Introduction

Bentonite is favored because it is low-cost. It has many good properties [1,2,3] e.g. high exchange capacity thanks to the high amount (> 50 mass%) of montmorillonite content. The montmorillonite is a layer silicate clay mineral. This mineral has permanent negative charge because of isomorphic substitution which means that the cations (e.g. Al3+ ion) in the layer may be exchanged by smaller charged cations e.g. Mg2+ or Fe2+ ions [4]. The ion exchange capacity depends on the number of these substitutions. The higher the clay mineral’s ion exchange capacity, the more it affects its physical properties by changing its water-absorbing capacity, and thus, its technological behavior [5]. The negative layer charge of the crystal lattice can be neutralized by cations attracted to the interlayer space. This interlayer space can call as nano laboratory’ where cation exchange takes place which may modify the properties of the bentonites. Interlayer cations may be exchanged to lanthanide cations in the so-called nano laboratory. Lanthanide cations are used as model ions during the study of interactions between soil and transuranium ions. These radionuclides are produced during the fission of 235U in nuclear power plants [6].

Bentonite is considered as candidate engineer barrier material—among others—for the safe disposal of high-level radioactive waste [7]. The structural analyses of bentonite have been investigated under different conditions to figure out the stability of the structure. The main points of these studies have been to characterize the clays before and after the modifying experiments and to determine the possible differences in nature or structural stability [8,9,10,11]. The important aspect of the interactions between lanthanides (Lns) and clays in the environment is the storage or disposal of nuclear waste. The studies on Ln-clay interactions may eminently be useful in evaluating the adequacy of proposed materials for the engineer barrier of nuclear waste.

Research papers talk about the dissolution of bentonite where the main point of the research is to modify the properties of the bentonite for different reasons [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. The treatment of the bentonite happens usually with inorganic acids (HCl, H2SO4, HNO3) to replace mainly divalent ions (Ca2+) to monovalent ion (H+) and to leach out Fe2+/Fe3+, Mg2+ and Al3+ ions, thus modifying the smectite layers and increase the specific surface area, and the porosity. The authors usually use inorganic acids in wide range of concentration (0.25–6 mol dm−3) and the treatment time is usually between 1 and 36 h at different temperature (from 25 °C, up to 100 °C). The results revealed that the dissolution of octahedral sheet is faster than tetrahedral sheets [23, 24]. Kooli and Yan [21, 22] observed about 5% of Mg2+, Fe3+ and Al3+ reduction. Upon acid treatments, protons enter the mineral layer and harms the structural OH-groups. Krupskaya et al. [28] treated bentonites with HCl and HNO3 solutions. The study concluded an interlayer modification which occurs even at early stages of the treatment. Further treatment leads to further modification of the montmorillonite structure accompanied by intense leaching of cations from the octahedral positions into interlayer space. These transformations changes layer charge and CEC.

The aim of this work was to prepare La, and Y exchanged bentonites and to characterize the possible changes in the structure of bentonite. In our previous study [32] La ions showed extreme values of sorption while iron quantity of La bentonite was less than that of initial Ca-bentonite. The conclusion was that lanthanum ions can somehow supersede iron from the octahedral positions of crystal lattice. We assume that the departure of positively charged iron ions from the lattice increases the negative layer charge and the cation exchange capacity. This can motivate the enhance sorption of lanthanum.

All the lanthanides releases iron, less than La, and Y. This observation is surprising in the scientific word, no similar scientific result have found about the releases of the structural iron during Ln modification of bentonite. Therefore, in this work the change of the composition in the Ca bentonite was studied depending on the pH during the Ca–H, Ca–Y and Ca–La cation exchange procedure. The change in the chemical composition (mainly the concentration of Y3+, La3+, Ca2+, Fe3+ ions) in the bentonite was analyzed by X-ray fluorescence spectrometry.

Experimental

The starting material Ca-montmorillonite was collected from Istenmezeje, Hungary. Elemental analysis of the original Ca-bentonite resulted in the following composition: 73.29% SiO2, 18.71% Al2O3, 2.98% Fe-oxide, 1.54% CaO and 3.48% MgO. The mineral composition was determined by means of X-ray powder diffraction (XRD) analysis using a Philips PW1710 diffractometer (Philips, (today PANalytical B.V.) De Schakel 18 5651 GH Eindhoven, Netherland), which is operating at 30 mA and 40 kV using Cu Kα radiation source and a graphite monochromator. The scanning rate was 2°2θ/min. The mineral composition was calculated on the basis of the relative intensities of the reflections characteristic of the minerals, applying the literature or experimental corundum factors on minerals [33,34,35,36]. The mineral composition was as it follows: 72% montmorillonite, 6 m/m% cristobalite, opal CT, 3 m/m% plagioclase, 4 m/m% mica, 2 m/m% K-feldspar, 2 m/m% quartz, 2 m/m% kaolinite, and 2 m/m% calcite. The basal space d(001) of montmorillonite was 1.465 nm showing the dioctahedral character of the smectites. The CEC, as determined by the ammonium acetate method [37], was 2.7 × 10−4 mol g−1 for trivalent cations.

Complexometric titration [32, 38] was used to determine the concentration of Ln(ClO4)3 solutions and energy dispersive X-ray fluorescence spectroscopy (XRF) was used to follow the chemical changes (mainly the concentration of Y3+, La3+, Ca2+, Fe3+ ions) in bentonite [39, 40]. Instrumental parameters were: Si(Li) detector with 20 mm2 surface and 3.5 mm evaporated layer (Atomki, Debrecen, Hungary),Canberra DSA 1000 digital spectrum analyzer (Camberra Industries, Meriden, CT 06450, USA), Canberra Genie 2000 3.0 spectroscopy software (Camberra Industries, Meriden, CT 06450, USA).

The Kα-lines of the lanthanide elements was excited with 185 MBq 241Am radioactive source. In addition to the lanthanides, samples were also analyzed for iron, and calcium using a tungsten X-ray tube (type: S6000, Oxford Instruments, Scotts Valley, CA 95066, USA), operating at 60 kV and 1 mA.

The structural modifying effect of lanthanum, and yttrium ions were investigated depending on the pH during cation exchange of Ca-bentonite procedure without and with Ln3+ ions. Therefore, the behavior of Ca bentonite was investigated in acidic medium without Ln3+ ions, i.e., Ca–H cation exchange procedure was carried out by suspending 200 mg of Ca-bentonite in 20 cm3 HClO4 solution at pH 2.8–5.2, and at 25 °C. The suspension was shaken for 2 h, filtered (0.45 µm, nylon, Labex Ltd. Filter-Bio), washed by tridistilled water and then air-dried. Equilibrium is set in half an hour based on our previous experiments [41].

The combined effect of pH and lanthanide concentration on the bentonite structure was studied by the addition of different concentrated lanthanide perchlorate solutions (1 × 10−4–2 × 10−1 mol dm−3), at pH 3.5–5.5. However, in the case of pH < 3, the structure of bentonite can be destroyed. Thus, the ion exchange experiments were carried out in the pH range of 3.5–5.5. In a series of experiments, either the pH or the lanthanide concentration were changed. Different pH was adjusted by 1 × 10−4 mol dm−3 Hexamethylenetetramine, and by 1 × 10−4 mol dm−3 HClO4.

The Ln-bentonites were produced by suspending 200 mg of Ca-bentonite and 20 cm3 Ln perchlorate solution [32]. The ion exchange procedure was carried out only one time, stirring time was 2 h, and different pH and initial concentration of Ln(ClO4)3 were used.

Results and discussion

Ca–H ion exchange

Based on our previous experience [32] during Ca–Ln cation exchange procedure structural iron loss was detected. Thereupon, the influence of the pH of the iron reduction was investigated in this study. In order to investigate the structural modifying effect of lanthanide ions, the effect of pH on cation exchange procedure had to be known. Therefore, the Ca–H cation exchange procedure was first examined in acidic medium without Ln3+ ions and then during Ca–Ln cation exchange procedure was examined with La3+ and Y3+ ions.

Figure 1 shows the concentration change of Ca2+ ions in mol g−1 as a function of pH of initial HClO4 solutions. As the concentration of hydrogen ion increases, more and more calcium ions are exchanged, thus, calcium-hydrogen bentonite is formed.

Fig. 1
figure 1

The change of concentration of Ca2+ ion depending on the pH in the bentonite during Ca–H ion exchange procedure

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Figure 2 shows the amount of iron ion on Ca bentonite after treatment of HClO4 solution of solid phase at different pH. Comparing the results with the Fe content of the starting Ca-bentonite (5.2 × 10−4 mol g−1), it can be concluded that the Fe content of bentonite does not change within acceptable error limits (< 5%) after the treatment with HClO4 solution. That is, since structural iron is not suppressed by the acidic effect, so lanthanide ions are responsible for the decrease of iron concentration.

Fig. 2
figure 2

The concentration of Fe3+ ion depending on the pH in the bentonite during Ca–H ion exchange procedure

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Ca–La ion exchange

Therefore, in a similar experiment, different pH values and different initial concentrations of La3+ ion were used to follow the structural change during Ca–La cation exchange procedure. The concentrations of these mentioned ions are shown in Figs. 3, 4 and 5 at three different initial concentrations of La(ClO4)3 after the ion exchange procedure at different pHs.

Fig. 3
figure 3

The concentration of Ca2+ ion depending on the pH, and concentration of La(ClO4)3 in the bentonite during Ca–La ion exchange procedure

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Fig. 4
figure 4

The concentration of La3+ ion depending on the pH, and concentration of La(ClO4)3 in the bentonite during Ca–La ion exchange

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Fig. 5
figure 5

The concentration of Fe3+ ion depending on the pH, and concentration of La(ClO4)3 in the bentonite during Ca–La ion exchange procedure

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Figure 3 shows that the calcium ion content of each sample is significantly reduced. The reduction is inversely proportional to the initial lanthanum ion concentration. Cca. 90% of Ca2+ ions are exchanged to La3+ ions.

Figure 4 shows that the exchange of La3+ ions are independent of pH, but the effect of the lanthanum concentration is robust.

The change in structural iron content (Fig. 5) is not influenced by the pH, only by the lanthanum concentration. The released iron content is about 15–25% compared to Ca-bentonite. So, the structural iron is suppressed by the trivalent lanthanum. This requires at least 5 × 10−3 mol dm−3 lanthanum ion concentration.

Ca–Y ion exchange

Similar experiment was performed with Y3+ ion exchange. The experiments were also carried out in the pH range of 3.5–5.5, but the concentration of yttrium ion was further increased. The results can be found in Figs. 6, 7 and 8. Similarly to lanthanum ion, the ion exchange between Ca2+–Y3+ ions is not affected by the pH, but with as the concentration of Y3+ ions increases more and more Y3+ ions are sorbed on the bentonite (Fig. 7).

Fig. 6
figure 6

The concentration of Ca2+ ion depending on the pH, and concentration of Y(ClO4)3 in the bentonite during Ca–Y ion exchange procedure

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Fig. 7
figure 7

The concentration of Y3+ ion depending on the pH, and concentration of Y(ClO4)3 in the bentonite during Ca–Y ion exchange procedure

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Fig. 8
figure 8

The concentration of Fe3+ ion depending on the pH, and concentration of Y(ClO4)3 in the bentonite during Ca–Y ion exchange procedure

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At the same time, with increasing concentration, more and more calcium ions are exchanged, using 0.2 mol dm−3 Y(ClO4)3 solution, almost 100% of the calcium ion is dissolved (Fig. 6).

Structural iron content (Fig. 8) shows that the pH has a slightly effect, but the concentration of yttrium ion has a significant effect. The released iron content is about 22–33% compared to Ca-bentonite. The higher release of iron content in case of yttrium ion compare to lanthanum ion is because the higher yttrium ion concentration during the acidic treatment.

Thus, in the case of both ions (lanthanum and yttrium ion), it can be concluded that the cation exchange of Ca–Ln is essentially independent of pH and depends on the concentration of Ln ion. Ln3+ ions at high concentrations almost exchange interlayer Ca2+ cations, displacing 30% of the structural iron from the octahedral layer. Some of the leaving iron ions enter the interlayer space [42], and the other part precipitates in the form of FeO–OH which is deposited on the wall of the laboratory vessels during the exchange procedure [32]. Therefore, Ln3+ ion is responsible for the effect, since there was no similar result during Ca–H cation exchange.

Conclusions

The aim of this study was to investigate the cause of the structural iron decrease during Ca–La, and Y cation exchange procedure. Therefore, the combined effect of pH and concentration on the bentonite structure was studied during Ca–H, Ca–Y, and Ca–La cation exchange procedure.

This study has proved that during Ca–Ln-cation exchange procedure (in case of Y, and La) iron released from octahedral position of montmorillonite which is not due to the pH but La, and Y concentration.

This study strongly confirms our previous study. However, further studies are needed to be aware of the mechanisms between the Ln and the structural composition of the bentonite. Kinetic studies planned to see the structural change by the time at different temperatures.