Environmental Pollution 52 (1988) 67-79

Caesium Selectivity and Fixation by Vermiculite in the Presence of Various Competing Cations*

C. A. Sikalidisfl P. Misaelides b & C. A. Alexiades"

" Department of Chemical Engineering, h Department of Chemistry, Aristotle University of Thessaloniki, GR-54006 Thessaloniki. Greece

(Received 1 July 1987; revised version received 17 November 1987: accepted 24 November 1987)

A BS TRA C T

Caesium selectivity coefficients (kG) that were determined with respect to Na-, Mg-, Ca-, Sr- and Ba-ions, for Kent and Libby vermiculites, range(l [?om 4"8 to 11 ( litres/mol) 1/2, much higher than those with respect to K- and NH 4- ions (0"01 to 0"06), indicating the high affinity of vermiculite for the weakh' hydrated cations. Higher K o values were alsojbund[br low and extremely low Cs concentrations. Vermiculite samples were veo' effective in removing ~ 37Cs J?om solutions containing traces oJ "137 Cs (4 ng litre- ~ ) and extremely higher quantities of K-, NH4-, Na-, Mg- and Ca-ions (500 mg litre ~ ). 0"5N chloride solutions of H, N H 4, K, Na, Mg and Ca were unable to remove CsJ)'om Cv- saturated vermiculite samples which had undergone heating at I10 C. Significant quantities (50 -65%) of f ixed Cs were removed j?om these samples only after treatment with strong acids. The high preference ant(fixing abili O' of vermiculiteJbr Cs suggests the use of this mineral as a radioactit:e Cs decontaminating agent.

INTRODUCTION

Caesium isotopes belong to the most soluble, long-living and hazardous nuclear fission products. Therefore, their fixation and immobilisation in natural environments has been the subject of various studies in the last decades (Rogowski & Tamura, 1970; Gillham et al., 1980; Gulis & Timulak, 1986). The role of clay mineral structure in caesium sorption from

* Part of the PhD Thesis of C. A. Sikalidis.

67 Environ. Pollut. 0269-7491/88/$03.50 (~~ 1988 Elsevier Applied Science Publishers Ltd. England. Printed in Great Britain

68 CI A. Sikalidis, P. Misaelides, C. A. Alexiades

radioactive waste solutions was recognised as early as 1960 (Tamura & Jacobs, 1960). Montmorillonite selective properties for caesium were slightly improved after heating (Tamura & Jacobs, 1961). Even better results were obtained with K-saturated bentonites (Maes et al., 1985). Sawhney (1964) made extensive studies on the sorption and fixation of Cs by clay minerals in relation to the saturating cations, to soil clay types (Sawhney & Frink, 1964) and to the lattice spacing and cation exchange capacity of biotite (Sawhney, 1967). The same author (Sawhney, 1970) studied K- and Cs-selectivity in relation to clay mineral structure and found that vermiculite has a dominating Cs sorption selectivity, in comparison with other clay minerals, such as montmorillonite, nontronite, biotite and illite. Later, Komarneni (1979) and Komarneni & Roy (1978, 1979) studied the effect of total negative layer charge and heat treatment on Cs fixation by layer silicate minerals and shales..The results showed that vermiculites of high negative layer charge fixed Cs strongly and therefore could be used for the decontamination of low and intermediate level radioactive waste. Additionally, heat treatment was found to increase Cs-fixation.

Komarneni & Roy (1979) showed that shales containing vermiculite fixed Cs in amounts of about 2 to 10% of their charge in the presence of competing Ca- and Na- ions in 100-fold higher concentrations. Misaelides et al. (1987) studied the distribution of radionuclides after the Chernobyl accident in the fine soil dust particles and suggested that vermiculite present in these materials was responsible for the retention of the various radionuclides.

The objectives of the present work were to study:

(a) Caesium selectivity of vermiculite, a well known Cs-fixing mineral, with respect to monovalent (H, NH 4, K, Na) and divalent (Mg, Ca, Sr, Ba) ions. The selection of these cations was motivated on the basis of their usual and abundant presence in natural waters and soil environments.

(b) Vermiculite selective adsorption of caesium at low concentrations, in the presence of a much higher concentration of the competing ion.

(c) Vermiculite ability to remove selectively 137Cs from extremely dilute solutions, in the presence of competing ions in concentrations usually encountered in natural systems.

(d) The reversibility of Cs fixation by vermiculite, using chloride solutions of H, NH4, K, Na, Mg and Ca as replacing agents, as well as 6N HC1.

It is hoped that this work will contribute to a better understanding of the distribution and behaviour of radioactive caesium isotopes in the soil environments and mineral deposits.

Caesium selectivity and fixation by vermiculite 69

MATERIALS AND METHODS

The standard vermiculite samples used were from Kent, Connecticut, and Libby, Montana, USA, both presumably alteration products of biotite, and supplied by Ward's Natural Science Establishment, Rochester, NY, USA. Their mineralogical composition (Table 1) was assayed on the basis of the Alexiades and Jackson (1966) method and verified by X-ray diffractograms of powdered samples, obtained by a Philips PW 1730 diffractometer, using Ni-filtered Cu-K~ radiation.

The vermiculite samples were ground in a water suspension with a rotary high speed stirrer for several hours (Jackson, 1974). The material that passed through a 300-mesh sieve was collected, centrifuged, air dried, and used for the subsequent experiments. Monoionic forms of vermiculite were prepared by saturation with different cations, such as H, NH 4, K, Na, Mg, Ca, Sr and Ba, according to the following procedure: 4 g of vermiculite samples in a 50- ml centrifuge tube were washed three times with 0"5y chloride solutions of the mentioned ions using the well known procedure of addition, centrifugation and decantation (Alexiades & Jackson, 1966). The samples were then washed free of excess salts three times with distilled water and two additional times with acetone to prevent hydrolysis of the exchangeable cations and to promote flocculation of the suspension. Cation exchange capacity (CEC) measurement of the two vermiculite samples was performed according to the procedures outlined by Alexiades & Jackson (1966).

The Gapon selectivity coefficients k6 (Fanning & Keramidas, 1977) were calculated using the formulae:

kc~, Cs/A + - CSads X ~A +, for monovalent ion exchange A + ads X ~Cs~o I

k~,Cs/B + + = CSad* X X~BL,+ for mono-divalent ion exchange Ba'd + X ~(Cs ....

where:

The subscripts 'sol' and 'ads! refer to ions in solution and solid phase, respectively, and ~ denotes activities. Ions on the solid phase were expressed in meqper 100g and those in solution in concentration units, mollitre-~, following Gapon's convention.

The experimental data used for the calculation of selectivities were obtained by the following procedure: 0"5 g of the monoionic form of each one of H-, NH4-, K-, Na-, Mg-, Ca-, Sr- and Ba-saturated vermiculites were

70 C. A. Sikalidis, P. Misaelides, C. A. Alex&des

TABLE 1 Minerals Present in the Standard Vermiculite Samples Used

Sample origin Quart- Mica Montmorillonite Chlorite Vermiculite (%) (%) (%) (%) (%)

Kent, Connecticut 3 17 5 20 54 Libby, Mon tana - - 52 13 - - 34

equilibrated (25°C, 60 min) with 50 ml of 0-01N CsC1 solution. Preliminary experiments had shown that equilibrium was rapidly established (within the first 15 min). The final Cs concentration in the equilibrium solution, which was obtained by 5min centrifugation at 2500rpm, was determined by Atomic Absorption Spectroscopy (AAS) using a Perkin Elmer Model 503 spectrophotometer. The amount of Cs adsorbed was calculated from the :lifference between the initial and final Cs concentrations. This calculation was verified by determining the actual amount of Cs on the solid phase by AAS, after dissolution with a HF-HC104-H2SO 4 solution. Concentration of the competing ions in the equilibrium solution was determined by AAS, and in the case of H- and NH4-ions by titration and NH4-selective electrode, respectively. The amount of the competing ion on the solid phase was calculated from the difference between the CEC of the sample and the amount of the ion in the equilibrium solution.

The caesium selective sorption by vermiculite with respect to each of the NH4-, K-, Na-, Mg- and Ca-ions was also ascertained using a low concentration of Cs and much higher concentrations of the competing ion. These experiments were conducted by equilibrating, at 25°C with 15 min continuous shaking, 0.2g samples of vermiculite with a 25ml chloride solution containing the competing ion (in concentrations ranging from 500 to 40000mgli t re -1) and l mglitre -1 of Cs. After equilibration the suspensions were centrifuged and Cs determined in the clear supernatant solution by AAS.

The radiocaesium (13VCs) selective sorption by vermiculite from extremely dilute solutions in the presence o f H-, NH4-, K-, Na-, Mg- and Ca- ions (in concentrations normally encountered in natural systems) was also studied. This involved the equilibration, at 25°C with 15min continuous shaking, of 0.2 g of vermiculite samples with a 25 ml of chloride solution containing the respective competing ion (500 mg litre - 1) and 10 nCi of 137Cs (the latter was supplied by Nucleus in the form of CsC1 solution in 0"5N HC1). After equilibration in the suspensions were centrifuged for separation of the two phases and for measurement of radiocaesium in the solution by means of a Ge(i) 7-ray detector (relative efficiency 18%) connected with a 4096-

Caesium selectivity and fixation by l~ermiculite 71

channel analyser (Canberra S-35 Plus). The amount of ~37Cs adsorbed was calculated from the difference between the initial and final radioactivity.

Desorption experiments were also performed according to the Alexiades & Jackson (1965) procedure, proposed for the potassium fixation studies. Cs-saturated vermiculites were heated overnight at l l0°C, in order to achieve interlayer dehydration and collapsing, and the consequent fixation of Cs. The unfixed Cs was then exchanged according to the following procedure: The heated Cs-vermiculite samples were treated three times with 0-5y chloride solutions of H, NH4, K, Na, Mg, Ca, and subsequently once with a 6N HC1 solution. Caesium released was determined by AAS in the clear supernatant solution, which was obtained by centrifugation. The difference between total Cs adsorbed by vermiculite and that released (in meq per unit cell) gives the interlayer charge of vermiculite. This value could be used as a criterion for quantitative vermiculite determination in soil clays (Sikalidis & Alexiades, In Press).

RESULTS AND DISCUSSION

Minerals present in vermiculites used

The minerals present in the two coarse grained (< 300 mesh) vermiculites used are given in Table 1. The CEC of Kent and Libby vermiculite samples was found to be 93"9 and 70.6 meq per 100 g, respectively. This low CEC of the samples is attributed to the low percentage of pure vermiculite present. The vermiculite content of Kent and Libby samples was 54% and 34%, respectively (Table 1). The other significant difference between these two minerals is their montmorillonite content. Montmorillonite cannot fix K or Cs, but possesses a high CEC of 110 meq per 100 g (Alexiades & Jackson. 1965). The rest of the minerals present in the two vermiculite samples, namely, mica, quartz and chlorite, possess a very low CEC ( < 5 meq per 100 g- 1). These minerals do not interfere with the vermiculite's ability to adsorb selectively and fix K and other poorly hydrated cations. such as NH4, Rb of Cs.

Cs selectivity coefficient values

Both vermiculites showed an almost identical Cs selectivity with respect to each competing ion studied. The Cs selectivity coefficients (kc) with respect to highly hydrated divalent cations ranged between 4.8 for the Cs-Mg pair and 11 (litres/mol) 1/2 for Cs-Ba (Table 2). Values for the rest of the cations lay between these two extremes. This demonstrates the selective adsorption

72 C. A. Sikalidis, P. Misaelides, C. A. Alexiades

TABLE 2 Cs Selectivity Coefficient Values (kG) in (litre/mol) 1/2 with Respect to Mono- and

Divalent Cations, for the Two Vermiculites Studied

Kent Libby Kent Libby

k~, Cs/H 0-20 0'45 k G, Cs/Mg 4"86 6"23 k G, Cs/NH4 0-01 0'06 k G, Cs/Ca 7.00 7'57 kG, Cs/K 0-01 0.01 k G, Cs/Sr 8-20 8.80 k G, Cs/Na 3.95 3"60 k G, Cs/Ba 8-40 11.00

of Cs by vermiculite in the presence of these cations. On the other hand, Cs selectivity coefficients with respect to the well known weakly hydrated cations, such as K and NH4, were found to be extremely low, i.e. 0"01 for Cs-NH4 and Cs-K in the case of both Kent and Libby vermiculites (Table 2), with an exception in the case of the Cs-NH4-value for the Libby sample which was 0.06. These results show that K and NH 4 ions compete with Cs for the same exchange sites or, in other words, Cs, K and NH4 ions behave more or less similarly, in the sense that vermiculite saturation with one of these three cations prevents the other two from entering the interlayers.

Potassium or ammonium saturation of vermiculite prior to Cs exchange causes interlayer collapse, and the apparently 'anomalously low' selectivity coefficients for Cs-K and Cs-NH 4 pairs simply indicate the inability of Cs ions to replace to a great extent K and NH4 in vermiculites already saturated with them. These results do not contradict the well known decreasing order of selectivity for monovalent cations: Cs > Rb > K = NH4 > Na > Li for the 2:1 type clay minerals. When the vermiculites studied were Cs-saturated, K and NH 4 ions were able to replace only less than 1% of the adsorbed Cs ions (Sikalidis, 1988), showing the high selectivity of Cs ions, whereas in the above-mentioned experiments Cs ions were able to replace about 15 % of the adsorbed K or NH 4 ions. Sodium, a highly hydrated ion, cannot compete with Cs and therefore the determined k G, Cs-Na values were in the intermediate region (3"9 for the Kent and 3"6 for the Libby vermiculite). These results reflect the high selective adsorption of Cs by vermiculites in the presence of hydrated cations, such as Na, Mg, Ca, Sr and Ba. This is in good agreement with earlier investigations, which showed that K-, NH4, Rb- or Cs-fixation by vermiculite is related to their low hydration, and the close fit of these cations within the hexagonal cavities of the tetrahedral sheets of vermiculite silicate layers (Douglas, 1977).

The abnormal low coefficient value k G of the Cs-H pair (0-20 for the Kent and 0-45 for the Libby vermiculite) could be explained by accepting a substitution of hydrogen for aluminium cations in the octahedral sheets. The released aluminium ions after hydration and/or hydroxylation can

Caesium selectivity and fixation by vermiculite 73

remain adsorbed on the exchange sites (Alexiades, 1986). As a consequence of these reactions, vermiculite loses part of its CEC and the resulting species behaves as chloritised vermiculite (Alexiades & Jackson, 1966).

Adsorption of Cs at low concentration

The determined Cs-selectivity coefficients with respect to non-hydrated and hydrated cations in the previous experiment made obvious the fact that the exchange properties of vermiculite depend primarily on vermiculite's preferential adsorption of certain ions relative to others.

This means that the structural and chemical properties of vermiculite favour the adsorption of certain weakly hydrated ions relative to other strongly hydrated ones. Vermiculite selectivity for Cs and other weakly hydrated cations originates from planar (external), wedge and interlayer sites, whose selectivity decreases in the order: interlayer >> wedge > planar. This order of selectivity cannot be clearly delineated in all cases, and depends on many factors, such as type of the clay mineral, layer charge concentration of the exchanging ion, and ionic strength of the bathing solution. Sawhney (1970) reported that the wedge sites are more selective for K in the presence of Ca than the interlayer sites. The latter, however, are more effective in fixing K at high concentrations. Since the previous experiments were conducted using Cs concentrations almost equinormal to the competing ion and to the CEC of the adsorbing material, it was considered worth while to determine vermiculite Cs selectivity under two different conditions. The first involved low Cs concentration in the presence of much higher concentrations of the competing ion, and the results obtained are discussed in the next paragraph, and the second, extremely low concentrations of ~37Cs. The second experiment was initiated because of the contamination of the environment with traces of radioactive Cs after the Chernobyl nuclear accident. The results of this experiment with 137Cs are discussed in the next section.

In the first experiments (where only the Kent vermiculite was used) the concentration ratio of Cs to the other cations varied between 1:500 and 1:40 000. The experimental results showed that 31% and 67% of the Cs ions in solution were adsorbed by Kent vermiculite in the presence of 500-fold higher concentrations of calcium and ammonium ions, respectively (Table 3, first column). The percentage of Cs adsorbed in the presence of the other cations studied (K, Na or Mg) varied between these two values. Cs adsorption decreased as the concentration of the competing species increased. Thus, in the case of a concentration ratio 1 : 5000, two groups of values can be distinguished (Table 3). The first group, representing about 20% of Cs adsorbed, was observed in the presence of low hydration energy cations, such as K and NH4, in solution. The second group, of about 30% Cs

74 C. A. Sikalidis, P. Misaelides, C. A. Alexiades

TABLE 3 Adsorption of Cs, in Per Cent, by 0.2g of Kent Vermiculite in the Presence of much Higher Concen-

trations of the Competing Ion

Compethlg Cs adsorbed (% of the amount added) ~ ion

Concentration ratio Cs:competing ion

1:500 1:5 000 1:20 000 1:40 000

NH 4 67 20 15 K 43 21 15 7 Na 57 26 l l 8 Mg 33 30 15 12 Ca 37 31 10 7

a 25 ml of an 1 mglitre 1 Cs solution.

adsorbed, was observed in the presence of high hydration energy cations, such as Na, Mg or Ca. The preferential adsorption of Cs by vermiculite, relative to the above-mentioned cations, can be explained on the basis of the well known fact that cations with low hydration energies, such as K, NH4, Rb or Cs, cause interlayer dehydration and are therefore selectively fixed in interlayer positions of vermiculite (Fanning & Keramidas, 1977). These ions can lose their shell of hydration more easily than the other cations and can enter the wedge zones of vermiculite preferentially. Once within the wedge zones, these ions can enter and occupy the hexagonal holes in the base of the tetrahedral silicate sheets of vermiculite and thus become fixed. When this happens, the wedges collapse and the vermiculite structure is essentially transformed to mica-like structure again. Conversely, cations with high hydration energy, such as Na, Mg, Ca, Sr and Ba, produced hydrated, expanded interlayers and are readily available to enter into exchange reactions. In the cases of extremely high concentration of the competing ions (1:20000 and 1:40000 ratio), Cs adsorption was reduced to 10-15% of the total Cs present in solution (Table 3). These findings indicate that cations in extremely high concentrations effectively compete with Cs for vermiculite adsorption sites, when Cs is in concentration of 1 mg litre- 1.

Adsorption of 137Cs

In these experiments, 137Cs concentration was kept extremely low (4 ng litre- ~) in order to simulate realistic concentrations of Cs-isotopes in the environment in the case of an eventual nuclear accident or releases from nuclear installations.

Caesium selectivity and fixation by vermiculite 75

It is noted here that this ~3VCs level was of the same order of magnitude as that observed in northern Greece after the Chernobyl accident (Misaelides et al., 1987). The competing ion concentration (mono- and divalent) was 500 mg litre- 1, resembling the concentration of these ions usually found in natural systems. In this way, the concentration ratio of 137Cs to the competing ions was near to 1:10 9 . The experimental results showed that, even at this extremely low Cs concentration, vermiculite exhibits a high Cs selectivity, removing it almost quantitatively from solutions containing the above mentioned ions (Table 4). This Table also shows that the competitive action of all cations tested was similar, with the exception of H-ions. The complications arising from the use of H-ions have already been discussed. This interesting observation, i.e. that traces of 137Cs are being selectively retained by vermiculite, stresses the fact that either wedges or interlayer sites of vermiculite are highly selective for this particular ion, even when a competing ion is present in much higher concentration. In addition, these findings suggest that vermiculite can be used for radiocaesium decontamin- ation purposes, either by preparing adsorbing columns or moving beds with this material through which liquid foodstuffs contaminated with 137Cs can be passed, or mixed with these (when practically feasible) and purified (Sikalidis et al., in press).

Cs selectivity coefficient values for low and extremely low Cs concentrations

It is well established that selectivity of an ion increases as the degree of saturation of the exchanger with this ion decreases (Talibudeen, 1981: Goulding, 1983). This means that the ability of the vermiculites tested to

TABLE 4 Adsorption of 137Cs, in Per Cent, by 0.2g of Kent and Libby Vermiculites in the Presence of Extremely High

Concentrat ions of the Competing Ion

Competing 137Cs adsorbed (% o[ the amount added)" ion

Kent Libhr

H 35 56 NH 4 88 98 K 96 97 Na 98 96 Mg 95 98 Ca 96 98

" 25 ml of a 4 ng 137Cs l i tre- 1 solution. Concentration ratio of 137Cs to competing ion =0"8 x 10 8.

76 c . A. Sikalidis, P. Misaelides, C. A. Alexiades

withdraw almost quantitatively even traces of Cs from solution would be reflected in high kc values under these conditions.

The calculated k C values at low and extremely low Cs concentrations (Table 5) were indeed higher than those determined in the commonly used manner (Table 2), and were also increasing as Cs saturation of the exchanger decreased in accordance with the theory. It must be remarked, however, that in this case (low and extremely low Cs concentrat ions and high concentration of the competing ion), determination of selectivity coefficients is, to a certain degree, ambiguous because of the large experimental error involved.

Cs-f ixat ion

The two vermiculite samples were Cs-saturated and, after the removal of the excess salts, they were heated at l l0°C overnight. This treatment causes dehydration and collapse of vermiculite layers and the consequent fixation of Cs. The amount of Cs which remains exchangeable was desorbed by three washings with 0.5 N chloride solutions of H, NH4, K, Na, Mg and Ca (Table 6). The amount of Cs desorbed, after the application of these washings, was found to be equivalent to the sum of the external charge of vermiculite and total charge of montmorillonite present (5-10 and 13-19 meq 100 g-~ for Kent and Libby samples, respectively). The high amounts of desorbable Cs in the Libby vermiculite reflect the presence of montmorillonite, a non-Cs- or K-fixing mineral (Alexiades & Jackson, 1965). The amount of Cs fixed was calculated from the difference between the CEC of the sample and the amount of Cs desorbed, and expressed as a percentage of the CEC of the corresponding vermiculite (Table 6). From these figures, it is clearly seen

TABLE 5 Cs Selectivity Coefficient Values (kc) in (litre/mol) 1/2 for Cs in Low and Extremely Low Concentrations with Respect to Mono- and Divalent Cations for the Kent

Vermiculite

Cs to cation ratio

1/500 1/5 000 1/1 x 10 s

kc, Cs/NH 4 5"9 9"0 20"0 kG, Cs/K 0-6 4'5 17'9 k~, Cs/Na 4-7 10-2 92.9 kG, Cs/Mg 8-7 26"8 323"7 kG, Cs/Ca 7.5 22.4 301.4

Caesium selectivity and fixation by vermiculite 77

TABLE 6 Desorption of Cs from the Heated at 110°C Cs-Saturated Vermiculites after Three Washings with Chloride Solutions of H, NH 4, K, Na, Mg and Ca, and after one subsequent Treatment

with 6N HC1

Washin~ Cs desorbed qJ'ter Total Cs j ixed Cs desorbed aJter one solution three washings with subsequent treatment

0"5,~ the solution used 6x HCl (meq per 100g) (% ~1" C EC) (meq per 100g)

Ken t Libby Ken t Libb)' Ken t Lihby

HC1 5"4 13'4 94"2 8 l'0 45"0 18"6 NH4C1 6"1 17"5 93"5 75'2 39"0 24"0 KCI 10"3 17"2 89"0 75"6 34"7 23"0 NaCI 7"4 18"0 92.1 74"5 38"2 25-0 MgCI 2 5"6 18'8 94'0 73'3 43-6 27.0 CaC12 4"7 19"0 94"6 73" 1 42" 1 26"5

that, in the case of the Kent vermiculite, all of the interlayer adsorbed Cs was fixed upon heating, due to the collapse of vermiculite high charge layers. Libby vermiculite showed an almost identical behaviour. It is noted that the higher amount ofCs desorbed from this mineral (Table 6, column 2) and the lower percentages of Cs fixed (column 4) are attributed to the considerable amount of montmorillonite present.

The results of this experiment showed that all the used salt solutions were unable to desorb Cs fixed in vermiculite. To ascertain Cs-fixation strength by vermiculite, an additional treatment with 6N HC1 was applied to the samples which were previously washed with the above-mentioned salt solutions. This treatment is considered as the most intensive that could be applied without destroying completely the vermiculite structure. After this intensive treatment, 35% to 50% of the total amount of fixed caesium still remained in the interlayer positions (Table 6). Thus the obtained results illustrate the strength of Cs fixation by vermiculite.

CONCLUSIONS

Studies on preferential adsorption of Cs by two selected vermiculites showed that Cs-selectivity coefficients with respect to strongly hydrated cations, namely, Na, Mg, Ca, Sr and Ba, were much higher than those with respect to K and NH 4 which are weakly hydrated and effectively compete with Cs for vermiculite adsorption sites. These selectivity coefficients were also found to increase as the degree of Cs saturation of vermiculite decreased. The high

78 c. A. Sikalidis, P. Misaelides, C. A. Alexiades

affinity of vermiculite for Cs and other monovalent cations is accounted for by the closer fit of these less hydrated ions into the interlayer hexagonal cavities of vermiculite tetrahedral rings.

The two vermiculite samples, when Cs saturated and heated at 110°C, held Cs tenaciously against depletion by 0.5N chloride solutions of H, NH 4, K, Na, Mg and Ca. Significant quantities of Cs were removed from the vermiculite samples only by treatment with a strong acid (6NHC1), indicating that Cs-desorption is accomplished after disruption of the lattice structure.

The vermiculite samples also had high affinity for 137Cs, being able to remove traces of 137Cs in solution in the presence of H, NH4, K, Na, Mg and Ca in concentrations usually found in natural systems. These findings, along with the Cs-fixing capacity of vermiculite, can contribute to a better understanding of the migrational behaviour of radioactive Cs isotopes in natural environments. In addition, they hint at the possibility of using vermiculite for Cs decontamination of liquid foodstuffs.

A C K N O W L E D G E M E N T S

We are grateful to Dr V. Z. Keramidas, Assistant Professor of Soil Science, for his helpful sugge'stions and comments.

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