Introduction

Energy is a basic assumption of civilization development. However, side products of nuclear energy production include production of nuclear waste and spent nuclear fuel. Currently Slovak Republic avails of two locations (Bohunice and Mochovce) with nuclear power plant (NPP) which make up to more than a half of primary sources of energy in the Slovakia (Fig. 1). These are the most significant sources of spent nuclear fuel and high-level radioactive waste.

Fig. 1
figure 1

Ratio of the primary energy sources in Slovakia for 2008: Nucleus 57.46%, Coal 18.31%, Gas 11.37%, Hydro 12.68%, Wind 0.19%

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A significant problem of the nuclear energy production industry in Slovakia is finding the final solution to the problem of the back side of nuclear cycle—problem of definite deposition of spent nuclear fuel and high-level radioactive waste.

“The suggestion of the final part of nuclear energy strategy” (Slovak government act No. 238/2008)—a programme of deep geological repository development in Slovakia defines the development of a deep geological repository as one of the three considered alternatives. It directly supports geological research and examination of potential localities possibly suited for placement of a deep geological repository [1].

A safe nuclear fuel cycle depends also on the waste storage and deep geological repository that are formed by natural and engineered barriers isolating long-lived radionuclides from the biosphere are recommended. Clay is the basic component of these barriers in many cases. Bentonite is generally considered as the most appropriate clay material [2–6].

Out of the group of Cs radio-isotopes the most significant one is 137Cs (T ~ 30.1 years, Eβ = 0.51 (92%), 1.18 (8%) MeV, Eγ(137mBa) = 0.6616 MeV, Fig. 2). Fission nuclear reactions, nuclear explosions and nuclear energetic are the main sources of its existence. When dispersed in the environment 137Cs migrates, sorbs onto natural matrices and also integrates in the food chain. Cesium enters human body most importantly by means of food (82–98%), drinking water (1–6%) and fallout inhalation (1–2%) and it consequently gets absorbed into the digestive tract. This element gets distributed by blood throughout the entire body and in case of continuous intake it accumulates in soft and muscle tissues. Presence of such radiotoxic metals in living organisms can lead to somatic as well as genetic changes and for this reason adsorption of that radionuclide is very carefully monitored [7–10].

Fig. 2
figure 2

Mode of decay 137Cs

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Bentonites present a significant group of natural materials mostly compounded from microcrystalline particles of minerals from the group dioctahedral smectite–montmorillonite (Fig. 3) [11–17]. Crystallochemical formula of montmorillonite:

$$ {\text{M}}_{\text{x}} \left[ {{\text{Al}}_{{ 2- {\text{x}}}} \left( {{\text{Fe}}^{ 2+ } {\text{Mg}}} \right)} \right]_{\text{x}} {\text{Si}}_{ 4} {\text{O}}_{ 10} \left( {\text{OH}} \right)_{ 2}\;\times\;{\text{nH}}_{ 2} {\text{O}}, $$

where X, which represents the layer charge, can change from 0.2 till 0.6. The formula expresses the characteristic feature of montmorillonite which is the non-existence of tetraedric charge. The entire charge is concentrated on the octaedric net. According to the presence of different octaedric cations we divide montmorillonites into:

Fig. 3
figure 3

Structure of smectite

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  • Al-montmorillonites,

  • Mg-montmorillonites, and

  • Fe-montmorillonites.

Significant characteristics of bentonites such as low permeability, high swelling ability absorption of ions predetermine their use as sealing barriers in multi-barrier system when underground geological repository for spent nuclear fuel and high-level radioactive wastes are constructed [18–22]. An extensive study of the sorption behaviour of the bentonite barriers is an inevitable part of deep geological repository concept [23–29]. Attention is being paid to the influence of various factors on sorption processes which take place in the area of deposited spent nuclear fuel and nuclear waste. An example of such factors can be influence of radiation and residual temperature, change in pH of environment, influence of the competitive ions and organic ligands, etc. [30–37].

There are several significant bentonite deposits in the Slovak republic. The best known and in long-term mined bentonite deposit containing Al–Mg montmorillonite is in the area of Slovak upland (“Slovenské stredohorie”) in the location of Stará Kremnička: Jelšový potok. This location features one more partly mined deposit of andesite bentonite consisting of Fe-montmorillonite: Lieskovec. In the area of East-Slovak neo-vulcanites we find two currently mined deposits of liparite bentonites: Kuzmice and Lastovce (Ag–Mg montmorillonite). Apart from these there are currently not mined deposits Nižný Hrabovec, Fintice, Nižný Žipov and Veľaty. An interesting accumulation of basaltic bentonites was discovered in basaltic maar in the vicinity of Jelšovec in Upland of Fiľakovo. The deposit of mixed-layer illite–smectite of Dolná Ves is situated on the south-western rim of the stratovolcano of Kremnica.

Experimental

Solid phase

The domestic bentonite samples were taken from five Slovak deposits and technologically processed by Envigeo a.s. (Table 1). From the deposit Dolná Ves was tested the sample NDV45, modified by sedimentation technique. Natural samples of bentonite were at in two different grain sizes, samples ground under 45 μm and under 250 μm (what means, that these are fractions of bentonite with size of particles smaller than 45 and 250 μm).

Table 1 Used samples of domestic bentonites
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Then samples of bentonites were compared from locality Jelšový potok and Lieskovec with their non-irradiated and irradiated forms. Samples were irradiated in natural state in Slovak Institute of Metrology, using 60Co source with energy of 1.17 and 1.33 MeV during cca 50 days period, with medium dose rate of 0.092 Gy.s−1. Total absorbed dose was 390 kGy. Samples of bentonite were dried in Petri dish in drying oven at the temperature 105 °C approximately 2–3 h. Then they were taken into small plastic containers and lay aside into a desiccator, where they were stored till next use.

Chemical and mineralogical analyses

Chemical composition of bentonites from deposits of Jelšový potok, Kopernica and Lieskovec is shown in Table 2.

Table 2 Chemical analysis of bentonites from Jelšový potok, Kopernica and Lieskovec
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Lieskovec, Kopernica and Jelšový potok have similar qualitative mineral composition (Table 3). Kopernica and Jelšový potok are similar quantitative as well. Lieskovec has significantly lower content of smectite (about 20% less), its representation is substitute mainly by kaolinite and cristobalite. Lastovce are significantly different from the other smectite deposits mainly by very low content of smectite, high content of cristobalite and not trivial presence of calcite. The sample NDV, K-bentonite from Dolná Ves represents in comparison with other used bentonites mineralogically different material. Its main clay phase is mixed-layering illite–smectite not present in other bentonites.

Table 3 Mineralogical composition
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Aqueous phase and radiotracer

Adsorption experiments were realized in the Cs concentration range 1 × 10−5–5 × 10−2 mol dm−3 solutions prepared from cesium chloride (Table 4). Effect of Na+ and Ca2+ ions on the adsorption of cesium was studied with its concentration 1 × 10−4 mol dm−3, while concentration of competitive ions was in the range 5 × 10−2–1 × 10−5 mol dm−3.

Table 4 Cesium ion concentration used in adsorption studies
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In experiments, chemicals of ordinary laboratory purity from supplies companies Slavus, s.r.o., Bratislava SR, or Lachema n.p., Brno CZ were used. Radioisotope 137Cs with the volume activity 1 MBq mL−1 was used as a radiotracer.

Batch method and measuring of radioactivity

Adsorption of Cs on samples of bentonites was studied through radioisotope indication using radioisotope of cesium-137 in static arrangement of experiment, in aerobic conditions at laboratory temperature. Adsorption parameters were determined after mixing in 0.05 g of adsorbent with 5 mL of water phase in plastic test tubes with internal diameter 14 mm and capacity 10 mL in laboratory extractor with constant speed of mixing both phases. After realization of adsorption and subsequent centrifugation (t = 15 min, 3,500 rpm) 1 mL was extracted from supernatant to measure radioactivity.

Radioactivity determination of 137Cs solutions was done with spectrometer Modumatic using NaI(Tl) detector.

Results and discussion

Adsorption properties of bentonite were calculated by following equations:

$$\text{Distribution\;coefficient} \,K_{\text{d}} = {\frac{{a_{0} - a}}{a}} \times {\frac{V}{m}}\,\left( {{\text{ml}}\,{\text{g}}^{ - 1} } \right) $$
(1)
$$ \text{Adsorption\;percentage}\,R = {\frac{{100 \times K_{d} }}{{K_{d} + {\frac{V}{m}}}}}\,\left( \% \right) $$
(2)
$$ \text{Adsorption\;capacity} \,\Upgamma = K_{\text{d}} \times c_{\text{eq}} \left( {{\text{mmol g}}^{ - 1} } \right) $$
(3)

where co—initial concentration (mol dm−3), ceq—equilibrium concentration (mol dm−3), V—aqueous phase volume (mL), m—adsorbent mass (g), ao—volume activity of initial solution (mL s−1), a—equilibrium volume activity of solution (mL s−1).

Kinetics

While testing kinetics adsorption, as a liquid phase, a solution of cesium with concentration cesium cations cCs+ = 1 × 10−3 mol dm−3 was used. As a solid phase a sample of bentonite from Jelšový potok locality J250 was used. The adsorption process was fast, maximum R was nearly reached within 1 min from the beginning of contact of solid and liquid phase. The comparable values of adsorption percentage were reached in a time interval of 1–480 min (Table 5).

Table 5 Effect of shaking time on the adsorption of Cs on bentonite J250
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A period of 2 h was chosen for the adsorption experiments.

Almost “instantaneous” capture of the cesium ions on the bentonite can be explained by adsorption and ions exchange on the surface for the cesium ions.

Adsorption experiments and cation exchange capacity

Cation exchange capacity (CEC) is defined by the ability of bentonites to adsorb the cations and keep them in the exchangeable positions—in smectites in the interlayer. During the characterisation of technological types of bentonites three methods of cation exchange determination were used. These are the Ba-method, Mg-method and Cu-triethylenetetramine method. Table 6 summarizes the CEC values [36].

Table 6 The values of cation exchange capacity
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The qualitative analysis of particular exchangeable cations (valid for all natural bentonite samples except the Dolná Ves sample) confirmed the dominion of Ca and Mg over Na and K elements (Table 6). The distribution of original cations indicates a match between Kopernica, Lieskovec and Jelšový potok bentonite samples, the only difference being that Lieskovec samples show a slightly higher content of Na. Dolná Ves samples are a separate group where the content of exchangeable Na and K cations is in majority (Fig. 4).

Fig. 4
figure 4

The average values of CEC

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The CEC values for particular deposits drop in the following order:

Jelšový potok > Kopernica > Lieskovec > Lastovce > Dolná Ves.

NDV45 bentonites show the lowest CEC values, which were four times lower than in Jelšový potok bentonites (Fig. 4).

While a comparison of cation exchange capacity values for individual bentonite samples of Jelšový potok, Kopernica, Lieskovec, Lastovce and Dolná Ves (Fig. 4) leads to assumption that the samples of bentonite from Jelšový potok would have the highest values of adsorption capacity (Γ), the highest values of adsorbed amount of Cs were gathered in adsorption experiments with samples of the bentonite of Kopernica.

As the basic adsorption mechanism is used cation exchange. Diversity of adsorption values between individual samples could be due to different cation exchange capacity, different mineralogical structure and difference in surface area of individual samples.

The highest adsorbed amounts were reached with the bentonite sample from locality Jelšový potok J45: adsorption percentage is 97.2% (Fig. 5). The lowest values of adsorbed amount were found for the bentonite sample Dolná Ves NDV45 (adsorption percentage is 83.7%). Poor adsorption values of bentonite NDV45 could be attributed to the mineralogical structure. The sample NDV45, K-bentonite from Dolná Ves ground under 45 μm represents in comparison with other used bentonites mineralogical different material. Its main clay phase is mixed-layering illite–smectite not present in other bentonites.

Fig. 5
figure 5

Dependency of adsorption percentage on initial concentration

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The values of adsorbed of Cs+ descend in line:

$$ J45 > L45 > K45 > LA45 > NDV45. $$

The highest values of adsorption capacity Γ were reached in the lowest concentration of Cs in solution. The highest value of distribution coefficient (Kd = 3,426 mL g−1) in comparison with other bentonites was reached at adsorption of univalent cations Cs+, bentonite ground under 45 μm from Jelšový potok (Table 7). The highest value of adsorption percentage (R = 97.2%) in comparison with other bentonites was also observed in the bentonite sample J45.

Table 7 Adsorption parameters of Cs on bentonite
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Effect of ionizing radiation

Adsorption of Cs on irradiated forms (J250-irr, L250-irr) was modelled using Langmuir isotherm (Fig. 6). Values of calculated adsorption capacity correspond with values of maximal adsorbed amount. The exposed samples of bentonites J250 and L250 showed higher values of adsorbed quantities than the forms which had not been exposed to radiation, where the J250 sample showed better adsorption qualities for Cs ions.

Fig. 6
figure 6

Langmuir isotherm, Jelšový potok (J250, J205-irr.) and Lieskovec (L250, L250-irr.)

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The studies of the radiation stability of bentonites are very important, due to their planned use as a sealing barrier in the vicinity of radioactive waste and spent nuclear fuel. The bentonite samples have been exposed to radiation in their natural form using a 60Co source. The bentonite exposure as a result of interaction of γ-rays led to expansion of the specific surface, increasing of the adsorption capacity, and to the change in the solubility of the clay materials.

Effect of particle shape

The influence of illites particle shape on adsorption of cesium was studied on Dolná Ves bentonite samples (Table 8) [38].

Table 8 Characteristic of sample Dolná Ves
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Different particle shape illite samples were compared, e.g., Le Puy (shape: “barrels”, France), Muloorina (shape: “barrels”, Australia), L-2A-2 (shape: “hairy”, Poland), RM30 (shape: “plates”) and SG4 (shape: “plates”; Fig. 7) as was shown in paper [37].

Fig. 7
figure 7

Three different shapes of illite particles used in the study: A barrel-like illite, B platy illite, and C “hairy” illite

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The Kd of illites depends strongly on their edge surface area which may be related to frayed edges. The Kd is unrelated to the total surface area of illites, which includes both basal surface area of illite fundamental particles and edge surface area. The highest Kd was found for illites with “barrel” shapes. Adsorption capacity of illites is related closely to their total surface area which is, in the most cases, equivalent to maximum expandability, which is a function of the mean thickness. The poor adsorption properties of “hairy” illite may be related to elongation of crystals along one axis, which may reduce the number of Cs specific sites.

Effect of adsorbent concentration

A change in ratio of liquid and solid phase on the distribution coefficients in bentonite sample J250 was studied (Fig. 8). Bentonite in the range of 50–500 mg at constant liquid phase volume 5 mL of CsCl with c = 1 × 10−4 mol dm−3 in experiments was used. A significant change of distribution coefficient in the respective phase was not observed.

Fig. 8
figure 8

Effect of adsorbate concentrations

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Effect of cations

The Fig. 9 presents the dependency of adsorption percentage from initial concentration of univalent and bivalent cations in solution c0 on the bentonite from Jelšový potok, Kopernica and Lieskovec localities.

Fig. 9
figure 9

Dependency of R from initial concentration of univalent and bivalent cations in solution (c0) on the J45, K45 and L45

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The influence of competitive cations on adsorption of Cs was observed on bentonites samples J45, K45 and L45. The higher values of adsorption percentage where achieved in the presence of univalent competitive cations then in the presence of bivalent competitive cations. The adsorption percentage values of cesium increase when initial concentration of competitive solution cations decreases. The highest values of R were achieved at the lowest concentration of ions 1 × 10−5 mol dm−3.

The Ca2+ bivalent cation has the most significant influence on Cs adsorption while the univalent Na+ cation has the least. The results indicate that adsorption of Cs can be heavily influenced by the presence of high cation concentrations of various salts, which can exist in, e.g., waste waters. The influence of cations on adsorption of Cs is possible to explain by competitive ion exchangeable reactions and by occupying of active adsorption centres of bentonites. Presence of other cations causes declining of adsorption percentage of cesium the higher concentration of competing ion.

Conclusion

Following conclusion can be drawn for the five bentonite samples of the five domestic deposits in Slovakia which are considered for use as barriers surrounding the banisters of spent nuclear fuel:

  1. (1)

    The bentonites of Jelšový potok, Kopernica, Lieskovec and Lastovce are characteristic for their good Cs-ion sorption properties. The least suitable mineral composition concerning Cs adsorption on bentonite is that from the repository Lastovce. Potassium containing minerals are present in all samples in the same level. Calcite is present only in Lastovce bentonite. The highest adsorption percentage values and highest adsorption capacity values for cesium ions were reached for the Jelšový potok and Kopernica bentonites. Bentonites from these two deposits had the highest CEC values.

  2. (2)

    The Dolná Ves bentonites have unsuitable sorption properties which are caused by their mineralogical content as well as CEC.

  3. (3)

    From the reason of sorption properties it is pointless to use bentonite fractions smaller than 250 μm.

  4. (4)

    The change in ration of solid and liquid phase did not show any effect on Cs adsorption.

  5. (5)

    During the comparison of influence of competitive univalent and bivalent cations a higher influence of Ca2+ on adsorption of cesium was shown.

  6. (6)

    The ionizing radiation significantly influences the sorption properties of bentonites. Higher values of adsorption capacity and adsorption percentage have been discovered with irradiated bentonite samples. The above is of great significance considering the inevitable exposition of bentonites to ionizing radiation.