Research of structure and sorption properties of montmorillonite modified with hexadecylthymethylammonium bromide

The object of research is a natural layered silicate – montmorillonite of the Cherkasy deposit (Ukraine) with the general formula (Ca,Na)(А1,Mg,Fe)2(OH)2[(Si,А1)4О10]×nH2O. The chemical composition of the mineral: SiO2 – 51.9 %, Аl2O3 – 17.10 %, Fe2O3 – 7.92 %, MgO – 1.18 %, Na2O, K2O and CaO up to 2 % and Н2О – 8.78 %. Montmorillonite is characterized by a significant dispersion of particles and the presence of a large number of sorption centers on its surface is capable of cation exchange. One of the most problematic places is that montmorillonite is practically incapable of removing pollutants present in water as anions. In order to obtain sorbents capable of removing heavy metal anions, the surface of montmorillonite was modified with the cationic surfactant hexadecyltrimethylammonium bromide.During the study, X-ray diffraction analysis, scanning electron microscopy, infrared spectroscopy, and thermal analysis were used to study the structure of the initial montmorillonite and its organomodified forms. The spectrophotocolorimetric method is used to study the sorption properties of composites.The work confirms that hexadecyltrimethylammonium bromide molecules are sorbed not only on the outer surface of the particles, but also migrate between the aluminosilicate packets of the layered structure of montmorillonite. Sorption studies have confirmed that the use of organomodified forms of montmorillonite has increased the degree of extraction of chromium (VI) ions from 32 % to 96 %. The resulting sorbents make it possible to purify contaminated water with a chromium (VI) concentration of 1 mg/dm3 to the maximum permissible concentrations. This is due to the fact that the organomodification of the surface of montmorillonite has a number of features and allows to change the structure of the original mineral, as well as recharge the clay surface from negative to positive. This makes it possible to use organoclay to remove inorganic toxicants in anionic forms. Compared with similar known ones, the obtained composites provide the removal of even trace amounts of heavy metal anions from aqueous media.


Introduction
For the extraction of heavy metal ions from aqueous media, especially if they are in trace amounts, sorption methods are promising. In this purification technology, activated carbon and synthetic ionites are most often used, but sorbents based on clay minerals are becoming increasingly important. Such materials are characterized by relative low cost, environmental friendliness and general availability of these raw materials. The high sorption properties of clay minerals are due to the significant dispersion of particles and the presence of a large number of sorption centers on their surface capable of cation exchange. Moreover, the montmorillonite minerals have the highest sorption properties, the large deposits of which are explored and developed in Ukraine. However, clay-based sorbents are practically incapable of removing pollutants present in waters in the form of anions, which in some cases are the most dangerous (complexes of chromium, uranium, arsenic, etc.) [1]. That is why the development of effective and safe sorbents with high selectivity will ensure the rational use of natural resources, compliance with environmental standards. This determines the relevance of scientific and technological developments to solve the problem of directed synthesis of effective sorbents based on natural silicates with studied properties and structure.
The object of research is a natural layered silicate -montmorillonite of the Cherkasy deposit (Ukraine) with the general formula (Ca,Na)(А1,Mg,Fe) 2 [2].
The subject of research is the structure of montmorillonite and synthesized organomontmorillonites, as well as their sorption properties. The structural cell of montmorillonite during adsorption of vapors of polar substances can increase along the C axis by 0.3-1.4 nm. As a result of the increase in the intermediate package space of layered silicates, gaps can be considered as micropores, the sizes of which can change during adsorption.
The aim of research is investigation of the structure of sorbents obtained on the basis of natural montmorillonite and their sorption properties with respect to heavy metal ions using chromium (VI) as an example.

Methods of research
To achieve the aim set in the work, let's use the methods of X-ray phase analysis (XRD), infrared (IR) spectroscopy, scanning microscopy, thermal analysis, as well as a spectrophotometric method for studying the sorption properties of synthesized materials.
Natural montmorillonite was previously converted to the Na form according to the procedure described in [3]. Modifications of Na-montmorillonite (MT) with a cationic surfactant were carried out according to the method described in [4]. Thus, the obtained samples with the ratio of the amount of surfactant in the cation of the exchange capacity of the mineral (surfactant/CEC) are 1 and 2, and are named OMT1 and OMT2, respectively.
X-ray analysis was carried out using a DRON-3M diffractometer (Russian Federation), with a computer scanning system and recording a diffraction pattern. Filtered Cu-K α radiation was also used.
Microscopic studies were performed on a REM-106I scanning electron microscope (Ukraine). This microscope receives images of the surface of an object with high spatial resolution and depth of field in reflected and secondary electrons. For research, let's use the samples in a powdered state, with a fraction of α<0.1 mm.
IR spectroscopic studies of the samples were carried out on a Spectrum-One Fourier spectrometer (Perkin-Elmer) in the region of 4000-450 cm -1 with a twenty-fold scan.
Thermogravimetric studies of sorbents were carried out on a derivatograph Q-1500 device (Hungary) of the Paulik-Paulik-Erdey system. Samples were analyzed in a dynamic mode with a heating rate of 10 °C/min to 1000 °C in an air atmosphere. The mass of the samples was 1500 mg. The reference substance is Al 2 O 3 . In the experiments, a platinum crucible was used. Sensitivity on a scale of change in mass (TG) is 50 mg, on a scale of differential heating (DTA) -250 μV.
In sorption experiments, a solution of potassium dichromate K Cr O 2 2 7 and weighed portions of the synthesized composites were used. Sorption was carried out under static conditions with continuous shaking of the samples for 1 h (the volume of the aqueous phase was 50 ml, the weight of the mineral was 0.1 g, the concentration of Cr (VI) in the sorption experiments was 1 mg/dm 3 ) according to the procedure [5]. Ionic strength (0.01 M) of solutions was established using NaCl. After the adsorption equilibrium was established, the aqueous phase was separated by centrifugation (5000 rpm) and the metal equilibrium concentration was determined in it by the spectrophotometric method (UNICO 2100UV, USA) using diphenylcarbazide reagent at a wavelength of 540 nm for chromium. Sorption of chromium ions was studied at pH = 6. Fig. 1 shows the diffraction patterns of non-oriented samples of montmorillonite and its organically modified form (OMT1). On the X-ray diffraction pattern, which corresponds to the initial montmorillonite (curve 1, Fig. 1), a series of basal reflexes is observed 1.240; 0.315; 0.253 nm and two-dimensional diffraction lines -0.447; 0.256; 0.224 nm. The presence of the indicated diffraction prints on powder X-ray diffraction patterns is characteristic of smectite [6,7], and narrow and intense lines with an interplanar distance of 0.425; 0.334; 0.245 nm relate to quartz. The X-ray diffraction pattern of the MT sample (curve 1, Fig. 1) shows a clear basal reflection of 1.24 nm with a peak center of gravity of 2θ = 6.8°, which is typical for this mineral in an air-dry state.

Research results and discussion
After surface modification of the montmorillonite HDT-MA (curve 2, Fig. 1), a shift of this reflection towards small angles is observed. In the X-ray diffraction pattern of the OMT1 sample, a peak is not observed at the indicated location, it shifts to the region 2θ = 4.55°, which corresponds to an interplanar distance of 1.87 nm. The lid of the structural packets of the mineral occurred as a result of the replacement of exchange cations in the interlayer space of montmorillonite with large volume HDTMA molecules [8][9][10]. Studies conducted in [3] showed that increasing the ratio of CEC/surfactants to 2 allows to increase the interplanar distance to 2.25 nm.
Montmorillonite and its organically modified form have been electronically studied. Electronic images of the MT surface are shown in Fig. 2, a. In these images, let's observe agglomerates of montmorillonite particles having the form of massive and curved plates. However, clay treated with cationic surfactant has a lower degree of aggregation and a more ordered structure (Fig. 2, b). The results of IR spectroscopy of montmorillonite, its organically modified form, and its iron-based composite are presented in Fig. 3. The IR spectra of the starting montmorillonite (curve 1, Fig. 3) have a typical appearance for this mineral with absorption bands at 694, 802, and 1030 cm -1 , due to vibrations of Si-O bonds, and bands at 530 and 906 cm -1 -vibrations of the Si-O-Al bond. A very intense band at 3625 cm -1 refers to stretching vibrations of O-H groups [11]. In the IR spectra of organomodified form montmorillonite (curve 2, Fig. 3), in addition to the main characteristic bands of montmorillonite, new bands appear at 2921 cm -1 and 2852 cm -1 , which correspond to the groups of (-CH 2 -) alkyl chains of HDTMA molecules that are sorbed on the surface of the particles. To determine the nature of the interaction of the applied modifier with the surface of a clay mineral, a thermal analysis was chosen as an additional method that will help to specify the results of XRD. According to the differential thermal analysis (DTA) data for the MT sample (Fig. 4, a), three endothermic effects are present on the curve (curve 3, Fig. 4, a). The first, an intense, low-temperature effect, is responsible for the release of adsorption and low-pack water, and the next two are responsible for the release of constitutional hydroxyl water [12,13].
To compare the research results, a thermal analysis of HDTMA is carried out (Fig. 4, b). For this sample, in the temperature range 93.9-112.5 °C, an endoeffect is observed without changing the mass of the sample. At this stage, a partial destruction of the structure of HDTMA occurs. At a temperature of 240-255 °C, let's observe an endoeffect, which corresponds to the melting point of HDTMA. With increasing temperature, let's observe the presence of endo and exeffective. It is impossible to analyze the DTA curve (curve 3, Fig. 4, b) in this form, since two opposite processes take place simultaneously: 1) melting, which is characterized by a significant absorption of heat (endoeffect); 2) combustion, accompanied by a large release of heat (exoefect).
Both processes are accompanied by weight loss. In the temperature range of 250-360 °C, a sharp weight loss is observed, which is associated with the burning out of surfactant in the sample. On the thermogavimetric analysis curve (TG) (curve 1, Fig. 4, b), each change in the slope (decomposition rate) corresponds to a change in the chemical composition of the substance, which is confirmed by the differential gravimetric analysis (DTG) curve (curve 2, Fig. 4, b). It can be argued that in the temperature range there are products of chemical transformation during heating. At a temperature of 595.0 °C, the substance completely burns out.
For the OMT1 sample, according to the DTA data (Fig. 4, c), in the temperature range 240-640 °С, let's observe an insignificant end-effect (240-270 °С), which corresponds to melting of the organic component. In the temperature range 270-620 °C, part of the organic component of the composite ignites. This process is accompanied by the appearance of a number of exothermic effects on the DTA curve. This thermal behavior of the samples can be explained by the pyrolysis of HDMA, which is adsorbed on a mineral. On the DTA curves, let's observe broad peaks, as a result of the presence of various forms of the introduced surfactant: monomers, micelles, which were formed as a result of thermal decomposition of HDTMA. In this temperature range, let's also observe the imposition of an endoefect with a maximum at 452 °C, which corresponds to the allocation of constitutional water. The burnout of organic matter ends at 620 °C.
Thermal analysis of the sample with organomontmorillonite with a twice as high HDTMA content was also carried out as compared with OMT1 (sample OMT2) (Fig. 4, d).
A comparison of the results of DTA and TG with organomontmorillonite with varying degrees of surface modification of montmorillonite of cationic surfactant allows to state that surfactant molecules are firmly bound to the surface of the mineral. The burning out of the organic component from the composite occurs gradually at a higher temperature than the melting and burning out of HDTMA, equal to 250 °C.

ISSN 2664-9969
The DTA curve (curve 3, Fig. 4, d) can't be analyzed in this form, since two opposite processes take place simultaneously: melting (endoeffect) and burning (exoefect). Both processes are accompanied by weight loss. In the temperature range of 250-360 °C, a sharp weight loss is observed, which is associated with the burning out of surfactant in the sample. On the thermogavimetric analysis curve (curve 1, Fig. 4, d), sections with different slopes corresponding to a change in the decomposition rate are confirmed by the differential gravimetric analysis (DTA) curve (curve 2, Fig. 4, d).
It can be argued that in the temperature range there are products of chemical transformation during heating. At a temperature of 595.0 °C, the substance completely burns out.
At a pH close to neutral, which corresponds to natural waters, chromium forms only anionic compounds. So, the dominant form of chromium is CrO 4 2− , in the presence of significantly lower amounts of HCrO 4 − ions and Cr O 2 7 2− ions in trace concentrations. Sorption studies have confirmed that the removal of the anionic forms of Cr(VI) by montmorillonite is negligible. At the same time, for the modified OMT1 and OMT2 samples, a significant increase in the sorption values is observed, which proves their efficiency with respect to Cr(VI) ions compared to the initial sample ( Table 1).
The results of checking the effectiveness of the developed sorbent in model waters with a Cr(VI) ion content of 1 mg/dm 3 at pH = 6 are presented in Fig. 5.
Studies confirm that the purification of aqueous media by synthesized OMT1 and OMT2 composites from Cr(VI) ions reached the maximum permissible concentration (MPC), which is 0.05 mg/dm 3 [14].

Conclusions
During the modification of the mineral, cationic surfactant molecules are sorbed not only on the outer surface of the particles, but also migrate between the aluminosilicate packets of the layered structure of montmorillonite, replacing ions of Na + , K + , Mg 2+ , Ca 2+ , etc. in ion exchange positions. This is evidenced by the shift of basal reflexes toward large interplanar distances in the diffraction patterns of the modified samples from 1.24 nm to 1.87 nm.
The results of IR spectroscopy confirmed that the groups of (-CH 2 -) alkyl chains of the HDTMA molecules ware successfully sorbed on the surface of montmorillonite particles.
Scanning microscopy results show that montmorillonite treated with cationic surfactant has a lower degree of aggregation and a more ordered structure compared to the starting material.
The study of the thermal properties of organomodified montmorillonite makes it possible to refine the X-ray phase analysis data and confirmed the formation of stable bonds between surfactant cations and the surface of montmorillonite. This makes it possible to carry out directed synthesis of organoclay with certain properties of the mineral surface.
The synthesized organomontmorillonites are capable of removing up to 96.25 % of Cr(VI) ions, which is 3 times higher than the starting montmorillonite and reach the MPC purification levels. Thus, the use of montmorillonite as a raw material for the production of sorbents contributes to a comprehensive solution of environmental issues, resource conservation and technology for the production of sorbents for the extraction of anionic forms of heavy metals from aqueous solutions.