Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 2, pp. 33-36, 2004

Review
Sorption of Strontium on Zirconia Modified Vermiculite
M. V. Sivaiah,a S. S. Kumar,a K. A. Venkatesan,b P. Sasidhar,*, c R. M. Krishna,a and G. S. Murthy aa

Nuclear Chemistry Section, Andhra University, Visakhapatnam-530 003, India
Fuel Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakam-603 102, Indiac
Safety Research Institute, Atomic Energy Regulatory Board, Kalpakkam-603 102, Indiab

Received: July 6, 2004; In Final Form: November 4, 2004
Zirconia-modified vermiculite (ZrMV) was prepared by mixing of zirconium oxycation with vermiculite followedby calcination at 523 K for two hours. Vermiculite and ZrMV were characterized by surface area and X-ray
diffraction patterns. Sorption of strontium as a function of pH, strontium ion concentration, time, and temperature
was studied on ZrMV, and the results were compared with those of vermiculite. The percentage sorption of strontium on vermiculite and ZrMV increased with increase in pH and lowering concentration of Sr 2+. The rate of
uptake of strontium by ZrMV was found to be much faster than that for vermiculite. The distribution coefficientof strontium on ZrMV is of the order of 104 mL/g at pH 3, and the amount of strontium sorbed on ZrMV wasalways found to be much higher than that on the parent clay. The sorption data were fitted to Langmuir adsorption model for obtaining the sorption capacity of the sorbent. Increase of temperature increases the distribution
coefficient of strontium on both ZrMV and vermiculite. Distribution of strontium on ZrMV was also determinedfrom the groundwater samples obtained from the waste disposal site at Kalpakkam.

1. Introduction

Tetrahedra sheet (Si [Al], O, OH)

Naturally occurring swelling clays (e.g. montmorillonite,
vermiculite) are regarded as the backfill material in the engineered barriers of high-level radioactive waste forms.1, 2 Huge
deposits of this clay material are abundant all over India, especially in the states of Andhra Pradesh and Tamil Nadu.3 The
swelling clays act as a barrier between the waste form and thebiosphere.2, 4 The characteristic properties1, 2 of these swelling
clays, such as high cation-exchange ability, low hydraulicpermeability, adequate thermal conductivity, etc., ensure thatthe long-lived radioactive fission products are retained in theclay for more than 103 years, such that the radioactivity of thefission products decays to very low levels.
Vermiculites are structurally similar to montmorillonite,
except that they have higher cation-exchange capacity. 5 It
consists of octahedral (aluminum oxide) and tetrahedral(silicon oxide) sheets, which are bonded together by sharing acommon oxygen atom as illustrated in Figure 1. Isomorphous
substitution by lower valent metal ion for Si4+ and Al3+ resultsin the generation of net negative charge to the layer, which isnormally compensated by cations such as Na+, Ca2+, or Mg2+.
These cations are situated between the layers and can beexchanged with any desired cations by simple ion exchange.
Vermiculites have higher cation-exchange capacity than smectite clays (e.g. montmorillonite) due to greater isomorphous
substitution by lower valence metal ions for Si4+ and Al3+ ionspresent in the lattice. Thus, it is expected for the vermiculite tosorb metal ions more efficiently than montmorillonite.
For the rapid sorption and enhancement of catalytic activity,swelling clays (bentonite and montmorillonite) were intercalatedwith polymeric oxocations such as [Al13O4(OH)24(H2O)12]7+, an
Al137+ Keggin ion,6 a tetrameric [Zr4(OH)8(H2O)16]8+ ion,7 or atrimeric chromium oxyformate [Cr3O(HCO2)6(H2O)6]+ ion8 etc.
into the lamella of swelling clay9−13 that results in propping of
the interlayer distance of the silica network. Dehydrating thisintercalated polyoxocation results in the formation of a metal*Corresponding author. E-mail: sasidhar@igcar.ernet.in. FAX:
+91-4114-280165.

Octahedral sheet (Al[Mg, Li, Fe], O, OH)
Tetrahedra sheet (Si [Al], O, OH)

M = hydrated
Na+, Ca2+, Mg2+

M

M

M

Tetrahedra sheet (Si [Al], O, OH)
Octahedral sheet (Al[Mg, Li, Fe], O, OH)
Tetrahedra sheet (Si [Al], O, OH)
Figure 1. The idealized structure of swelling phyllosilicate minerals.
The elements shown in square brackets are isomorphous substitutedions.

oxide link acting as pillars between the interlayer. However, noreports were available for pillaring of vermiculites by classicalintercalation procedures, and it has been reported 6 that it isrelatively easy to pillar with Al137+ Keggin ion or zirconium
oligomers in smectites rather than vermiculites. It was also
reported, even in some cases of smectite clays, that pillaring
can also be resulting from oligomeric aluminum species otherthan Al137+ Keggin ion.12 This has resulted in a decrease inlattice distance rather than the increase normally observed due
to pillaring. Nevertheless, these pillared smectite type clays,
especially montmorillonite, were shown to exhibit good selectivity and rapid kinetics of sorption of fission products such as
137
Cs and 90Sr.14−17
In the present study, we have adopted the classical methodof pillaring of clays, that is, we have attempted15 to intercalate
with smaller Zr48+ tetramer rather than bigger Al137+ polymer inthe lamella of vermiculite to observe if pillaring is possible.
This zirconia modified vermiculite (ZrMV) was prepared bymixing partially hydrolyzed zirconium oxychloride (results in© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/24/2004

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
8+

tetrameric [Zr4(OH)8(H2O)16] ion) with vermiculite followedby calcination. Sorption behavior of strontium on ZrMV and theparent clay, vermiculite, was reported in this paper. Sorption ofstrontium on ZrMV and vermiculite was studied as a function ofpH, time, temperature, and the concentration of strontium. The
suitability of ZrMV as a potential backfill material was also
ascertained by obtaining the basic distribution data for the sorption of 85+89Sr on ZrMV from the groundwater samples procured
from the radioactive waste-disposal site at Kalpakkam.
2. Experimental
Materials. All the reagents used were of Analytical AR
grade. Vermiculite (CEC 0.9 meq/g) was procured from M/s
Sigma Enterprises, Chennai, India and it is composed of SiO2
53.3%, Al2O3 9.2%, TiO2 2.5%, FeO 3.0%, Fe2O3 5.3%, MnO
0.14%, Na2O 1.16%, K2O 1.53%, CaO 0.07%, MgO 2.6%, andrest H2O. The vermiculite was ground and sieved to particles
of diameter 400 – 500 µm. This fraction was used for intercalation and subsequent sorption studies. Radioactive isotope
85+89
Sr was obtained from the Board of Radiation and Isotope
Technology (BRIT), Mumbai, India.
Preparation of zirconia modified vermiculite (ZrMV).
ZrMV was prepared by an analogous method15 used for pillaringof zirconium oxide in montmorillonite. Initially, vermiculite
was converted to the sodium form by stirring 50 g of vermiculitein 500 mL of 1 M NaCl for 12 hours. This procedure wasrepeated twice. One litre of 0.1 M zirconium oxychloride wasadjusted to pH 4 and added to 37 g of Na-vermiculite. The
mixture was stirred for 24 hours at 303 K. The product obtained
was filtered and washed with distilled water until the filtrate wasfree from chloride ion. The modified vermiculite was dried at
373 K for 24 hours and further calcined at 523 K for two hours
to obtain ZrMV. This final product and the parent vermiculite
were used for the subsequent sorption studies.
Characterization. X-ray diffraction pattern of vermiculiteand ZrMV were obtained using a Philips 1011 X-ray diffractometer with Cu Kα radiation. Surface area of the vermiculiteand ZrMV was determined by the BET method using a
Micromerities Pulse Chemisorb 2700 System.
Sorption of strontium on vermiculite and ZrMV.
Effect of pH. Sorption of strontium as a function of equilibrium
pH was studied by equilibrating 10 mL of solution containing
85+89
Sr tracer with 0.05 g of the sorbent at 301 K. The pH of
the solution was adjusted, using dilute nitric acid, to the pH
range 1 – 6. After six hours of equilibration, the solution wascentrifuged and the radioactivity of the supernatant was measured
using a well-type NaI(Tl) scintillation counter. The distribution
coefficient (Kd) and percentage sorption of strontium (%Sorb)
on the sorbent were calculated using the following eq
1368

Ai – Af V
A A
or %Sorb = i – f × 100, (1)
Af
Af
m
68
13

Kd (mL/g) =where Ai and Af are the initial and final activity of the solution
phase, respectively. V and m are the volume of the solution(mL) and the mass of the sorbent (g), respectively, taken forequilibration.
Kinetics of sorption. Sorption of strontium as a function oftime was studied at 301 K by equilibrating 0.05 g of thesorbent with 10 mL of the solution containing 0.001 M nitricacid and 1.14 × 10−4 M Sr2+ ions added in the form of strontium nitrate spiked with 85+89Sr. The equilibration was stopped
at various intervals of time and the percentage of strontium
sorbed by the sorbent was calculated from the radioactivity
measurements. A similar experiment was also performed with
the initial strontium concentration of 1.14 × 10−3 M.
Effect of Sr2+ ion concentration. Effect of strontium ion
concentration on the sorption of strontium was studied by equi-

Sivaiahlibrating 0.05 g of the sorbent with 10 mL of the solution
containing 0.001M nitric acid spiked with 85+89Sr tracer andvaried concentrations of strontium ion at 301 K for six hours.
The strontium ion concentration in the solution was varied
from 5.7 × 10−5 M to 1.14 × 10−2 M. The amount of strontium
sorbed by the sorbent was calculated from the radioactivity
measurements.
Effect of Na+ and Ca2+ ion concentration. The effect of
sodium and calcium ion on Kd of strontium was studied at 301
K by equilibrating 0.05 g of the sorbent with 10 mL of thesolution containing 0.001 M nitric acid, 1.14 × 10−4 M Sr2+
spiked with 85+89Sr tracer, and varied concentration of additives
for six hours. The concentration of additives (Na+ and Ca2+)
was varied from 0.001 M to 0.1 M and they were added as
nitrate salts.
Effect of temperature. The influence of temperature on the
distribution coefficient was studied by equilibrating 0.05 g of
the sorbent with 10 mL of the solution containing 0.001 M
nitric acid and 1.14 × 10−3 M strontium ion spiked with 85+89Sr
tracer at 301 K. The value of Kd was calculated using eq 1 andsimilar experiments were also performed at 307 and 313 K.
Sorption of strontium from groundwater (Kalpakkam).
Groundwater samples were obtained from three different locations of shallow land radioactive waste-disposal sites at
Kalpakkam. The water sample was spiked with 85+89Sr tracer,and 10 mL of the solution was equilibrated with 0.05 g ofvermiculite and ZrMV for 24 hours. Kd was calculated fromthe radioactivity measurements.
3. Results and Discussion
X-ray diffraction patterns of vermiculite (as obtained andsodium form) and zirconia modified vermiculite (ZrMV) are
shown in Figure 2. Vermiculite exhibited several reflections in
the 2θ range 7.3˚ – 5.9˚ indicative of the presence of various
hydrated monovalent and divalent cations in the interlayer.
Conversion of vermiculite into the sodium form shifts the d(001)peak of vermiculite from 14 Å (2θ = 6.3) to 12.6 Å (2θ =7.0)
as shown in Figure 2. Further this peak position gets shifted to14.5 Å (2θ = 6.0) after zirconia modification. The shift of d(001)spacing from 12.6 Å to 14.5 Å is the indication that vermiculite was swollen by the presence of zirconium oxide in theinterlayer. However, pillaring in smectite clays e.g. montmorillonite or bentonite, normally results in propping of interlayer
distance from 4 to 14 Å depending upon the method of preparation. Even though, vermiculites belong to swelling clays,
structurally similar to smectites, however, did not prop apart tothe anticipated expansion by the current procedure adopted forpillaring which can be attributed to the higher basal charge ofvermiculites than smectites.6 It was estimated by ICP-MS thatnearly 3.8 mg of zirconium is present per gram of ZrMV. Further,very marginal increase of surface area from 11 m2/g for vermi-

6000

d(001)
5000

Relative Intensity

34

4000

ZrMV
3000

2000

Na-form of vermiculite (uncalcined)
1000

Vermiculite as obtained
0

6

8

10

12

14

2θ

Figure 2. XRD pattern of ZrMV and vermiculite.

16

18

20

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Sorption of Strontium on Zirconia Modified Vermiculite2

culite to 49 m /g for ZrMV was observed indicating that it wasdifficult to intercalate or provide pillars in vermiculites than insmectites by classical procedures. However, the presence ofminimal amount of zirconia in the interlayer have created moreroom for the diffusion of strontium ion in ZrMV that seems tobe adequate for the rapid sorption of strontium as it can beseen from the following results.
Figure 3 shows the amount of strontium sorbed as a functionof time. It can be seen that there is a rapid sorption of strontium
in the initial stages of equilibration followed by saturation. In
all the cases, equilibrium was established within 5 hours andthe rate of sorption of strontium on ZrMV was always higher
than that on the parent vermiculite. This could be due to thepartial propping of vermiculite by zirconia that facilitates the
diffusion of strontium at a much faster rate compared to vermiculite. In vermiculites, the negatively charged lamella are
attracted towards each other by the presence of positively
charged cations in the interlayer. Diffusion of Sr2+ ion towards
the sorption site is hindered in this case, and it occurs bypushing the lamella apart while moving towards the sorption
site. However, the presence of zirconia in the interlayer,propped the interlayer apart and created more room for the
diffusion of Sr2+. Sorption of strontium that emanates fromzirconia can be ruled out by the fact18, 19 that zirconium oxide
sorbs Sr2+ only above pH 9. Thus, the enhancement in the rateof sorption can only be attributed to the increase in the diffusionof metal ions such that it reaches the interior sorption sites in ashort time.
The percentage of strontium sorbed by the sorbent as a function of pHeq is presented in Table 1. Sorption of strontium
increased with increase in pH in both the sorbents, and thesorption was always found to be higher in the case of ZrMV.
More than 99% of strontium was sorbed by ZrMV when theequilibrium pH was 3, but only 70% was sorbed by vermiculiteat this pH. K d values of strontium ion in the presence of
sodium and calcium ion are shown in Table 2. The distribution coefficient of strontium decreased with increase in theconcentration of additive (Na+ or Ca2+), and sorption was negligible when the concentration of the additive exceeded 0.1 M in
all the cases.
The classical Langmuir equation relating the strontium ion
concentration in sorbent and in solution is given by
Cf
Cf
1
=
+
,
Cs Kb
b

(2)

where Cf is the equilibrium concentration of Sr2+ (mg/L), Cs isthe amount of strontium sorbed on the sorbent (mg/g), K is theLangmuir adsorption constant (L/mg), and b (mg/g) is the
maximum amount of strontium that can be sorbed. The magnitudes of K and b are obtained from the slope and intercept of
the plot shown in Figure 4, as given in the same figure. The

TABLE 1: Variation of Percentage Sorption of Strontium
Tracer on Vermiculite and ZrMV as a Function of pH
Percentage sorption, %Vermiculite
ZrMV

pHeq
1.00
2.00
3.00
4.00
4.95
5.86
6.67

0.00
20.2
70.6
87.5
87.5
88.0
89.1

3.4
37.3
99.2
99.3
99.7
99.8
99.9

V/m = 200 mL/g, T = 301 K, equilibrium time = 6 h,
[Sr2+] = trace.

TABLE 2: Variation of Distribution Coefficient (K d ) of
Strontium in the Presence of Sodium and Calcium Ion
Kd of strontium, mL/g, in the presence of
Na+ ion
Ca2+ ion
Vermiculite
ZrMV
Vermiculite
ZrMV
42
55
2
8
120
225
51
64
411
2896
114
220
566
22178
301
1242

[Mn+]
0.1
0.05
0.01
0.001

[Sr 2+] = 1.14 × 10−4 M, V/m = 200 mL/g, [HNO3] = 0.001 M,
equilibrium time = 6 h, T = 301 K.

TABLE 3: Variation of Distribution Coefficient of Strontium from 0.001 M Nitric Acid on Vermiculite and ZrMV
Kd of strontium, mL/g
Vermiculite
ZrMV

Temperature, K
298
307
313

230
243
250

1975
2440
2574

[Sr 2+] = 1.14 × 10−3 M, V/m = 200 mL/g, equilibrium time = 6 h.

sorption capacity of strontium on vermiculite and ZrMV wasfound to be 32.5 and 31.5 mg/g, respectively. It is interesting
to note that, though the sorption capacities are almost thesame, the rate of sorption of strontium on ZrMV was always
higher than that on vermiculite as shown in Figure 3. This
could be possible only when the sorption sites that were
hindered in vermiculite can be accessed without much difficulty while the strontium ion was diffusing in modified vermiculite. Table 3 shows the distribution coefficient of strontium
from 0.001 M nitric acid on ZrMV and vermiculite at three
30

100
25

80

ZrMV, b = 31.53 mg/g, K = 0.3359 L/g, R2 = 0.998
2
Vermiculite, b = 32.52 mg/g, K = 0.0155 L/g, R = 0.995
V/m = 200 mL/g, equilibrium time = 6 h, T = 301 K[HNO3] = 0.001 M

20

C f /CS /g L-1% Sorb35

60

ZrMV, [Sr 2+] = 1.14 × 10-4 M

40

Vermiculite, [Sr 2+] = 1.14 × 10-4 M
ZrMV, [Sr ] = 1.14 × 10 M
2+

20

15

10

-3

5

Vermiculite, [Sr 2+] = 1.14 × 10-3 M
V/m = 200 mL/g, T = 301 K

0
0

200

400

600

800

1000

0

1200

1400

1600

Time /minFigure 3. Variation of percentage sorption of strontium from 0.001 M
nitric acid as a function of time.

0

200

400

600

800

1000

C f /mg L-1

Figure 4. Langmuir plot for the sorption of strontium on vermiculiteand ZrMV.

36

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Sivaiahdifferent temperatures. It is seen that Kd values increases withincrease of temperature indicating that sorption process isendothermic.18, 20, 21
The mobility of groundwater at the radioactive waste
disposal sites is the mode of transport for the migration ofradionuclides from the waste form to the geosphere. To avoid
the contamination of the environment and the hazards tohuman beings, it was proposed to use the naturally occurringswelling clays (e.g. montmorillonite, vermiculite) as the backfill material in the engineered barriers of high-level waste
forms.1, 2, 22 To study the retention ability of strontium on both
ZrMV and vermiculites, equilibration was performed with thegroundwater samples obtained from three different locations ofwaste-disposal sites at Kalpakkam. The samples were spiked
with 85+89Sr tracer and the distribution coefficients of strontium
obtained on both sorbents are shown in Table 4. The data
reveal that the modified vermiculite is a superior candidate
than vermiculite as a backfill material for retaining strontium
for a sufficiently long time and thus can avoid the migrationand contamination of the environment.
TABLE 4: The Composition of Groundwater and the Kd of
85+89
Sr from Groundwater onto Vermiculite and ZrMV
SampleNo

ConstituentsKd, mL/g
Vermiculite ZrMV

1

pH = 6.32, Na+ = 48 mg/L, K+ = 6 mg/L,
Ca2+ = 9.62 mg/L, Mg2+ = 2.92 mg/L,
dissolved solids = 110 mg/L

1119

7761

2

pH = 7.70, Na+ = 36 mg/L, K+ = 12 mg/L,
Ca2+ = 6 mg/L, Mg2+ = 3.8 mg/L,
dissolved solids = 154 mg/L

1523

9869

3

pH = 7.32, Na+ = 9 mg/L, K+ = 3.7 mg/L,
Ca2+ = 26.5 mg/L, Mg2+ = 5.5 mg/L,
dissolved solids = 80.3 mg/L

1022

18686

T = 301 K, V/m = 200 mL/g, equilibrium time = 24 h.

4. Conclusions
In this study, we have shown that it is difficult to place
pillars completely in the interlayer of vermiculites by classicalmethods. However, it is possible to partially swell or prop
apart the lamella of vermiculite to some extent by the tetrameric[Zr 4 (OH) 8 (H 2 O) 16 ] 8+ ion that results in a zirconia modified
product, ZrMV, which seems to be adequate for the rapid sorption of strontium than the parent clay. The percentage sorptionof strontium increased with increase in pH. The presence of
sodium and calcium ion decreased the sorption of strontium inboth vermiculites, and when the concentration of sodium ion in
the liquid phase exceeds 0.1 M sorption of strontium was notfavored. Though the sorption capacities of strontium on bothmodified and parent vermiculites were nearly identical, the rateof the sorption and the percentage of the sorption (= Kd > 104
mL/g at pH 3) of strontium in modified vermiculite were muchhigher. The sorption of strontium was favored by an increasein temperature. ZrMV can be considered as a superior candidate over vermiculite as a backfill material, and it can ensure

the retention of radioactive strontium leached out from thewaste forms.
Acknowledgements. The authors (M. V. Sivaiah and S. S.
Kumar) are grateful to Atomic Energy Regulatory Board forproviding fellowship and thankful to Dr. P. R. Vasudeva Rao,
Associate Director, Fuel Chemistry Division, IGCAR for
providing the facilities and the suggestions while preparing themanuscript.
References(1) G. Bruno, A. Decarreau, D. Proust, and A. Lajudie, Applied
Clay Science 7, 169 (1992).
(2) E. J. Nowak, Scientific Basis for Nuclear Waste Management,
ed. C. J. Northrup, Plenum Press, Vol. 2, New York, 1980,
p403.
(3) S. Krishnaswamy, India’s Mineral Resources, Oxford &IBH Publishing Co. Pvt. Ltd. New Delhi, 1988, p450.
(4) H. Mott and N. Anderson, The Backfilling and sealing ofradioactive waste repositories, Report EUR 9115, 1984.
(5) R. M. Barrer, Zeolite and clay minerals as sorbents andmolecular sieves, Academic Press, London, 1978, p339.
(6) F. J. DelEry-Perez-Caballero and G. Poncelet, Micropor.
Mesopor. Mater. 37, 313 (2000).
(7) R. Toranzo, M. A. Vicente, M. A. Banares-Munoz, L. M.
Gandia, and A. Gil, Micropor. Mesopor. Mater. 24,173(1998).
(8) J. B. Yoon, S. H. Hwang, and J. J. Choy, Bull. Korean
Chem. Soc. 21, 989 (2000).
(9) T. J. Pinnavaia, Science 220, 365 (1983).
(10) T. J. Pinnvaia, Expanded Clays and Other Microporous
Solids, ed. M. L. Occelli and H. E. Robson, Van Nostrand
Reinhold, New York, 1992, p1.
(11) A. Gil, M. A. Vicente, and L. M. Gandia, Micropor.
Mesopor. Mater. 34, 115 (2000).
(12) A. Sanchez and M. Montes, Micropor. Mesopor. Mater.
21, 117 (1998)(13) N. D. Hutson, M. J. Hoekstra, and R. T. Yang, Micropor.
Mesopor. Mater. 28, 447 (1999).
(14) P. Sylvester and A. Clearfield, Sep. Sci. Technol. 33, 1605
(1998).
(15) P. Sylvester, A. Clearfield, and R. J. Diaz, Sep. Sci. Technol.
34, 2293 (1999).
(16) D. T. Karamanis, P. A. Assimakopoulos, and N. H. Gangas,
J. Radioanal. Nucl. Chem. 242, 189 (1999).
(17) C. A. Papachristodoulou, P. A. Assimakopoulos, N. H.
Gangas, and D. T. Karamanis, Micropor. Mesopor. Mater.
39, 367 (2000).
(18) K. A. Venkatesan, G. Pannerselvam, and P. R. Vasudeva
Rao, Sep. Sci. Technol. 35, 2343 (2000).
(19) K. A. Venkatesan, P. R. Vasudeva Rao, and K. Stamberg,
J. Radioanal. Nucl. Chem. 250, 477 (2001).
(20) L. Kullberg and A. Clearfield, J. Phys. Chem. 85, 1578
(1981).
(21) H. Kaneko, M. Tsuji, and Y. Tamaura, Solv. Extr. Ion
Exch. 11, 693 (1993).
(22) P. Sasidhar, Safety Asessment of Low Level Radioactive
Waste Disposal Facility at Kalpakkam, Ph. D Thesis, Anna
University, 1995.