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Uranium and radium sorption on amorphous ferric oxyhydroxide
Chemical Geology, 4 0 ( 1 9 8 3 ) 1 3 5 - - 1 4 8
135
Elsevier S c i e n c e P u b l i s h e r s B.V., A m s t e r d a m - P r i n t e d in T h e N e t h e r l a n d s
URANIUM AND RADIUM SORPTION ON AMORPHOUS FERRIC OXYHYDROXIDE*
L L O Y D L. A M E S I, J E F F E. M c G A R R A H P A T R I C I A F. S A L T E R 2
I, B E C K Y
A. W A L K E R
I and
IPacific Northwest Laboratory, Richland, W A 99352 (U.S.A.) 2 Rockwell Hanford Operations, Richland, W A 99352 (U.S.A.) (Received March 15, 1982; revised and accepted November 26, 1982)
ABSTRACT A m e s , L.L., M c G a r r a h , J.E., Walker, B.A. a n d Salter, P.F., 1 9 8 3 . U r a n i u m a n d r a d i u m s o r p t i o n o n a m o r p h o u s ferric o x y h y d r o x i d e . C h e m . Geol., 40: 1 3 5 - - 1 4 8 . S o r p t i o n o f u r a n i u m o n a m o r p h o u s ferric o x y h y d r o x i d e was i n v e s t i g a t e d a t 25 ° a n d 60°C f r o m 0.01 M NaCl a n d 0.01 M N a H C O 3 s o l u t i o n s over a n initial U c o n c e n t r a t i o n range o f ~ 1 0 -4 M t o 5 • 10 -7 M ( 2 3 , 8 0 0 - - 9 3 . 2 p p b U). U r a n i u m d i s t r i b u t i o n c o e f f i c i e n t s ranged f r o m m o r e t h a n 2 • 106 ml g-i f r o m 0.01 M NaC1 at 25°C t o ~ 3 o 104 m l g-I f r o m 0.01 M N a H C O 3 at 25°C a n d fell r a p i d l y w i t h increasing initial U s o l u t i o n c o n c e n t r a t i o n . T h e u r a n i u m s o r p t i o n d a t a fit a D u b i n i n - - R a d u s h k e v i c h s o r p t i o n i s o t h e r m . S o r p t i o n o f r a d i u m o n a m o r p h o u s ferric o x y h y d r o x i d e also was i n v e s t i g a t e d at 25 ° a n d 60°C f r o m 0.01 M NaC1 over a n initial R a c o n c e n t r a t i o n range o f ~ 5 o 10-7--5 o 10 -1° M ( 1 1 3 - - 0 . 1 1 3 p p b Ra). T h e r a d i u m s o r p t i o n d a t a fit a F r e u n d l i c h s o r p t i o n i s o t h e r m w i t h r a d i u m d i s t r i b u t i o n c o e f f i c i e n t s ranging f r o m a l o w o f ~ 1 1 0 0 ml g-i at 60°C t o a high o f > 2 0 , 0 0 0 ml g-i a t 25°C, m u c h l o w e r t h a n c o m p a r a b l e u r a n i u m d i s t r i b u t i o n coefficients.
INTRODUCTION
Uranium and radium sorption on and migration through iron precipitates associated with natural environments are of interest to several disciplines including geochemistry o f uranium deposits, storage and isolation of uranium mill tailings and safety assessments of geologic radioactive waste storage sites. Starik et al. (1958) first studied the sorption of trace concentrations of uranium on Fe-oxyhydroxide. Sorption was optimal at ~ pH 5, and declined above and below pH 5. U was desorbed with a carbonate solution. Boyle (1970) discussed the two generalized hydrothermal pitchblende wall-rock zoning sequences (Table I). Hematite associations figure prominently in both zoning types. Rich et al. (1977) reported that in 77% of the hydrothermal *Work p e r f o r m e d f o r t h e U.S. D e p a r t m e n t o f E n e r g y u n d e r C o n t r a c t D E - A C 0 6 - 7 6 R L O 1830. 0009-2541/83/$03.00
© 1 9 8 3 Elsevier Science P u b l i s h e r s B.V.
136 TABLE I Alteration zoning around hydrothermal pitchblende veins (Boyle, 1970) Zone type I
Zone type II
(1) Pitchblende vein (2) Argillic alteration with quartz, sericite, carbonate and abundant hematite (3) Weak hematitization (4) Unaltered rock
(1) Pitchblende vein (2) Albitization with chloride, sericite, quartz and abundant hematite (3) Weak hematitization (4) Unaltered rock
uranium deposits examined, pitchblende was first deposited with or after the initial deposition of hematite. They state that the frequent occurrence of primary hematite in hydrothermal uranium deposits serves to distinguish them from the great majority of other hydrothermal ore deposits. Langmuir (1978) reported several uranium enrichment factors based on the work of Schmidt-Collerus (1967) including an enrichment factor on amorphous Fe(III)-oxyhydroxides that varied from 1.1 • 106 to 2.7 • l 0 s. Hsi and Langmuir (1980) also have investigated uranium sorption on several iron compounds, including amorphous ferric oxyhydroxides. Carbonate complexing of the uranyl ion was found to inhibit sorption on ferric oxyhydroxides, but Ca 2÷ or Mg2÷ ions at 10 -3 M concentrations did not affect sorption of the uranyl ion. Decreases in uranium sorption were proportional to carbonate solution concentrations. Ames et al. (1982) studied the sorption behavior of U on a secondary smectite associated with the Columbia River basalts. The Dubinin--Radushkevich sorption equation described uranium sorption on the secondary smectite before ferric o x y h y d r o x i d e removal and the Freundlich sorption equation described uranium sorption after ferric o x y h y d r o x i d e removal. Most of the uranium sorption was due to the presence of the ferric oxyhydroxides in the secondary smectite. Ra also was found to be efficiently sorbed by ferric oxyhydroxides but only under certain conditions, suggesting the possibility for Ra--U separations during migration in solution. There is widespread evidence in field studies for uranium and radium differential migration rates. For example, Granger et al. (1961) reported that Ra from the Ambrosia Lake, New Mexico, sandstone deposit had migrated a short distance, but rarely had a significant a m o u n t escaped from the immediate vicinity of the uranium ore body. U, in some cases, had migrated completely out of the area. Ra was found precipitated with barite in, or near, adjacent postfault ore bodies which may have formed from migrating U. Granger (1963) later reported considerable short-distance migration of Ra from the uranium ore bodies of the above deposit. 226Ra was nearly 41% deficient in the ore and ~220% in excess in the sandstone adjacent to the ore. He reported enrichment of 226Ra in adjacent barite, cryptomelane and mudstone lenses within the sandstone. Kaufmann et al. (1976) extensively rumpled groundwaters in the Grants, New Mexico, area. Their radium results
137 show the tendency of the Ra daughter to become separated from U, due initially to uranium milling operations, and to continue migrating due to inherent geochemical differences between U and Ra. Langmuir and Chatham (1980) also reported Ra displaced in the direction of groundwater flow at the Oakville, Texas, deposit in a sandstone. Several laboratory studies of uranium and radium mill tailings leaching were reviewed in Ames and Rai (1978). Komer and Rose (1977) in a study of the Rn contents of Pennsylvania groundwaters suggested that the increase of Ra with depth was due primarily to dissolution or reduction of ferric hydroxides on rock surfaces, removing a major site for radium sorption. Dyck (1978) reviewed the several factors involved in U and Ra migration, including sorption on clays and Feand Mn-oxyhydroxides. Hsi (1981) has investigated the sorption of U on several Fe-oxides including amorphous ferric oxyhydroxide. Several simple systems were investigated with a surface ionization and complexation model and a power exchange function model, both successfully. The present study was undertaken to demonstrate quantitatively the effects of temperature, uranyl cation concentration and solution bicarbonate concentration on the uranyl cation sorption efficiency of freshly precipitated ferric oxyhydroxides, and to compare uranium sorption to radium sorption under the same experimental conditions. METHODS OF INVESTIGATION One ml of reagent grade 0.1 M FeCla was added to 30 ml of 0.01 M NaOH in 50-ml polypropylene centrifuge tubes. The resulting ferric oxyhydroxide precipitates were centrifuged at 104 g, and the solution discarded. The precipitates were then washed, centrifuged three times in their respective 0.01 M NaC1 or 0.01 M NaHCO3 solutions and used within 4 hr. of synthesis. Two hundred lambda of 5 • 10 -a M NaOH was added to the 0.01 M NaC1 solution in one case to ascertain effects of higher pH on uranium sorption efficiency. Four aliquots of the initial solutions were taken, sealed in polycarbonate tubes without the precipitate and treated the same as the solutions containing ferric oxyhydroxide. 20 ml of solution containing radiochemically pure and carrier-free 2aaU as uranyl nitrate as a tracer plus depleted uranyl nitrate for the higher initial U concentrations were added to the precipitate from the 1 ml 0.1 M FeC13 solution, yielding an iron weight to solution volume of 0.279 g 1-1 Fe. Solutions and ferric oxyhydroxide were sealed in the polypropylene tube, contacted with agitation for seven days at a temperature of 25 ° or 60°C, centrifuged, unsealed, sampled and the 233U remaining in solution determined by scintillation counting. A count of the initial solution samples allowed determination of the U sorbed on the ferric oxyhydroxide by difference. All tubes were checked for wall sorption. Uranium tube-wall sorption was relatively high from the 0.01 M NaC1 initial solutions. However, all other solutions, including the 0.01 M NaC1 solutions containing ferric oxyhydroxide, showed less than 2% tube-wall sorption.
138 A final pH and dissolved oxygen measurement were taken following pipeting of the sample for counting. The 60°C solutions were resealed and cooled to 25°C before pH and dissolved oxygen were measured. The pH variation of aqueous solutions due to temperature changes is generally limited by buffering due to changes in the dissociation constant (Kw) of water (Krauskopf, 1979). A pH change of less than - 0 . 6 was measured in several instances between the 25°C and 60°C pH's. Dissolved oxygen measurements averaged 8.25 and 5.30 mg 1-' at 25 ° and 60°C, respectively. The 60°C samples were sealed originally to avoid outgassing of the 0.01 M NaHCO3 solutions. Temperatures were controlled in a New Brunswick ® incubator--rotator to within +0.5°C. From the scintillation counting efficiency, initial and equilibrium solution counts per unit volume and specific activity of the 233U, the concentration of U remaining in the equilibrium solution and on the solid were calculated. Experiments were in triplicate. Experimental values given in Table I for the various quantities are mean values for the triplicate samples. The ferric oxyhydroxide was not characterized other than to determine that it was noncrystalline, as indicated from X-ray diffraction tracings. RESULTS AND DISCUSSION Uranium sorption data on ferric o x y h y d r o x i d e are listed in Table II for two solutions (0.01 M NaC1 and 0.01 M NaHCO3), two temperatures (25 ° and 60°C), two pH-values at 60°C and four U concentrations (1.00 • 10 - 4 5.00 • 10-TM ). Twelve experiments were completed to determine each isotherm. These uranium sorption data were found to best fit the Dubinin--Radushkevich (DR) sorption isotherm (Dubinin and Radushkevich, 1947). In its linearized form, the DR equation is: i n (x/m) = In Xm + Be 2
where x / m is the a m o u n t of U sorbed on the ferric o x y h y d r o x i d e in mol g-l; Xm is the maximum amount of U that can be sorbed on the ferric oxyhydroxide for the experimental conditions in mol g - ' , B is a constant with the dimensions of energy; and: e = R T ln(1 + 1/C)
where R is the gas constant in kJ deg-' mol-1; T is the absolute temperature; and C is the equilibrium uranium solution concentration in molarity. Plotting l n ( x / m ) vs. e 2 results in a straight line as shown in Fig. 1. A linear regression by the least-squares method on paired values of l n ( x / m ) and e 2 resulted in the derivation of X m- and B-values as shown in Table III. The correlation coefficient (r) is a statistical measure of how well the experimental data points fit the regression line. Sy • x, a standard deviation from regression, is a measure of dispersion applied to differences between estimated and
139 TABLE II Mean experimental uranium sorption data on ferric oxyhydroxide
Temperature
InitialU
Final U
Solids loading
Average
(°C)
(10 -s M)
(10 -8 M)
(10 -~ mol g-~ U)
final pH, 25°C
(A) 0.01 M NaCI: 25
10.05 1.053 0.1053 0.04891
74.84 4.588 0.3087 0.1034
357.2 37.54 5.371 1.748
6.85 6.80 6.90 6.90
60
10.06 1.053 0.1053 0.04891
133.0 3.480 0.4987 0.1471
355.5 37.58 5.364 1.746
6.80 6.85 6.95 6.90
60
10.06 1.056 0.1546 0.05461
359.4 25.34 5.535 1.955
8.15 8.15 8.25 8.35
23.17 0.2268 0.03376 0.01193
(B) 0.01 M NaHCO 3 : 25
10.06 1.053 0.1529 0.05428
4,563 213.3 21.95 6.479
196.8 30.07 4.689 1.702
8.60 8.65 8.65 8.80
60
10.05 1.053 0.1530 0.05377
4,110 157.7 16.39 5.064
212.7 32.06 4.892 1.744
8.70 8.75 8.75 8.80
I
i
I ' ' ' ' I
....
,~
I'
' I ' I ' '
~
''I
' ' ' ' i
''
001_~ ~c, o
' _
oc
• 25°C ~.
~
~
~
.•
-10
0,01 M NaHCO3 [3 ~ c
-
E
5 -12
-14 i
I
I £2
Fig. 1. Uranium sorption on ferric oxyhydroxide plotted as Dubinin--Radushkevich (DR) sorption isotherms.
140 T A B L E III
Dubinin--Radushkevich equation constants and statistical measurements for uranium sorption by ferric oxyhydroxide Solution type
Temperature
xlm = X m e x p ( B e 2)
(°C)
Xm
pH range, 25°C
B
r
Sy • x (ln units)
(tool U/
kg solid) NaC1 NaC1 NaC1 + NaOH
25 60 60
29.869 28.055 25.131
0.00376 --0.00303 0.00232
....0.9983 --0.9976 --0.9980
± 0.1680 -+ 0.1998 ± 0.1816
6.80-6.90 6.80--6.95 8.15--8.35
NaHCO 3 NaHCO 3
25 60
3.116 3.457
-0.00454 -0.00349
0.9998 -0.9995
± 0.0451 ± 0.0860
8.60--8.80 8.70--8.80
observed values of Y[ln(x/m)] in linear regression. Xm-values supposedly represent uranium loading maxima. However, the uranium Xm-values for the 0.01 M NaC1 solution would suggest that > 7 kg U would sorb on 1 kg Fe(III) as the oxyhydroxide, an unlikely situation. A ferric oxyhydroxide equilibration with an initial U concentration of 1.00 • 10 -3 M showed that '
I
-9
g E . -11 0.01 M NaCI pH 7~0 OR LESS
/ /
/ /
5
//
o ~oc
J -13
-15
-15
• 25°C
_ /
0.01 M NaHCO3 60oc
/,,
I
I -13
i
I -11
=
I -9
i
-7
Ln C], M
Fig. 2. Natural logarithm of equilibrium ferric oxyhydroxide uranium loading
natural logarithm o f the initial uranium solution concentration (CI).
(x/m) vs.
141
uranium sorption m uc h above 1.00 • 10 -4 M in initial uranium solution concentration was n o t described by the DR sorption isotherm. However, th e DR constants are useful in t hat t h e y can be used in conjunct i o n with Freundlich-like constants to describe uranium sorption efficiency on ferric o x y h y d r o x i d e as a f unc t i on of temperature, solution com posi t i on and U c o n c e n t r a t i o n at 1.00 • 10 -4 M Cx or less. The Freundlich-like sorption isotherms are shown in Fig. 2 where In CI, the initial uranium solution conc e n tr atio n in molarity is pl ot t ed vs. l n ( x / m ) defined above. A normal linearized Freundlich {1922) sorption equation is: l n ( x / m ) = In K + n In C
where C is the equilibrium uranium solution concentration; and K and n are constants. Th e linearized relationship shown in Fig. 2 is: i n ( x / m ) = In L + m In CI
These Freundlich-like constants L and m for uranium sorption on ferric oxyh y d r o x i d e and associated statistical parameters are given in Table IV. The Freundlich-like constants can be used t o c o m p u t e an x / m - v a l u e f r o m a given Crvalue. Th e c o m p u t e d x / m is then used with the DR constants to generate a C-value. A uranium distribution coefficient (D) can t h e n be determined from the relationship: x/m
D - 0.001 C
(ml g-l)
T h e D-value is similar to a Kd-value which is, however, based directly on the mean o f usually three experimental replicates. The D is based on four o f t he means usually used to determine a single Kd. Thus the effects of temperature, solution bicarbonate c o n t e n t and uranium c o n c e n t r a t i o n on D can be d e ter min ed and c o m p a r e d as a f u n c t i o n of CI, as shown in Fig. 3. The presence o f bicarbonate in solution has the greatest effect on uranium
TABLE IV Freundlich-like constants for uranium sorption on ferric oxyhydroxide Solution type
Temper- x / m ffi LCI m
pH range,
ature
(°C)
25 °C
L
m
r
Sy • x (In units)
NaCl NaCl NaCl + N a O H
25 60 60
3.51527 3.47696 2.63239
0.99880 0.99806 0.98293
+1.0000 +1.0000 +0.9963
± 0.0007 ± 0.0031 ± 0.2777
6.80---6.90 6.80--6.95 8.15--8.35
NaI-ICO 3 NaHCO 3
25 60
0.92206 1.08405
0.91113 0.92003
+0.9992 +0.9992
± 0.1007 ± 0.1037
8.60--8.80 8.70--8.80
142 I
106
10 5
60oc
25°C " ~ 0.01 M NaHCO 3
lO4
103 -18
I
I -16
I
I
I
-14
1 -12
J
I -10
L -8
Ln C I, M__
Fig. 3. Natural logarithm o f the initial uranium concentration in solution (CI) vs. the uranium equilibrium coefficient (D) for ferric oxyhydroxide at two temperatures and two solution compositions.
sorption efficiency, lowering the 23°C D-values at a ln(Ci)-value of -16.00 from greater than 106 in 0.01 M NaC1 to ~ 4 • 104 in 0.01 M NaHCO3, due to the formation of uranyl carbonate complexes. The ferric o x y h y d r o x i d e is primarily a cation-exchange material at these pH-values. The heat-sensitivity of these carbonate complexes is the cause o f the 25 ° and 60°C reversals in going from a 0.01 M NaC1 to a 0.01 M NaHCO3. Normally, a rise in
143
temperature would lower the D-value in a cation-exchange system since solution to solid uranium exchange is an exothermic process. The D-values fall rapidly with increasing initial uranium solution concentration. These D curves are given, and are valid, only within the experimental Crvalues shown in Table II. The D-values of Fig. 3 are similar in magnitude to those reported b y Langmuir (1978), partially based on the work of Schmidt-Collerus (1967), for uranium sorption on ferric oxyhydroxide. With D-values of > 106 at low Crvalues, relatively minor amounts of ferric oxy-
'
I
'
I
'
I
'
I
'
10 7
108
60°C
le
-18
,
I
-16
*
I
J
I
-14 -12 LnCI,M
i
,
I
-10
i
-8
Fig. 4. Natural logarithm of the initial uranium concentration in solution (CI) vs. the uranium equilibrium coefficient (D) for ferric oxyhydroxide at two pH ranges.
144
hydroxide could sorb and concentrate a significant a m o u n t of U from solution. Though the relationship between the experimental conditions of this study and the sorption of U on ferric oxyhydroxides in nature are probably dissimilar, the occurrence of hematite in the majority of hydrothermal uranium deposits (Rich et al., 1977) m a y indicate the existence of a natural U-Fe(III)-correlation. Uranium removal efficiencies (D-values) were compared as a function of pH as shown in Fig. 4. The pH 8.15--8.35 range is close to the pH of zero point of charge for amorphous ferric o x y h y d r o x i d e (Stumm and Morgan, 1981). The higher-pH 0.01 M NaC1 solution showed an enhanced sorption efficiency over the U concentration range studied. The higher-pH 0.01 M NaC1 uranium sorption efficiencies cannot be directly compared to those in the 0.01 M NaHCO3 solution because there is still a significant difference in pH between the two solutions. However, these data do tend to indicate that the major differences in uranium sorption efficiencies between NaC1 and NaHCO3 solutions are primarily due to uranium carbonate complexing rather than due to pH difference. Radium sorption on ferric o x y h y d r o x i d e also was examined in 0.01 M NaCl solutions. The initial 0.01 M NaHCO3 solution radium sorption results showed little, if any, change from the 0.01 M NaC1 results, and so were not completed. The radium sorption results are given in Table V. The sorption data were found to fit a Freundlich sorption isotherm. The constants for both Freundlich and Freundlich-like plots are given in Table VI. The usual Freundlich plots are shown in Fig. 5. Using the Freundlich-like and Freundlich constants as outlined above, a radium distribution coefficient (D) was generated and plotted against the natural log of the initial radium solution concentration (ln CI) as shown in Fig. 6. The 25 ° and 60°C, pH ~ 7, D curves for Ra sorbed on ferric o x y h y d r o x i d e can be compared to those obtained for U in pH ~ 7 0.01 M NaC1 solution sorbed on ferric oxyhydroxide. The radium D curves are more than two orders of magnitude lower than comparable uranium sorption curves. A few exploratory experiments with addiTABLE V Mean experimental radium s o r p t i o n data on ferric oxyhydroxide from 0.01 M NaC1 Temperature (°C)
Initial Ra ( 1 0 - ' M)
Final Ra ( 1 0 - ' M)
Solids loading (10 -8 mol g-' Ra)
Average final pH, 25°C
25
47.55 5.153 0.5209
26.4 1.998 0.1076
75.74 11.3 1.064
7.00 7.00 6.95
60
47.95 4.973 0.5336 0.0508
40.72 3.563 0.4275 0.0387
25.89 2.131 0.3799 0.04321
6.85 6.80 6.85 6,80
145 TABLE VI F r e u n d l i c h a n d F r e u n d l i c h - l i k e c o n s t a n t s f o r r a d i u m s o r p t i o n o n ferric o x y h y d r o x i d e Solution type
Temperature (°C)
NaC1 NaCI
25 60
x/m
= KC n
K
n
r
Sy • x (In u n i t s )
0.099773 0.152836
0.775948 0.910189
+0.9996 +0.9990
± 0.0609 ± 0.1491
L
m
r
Sy
0.770547 0.139857
0.945511 0.917793
+0.9986 +0.9977
± 0.1225 ± 0.2224
'
I
'
x/m
NaCI NaCI
-14
25 60
,-,_ i
-15
'
I
= LCI m
I
'
'
I
I
'
o
I
x (ln u n i t s )
'
-16
-17
-- Y / 7
"
E -18
-19
-20
-21
-221-22
-21
-20
-19
-18 LnC, M
-17
-16
-15
-14
Fig. 5. R a d i u m s o r p t i o n o n ferric o x y h y d r o x i d e p l o t t e d as F r e u n d l i c h s o r p t i o n i s o t h e r m s .
146 '
I '''~'
I
'
I
,
I
r'
15,000
10,000
5000
H
o -22
,
I -21
,
I -20
~
I
,,i
- 19
L -18
,
-1"/
I - 16
I - 15
Ln CI,
Fig. 6. Natural logarithm of the radium concentration in solution (CI) vs. the radium equilibrium coefficient (D) for ferric oxyhydroxide.
tion of Ca 2+ or Ba ~÷ to Ra-containing systems showed that these constituents can have significant effects on radium sorption. A recent article by Benjamin and Leckie (1981) has also examined the sorption of ions on amorphous iron oxyhydroxide. Their sorption isotherms for Cu and Zn appear to fit DR isotherms reasonably well, as did the ttranyl ion sorption of this study, although their results are presented as multiplesite population Freundlich isotherms. Their Freundlich lead sorption isotherm, on the other hand, was comparable to our Freundlich radium sorption isotherms over the concentration ranges investigated. Natural solutions containing both U and Ra migrating over amorphous Fe-oxides could result in several events: (1) Total concentration high and alkaline-earth metals low: minor U sorption and major Ra sorption on Fe-oxides.
147
(2) Total concentration low and alkaline-earth metals low: minor to major U and Ra sorption on Fe-oxides. (3) Total concentration low and alkaline-earth metals high: major U sorption and minor Ra sorption on Fe-oxides. (4) Total concentration high and alkaline-earth metals high: minor U and Ra sorption on Fe-oxides. The presence of secondary Fe-oxides on rock surfaces and joints could result in enhanced U and Ra sorption and hence contribute to the safe geologic storage of radioactive wastes containing them. The unaltered primary rocks often exhibit relatively poor sorption characteristics for U (Langmuir, 1978). ACKNOWLEDGEMENT
The authors would like to acknowledge the encouragement and financial assistance of the Rockwell Hanford Operations, Basalt Waste Isolation Program, U.S. Department of Energy.
REFERENCES Ames, L.L. and Rai, D., 19.78. Radionuclide interactions with soil and rock media, Vol. I. Processes influencing radionuclide mobility and retention, element chemistry and geochemistry, conclusions and evaluations. Environ. Prot. Agency, Washington, D.C., Vol. 1 of two vols. E P A 520/6-78-007, pp. 3-142--3-149. Ames, L.L., McGarrah, J.E., Walker, B.A. and Salter, P.F., 1982. Sorption of uranium and cesium by Hanford basalts and an associated secondary smectite. Chem. Geol., 35: 205--225. Benjamin, M.M. and Leckie, J.O., 1981. Multiple-site adsorption of Cd, Cu, Zn and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci., 79: 209--221. Boyle, R.W., 1970. Regularities in wall rock alteration phenomena associated with epigenetic deposits. In: Z. Pouba and M. Stemprok (Editors), Problems of Hydrothermal Ore Deposition, Akad~mia KiadS, Budapest, pp. 233--260. Dubinin, M.M. and Radushkevich, L.V., 1947. Equation of the characteristic curve of activated charcoal. Proc. Acad. Sci., Phys. Chem. Sect., U.S.S.R., 55: 331--333. Dyck, W., 1978. The mobility and concentration of uranium and its decay products in temperate surficial liners. In: Uranium Deposits, Their Mineralogy and Origin. Short Course Handbook, Vol. 3. University of Toronto Press, Toronto, Ont., pp. 57--100. Freundlich, H., 1922. Colloid and Capillary Chemistry. Methion and Co., London, pp. 172--179. Granger, H.C., 1963. Radium migration and its effect on the apparent age of uranium deposits at Ambrosia Lake, New Mexico. U.S. Geol. Surv., Prof. Pap., 475-B: B60-B62. Granger, H.C., Santos, E.S., Dean, B.G. and Moore, F.B., 1961. Sandstone-type uranium deposits at Ambrosia Lake, New M e x i c o - An interim report. Econ. Geol., 56: 1179-1210. Hsi, D.C., 1981. Sorption of uranium(VI) by iron oxides. Ph.D. Thesis, Colorado School of Mines, Golden, Colo., 154 pp. Hsi, D.C. and Langmuir, D., 1980. The effects of carbonate complexing on the adsorption of uranyl ion by ferric oxyhydroxides. Prog. 1980 Annu. Meet., Geol. Soc. Am., Nov. 17--20, 1980, Atlanta, Ga., Abstr., p. 452.
148 Kaufmann, R.D., Eadie, G.G. and Russell, C.R., 1976. Effects of uranium mining and milling on groundwater in the Grants Mineral Belt, New Mexico. Ground Water, 14: 296--308. Korner, L.A. and Rose, A.W., 1977. Radon in streams and groundwaters of Pennsylvania as a guide to uranium deposits. U.S. Dep. Energy, Washington, D.C., Rep. GJ0-165920: 67--73. Krauskopf, K.B., 1979. Introduction to Geochemistry. McGraw-Hill, New York, N.Y. (see especially pp. 26--28). Langmuir, D., 1978. Uranium s o l u t i o n - m i n e r a l equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta, 42: 547--569. Langmuir, D. and Chatham, J.R., 1980. Groundwater prospecting sandstone-type uranium deposits: a preliminary comparison of the merits of mineral--solution equilibria, and single-element tracer methods. J. Geochem. Explor., 13: 201--219. Rich, R.A., Holland, H.D. and Petersen, U., 1977. Hydrothermal Uranium Deposits. Elsevier, Amsterdam, 264 pp. (see especially pp. 10--18). Schmidt-Collerus, J.J., 1967. Research in uranium geochemistry -- Investigations of the relationship between organic matter and uranium deposits, Denver Research Institute, Denver, Colo., Open-File Rep. GJO-933-I, 141 pp. Starik, I.Ye., Starik, F.Ye. and Apollonova, A.N., 1958. Adsorption of traces of uranium on iron hydroxide and its desorption by the carbonate method. Zh. Neorg. Khim., 3 : 1. Stumm, W. and Morgan, J.J.,1981. Aquatic Chemistry. Wiley, N e w York, N.Y., 631 pp.
135
Elsevier S c i e n c e P u b l i s h e r s B.V., A m s t e r d a m - P r i n t e d in T h e N e t h e r l a n d s
URANIUM AND RADIUM SORPTION ON AMORPHOUS FERRIC OXYHYDROXIDE*
L L O Y D L. A M E S I, J E F F E. M c G A R R A H P A T R I C I A F. S A L T E R 2
I, B E C K Y
A. W A L K E R
I and
IPacific Northwest Laboratory, Richland, W A 99352 (U.S.A.) 2 Rockwell Hanford Operations, Richland, W A 99352 (U.S.A.) (Received March 15, 1982; revised and accepted November 26, 1982)
ABSTRACT A m e s , L.L., M c G a r r a h , J.E., Walker, B.A. a n d Salter, P.F., 1 9 8 3 . U r a n i u m a n d r a d i u m s o r p t i o n o n a m o r p h o u s ferric o x y h y d r o x i d e . C h e m . Geol., 40: 1 3 5 - - 1 4 8 . S o r p t i o n o f u r a n i u m o n a m o r p h o u s ferric o x y h y d r o x i d e was i n v e s t i g a t e d a t 25 ° a n d 60°C f r o m 0.01 M NaCl a n d 0.01 M N a H C O 3 s o l u t i o n s over a n initial U c o n c e n t r a t i o n range o f ~ 1 0 -4 M t o 5 • 10 -7 M ( 2 3 , 8 0 0 - - 9 3 . 2 p p b U). U r a n i u m d i s t r i b u t i o n c o e f f i c i e n t s ranged f r o m m o r e t h a n 2 • 106 ml g-i f r o m 0.01 M NaC1 at 25°C t o ~ 3 o 104 m l g-I f r o m 0.01 M N a H C O 3 at 25°C a n d fell r a p i d l y w i t h increasing initial U s o l u t i o n c o n c e n t r a t i o n . T h e u r a n i u m s o r p t i o n d a t a fit a D u b i n i n - - R a d u s h k e v i c h s o r p t i o n i s o t h e r m . S o r p t i o n o f r a d i u m o n a m o r p h o u s ferric o x y h y d r o x i d e also was i n v e s t i g a t e d at 25 ° a n d 60°C f r o m 0.01 M NaC1 over a n initial R a c o n c e n t r a t i o n range o f ~ 5 o 10-7--5 o 10 -1° M ( 1 1 3 - - 0 . 1 1 3 p p b Ra). T h e r a d i u m s o r p t i o n d a t a fit a F r e u n d l i c h s o r p t i o n i s o t h e r m w i t h r a d i u m d i s t r i b u t i o n c o e f f i c i e n t s ranging f r o m a l o w o f ~ 1 1 0 0 ml g-i at 60°C t o a high o f > 2 0 , 0 0 0 ml g-i a t 25°C, m u c h l o w e r t h a n c o m p a r a b l e u r a n i u m d i s t r i b u t i o n coefficients.
INTRODUCTION
Uranium and radium sorption on and migration through iron precipitates associated with natural environments are of interest to several disciplines including geochemistry o f uranium deposits, storage and isolation of uranium mill tailings and safety assessments of geologic radioactive waste storage sites. Starik et al. (1958) first studied the sorption of trace concentrations of uranium on Fe-oxyhydroxide. Sorption was optimal at ~ pH 5, and declined above and below pH 5. U was desorbed with a carbonate solution. Boyle (1970) discussed the two generalized hydrothermal pitchblende wall-rock zoning sequences (Table I). Hematite associations figure prominently in both zoning types. Rich et al. (1977) reported that in 77% of the hydrothermal *Work p e r f o r m e d f o r t h e U.S. D e p a r t m e n t o f E n e r g y u n d e r C o n t r a c t D E - A C 0 6 - 7 6 R L O 1830. 0009-2541/83/$03.00
© 1 9 8 3 Elsevier Science P u b l i s h e r s B.V.
136 TABLE I Alteration zoning around hydrothermal pitchblende veins (Boyle, 1970) Zone type I
Zone type II
(1) Pitchblende vein (2) Argillic alteration with quartz, sericite, carbonate and abundant hematite (3) Weak hematitization (4) Unaltered rock
(1) Pitchblende vein (2) Albitization with chloride, sericite, quartz and abundant hematite (3) Weak hematitization (4) Unaltered rock
uranium deposits examined, pitchblende was first deposited with or after the initial deposition of hematite. They state that the frequent occurrence of primary hematite in hydrothermal uranium deposits serves to distinguish them from the great majority of other hydrothermal ore deposits. Langmuir (1978) reported several uranium enrichment factors based on the work of Schmidt-Collerus (1967) including an enrichment factor on amorphous Fe(III)-oxyhydroxides that varied from 1.1 • 106 to 2.7 • l 0 s. Hsi and Langmuir (1980) also have investigated uranium sorption on several iron compounds, including amorphous ferric oxyhydroxides. Carbonate complexing of the uranyl ion was found to inhibit sorption on ferric oxyhydroxides, but Ca 2÷ or Mg2÷ ions at 10 -3 M concentrations did not affect sorption of the uranyl ion. Decreases in uranium sorption were proportional to carbonate solution concentrations. Ames et al. (1982) studied the sorption behavior of U on a secondary smectite associated with the Columbia River basalts. The Dubinin--Radushkevich sorption equation described uranium sorption on the secondary smectite before ferric o x y h y d r o x i d e removal and the Freundlich sorption equation described uranium sorption after ferric o x y h y d r o x i d e removal. Most of the uranium sorption was due to the presence of the ferric oxyhydroxides in the secondary smectite. Ra also was found to be efficiently sorbed by ferric oxyhydroxides but only under certain conditions, suggesting the possibility for Ra--U separations during migration in solution. There is widespread evidence in field studies for uranium and radium differential migration rates. For example, Granger et al. (1961) reported that Ra from the Ambrosia Lake, New Mexico, sandstone deposit had migrated a short distance, but rarely had a significant a m o u n t escaped from the immediate vicinity of the uranium ore body. U, in some cases, had migrated completely out of the area. Ra was found precipitated with barite in, or near, adjacent postfault ore bodies which may have formed from migrating U. Granger (1963) later reported considerable short-distance migration of Ra from the uranium ore bodies of the above deposit. 226Ra was nearly 41% deficient in the ore and ~220% in excess in the sandstone adjacent to the ore. He reported enrichment of 226Ra in adjacent barite, cryptomelane and mudstone lenses within the sandstone. Kaufmann et al. (1976) extensively rumpled groundwaters in the Grants, New Mexico, area. Their radium results
137 show the tendency of the Ra daughter to become separated from U, due initially to uranium milling operations, and to continue migrating due to inherent geochemical differences between U and Ra. Langmuir and Chatham (1980) also reported Ra displaced in the direction of groundwater flow at the Oakville, Texas, deposit in a sandstone. Several laboratory studies of uranium and radium mill tailings leaching were reviewed in Ames and Rai (1978). Komer and Rose (1977) in a study of the Rn contents of Pennsylvania groundwaters suggested that the increase of Ra with depth was due primarily to dissolution or reduction of ferric hydroxides on rock surfaces, removing a major site for radium sorption. Dyck (1978) reviewed the several factors involved in U and Ra migration, including sorption on clays and Feand Mn-oxyhydroxides. Hsi (1981) has investigated the sorption of U on several Fe-oxides including amorphous ferric oxyhydroxide. Several simple systems were investigated with a surface ionization and complexation model and a power exchange function model, both successfully. The present study was undertaken to demonstrate quantitatively the effects of temperature, uranyl cation concentration and solution bicarbonate concentration on the uranyl cation sorption efficiency of freshly precipitated ferric oxyhydroxides, and to compare uranium sorption to radium sorption under the same experimental conditions. METHODS OF INVESTIGATION One ml of reagent grade 0.1 M FeCla was added to 30 ml of 0.01 M NaOH in 50-ml polypropylene centrifuge tubes. The resulting ferric oxyhydroxide precipitates were centrifuged at 104 g, and the solution discarded. The precipitates were then washed, centrifuged three times in their respective 0.01 M NaC1 or 0.01 M NaHCO3 solutions and used within 4 hr. of synthesis. Two hundred lambda of 5 • 10 -a M NaOH was added to the 0.01 M NaC1 solution in one case to ascertain effects of higher pH on uranium sorption efficiency. Four aliquots of the initial solutions were taken, sealed in polycarbonate tubes without the precipitate and treated the same as the solutions containing ferric oxyhydroxide. 20 ml of solution containing radiochemically pure and carrier-free 2aaU as uranyl nitrate as a tracer plus depleted uranyl nitrate for the higher initial U concentrations were added to the precipitate from the 1 ml 0.1 M FeC13 solution, yielding an iron weight to solution volume of 0.279 g 1-1 Fe. Solutions and ferric oxyhydroxide were sealed in the polypropylene tube, contacted with agitation for seven days at a temperature of 25 ° or 60°C, centrifuged, unsealed, sampled and the 233U remaining in solution determined by scintillation counting. A count of the initial solution samples allowed determination of the U sorbed on the ferric oxyhydroxide by difference. All tubes were checked for wall sorption. Uranium tube-wall sorption was relatively high from the 0.01 M NaC1 initial solutions. However, all other solutions, including the 0.01 M NaC1 solutions containing ferric oxyhydroxide, showed less than 2% tube-wall sorption.
138 A final pH and dissolved oxygen measurement were taken following pipeting of the sample for counting. The 60°C solutions were resealed and cooled to 25°C before pH and dissolved oxygen were measured. The pH variation of aqueous solutions due to temperature changes is generally limited by buffering due to changes in the dissociation constant (Kw) of water (Krauskopf, 1979). A pH change of less than - 0 . 6 was measured in several instances between the 25°C and 60°C pH's. Dissolved oxygen measurements averaged 8.25 and 5.30 mg 1-' at 25 ° and 60°C, respectively. The 60°C samples were sealed originally to avoid outgassing of the 0.01 M NaHCO3 solutions. Temperatures were controlled in a New Brunswick ® incubator--rotator to within +0.5°C. From the scintillation counting efficiency, initial and equilibrium solution counts per unit volume and specific activity of the 233U, the concentration of U remaining in the equilibrium solution and on the solid were calculated. Experiments were in triplicate. Experimental values given in Table I for the various quantities are mean values for the triplicate samples. The ferric oxyhydroxide was not characterized other than to determine that it was noncrystalline, as indicated from X-ray diffraction tracings. RESULTS AND DISCUSSION Uranium sorption data on ferric o x y h y d r o x i d e are listed in Table II for two solutions (0.01 M NaC1 and 0.01 M NaHCO3), two temperatures (25 ° and 60°C), two pH-values at 60°C and four U concentrations (1.00 • 10 - 4 5.00 • 10-TM ). Twelve experiments were completed to determine each isotherm. These uranium sorption data were found to best fit the Dubinin--Radushkevich (DR) sorption isotherm (Dubinin and Radushkevich, 1947). In its linearized form, the DR equation is: i n (x/m) = In Xm + Be 2
where x / m is the a m o u n t of U sorbed on the ferric o x y h y d r o x i d e in mol g-l; Xm is the maximum amount of U that can be sorbed on the ferric oxyhydroxide for the experimental conditions in mol g - ' , B is a constant with the dimensions of energy; and: e = R T ln(1 + 1/C)
where R is the gas constant in kJ deg-' mol-1; T is the absolute temperature; and C is the equilibrium uranium solution concentration in molarity. Plotting l n ( x / m ) vs. e 2 results in a straight line as shown in Fig. 1. A linear regression by the least-squares method on paired values of l n ( x / m ) and e 2 resulted in the derivation of X m- and B-values as shown in Table III. The correlation coefficient (r) is a statistical measure of how well the experimental data points fit the regression line. Sy • x, a standard deviation from regression, is a measure of dispersion applied to differences between estimated and
139 TABLE II Mean experimental uranium sorption data on ferric oxyhydroxide
Temperature
InitialU
Final U
Solids loading
Average
(°C)
(10 -s M)
(10 -8 M)
(10 -~ mol g-~ U)
final pH, 25°C
(A) 0.01 M NaCI: 25
10.05 1.053 0.1053 0.04891
74.84 4.588 0.3087 0.1034
357.2 37.54 5.371 1.748
6.85 6.80 6.90 6.90
60
10.06 1.053 0.1053 0.04891
133.0 3.480 0.4987 0.1471
355.5 37.58 5.364 1.746
6.80 6.85 6.95 6.90
60
10.06 1.056 0.1546 0.05461
359.4 25.34 5.535 1.955
8.15 8.15 8.25 8.35
23.17 0.2268 0.03376 0.01193
(B) 0.01 M NaHCO 3 : 25
10.06 1.053 0.1529 0.05428
4,563 213.3 21.95 6.479
196.8 30.07 4.689 1.702
8.60 8.65 8.65 8.80
60
10.05 1.053 0.1530 0.05377
4,110 157.7 16.39 5.064
212.7 32.06 4.892 1.744
8.70 8.75 8.75 8.80
I
i
I ' ' ' ' I
....
,~
I'
' I ' I ' '
~
''I
' ' ' ' i
''
001_~ ~c, o
' _
oc
• 25°C ~.
~
~
~
.•
-10
0,01 M NaHCO3 [3 ~ c
-
E
5 -12
-14 i
I
I £2
Fig. 1. Uranium sorption on ferric oxyhydroxide plotted as Dubinin--Radushkevich (DR) sorption isotherms.
140 T A B L E III
Dubinin--Radushkevich equation constants and statistical measurements for uranium sorption by ferric oxyhydroxide Solution type
Temperature
xlm = X m e x p ( B e 2)
(°C)
Xm
pH range, 25°C
B
r
Sy • x (ln units)
(tool U/
kg solid) NaC1 NaC1 NaC1 + NaOH
25 60 60
29.869 28.055 25.131
0.00376 --0.00303 0.00232
....0.9983 --0.9976 --0.9980
± 0.1680 -+ 0.1998 ± 0.1816
6.80-6.90 6.80--6.95 8.15--8.35
NaHCO 3 NaHCO 3
25 60
3.116 3.457
-0.00454 -0.00349
0.9998 -0.9995
± 0.0451 ± 0.0860
8.60--8.80 8.70--8.80
observed values of Y[ln(x/m)] in linear regression. Xm-values supposedly represent uranium loading maxima. However, the uranium Xm-values for the 0.01 M NaC1 solution would suggest that > 7 kg U would sorb on 1 kg Fe(III) as the oxyhydroxide, an unlikely situation. A ferric oxyhydroxide equilibration with an initial U concentration of 1.00 • 10 -3 M showed that '
I
-9
g E . -11 0.01 M NaCI pH 7~0 OR LESS
/ /
/ /
5
//
o ~oc
J -13
-15
-15
• 25°C
_ /
0.01 M NaHCO3 60oc
/,,
I
I -13
i
I -11
=
I -9
i
-7
Ln C], M
Fig. 2. Natural logarithm of equilibrium ferric oxyhydroxide uranium loading
natural logarithm o f the initial uranium solution concentration (CI).
(x/m) vs.
141
uranium sorption m uc h above 1.00 • 10 -4 M in initial uranium solution concentration was n o t described by the DR sorption isotherm. However, th e DR constants are useful in t hat t h e y can be used in conjunct i o n with Freundlich-like constants to describe uranium sorption efficiency on ferric o x y h y d r o x i d e as a f unc t i on of temperature, solution com posi t i on and U c o n c e n t r a t i o n at 1.00 • 10 -4 M Cx or less. The Freundlich-like sorption isotherms are shown in Fig. 2 where In CI, the initial uranium solution conc e n tr atio n in molarity is pl ot t ed vs. l n ( x / m ) defined above. A normal linearized Freundlich {1922) sorption equation is: l n ( x / m ) = In K + n In C
where C is the equilibrium uranium solution concentration; and K and n are constants. Th e linearized relationship shown in Fig. 2 is: i n ( x / m ) = In L + m In CI
These Freundlich-like constants L and m for uranium sorption on ferric oxyh y d r o x i d e and associated statistical parameters are given in Table IV. The Freundlich-like constants can be used t o c o m p u t e an x / m - v a l u e f r o m a given Crvalue. Th e c o m p u t e d x / m is then used with the DR constants to generate a C-value. A uranium distribution coefficient (D) can t h e n be determined from the relationship: x/m
D - 0.001 C
(ml g-l)
T h e D-value is similar to a Kd-value which is, however, based directly on the mean o f usually three experimental replicates. The D is based on four o f t he means usually used to determine a single Kd. Thus the effects of temperature, solution bicarbonate c o n t e n t and uranium c o n c e n t r a t i o n on D can be d e ter min ed and c o m p a r e d as a f u n c t i o n of CI, as shown in Fig. 3. The presence o f bicarbonate in solution has the greatest effect on uranium
TABLE IV Freundlich-like constants for uranium sorption on ferric oxyhydroxide Solution type
Temper- x / m ffi LCI m
pH range,
ature
(°C)
25 °C
L
m
r
Sy • x (In units)
NaCl NaCl NaCl + N a O H
25 60 60
3.51527 3.47696 2.63239
0.99880 0.99806 0.98293
+1.0000 +1.0000 +0.9963
± 0.0007 ± 0.0031 ± 0.2777
6.80---6.90 6.80--6.95 8.15--8.35
NaI-ICO 3 NaHCO 3
25 60
0.92206 1.08405
0.91113 0.92003
+0.9992 +0.9992
± 0.1007 ± 0.1037
8.60--8.80 8.70--8.80
142 I
106
10 5
60oc
25°C " ~ 0.01 M NaHCO 3
lO4
103 -18
I
I -16
I
I
I
-14
1 -12
J
I -10
L -8
Ln C I, M__
Fig. 3. Natural logarithm o f the initial uranium concentration in solution (CI) vs. the uranium equilibrium coefficient (D) for ferric oxyhydroxide at two temperatures and two solution compositions.
sorption efficiency, lowering the 23°C D-values at a ln(Ci)-value of -16.00 from greater than 106 in 0.01 M NaC1 to ~ 4 • 104 in 0.01 M NaHCO3, due to the formation of uranyl carbonate complexes. The ferric o x y h y d r o x i d e is primarily a cation-exchange material at these pH-values. The heat-sensitivity of these carbonate complexes is the cause o f the 25 ° and 60°C reversals in going from a 0.01 M NaC1 to a 0.01 M NaHCO3. Normally, a rise in
143
temperature would lower the D-value in a cation-exchange system since solution to solid uranium exchange is an exothermic process. The D-values fall rapidly with increasing initial uranium solution concentration. These D curves are given, and are valid, only within the experimental Crvalues shown in Table II. The D-values of Fig. 3 are similar in magnitude to those reported b y Langmuir (1978), partially based on the work of Schmidt-Collerus (1967), for uranium sorption on ferric oxyhydroxide. With D-values of > 106 at low Crvalues, relatively minor amounts of ferric oxy-
'
I
'
I
'
I
'
I
'
10 7
108
60°C
le
-18
,
I
-16
*
I
J
I
-14 -12 LnCI,M
i
,
I
-10
i
-8
Fig. 4. Natural logarithm of the initial uranium concentration in solution (CI) vs. the uranium equilibrium coefficient (D) for ferric oxyhydroxide at two pH ranges.
144
hydroxide could sorb and concentrate a significant a m o u n t of U from solution. Though the relationship between the experimental conditions of this study and the sorption of U on ferric oxyhydroxides in nature are probably dissimilar, the occurrence of hematite in the majority of hydrothermal uranium deposits (Rich et al., 1977) m a y indicate the existence of a natural U-Fe(III)-correlation. Uranium removal efficiencies (D-values) were compared as a function of pH as shown in Fig. 4. The pH 8.15--8.35 range is close to the pH of zero point of charge for amorphous ferric o x y h y d r o x i d e (Stumm and Morgan, 1981). The higher-pH 0.01 M NaC1 solution showed an enhanced sorption efficiency over the U concentration range studied. The higher-pH 0.01 M NaC1 uranium sorption efficiencies cannot be directly compared to those in the 0.01 M NaHCO3 solution because there is still a significant difference in pH between the two solutions. However, these data do tend to indicate that the major differences in uranium sorption efficiencies between NaC1 and NaHCO3 solutions are primarily due to uranium carbonate complexing rather than due to pH difference. Radium sorption on ferric o x y h y d r o x i d e also was examined in 0.01 M NaCl solutions. The initial 0.01 M NaHCO3 solution radium sorption results showed little, if any, change from the 0.01 M NaC1 results, and so were not completed. The radium sorption results are given in Table V. The sorption data were found to fit a Freundlich sorption isotherm. The constants for both Freundlich and Freundlich-like plots are given in Table VI. The usual Freundlich plots are shown in Fig. 5. Using the Freundlich-like and Freundlich constants as outlined above, a radium distribution coefficient (D) was generated and plotted against the natural log of the initial radium solution concentration (ln CI) as shown in Fig. 6. The 25 ° and 60°C, pH ~ 7, D curves for Ra sorbed on ferric o x y h y d r o x i d e can be compared to those obtained for U in pH ~ 7 0.01 M NaC1 solution sorbed on ferric oxyhydroxide. The radium D curves are more than two orders of magnitude lower than comparable uranium sorption curves. A few exploratory experiments with addiTABLE V Mean experimental radium s o r p t i o n data on ferric oxyhydroxide from 0.01 M NaC1 Temperature (°C)
Initial Ra ( 1 0 - ' M)
Final Ra ( 1 0 - ' M)
Solids loading (10 -8 mol g-' Ra)
Average final pH, 25°C
25
47.55 5.153 0.5209
26.4 1.998 0.1076
75.74 11.3 1.064
7.00 7.00 6.95
60
47.95 4.973 0.5336 0.0508
40.72 3.563 0.4275 0.0387
25.89 2.131 0.3799 0.04321
6.85 6.80 6.85 6,80
145 TABLE VI F r e u n d l i c h a n d F r e u n d l i c h - l i k e c o n s t a n t s f o r r a d i u m s o r p t i o n o n ferric o x y h y d r o x i d e Solution type
Temperature (°C)
NaC1 NaCI
25 60
x/m
= KC n
K
n
r
Sy • x (In u n i t s )
0.099773 0.152836
0.775948 0.910189
+0.9996 +0.9990
± 0.0609 ± 0.1491
L
m
r
Sy
0.770547 0.139857
0.945511 0.917793
+0.9986 +0.9977
± 0.1225 ± 0.2224
'
I
'
x/m
NaCI NaCI
-14
25 60
,-,_ i
-15
'
I
= LCI m
I
'
'
I
I
'
o
I
x (ln u n i t s )
'
-16
-17
-- Y / 7
"
E -18
-19
-20
-21
-221-22
-21
-20
-19
-18 LnC, M
-17
-16
-15
-14
Fig. 5. R a d i u m s o r p t i o n o n ferric o x y h y d r o x i d e p l o t t e d as F r e u n d l i c h s o r p t i o n i s o t h e r m s .
146 '
I '''~'
I
'
I
,
I
r'
15,000
10,000
5000
H
o -22
,
I -21
,
I -20
~
I
,,i
- 19
L -18
,
-1"/
I - 16
I - 15
Ln CI,
Fig. 6. Natural logarithm of the radium concentration in solution (CI) vs. the radium equilibrium coefficient (D) for ferric oxyhydroxide.
tion of Ca 2+ or Ba ~÷ to Ra-containing systems showed that these constituents can have significant effects on radium sorption. A recent article by Benjamin and Leckie (1981) has also examined the sorption of ions on amorphous iron oxyhydroxide. Their sorption isotherms for Cu and Zn appear to fit DR isotherms reasonably well, as did the ttranyl ion sorption of this study, although their results are presented as multiplesite population Freundlich isotherms. Their Freundlich lead sorption isotherm, on the other hand, was comparable to our Freundlich radium sorption isotherms over the concentration ranges investigated. Natural solutions containing both U and Ra migrating over amorphous Fe-oxides could result in several events: (1) Total concentration high and alkaline-earth metals low: minor U sorption and major Ra sorption on Fe-oxides.
147
(2) Total concentration low and alkaline-earth metals low: minor to major U and Ra sorption on Fe-oxides. (3) Total concentration low and alkaline-earth metals high: major U sorption and minor Ra sorption on Fe-oxides. (4) Total concentration high and alkaline-earth metals high: minor U and Ra sorption on Fe-oxides. The presence of secondary Fe-oxides on rock surfaces and joints could result in enhanced U and Ra sorption and hence contribute to the safe geologic storage of radioactive wastes containing them. The unaltered primary rocks often exhibit relatively poor sorption characteristics for U (Langmuir, 1978). ACKNOWLEDGEMENT
The authors would like to acknowledge the encouragement and financial assistance of the Rockwell Hanford Operations, Basalt Waste Isolation Program, U.S. Department of Energy.
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