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Article

Mechanochemical-Activated Organomontmorillonite for Uranium Pollution Protection

by
Iryna Kovalchuk
1,2,*,
Iryna Farbun
3,
Volodymyr Sydorchuk
4,
Andrey Lakhnik
5 and
Olena Diyuk
4
1
Laboratory of Environmental Chemistry, Institute for Sorption and Problem of Endoecology, National Academy of Science of Ukraine, 03164 Kyiv, Ukraine
2
Department of Chemical Technology of Ceramics and Glass, Igor Sikorsky Kyiv Polytechnic Institute, 03056 Kyiv, Ukraine
3
Department of Sorption and Fine Inorganic Synthesis, Institute for Sorption and Problem of Endoecology, National Academy of Science of Ukraine, 03164 Kyiv, Ukraine
4
Department of Oxidative Heterogeneous Catalytic Processes, Institute for Sorption and Problem of Endoecology, National Academy of Science of Ukraine, 03164 Kyiv, Ukraine
5
Department of Physics of Dispersed Systems, H.V. Kurdyumov Institute of Metal Physics, National Academy of Science of Ukraine, 02000 Kyiv, Ukraine
*
Author to whom correspondence should be addressed.
AppliedChem 2025, 5(1), 5; https://doi.org/10.3390/appliedchem5010005
Submission received: 13 January 2025 / Revised: 18 February 2025 / Accepted: 20 February 2025 / Published: 24 February 2025
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Abstract

:
The modification of the layered silicate with a structural type 2:1 montmorillonite by the cationic surfactant hexadecyltrimethylammonium bromide was carried out. The obtained organomontmorillonite was milled for 2–25 min in a high-energy planetary ball mill. The structural and physicochemical characteristics of the modified montmorillonite and the mechanochemically activated montmorillonite were investigated using various methods such as X-ray diffraction, thermal analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and determination of the specific surface area as well as the parameters of the porous structure by the low-temperature adsorption–desorption of nitrogen. The modification of montmorillonite with the quaternary ammonium salt led to a slowdown of deformation and subsequent amorphization of the montmorillonite structure during the high-energy milling. Mechanochemical activation of the modified montmorillonite increased its sorption capacity nine times, with the maximum uranium sorption achieved after mechanochemical treatment for 10 min.

1. Introduction

Sorption methods play an important role in solving the environmental problems of purifying polluted waters. Recently, many new organic and inorganic sorbents with increasing, and sometimes unique, characteristics about certain toxicants or their groups have been obtained [1]. Natural minerals, coal, carbon materials, etc., are considered as promising sorbents [2]. The economy and application of clay’s uses are widely known in different spheres due to the availability of raw materials and their physicochemical properties. Natural layered silicates are occupied in environmental protection practices as cheap sorbents for the removal of various toxicants from polluted waters, mainly in cationic forms [3,4,5,6]. Acid activation, thermal and hydrothermal treatment, modification with surface-active substances, etc., are used to improve the sorption characteristics of natural silicates [7,8,9]. The application of the latter method makes it possible to change in a directed order the lyophilicity of layered silicates’ surfaces, and, thus, to increase their sorption capacity [10,11,12,13].
Currently, mechanochemical methods are regularly used in the production of catalysts, functional ceramics, various special-purpose materials, metallurgy, pharmaceuticals, etc. [14,15]. During the mechanochemical activation, structural disorder of the material by intensive grinding occurs, which leads to an increase in the chemical reactivity of the processed material. The mechanochemical activation of clays is actively selected to change the interspace distance, introduce various cations, and, accordingly, change the operational characteristics [16,17]. In [18], the gradual destruction of the crystalline phase, changes in the morphology, and an increase in the specific surface area from 80.3 m2/g to 146.8 m2/g at 2 h of mechanochemical activation of montmorillonite were shown. Mechanochemical processing of clay minerals is used to remove polar organic compounds [19], phenathrene [20], phenol [21] from aqueous solutions and soils, fluorides [22], and for the purification of polluted waters from radionuclides [18,23].
The intercalation of surfactants between the clay layers changes the surface properties from hydrophilic to hydrophobic. For the swelling clay, in addition, modification by the surfactant leads to an increase in the basal spacing between the layers. The sorption values for montmorillonite with a high degree of surface coverage (ratio cationic exchange capacity montmorillonite: the molar mass of the surfactant consisted of 1:5) and the formation of predominantly double layers of surfactants were 31 mg/g [12]. Separate works are devoted to the study of the mechanochemical processing of clay minerals modified by organic substances [24,25,26]. In particular, in the article devoted to the mechanochemical modification of bentonite [27], the structural stability of the obtained materials was studied, but their sorption characteristics were not determined. It was shown that carrying out the mechanochemical treatment of bentonite for 1–20 min in the presence of surfactants (trimethylammonium cations with different chain lengths) increased the structural stability of bentonite [27]. The preliminary addition of intercalants prevents the destruction of the layered structure of clay during mechanochemical treatment as, for example, alkylammonium ions, which are part of organomontmorillonite [28], and potassium acetate, which is part of kaolinite [29], stabilize the structure of these clay minerals.
Considering that Ukraine ranks 10th in the world in terms of uranium reserves [30], mining and processing of uranium in the Dnipropetrovsk region raise the danger of uranium leaking into surface and underground waters, and later into drinking water sources [31]. Therefore, the question of protecting the water basin from uranium compounds using effective sorption materials based on natural raw materials is quite urgent. Modified layered silicates are suitable for use in barriers and injection wells in the field of environmental protection. The mechanochemical modification of clay minerals by their energy-intensive dispersion with the simultaneous addition of surfactants will save time and the cost of clay mineral modification.
The aim of this work was to study the effect of the mechanochemical treatment of organomontmorillonite on its surface and sorption properties, as well as to establish the mechanisms of sorption. The novelty of the work lies in the study of the features of sorption of such a dangerous toxicant as uranium on the active centers, radicals, microcracks, pores, channels, and other structural defects formed during the mechanochemical modification of montmorillonite, as well as on the surface centers obtained by surfactants’ modification.

2. Materials and Methods

2.1. Materials and Chemicals

Layered silicate montmorillonite (MMT) with structural type 2:1 coming from the Dashukivske deposit, Cherkasy region, Ukraine, was provided by PJSC “Dashukiv Bentonites”. The structural formula of montmorillonite is (Ca0.12Na0.03K0.03)0.18 (Al1.39Mg0.13Fe0.44)1.96 (Si3.88Al0.12)4O10 (OH)2·nH2O), specific surface area (SSA) 89 m2/g, and cationic exchange capacity (CEC) 0.71 meq/100 g [32]. Coarse mineral impurities such as feldspars, quartz, aluminum and iron oxides, carbonates, etc., were withdrawn from the rock. Purified clay minerals consisting of montmorillonite as the main mineral, and used in the next experiments, were obtained by way of sedimentation from an aqueous–clay suspension, centrifugation, drying at 105 °C, and grounding to the fraction <0.1 mm.
Hydrochloric acid (HCl), sodium hydroxide (NaOH), salts of sodium chloride (NaCl), uranyl sulfate (UO2SO4·3H2O), and hexadecyltrimethylammonium bromide (HDTMA (C16H33) N (CH3)3Br)) were obtained from Sigma–Aldrich, St. Louis, MO, USA. All the chemicals were analytical reagent grade and used without further purification. The distilled water was used for the solution of uranium and sodium salt preparation.

2.2. Synthesis

Cationic surfactant hexadecyltrimethylammonium bromide (C16H33N (CH3)3Br) was used to obtain an organomodified (MMT-HDTMA) sample of montmorillonite. Montmorillonite was mixed with HDTMA in a water–ethanol (50:50) medium at 60 °C for 4 h with constant stirring. Ratio CEC montmorillonite: the molar mass of the surfactant consisted of 1:2, the mass of clay was 50.0 g, the mass of HDTMA was 25.9 g, and the volume was 500 mL. The sample was washed with distilled water to a negative reaction to Br−-ions and was dried at 105 °C.
Planetary ball mill Pulverisette-6 (Fritsch Milling & Sizing, Inc., Pittsboro, NC, USA) was used for the mechanochemical treatment of MMT-HDTMA samples. Si3N4 balls (d = 20 mm, n = 12) with a total weight of 160 g were used as working bodies. The mass ratio of balls and MMT-HDTMA was 1:10. The milling process was continued for 2–25 min. The rotation speed was equal to 300 rpm. Milling was performed in 10 min cycles in the case where it was needed; subsequently, a reverse was carried out after each cycle. Obtained samples were marked as MMT-HDTMA-MCA.

2.3. Characterization Methods

X-ray powder diffraction (XRD) patterns of the natural montmorillonite, organomontmorillonite, and mechanochemically activated organomontmorillonite were recorded with a DRON-4-07 diffractometer (Burevestnik, St. Petersburg, Russia), using CuKα (0.154 nm) in the range between 3 and 60 grad (2θ). The constant pass energies of U = 30 kV and I = 20 mA were used [33]. The average size of the coherent scattering region (CSR) was calculated from the broadening of the diffraction peak of montmorillonite, organomontmorillonite, and mechanochemical-activated organomontmorillonite (001) according to the Scherrer equation [34].
The scanning electron microscopy (SEM) method was used for the surface morphology investigation of the mechanochemically activated organomontmorillonite. The samples were sputter coated with gold for 3 min. In the conditions of analysis, secondary images were used with an acceleration voltage of 30 kV and a magnification of 5000-fold. The samples were analyzed using a JED-2300 X-ray spectrometer integrated with a JSM 6060 LA scanning electron microscope (Jeol JSM-6060, Tokyo, Japan). An energy-dispersive X-ray (EDS) analysis was performed with an accelerating voltage of 15 kV and a magnification of ×500 using standard software.
Transmission electron microscopic (TEM) studies were performed using a JEM-2100F electron microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV.
Curves of thermogravimetric (TG) and differential thermal (DTG) analyses were obtained using apparatus Derivatograph–C (F. Paulik, J. Paulic, L. Erdey, Budapest, Hungary) in the range of temperature 20–800 °C at a heating rate of 10 min−1.
Nitrogen adsorption–desorption isotherms were measured at −196 °C and previously dried at 130 °C for 6 h under high vacuum, using a Quantachrome NOVA-2200e Surface Area and Pore Size Analyzer (Boynton Beach, FL, USA). The pressure measurement error was 0.016 mmHg. The ASiQwinTM V 3.0 software (developed by Quantachrome Instruments, Boynton Beach, FL, USA) was used for the results processed. The specific surface area (SSA, m2/g), the total pore volume (V, cm3/g), and the pore radius (r, nm) were estimated using the BET and BJH methods. The pore size distributions (dV/dlog (r)) were determined using BJH and DFT models [35]

2.4. Adsorption Experiments

The sorption of uranium ions on MMT, MMT-HDTMA, and MMT-HDTMA-MCA was performed in batch conditions (V = 50 mL, T = 25 °C, contact time 1 h, pH = 6, solid/liquid ratio = 2 g/L) varying the uranium concentrations from 50 μmol/L to 800 μmol/L according to previously established optimal conditions [7]. After establishing the adsorption equilibrium, the liquid phase was separated by centrifugation (6000 rpm) for 30 min. All sorption experiments were performed in duplicate. The spectrophotometric method (UNICO 2100UV, United Products, and Instruments, Dayton, NJ, USA) was used for the determination of the equilibrium concentration of U (VI) after adsorption. The wavelength was 665 nm, and Arsenazo III was used as a reagent.
The adsorption capacity (qeq, µmol/g) of radionuclide ions was estimated by Equation (1):
qeq = (Cin − Ceq)·V/m
where Cin and Ceq represent the initial and equilibrium uranium concentrations (µmol/L), V is the solution volume (L), and m is the weight of the adsorbent (g).
The experimental isotherms of uranium adsorption were processed using the Langmuir mathematical model for a homogeneous surface, and the Freundlich model and Temkin model for a heterogeneous surface [36,37].

3. Results and Discussion

The diffraction pattern of MMT (Figure 1 curve 1) is typical for air-dried samples’ Ca-form of montmorillonite (pdf card № 00-013-0135). The most important factor reflecting the structural characterization of modified montmorillonite is basal spacing. The intercalation of HDTMA molecules into the interlayer space of montmorillonite clay is indicated by the results of the X-ray investigation (Figure 1: curve 2) [27,38].
This can be seen with an increase in the basal reflex d001 from 1.52 nm for the natural montmorillonite sample to 1.86 nm for the modified MMT-HDTMA sample. A concomitant decrease in the intensity of all reflections and their broadening is observed, which indicates some disordering in the structure. Mechanochemical treatment for the 10 min MMT- HDTMA sample (Figure 1: curve 3) leads to a further shift of the 001 peak (to 1.96 nm), but the intensity of all peaks does not change. The latter suggests that no additional destruction of the crystal structure occurs during grinding, which is consistent with the results of the works [23,28].
The SEM microphotographs of the natural MMT sample, organomontmorillonite MMT-HDTMA, and the most reactive sample 10 min MMT-HDTMA-MCA are shown in Figure 2. Particles of natural MMT have the shape of plates, which can be seen in Figure 2a. As a result of the modification, the surface of MMT is covered with a surfactant layer (Figure 2b). The particles of clay with organic cations sites in the surface are noticed. The microstructure of the milled organomontmorillonite sample is changed. Fine milling of MMT-HDTMA for 10 min leads to morphological changes (Figure 2c). The mechanochemical treatment destroys the particle shape of MMT-HDTMA and produces particles with a predominantly spherical shape, and the flaky-edge particles of organomontmorillonite form mainly globular ragged agglomerates.
Using the SEM/EDS analysis method, it was found that peaks at 0.53 keV (Kα); 1.83 keV (Kα); 1.55 keV (Kα); 7.08 keV (Kα), and 0.73 keV (Lα); 4.02 keV (Kβ); 1.29 keV (Kα) are observed in the spectrum of the initial montmorillonite. According to [39], these reflexes are attributed to the line energies of oxygen, silicon, aluminum, iron, calcium, and magnesium, respectively. In the EDS spectrum of modified montmorillonite, in addition to the oxygen, silicon, aluminum, iron, calcium, and magnesium peaks, a peak at 0.29 keV (Kα) is observed. According to [39], this reflex is attributed to the energy of the carbon line. These data confirm the surface modification of montmorillonite with hexadecyltrimethylammonium bromide.
The TEM images of the MMT-HDTMA sample are shown in Figure 3. Generally, MMT-HDTMA is mainly composed of regularly intercalated layers (Figure 3a). The clay stacking in a number of silicate layers’ particles is preserved with an intercalated amount of HDTMA cations, with most of them situated at the outer surface. However, a few inequalities in the layer-to-layer distance for different intercalated clay particles are observed (Figure 3b,c) that may be conditioned by defects of the clay structure [40].
Surfactant modification and mechanochemical activation have a significant impact on the character and temperature intervals of transformations occurring in the processes of thermolysis of montmorillonite. First of all, this is manifested by the appearance of new and shifting thermal effects on the heating curves of montmorillonite. Figure 4 shows the thermogravimetric (TG), derivative thermogravimetric (DTG), and differential thermal analysis (DTA) curves of montmorillonite and the obtained samples of organoclay and mechanochemically treated organoclay. The TG curve of MMT (curve 1) has two mass loss steps. They correspond to the endothermic peak in the temperature range of 100–220 °C, which occurs due to the loss of interlayer water. The endoeffect in the range of 450–600 °C is due to the loss of structural hydroxyl groups, which corresponds to the peaks on the DTG curves [17].
Montmorillonite modified with the quaternary ammonium surfactant takes place in more mass loss steps. Curve 2 (MMT-HDTMA) has three endothermic picks. The presence of a fairly large endothermic effect in the range of 200–370 °C with a weight loss of 15–20% for organomontmorilonite is associated with the destruction of organic matter. The results of the thermal method confirm the assumption that hydration–ionic layers of water are displaced from the interlayer space by organic cations during modification. Curve 3 (10 min MMT-HDTMA-MCA) demonstrates endothermic picks are also in the range of 150–220 °C similar to MMT and 200–370 °C similar to the MMT-HDTMA endothermic pick. At the milling, the dehydroxylation peaks are shifted and decrease in intensity. This suggests that mechanochemical treatments reduce the energy required for decomposing the hydroxyl groups and increase part of the hydroxyl groups during the high-impact milling process. Table 1 highlights the main thermogravimetric parameters that confirm the effect of mechanochemical treatment on moisture loss, on residual mass, and on the shift of DTG peaks.
Nitrogen adsorption/desorption isotherms for the natural MMT belong to type IV isotherms with a hysteresis loop of type H3 according to the IUPAC classification [41] (Figure 5a).
A sharp rise at low values indicates the presence of micropores (about 0.01 cm3/g). A hysteresis loop on the isotherm in the range of medium and high pressures shows that this sample has a uniform mesoporous fracture. The latter is confirmed by the PSD curves (Figure 5a,b). The presence of micropores in the structure of the MMT sample is confirmed by calculations using the t-method, as well as by the excess of the specific surface area calculated using the BET method over its value obtained using the BJH method.
The mechanochemical treatment of organomontmorillonite (10 min MMT-HDTMA-MCA) results in a three-fold increase in the specific surface area and volume of mesopores (Table 2). This may be due to two reasons: (i) the partial removal of the modifier during milling, as indicated by the decrease in mass loss for the milled sample compared to the modifier with HDTMA (Figure 4c); (ii) an increase in the interlayer distance from 1.86 nm to 1.96 nm, which entails its accessibility for nitrogen molecules during measurements.
It can be seen from the differential curves of the distribution of pore volumes by size (Figure 5b,c) that a narrow peak (r = 1.5–3 nm) is observed for the MMT and 10 min MMT-HDTMA-MCA samples, which indicates a narrow distribution of the pore volume by size.
The set of above-described data makes it possible to detect changes in the physical and chemical properties of organomontmorillonite during its mechanochemical activation. The mechanical activation of MMT leads to structural disturbances due to the disordering of the original crystal lattices by shifting the layers and meshes relative to each other.
The sorption isotherms of uranium ions at pH 6 show that the sorption characteristics of samples of montmorillonite (Figure 6) modified with surfactant and mechanochemically treated organomontmorillonite are much higher as compared to those for natural mineral MMT (qmax = 87 µmol/g U (VI)). The maximum U (VI) sorption capacity of modified montmorillonite is 111 µmol/g for MMT-HDTMA.
The results indicate a significant increase in the sorption capacity and high selectivity of the synthesized materials for U (VI) ions. After 10 min of milling, a large number of agglomerates are not yet formed, so the sorption of uranium (VI) is at a maximum. The modification of montmorillonite with cationic surfactant molecules not only softens the friction during milling and significantly reduces the milling time (2 h is the optimal time of milling for natural montmorillonite [18]), but also makes it possible to remove not only the cationic (UO22+), but also the negatively charged products of uranium hydrolysis and their interaction with dissolved CO2: [UO2OH]+, [(UO2)3 (OH)5]+, UO2 (OH)2, UO2CO3, [(UO2)2CO3 (OH)3]−, etc. [42].
At the initial stages of the modification process, the intercalation of HDTMA molecules into the interlayer space of montmorillonite occurs first. According to [43], the main exchanging ion to HDTMA is sodium ion in the case of Na-bentonite. Inside the remaining metals, Mg, Fe, and Al remain unexchanged. In the case of natural montmorillonite [32], the ion-exchange cations are calcium, magnium, sodium, and potassium. As the modification process progresses, the hydrophobicization of the aluminosilicate surface, and, accordingly, the flocculation of the dispersion occur, as exchangeable inorganic cations are replaced by modifier molecules and an organic monolayer is formed. With a further increase in the HDTMA content in the solution to values close to the critical concentration of micelle, the formation of double layers of adsorbed surfactants, the so-called hemimicelles, occurs on the basal surfaces of the particles. In this case, the non-polar hydrocarbon parts of the surfactant molecules are oriented towards each other with the formation of two-dimensional associates, which causes the secondary hydrophilization of the surface [12]. However, even with such a sufficiently large excess of surfactants in the modifying solution, the formation of a continuous double layer of HDTMA on the surface of clay particles is not achieved.
Alkylammonium ions, contained in the organomontmorillonite, stabilize its structure during mechanochemical activation. The XRD results together with the SEM analysis of the MCA organomontmorillonite indicate that layers of the exfoliated particles are not deteriorated in the structure by ball milling, which correlates with [28]. Changing the XRD basal reflection is related to the increased disorder and widespread delamination of the clay mineral particles, together with some fragmentation and distortions.
It is important to analyze the forms in which uranium exists under the sorption processes. At pH values close to neutral and in conditions of contact with air, which correspond to surface waters, the dominant forms of U (VI) in solutions are the uranyl ion UO22+ and its positively charged (Catx+) hydroxo complexes UO2OH+, (UO2)3 (OH)5+, (UO2)4 (OH)7+, etc. The neutral UO2 (OH)2, UO2CO3 and anionic forms (Anx−) of uranium (UO2) (OH)3−, (UO2) (OH)42−, (UO2)3 (OH)7−, [(UO2)2CO3 (OH)3]−, (UO2 (CO3)22−, UO2 (CO3)43−, etc., exist too [7,44].
The sorption of negatively charged uranium forms occurs due to the interaction with double layers of surfactant particles adsorbed on the surface through the Van der Waals forces according to the reaction:
Any− + y(HDTMA+)–surface→ Any−[yHDTMA+]–surface
In return, the interaction of positively charged ions (Catx+) of U(VI) with the surface of layered silicates occurs primarily with the formation of strong surface complexes due to the Si−OH, Al−OH, and Mg−OH groups localized on the lateral faces of minerals [7], which are not covered with surfactant hemimicelles.
Catx+ + x ≡ Si (Al)−OH → (≡ Si (Al) − O)xCat + xH+
The bonds of uranium and organic compounds confirm the data [45] of uranium desorption from modified HDTMA diatomite (SiO2):,uranium sorbed on the surface is removed by 0.1 M HCl or 0.05 M Na2CO3 on 86% and 69%, respectively.
The increase in the sorption characteristics of mechanochemically activated organomontmorillonite is influenced by two main factors: the positive surface charge formed as a result of surfactant modification (ζ-potential consists of 30.1 mV at pH 6 for CEC montmorillonite: surfactant 1:1 [12]), and the decrease in particle size as a result of mechanochemical treatment. HDTMA molecules almost completely cover the montmorillonite surface, filling the access to micropores, which is evident from the results of nitrogen adsorption. In this case, the formation of a double layer of surfactants on the surface leads to a recharge of the surface from a negative to a positive charge with the formation of hemimicelles [12]. Increasing the specific surface area and mesopore volume may also contribute to increased sorption by the milled samples (MMT-HDTMA-MCA). The optimal milling conditions for obtaining a sorbent with the best sorption characteristics relative to uranium are 10 min. According to the mechanochemical activation theory, the main processes occurring during the action of mechanical forces on solids are the initial stage, metastable stage, and final stage [46]. According to the Avakumov theory, the final stage is characterized by the agglomerations of particles, and as result, the sorption will be decreased. At 25 min of mechanochemical treatment, the agglomeration of particles and their adhesion already occur.
The applicability of the Freundlich equation (Table 3) to the description of uranium sorption on samples of mechanochemically activated montmorillonite data indicates the significant energy heterogeneity of the sorption centers located on the surface. For mechanochemically treated samples of 2 min, 5 min, and 10 min, the Freundlich correlation coefficients are very high: (R2 = 0.846–0.999).
For the Langmuir monomolecular sorption equation, which indicates the energy homogeneity of active centers, and, accordingly, the proximity of ion sorption energies as the surface is filled, the correlation coefficients for mechanochemically treated samples are R2 = 0.926–0.999.
According to the Temkin model, KT, as the sorption equilibrium constant which corresponds to the maximum binding energy, has a higher value for 5 and 10 min mechanochemical treatments of organomontmorillonite, and these are consistent with the obtained experimental data.

4. Conclusions

The montmorillonite was treated by cationic surfactant hexadecyltrimethylammonium bromide and milled at a high-energy planetary ball mill. Organoclay modified by long-chain alkylammonium cations acted as a lubricant and protected the material before friction during milling in a high-energy planetary ball mill. Changing the XRD basal reflection from 1.54 nm for montmorillonite to 1.96 nm for mechanochemical-activated organomontmorillonite was related to the increased disorder and widespread delamination of the clay mineral particles, together with some fragmentation and distortions. According to the SEM analysis, the mechanochemical treatment destroyed the particle shape of MMT-HDTMA and produced particles with a predominantly spherical shape, and the flaky-edge particles of organomontmorillonite formed mainly globular ragged agglomerates. The mechanochemical treatment of organomontmorillonite (10 min MMT-HDTMA-MCA) resulted in a three-fold increase in the specific surface area and volume of mesopores. Modification of montmorillonite with quaternary ammonium salts led to deceleration of deformation and amorphization of its structure.
The mechanochemical treatment of the organomontmorillonite improved its sorption properties due to an increase in active centers, and an increase in structural stability. The obtained results suggest that structural changes induced by milling organoclay have a high effect on sorption properties. The optimal milling conditions for obtaining a sorbent with the best sorption characteristics relative to uranium are 10 min and qmax consisting of 769.2 μmol/L. The increase in the sorption characteristics of mechanochemically activated organomontmorillonite is influenced by the positive surface charge formed as a result of surfactant modification and the decrease in particle size as a result of mechanochemical treatment.
Thus, the obtained sorption material with a stabilized structure and high sorption characteristics could be used in environmental protection technologies. This is particularly relevant for treating surface and underground waters contaminated with uranium compounds, which can exist in various forms in the surrounding water environment. The obtained sorbents allow the purification of natural water, wastewater, and technical waters from pollution to the required standards; improving the ecological situation of basins and territories; and, in some cases, concentrating and extracting useful components from water and liquid ores into marketable products.

Author Contributions

Conceptualization, I.K.; methodology, I.K.; software, I.K. and I.F.; validation, I.K., V.S. and I.F.; formal analysis, I.K. and I.F.; investigation, V.S., O.D. and A.L.; writing—original draft preparation, I.K.; writing—review and editing, I.K.; visualization, I.F.; supervision, I.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors are grateful to the staff of the National Academy of Sciences of Ukraine for their support. Special thanks to coworkers from the Institute for Sorption and Problems of Endoecology M. Tsyba for obtaining the nitrogen adsorption–desorption isotherms and L. Kuznietsova for the thermogravimetric analysis; O. Oranska from the Institute of Surface Chemistry for obtaining patterns for X-ray powder diffraction; and M. Danylenko from the Institute for Problems of Material Sciences for the photos of transmission electron microscopy.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

MMT Montmorillonite
CECCationic exchange capacity
HDTMAHexadecyltrimethylammonium bromide
MMT-HDTMA Organomodified montmorillonite
MMT-MCAMechanochemically activated montmorillonite
MMT-HDTMA-MCAMechanochemically activated organomontmorillonite
SEMScanning electron microscopy
TEMTransmission electron microscopy
XRDX-ray powder diffraction
EDSEnergy-dispersive X-ray
TGThermogravimetric analysis
DTGDerivative thermogravimetric analysis
DTADifferential thermal analysis

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Figure 1. X-ray diffractograms of montmorillonite: 1-MMT, 2-MMT-HDTMA, 3–10 min MMT-HDTMA-MCA.
Figure 1. X-ray diffractograms of montmorillonite: 1-MMT, 2-MMT-HDTMA, 3–10 min MMT-HDTMA-MCA.
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Figure 2. SEM microphotographs of montmorillonite particles at 5000× magnification: (a) MMT, (b) MMT-HDTMA, (c) 10 min MMT-HDTMA-MCA.
Figure 2. SEM microphotographs of montmorillonite particles at 5000× magnification: (a) MMT, (b) MMT-HDTMA, (c) 10 min MMT-HDTMA-MCA.
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Figure 3. TEM micrographs of MMT-HDTMA particles at three different magnifications: (a) scale bar 200 nm, (b) scale bar 50 nm, (c) scale bar 20 nm.
Figure 3. TEM micrographs of MMT-HDTMA particles at three different magnifications: (a) scale bar 200 nm, (b) scale bar 50 nm, (c) scale bar 20 nm.
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Figure 4. TG (a), DTG (b), and DTA (c) analysis: 1–MMT, 2–MMT-HDTMA, 3–10 min MMT-HDTMA-MCA.
Figure 4. TG (a), DTG (b), and DTA (c) analysis: 1–MMT, 2–MMT-HDTMA, 3–10 min MMT-HDTMA-MCA.
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Figure 5. Nitrogen adsorption–desorption isotherms (a) and pore size distributions obtained by the BJH (b) and DFT (c) methods for MMT (●), MMT-HDTMA (■), and 10 min MMT-HDTMA-MCA (◄).
Figure 5. Nitrogen adsorption–desorption isotherms (a) and pore size distributions obtained by the BJH (b) and DFT (c) methods for MMT (●), MMT-HDTMA (■), and 10 min MMT-HDTMA-MCA (◄).
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Figure 6. Sorption isotherms of uranium for the following: 1—MMT, 2—MMT-HDTMA, 3—MMT-MCA, 4—25 min MMT-HDTMA-MCA, 5—2 min MMT-HDTMA-MCA, 6—5 min MMT-HDTMA-MCA, 7—10 min MMT-HDTMA-MCA.
Figure 6. Sorption isotherms of uranium for the following: 1—MMT, 2—MMT-HDTMA, 3—MMT-MCA, 4—25 min MMT-HDTMA-MCA, 5—2 min MMT-HDTMA-MCA, 6—5 min MMT-HDTMA-MCA, 7—10 min MMT-HDTMA-MCA.
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Table 1. The main thermogravimetric parameters.
Table 1. The main thermogravimetric parameters.
SampleMoisture Loss, %Residual Mass
at 739 °C, g		Temperature
at the DTG Peaks, °C
MMT12.9174.2133.6
MMT-HDTMA28.5143.0282.5
10 min MMT-HDTMA-MCA24.2151.7172.8; 298.9
Table 2. Characteristics of the porous structure.
Table 2. Characteristics of the porous structure.
SampleSSSA(BET), m2/gSSSA(BJH), m2/gVtot, cm3/gVmeso (BJH), cm3/gMeso-Pores, %raver., nm
MMT89.139.50.080.0562.51.8
MMT-HDTMA0.91.90.0050.00510010.1
10 min MMT-HDTMA-MCA3.14.60.0120.0131007.8
Table 3. Parameters of the Langmuir, Freundlich, and Temkin expressions fitted to the sorption isotherms of uranium.
Table 3. Parameters of the Langmuir, Freundlich, and Temkin expressions fitted to the sorption isotherms of uranium.
AdsorbentLangmuirFreundlichTemkin
qmax,
μmol/g		KL,
L/μmol		R21/nKF,
L/μmol		R2KT L/μmolbTR2
MMT87.06.10.9940.9712.90.9959.5182.10.957
MMT-HDTMA111.145.00.9990.268.70.8461.0134.50.916
2 min MMT-HDTMA-MCA476.2210.9531.5596.90.9942.280.80.993
5 min MMT-HDTMA-MCA526.3190.9261.3164.10.99911.212.80.968
10 min MMT-HDTMA-MCA769.26.50.9790.8327.80.9958.319.00.950
25 min MMT-HDTMA-MCA434.8230.9420.9937.30.9702.218.40.964
MMT-MCA123.581.00.9960.3210.50.9146.225.10.905
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Kovalchuk, I.; Farbun, I.; Sydorchuk, V.; Lakhnik, A.; Diyuk, O. Mechanochemical-Activated Organomontmorillonite for Uranium Pollution Protection. AppliedChem 2025, 5, 5. https://doi.org/10.3390/appliedchem5010005

AMA Style

Kovalchuk I, Farbun I, Sydorchuk V, Lakhnik A, Diyuk O. Mechanochemical-Activated Organomontmorillonite for Uranium Pollution Protection. AppliedChem. 2025; 5(1):5. https://doi.org/10.3390/appliedchem5010005

Chicago/Turabian Style

Kovalchuk, Iryna, Iryna Farbun, Volodymyr Sydorchuk, Andrey Lakhnik, and Olena Diyuk. 2025. "Mechanochemical-Activated Organomontmorillonite for Uranium Pollution Protection" AppliedChem 5, no. 1: 5. https://doi.org/10.3390/appliedchem5010005

APA Style

Kovalchuk, I., Farbun, I., Sydorchuk, V., Lakhnik, A., & Diyuk, O. (2025). Mechanochemical-Activated Organomontmorillonite for Uranium Pollution Protection. AppliedChem, 5(1), 5. https://doi.org/10.3390/appliedchem5010005

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