in ~/~/as a f u n c t i o n o f surface coverage x / m .
l
I - 20
I -18
301
ature as sorption if a large activation energy barrier had to be traversed during desorption in addition to the desorbed- and sorbed-state energy difference. It can be stated that the use of a proposed high-level waste repository in basalt at ~ 1 0 0 0 m in depth and a 55--60°C ambient temperature would result in greatly retarded potential selenate-selenium migration rates under both oxidizing and reducing conditions, even though the exact chemisorption mechanism has not been determined. ACKNOWLEDGEMENT
This work was funded by the U.S. Department of Energy under Contract DE-AC06-76RL0 1830.
REFERENCES Ames, L.L., 1980. Hanford basalt flow mineralogy. Pac. Northw. Lab., Richland, Wash., PNL-2847. Barney, G.S. and Wood, B.J., 1980. Identification of key radionuclides in a nuclear waste repository in basalt. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-ST-9. Cary, E.E. and Allaway, W.H., 1969. The stability of different forms of selenium added to low-selenium soils. Soil Sci. Soc. Am. Proc., 33: 571--574. Cutter, G.A., 1982. Selenium in reducing waters. Science, 217: 829--831. Davies, E.E. and Watkinson, J.H., 1966. Uptake of native and applied selenium by pasture species, I. Uptake of Se by browntop, ryegrass, cocksfoot and white clover from Ataimure sand. N.Z.J. Agric. Res., 9: 317--327. Deju, R.A., Ledgerwood, R.K. and Long, P.E., 1978. Reference waste form, basalts and groundwater systems for waste interaction studies. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-LD-11 Farrington, R.A. and Daniels, A., 1979. Physical Chemistry. Wiley, New York, N.Y., 5th ed., pp. 151--153. Gissel-Nielsen, G., 1973. Uptake and distribution of added selenite and selenate by barley and red clover as influenced by sulfur. J. Sci. F o o d Agric., 24: 649---655. Hayes, M.H.B., Isaacson, P.J., Chia, K.Y. and Lees, A.M., 1982. Interactions of hydrazine and of hydrazine derivatives with soil constituents and soils. Defense Tech. Info. Cent. (D.T.I.C.), Cameron Stn., Alexandria, Va., Rep. No. AFOSR-TR-82-0231. Heilman, M.D. Carter, D.L. and Gonzalez, C.L., 1965. The ethylene glycol monoethyl ether (EGME) technique for determining soil-surface area. Soil Sci., 100: 409--413. Hodge, C.E. and Grutzeck, M.W., 1978. Preparation of standard Umtanum sample by Battelle-Northwest Laboratories. Rockwell Hanford Operat. Richland, Wash, RHOBWI-LD-3. Hudson, G.H., 1967. Supplement II, Nitrogen (Part II). In: J.W. Mellor (Editor), Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VIII. Longmans, Green and Co. Ltd., London, pp. 69--113. John, M.K., Saunders, W.M.H. and Watkinson, J.H., 1976. Selenium adsorption by New Zealand soils, I. Relative adsorption of selenite by representative soils and the relationship to soil properties. N . Z . J . Agric. Res., 19: 143--151. Ledgerwood, R.K., Myers, C.W. and Cross, R.W., 1978. Pasco Basin stratigraphic nomenclature. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-LD-1. Lowell, S., 1979. Introduction to Powder Surface Area. Wiley, New York, N.Y., pp. 16-39.
302
Riese, W.C., Lee, M.J., Brookins, D.G. and Della Valle, R., 1970. Application of trace element geochemistry to prospecting for sandstone type uranium deposits. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration 1978, Proc. 7th Int. Geochem. Explor. Syrup., Golden, Colo., pp. 47---63. Tompkins, F.C., 1978. Chemisorption of Gases on Metals. Academic Press, N e w York, N.Y., pp. 1--9. TrapneU, B.M.W., 1955. Chemisorption. Butterworths, London, p. 2. Trotman-Dickenson, A.F., 1973. Comprehensive Inorganic Chemistry. Pergamon, N e w York, N.Y., p. 947. Warnant, P., Martin, H. and Herbillon, A.J., 1981. Kinetics of the selective extraction of iron oxides in geochemical samples. Association between Fe and Cu in acid brown soils.J. Geochem. Explor., 15: 635--644.
287
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
SELENATE-SELENIUM SORPTION ON A COLUMBIA RIVER BASALT (UMTANUM BASALT, WASHINGTON, U.S.A.)*
L.L. AMES', P.F. S A L T E R 2, J.E. M c G A R R A H '
and B.A. W A L K E R '
'Pacific Northwest Laboratories, Richland, W A 99352 (U.S.A.) 2Rockwell Hanford Operations, Richland, W A 99352 (U.S.A.) (Received March 7, 1983; revised and accepted September 23, 1983)
ABSTRACT Ames, L.L., Salter, P.F., McGarrah, J.E. and Walker, B.A., 1984. Selenate-selenium sorption on a Columbia River basalt (Umtanum basalt, Washington, U.S.A.). Chem. Geol., 43: 287--302. Sodium selenate and Na 27sSeO4 were used with a synthetic groundwater and hydrazine to determine selenate-selenium sorption characteristics of crushed Umtanum basalt between 40 ° and 60°C. Selenium sorption kinetics from both oxidizing and reducing solutions followed an equation of the type: CI-
C = Kt b
where CI is the initial selenium concentration in molarity; C is the selenium concentration in molarity at time t in hours; and K and b are constants. Relatively constant b-values of a wide CI range allowed K determination over the whole initial concentration range investigated (CI ffi ~ 1 0 - ' - - 1 0 -'2 M). Selenium desorption kinetics in 60°C oxidizing systems were described by:
c =Ad where A and d are constants and other quantities defined above. Equilibrium results conformed to a Freundlich sorption isotherm. The enthalpy change was independent of temperature within the range studied and yielded AH-values of from +27.23 kcal. m o l - ' at 1.266 • 10 -'4 tool Se/g basalt to +28.47 kcal. tool-' at 2 . 0 6 1 . 1 0 -9 mol Se/g basalt. Chemisorption was indicated by the magnitude of the enthalpy change. Desorption kinetics at 60°C were only modestly less rapid than sorption kinetics, suggesting a relatively small sorption activation energy. Reduced (Eh ffi --150 to --200 mV) selenium did not desorb.
INTRODUCTION High-level nuclear reactor wastes contain a variety of radionuclides of varying toxicities, concentrations and transport hazards. Efforts have been *Prepared for Rockwell Hanford Operations, Prime Contractor to U.S. Department o f Energy under Contract DE-AC06-77RL01030. 0009-2541/84/$03.00
© 1984 Elsevier Science Publishers B.V.
288 made to obtain an overall ranking o f radionuclides that include the above variables (Barney and Wood, 1980). Selenium can usually be found in the top ten of such key radionuclide rankings. Dissolved selenium exhibits the three oxidation states of selenate (6+), selenite (4+) and selenide ( 2 - ) as a function of oxygen partial pressure. However, kinetic effects often result in the persistence of thermodynamically unstable species. Recent work by Cutter (1982) in sampling Saanich Inlet waters by Vancouver Island, British Columbia, Canada, showed a steady decrease of the Se(VI) to Se(IV) ratio with increasing depth. Though the Se(IV) oxidation to Se(VI) may be a relatively sluggish transition, it does occur in natural environments that range from oxidizing to reducing conditions. Thus selenate-selenium may be generated during the transient oxidizing environment present in the repository following closure. The present work was confined to selenate-selenium sorption reactions with U m t a n u m flow basalt (Washington, U.S.A.) and was undertaken as a step in assessing the migration potential of selenate-selenium through the basalt geohydrologic system. However, the results may be of interest to agronomists (Davies and Watkinson, 1966; Cary and Allaway, 1969; Gissel-Nielsen, 1973), who have applied various forms of selenium-bearing to selenium-deficient softs sometimes containing basalt particles, and to geochemists involved in exploration for various ores including uranium (Riese et al., 1979). MATERIALS USED A synthetic groundwater composition, based on chemical analyses of natural groundwaters found just above the U m t a n u m basalt, was used in the sorption work. The analysis of this synthetic groundwater (GR-3) is given in Table I. Reagents used in makeup of GR-3 included deionized water, Na2CO3, Na2SiO3.9H~O, NaOH, KC1, Na2SO4, NaF, HC1, CaC12 and MgC12. The synthetic GR-3 was made up as required and n o t stored. Radioactive ~SSe with a 120-day half-life was used as a selenium tracer. It was present as sodium selenate. All counting data older than two days were TABLEI Synthetic groundwater composition l- 1)
Constituent
(mg
Na ÷ K÷ Ca:÷ Mg2+
358 3.43 2.78 0.032
SiO 2
Constituent
(mg 1-1)
FC1SO~Alkalinity (as HCO~)
33.4 312 173 1.82 meq 1-1
76.2
pH -- 9.77 at 25°C; pH = 9.50 at 45°C; pH = 9.20 at 65°C. Ionic strength -- 0.03425.
289
decay-corrected due to the relatively short half-life of 7SSe. Nonradioactive sodium selenate was used with the selenium tracer to augment initial solution selenium concentrations above that of the 7SSe tracer (~5 • 10 -12 M 7SSe). Hydrazine (N2H4) was added to the GR-3 synthetic groundwater to create a reducing environment. Hydrazine reacts with oxygen to form nitrogen and water, N2H4 + O2 -~ N2 + 2H20, and is used extensively as a corrosion control agent in boilers and hot-water systems. The reaction is relatively slow at room temperature. Dibasic hydrazine is a slightly weaker base than ammonium, forming salts with inorganic acids. Hydrazine and its salts are good reducing agents as indicated by their standard redox potentials: N2H4 + 4OH- -~ N2 + 4H20 + 4e-,
E ° = +1.16 V
and N2Hs ÷ -+ N2 + 5H ÷ + 4e-,
E ° = +0.23 V
although hydrazine is clearly a better reducing agent in alkaline solutions. Hence, hydrazine is able to reduce Se(VI) and Se(IV) to Se metal. One of the disadvantages of using hydrazine for potential control is that it dissociates in basic aqueous solutions producing hydrazinium ions as follows (Hudson, 1967): [N2H~] [OH-] N=H4 ~ N2H; + OH-,
N2H~ -* N2H~÷ + OH-,
[N2H4] [N2H~÷] [OH-]
= 8.6 • I0 -~
= ~ 9 . 4 • 10 -is
[N2H:] Hence, if an ion-exchange reaction is under study, the hydrazinium ions m a y themselves compete with the lower radionuclide concentrations for cation-exchange sites on the basalt (Hayes et al., 1982). Measurements of the basalt--GR-3 groundwater system in a low-oxygen atmosphere chamber (< 0.1 ppm O2) with a Pt electrode consistently yielded potentials of from - 3 0 0 to - 4 0 0 mV at pH 9.0 and 25°C. Quinhydrone was used as a standard. The oxygenated solutions, on the other hand, contained from an average of 5.30 mg dissolved O2/1 at 60°C to 8.25 mg dissolved O2/1 at 25°C as measured with a calibrated oxygen diffusion membrane, and changed little over a 31-day period. Thus the potential at 25°C and pH 9.0 ranged from +550 to +650 mV. U m t a n u m basalt, a flow from the Columbia River Group (Ledgerwood et al., 1978), was collected at a field outcrop, crushed and screened to 0.85-0.30 m m in size (Hodge and Grutzeck, 1978). This fraction was washed, contacted three times with nonradioactive GR-3, dried at room temperature and stored in a low-oxygen (< 0.1 ppm 02) environment for later use. The properties o f the U m t a n u m basalt are summarized in Table II. For further in-
290
formation, consult Deju et al. (1978). The surface area was derived by nitrogen sorption at four relative nitrogen pressures and extrapolation to zero relative pressure (Lowell, 1979). The m e t h o d proposed by Heilman et al. (1965) was used to obtain an ethyleneglycolmonoethyl ether surface area on the same material. The t w o specific surface areas apparently do not measure the same surfaces because the BET measurement is usually considerably lower than the ethylene glycol m o n o e t h y l ether measurement. The Cs ÷ cation-exchange capacity is relatively low in comparison to smectites (60--90 meq/100 g) and other layer-silicates and zeolites. The groundmass was defined as the residual system remaining after the basalt cooled. It consists mainly of crystaUites of the more sodic plagioclases embedded in a glassy matrix composed of silica (50--75 wt.%), alumina (12--15 wt.%), potassium (2--8 wt.% K20) and other minor constituents (Ames, 1980). TABLE II Characterization of the Umtanum basalt used in the sorption work Size range (mm)
Mineralogy (vo1.%)
0.85--0.30
groundmass 38 plagioclase 26 pyroxenes 21 secondary minerals 8 metallic oxides 7
Specific surface (m ~ g-l)
Cesium cationexchange capacity, pH 7.0, 25°C (meq/100 g)
2.67
1.83 ± 0.02 ( l a )
Ethylene glycol monoethyl ether specificsurface
17.7 + 3.8 (lo)
Typical chemical analysis Oxide
(wt.%)
Oxide
SiO~ A1203 F%O 3 FeO MnO CaO MgO
55.64 13.62 2.00 10.68 0.20 7.17 3.33
Na20 K~O TiO 2 P205
3.30 1.62 2.05 0.39
Total
100.00
METHODS
(wt.%)
OF INVESTIGATION
To determine selenium sorption kinetics, 10 g of 0.85- to 0.30-mm Umtanum basalt were placed in a 125-ml Wheaton ® borosilicate glass serum bottle containing 100 ml of GR~q synthetic groundwater plus the 75Se and nonradioactive NacSeO4, b o t h at the selected system temperature. The Wheaton ® serum bottle was sealed with a Teflon ® -faced silicone septum and
291
secured with a crimped aluminum seal. Reducing solutions also contained ~ 0 . 0 5 M N2H4 and were made up and sealed in an anoxic (<0.1 ppm O~) chamber. Timed samples were taken after basalt--solution contact at temperature with a stainless-stee ! needle and syringe through the septum that resealed upon withdrawal of the needle, thus minimizing atmospheric contact. All samples were immediately filtered through a 0.22-#m filter unit that fit onto the end of the syringe. New Brunswick ® incubator--rotators were used for agitation and temperature control between samples. Sampling of solution for radionuclide removal rates was carried out at timed intervals. Upon approach to equilibrium, a last solution sample was taken to establish the radionuclide load on the basalt. The remaining solution was then removed and discarded, the basalt rinsed with methanol, 100 ml of GR-3 groundwater containing no radionuclide added at system temperature and the Wheaton ® bottle resealed. This operation was carried out in the anoxic chamber for the reducing systems. A sample for comparative counting was removed from the solutions before they were contacted with the basalt. All 75Se samples were gamma-counted on a sodium iodide crystal whose geometry was known. From the specific activity, a concentration of selenium could be obtained, assuming that the sorption behavior of the tracing 7SSeO~- and nonradioactive SeO~- was identical. The various solution containers were checked for selenium sorption with KOH. Container sorption was generally less than 2% of the contacting solution in the presence of basalt. Because all samples for counting were filtered, filtered and unfiltered samples of 40°C reduced selenium experiments at 1200 hr. of solution--basalt contact were taken and counted with the results shown in Table III. Unfiltered samples averaged ~3.72% higher than the 0.22-pm filtered samples. Hence, very little of the Se ° existed in solution as a colloid greater than 0.22 g m in size, and the selenium, filterable or unfilterable, was n o t in solution. T A B L E III C o m p a r i s o n o f filtered ( 0 . 2 2 u m ) a n d u n f i l t e r e d s o l u t i o n s a m p l e c o u n t i n g d i f f e r e n c e s for s e l e n i u m s o r p t i o n o n 20- t o 5 0 - m e s h U m t a n u m b a s a l t f r o m r e d u c i n g GR-3 g r o u n d w a t e r at 40°C at 1 2 0 0 hr. o f c o n t a c t C!
(100)(C, unfiltered)-
(C, filtered)] % d i f f e r e n c e
(M) 1.000
• 10-'
1.000 • 1 0 -8 1.440 o 10 -1' 4 . 4 3 7 • 10 - ' : M e a n -+ l o
+5.51 +3.51
+1.10 +4.75 + 3 . 7 2 -+ 1.93
RESULTS AND DISCUSSION
Selenate kinetic sorption isotherms on 0.85- to 0.30-mm U m t a n u m basalt were determined at 40 ° and 60°C for four selenate ion concentrations under
292 oxidizing and reducing conditions. The sorption kinetics for oxidizing and reducing conditions at 60°C are given in Figs. 1 and 2, respectively. The oxidizing kinetic sorption data show that equilibrium was approached and that the percentages of selenium CI sorbed with time are approximately the same over nearly a seven-fold selenate ion concentration range. -IO
'
r
'
I
'
I
'
u
'
I
'
)
'
l
=
]
m .__.._._m ..------m ..._._.m _ _ m
-14
/
' •
•
•
•
m m--
_ mm_.__m_..._ m_._.- m CI= 1.000 x 10-5M Se
=~i - 1 8 A ~
5~
c -22
" .----- A
A- - ' - ' A ' - - A - ' - - j
~
A
~
A
A~
A_.._...~ & -
CI= 1.000 x 10-VM Se
CI= 4.393 x 10-12M Se CI= 1 . 4 3 2 x 10-11M Se \ _.._ x ~ x
-26
X .....=..X ...~... X..........-X
-30
"rO"d" -2 -1
i
T 0
l
....= ~
l 1
=
xX
~
~
x,_~...,x~ x , . . ~ o------- o
x x • •
• •~O
O ~
I 2
,
l 3
'
l 4
,
I 5
=
l 6
,
7
In t, hr Fig, 1. Sorption kinetics of selenium on Umtanum basalt in oxidizing GR-3 groundwater
at 60°C. -11
'
I
'
I
'
I
'
I
'
]
' I • n o , ~
' I ' I. u ''-'-" n" u - ' -
m~
-15
• l
m ~
m ~ m "
(b)
CI= 1 . 0 0 0 x 10-SM Se
(a)
-19
AT"--
A'A~CI _= - 2 3
m'
A'A~
A--A
1.000 X 10"SM Se
& ~ & ~ A Cl= 1.418 x 10-11M Se
x ~ X " - - - ~ . x _ _ x -, x e: •
-
_x~
x~x~e~
e'~
-31 -2
-1
0
1
CI= 4,215 x 10-12M Se i , h, 2 3 In t, hr
. 4
, ,
, 5
6
7
Fig. 2. Sorption kinetics of selenium on Umtanum basalt in reducing GR-3 groundwater at 60°C.
293
The reducing selenium kinetic sorption results indicate that 100% selenium removal was probable even at 40°C, presumably as selenium metal at the potential of the system. After ~ 4 6 days, the U m t a n u m basalt used for sorption was desorbed after removing all of the radioactive equilibrium solution and adding a 60°C, nonradioactive solution of the same composition. The results are given in Fig. 3. I
'
I
'
I
'
I
'
I
'
__._ n.___ n - - n-----'_=
-15
INITIAL (x/m) = 3 . 9 7 2 x 10-8M Se/g B A S A L T -19
INITIAL (x/m) = 4 . 2 6 5 x 10-11M Se/g B A S A L T
=~1 d -23 _c
INITIAL (x/m) = 6 . 4 4 1 x 10-14M Se/g B A S A L T -27 INITIAL (x/m) = 1.791 x 1 0 1 4 M Se/g B A S A L T ,...,....=..\~ X~
~ x ~ X -31
-...~m-
1
I
2
J
~
~k
__0~0-I i
l
3
4
I
x~X-----
^ ===~ O~
• I 5
J
1 6
0 =--~l
L 7
In t, hr
Fig. 3. Desorption of selenium from Umtanum basalt in oxidizing GR-3 groundwater at 60°C.
A comparison of sorption and desorption regression line slopes is shown in Fig. 4. Similar to sorption kinetics, the percentage of selenium desorbed with time remained nearly the same over a wide range in U m t a n u m basalt selenium loading. Attempts at desorbing the reduced selenium on Umtanum basalt were unsuccessful, suggesting once again t h a t the selenium was reduced to the metal and remained as metal as long as the low potential conditions prevailed. Treatment of the above kinetic data indicated that selenium sorption followed an equation of the type: Cz-
C = Kt b
where (CI -- C) is the selenium removed from solution onto the basalt in mol 1-1 (M); t is the solution--basalt contact time in hours; and K and b are constants. The above expression is the kinetic equivalent of the Freundlich sorp-
294
-25
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
' •
-
26
/ o J SORPTION SLOPE = 0.4426
:;t
•
°
• •
-27 J • J • "
e-
-29
-
DESORPTION SLOPE = 0.3250
28
/&
-
30
I
-2
I
-1
J
1
0
L
I
1
~l
I
~
L
2
3 In t,
~
I
4
i
I
5
i
I
6
L
7
hr
Fig. 4. Comparison of s o r p t i o n and desorption slopes for the system with C! ffi 1.432 • 10 -11 M Se.
tion isotherm. Thus, if ln(C~ - C) is plotted vs. In t, a straight line should result with K as the y-axis intercept when x -- 0 and b the regression line slope. There were two distinct slopes to the 60°C reducing regression line. The constants for the oxidizing and reducing 40°C results are listed in Table IV while the 60°C oxidizing and reducing results are given in Table V. Unlike the 40°C b-values, the b-values of the 60°C results are relatively constant for both the oxidizing and reducing results. This condition allows extrapolation of K-values over the initial selenium concentration (CI) range investigated (1.0.10 -s --4.2 • 10 -~2 M Se) in the 60°C U m t a n u m basalt--GR~3 solution system with the relationship: K = aCi d
where K is the y-axis intercept at x = 0; a and d are constants; and C~ is the initial selenium concentration in mol 1-~ (M). The constants for the above expression are given in Table VI. In similar fashion, the oxidizing selenium desorption kinetics at 60°C were found to follow an equation o f the type: C = At b
where C is the selenium concentration appearing in solution in mol 1-1 (M); A and b are constants; and t is the time in hours. The constants are listed in Table VII for the 60°C oxidizing desorption kinetic data. The b-values here also are approximately the same, enabling determination of an A-value over the range of selenium basalt loading x / m of the experiment (~1.8 • 10 -~4 to
295 TABLE IV C o n s t a n t s f o r s e l e n i u m s o r p t i o n at 4 0 ° C f r o m G R - 3 g r o u n d w a t e r o n 20- t o 5 0 - m e s h U m t a n u m b a s a l t - - t h e d e s c r i p t i v e e q u a t i o n is (CI -- C) = Kt b C I (M)
K
b
r
Oxidizing: • 10 -s 1 . 0 0 0 • 10 -8 1 . 4 5 0 o 10 -11 4 . 5 7 4 • 10 -12 1.000
7.1424 6.3443 2.5841 5.0443
• • • •
10 10 10 10
-9 -11 -13 -l'
0.8955 0.4603 0.2873 0.4298
+0.9920 +0.9946 +0.9953 +0.9982
3.86473.1478 • 2.7249 • 1.0143.
10 10 10 10
-9 -12 -14 -'4
1.0673 1.1311 0.8277 0.8150
+0.9965 +0.9951 +0.9949 +0.9936
Reducing: 1.000. 1.000 • 1.4404.473 •
10 10 10 10
-s -8 -11 -12
T h e r is a c o r r e l a t i o n c o e f f i c i e n t t h a t is a m e a s u r e o f h o w well t h e e x p e r i m e n t a l p o i n t s fit t h e r e g r e s s i o n line. A p e r f e c t fit is 1 . 0 0 0 . TABLE V C o n s t a n t s f o r s o r p t i o n k i n e t i c s o f s e l e n i u m o n U m t a n u m b a s a l t at 6 0 ° C f r o m G R - 3 g r o u n d w a t e r - - t h e d e s c r i p t i v e e q u a t i o n is ( C I - - C) = K t b C I (M)
K
b
r
Umtanum basalt, oxidizing, whole loading curve: 1.000 • 1.000 • 1.432. 4.393.
10 10 10 10
-5 -8 -11 -12
4.9888 5.2839 7.5568 2.1569
• • • •
10 10 10 10
-7 -1° -13 -13
Mean
0.3802 0.3819 0.4075 0.3785
+0.9975 +0.9980 +0.9962 +0.9922
0.3870
Umtanum basalt, reducing, lower loading curve (a): 1.000.
10 10 1 . 4 1 8 • 10 4 . 2 1 5 . 10 1.000
•
-s -8 -11 -12
3.0229 2.9124 4.7391 1.5635
• o • •
10 10 10 10
-7 -1° -13 -13
Mean
0.7563 0.7680 0.7449 0.7649
+0.9976 +0.9931 +0.9957 +0.9939
0.7585
Umtanum basalt, reducing, upper loading curve ( b ) : 1.000. 1.000 o 1.418 • 4.215. Mean
10 10 10 10
-s -8 -11 -12
1.2330. 10-' 1 . 2 3 7 1 • 10 `9 1 . 7 5 6 8 • 10 -12 7 . 1 2 1 3 • 10 -13
0.3057 0.3172 0.3219 0.2713 0.3040
+0.9727 +0.9810 +0.9704 +0.9633
296 TABLE VI Constants for the relationship between selenium C! and K in the equation K = aCi d for selenium sorption on Umtanum basalt in GR-3 groundwater at 60°C a
d
r
Oxidizing, whole loading curve:
0.050657
0.9995
+0.9999
Reducing, lower loading curve (a):
0.024960
0.9873
+0.9999
Reducing, upper loading curve (b):
0.101314
0.9863
+0.9998
TABLE VII Constants for desorption of selenium from Umtanum basalt in oxidizing GR-3 groundwater at 60°C Initial x / m (mol Se/g)
A
b
r
2.3521 • 10 -'4 7.3904 • 10 -14 5.3069 • 10 -11 5.0854 • 10 -g
0.3090 0.3223 0.3485 0.3065
+0.9936 +0.9919 +0.9961 +0.9964
C =Atb:
1.791.
I 0 -'4
6.441"
1 0 -'4
4.265- 10 -1' 3.972" 10-' Mean
0.3216
A = a(x/m) b :
1.31917
1.00225
+0.9999
4.0 - 10 -8 tool Se/g basalt). H e n c e , given an x / m - v a l u e w i t h i n t h e a b o v e range, the Table V I I c o n s t a n t s can be used t o d e t e r m i n e an a p p r o p r i a t e Avalues a n d t h e n t o c o n s t r u c t the kinetic selenium d e s o r p t i o n curve. I n an e f f o r t t o learn m o r e a b o u t h o w t h e selenium was held o n the basalt, T a m m ' s s o l u t i o n ( 0 . 1 7 5 M a m m o n i u m o x a l a t e - - 0 . 1 0 0 M oxalic acid, p H 3.3) was used in darkness at 60°C t o d e t e r m i n e the relationship b e t w e e n a m o r p h o u s m e t a l oxides p r e s e n t o n t h e w e a t h e r e d basalt surfaces and conc o m i t a n t selenium d e s o r p t i o n (Warnant et al., 1 9 8 1 ; J o h n et al., 1976). T h e r e was little c o r r e l a t i o n b e t w e e n a m o r p h o u s F e r e m o v a l a n d c o n c o m i t a n t Se removal. A certain a m o u n t o f A1 a n d Ti were r e m o v e d f r o m t h e basalt at the same t i m e as t h e a m o r p h o u s Fe, also with n o a p p a r e n t relationship bet w e e n c o n c o m i t a n t Se and A1 o r Ti r e m o v a l w i t h time. T h e Fe and Se re-
297
moval relationship is shown graphically in Fig. 5. Amorphous Fe removal from basalt was reasonably linear when plotted as a natural log function of In t. Little, if any, o f the Se was sorbed on the amorphous Fe. As mentioned previously, reduced selenium desorption did not occur. In one case, a reduced system was allowed to become partially oxidized and again reduced. U p o n oxidation, a portion o f the selenium was solubilized, but removed from solution again when reducing conditions were reestablished.
'
I
'
I
'
1
'
4 •
_
-
-
¢/1 z
~2
-- ~
AN
--
t-
/ ~df
I
0
L 2
'J 3
I
'J 4
i
1 5
, 6
In t, min Fig. 5. R e l a t i o n s h i p b e t w e e n a m o r p h o u s F e a n d Se r e m o v a l b y T a m m ' s s o l u t i o n at 60°C w i t h time. x / m at t i m e zero was 5 . 6 4 2 . 1 0 -14 M Se/g of U m t a n u m basalt.
The slopes b o f the 60°C sorption and desorption regression lines can be used to compare relative selenium loading and unloading rates onto U m t a n u m basalt as shown in Fig. 4. Selenium loading is only slightly more rapid than unloading. Selenium removal from solution at 60°C under reducing conditions is initially about twice as fast as from oxidizing solutions, b u t they eventually become about equal in reaction rate. The lower selenium concentration values of the 40°C experiments also show removal rates that are approximately twice as rapid under reducing conditions. The upper 40°C
298
selenium concentrations approach linear selenium removal rates. The 40°C kinetic sorption regression lines are n o t doubly sloped as the 60°C lines are. It should be noted that nearly all of the regression coefficients (r) for these data sets are 0.99 or higher, statistically significant at a very high probability level. As noted previously, the 40 ° and 60°C oxidizing selenate kinetic sorption results had approached equilibrium. By taking a mean of the last three or four C and x/m kinetic values (Table VIII), equilibrium isotherms can be constructed as shown in Fig. 6. These are Freundlich isotherms with the constants given in Fig. 6. The three isotherms allow derivation of a sorption reaction enthalpy change (~H). To determine AH, a derivative of the Gibbs-Helmholtz equation was used, because within the temperature range studied AH was independent of temperature (Fig. 7). The equation is:
log(KdJKd,) = [,~H(T2 -- TI)]/(2.303RTIT2) where Kd2 is the 60°C selenium equilibrium distribution coefficient at a given x/m-value; Kd, is the 40°C selenium equilibrium distribution coefficient at the same given x/m-value; R is the gas constant (1.987 cal. mol -~ K-~); 2.303 is a conversion factor for natural to base 10 logarithmic units; and T1 and T2 are in kelvins (Farrington and Daniels, 1979). The Kd is de fined as (x/m)/O.O01C, with units ml g-~, and was determined utilizing the applicable Freundlich constants. TABLE
VIII
Equilibrium groundwater
sorption of selenium on 20- to 50-mesh a t 4 0 °, 4 5 ° a n d 6 0 ° C
CI
C
x/m
(M)
(M)
(mol g ' )
Temperature
Umtanum
40°C:
1.000 1.000 1.450
• 1 0 -2 • 10 8 • 10 -'i
9.249 9.294 1.324
• I0-' • 1 0 -9 • 1 0 -11
7.510 7.066 1.253
• 1 0 -9 • 1 0 -I~ • 1 0 -14
4.574
• 10 -l:
4.115
• 1 0 -I~
4.588
• 10 -Is
• 10-' • 1 0 -9 • 1 0 -1~
8.272. 1 0 -9 8 . 3 3 8 • 1 0 -l~ 1 . 5 2 0 • 1 0 -15
Temperature 1.000. 1 0 -s 1 . 0 0 0 • 1 0 -8 1 . 4 5 8 • 10112
Temperature 1.000. 1.000. 1.432. 4.393 •
10 10 10 10
-5 -~ -11 -12
45°C: 9.173 9.166 1.306 60°C: 4.681 • 10-' 4 . 1 3 4 • 1 0 -9 5.507. 1 0 -1~ 2 . 0 3 3 • 1 0 -1~
5.185 5.688 8.551 2.301
• • • •
10-' 1 0 -11 10 -1' 10 -'4
basalt from oxidized
GR-3
299
1
{
I
-18
60oc
-22
45oc
40oc 0
E
- 26
x C
- 30
(x/m)= KC N TEMPERATURE, °C
K
N
r
40
6.0051 x 10 .4
0.9802
+0.999
45
7.1510 x 104
0.9833
+0,999
60
1.0388 x 102
0.9901
+0,999
- 34
-28
-24
{
I
I
-20
-16
-12
-8
In C , M
Fig. 6. Equilibrium isotherms for sorption of selenium from oxidized GR-3 groundwater.
It was assumed that the basalt surface coverage remained constant for the same x/m-values. The change of AH with x/m is shown in Fig. 8. No change in the selenium species sorbed over the investigated selenium concentration range was indicated although the species sorbed and the specific sorption mechanism were not determined. The best single criterion for distinguishing between chemisorption and physical adsorption is the heat of sorption (Trapnell, 1955). The rather high AH of oxidized selenium sorption indicates that chemisorption has occurred, with selenium chemically bonded rather than the physical bonding of sorption reactions. These conclusions were supported by the results of selenium sorption on Umtanum basalt at 25°C which was minimal. Because initial desorption apparently takes place with little difficulty (Fig. 3), it may also be concluded that the activation energy involved in the sorption reaction is probably relatively small in magnitude (Tompkins, 1978). Desorption would only occur with difficulty at the same temper-
300
2,0
E _= 1.0
3.00
3.05
3.10
3.15
3.20
1 / T x 103, K
Fig. 7. In K d p l o t t e d against reciprocal a b s o l u t e t e m p e r a t u r e at x / m = 1 . 2 6 6 . 1 0 -14 M Se/ g U m t a n u m basalt. 29.0
I
l
I
I
I
1
1
I
I
I
l
I
I
e
I
I
.J
O
E "-28.5 == ¢.1
",r-
Z
28.0
'I-
,.I
•.r 2 7 . 5
ee
j ~
=
.
+ 0.1035
[In(x/m)]
I--
z
I&.l
27.0
I'~"1
- 34
1 - 32
I - 30
I
1 - 28
I
I - 26
I
I - 24
I
I - 22
In(x/m), moles/g
Fig. 8. T h e c h a n g e in ~/~/as a f u n c t i o n o f surface coverage x / m .
l
I - 20
I -18
301
ature as sorption if a large activation energy barrier had to be traversed during desorption in addition to the desorbed- and sorbed-state energy difference. It can be stated that the use of a proposed high-level waste repository in basalt at ~ 1 0 0 0 m in depth and a 55--60°C ambient temperature would result in greatly retarded potential selenate-selenium migration rates under both oxidizing and reducing conditions, even though the exact chemisorption mechanism has not been determined. ACKNOWLEDGEMENT
This work was funded by the U.S. Department of Energy under Contract DE-AC06-76RL0 1830.
REFERENCES Ames, L.L., 1980. Hanford basalt flow mineralogy. Pac. Northw. Lab., Richland, Wash., PNL-2847. Barney, G.S. and Wood, B.J., 1980. Identification of key radionuclides in a nuclear waste repository in basalt. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-ST-9. Cary, E.E. and Allaway, W.H., 1969. The stability of different forms of selenium added to low-selenium soils. Soil Sci. Soc. Am. Proc., 33: 571--574. Cutter, G.A., 1982. Selenium in reducing waters. Science, 217: 829--831. Davies, E.E. and Watkinson, J.H., 1966. Uptake of native and applied selenium by pasture species, I. Uptake of Se by browntop, ryegrass, cocksfoot and white clover from Ataimure sand. N.Z.J. Agric. Res., 9: 317--327. Deju, R.A., Ledgerwood, R.K. and Long, P.E., 1978. Reference waste form, basalts and groundwater systems for waste interaction studies. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-LD-11 Farrington, R.A. and Daniels, A., 1979. Physical Chemistry. Wiley, New York, N.Y., 5th ed., pp. 151--153. Gissel-Nielsen, G., 1973. Uptake and distribution of added selenite and selenate by barley and red clover as influenced by sulfur. J. Sci. F o o d Agric., 24: 649---655. Hayes, M.H.B., Isaacson, P.J., Chia, K.Y. and Lees, A.M., 1982. Interactions of hydrazine and of hydrazine derivatives with soil constituents and soils. Defense Tech. Info. Cent. (D.T.I.C.), Cameron Stn., Alexandria, Va., Rep. No. AFOSR-TR-82-0231. Heilman, M.D. Carter, D.L. and Gonzalez, C.L., 1965. The ethylene glycol monoethyl ether (EGME) technique for determining soil-surface area. Soil Sci., 100: 409--413. Hodge, C.E. and Grutzeck, M.W., 1978. Preparation of standard Umtanum sample by Battelle-Northwest Laboratories. Rockwell Hanford Operat. Richland, Wash, RHOBWI-LD-3. Hudson, G.H., 1967. Supplement II, Nitrogen (Part II). In: J.W. Mellor (Editor), Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VIII. Longmans, Green and Co. Ltd., London, pp. 69--113. John, M.K., Saunders, W.M.H. and Watkinson, J.H., 1976. Selenium adsorption by New Zealand soils, I. Relative adsorption of selenite by representative soils and the relationship to soil properties. N . Z . J . Agric. Res., 19: 143--151. Ledgerwood, R.K., Myers, C.W. and Cross, R.W., 1978. Pasco Basin stratigraphic nomenclature. Rockwell Hanford Operat., Richland, Wash., RHO-BWI-LD-1. Lowell, S., 1979. Introduction to Powder Surface Area. Wiley, New York, N.Y., pp. 16-39.
302
Riese, W.C., Lee, M.J., Brookins, D.G. and Della Valle, R., 1970. Application of trace element geochemistry to prospecting for sandstone type uranium deposits. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration 1978, Proc. 7th Int. Geochem. Explor. Syrup., Golden, Colo., pp. 47---63. Tompkins, F.C., 1978. Chemisorption of Gases on Metals. Academic Press, N e w York, N.Y., pp. 1--9. TrapneU, B.M.W., 1955. Chemisorption. Butterworths, London, p. 2. Trotman-Dickenson, A.F., 1973. Comprehensive Inorganic Chemistry. Pergamon, N e w York, N.Y., p. 947. Warnant, P., Martin, H. and Herbillon, A.J., 1981. Kinetics of the selective extraction of iron oxides in geochemical samples. Association between Fe and Cu in acid brown soils.J. Geochem. Explor., 15: 635--644.