PREPARATION OF POROUS SILICA NANOCOMPOSITES FROM MONTMORILLONITE USING SOL-GEL APPROACH

Об’єктом дослiдження є монтморилонiт, який завдяки своїм властивостям i будовi проявляє високi сорбцiйнi характеристики. Однак суттєвою перешкодою його використання в промислових технологiях водоочищення є схильнiсть монтморилонiту самочинно диспергуватись в водних розчинах на елементарнi структурнi пакети i утворювати стiйкi зависi. Це призводить до виникнення складнощiв при роздiленнi твердої i рiдкої фаз пiсля проведення процесу сорбцiї. Авторами був використаний золь-гель метод синтезу нанокомпозитних матерiалiв на основi монтморилонiту з використанням тетраетоксисилану в якостi гелеутворюючої речовини. Синтез включає в себе реакцiю гiдролiзу тетраетоксисилану i наступною полiконденсацiєю молекул кремнекислоти з гiдроксильними групами монтморилонiту. Отриманi зразки поєднують в собi хорошi сорбцiйнi властивостi шаруватого мiнералу та мiцну каркасну структуру. Така структура синтезованих нанокомпозитiв забезпечується наявнiстю силоксанових зв’язкiв, якими елементарнi пакети монтморилонiту з’єднанi мiж собою. Це, в свою чергу, сприяє збiльшенню водостiйкостi експериментальних зразкiв. Згiдно з результатами реологiчних дослiджень, основнi процеси структуроутворення у вихiдних водно-спиртових суспензiях продуктiв гiдролiзу тетраетоксисилану та монтморилонiту вiдбуваються при концентрацiї кремнезему 1 %, що пов'язано з колоїдно-хiмiчними властивостями дослiджуваних систем. Обробка монтморилонiту продуктами гiдролiзу тетраетоксисилану приводить до утворення матерiалу з нижчою здатнiстю до набухання та кращим роздiленням рiдкої та твердої фаз. Оптимальний вмiст кремнезему у зразку, який знаходиться в iнтервалi 0,1–14 %, дає змогу зменшити оптичну густину розчинiв у 2,5 рази у порiвняннi з вихiдним монтморилонiтом. Синтезованi матерiали зберiгають достатньо високу сорбцiйну ємнiсть щодо вилучення катiонного барвника метиленового блакитного (до 158 мг/г), яка зростає при збiльшеннi вмiсту глинистого мiнералу. А також мають вищу селективнiсть (до 3,4 дм3/мг). Ключовi слова: золь-гель синтез, модифiкацiя поверхнi, iзотерма сорбцiї, монтморилонiт, тетраетоксисилан, метиленовий блакитний. Doroshenko D., Pylypenko I., Kornilovych B., Subbota I.


Introduction
One of the most important problems of our time is the removal of toxicants of organic and inorganic origin (ions of heavy metals, dyes and other compounds) from the aquatic environment, which are in large quantities in industrial wastewater. When entering the hydrosphere, these toxicants adversely affect the living organisms of our planet, as well as the state of the terrestrial ecosys tem as a whole.
When purifying industrial wastewater, or when obtai ning drinking water from contaminated sources of water supply, sorption methods are indispensable. In recent de cades, sorption studies of heavy and radioactive metals, as well as synthetic dyes of organic or inorganic origin in natural materials, for example, clay minerals (montmoril lonite, kaolinite, illite, etc.) have been widely conducted. The latter are cheaper and easily accessible compared to much more expensive activated carbon or ion exchange resins, traditionally used in chemical technology.
Layered silicates are characterized by complex surface chemistry due to the presence of active centers of diffe rent nature. These may be hydroxyl groups on the side faces of the mineral, silanol groups in places of crystal defects on the basal surfaces of planar particles, non stoichiometric isomorphous substitutions in the mineral structure, etc. Such set of active centers opens the pos sibility of applying many strategies for surface functiona lization [1].
Therefore, research in this area should be considered relevant, namely, the search for new methods for modi fying the surface of clay minerals to obtain modern ef fective sorption materials, but with the condition that the economic expediency of producing such sorbents is preserved.

The object of research and its technological audit
The object of research is montmorillonite. This is a na tural layered silicate with structural packets of type 2:1, in which one layer of alumina octahedra is located between two layers of siliceous tetrahedra.
Among other natural clay minerals, montmorillonite is characterized by a large specific surface area, as well as high values of cation exchange capacity. However, tech nological audit indicates that the obstacle for its wide application in water treatment technologies is the abi lity of clay minerals to disperse in an aqueous medium with the formation of stable slurry. As a result, after the sorption processes, difficulties arise with the separation of the liquid and solid phases. TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 4/3(42), 2018 ISSN 2226-3780

The aim and objectives of research
The aim of research is studying the structural and sorp tion characteristics of a synthesized semisynthetic nano composite material.
To achieve this aim, it is necessary to perform the following tasks: 1. Synthesize experimental samples by modifying the montmorillonite surface with solgel method using tetraeth oxysilane (TEOS) as a structurant reagent and investigating the rheological properties of the initial systems, as well as the structure formation processes in them, depending on the silica content.
2. Determine the effect of the composition of nano composites on the sorption properties with respect to the cationic dye of methylene blue.
3. Determine the effect of the composition of the ma terial on the value of the separation index of the solid and liquid phases.

Research of existing solutions of the problem
Recently, much attention has been paid to studying the methods of structural modification of clay minerals as promising sites for the creation of catalysts or adsorbents for various purposes. Both surfacelayered and layered minerals (montmorillonite, saponite, kaolinite) are used to modify the surface, the structural features of which determine the usefulness of the method of modification.
In particular, montmorillonite, due to its developed specific surface and sufficiently high cationexchange capac ity (up to 120-140 mmol/g), due to the presence of non stoichiometric isomorphic substitutions in the structure, is often chosen as an object of additional modification. Among such methods, mechanochemical or hydrothermal treatment is isolated during which the structure of the mineral is partially destroyed and new active centers ap pear [2,3]. Acid activation also makes it possible to improve cer tain properties of clay minerals. For example, the interac tion of montmorillonite with concentrated sulfuric acid leads to leaching of aluminum from the octahedral layer and improvement of the pore structure of the resulting material, which in turn improves sorption properties [4].
A separate method of structural modification is the pillaring based on intercalation of the polynuclear cationic hydroxocomplexes of metals to the interlayer space of laye red aluminosilicates with subsequent heat treatment. Such materials have an increased specific surface area, they do not swell in aqueous solutions and can selectively remove inorganic and organic toxicants of aqueous solutions [5][6][7].
Cationic surfactants are often used to modify the surface of clay minerals. Positively charged surfactant molecules replace exchange cations in interlayer space and on the surface of the mineral, creating a hydrophobic layer or a positively charged bilayer, which allows the removal of anionic dyes from aqueous solutions [8,9].
The presence of hydroxyl groups (≡Si-OH and = Al-OH) on the side surfaces of the mineral particles makes it pos sible to carry out their additional functionalization due to the formation of chemically bound molecules on the surface of the mineral. Such reagents include g amino propyltriethoxysilane and its derivatives, which make it possible to obtain highly selective sorption materials to extract, as a rule, anionic toxicants [10][11][12].
Composite sorption materials, which include not only clay minerals but also other components, for example, metal hydroxides, activated carbon, ion exchange resins, are also quite common [13].
Silica gel and other silica derivatives have found wide application in sorption processes [14]. The preparation of composite materials based on clay minerals together with silica opens the way to obtaining new functional materials, which is the aim of this work.
To modify the surface of montmorillonite, a rather simple and cheap reagent, sodium silicate, was previously used [15]. The obtained composite material is characterized by suf ficiently high sorption characteristics. However, when using tetraethoxysilane (TEOS) as a modifier, it is likely to produce composite materials with better properties, since TEOS hydrolysis processes result in the formation of sili cic and polysilicic acids of low molecular weight. The latter have an increased reactivity compared to the large molecules of silicic acid formed by the use of sodium silicate [16].
The rheological data were obtained with a Rheotest2 rotary viscosimeter (Germany) at 25 °C, and the experimen tal data were processed using the BinghamShvedov equa tion, which allows one to characterize the dispersion of clay mine rals. The BinghamShvedov equation has the form [17]: where D -strain rate; τ and τ 0 -stress and limiting shear stress respectively; η -plastic viscosity. The sorption properties of the samples were studied with the sorption of the cationic dye -methylene blue under static conditions, the concentration of which was de termined spectrophotometrically using a UNICOUV 2100 spectrophotometer (USA) at a wavelength of 665 nm. The weight of the adsorbent was 0.1 g per 50 cm 3 solu tion. The selected duration of contact between the solid and liquid phases was one hour, which corresponds to the establishment of adsorption equilibrium.
Determination of the optical density of the solutions was carried out according to the following procedure. A 0.1 g sample of sorbent was introduced into a conical flask of 100 cm 3 and 50 cm 3 of a 0.1 M NaCl solution was added and the flasks were shaken on the shaker for 60 min. The contents of the flasks were transferred into beakers per 100 cm 3 , and after settling for 10 minutes, solution was decanted with following measurement of its optical density at a wavelength of 400 nm and a cell length of 1 cm. 0.1 M NaCl was the reference solution.
To study the effect of the TEOS content of structural and sorption properties, a synthesis of experimental samples with different ratios NaMMT/TEOS /(H 2 O+C 2 H 5 OH) was carried out. The list of samples obtained is shown in Table 1. To prepare samples of the composite with an aqueous suspension of montmorillonite pretreated for 5 minutes with an ultrasonic dispersant, an appropriate amount of TEOS and ethanol was added, and the obtained mixture was treated with an ultrasonic disperser for 1 minute. After that, the pH of the mixture was adjusted to level 2 with sulfuric acid followed by ultrasonic treatment for 1 minute. After TEOS hydrolysis, the mixture was transferred to a magnetic stirrer, the pH was adjusted to 7 with sodium hydroxide solution and, with vigorous stirring under hea ting (60 °C), the mixture was held for hours. The resulting suspension was transferred to an evaporation bowl, held for 48 hours at 25 °C to complete gelation, washed from the salts with a 60 % ethanol solution in a vacuum fil ter and dried at 105 °C to constant weight. At the final stage, the sample was ground, screened and a fraction of 0.063-0.16 mm was selected for further studies. Samples for rheological studies were selected at the synthesis stage, after the addition of sodium hydroxide solution.

Research results
The processes of structure formation in dispersions of montmorillonite with TEOS in aqueous media with a high content of ethanol proceed by a different mechanism than sodium silicate in water [15]. Rheological studies have shown that the curves for the flow of such disper sions ( Fig. 1, a) have a less pronounced tendency to form hysteresis loops, indicating that there is practically no thixotropic properties in them. Such changes are due to a lower polarity of ethanol, enters into reaction mixtures with TEOS compared to water (ethanol dielectric con stant -24.6, water -81.0). Replacing part of the water with ethanol leads to a decrease in the intensity of in teraction between the molecules of the dispersion medium and the particles of the dispersed phase. According to the data [18,19], in solutions of ethanol or other alcohols, the selfdispersion of clay minerals is poorly expressed, which causes a decrease in the volume concentration of the dispersed phase.
In addition, there is a decrease in the values of the limiting shear stress and plastic viscosity. This is due to the fact that during the TEOS hydrolysis, in an acidic environment, in contrast to sodium silicate, polyconden sation processes predominate with the formation of low molecular weight silicic and polysilicic acids that have an increased reactivity. These silicic acid molecules interact predominantly with hydroxyl groups of the edge surface of montmoril lonite (≡Si-OH) due to polycondensation reactions [16]. The maxima on the curves of the limiting shear stress and plastic viscosity at a 1 % silica content (Fig. 1, b) correspond to the formation of the maximum amount of silica nanoparticles on the surface of clay particles. The latter are combined with each other due to interaction with the «side surface -silica globule -lateral surface» scheme. In most cases, the formation of loworder «house of cards» structures, in which clay mineral particles are placed chaotically, is most likely [20]. When the TEOS content in the system increases, silica globule grows, which is accompanied by an increase in internal stresses in the existing coagulationcondensation structure and, corres pondingly, a decrease in its strength.
This interpretation of the results is confirmed by electron microscopy and Xray analysis. Diffractograms of nanocom posite samples (Fig. 2) indicate a gradual decrease in the intensity of the basal reflex of montmorillonite (1.26 nm) with increasing silica content. For the raw montmorillonite (curve 100 %), the basal reflex is most clearly expressed. For pure silica (curve 0 %), the diffractograms have no pronounced reflexes, except two at 27 and 31⋅2Θ. The latter may belong to the residues of sodium sulphate, which is formed during the synthesis of samples and is ISSN 2226-3780 difficult to remove from the samples. The decrease in the intensity of the basal reflex of montmorillonite indicates a gradual transition of the system to a less ordered state with a chaotic arrangement of the mineral particles. In addition, a decrease in the mass fraction of montmoril lonite in nanocomposites can also reduce the intensity of the reflex, since the fraction of one of the crystalline phases is decreased. .

Fig. 2. Diffractograms of samples of nanocomposites with different contents of montmorillonite
The sorption capacity of nanocomposites was studied by the sorption of methylene blue (MB) (Fig. 3), one of the most common organic dyes in wastewater from industrial enterprises [21]. In chemistry of clay minerals, MB is also used as a standard reagent to determine the characteris tics of the surface. Specific surface of the samples (m 2 /g) was determined by the formula [22]: where a ∞ -the capacity of the monolayer (limiting adsorp tion) mol/g; N A -Avogadro number (mol -1 ); s m -the area occupied by one MB molecule on the surface (75 2 ). Isotherms of MB sorption with nanocomposite samples are shown in Fig. 4.
The experimental data are described quite well by the Langmuir equation with correlation coefficients from 0.934 to 0.998 (Table 2).
This dependence is primarily due to the suppressive sorption of MB ions by the surface of a clay mineral -at cation exchange centers (approximately equal in energy), while the silica phases contribute insignificantly to the total MB sorption. This is also confirmed somewhat lower by the values of the correlation coefficients according to the Freundlich model (0.817-0.984). An increase in the silica content leads to a decrease in the sorption capa city, which is due to the covering of the cation exchange centers by globules of silica gel on the basal surface of montmorillonite.  (Table 1) It has been established that the values of limiting ad sorption, as well as the calculated specific surface, for MB are reduced in the concentration range 0-3 % after silica ( Table 2). A further increase in the SiO 2 content has little effect on the value of the limiting adsorption. This depen dence may indicate the formation of the basic structure of the nanocomposite at silica concentrations of up to 3 %. A further increase in the silica content leads to precipitation of the latter on the free surface of the composite, which has little effect on the value of the limiting adsorption. Sufficiently high values of the limiting MB adsorption in the range 0-0.5 % SiO 2 are related, first of all, to the MB dimerization. The molecules (ions) of methylene blue, both in solutions and on the surface, are characterized by the formation of dimers even at concentrations of 2 mg/dm 3 [23], and increasing the ionic strength of the solution increases the content of dimers [24].
To determine the effectiveness of synthesized nano composites for use in sorption technologies, the optical density of solutions was studied after their contact with the adsorbent. This value is proportional to the concen tration of the dispersed phase in the solution and can serve to evaluate the efficiency of phase separation after the sorption process.
The results show that nanocomposites have a much higher separation index, lower swelling capacity and are more easily separated from aqueous solutions, unlike the original montmorillonite. The change in the optical density of the solutions with 1.64 for the original montmorillonite at 0.069 for the sample with 23 % montmorillonite (Fig. 5) indicates the formation of sufficiently strong aggregates from the sorbent particles after the TEOS treatment. There is a sharp (2.5 times) decrease in optical den sity, even with a small amount (3 %) of silica in the sample. Thus, treatment of montmorillonite with even a small amount of TEOS (from 3 % by weight) allows a significant increase in the separation index.
The obtained results indicate the prospect of develop ment for extraction of cationic dyes from aqueous solutions of sorption materials based on cheap natural raw materialmontmorillonite and the available silica compound -tetra ethoxysilane. At the same time, using even a small amount of the latter, it is possible to obtain economically advan tageous semisynthetic sorbents.

SWOT analysis of research results
Strengths. The main advantage of such sorption ma terials is low cost and environmental friendliness, since the main component is a natural mineral, whose reserves in Ukraine are quite large. Similar sorption materials are often obtained using rather expensive reagents and so phisticated hardware design of technologies, significantly increases their cost price.
Weaknesses. The use of tetraethoxysilane and ethanol as reagents for synthesis significantly increases the cost of the material.
Opportunities. A promising direction for further research is the development of a technology for granulating the nanocomposite obtained and investigating the possible re placement of valuable tetraethoxysilane and ethanol with cheaper analogues.
Threats. Availability of sufficiently effective sorption materials on the world market based on activated carbon or ionexchange resins creates a sufficiently strong com petitive environment.

Conclusions
1. The rheological properties of the initial systems based on the products of the hydrolysis of tetraethoxysilane and montmorillonite in waterethanol media were investigated. It has been established that the maxima on the curves of the limiting shear stress and plastic viscosity are observed with a content of 1 % silica in the samples.
2. It is established that the sorption properties with respect to the cationic methylene blue dye depend sig nificantly on the composition of the nanocomposites. It is shown that the limiting adsorption increases monotoni cally with increasing montmorillonite content. Taking into account other parameters, the optimum silica content in dry nanocomposites is in the range of 3-14 %.
3. It is found that the minimum silica content, which provides an effective separation of the solid and liquid phases after the sorption process, is 0.5 % SiO 2 in the system, or 14 % in terms of the dry sample.