Clays and Clay Minerals, Vol. 31, No. 5, 321-334, 1983. SORPTION OF TRACE CONSTITUENTS FROM AQUEOUS SOLUTIONS ONTO SECONDARY MINERALS. I. URANIUM L. L. AMES, J. E. MCGARRAH, AND B. A. WALKER Battelle, Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352 Abstract--Well-characterized American Petroleum Institute clay standards, source clays from The Clay Minerals Society, and other secondary minerals were used to determine the effects of U concentration, temperature, and solution composition on U-sorption properties. Uranium concentrations ranged from about 1.00 x 10 -4 M to 4.00 x 10 7M, temperatures from 5 ~ to 65~ and solution compositions containing 0.01 M NaCl and 0.01 M NaHCO3. Silica gel efficiently sorbed uranyl carbonate anion complexes. The higher cation-exchange capacity materials most readily sorbed uranyl ions from the 0.01 M NaCl solution. Temperature increases tended to affect uranyl ion sorption adversely except when the U was present as carbonate complexes. Noncrystalline ferric oxyhydroxides sorbed uranyl ions much more efficiently than any of the secondary crystalline minerals studied. A method for accurately extrapolating U-sorption efficiencies between experimental points based on the Freundlich equation is presented. Key Words--Cation exchange, Clinoptilolite, Freundlich isotherm, Glauconite, lllite, Montmorillonite, Opal, Sorption, Uranium. INTRODUCTION Uranium is a relatively mobile element in temperate surface environments; its mobility as controlled by U-mineral solubility equilibria in low temperature waters is well documented (Langmuir, 1978a, 1978b). An additional, relatively unknown factor in U mobility is, however, that of sorption on secondary minerals commonly found on joint surfaces of rocks and in sed- iments normally encountered during aqueous trans- port. Quantitative sorption data are scarce. Goldsztaub and Wey (1955) reported that 7.5 g of U was sorbed from a 1% uranyl nitrate solution onto 100 g of calcined kaolinite. Starik et al. (1958) found that the sorption of trace concentration of U on iron hydroxide was optimum at about pH 5, and declined above and below this value. Uranium was desorbed with a carbonate solution. Rancon (1973) studied the sorption of U using four soils described as: (1) a river sediment containing a mixture of quartz, clay, calcite, and organic matter, (2) a river peat, (3) a sediment from Cadarache containing a mixture of quartz, clay, and calcite with no organic matter, and (4) a soil developed on an altered schist from near LaHague containing a mixture of quartz and clay but no calcite or organic matter. The first two soils were equilibrated with their river waters containing 10 ppm U and the last two soils were equilibrated with their respective groundwaters also containing 10 ppm U. The resulting U distribution coefficients are shown in Table 1, which also includes the Kd values on pure quartz, calcite, and illite. The clay minerals in Soils 1, 3, and 4 were not identified, nor were the soils further characterized. Rancon also examined the effects ofini- Copyright 1983, The Clay Minerals Society tial U concentration on Kd values. Both the U con- centration and solution pH changed as U was added to the solution. At 0.1 mg U/liter, the pH was 7.6, for example, and at 1.0 g U/liter, the pH was 3.5. Because the pH changes were a function of U concentration changes, the results are not easily interpreted. In ad- dition, the Kd concept is invalid above the trace U concentration (~ 1.0 mg U/liter). Uranium adsorption data at 1 ppm vs. Kd also were presented. For Soil 4, three peaks were observed: Kd 300 ml/g at about pH 5.5, Kd 2000 ml/g at pH 10, and Kd 270 ml/g at pH 12. Rancon suggested that the adsorption maxima rep- resented by the three peaks also represent electrokinetic potential maxima. Quartz was characterized as inert, calcite as a poor U adsorber, and clays as the best adsorbers of U from solution. Acid, organic-rich soils show much higher U Kd values than the alkaline peat (Soil 2) of Ranron's (1973) study. Langmuir (1978b) reported several U-enrichment materials based on the work of Schmidt-Collerus (1967) including noncrystalline titanium oxide (8 • 104 to 1 X 106), noncrystalline Fe(lII) oxyhydroxides (1.1 X 106 to 2.7 X 106), peat (104 to 106), fine-grained goethite (4 X 103), phosphorites (15), montmorillonite (6), and kaolinite (2). Giblin (1980) studied U sorption from a simulated groundwater containing 100 ~g U/liter (4.202 X 10-7 M) onto kaolinite over a pH range of 3.5 to 10. A maximum U distribution of 35,000 ml/g was attained at a pH of 6.5. Hsi (198 l) examined the sorption of U on hematite, goethite, and noncrystalline ferric oxyhydroxide as af- fected by pH, U concentration, and carbonate com- plexing. Tsunaskima et al. (1981) examined the sorp- 321
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Clays and Clay Minerals, Vol. 31, No. 5, 321-334, 1983.
SORPTION OF TRACE CONSTITUENTS FROM AQUEOUS SOLUTIONS ONTO SECONDARY MINERALS.
I. U R A N I U M
L. L. AMES, J. E. MCGARRAH, AND B. A. WALKER
Battelle, Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352
Abstract--Well-characterized American Petroleum Institute clay standards, source clays from The Clay Minerals Society, and other secondary minerals were used to determine the effects of U concentration, temperature, and solution composition on U-sorption properties. Uranium concentrations ranged from about 1.00 x 10 -4 M to 4.00 x 10 7 M, temperatures from 5 ~ to 65~ and solution compositions containing 0.01 M NaCl and 0.01 M NaHCO3. Silica gel efficiently sorbed uranyl carbonate anion complexes. The higher cation-exchange capacity materials most readily sorbed uranyl ions from the 0.01 M NaCl solution. Temperature increases tended to affect uranyl ion sorption adversely except when the U was present as carbonate complexes. Noncrystalline ferric oxyhydroxides sorbed uranyl ions much more efficiently than any of the secondary crystalline minerals studied. A method for accurately extrapolating U-sorption efficiencies between experimental points based on the Freundlich equation is presented. Key Words--Cation exchange, Clinoptilolite, Freundlich isotherm, Glauconite, lllite, Montmorillonite, Opal, Sorption, Uranium.
INTRODUCTION
Uranium is a relatively mobile element in temperate surface environments; its mobility as controlled by U-mineral solubility equilibria in low temperature waters is well documented (Langmuir, 1978a, 1978b). An additional, relatively unknown factor in U mobility is, however, that of sorption on secondary minerals commonly found on joint surfaces of rocks and in sed- iments normally encountered during aqueous trans- port. Quantitative sorption data are scarce. Goldsztaub and Wey (1955) reported that 7.5 g of U was sorbed from a 1% uranyl nitrate solution onto 100 g of calcined kaolinite. Starik et al. (1958) found that the sorption of trace concentration of U on iron hydroxide was opt imum at about pH 5, and declined above and below this value. Uranium was desorbed with a carbonate solution.
Rancon (1973) studied the sorption of U using four soils described as: (1) a river sediment containing a mixture of quartz, clay, calcite, and organic matter, (2) a river peat, (3) a sediment from Cadarache containing a mixture of quartz, clay, and calcite with no organic matter, and (4) a soil developed on an altered schist from near LaHague containing a mixture of quartz and clay but no calcite or organic matter. The first two soils were equilibrated with their river waters containing 10 ppm U and the last two soils were equilibrated with their respective groundwaters also containing 10 ppm U. The resulting U distribution coefficients are shown in Table 1, which also includes the Kd values on pure quartz, calcite, and illite. The clay minerals in Soils 1, 3, and 4 were not identified, nor were the soils further characterized. Rancon also examined the effects of ini-
Copyright �9 1983, The Clay Minerals Society
tial U concentration on Kd values. Both the U con- centration and solution pH changed as U was added to the solution. At 0.1 mg U/liter, the pH was 7.6, for example, and at 1.0 g U/liter, the pH was 3.5. Because the pH changes were a function of U concentration changes, the results are not easily interpreted. In ad- dition, the Kd concept is invalid above the trace U concentration (~ 1.0 mg U/liter). Uranium adsorption data at 1 ppm vs. Kd also were presented. For Soil 4, three peaks were observed: Kd 300 ml/g at about pH 5.5, Kd 2000 ml/g at pH 10, and Kd 270 ml/g at pH 12. Rancon suggested that the adsorption maxima rep- resented by the three peaks also represent electrokinetic potential maxima. Quartz was characterized as inert, calcite as a poor U adsorber, and clays as the best adsorbers of U from solution. Acid, organic-rich soils show much higher U Kd values than the alkaline peat (Soil 2) of Ranron's (1973) study.
Langmuir (1978b) reported several U-enrichment materials based on the work of Schmidt-Collerus (1967) including noncrystalline t i tanium oxide (8 • 104 to 1 X 106), noncrystalline Fe(lII) oxyhydroxides (1.1 X 106 to 2.7 X 106), peat (104 to 106), fine-grained goethite (4 X 103), phosphorites (15), montmoril lonite (6), and kaolinite (2). Giblin (1980) studied U sorption from a simulated groundwater containing 100 ~g U/liter (4.202 X 10 -7 M) onto kaolinite over a pH range of 3.5 to 10. A maximum U distribution of 35,000 ml/g was attained at a pH of 6.5.
Hsi (198 l) examined the sorption of U on hematite, goethite, and noncrystalline ferric oxyhydroxide as af- fected by pH, U concentration, and carbonate com- plexing. Tsunaskima et al. (1981) examined the sorp-
321
322 Ames, McGarrah, and Walker Clays and Clay Minerals
tion of U by Volclay over the concentration range of 1-300 ppm U. The sorption isotherms were reported to follow Langmuir-type curves at higher U-solution concentrations. Giblin et al. (1981) examined the mo- bility of U as affected by Eh and pH at 25~ Hydrous ferric oxide and kaolinite were used as the solid phases. High mobility in a pH-Eh range where U was ther- modynamically insoluble suggested that U was present in this region as a colloid.
Walton et al. (1981) examined the release of U from two volcanic glass sedimentary sequences in Texas dur- ing diagenesis. Uranium was not significantly mobi- lized during the solution of the glass. They reported that once U was effectively trapped by secondary phas- es, it did not move for about 30 million years. Nash et al. (1981) recently reviewed the few reported field occurrences of U deposition by sorption and concluded that sorption could be an important concentrating step prior to the formation of uranyl or uranous minerals. The lack of rigorous and comparable U-sorption data on characterized geological materials, however, made the role of sorption in U migration difficult to assess.
In addition to the migration and sorption of U and Ra in natural deposits, the subject also is pertinent to high-level radioactive waste disposal where certain clays are under consideration as a portion of the backfill component of the multiple barrier waste package (Wood and Aden, 1982). The primary function of the waste package backfill is to assist in meeting the Nuclear Regulatory Commission criteria that requires a radio- nuclide release rate of no more than one part in 105 of
the inventory from the engineered barrier system (waste package plus repository) after 1000 years. All other things being equal, radionuclide breakthrough time is a function of the backfill material equilibrium radio- nuclide distribution coefficient. The present investi- gation was conducted to determine the sorption of ura- nium on various clays and other secondary minerals as a function of U concentration, temperature, and solution composition.
MATERIALS AND METHODS
Characterization of the secondary minerals
The identification and location of the secondary minerals used in the sorption studies are given in Table 2. The clay minerals were either American Petroleum Institute clay standards (Kerr, 1950) or source clays from The Clay Minerals Society (van Olphen and Fri- plat, 1979). The various size ranges selected for use were obtained by grinding or elutriation. Chemical analyses of these materials for major components were made by Teflon-bomb digestion and inductively cou- pled argon-plasma before sodium solution contact (Ta- ble 3). Thorium analysis was by neutron activation and U, by atomic fluorescence. X-ray powder diffraction results showed that most of the minerals were free of detectable impurities with the exception of the illite and opal samples. The major constituent of the opal was a-cristobalite. Minor quartz contamination was carried over during recovery of the < 2-/~m illite frac- tion.
Cation-exchange capacities (CEC), measured with Cs and a Cs radionuclide (Cs 137) as chlorides at pH 7.0 and 25~ are given in Table 4. The procedure used was similar to that given by Routson et al. (1973). An ethylene glycol-monoethyl ether sorption method (Heilman et al., 1965) was utilized to measure the sur- face area of the secondary minerals (Table 4). The first number given represents a mean of three values; the following number is a standard deviation.
Following separations, the secondary minerals were contacted three times with 3 M NaC1 solutions, washed with methanol, and centrifugation to remove excess NaC1. Some care had to be taken with the kaolinite to avoid pH values below 5 in the NaC1 solutions.
Table 2. Identification and origin of secondary minerals.
Mineral Location Origin
Illite, <2 um Kaolinite, < 2 ~m Montmorillonite, <2 um
Nontron i t e , < 2 # m Glauconite, 20-50 mesh Clinoptilolite, 50-100 mesh Opal, 20-40 mesh Silica gel, 100-200 mesh, reagent grade
Fithian, Illinois Macon County, Georgia Apache County, Arizona
Garfield, Washington New Jersey coastal plain Death Valley Junction, California Virgin Valley, Nevada Synthetic
A.P.I. Clay Standard 35 A.P.I. Clay Standard 4 The Clay Minerals Society Source Clay
SAz- 1 A.P.I. Clay standard 33A Ward's Natural Science Establishment Anaconda Minerals Company, 1010 A Ward's Natural Science Establishment Fisher Scientific Company
Vol. 31, No. 5, 1983 Sorption of uranium onto clays and clinoptilolite
Table 3. Chemical analyses of the secondary minerals used in the uranium sorption work.
Two solut ions con ta in ing four U concen t ra t ions were used wi th the a b o v e minera l s . T h e uranyl concen t r a - t ions are g iven in Tab les 5 -7 wi th the u r a n i u m in 0.01 M NaC1 or 0.01 M NaHCO3. T h e two types o f so lu t ions a l lowed e x a m i n a t i o n o f the effects o f U - c a r b o n a t e c o m p l e x i n g on U so rp t ion a n d its in t e rac t ions wi th t e m p e r a t u r e a n d U concen t r a t i on .
Rad iochemica l l y pure and carr ier - f ree U 233 as uranyl n i t ra te was used to t race U so rp t ion on the minera ls . The use o f U 233 a l lowed accura te sc in t i l la t ion coun t ing o f U in to the pa r t per b i l l ion c o n c e n t r a t i o n range. De- p le ted u rany l n i t ra te (U 23s) was used a long wi th the U 233 to m a k e up the solut ions . T he specific ac t iv i ty o f U 233 was too h igh (9.48 • 10 3 Ci/g) to a l low its pract ical use m u c h a b o v e a so lu t ion c o n c e n t r a t i o n o f 1.0 • 10 -6 M.
Methods
T h r e e a l iquo ts o f each or ig inal so lu t ion before they were con tac t ed wi th the mine ra l s were set as ide for la te r coun t ing wi th the e q u i l i b r i u m so lu t ion al iquots . All so lu t ion a l iquo ts were fi l tered t h r ough 15-]k m e m - branes . Ten mil l i l i ters o f each or iginal so lu t ion was a d d e d for each g r am of minera l . Each so lu t ion-so l id e q u i l i b r i u m e x p e r i m e n t was c o n d u c t e d in t r ip l ica te in sealed, po lyp ropy lene tubes . T h e tubes c o n t a i n i n g the r ad ioac t ive so lu t ions were la ter rap id ly r insed wi th a few mil l i l i ters o f m e t h a n o l to r e m o v e all sol ids a n d r e c o u n t e d to ver i fy t ha t tube so rp t ion was less t h a n 2% of the total act ivi ty , or to a l low cor rec t ions for tube- wall sorp t ion .
T h e slurrys were gently ro ta t ed to assure un i fo rmi ty for a 30-day e q u i l i b r i u m period, A m i n i m u m of 30
days o f m i n e r a l - s o l u t i o n con tac t was r equ i red for the sys tem to a p p r o a c h chemica l equ i l ib r ium. Exper i - m e n t s were c o n d u c t e d at 5 ~ 25 ~ a n d 65~ in e n v i r o n - m e n t a l c h a m b e r s wh ich c o n t a i n e d facili t ies for agi tat- ing the samples a n d con t ro l l ed t e m p e r a t u r e s to wi th in _ 2~
Af te r a 30-day so lu t ion-so l id con tac t per iod, a 15- fi l tered a l iquo t o f each so lu t ion was c o u n t e d a long
wi th in i t ia l so lu t ion samples at the i r respec t ive t em- pera tures . F r o m the sc in t i l la t ion coun t ing efficiency, the ini t ia l a n d e q u i l i b r i u m so lu t ion counts , a n d the specific ac t iv i t ies o f the rad ionuc l ides , the concen t r a - t ions o f U in the e q u i l i b r i u m so lu t ion a n d on the m i n - eral were calculated. D i s so lved oxygen m e a s u r e m e n t s on final so lu t ions yie lded an average o f 8.3 mg O2/liter, or a c o m p u t e d Eh o f + 6 5 0 mY. Whereas , such solu- t ions were no t s a tu ra t ed in a t m o s p h e r i c 02, the env i - r o n m e n t s were no t sufficiently anox ic to reduce U(VI)
Table 4. Surface areas, ignition losses, and cation-exchange capacities of the based, freeze-dried minerals.
Cesium cation- exchange
900~ capacity ignition (meq/100 g,
Surface area (% wt. pH 7.0, Mineral (mVg _+ 1 s) loss) 25~
324 Ames, McGarrah , and Walker Clays and Clay Minerals
Table 5. Exper imenta l mean values for sorpt ion of u ran ium by secondary minerals at 65~ from 0.01 M NaC1 and 0.01 M NaHCO3 solutions.
............................................... 0.01 M NaCI ......................................................................................... 0.01 M NaHCO3 ............................................
Initial U Equilibrium U U on solid Initial U Equilibrium U U on solid Solid (mole/liter) (mole/liter) (mole/g) (mole/liter) (mole/liter) (mole/g)
l l l i te 1.005 X 10 -4 1.585 • 10 -5 9,307 x 10 7 1,004 x 10 4 4.047 x 10 -6 1.059 x 10 -6 1,039 • 10 -5 1.815 x 10 -8 9.428 • 10 -8 1,041 X 10 -5 3,748 X 10 -7 1.104 X 10 -7 1.393 • I0 6 1,754 x 10 -7 1.339 • 10 8 1,353 x 10 6 6.282 • 10 -8 1.418 • 10 s 3,458 x 10 7 4.309 X 10 -8 3,328 x 10 9 3.508 X 10 -7 1,683 • 10 -8 3.672 • 10 -9
Kaol ini te 1.005 X 10 4 3.545 X 10 -6 1.083 • 10 6 1,004 • 10 4 7.130 • 10 -s 2.447 X 10 -7 1,039 X 10- ' 3.775 • 10 -7 1.118 X 10 -7 1.041 X 10 -5 4.478 • 10 -6 6.629 • 10 8 1.393 • 10 -6 5.069 X 10 -8 1,499 • 10 8 1.353 X l0 6 2.433 X 10 -7 1.240 • 10 8 3.458 • 10 7 1.288 X 10 -5 3.720 • 10 9 3,508 • 10 -7 4.143 • 10 -9 3.874 • 10 -9
Montmor i l lon i t e 1,005 • 10 4 5.845 • 10 -6 2.129 • 10 6 1,004 • 10 -4 2.085 • 10 -5 1.790 X 10 -6 1.039 • 10 -5 3.540 • 10 -7 2.258 X 10 7 1,041 • 10 5 1.568 X 10 -6 9.946 • 10 8 1.393 • 10 6 4.787 • 10 -5 3.026 X 10 8 1.353 • 10 -6 2.955 X 10 -7 2.380 • 10 -8 3.458 X 10 -7 1,399 • 10 -8 7,465 X 10 9 3,508 • 10 7 8.764 • 10 -5 5,918 X 10 -9
Nont roni te 1.005 • 10 -4 3,918 X 10 -6 2 . [37 X I0 6 1,004 • 10 4 9.064 X 10 -5 2,159 • 10 -7 1.039 • 10 -5 4.031 • 10 -7 2,209 X 10 -7 1,041 X 10 -5 7.752 X 1.0 -6 5.881 • 10 8 1,393 X 10 6 4,843 X 10 -s 2.975 • 10 -8 1,353 X 10 -6 5.330 X 10 -7 1.770 • 10 -8 3,458 X l0 -7 1.415 • 10 -8 7.336 X 10 9 3,508 • 10 7 6.495 • 10 -8 6.325 X 10 -9
Glauconi te 1.005 • 10 -4 2.982 • 10 -6 1.057 • 10 6 1.004 • 10 4 4.355 X 10 -6 1.041 x 10 6 1.039 x 10 -5 4.783 x 10 -7 1.074 x 10 -7 1.041 X 10 -5 6.887 x 10 -7 1.054 X 10 7 1,393 • 10 -6 6.251 • 10 -5 1.442 x 10 8 1.353 • 10 6 1.003 • 10 -7 1.358 x 10 -8 3.458 • 10 7 1.956 • 10 -8 3.536 • 10 -9 3,508 • l0 -7 2,917 • 10 -8 3.432 • 10 9
Clinopt i lol i te 1.005 x 10 4 4.190 • 10 -6 6,831 X 10 -7 1,004 X 10 -4 7.120 • 10 -5 3,404 X 10 7 1.039 X 10 8 6,440 • 10 -7 1.136 • 10 7 1.041 x 10 -5 7,027 • 10 -6 3.944 X 10 -8 1,393 • 10 -6 6.645 x 10 -8 1,547 x 10 , 1,353 x 10 6 8.489 x 10 -7 5,877 • 10 -9 3.458 x 10 7 1.765 x 10 -8 3.826 X 10 -9 3,508 • 10 -7 3.304 X 10 -7 2.379 X 10 9
fo r t h e s e s o r p t i o n d a t a by t h e l i n e a r i z e d F r e u n d l i c h
e q u a t i o n , a F r e u n d l i c h - l i k e g r o u p o f c o n s t a n t s c a n a l s o
be g e n e r a t e d b y t he l i n e a r i z e d e q u a t i o n , l n ( x / m ) = In
L + p i n CI, w h e r e Ct is t he i n i t i a l U c o n c e n t r a t i o n in
s o l u t i o n a n d L a n d m a r e c o n s t a n t s . T h e s e c o n s t a n t s
a r e g i v e n in T a b l e 9 a l o n g w i t h t h e i r r e s p e c t i v e cor -
r e l a t i o n coe f f i c i en t (r) a n d s t a n d a r d d e v i a t i o n f r o m
r e g r e s s i o n (Sy. x) v a l u e s . T h e y a r e p o t e n t i a l l y m o r e use -
ful t h a n t h e a c t u a l F r e u n d l i c h c o n s t a n t s o f T a b l e 8
b e c a u s e t h e y c a n be u s e d to d e t e r m i n e a U l o a d i n g o n
a g i v e n s e c o n d a r y m i n e r a l w h e n o n l y t he i n i t i a l s o l u -
t i o n c o n c e n t r a t i o n (CI) is k n o w n .
D I S C U S S I O N
T h e 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 h a s b e e n w i d e l y
u s e d to fit s o r p t i o n o f v a r i o u s s u b s t a n c e s o n t o so i l s
( H a m a k e r a n d T h o m p s o n , 1972; R e i n b o l d et al., 1979),
a n d i t s u se h e r e is n o t u n i q u e . H a l s e y (1952 ) a n d A d -
a m s o n ( 1 9 7 6 ) s h o w e d t h e o r e t i c a l l y t h a t t he K a n d n
c o n s t a n t s i n t h e F r e u n d l i c h e q u a t i o n d e p e n d on t h e
e n e r g y a n d e n t r o p y o f s o r p t i o n a n d o n t he e n e r g y o f
i n t e r a c t i o n b e t w e e n t h e s o r p t i o n s i tes . T h e e q u a t i o n
d e s c r i b e s a s o r p t i o n i s o t h e r m f r o m a n i d e a l s o l u t i o n
by an e n e r g e t i c a l l y h e t e r o g e n e o u s se t o f s o r p t i o n s i tes ,
w i t h t h e s o r p t i o n e n e r g y v a r y i n g e x p o n e n t i a l l y . T h e
o r i g i n a l F r e u n d l i c h e q u a t i o n , h o w e v e r , for w h i c h t h e
s o r p t i o n i s o t h e r m w a s n a m e d , w a s p u r e l y e m p i r i c a l .
T h e i n s t a n c e i n w h i c h Ct is u s e d in t h i s w o r k r a t h e r
t h a n C is m e r e l y a u s e f u l v a r i a t i o n on F r e u n d l i c h ' s
o r i g i n a l s o r p t i o n i s o t h e r m .
O n e o f t he m o r e p r a c t i c a l m e a s u r e s o f t h e e f f i c i ency
o f a s o l i d in s o r b i n g a d i s s o l v e d s u b s t a n c e f r o m so lu -
t i o n is t h e d i s t r i b u t i o n coef f i c ien t , D. I f D v a l u e s a r e
o b t a i n e d a t s o l u t i o n - m i n e r a l e q u i l i b r i u m a n d u n d e r
c o m p a r a b l e e x p e r i m e n t a l c o n d i t i o n s , t h e y a l l o w a
c o m p a r i s o n to be m a d e o f t h e e f f i c i ency o f s e v e r a l
m i n e r a l s i n s o r b i n g a d i s s o l v e d c o m p o n e n t f r o m t h e
s a m e s o l u t i o n . A D v a l u e is d e f i n e d as t he e q u i l i b r i u m
c o n c e n t r a t i o n o f U o n t he m i n e r a l in m o l e / g d i v i d e d
by t he e q u i l i b r i u m s o l u t i o n c o n c e n t r a t i o n in m o l e / m l
a n d h a s the d i m e n s i o n s o f m l / g .
U s e o f t he s a m e C v a l u e t o c o m p u t e U d i s t r i b u t i o n
coe f f i c i en t s a l s o a l l o w s a s o r p t i o n c o m p a r i s o n b e t w e e n
m i n e r a l s . T h i s u se is s o m e w h a t a r t i f i c i a l b e c a u s e d i f -
3 2 6 A m e s , M c G a r r a h , a n d W a l k e r Clays and Clay Minerals
-19 I ' I ' L ' ' I ' I '
-20 5 ~ 1 7 6 o / / 25 C 65oC
2-
= -23
-24
-25, -27 -26 -25 -24 -23 -22 -21 -20 -19
InC, M
F i g u r e 1. F r e u n d l i c h e q u a t i o n p l o t s o f t he n a t u r a l l o g a r i t h m o f t he e q u i l i b r i u m U c o n c e n t r a t i o n in s o l u t i o n (C) vs. t he n a t u r a l l o g a r i t h m o f e q u i l i b r i u m U l o a d i n g o n il l i te (x /m) .
f e r e n t C t v a l u e s f o r e a c h m i n e r a l a r e r e q u i r e d t o p r o -
d u c e t h e s a m e C . A b e t t e r c o m p a r i s o n c a n b e m a d e b y
u s i n g t h e s a m e CI v a l u e f o r a l l m i n e r a l s .
F i t h i a n i l l i t e i s u s e d h e r e t o i l l u s t r a t e t h e s o r p t i o n
d a t a . T h e e x p e r i m e n t a l s o r p t i o n v a l u e s a n d t h e r e -
s u i t i n g r e g r e s s i o n l i n e s ( s h o w n i n F i g u r e 1 f o r i l l i t e ) a r e
n o r m a l 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 o f I n ( x / m ) p l o t -
t e d v s . In C a t e q u i l i b r i u m .
C o m p a r i n g D v a l u e s b e t w e e n m i n e r a l s e x p o s e d t o
t h e s a m e i n i t i a l U s o l u t i o n i s p o s s i b l e b y u s e o f t h e
d a t a i n T a b l e s 8 a n d 9 . T h e D v a l u e d a t a w e r e g e n e r a t e d
b y a s s u m i n g a n i n i t i a l U c o n c e n t r a t i o n , d e t e r m i n i n g
t h e ( x / m ) v a l u e w i t h t h e F r e u n d l i c h - l i k e c o n s t a n t s g i v -
e n i n T a b l e 9 a n d u s i n g t h e ( x / m ) v a l u e t o c o m p u t e a
C v a l u e w i t h t h e F r e u n d l i c h c o n s t a n t s g i v e n i n T a b l e
8. W i t h i l l i t e a s a n e x a m p l e , D v a l u e s d e c r e a s e w i t h
i n c r e a s i n g t e m p e r a t u r e s i n N a C 1 s o l u t i o n , w h e r e a s t h e y
i n c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e f r o m a N a H C O 3
s o l u t i o n d u e t o t h e h e a t s e n s i t i v i t y o f t h e u r a n y l c a r -
b o n a t e c o m p l e x ( L a n g m u i r , 1 9 7 8 a ) . F u r t h e r , D v a l u e s
T a b l e 7. E x p e r i m e n t a l m e a n v a l u e s fo r s o r p t i o n o f u r a n i u m b y s e c o n d a r y m i n e r a l s a t 5~ f r o m 0.01 M NaC1 a n d 0 .01 M N a H C O 3 s o l u t i o n s .
..................................... 0.01 M NaCI ........................................................................................ 0.01 M NaHCOs ........................................... Initial U Equilibrium U U on solid Initial U Equilibrium U U on solid
increase as U concen t ra t ions decrease in NaCI solu- t i ons . In NaHCO3 solut ions, D values also increase with t empera tu re at 5~ but at 25 ~ and 65~ they decrease with increasing tempera ture . In general,
N a H C O s solut ion D values are less than those f rom NaC1 solut ions for the ca t ion-exchange mater ia ls (illite,
kaolinite, smecti tes , zeolites, and opal) due to carbon- ate eomplex ing o f the U. The effects o f t empera ture ,
solut ion compos i t ion , and initial solut ion concent ra- t ion o f U on U sorp t ion on illite are more easily seen
graphically in Figures 2 and 3. The D values represent a response to complex in terac t ions be tween carbona te complexing, the exo the rmic nature o f the sorp t ion , the type o f sorpt ion sites on the minera l and t empera tu re effects on the aqueous species.
The compar i son be tween opal and silica gel sorp t ion
o f U is o f interest because o f the pr ior work by Zielinski (1980) on u ran ium in secondary silica. He found that dr ied silica gel con ta ined f rom 400 to 1000 t imes the U concen t ra t ion in solution. Ziel inski uti l ized a some-
328
Table 9. In CI.
Ames, McGarrah, and Walker Clays and Clay Minerals
Freundlich-like constants for the uranium sorption isotherms of Tables 5-7 for the equation In(x/m) = In K + p.
Temperature Sy.x Solid (~ Solution K p r (In units)
wha t different me thodo logy in that his silica gel was prec ip i ta ted in the U-con ta in ing solut ion f rom a Na- s tabi l ized silica sol. The silica gel used in the present s tudy was added to the solut ion as a 100 to 200 mesh solid originally conta in ing 9.1 wt. % water. It was also a high surface area (626.3 _+_ 25.0 mVg), low cat ion- exchange capaci ty (1.28 meq /100 g) material . The D values p lo t ted vs. In C~ for opal and silica gel are given
in Figures 4-7 . Note that opal behaves much like illite in its response to t empera tu re dur ing U sorpt ion. Ura-
n ium D values were not very large f rom sod ium bi- ca rbona te solut ions, but the order o f magn i tude o f D was reversed with t empera tu re as c o m p a r e d to NaC1 solutions. Silica gel, on the o ther hand, showed the
same order and type o f U sorp t ion wi th changing t em- perature. Uran ium sorpt ion was somewha t greater when complex ing b icarbonate was present with D values o f about 1800 ml /g at the lower end o f the concen t ra t ion range studied. Illite, however , a t ta ined D values > 3000 ml/g in 0.01 M NaC1 solutions. An i m p o r t a n t aspect
Vol. 31, No. 5, 1983 Sorption of uranium onto clays and clinoptilolite 329
3500 ' I ' I ' I ' I ' I ' I '
3000
2500
1000 5 ~
500 25 ~
65 ~ /
0 , I , I , I t , I L I , I , -15 -14 -13 -12 -11 -10 -9 -8
InC[, M
Figure 2. Natural logarithm of the initial U concentration in the solution (CO vs. the U equilibrium distribution coef- ficient (D) for illite in 0.01 M NaCI.
2O00
1500
of silica gel is that it is able to sorb both uranyl cations to some extent and anionic carbonate complexes from solution. This also is true for glauconite at the lower U concentrat ions, but to a lesser extent.
350_ , 1 ' I ' I ' I ' I 2'5oC I '
300 "--
-g"= 2o0
1 5 0 1 ~
100
50
0 -15
0.01 M NaCI
, I , I L I J I J I J I , -14 -13 -12 -11 -10 -9 -8
InCI, M
Natural logarithm of the initial U concentration Figure 4. in solution (CI) vs. the U equilibrium distribution coefficient (D) for opal in 0.01 M NaC1.
Gal loway and Kaiser (1980) reported in a study o f u ran ium deposits o f the Catahoula Fo rma t ion on the Texas coastal plain that groundwaters were more closely associated with the U minera l iza t ion plot on mont -
300 ' ] ' I ' I ' I ' I ' I '
~ . . . . ~ . ~ ' ~ ' ~ 5 ~
250 J
200 0.01 M NaHC03
15o a"
100
5 0 1 5oc
OF , I , I ; I i I i I , I , -15 -14 -13 -12 -11 -10 -9 -8
InCI, M
Figure 3. Natural logarithm of the initial U concentration in the solution (CO vs. the U equilibrium distribution coef- ficient (D) for illite in 0.01 M NaHCO3.
..=
0
30
25
20
151
10!
03
-15 -14 -13 -12 -11 -10 -9 -8 In C[, M
Figure 5. Natural logarithm of the initial U concentration (C,) vs. the U equilibrium distribution coefficient (D) for opal in 0.01 M NaHCQ.
330 Ames, McGarrah, and Walker Clays and Clay Minerals
1500 _ ' I ' I ' I ' I ' I ~ I '
1250
500 " ' - ' ' ' " ~ 2 5 ~ C
. , . . . , . . . . . - - , - -
65oc 250
, l L ] , I , I , l , I , -15 -14 -13 -12 -11 -10 -9 -8
InC[ , M
Figure 6. Natural logarithm of the initial U concentration in solution (C0 vs. the U equilibrium distribution coefficient (D) for silica gel in 0.01 M NaC1.
1000
O3
-~ 750
d
morillonite-clinoptilolite activity diagrams deepest into the montmoril lonite field. Implied was an important role for montmoril lonite in U concentration and an unimportant role for clinoptilolite. Assuming that ini-
1750
1500
125C
10001 O~
E
75C
500
25(?
0.01 MNaHCO3
25oC
65oc . ._ . , . , . . . - . - . . - - -
0 , ! I I , I ~ L i l ~ P z -15 -14 -13 -12 -11 -10 -9 -8
In CI, M
Figure 7. Natural logarithm of the initial U concentration in solution (CO vs. the U equilibrium distribution coefficients (D) for silica gel in 0.01 M NaHCO3.
250
200
150
~ 1 o o
50
- a _ _ L _ _ L _ 2 I I ' I I ' I /
0 ' I 5.C [ , I , I , I , 1 [ {3
-17 -16 -15 -14 -13 -12 -11 -10 -9
InCI, M
Figure 8. Natural logarithm of the initial U concentration in solution (C]) vs. the U equilibrium distribution coefficient (D) for clinoptilolite in 0.01 M NaC1.
tial U concentration was by sorption, an examination of the comparative U-sorption efficiencies of mont- morillonite and clinoptilolite should confirm the above suggested U-montmori l l ini te association.
Comparable D curves for clinoptilolite are shown in Figures 8 and 9 for 0.01 M NaCl and 0.01 M NHCO3 solutions, respectively, and for montmoril lonite in Fig- ures l0 and 11 for the same solutions. As usual, the presence of anionic uranyl carbonate complexes in the 0.01 M NaHCO3 solution greatly diminished uranyl sorption by the clinoptilolite and montmorillonite, both of which are cation-exchange materials. At 65~ U sorption was greatest due to uranyl carbonate complex heat sensitivity and a resulting higher uncomplexed uranyl ion concentration. Montmorillonite is obvious- ly a much more efficient U sorbent than clinoptilolite over the initial U concentration shown in Figures 8 - 11. Hence, the laboratory results here tend to support
10
d
' I ' I ' I ' I ' 1 ' I ' I
-,----___._ 65oc
0 , I , I { I -17 -16 -15 -14 -13
InCl , M
25 ~ 5oc
-12 -11 -10 -9
Figure 9. Natural logarithm of initial U concentration in solution (C~) vs. the U equilibrium distribution coefficient (D) for clinoptilolite in 0.01 M NaHCO3.
Vol. 31, No. 5, 1983 Sorption of uranium onto clays and clinoptilolite 331
700
600
500
400
300
2O0
100
0 i 17
I ~ , ~ ' I ' I ' I ' I ' I ' _
25~
I , I J l J I , I J I ~ I , -16 -15 -14 -13 -12 -11 -10 -9
In CI, M
Figure 10. Natural logarithm of the initial U concentration in solution (C,) vs. the U equilibrium distribution coefficient (D) for montmorillonite in 0.01 M NaC1.
the field o b s e r v a t i o n s o f Ga l loway a n d Kaise r (1980) conce rn ing the U m o n t m o r i l l o n i t e assoc ia t ion in Ca- t ahou l a F o r m a t i o n U deposi ts .
G ib l in (1980) repor ted some ra ther large (up to 35,000 ml /g at p H 6.5) U d i s t r i bu t i on coefficients on a ka- ol ini te , whereas L a n g m u i r (1978a, 1978b) r epor t ed some very low d i s t r i bu t i on coefficients (as low as 2 m l / g). D curves at 5 ~ 25 ~ a n d 65~ for 0.01 M NaC1 a n d kao l in i t e are s h o w n in Figure 12. T he cu rve at 25~ a n d Ct o f a b o u t 4.2 X 10 _7 M U ( - 1 4 . 6 8 3 ) a n d ap- p rox ima te ly p H 7 shou ld be close to G i b l i n ' s 35 ,000 ml /g U d i s t r i b u t i o n coefficient, bu t it is not . T h e rea- sons for the d ive rgence be tween the a b o v e two repor ted d i s t r i bu t i on coefficients a n d those r epor ted in Figure 12 are u n k n o w n , bu t m a y be due to any o f several expe r imen t a l factors, no t all o f wh ich can be deduced
10o
5c
0 17
' I ' I , I ' I T ] ' I ' I '
25oC 5~ / /
I ~1 , ~ 1 , l , I L 1 , J , I , -16 -15 -14 -13 -12 -11 -10
InCl , M
- 9
Figure 11. Natural logarithm of the initial U concentration in solution (CO vs. the U equilibrium distribution coefficient (D) for montmorillonite in 0.01 M NaHCO3.
2000 o~
~ 15oo
3000 f ' X ' r ' I ' I ' I " I ' I ' _ _
I k 2500f
1ooo --
500 65 ~
D "-17 -16 -15 -14 -13 -12 -11 -10 -9
In CI, M
Figure 12. Natural logarithm of the initial U concentration in solution (C,) vs. the U equilibrium distribution coefficient (D) for kaolinite in 0.01 M NaC1.
f rom the G i b l i n a n d L a n g m u i r reports . Fo r example , U tube-wal l so rp t ion cor rec t ions were f o u n d to be re- qu i r ed in some cases. Fa i lure to do so resu l ted in dis- t r i bu t i on coefficients t ha t were too low w h e n the tube- wall so rp t ion occur red in the or iginal so lu t ion samples a n d too high w h e n it occur red on the equ i l ib ra t ing so lu t ion-so l id s ample tubes.
Near ly all s econdary clay mine ra l s show surficial coat ings o f nonc rys t a l l i ne Fe, Mn , A1, a n d Si oxyhy- d rox ides t ha t can po ten t ia l ly resul t in con t ro l o f sorp- t ion o f some d i s so lved subs tances , inc lud ing U. O f the a b o v e nonc rys t a l l i ne surficial coat ings, ferric oxyhy- d rox ide p r o b a b l y has the m o s t po ten t i a l for increas ing U so rp t ion a b o v e t h a t o f the seconda ry clay m i n e r a l itself.
As m e n t i o n e d above , no a t t e m p t was m a d e to re- m o v e the smal l quan t i t i e s o f nonc rys t a l l i ne oxyhy- d rox ides o f F e , Mn, A1, a n d Si tha t are usual ly present , even on reference or source clays. Fo r example , A n - de r son a n d J e n n e (1970) r epo r t ed tha t the A.P.I . ka-
Table 10. Experimental mean values for sorption of ura- nium by ferric oxyhydroxide at 60~ from the solution com- position given in Table 11.
U on ferric Initial U Equilibrium U oxyhydroxide
(mole/liter) (mole/liter) (mole/g)
1.005 X 10 -4 4.113 X 10 -6 3.452 X 10 -4 1.051 X 10 -s 1.724 X 10 -7 3.702 X 10 -s 1.513 X 10 _6 1.805 X 10 -8 5.353 X l0 -6 5.041 X l0 -7 6.756 X 10 -9 1.781 • l0 -6
332 Ames, McGarrah, and Walker Clays and Clay Minerals
-7 ' I ' [ ' I ' I
-9
o~ O
E. -11 E
-13
-15 , I J I J I , I I -20 -18 -16 -14 -12 -10
In C, M
Figure 13. Naturallogarithmofthe equilibrium U concen- tration in the solution (C) vs. the natural logarithm of equi- librium U loading on ferric oxyhydroxide (x/m).
olinite 4 used in the present study contained 0.01 wt. % Fe203 and 0.001 wt. % MnO2. The most important noncrystalline sorbent of uranium was ferric oxyhy- droxide with distribution coefficients in excess of 2 • 106 ml/g. It always produced U isothermal sorption data that fit the Dubinin-Radushkevich sorption iso- therm (Ames et aL, 1983). A ferric oxyhydroxide-coat- ed dioctahedral smectite (0.68 wt. % Fe) produced Du- binin-Radushkevich sorption isotherms. Removal of the ferric oxyhydroxide coating and reoxidation of the dioctahedral smectite not only reduced U sorption by an order of magnitude, but the sorption data then fit a Freundlich sorption isotherm (Ames et al., 1982). Hence, if ferric oxyhydroxide U sorption comprises a substantial portion of the total clay plus ferric oxy- hydroxide U sorption, the data fit a Dubinin-Radush- kevich sorption isotherm. If ferric oxyhydroxide made a minimal contribution to total U sorption, the data fit a Freundlich sorption isotherm. Because all of the U-sorption data for the minerals in the present study fit the Freundlich sorption isotherm, U-sorption on ferric oxyhydroxide was considered to be minimal.
Table 11. Solution composition used in the ferric oxyhy- droxide experiments.
NaHCO3 112.2 Na + 30.7 KzSO4 20.1 K + 9.0 CaClz. 2H20 23.8 Ca 2+ 6.5 MgCI:. 6H20 8.4 Mg z+ 1.0 SiO2 (noncrystalline) 22.5 HCO3- 81.5
SO42 11.1 C1- 14.4
pH = 8.0; ionic strength = 0.002132.
An example of the efficiency of ferric oxyhydroxide for sorption of U is given in Table 10. One milliliter of 0.1 M FeC13 solution was added to 40 ml of distilled water and titrated to pH 7 in 50-ml polypropylene centrifuge tubes. The X-ray amorphous precipitate was washed three times with the solution composition giv- en in Table 11. Uranium was added to the solution, and the ferric oxyhydroxide and solution were contact- ed with agitation for seven days. These sorption data yielded a straight line with the Dubinin-Radushkevich sorption isotherm (Dubinin and Radushkevich, 1974), but are presented as Freundlich sorption isotherms in Figure 13. The relationship between the Freundlich and Dubinin-Radushkevich sorption isotherms was given by Sokolowska and Szczypa (1980). The D values for ferric oxyhydroxide varied from 85,000 ml/g at a C~ value of 1.005 • 10 _4 M U to 300,000 ml/g at a C~ of 5.041 • 10 -7 M U, about two orders of magnitude above comparable D values for the most efficient U-sorbing secondary minerals in a similar environ- ment. Uranium sorption on crystalline secondary min- erals probably should not be considered as a process leading directly to the formation of U ore deposits. Although sorption may contribute to other U concen- trating processes leading to ore deposits, instances of sorption on crystalline secondary minerals as the prin- cipal U concentration process would be rare.
A C K N O W L E D G M E N T
The authors are grateful to the U.S. Department of Energy for sponsoring this work.
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Zielinski, R. A. (1980) Uranium in secondary silica: a pos- sible exploration guide: Econ. Geol. 75, 592-602.
(Received t October 1982; accepted 1 April 1983)
Pe31oMe---Xopouao cxapaKTeptl3OBaHHbIe o6pa3tlbl CTaH~apTHblX r~14n H3 AMepnKancKoro HeqbTmioro I/IHcTtlTyTa, o6pa311OBhlC r31IIHh! 143 OOuleCTBa no FJndHIfCTblM MHHepaJIaM 14 ~pyrne BTOptlqHbIe Mn14epa31bl 14cno~b3oBarlncb ~IJDI onpe]Ie~enns BaHsnnn KOHLIeHTpat114n ypaaa, TeMrlepaTypbl 14 COCTasa pacTBopa na CBOfiCTBa copfRn14 ypaHa. KonResTpatlrln ypana naxo~4~14Cb B ~nanasone OT OKO~O 1,00 • 10 -4 M ~o 4,00 • 10 -7 M, TeunepaTypbi n3uenn~nc~, OT 5 ~ ~Io 65~ ri pacTaOpbI co;aep;~a3n 0,01 M NaCI n 0,01 M NaHCO3. KpeMne3eMnhxfi renb xqbdpercTnnno cop6apoaa.a annonnbie KOM- nneKcbt ypannaoBoro Kap6onara. M14nepam, l c noabltuennofi KaTnono-o6Mennofi cnoco6~tOCTbrO nan6o~ee OXOTnO cop6npoBann ypannnoab~e nonbi nx 0,01 M pacTBopa NaCI. YBe~14qe14ne TeMne- paTypbI BJIn~l~O o6paTHOnponopllnoHa~bHO na cop6ttnro ypa8n.rloBbxx HOHOB, 3a ncrJnoqeHneM c~yqan, Korea U npncyTcTaoaa~ B B14jle gap6onaTHhlX KoMn3eKcom HeKpncTa~anqecKne ~<eae314bxe r14;IpooKnc14 cop6npoBa~14 ypaHriaOBbIe 14OHbl 60ace 3qbqbeKTnB140, aeM Bce riBB3e~oaanHh~e BTOpt'lqHhIe KpncTa~n- qecrdte Mnnepam+ Flpe~acTaa~ea, pa3pa60TanHbifi na OCHORe ypaaHennfl qbpefiu~nxa, MeTo~ ~a3z TOqHOfi 3~cTpano~mtn14 3dpqbeKTnaHOCT14 cop6Un14 U Me:m4Iy 3KcnepnMeHTam, HblMn ToqKaM14. [E.G.]
Resiimee--Gut bestimmte Tonstandards des American Petroleum Institute und der Clay Minerals Society sowie andere sekund~ire Minerale wurden verwendet, um die Auswirkungen der U-Konzentration, der Temperatur und der L6sungszusammensetzung auf die U-Adsorption zu bestimmen. Die U-Konzentra- tionen reichten von etwa 1,00 • l0 -4 M bis 4,00 • 10 -7 M, die Temperatur von 5 ~ bis 65~ Die L6- sungszusammensetzung war 0,01 M NaC1 und 0,01 M NaHCO 3. Silikagel adsorbierte Uranylkarbona- tanionenkomplexe sehr gut. Die Substanzen mit h6herer Kationenaustauschkapazit~t adsorbierten sehr leicht Uranylionen aus der 0,01 M NaC1-LOsung. Ein Temperaturanstieg zeigte einen negativen Effekt aufdie Uranyladsorption, aut3er das U war in Form eines Karbonatkomplexes vorhanden. Nichtkristalline Eisenoxyhydroxide adsorbierten Uranylionen vie1 wirksamer als alle andere untersuchte sekund~ire kristal- line Minerale. Es wird eine Methode zur genauen Extrapolation zwischen experimentell bestimmten Punkten der U-Adsorptionseffizienz angegeben, die auf der Freundlich-Gleichung beruht. [U.W.]
334 Ames, McGarrah, and Walker Clays and Clay Minerals
R~sum~--Des standards d'argile bien caracteris6s de l'American Petroleum Institute, des argiles de source du Clay Minerals Society, et d'autres min6raux secondaires ont 6t6 employ6s pour d6terminer les effets de la concentration d'U, de la temp6rature, et de la composition de la solution sur les propri6t6s de la sorption d'U. Les concentrations d'uranium s'6tageaient d'~t peu pr6s 1,00 • 10 4/~ 4,00 X 10 7 M, les temp6ratures de 5~ ~ 65~ et les compositions des solutions contenant 0,01 M NaCI et 0,001 M NaHCO3. Le gel de silice a sorb6 de mani6re efficace les complexes anion de carbonate uranyl. Les mat6riaux ayant la capacit6 d'6change de cations la plus elev6e ont sorb6 le plus facilement les ions uranyls de la solution 0,01 M NaCI. Des augmentations de temp6rature tendaient ~ affecter advers6ment la sorption de l'ion uranyl, sauf lorsque I'U 6tait pr6sent en tant que complexes carbonates. Des oxyhydrides ferriques non- cristallins ont sorb6 les ions uranyls de mani6re beaucoup plus efficace qu'aucun des min6raux cristalline secondaires 6tudies. Une m6thode est pr6sent6e pour extrapoler pr6cisement les efficacit6s de sorption d 'U entre des points exp6rimentaux bas6e sur l'6quation de Freundlich. [D.J.]