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Sorption and desorption behavior of cesium on soil components
Appl. Rodiaf. Isor. Vol. 45, No. 4, pp. 433-437, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 096978043/94 $6.00 + 0.00
Pergamon
of
Sorption and Desorption Behavior Cesium on Soil Components CHUN-NAN Institute
of Nuclear
Science, National
HSU and KWO-PING Tsing Hua University,
Hsinchu
CHANG 30043, Taiwan,
Republic
of China
(Received 18 June 1993; in revised form 4 October 1993) The sorption desorption behaviors of Cs ion in the concentration region of 10m9 to 10m4 m-equiv mL_’ were studied using bentonite, humic acid and sand as well as their mixtures. The Cs cation was indicated from the results of desorption studies to have, to a large extent, been sorbed to the interlayer sites of bentonite in an irreversible manner, while the sorption of Cs on humic acid and sand was reversible. In the experiments with mixed sorbents, Kd value decreased with increasing quantities of humic acid in the region where humic was below 6%; in addition, some sorption sites of bentonite were confirmed to have been blocked by humic acid. Humic acid, in the region where it is over 6%, would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid.
Introduction Sorption
studies
of radionuclides
on soils are import-
in light of soil’s being a natural barrier to radwaste disposal in geologic media. The interaction of nuchdes with soil directly affects the migration of radionuclides. The distribution coefficient K,, is a function of many factors, e.g. the nuclides in question, the mineralogical and chemical composition of the solid geologic media, and the chemical composition of the liquid transport media (Relyea et al., 1979). Many studies (Comans and Hockley, 1992; Erten et al., 1988a, b; Grutter ef al., 1990; Konishi et al., 1988; Yanagi et al., 1989) in which the distribution coefficient Kd value was derived placed emphasis on clay minerals for their negative electricity and high cation exchange capacity (CEC) (Yong and Warkentin, 1975). Humic substances, which are collections of organic polyelectrolyte macromolecules, are the principal organic components of soils. Livens et al. (1991) concluded that organic materials can take up radiocesium in competition with minerals, or they may react with the minerals and block the Cs uptake sites. The relative contribution of clay mineral and organic fractions to the overall sorption cannot be accurately evaluated from studies based on natural soils as a result of several factors varying simultaneously, and the effect of individual parameters is also difficult to isolate. This difficulty is circumvented in the present study by using synthetic soils of known and controlled compositions. This has allowed sorption isotherms to be. run on soils of identical mineral content but with varied humic acid content. Clay ant
433
(bentonite), organic matter (humic acid) and sand (standard sand) as well as their mixtures were employed here for investigating the sorption behavior of radionuclides on soil. ‘.“Cs was studied here because of its long half-life (30.17 y) and also because of the high solubility of most cesium compounds. Desorption experiments were also performed to obtain further information about the sorption mechanism.
Experimental I. The solid phase Bentonite (Wyoming, U.S.A.) was used for both its high content and its purity of clay (montmorillonite). It was sieved to under 0.0074mm size. The sieved bentonite was stirred with 1 M CaCI, solution for 24 h and then washed with deionized water until almost all of the chloride was removed (AgN03 test). Humic acid (Nacalai Tesque, Inc. Kyoto, Japan) and standard sand (ASTM C109, U.S.A.) were used. 2. The aqueous phase Deionized water was used. The sorption experiments were carried out at the initial cesium concentrations listed in Table 1. The initial 13’Cs (tracer) activity of 1 mL solution is about 70 counts per second. 3. Batch method The mixture consisted of 20% Ca-saturated bentonite and (O-10%) humic acid and (80-70%) standard sand. The individual components were also used for
CHUN-NANHsu and KWO-PINGCHANG
434
Table I. C&urn ion concentration used in sorption/ desorption studies
where Aw,, , the amount of liquid remaining in the tube after sorption and decantation; A,,,,, count rate of 5 mL of solution after desorption. The rest of the terms in equations (3) and (4) have been defined earlier.
Co (m-equiv mL_ ’ ) 2.34 x 1.11 x 1.01 x 1.00 x 1.00x 1.00 x
IO-’ 10-6 10-7 10~6 IO 5 10-4
Results and Discussion 1. Kinetics
sorption and desorption experiments. All experiments were carried out in duplicate. The solid sorbent (0.1 g) samples were gently shaken with the aqueous solution (8 mL) in closed centrifuge tubes. These samples were shaken for four days with deionized water for preequilibrium. The phases were separated by centrifuging at 15,000 rpm for 20 min. Following an addition of 8 mL Cs solution, the samples were shaken again and centrifuged. 5 mL of the supernatant was counted using a high purity germanium detector (HPGe) connected to a multichannel analyzer. No filtration was done prior to counting. For desorption studies, 8 mL of deionized water was added to the sample tube following the sorption step, after which the tube was shaken for four days, centrifuged and decanted. The activity of the liquid phase was then determined. All experiments were performed at an ambient temperature of 27 _+ 5°C. The concentrations of Cs in the solid phase Qad and liquid phase C,, after sorption were calculated from the measured activities before and after shaking using the following relations:
Qad=
I’.C,-C,,.(v+Awp,) W5
(m-equiv g-i)
(1)
Batch kinetics experiments were performed on the bentonite and humic acid to determine how rapidly the cesium would associate with them and the time to steady-state (assumed equilibrium). The Cs is illustrated in Fig. I to have been rapidly sorbed by bentonite, reaching a virtual steadystate within 24 h. Cs was sorbed by humic acid somewhat slowly (roughly 3 days to reach equilibrium). Moreover, the sorption fractions of Cs in humic acid decreased with time during the initial period. Some of the humus substance apparently dissolved and was unavailable for sorption as indicated by the solution’s having turned yellow in the batch kinetics on humic acid. Four days was a sufficient amount of time for obtaining equilibrium in sorption and desorption experiments on the sorbents. 2. Sorption and desorption isotherms All experiments isotherms (Travis,
conformed to 1978) (Fig. 2).
the
Freundlich
Q = ,I& ‘,ln
(5)
where: Q, nuclide sorbed on solid in equilibrium (m-equivg-‘); C, nuclide in liquid phase in equilibrium (m-equiv mL_‘); and k, n are constants to be determined experimentally. The distribution coefficient Kd is defined as Kd = Q/C (mL g-.‘)
where C,,, initial cesium ion concentration (mequivmL-‘); V, volume of solution added (mL); Awr,, amount of liquid remaining in the tube after pretreatment and decantation, determined by weight difference; A,, initial count rate of 5 mL of solution added for sorption; A,.,, , count rate of 5 mL of solution after sorption; ws, weight of solid material
(6)
‘zr----1 .o
b
1 Bentomte
-_o-O--c:
-0
I
(g). The concentration of Cs in the solid phase Qdc and liquid phase Cd, after desorption were calculated as fo1lows: Qdc=
V-+(Y+Aw,,-Aw.,) [
0
-(?‘+Aw.~)%
1
.z(m-equivg-‘)
0
. C, (m-equiv mL- ’ )
C de = + 0
(3)
0
50
100
150
200
Tome (hj
(4)
Fig. 1. The sorption kinetics of cesium on bentonite and
humic acid.
Sorption and desorption behavior of cesium on soil components
lo-'
(4
r
I
I
!
senton1te 0
Sorption
l
Desorptmn
03
10-z
I
-
I
0
,‘. 4 ,!' ,/
I
I
Sand
,.i’O ,.fl'
435
10-S
P
0 1o-4
/
0
Sorption
0
Desorption
l,:.
F $
10-5
5 0 10-6
lo-'
,o-‘2
,o-ll
,o-‘o
,o-9
10-8
10-7
10-e
10-5
10.' 10-a
1 o-9
C (meq/mL)
10-s
10-7
10-e C
Humic 0
10-5
lo-'
to-'
(meq/mL)
acid
20% bentonite+BO%
1o-2
sorption
0 1
Sorption
.:‘/
lo-'
-
1o-5
-
1o-6
-
:’
a
1
l ,..I"
z L
i
0 Desorption
o-’
t 2
sand
l
,:'
,....."
:' 0
::l
tL_____J
10-‘1
,o-‘0
10-Q
,0-a
10-7
10-6
10-5
10.'
lo-'
C (meq/mL)
Fig. 2. (a) The sorption and desorption isotherm of cesium on bentonite. (b) The sorption and desorption isotherm of cesium on humic acid. (c) The sorption and desorption isotherm of cesium on standard sand. (d) The sorption isotherm of cesium on 20% bentonite + 80% sand.
Combining
equations
(5) and (6) gives equation
Kd = kc”-‘(mL
g-l)
(7) (7)
where m = l/n.
The m-value obtained from sorption and desorption experiments is summarized in Table 2, where Kds are calculated by assuming the concentration of Cs to be 10-8m-equivmL-‘. Bentonite-sorbed Cs was found from a comparison of the K,-value from Table 2 to be stronger than humic acid, while sand sorbed Cs to a small extent. Desorption-K, value is seen from Fig. 2(a) and Table 2 to be significantly greater than the sorptionKd value on bentonite. Such phenomena revealed that a substantial amount of Cs was sorbed to the interlayer sites of bentonite and that the sorption was predominantly irreversible in deionized water. Cs sorbed to interlayer sites of clay could explain why Cs cannot easily migrate in soil (Amalks et al.,
1989). The sorption on humic acid and sand was found from Table 2 to be reversible. Such phenomena indicated that surface sites have dominated the sorption on humic acid and sand.
Table 2. The m-value in equation (7) and Kd of sorption and desorution Sorption Solid sorbent Bentonite Humic acid Standard sand B(20%) + S(80%) B(20%) + S(78%) B(ZO%) + S(76%) B(20%) + S(74%) B(20%) + S(72%) B(20%) + S(70%)
+ + + + +
H(2%) H(4%) H(6%) H@%) H(lO%)
Desorption
m
KA
m
KA
0.918 0.964 0.964 0.881 0.843 0.878 0.886 0.894 0.901
2970 317 15.0 748 567 447 374 408 387
I .05 1.06 0.909 0.896 0.749 0.891 0.867 0.874 0.824
19400 462 39.0 2240 1290 955 686 1020 1190
8, Bentonite; S, standard sand; H, humic acid; &, distribution coefficient calculated by assuming the concentration of Cs being IO-‘m-equivmL_‘.
436
CHUN-NANHsu and KWO-PINGCHANG
300 L C
4
2
Percentage
of
1 6
8
I
numc
acid
Fig. 3. Distribution coefficient vs quantities (C = 1.O E - 8 m-equiv mL-‘).
10
I 12
(%)
of different concentrations of many varieties of cations, the block function would vary depending on the type of groundwater. The value of Kd would therefore differ with the type of groundwater which is present. The results in some studies which used different groundwater and had differing values for K, can be explained post hoc according to this analysis. In the region where humic acid is over 6%, humic acid would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid, and K, value would possibly increase with humic acid (Fig. 3). However, KD decreased in the region of 8-10% of humic acid, indicating that another phenomenon such as complex formation apparently occurred.
of humic acid
Conclusions The following conclusions could be drawn on the basis of the above discussion:
3. Block phenomena
When the weight fraction of bentonite was fixed at 20%, Kd was found to have decreased with increasing humic acid in the region below 6% (Table 2 and Fig. 3). Some of the sorption sites of bentonite were apparently blocked by humic acid. Some models have been proposed in the following so as to more clearly clarify this situation. If no blocking occurs, the total sorption Kd is considered to be expressed as a summation of Kd for each component, i.e.
& = 1 wc4i.c where: w,, weight fraction of component c; Kd,C, Kd value of component c. The equation is modified in this study by a term of block functionf, C . eebr for description of the data. Thus,
Ko = &.I, -fx .J&s+ fr &.h -Cf,.C.e-bx).Kd,b
(9)
where: K,, Total distribution coefficient (mL g-l); Kd,h, 20% bentonite distribution coefficient (mL g-r); Kd,$, standard sand distribution coefficient (mL g-r); K d.h) humic acid distribution coefficient (mL g-l); f,, weight fraction of humic acid = x%; x, weight percent of humic acid; b,C, parameters of block function. Pairwise data K,, f, were input into equation (9). Parameters b and C were determined as 11.7 and 16.8, respectively. The b and C values are used in equation (9) for calculating the relationship between the equation and the experimental data. Clay is well known to be capable of being linked with humic acid since cations function as a bridge in completing this link. Cations have been derived from the original cations in bentonite in light of the use of deionized water in this study. As a result
1. All solid sorbents (bentonite, humic acid and standard sand) conformed to Freundlich isotherms under initial concentrations ranging from 10m9 to 10e4 m-equiv mL-’ of Cs. 2. Desorption-K, value was observed to be significantly greater than sorption-i(, value on bentonite. This phenomenon revealed that a substantial amount of Cs was sorbed to the interlayer sites of bentonite and the sorption was found to be predominantly irreversible. Meanwhile, the sorption was found to be reversible and Cs to be sorbed on organics, and sand could be easily desorbed. 3. Kd value decreased with increasing quantities of humic acid in the region where humic acid content is below 6%. Some sorption sites of bentonite were proved to be blocked by humic acid. In the region where humic acid is over 6%, humic acid would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid. Another phenomenon, e.g. complex formation, would be considered to occur when the quantities of humic acid are substantially greater. Acknowledgements-The authors wish to express their gratitude to Mr Chia-Lian Tseng for his help on the experiments, as well as to Mr Chin-Nan Ke for his assistance in the analyses of samples.
References Arnalds O., Cutshall N. H. and Nielsen G. A. (1989) Cesium-137 in Montana soils. Hlth Phys. 57, 955. Comans R. N. J. and Hocklev D. E. (1992) Kinetics of cesium sorption on illite. G&him. Acfa 56, 1157. Erten H. N., Aksovoglu S. and Gokturk H. (1988a) Sorption of cesium and strontium on montmorillonite and kaolinite. Radiochim. Acta 44145, 147. Erten H. N., Aksoyoglu S. and Gokturk H. (1988b) Sorption/desorption of Cs on clay and soil fractions from various regions of Turkey. Sci. Total Environ. 69, 269.
Sorption
and desorption
behavior
Grutter A., Gunten H. R. V., Kohler M. and Rossler E. (1990) Sorption, desorption and exchange of cesium on glaciofluvial deposits. kadiochim. Acfa %, 177. Konishi M.. Yamamoto K.. Yanaei T. and Okaiima Y. (1988) Sorption behavior of c&ium, strontium and americium ions on clay materials. J. Nucl. Sci. Techn. 25, 929. Livens F. R., Horrill A. D. and Singleton D. L. (1991) Distribution of radiocesium in the soil-plant systems of upland areas of Europe. Hlth Phys. 60, 539. Relyea J. F., Ames L. L., Serne R. J., Fulton R. W.
of cesium
on soil components
437
and Washburne C. D. (1979) Batch Kd determination with common minerals and representative groundwaters. PNL-SA - 7352. Travis C. C. (197X) Mathematical description of adsorption and transport of reactive solutes in soil: a review of selected literature. ORNL-5403. Yanagi T., Watanabe M. and Yamamoto K. (1989) Sorption behavior of cesium and strontium ions on mixtures of clay sorbents. J. Nucl. Sci. Techn. 26, 861. Yong R. N. and Warkentin B. P. (1975) Soil Properties and Behavior. Elsevier, New York.
Pergamon
of
Sorption and Desorption Behavior Cesium on Soil Components CHUN-NAN Institute
of Nuclear
Science, National
HSU and KWO-PING Tsing Hua University,
Hsinchu
CHANG 30043, Taiwan,
Republic
of China
(Received 18 June 1993; in revised form 4 October 1993) The sorption desorption behaviors of Cs ion in the concentration region of 10m9 to 10m4 m-equiv mL_’ were studied using bentonite, humic acid and sand as well as their mixtures. The Cs cation was indicated from the results of desorption studies to have, to a large extent, been sorbed to the interlayer sites of bentonite in an irreversible manner, while the sorption of Cs on humic acid and sand was reversible. In the experiments with mixed sorbents, Kd value decreased with increasing quantities of humic acid in the region where humic was below 6%; in addition, some sorption sites of bentonite were confirmed to have been blocked by humic acid. Humic acid, in the region where it is over 6%, would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid.
Introduction Sorption
studies
of radionuclides
on soils are import-
in light of soil’s being a natural barrier to radwaste disposal in geologic media. The interaction of nuchdes with soil directly affects the migration of radionuclides. The distribution coefficient K,, is a function of many factors, e.g. the nuclides in question, the mineralogical and chemical composition of the solid geologic media, and the chemical composition of the liquid transport media (Relyea et al., 1979). Many studies (Comans and Hockley, 1992; Erten et al., 1988a, b; Grutter ef al., 1990; Konishi et al., 1988; Yanagi et al., 1989) in which the distribution coefficient Kd value was derived placed emphasis on clay minerals for their negative electricity and high cation exchange capacity (CEC) (Yong and Warkentin, 1975). Humic substances, which are collections of organic polyelectrolyte macromolecules, are the principal organic components of soils. Livens et al. (1991) concluded that organic materials can take up radiocesium in competition with minerals, or they may react with the minerals and block the Cs uptake sites. The relative contribution of clay mineral and organic fractions to the overall sorption cannot be accurately evaluated from studies based on natural soils as a result of several factors varying simultaneously, and the effect of individual parameters is also difficult to isolate. This difficulty is circumvented in the present study by using synthetic soils of known and controlled compositions. This has allowed sorption isotherms to be. run on soils of identical mineral content but with varied humic acid content. Clay ant
433
(bentonite), organic matter (humic acid) and sand (standard sand) as well as their mixtures were employed here for investigating the sorption behavior of radionuclides on soil. ‘.“Cs was studied here because of its long half-life (30.17 y) and also because of the high solubility of most cesium compounds. Desorption experiments were also performed to obtain further information about the sorption mechanism.
Experimental I. The solid phase Bentonite (Wyoming, U.S.A.) was used for both its high content and its purity of clay (montmorillonite). It was sieved to under 0.0074mm size. The sieved bentonite was stirred with 1 M CaCI, solution for 24 h and then washed with deionized water until almost all of the chloride was removed (AgN03 test). Humic acid (Nacalai Tesque, Inc. Kyoto, Japan) and standard sand (ASTM C109, U.S.A.) were used. 2. The aqueous phase Deionized water was used. The sorption experiments were carried out at the initial cesium concentrations listed in Table 1. The initial 13’Cs (tracer) activity of 1 mL solution is about 70 counts per second. 3. Batch method The mixture consisted of 20% Ca-saturated bentonite and (O-10%) humic acid and (80-70%) standard sand. The individual components were also used for
CHUN-NANHsu and KWO-PINGCHANG
434
Table I. C&urn ion concentration used in sorption/ desorption studies
where Aw,, , the amount of liquid remaining in the tube after sorption and decantation; A,,,,, count rate of 5 mL of solution after desorption. The rest of the terms in equations (3) and (4) have been defined earlier.
Co (m-equiv mL_ ’ ) 2.34 x 1.11 x 1.01 x 1.00 x 1.00x 1.00 x
IO-’ 10-6 10-7 10~6 IO 5 10-4
Results and Discussion 1. Kinetics
sorption and desorption experiments. All experiments were carried out in duplicate. The solid sorbent (0.1 g) samples were gently shaken with the aqueous solution (8 mL) in closed centrifuge tubes. These samples were shaken for four days with deionized water for preequilibrium. The phases were separated by centrifuging at 15,000 rpm for 20 min. Following an addition of 8 mL Cs solution, the samples were shaken again and centrifuged. 5 mL of the supernatant was counted using a high purity germanium detector (HPGe) connected to a multichannel analyzer. No filtration was done prior to counting. For desorption studies, 8 mL of deionized water was added to the sample tube following the sorption step, after which the tube was shaken for four days, centrifuged and decanted. The activity of the liquid phase was then determined. All experiments were performed at an ambient temperature of 27 _+ 5°C. The concentrations of Cs in the solid phase Qad and liquid phase C,, after sorption were calculated from the measured activities before and after shaking using the following relations:
Qad=
I’.C,-C,,.(v+Awp,) W5
(m-equiv g-i)
(1)
Batch kinetics experiments were performed on the bentonite and humic acid to determine how rapidly the cesium would associate with them and the time to steady-state (assumed equilibrium). The Cs is illustrated in Fig. I to have been rapidly sorbed by bentonite, reaching a virtual steadystate within 24 h. Cs was sorbed by humic acid somewhat slowly (roughly 3 days to reach equilibrium). Moreover, the sorption fractions of Cs in humic acid decreased with time during the initial period. Some of the humus substance apparently dissolved and was unavailable for sorption as indicated by the solution’s having turned yellow in the batch kinetics on humic acid. Four days was a sufficient amount of time for obtaining equilibrium in sorption and desorption experiments on the sorbents. 2. Sorption and desorption isotherms All experiments isotherms (Travis,
conformed to 1978) (Fig. 2).
the
Freundlich
Q = ,I& ‘,ln
(5)
where: Q, nuclide sorbed on solid in equilibrium (m-equivg-‘); C, nuclide in liquid phase in equilibrium (m-equiv mL_‘); and k, n are constants to be determined experimentally. The distribution coefficient Kd is defined as Kd = Q/C (mL g-.‘)
where C,,, initial cesium ion concentration (mequivmL-‘); V, volume of solution added (mL); Awr,, amount of liquid remaining in the tube after pretreatment and decantation, determined by weight difference; A,, initial count rate of 5 mL of solution added for sorption; A,.,, , count rate of 5 mL of solution after sorption; ws, weight of solid material
(6)
‘zr----1 .o
b
1 Bentomte
-_o-O--c:
-0
I
(g). The concentration of Cs in the solid phase Qdc and liquid phase Cd, after desorption were calculated as fo1lows: Qdc=
V-+(Y+Aw,,-Aw.,) [
0
-(?‘+Aw.~)%
1
.z(m-equivg-‘)
0
. C, (m-equiv mL- ’ )
C de = + 0
(3)
0
50
100
150
200
Tome (hj
(4)
Fig. 1. The sorption kinetics of cesium on bentonite and
humic acid.
Sorption and desorption behavior of cesium on soil components
lo-'
(4
r
I
I
!
senton1te 0
Sorption
l
Desorptmn
03
10-z
I
-
I
0
,‘. 4 ,!' ,/
I
I
Sand
,.i’O ,.fl'
435
10-S
P
0 1o-4
/
0
Sorption
0
Desorption
l,:.
F $
10-5
5 0 10-6
lo-'
,o-‘2
,o-ll
,o-‘o
,o-9
10-8
10-7
10-e
10-5
10.' 10-a
1 o-9
C (meq/mL)
10-s
10-7
10-e C
Humic 0
10-5
lo-'
to-'
(meq/mL)
acid
20% bentonite+BO%
1o-2
sorption
0 1
Sorption
.:‘/
lo-'
-
1o-5
-
1o-6
-
:’
a
1
l ,..I"
z L
i
0 Desorption
o-’
t 2
sand
l
,:'
,....."
:' 0
::l
tL_____J
10-‘1
,o-‘0
10-Q
,0-a
10-7
10-6
10-5
10.'
lo-'
C (meq/mL)
Fig. 2. (a) The sorption and desorption isotherm of cesium on bentonite. (b) The sorption and desorption isotherm of cesium on humic acid. (c) The sorption and desorption isotherm of cesium on standard sand. (d) The sorption isotherm of cesium on 20% bentonite + 80% sand.
Combining
equations
(5) and (6) gives equation
Kd = kc”-‘(mL
g-l)
(7) (7)
where m = l/n.
The m-value obtained from sorption and desorption experiments is summarized in Table 2, where Kds are calculated by assuming the concentration of Cs to be 10-8m-equivmL-‘. Bentonite-sorbed Cs was found from a comparison of the K,-value from Table 2 to be stronger than humic acid, while sand sorbed Cs to a small extent. Desorption-K, value is seen from Fig. 2(a) and Table 2 to be significantly greater than the sorptionKd value on bentonite. Such phenomena revealed that a substantial amount of Cs was sorbed to the interlayer sites of bentonite and that the sorption was predominantly irreversible in deionized water. Cs sorbed to interlayer sites of clay could explain why Cs cannot easily migrate in soil (Amalks et al.,
1989). The sorption on humic acid and sand was found from Table 2 to be reversible. Such phenomena indicated that surface sites have dominated the sorption on humic acid and sand.
Table 2. The m-value in equation (7) and Kd of sorption and desorution Sorption Solid sorbent Bentonite Humic acid Standard sand B(20%) + S(80%) B(20%) + S(78%) B(ZO%) + S(76%) B(20%) + S(74%) B(20%) + S(72%) B(20%) + S(70%)
+ + + + +
H(2%) H(4%) H(6%) H@%) H(lO%)
Desorption
m
KA
m
KA
0.918 0.964 0.964 0.881 0.843 0.878 0.886 0.894 0.901
2970 317 15.0 748 567 447 374 408 387
I .05 1.06 0.909 0.896 0.749 0.891 0.867 0.874 0.824
19400 462 39.0 2240 1290 955 686 1020 1190
8, Bentonite; S, standard sand; H, humic acid; &, distribution coefficient calculated by assuming the concentration of Cs being IO-‘m-equivmL_‘.
436
CHUN-NANHsu and KWO-PINGCHANG
300 L C
4
2
Percentage
of
1 6
8
I
numc
acid
Fig. 3. Distribution coefficient vs quantities (C = 1.O E - 8 m-equiv mL-‘).
10
I 12
(%)
of different concentrations of many varieties of cations, the block function would vary depending on the type of groundwater. The value of Kd would therefore differ with the type of groundwater which is present. The results in some studies which used different groundwater and had differing values for K, can be explained post hoc according to this analysis. In the region where humic acid is over 6%, humic acid would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid, and K, value would possibly increase with humic acid (Fig. 3). However, KD decreased in the region of 8-10% of humic acid, indicating that another phenomenon such as complex formation apparently occurred.
of humic acid
Conclusions The following conclusions could be drawn on the basis of the above discussion:
3. Block phenomena
When the weight fraction of bentonite was fixed at 20%, Kd was found to have decreased with increasing humic acid in the region below 6% (Table 2 and Fig. 3). Some of the sorption sites of bentonite were apparently blocked by humic acid. Some models have been proposed in the following so as to more clearly clarify this situation. If no blocking occurs, the total sorption Kd is considered to be expressed as a summation of Kd for each component, i.e.
& = 1 wc4i.c where: w,, weight fraction of component c; Kd,C, Kd value of component c. The equation is modified in this study by a term of block functionf, C . eebr for description of the data. Thus,
Ko = &.I, -fx .J&s+ fr &.h -Cf,.C.e-bx).Kd,b
(9)
where: K,, Total distribution coefficient (mL g-l); Kd,h, 20% bentonite distribution coefficient (mL g-r); Kd,$, standard sand distribution coefficient (mL g-r); K d.h) humic acid distribution coefficient (mL g-l); f,, weight fraction of humic acid = x%; x, weight percent of humic acid; b,C, parameters of block function. Pairwise data K,, f, were input into equation (9). Parameters b and C were determined as 11.7 and 16.8, respectively. The b and C values are used in equation (9) for calculating the relationship between the equation and the experimental data. Clay is well known to be capable of being linked with humic acid since cations function as a bridge in completing this link. Cations have been derived from the original cations in bentonite in light of the use of deionized water in this study. As a result
1. All solid sorbents (bentonite, humic acid and standard sand) conformed to Freundlich isotherms under initial concentrations ranging from 10m9 to 10e4 m-equiv mL-’ of Cs. 2. Desorption-K, value was observed to be significantly greater than sorption-i(, value on bentonite. This phenomenon revealed that a substantial amount of Cs was sorbed to the interlayer sites of bentonite and the sorption was found to be predominantly irreversible. Meanwhile, the sorption was found to be reversible and Cs to be sorbed on organics, and sand could be easily desorbed. 3. Kd value decreased with increasing quantities of humic acid in the region where humic acid content is below 6%. Some sorption sites of bentonite were proved to be blocked by humic acid. In the region where humic acid is over 6%, humic acid would become the primary sorbent for Cs instead of bentonite, which would be blocked with humic acid. Another phenomenon, e.g. complex formation, would be considered to occur when the quantities of humic acid are substantially greater. Acknowledgements-The authors wish to express their gratitude to Mr Chia-Lian Tseng for his help on the experiments, as well as to Mr Chin-Nan Ke for his assistance in the analyses of samples.
References Arnalds O., Cutshall N. H. and Nielsen G. A. (1989) Cesium-137 in Montana soils. Hlth Phys. 57, 955. Comans R. N. J. and Hocklev D. E. (1992) Kinetics of cesium sorption on illite. G&him. Acfa 56, 1157. Erten H. N., Aksovoglu S. and Gokturk H. (1988a) Sorption of cesium and strontium on montmorillonite and kaolinite. Radiochim. Acta 44145, 147. Erten H. N., Aksoyoglu S. and Gokturk H. (1988b) Sorption/desorption of Cs on clay and soil fractions from various regions of Turkey. Sci. Total Environ. 69, 269.
Sorption
and desorption
behavior
Grutter A., Gunten H. R. V., Kohler M. and Rossler E. (1990) Sorption, desorption and exchange of cesium on glaciofluvial deposits. kadiochim. Acfa %, 177. Konishi M.. Yamamoto K.. Yanaei T. and Okaiima Y. (1988) Sorption behavior of c&ium, strontium and americium ions on clay materials. J. Nucl. Sci. Techn. 25, 929. Livens F. R., Horrill A. D. and Singleton D. L. (1991) Distribution of radiocesium in the soil-plant systems of upland areas of Europe. Hlth Phys. 60, 539. Relyea J. F., Ames L. L., Serne R. J., Fulton R. W.
of cesium
on soil components
437
and Washburne C. D. (1979) Batch Kd determination with common minerals and representative groundwaters. PNL-SA - 7352. Travis C. C. (197X) Mathematical description of adsorption and transport of reactive solutes in soil: a review of selected literature. ORNL-5403. Yanagi T., Watanabe M. and Yamamoto K. (1989) Sorption behavior of cesium and strontium ions on mixtures of clay sorbents. J. Nucl. Sci. Techn. 26, 861. Yong R. N. and Warkentin B. P. (1975) Soil Properties and Behavior. Elsevier, New York.