Introduction

Organoclays are widely used as adsorbents in the industry and in waste treatment [1,2,3,4,5]. The potential application of organically modified clays in the removal of U(VI) ions from the aqueous phase has been reported by many authors [6,7,8,9,10,11,12] and is very important from the practical viewpoint, especially with respect to decontamination of nuclear wastes [13,14,15,16,17,18,19,20,21,22] and use of barrier materials in radioactive waste repositories [23, 24]. The use of unmodified clays in repositories is limited due to the fact that waste disposed in those facilities releases not only cationic nuclides (Cs-137, Sr-90, Co-60) but also anionic ones (Se-79, Tc-99, I-129), whose retention by clay minerals is reduced due to electrostatic repulsion. Surface modification with organic surfactants markedly improves the ability of clays to absorb anionic contaminants [25].

Intercalation of cationic surfactants changes the surface properties of clays from hydrophilic to hydrophobic and enables their industrial application, dependent on their structural properties [26, 27]. Worth mentioning is a study by Bickford and Foote [28]. Those authors synthesized sorbents capable of solidifying Purex, Pubex, and Cleanex organic wastes from the nuclear industry. Two of these sorbents, Petroset II and organoclay, were very easy to work with, as they had been designed to solidify liquid waste without the need for mixing. The Petroset II-G sorbent testing resulted in an optimized waste-loading ratio of 3.44:1 (sorbent to surrogate), and the optimized ratio for organoclay was 2.53:1.

The aim of this present study was to examine the influence of phosphate ions on the thermal stability of complexes obtained through the adsorption of uranium (VI) on natural red clay modified with hexadecyltrimethylammonium bromide (HDTMA). In our recent paper, we showed that HDTMA-modified clay effectively sorbed U(VI) ions in the presence of phosphates [29]. In that study, we tried to characterize the complexes formed during the interaction of U(VI) ions with P(V) ions on the sorbent surface. To complement these data, we now used the thermogravimetric method coupled with MS and FTIR spectra of the gaseous phase to analyze the U(VI) sorption products, expecting to find an explanation for the mechanism of interaction of the surfactant with P(V) ions and its gradual decomposition with rise in temperature. Our investigation was also intended to update the excellent studies by Yariv et al. [30,31,32,33,34] and Frost [35,36,37] concerning thermal analysis of organoclays, with information on new ligands complexing U(VI) ions.

Experimental

Preparation of the clays

A sodium form of red clay, designated as Na-clay, was prepared by equilibration of a raw clay product (R-clay) kindly provided by Geolmin (composition: SiO2 64.79%, Al2O3 16.26%, Fe2O3 7.22%, MgO 2.38%, K2O 2.68%, CaO 0.4%, TiO2 0.91%, MnO 0.09%) with a NaCl (Sigma-Aldrich, 99.99% purity) solution [38]. Organoclays were prepared by equilibration of 1 g Na-clay with aqueous solutions of hexadecyltrimethylammonium bromide (HDTMA-Br, Sigma-Aldrich, 98% purity) for 6 h at 60 °C. The initial concentrations of HDTMA-Br ([HDTMA]in) in the aqueous phase were 0.0001, 0.001, and 0.01 mol dm−3. The concentration of C in 2-mg samples of organoclays was determined using a Perkin-Elmer 2400 C H N analyzer. The molar concentration of the HDTMA+ cation in the samples (cHDTMA % CEC), corresponding to the number of HDTMA+ moles per 1 g of the inorganic part of the samples, was calculated [12]. The cation exchange capacity of Na-clay, determined by the cobalt hexamine complex method [39], was 0.0005 mol g−1.

FTIR analysis

FTIR spectra of HDTMA-clay samples were recorded in the transmission mode at room temperature by means of a 1725X Perkin-Elmer instrument using the KBr pellet technique (1:20) at a resolution of 2 cm−1. KBr was dried at 200 °C for 24 h, and then, 560 mg KBr was homogenized with a HDTMA-clay sample in a ball grinder (Narva Brand-Erbisdorf, Germany). Tablets (radius, 1 cm; thickness, 0.1 cm) were prepared using a hydraulic press.

Thermal decomposition spectra

The thermal stability of samples was determined using the device of STA 449 Jupiter F1 (Netzsch, Germany) coupled online with mass spectrometer QMS 403D Aeölos (Netzsch, Germany) and FTIR spectrometer (Bruker, Germany). The measurement conditions were described in detail in our earlier work [29, 40]. The samples (mass ~ 21 mg) were heated at the rate 10 °C min−1 in the temperature range of 30–950 °C in atmosphere of synthetic air (50 mL min−1) in an alumina crucible. An empty Al2O3 crucible was used as a reference. Data were collected and processed using the NETZSCH Proteus® software, version 6.1. The NIST library database was used for mass spectra analysis.

U(VI) sorption in the presence of P(V) ions under altered pH conditions

0.1 g of HDTMA-clay was contacted for 6 h, using a mechanical shaker, with 0.1 dm3 aqueous phase composed of 0.0005 mol dm−3 (UO2)3(CH3COO)22H2O (98% purity, Lacheman, p. Brno), 0.0001 mol dm−3 Na2HPO4 (p.a., POCH, Poland), and 0.05 mol dm−3 CH3COONa 3H2O (p.a., POCH, Poland). The initial pH of the aqueous phase was controlled by the addition of HCl or NaOH. After 6 h, the equilibrated phases were left to separate for 12 h, and then, the equilibrium pH (pHeq) was measured with a glass electrode after centrifugation of the aqueous phase. The concentration of U(VI) in the aqueous phase was measured spectrophotometrically according to the Arsenazo III method [41].

Results and discussion

The conditions under which the organoclays were prepared are given in Table 1.

Table 1 Conditions of synthesis of the analyzed organoclays
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Thermal characterization of sorption products

In the case of unmodified materials, three endothermic peaks on DSC curves are observed (Fig. 1). The first peak representing a mass loss of 0.68%, with a minimum at about 60.4 °C, is due to the removal of physically adsorbed water [38]. In the second stage (300–750 °C, a minimum at 516.6 °C), the mass loss of 3.8% corresponds to the dehydroxylation of the clay and the thermal polymorph transition (α → β) of quartz [42, 43].

Fig. 1
figure 1

TG, DTG, and DSC curves of red clay

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TG/DTG/DSC curves of HDTMA-clay for the initial HDTMA concentrations of 0.0001 [HDTMA(0.0001)], 0.001 [HDTMA(0.001)], and 0.01 [HDTMA(0.01)] mol dm−3 are shown in Fig. 2. Endothermic dehydration of the clay is observed in the first region of the curves up to 200 °C. For HDTMA (0.01), a sharp exothermic peak evidences a phase transition of the surfactant, known as chain melting [35, 44]. The second region of the curves, from 200 to 450 °C, probably represents exothermic defragmentation/oxidation of the surfactant. For HDTMA (0.0001), only one peak is visible, at 334.5 °C, whereas for HDTMA (0.001), two peaks are observed, at 252.8 and 334.1 °C: The first of these two peaks is probably related to nonelectrostatically bound HDTMA+ cations forming the bilayer of the sorbent surface, whereas the second one originates from surfactant cations attracted via electrostatic forces by the clay’s aluminosilicate framework. For HDTMA (0.01), three peaks are visible, at 237.5, 264.2, and 320.6 °C, which are probably the result of a very complex aggregation of the surfactant accompanying the formation of the bilayers and micelles on the sorbent surface. The third region in the curves, extending from 400 to 900 °C, refers to endothermic oxidation/dehydroxylation of the clay and the α- to β-form quartz polymorph transition (only for the samples with low concentration of surfactant). The temperature of dehydroxylation shifts from 520 to 515 to 410 °C, which is a reflection of the expansion of the structural skeleton of the clay by spaciously extended HDTMA+. There is an overlap between dehydroxylation and oxidation of the surfactant in the case of HDTMA (0.01), because the overall process is exothermic. For U(VI)–HDTMA-clay (Fig. 3), the arrangement of the peaks is very similar to that for HDTMA-clay, i.e., the region of defragmentation/oxidation consists of two peaks for HDTMA (0.001)–U(VI) and three peaks for HDTMA (0.01)–U(VI). There is, however, a clear drop in the dehydration temperature from 91.5 to 72.9 °C and in the defragmentation/oxidation temperature from 318.1 to 235.6 °C. Moreover, the dehydroxylation temperature decreases from 517.5 to 423.8 °C as a result of the expansion of the aluminosilicate framework of the clay under the influence of the increasing amount of surfactant cations filling the interior of the clay structure. Endothermic dehydroxylation is observed for HDTMA (0.0001)–U(VI) and HDTMA(0.001)–U(VI) and exothermic dehydroxylation (due to overlapping of the dehydroxylation with the surfactant oxidation) for HDTMA(0.01)–U(VI). For U(VI)–P(V)–HDTMA-clay (Fig. 4), the endothermic dehydration temperature decreases steadily from 107.2 to 88.8 °C with increasing surfactant concentration. The dehydration temperature is evidently higher when compared with that for the HDTMA-clay and HDTMA–U(VI)-clay systems, owing to the strong hydrophilicity of both U(VI) and P(V) ions. The defragmentation/oxidation process for the U(VI)–P(V)–HDTMA-clay system is more complex than in the case of U(VI)–HDTMA-clay, with three/four peaks observed in the region from 200 to 450 °C, at 246.5, 321.7, 439.1; 222.5, 270.8, 324.7, 437.2; and 247.3, 321.8, 437.5 °C. These peaks appear at higher temperatures than those observed for U(VI)–HDTMA-clay. This is presumably, because U(VI) phosphate complexes interact with the surfactant intercalated in the clay structure, which also manifests itself in the weak decrease in the dehydroxylation temperature from 520.5 to 515.9 °C. It seems that the interaction of the U(VI) phosphate complex with the surfactant counteracts the interaction between the surfactant and the clay matrix. The curve of P(V)–HDTMA-clay shows clearly that the interaction of P(V) ions with the surfactant cations is stronger than the interaction between surfactant cations and the clay matrix. The position of the dehydroxylation peak is unaltered (Fig. 5). The endothermic character of the dehydroxylation reaction may be a response to the fact that dehydroxylation of the clay is not accompanied by oxidation of the surfactant. The data obtained from thermal analysis of all samples are summarized and compared in Table 2.

Fig. 2
figure 2

TG (a), DTG (b), and DSC (c) curves of HDTMA-clay for different the initial concentrations of HDTMA: Co = 0.0001 mol dm−3 (green line), Co = 0.001 mol dm−3 (red line), Co = 0.01 mol dm−3 (black line). (Color figure online)

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Fig. 3
figure 3

TG (a), DTG (b), and DSC (c) curves of HDTMA–U(VI)-clay for different the initial concentrations of HDTMA: Co = 0.0001 mol dm−3 (green line), Co = 0.001 mol dm−3 (red line), Co = 0.01 mol dm−3 (black line). (Color figure online)

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Fig. 4
figure 4

TG (a), DTG (b), and DSC (c) curves of HDTMA–U(VI)–P(V)-clay for different the initial concentrations of HDTMA: Co = 0.0001 mol dm−3 (green line), Co = 0.001 mol dm−3 (red line), Co = 0.01 mol dm−3  (black line). (Color figure online)

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Fig. 5
figure 5

TG (a), DTG (b), and DSC (c) curves of HDTMA–P(V)-clay for different the initial concentrations of HDTMA: Co = 0.0001 mol dm−3 (green line), Co = 0.001 mol dm−3 (red line), Co = 0.01 mol dm−3 (black line). (Color figure online)

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Table 2 Thermal behavior of studied samples
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The MS spectra of the decomposition of U(VI) sorption products are shown in Fig. 6. The peaks originating from the –CH3 radical (M/Z = 15), amines (43), –N(CH3)2CH2(58), and CH3Br(58), appearing in the region between 200 and 450 °C, provide evidence for the defragmentation of surfactant molecules [45,46,47]: via Hoffman rearrangement and S 2N nucleophilic substitution. The peaks observable in the region 200–800 °C characteristic of H2O(M/Z = 18), NO(30), CO2(44), and NO2(46) represent surfactant oxidation, which proceeds in two steps, between 200 and 400 °C and between 400 and 800 °C. The first step regards the surfactant sorbed on the mineral surface via electrostatic and covalent forces, whereas the second one relates to the surfactant present in the interior of the mineral structure, bound by electrostatic forces.

Fig. 6
figure 6

MS profiles of gaseous products of defragmentation [15 (CH3); 43 (amine); 58 (NCH2(CH3)2); 94 (CH3Br)] and oxidation [18 (H2O); 30 (NO); 44 (CO2); 46 (NO2)] characteristic for organoclay decomposition vs temperature

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Spectral characterization of sorption products

FTIR spectra of the sorption products in the gaseous phase are given in Fig. 7 The increase in temperature results, at first, in the appearance of surfactant defragmentation products. The band at 2933 cm−1 indicates stretching vibrations of CH2 groups, and the band at 1460 cm−1 originates from scissoring vibrations of methylene groups. Apart from that, there is a prominent band at 2360 cm−1 characteristic of CO2 and a peak at 1750 cm−1 characteristic of stretching vibrations of C=O (carbonyl) groups or N=O stretching vibrations of NO2. Both of these bands are evidence for a stepwise transition from defragmentation to oxidation of HDTMA. For temperatures above 400 °C, oxidation clearly prevails over defragmentation.

Fig. 7
figure 7

FTIR–TG space image of HDTMA(0.01)- (a), HDTMA(0.01)–U(VI)- (b), HDTMA(0.01)–U(VI)–P(V)- (c), HDTMA–P(V)-clay (d) in air atmosphere

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Detailed data on changes in the position of the peaks with temperature are given in Fig. 8 and Table 3. An analysis of the peaks at 1464, 1750, 2933 leads to the conclusion that the pure surfactant (HDTMA-Br) decomposes weakly, as the temperature of the peak maximum is lower than the decomposition temperatures of the clay modifications. This observation demonstrates that interaction in HDTMA-Br is exclusively covalent.

Fig. 8
figure 8

FTIR evolution profiles of the characteristic signal: 2937 cm−1 (CH2), 2366 cm−1 (CO2), 1181 cm−1 (CO) 1750 cm−1 (C=O in carbonyl groups), 1464 cm−1 (CH2)

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Table 3 Peak temperatures on the infrared spectrums
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The peak at 2181 cm−1, characteristic of carbon monoxide, changes its position in a fashion indicating that the oxidation of the surfactant is a multistage process. It seems that there are different sorption sites in the structure of the clay, with the surfactant molecules either deposited on the mineral surface or hidden in the structural pores. This means that the surfactant molecules less strongly bound by the clay matrix are more easily oxidized. Also, the enthalpy of CO formation is lower than for CO2; therefore, oxidation of the surfactant in the case of CO formation is less spontaneous than when CO2 is formed. The temperature sequence HDTMA–U(VI)–P(V)-clay < HDTMA–P(V)-clay for the peak maxima at 2181 and 2360 is intriguing. It seems possible that in the case of HDTMA–P(V)-clay some part of the surfactant occurs in the form of alkylammonium phosphate, unlike in the HDTMA–U(VI)–P(V)-clay system, where phosphate ions are “exploited” to form complexes with U(VI) ions. Apart from that, U(VI) ions interact with the oxygens of the sorbent framework, and therefore, the temperatures of defragmentation and oxidation (peaks at 1464, 2181, and 2360 cm−1) of HDTMA–U(VI)–P(V)-clay are lower than those of HDTMA–P(V)-clay.

FTIR spectra of the sorption products in the solid phase (clay phase) are given in Fig. 9. There is a remarkable downfall shift of the Si–O stretching vibration band from 1111 to 1032 cm−1 with an increase in HDTMAin concentration from 0.0001 to 0.01 mol dm−3 for the P(V)–HDTMA-clay system. This probably is a consequence of the replacement of the weak interaction between hydrophilic P(V) ions and negatively charged siloxane oxygens by the strong interaction of hydrophobic HDTMA+ cations with Si–O structural units of clay. For the U(VI)–P(V)–HDTMA-clay system, the position of the Si–O vibration band remains stable. This means that U(VI) and P(V) ions interact on the clay surface to form a phosphate complex that strongly interacts with Si–O units.

Fig. 9
figure 9

FTIR spectra of solid phase of sorption products

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HDTMA sorption equilibrium data

The percentages of sorption of HDTMA [cCEC(surf)] from the aqueous to the sorbent phase are given in Fig. 10. For HDTMAin (0.0001), cCEC(surf) increases when going from HDTMA- to HDTMA–U(VI) to HDTMA–U(VI)–P(V)- to HDTMA–P(V)-clay. This demonstrates that HDTMA interacts with P(V) ions in the sorbent phase. For HDTMAin (0.001), CCEC(surf) increases when going from HDTMA- to HDTMA–U(VI) to HDTMA–P(V)- to HDTMA-clay. This sequence does not include the HDTMA–U(VI)–P(V)-clay system because the presence of U(VI) ions interacting with P(V) ions prevents interaction between HDTMA and P(V) in the sorbent phase. For HDTMAin (0.01), the sudden decrease in CCEC(surf) results from the decomposition of the bilayer of surfactant cations (clay-(HDTMA) +2 ) by both P(V) ions and the phosphate complex of U(VI).

Fig. 10
figure 10

Changes in sorption of HDTMA with initial concentration HDTMAin

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U(VI) sorption equilibrium data: the influence of pH

The distribution constants of U(VI) between the aqueous and the sorbent phase, defined as Kd = cs/caq, are given in Fig. 11a. There is a visible improvement in U(VI) sorption over the entire pH range in the presence of P(V) ions in the aqueous phase. The general trend of the increase in logKd is very complex as it reflects the existence of a vast array of different species formed in both the aqueous and the solid phases (Fig. 11b). The hypothetical composition of the aqueous and solid phases was estimated using Medusa software [48]. The results suggested that the solid phase was composed, among others, of (UO2)3(PO4)24H2O and UO2(OH)2H2O and that the equilibrium aqueous phase contained several acetate and carbonate complexes. The species distribution diagrams obtained are idealized, however. In our recent paper [29], we strongly emphasized that the sorbent phase contained at least two different U(VI) phosphate complexes with a U(VI)/P(V) ratio in the range of 1.1–1.5 or 3.3–3.8. At the same time, we excluded the possibility of (UO2)3(PO4)24H2O being formed, based on the analysis of the concentrations of U(VI) and P(V) ions in the equilibrium aqueous phase. By contrast, the presence of anionic acetate and carbonate complexes in the aqueous phase is unquestionable, and these species probably interact with the positively charged bilayer of surfactant cations anchored on the mineral surface in the form of clay-(HDTMA) +2 . The ‘mysterious’ minimum logKd value at pH 8 may be related to the lower distribution constant of (UO2)2CO3(OH) −3 when compared with the UO2(CO3) 4−3 species. In turn, the increase in logKd at pH 9 is a result of the formation of sodium uranate Na2U2O7 in the sorbent phase [6].

Fig. 11
figure 11

Influence of pH on U(VI) sorption on HDTMA-clay

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Conclusions

The thermal analysis of the products of modification of red clay with HDTMA revealed the following temperature regions in the DTG curves: dehydration, defragmentation/oxidation, oxidation/dehydroxylation. The HDTMA–U(VI)–P(V)-clay system differs from the remaining clay modifications in the defragmentation/oxidation region, which shows more DTG peaks, indicating that there is mutual interaction among U(VI), P(V), and HDTMA+ cations, an observation that is supported by the differences in the FTIR spectra of HDTMA–P(V)- and HDTMA–U(VI)–P(V)-clay.

The MS and FTIR gaseous phase spectra of thermal decomposition of U(VI) sorption products reveal that oxidation of the surfactant is a multistage process. Moreover, the higher temperature of defragmentation/oxidation observed for HDTMA–P(V)- compared with HDTMA–U(VI)–P(V)-clay indicates the probable formation of alkylammonium phosphates in the first case and interaction of U(VI) ions with oxygen atoms of the sorbent framework in the second case.