Sorption behavior of cesium and strontium in selected soilsof semiarid and arid regions of Iran

Rayehe Mirkhani • Mohammad Hassan Roozitalab •

Naser Khaleghpanah • Abbas Majdabadi

Received: 8 February 2012 / Published online: 26 April 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract Sorption–desorption phenomena of contami-

nants by soils are of great importance from the viewpoint

of environmental and agricultural concerns. The knowl-

edge of sorption–desorption characteristics of contaminants

by soil is useful in simulation and prediction of contami-

nants, transport and diffusion in soil–water systems and

uptake by plants. The adsorption behavior of Cs and Sr on

two selected calcareous soils (clay and silty clay loam)

with contrasting physical and chemical characteristics was

studied in different concentrations of Cs and Sr by a batch

method. The selected chemical and physical properties of

the soils were determined and the dominant clay minerals

of the soils were identified by XRD. Cs and Sr sorption

characteristics were determined at 22.5 ± 0.5 �C in the

presence of 0.01 M CaCl2. Freundlich isotherms were

found to fit well with experimental data obtained for

adsorbed Cs and Sr. This experiment showed that using the

relative ratio of cations in the soil solution was better index

to understand the phenomenon of the adsorption of Sr and

Cs in saline-sodic soils. The higher amount of illite and

vermiculite clay minerals in the soils contributed signifi-

cantly to the higher affinity of Cs by the soils.

Keywords Strontium � Cesium � Freundlich isotherm �Calcareous soils

Introduction

The disposal problem of radioactive wastes and reclama-

tion of contaminated soils by radioactive fallout has

become increasingly important with the advent of nuclear

detonations and reactors [1]. Radionuclides reach humans

and other living organisms through several paths after their

release into the environment from nuclear facilities. One of

the important paths is by the root uptake from soil solution

to an edible part of a food crop [2]. In view of radiological

safety, therefore, it is necessary to study behaviors of the

radionuclides in soil [3]. The contaminated soils may cause

an immediate danger to human health as well as a chronic

hazard to environment [4]. Cesium (137Cs) and strontium

(90Sr) are particularly of concern for soil contamination

with a long half-life time (30 years for Cs and 28 years for

Sr) [4].

The sorption and transfer of radionuclides in soil could

be influenced by many factors, e.g., pH, chemical species

and the nature of radionuclides in the solution as well as

chemical and physical nature of the soil [3]. Sorption–

desorption behavior at interfaces between soil particles and

solution is extremely important in transport, bioavailabil-

ity, and fate of different metals and other inorganic matter

in soils and related environments [5]. Soil has the ability to

restrict movement of Cs and Sr in the environment and

retard their risk to the food chain through regulating the

concurrent sorption–desorption processes. Therefore, it is

critical to understand the kinetics and mechanisms of 137Cs

and 90Sr sorption on soil colloidal surfaces [4]. Cesium has

very small hydration energy, thus the electrostatic attrac-

tion of Cs ions by clay particles is large and, therefore, the

ions are preferentially sorbed by the clay fraction [6]. Cs

sorption is highly dependent on phyllosilicate clay miner-

als. Several factors control Cs sorption to soil such as metal

R. Mirkhani (&) � N. Khaleghpanah � A. Majdabadi

Agricultural, Medical and Industrial Research School,

P.O. Box 31485-498, Karaj, Iran

e-mail: rmirkhani@nrcam.org

M. H. Roozitalab

ICARDA-Office, Tehran, Iran

123

J Radioanal Nucl Chem (2012) 293:587–594

DOI 10.1007/s10967-012-1779-x

concentration, pH, cation exchange capacity (CEC), ionic

strength and temperature [7–9].

Many studies have demonstrated that the transport of

radiostrontium in soil is faster than that of other elements

such as cesium, cobalt and plutonium [3]. Some reports

have been made about factors influencing Sr sorption in

soil. The Sr sorption mechanism in soil is mainly an ion

exchange reaction, and sorbed Sr could not exist in the

fixation fraction, as illustrated by the findings that all of

the 90Sr in Chernobyl-contaminated soil could be extrac-

ted by strong acid. In addition, sorption of Mg2? and

Ca2? are in competition with sorption of Sr2? [10, 11].

Indeed, the amount of sorbed Sr was shown to decrease

with increasing Mg and Ca concentrations in soil solution

[12].

The sorption and desorption of radiocesium on a cal-

careous soil were studied by Xiangke et al. [13]. They

found that the sorption and retention of cesium are

mainly determined by the clay minerals. They found also

organic matter has a little positive contribution and the

calcium carbonate has a low negative contribution on the

sorption of cesium on the whole soil. Xiongxin and Zuyi

[3] found that radiostrontium is a relatively mobile

nuclide in calcareous soil and removal of CaCO3 from

the soil slightly increases the retention ability for radio-

strontium. Hakem et al. [6] studied the sorption of cesium

and strontium on a soil in Washington State under batch

experiment. They reported that cesium and strontium

were extensively sorbed on the soil particles and the

sorption data were well described by a Freundlich iso-

therm. Cesium and strontium sorption by selected tropical

and subtropical soils were studied by Chiang et al. [4].

They found that the reactive components of the soils for

Cs and Sr are significantly correlated with Langmuir

sorption.

The main objective of this research was to study the

sorption of cesium and strontium on calcareous soils

developed under dry climatic condition in Karaj and Es-

htehard located in central part of Iran and to identify

suitable models, which best fit the sorption isotherms of

these elements. Calcareous soils are widely distributed and

developed in different regions of Iran under varied climatic

conditions and various parent materials. However, studies

on sorption and transport behavior of cesium and strontium

are very limited.

This research was carried out on highly calcareous

soils developed in arid region of Iran representing the

countries in the Near East and North Africa where very

little information is available on the adsorption of Cs and

Sr by the soils. Also, the research presented new infor-

mation on the status of adsorption of these elements on

a highly saline-sodic soil, which was not previously

studied.

Experimental

Soil sampling

Thirty one soil samples from Karaj and Eshtehard regions

in Iran were selected from eight dominant soil series

developed under arid and semiarid conditions. The soils

were classified under Aridisols and Entisols orders. The

composite soil samples were taken from 0 to 25 cm depth.

The soils were air-dried at room temperature, and crushed

to pass through a 2 mm sieve and stored before analysis.

Soil analysis

Particle sizes analyses were made using the hydrometer

method with reading during 48 h (15 time interval) and

perform all of corrections for determining particle size

distribution [14]. Sand fraction (0.05–2 mm) was deter-

mined by 0.05 mm sieve (270 meshes).

Soil pH and EC were measured in a saturation extract

using an EC meter and pH meter, respectively. Organic

matter was determined using Walkley–Black wet digestion

method [14]. CEC was measured according to the ammo-

nium acetate (pH = 7.0) method [14]. Equivalent calcium

carbonate was measured using titration method (HCl 1 M)

[14]. Water soluble Na and K were determined in satura-

tion extract by Flame Photometer. Water soluble Ca and

Mg were measured by Atomic Absorption Spectrometer

[14].

Clay mineralogy analysis

The pretreatments of the soil samples were conducted

according to Kunze and Dixon [15]. The soil samples were

washed with distilled water and checked with silver nitrate

(AgNO3) for removal of soluble salts. Then, samples were

treated with 30 % H2O2 and heated on a water bath

(60–70 �C) to remove organic matter. In order to improve

the identification of soil minerals by X-ray Diffraction

(XRD), the soil samples were treated with 1 M sodium

acetate (pH = 5) solution, and heated on a water bath

(60–70 �C) to remove CaCO3. Free iron oxides removal

from soils was done by dithionate-citrate buffered with

sodium bicarbonate.

The samples were dispersed and fractionated into clay

fractions (\2 mm) according to the Stokes’ Law. The clay

fraction was concentrated by putting the clay solutions in

an oven and heated to 60 �C. The fractionations and

treatments of clay minerals were then followed by XRD

analysis [15, 16]. The clay samples were saturated with Mg

and K ions, and mounted as slurries on glass slides for

XRD analysis. The Mg-saturated clays were examined at

25 �C before and after glycerol solvation. The K-saturated

588 R. Mirkhani et al.

123

clays were examined at 25 �C and after heating at 300 and

550 �C for 2 h. The oriented clay mineral aggregates were

studied using an X-Ray Diffractometer (Philips, PW 1800

model). The XRD patterns were recorded in the range of

2–45� 2h.

Sorption experiments

In order to give a range of physical and chemical proper-

ties, e.g., clay content, CEC, organic matter and calcium

carbonate, which may affect Cs and Sr sorption, two soil

samples (a saline-sodic soil and a non-saline soil) were

selected from the initial study on the basis of physical and

chemical properties for cesium and strontium sorption.

The soil sorption was investigated in this study by batch

method, at room temperature (22.5 ± 0.5 �C). Sorption

experiments were performed in 50 mL screw cap centri-

fuge tubes. The Cs sorption isotherms were determined

using eight concentrations of CsNO3 in the range

2–16 mg L-1, in the presence of 0.01 mol L-1 calcium

chloride as a background electrolyte. The Sr sorption iso-

therms were determined using eight concentrations of

SrNO3 in the range 2–30 mg L-1, in the presence of

0.01 mol L-1 calcium chloride.

To determine the time required for equilibration of the

Cs and Sr solutions of the soils, a study was conducted

for both soils. The solution concentrations were 8 and

14 mg L-1 for Cs, Sr respectively and 0.5 g soil samples

were shaken with 25 mL of each solution for specific

length of time ranging from 30 min to 24 h. The filtration

method of separating the solutions from the solids was

rapid enough for precise timing to within a few minutes.

The result of the preliminary kinetic study showed that

the most rapid increase in the amount of Cs and Sr sorbed

onto both soils occurred in the first 3 h. At 6 h, the

amounts of sorption onto the both soils were no different

from the amount at 4 h. Furthermore, it did not change

with any other increase in time interval up to the maxi-

mum of 24 h.

The ratio of mass of soil to volume of solution was 1:50

(25 mL of each solution was mixed with 0.5 g of soil) and

shaken for 3 h using reciprocating shaker. The suspensions

of soil were centrifuged at 4,500 rpm for 15 min and the

supernatant was filtered through Whatman No. 42

(0.45 lm pore diameter). The solutions from the above

treatment were analyzed for Cs and Sr using an Atomic

Absorption Spectrometer. The amount of Cs and Sr sorbed

were deduced from the difference between the solution

concentration added and concentration remaining in

supernatant. The sorption experiments were conducted in

duplicate and the means reported.

It is assumed that the fate of radionuclides in the soils

follows the behavior of stable elements. Therefore, to

facilitate experimental studies on radiocesium and radio-

strontium sorption in the environment, the utilization of

stable Cs and Sr were applied.

Results and discussion

The geometric means of selected physicochemical prop-

erties of 31 soils studied were presented in Table 1. Geo-

metric means for non-saline and saline-sodic soils were

separately reported in Table 1. The soil sites represented a

wide range of soil texture, salinity and sodicity. The soils

were alkaline and contained various amounts of calcium

carbonate. The soil sites were also representing both cul-

tivated and barren soils. To assess the sorption behavior of

Sr and Cs, two soils with different land use and level of

salinity were selected for this study.

Physicochemical properties of the selected soils were

presented in Table 2. Soil 1 was a saline-sodic soil with

high EC, SAR, CEC and clay content and Soil 2 was a non-

saline soil with lower EC, SAR, CEC and clay content.

XRD data have shown that the mineralogical composition

of two soils dominated by the presence of 2:1 phyllosilicate

clay minerals and vermiculite and illite were dominant clay

minerals in both soils (see Fig. 1).

Sorption isotherms

The Freundlich and Langmuir adsorption isotherm equa-

tions were used to model cesium and strontium sorption in

selected soils. The sorption isotherms of cesium and

strontium were shown in Figs. 2, 3, 4, 5, 6 and 7.

Freundlich isotherm can be expressed as:

q ¼ KdC1=n ð1Þ

where q is the amount of adsorption and C is the equilib-

rium concentration in solution, Kd is the distribution

coefficient and n is a correction factor.

The linearized Freundlich equation is given as [5]:

Log q ¼ logKd þ 1=nlogC ð2Þ

The Langmuir adsorption equation can be expressed as

q ¼ kCb= 1þ kCð Þ ð3Þ

where q and C were defined above, k is a constant related to

the binding strength and b is the maximum amount of

adsorptive that can be adsorbed.

By rearranging to a linear form, the equation could be

expressed as

C=q ¼ 1=kb þ C=b ð4Þ

Plotting C/q vs C, the slope is 1/b and the intercept is

1/kb [5].

Sorption behavior of cesium and strontium 589

123

Cs sorption isotherm

The sorption data was found to demonstrate Freundlich-

type behavior with high correlation coefficient of

R2 C 0.98 (Fig. 2) because Freundlich model is appropri-

ate for adsorption on heterogeneous surfaces. The distri-

bution coefficient (Kd) of cesium was calculated from the

linear equation of Freundlich isotherm. The results

Table 1 Range of soil properties (31 soil samples)

Soil properties Sand (%) Silt (%) Clay (%) dg (mm) dg CaCO3 (%)

All soils (n = 31)

GM 23.39 33.26 31.04 0.0274 12.796 16.763

Min 2.78 8.88 13.76 0.0043 6.252 7.93

Max 69.39 63.28 58.85 0.1913 23.593 31.47

Non-saline soils (n = 22)

GM 31.11 30.12 29.65 0.0352 14.486 15.51

Min 10.34 8.88 16.70 0.0082 8.958 7.93

Max 69.39 51.87 54.11 0.1913 23.593 31.47

Saline-Sodic Soils (n = 9)

GM 11.64 42.38 34.70 0.0148 9.448 20.263

Min 2.78 23.81 13.76 0.0043 6.252 16.49

Max 46.28 63.28 58.85 0.0860 19.738 26.84

Soil properties OC (%) CEC (cmolc ?/kg) ECe (dS/m) SAR (mmol/L)0.5 pH (paste saturation)

All soils (n = 31)

GM 0.605 13.79 3.02 4.69 8.3

Min 0.166 7.74 0.575 0.267 7.19

Max 2.43 27.79 134.5 241.232 8.93

Non-saline soils (n = 22)

GM 0.694 13.83 1.18 1.81 8.56

Min 0.166 7.74 0.575 0.267 8.34

Max 2.43 27.79 3.45 11.967 8.93

Saline-Sodic Soils (n = 9)

GM 0.432 13.69 30.01 48.26 7.71

Min 0.186 9.086 4.66 14.905 7.19

Max 0.604 17.65 134.5 241.232 8.57

Table 2 Chemical and physical properties of selected soils

Properties Clay (%) Silt (%) Sand (%) Texture dg (mm) dg Na? (mg/L) K? (mg/L) Ca2? (mg/L) Mg2? (mg/L)

Selected soils

Soil 1 58.85 38.20 2.95 Clay 0.0043 6.276 17627.9 70.05 1372.67 436.27

Soil 2 28.75 51.87 19.38 Silty Clay Loam 0.0208 10.698 79.3 19.92 90.18 37.67

Properties KCa + Mg + Na + K

CaCa + Mg + Na + K

Ca + MgCa + Mg + Na + K

CEC (cmolc ?/kg) CaCO3 (%) OC (%)

Selected soils

Soil 1 0.00359 0.07 0.093 17.65 21.15 0.51

Soil 2 0.088 0.397 0.563 11.69 22.03 0.97

Properties ECe (dS/m) SAR (mmol/L)0.5 ESP (%) pH (paste saturation) Cs-Kd (L/kg) Sr-Kp (L/kg) Soil type

Selected soils

Soil 1 60.1 106.13 60.83 7.73 253.98 23.43 Saline-Sodic

Soil 2 1.003 1.77 1.33 8.60 213.65 15.98 Non-saline

590 R. Mirkhani et al.

123

indicated that Soil 1 (saline-sodic soil) adsorbed Cs higher

than Soil 2 (non-saline soil) at same Cs concentration

(Fig. 3). The distribution coefficient value was higher in

clay soil (253.98 L kg-1) than in the silty clay loam soil

(213.65 L kg-1). Comparison of both soils showed that

clay soil had higher CEC and very low dg (Geometric

Mean Diameter), whereas silty clay loam soil had lower

CEC and higher dg than clay soil. Since clay fraction has a

high affinity for sorption of cesium, Soil 1 adsorbed Cs

higher than Soil 2. Giannakopoulou et al. [17] showed that

particle size fractions and especially clay content plays

predominant role on sorption of Cs. The affinity of illite

and vermiculite for Cs is generally assumed to be due to the

presence of frayed edges sites and wedge zones. Their

strong affinity for Cs is due to very small hydration energy

of the cation and higher electrostatic attraction between Cs

and the clay particles, preferentially adsorbed on these

sites. This study indicated that Cs sorption in two selected

soils was very high. The distribution coefficient of Cs was

higher in the saline-sodic soil due to higher amount of clay

content (58.85 %) and more fine clay fraction (Table 2).

The results showed that the concentrations of Na, Ca,

Mg and K in Soil 1 with a high EC (60.1 dS m-1) were,

respectively 222, 15, 11 and 3.5 times more than those in

Soil 2 (Table 2). This soil had also higher Cs adsorption in

Fig. 1 XRD patterns of clay fraction in Soil 1 (left) and Soil 2

(right). Mg-EG Mg-saturated and EG solvated clay samples, Mg Mg-

saturated clay samples, K-25 K-saturated clay samples dried at 25 �C,

K-300 K-saturated clay samples heated at 300 �C, K-550 K-saturated

clay samples heated at 550 �C

Fig. 2 Freundlich isotherm (linear form) of cesium

Fig. 3 Comparison of Freundlich isotherms of cesium in two soils

Sorption behavior of cesium and strontium 591

123

different concentrations. In initial Cs concentrations, the

difference in the amount of adsorption by two soils was

relatively lower, but at the higher Cs concentration, Soil 1

relatively adsorbed higher amount of Cs from the solution.

Bangash [18] has shown that the ability of some cations to

depress the adsorption of cesium follow the order

K [ Ca [ Mg [ Na. The presence of K ion strongly

constrains the Cs adsorption because cations with similar

radius and hydration energy compete more effectively

against Cs ions on illite and vermiculite surfaces. Fur-

thermore, evidence shows that because of its lower

hydration energy, Cs is adsorbed stronger than K by clay

minerals. Since the ratio of K to ‘‘Mg ? Ca ? Na ? K’’

ions in Soil 1 was relatively lower compared to Soil 2, this

prevailing condition had limited the role of K in con-

straining the adsorption of Cs in Soil 1. Bangash [18]

reported that the Freundlich isotherm adequately describes

the experimental data and the sorption behavior of Cs on

soil samples containing illite as a dominant clay mineral.

Also, Hakem et al. [6] found that the sorption behavior of

Cs was well described by a Freundlich isotherm.

As with the Freundlich model, both soil samples tested

were followed the Langmuir model of soil adsorption. The

Fig. 4 Langmuir isotherm of cesium in the Soil 1 (left) and Soil 2 (right)

Fig. 5 Freundlich isotherm of strontium in the Soil 1 (left) and Soil 2 (right)

Fig. 6 Comparison of Kp in two soils

Fig. 7 Langmuir isotherm of Sr in the Soil 2

592 R. Mirkhani et al.

123

Langmuir isotherm was conducted by plotting C/q against

C; if data lie on a straight line then the Langmuir model

might be appropriate. But experimental measurements in

both soils were not described by a single straight line in all

Cs concentrations. The Langmuir model was best fitted

(R2 = 0.979 and 0.901 for Soil 1 and 2, respectively) the

data in lower Cs concentrations up to 10 mg/L (Fig. 4). In

both soils the Langmuir isotherm described the sorption of

Cs with lower R2 and higher standard error than Freundlich

isotherm. It could be concluded that the Freundlich model

better defined the Cs sorption phenomena in both soils.

Langmuir model is based on theoretical assumptions such

as the monolayer adsorption on homogenous surfaces

without adsorbate–adsorbate interactions. Therefore, this

may explain why at low concentrations the Langmuir was

well fitted. But with increasing concentration of Cs

([10 ppm) there were certainly more layers, hence the

Langmuir model became inappropriate.

Sr sorption isotherm

The Freundlich adsorption equation was used to model Sr

sorption in two soils. In Soil 1, 1/n value was equal to 1.2

(1/n [ 1) (Fig. 5), and therefore, adsorption isotherm

described by S-type isotherm. This type of isotherm indi-

cates that at low concentrations the surface has a low

affinity for the strontium, which will be increased at higher

concentrations. In Soil 2, 1/n value was equal to 0.74 (1/

n \ 1) (Fig. 5), hence adsorption isotherm described by

L-type isotherm. This adsorption behavior could be

explained by the high affinity of strontium by the soil,

which then decreases as concentration increases [5]. S-type

isotherm is prevalent in soils with fine texture such as Soil

1, which had higher clay content and low dg [19].

To compare the Kd values of the 2 soils studied, a par-

tition coefficient, Kp, can be obtained from the slope of a

linear adsorption isotherm. The value of Kp in Soil 1 was

higher than the one obtained from Soil 2 (Fig. 6). There-

fore, at similar concentration, sorption of Sr by Soil 1 was

higher than that by Soil 2 and at higher Sr concentrations,

the difference between sorption of Sr by the soils were

increased (Fig. 6). The higher sorption of Sr in Soil 1 (with

high SAR of 106.13 (mmol L-1)0.5 and ESP of % 60.83)

may be explained by easier replacement of exchangeable

Na by the Sr ions in the solution in spite of a higher

absolute concentration of Ca and Mg in the soil solution.

The amount of clay and fine clay content in this soil was

also much higher (Table 2) which contribute to higher Sr

sorption. Furthermore, the higher amount of vermiculite-

type clay mineral with high charge density was another

reason for the higher affinity of Sr by the saline-sodic soil.

The sorption data in Soil 1 were not defined by the

Langmuir equation. The results also showed that in Soil 2,

Langmuir model was best correlated (R2 = 0.95) at the

higher concentration of Sr of more than 14 ppm (Fig. 7).

Therefore, the sorption study of the 2 soils indicated that

Freundlich model described more appropriately the Sr

sorption in saline-sodic and non-saline soils.

Comparison of Cs and Sr sorption

As may be seen in Fig. 8, at a similar Cs and Sr concen-

tration, the amount of sorption of Cs was significantly

higher than Sr in both soils. For example, at 12 ppm con-

centration of Cs and Sr added to both soils, the amounts of

Cs adsorbed per unit weight of soil (q) were 469 and

453.25 mg kg-1 in soils 1 and 2, respectively. Whereas for

Sr were 181.25 and 163.5 mg kg-1 in Soil 1 and 2,

respectively (Fig. 8). Also, the Kd values of Cs in both soils

were higher than Sr. This showed that the Cs sorption is

very selective compared to Sr. This finding was compatible

with the results obtained by Price [20], indicating that

radiostrontium has more mobility than other ions such as

Cs, Co and Pt.

Fig. 8 Comparison of Cs and Sr sorption in two studied soils

Sorption behavior of cesium and strontium 593

123

Conclusion

The sorption of Cs and Sr in soils may be influenced by

several factors, e.g. quantity and quality of clay fraction,

EC, SAR, dg and CEC. The Freundlich isotherm ade-

quately described the Cs and Sr sorption for the saline-

sodic and non-saline soils studied, and the Langmuir model

was not applicable to describe the soil behavior for Cs and

Sr adsorption.

Furthermore, this study may indicate that in saline-sodic

soils (soils with high EC and SAR) using the relative ratio

of K to total soluble cations, i.e. Ca, Mg, Na and K was

more appropriate in identifying the process of Cs adsorp-

tion in the soils. The higher amount of vermiculite and illite

clay minerals in soils contributed significantly to the higher

affinity of Cs by soils.

This experiment also showed that using the relative ratio

of divalent to monovalent cation was a better index to be

used to understand the phenomenon of the Sr adsorption in

saline-sodic soils.

Acknowledgments The authors would like to express their thanks

and appreciation to Dr. Hassan Tofighi, Associate Professor, Uni-

versity of Tehran, Department of Soil Science and Dr. Shahin Oustan,

Assistant Professor, University of Tabriz, Department of Soil Science

for their advice and providing assistance during the course of this

study.

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