FEASIBILITY STUDY FOR THE USE OF NATURAL MORDENITE IN PURIFICATION PROCESSES OF WASTEWATERS FROM PHARMACEUTICAL POLLUTANTS

The present research concerns the possibility of adsorptive removal of the frequently used antibiotics – moxifloxacin and norfloxacin on natural zeolites – mordenite and its acid-modified form from aqueous solutions. The adsorption of the above-mentioned antibiotics on the selected natural zeolite samples was investigated under static and dynamic conditions. Adsorption experiment under dynamic conditions carried out using the specially constructed dynamic type of laboratory equipment. Based on the data of chemical, IR spectroscopic, X-ray diffraction analyses, it has shown that when the zeolite adsorbent is treated with an acid, an equivalent exchange of extra framework cations for a hydrogen ion and dealumination occurs and silanol groups are formed at the same time. The

Fluoroquinolone antibiotics are synthetic antibacterial drugs with a 4-quinolone basic structure ( Figure 1) which can inhibit the proliferation of many Gram-negative and Gram-positive bacteria [4,5]. The presence of fluorine atoms is a critical factor for high activity; the activity of this antibiotic increased by approximately tenfold. So, they have been widely used in medical and veterinary practice. However, as a result of antibiotic overuse, public concern about FQs has significantly been increasing in the past decades. The environmental concern of FQ antibiotics residues in the aquatic environments is not only on their potential to increase antibiotic resistance but also on their unfavourable ecotoxicity profile [6]. The presence of FQs residues in effluents from households, hospitals, and pharmaceutical industries is a significant cause of acute and chronic toxicity, as well as the emergence of resistant bacteria. Consequently, removal of FQ residues from the environment is a crucial issue [7,8].
Adsorption method is a well-controlled process that allows to removing various contaminants up to almost any residual concentration, regardless of the chemical stability of pollutants, the absence of secondary contaminants, and the cost efficiency associated with the repeated use of the sorbent [9].
The adsorption method for removal of organic pollutants from wastewaters is based on the difference variety in energies of Van der Waals interaction of dissolved substance molecules and solvent with solid adsorbent surface or on formation of chemical bonds between dissolved substance and surface groups of adsorbent. The possibility of adsorption of porous structure materials is also associated with the developed adsorption surface and its availability. The competition of water binding and adsorption processes in aqueous solutions is caused by differentiation and selection of adsorbents for removal of organic and inorganic substances from wastewaters [10]. Mordenite has a tetrahedral structure in which a silicon or aluminum atom is surrounded by four oxygen atoms. Tetrahedrons are grouped in chains ( Figure 2). Inside the chains, there are adsorption cavities [11,12]. Mordenite structure is penetrated by a two-dimensional system of 8-and 12-membered channels. Some cations are localized in eight-membered channels, and some -in large channels. Cations block narrow 8-membered channels, and only large channels with diameter of 0.67 nm are open for the diffusion of molecules; Na-, Ca-mordenites are unblocked after treatment with hot hydrochloric acid. Chemical modification provides ample opportunities for regulating the molecular sieve and catalytic properties of mordenite. The limiting adsorption volume of both narrowporous and wide-porous mordenite varies between 0.076-0.09 cm 3 g -1 . The difference in the molecular-sieve properties caused by the different arrangement of sodium cations, which in narrow-pore mordenite are located in large twelve-membered channels (0.59-0.47 nm) and thus, narrow their effective diameter. In wide-pore ones they are localized in the depths of niches composed double eight-membered rings (0.39-0.47 nm) and do not prevent the penetration of large molecules into large channels [11][12][13][14][15]. The projection of the mordenite structure in the direction of the main channels is shown in Figure 3.

Experimental:-
Natural mordenite with zeolite-phase content of 50-60% from Bolnisi-Ratevani deposit (Georgia) was used in this study. The chemical composition of mordenite varies within narrow range, the ratio of Si/Al is within 4.5-5.5. Chemical composition of the studied samples of mordenite is presented in Table 1. The analysis of the chemical composition of natural and treated mordenite samples was carried out using Spectroscout XEP-04 (Germany). Exchange cations are represented by calcium and sodium; potassium and magnesium are present in smaller quantities. Chemical modification provides ample opportunities to regulate the molecular-sieve properties of mordenite. The high value of the silicate module (Si/Al) determines the high thermal-and acid-resistance of the mineral.
The X-ray diffraction analysis of the studied samples of mordenite (initial and acid-modified) was carried out using X-ray diffractometer -Drone-4 (Russia). The obtained diffractograms are presented in Figure 4. The diffractogram of the initial natural mordenite treated by boiling acids of different concentrations show that despite of changes in the chemical composition, the crystal lattice of zeolite remains unchanged after acid treatment. The diffractogram of natural (native) and treated boiling 5N solution of hydrochloric acid (HC1) zeolite is similar to the diffractogram of original mordenite. Leaching of zeolites with acids is characterized by the fact that acid exposes the mineral frame, which preserves the architecture of the zeolite. At the same time, the value of the silicate module increases. It should be noted that treatment of mordenite with 10N HC1 increases this value to 24.45 ( Figure 4), but the diffractogram of this sample indicates the destruction of the crystal structure and the mordenite lattice becomes more defective, which is probably a consequence of leaching. Figure 5 presents IR spectra of mordenites -initial and acid-modified with 5N HC1 and 10N HCl. IR-spectrometric study was performed using Agilent Cary 630 FTIR Spectrometer (USA) in the range of 350-1300 cm -1 . Comparison of the IR spectra of natural mordenite with an acid-treated sample (5N HCl) showed the presence of all characteristic absorption bands of the aluminosilicon-oxygen framework of the zeolite. A somewhat different spectrum is observed for the sample modified with 10N HCl, which is probably caused by the deformation of the zeolite structure [13].  Along with the chemical and mineral compositions of natural zeolites, textural and structural characteristics are fundamental in the study and development of adsorbents involved in various adsorption processes. Nanomodification of natural zeolites with solutions of hydrochloric acid or alkali leads to a significant increase in the values of physical-mechanical and textural characteristics: the specific surface area of the zeolite increases on average up to five times, and the total pore volume doubles. To characterize the texture parameters of the studied samples (specific surface area, volume of mesopores and total pore volume, pore-size distribution), the method of low-temperature adsorption-desorption of nitrogen at 77.2 K was used. The procedure was carried out on the equipment -Micromeritics Asap 2020 Plus Physisorption Analyzer (USA). The obtained isotherms were processed by the Barrett-Joyner-Hallend (BJH) method [16,17].
Among the studied mordenite samples, the natural mordenite sample is characterized by the lowest specific surface area, density, porosity and total pore volume. Nano-modification of samples with acid solution is accompanied by an increase in specific surface area (to 70.84 from 162.33 m 2 /g), volumes (to 0.086 from 0.097 cm 3 g -1 ), pore diameters, porosity and a decrease in true density. The most significant changes in the structure of the zeolite occurred during its treatment with 10N boiling acid solution. By varying the type of modifying agent, it is possible to form a porous structure at the level of micro-, meso-and macro-pores.
The adsorption capacity of zeolites was studied under static and dynamic conditions. Adsorption capacity of mordenite samples by water vapour was studied by excitation (desiccant) method (pre-activation temperature 400 °C ) ( Table 2). As it is obvious from the above data, modification of natural mordenite with 5N acid solution increases its adsorption capacity by 7.3%, while treatment with 10N acid solution leads to its decrease ( Figure 6) [18][19][20]. Investigation of adsorption properties of natural and modified samples of mordenite in relation to selected FQ antibiotics was carried out under dynamic conditions on the specially designed dynamic type equipment. The change of FQ antibiotic sorbate concentration at the outlet of adsorption column was determined by the validated HPLC method [5]. The HPLC analysis was performed using LC-20AD Prominence Shimadzu HPLC System (Japan) and HPLC column -Agilent SB-C18 4.6×250 mm, 5 µm (USA). To prepare model aqueous test solutions of each FQ antibiotic -moxifloxacin and norfloxacin, the analytical standards (MilliporeSigma) and "Aqua FX" water purification system, analytical balance -LEX-210 were used. All the measuring equipment have been properly calibrated. Figure 6:-Modification of the initial mordenite with HCl [19].
The concentration of the sorbate (FQ antibiotic) in the adsorbent before the start of the process is zero, the concentration of the sorbate in the input stream is constant.
The dynamics of sorption processes considers the spatio-temporal distribution of components between the phases of the system (one of which is solid), which occurs when these phases move relative to each other.
Experimental study of adsorption dynamics usually has two purposes: to check the adequacy of the model of the experiment and to obtain information from the output curves of adsorption needed to select adsorbents, their sizes and design. Adsorption process is characterized by probabilistic functions of particle distribution between mobile and solid phases, on the basis of which concentration distribution curves are found. For practical calculation of adsorption processes, a number of generally accepted simplifications were used in the work: 1) only one component of the flow is adsorbed; 2) The mobile phase is incompressible, and the concentration of adsorbed substance is so low that changes in flux density due to adsorption loss can be ignored; 3) The flow is moving in one direction at a constant flow rate [11,14].
The dynamic characteristics of the adsorption process were studied using the above-mentioned equipment which consists of three parts: 1) a glass adsorption column, 2) a cylindrical electric furnace, 3) a high-pressure pump. In dynamic condition through the prepared zeolite samples placed in a glass adsorption column, the studied FQ antibiotic sorbate solution at a concentration of 1.0 mg mL -1 was passed. The experiment was carried out at room temperature (22±3 °C), with the flow rate of 1.0 mL min -1 . When analysing the results obtained with the experiment, approximate test solutions for problems of equilibrium and non-equilibrium frontal dynamics of adsorption were used.

Results And Discussion:-
On the basis of experimental data, adsorption output curves describing the dependencies of adsorbed sorbate concentration growth behind the zeolite layer in time were constructed (the ratio of effluent concentration to influent concentration of the sorbate -C/C 0 versus time) which are shown in Figure 7.
The experiment was carried out until the adsorption capacity of the zeolite layer was fully worked out, i.e. until the sorbate concentration equal to its concentration in the initial solution (influent solution) appeared at the outlet of the adsorption column. The dynamic characteristics of adsorption of moxifloxacin and norfloxacin calculated from experimental data are summarized in Table 3 The dynamic adsorption capacity of the adsorbent layer (a d ) characterizes the amount of substance absorbed before the moment of "breakthrough" (τ b ) and attributed to the mass of loading of the entire adsorbent. From the "moment of breakthrough", extracted substance appears in the outgoing stream and its concentration begins to increase progressively. Between the time of protective action of the sorbent layer and the length of the adsorbent (L) there is a dependence determined by the formula of N. A. Shilov [11]: where K is coefficient of the protective action, showing the time taken by the 1 cm high adsorbent layer retains the absorbed substance in a stationary mode, cm, τ o is time loss of protective action associated with the initial period of formation of the sorbate distribution curve, min; L is height of loading of the adsorbent layer (in this case 24 cm).
The equilibrium adsorption capacity of the sorbent layer depends on the nature of the sorbate and adsorbent (a eq ) and was calculated by the formula [11]: where, W is the volumetric flow rate of the sorbate influent solution, cm 3 min -1 ; τ b is time of protective actions of the sorbent layer, min; C o -initial concentration of pollutant (influent concentration), mg mL -1 ; Δτ is the difference in sorbent layer performance time to complete saturation -τ eq , i.e., the time of occurrence of the maximum (equilibrium) concentration and breakthrough concentration -τ b through the sorbent layer, min; φ is the coefficient of the symmetry of the output curves or the distribution curves, V -the mass of the loaded adsorbent.
The height of the working layer -L o , determined from the output curves reflecting pollutant concentration rises behind the adsorbent layer with time, is an important characteristic of the dynamics of the adsorption process. The value of L o , the length of the mass-transfer zone, was determined on the basis of the obtained output curves according to the equation reported in the papers. The efficiency equilibrium adsorption capacity of the adsorbent layer (the degree of utilization of the adsorbent) -η, % is determined by the ratio of the dynamic adsorption capacity of the layer to the equilibrium adsorption capacity of the sorbent layer (Table 3, 4) [4,11].  Experimental data obtained FQ antibiotics adsorption on the studied mordenite samples in dynamic conditions indicates an improvement in the process of adsorption of NOR and MOX by the sample of mordenite modified with 5N hydrochloric acid solution. The efficiency of the equilibrium adsorption capacity of the sorbent layer (η) increases. The height of the working layer (L o ) decreases, which increases the repeatability of sorbent "elementary" volumes in the process of repeated acts of sorption-desorption. The obtained data clearly demonstrates the influence of the pharmaceutical pollutant nature and chemical structure on the process of its extraction from the aqueous solution by zeolite.

Conclusion:-
It has been established that natural and acid-modified (5N HCl) mordenite can be proposed as an eco-friendly, highly efficient adsorbent for the extraction of a fairly wide range of organic substances from aqueous medium, including pharmaceutical pollutants. Most adsorption processes are carried out in equipment with a fixed dense layer. It should be noted that the use of granular zeolites (mordenite) in close-packed adsorbers is very effective for removing ecologically significant concentrations of many pharmaceutical pollutants and many other toxic organic compounds. From the economic point of view, it is very important that it is possible to recycle the used high-silica sorbents after their regeneration by the oxidation method, because such zeolites are resistant to oxidizing agents (such as hydroxyl radicals, ozone, etc.), and can be regenerated without removing them from the adsorber.