INTRODUCTION

Residues from pharmaceuticals have been considered a major problem affecting the environment and living organisms especially antibiotics and steroids which cause resistance in natural bacteria [1]. Many techniques have recently been proposed and used for removing these pharmaceuticals from wastewater such as activated sludge, osmosis, zonation and membrane bioreactor system. Although such techniques have been used to date, they showed poor removal efficiencies for some pharmaceutical compounds [2, 3]. Physiochemical techniques such as adsorption have shown high efficiency in removing pharmaceutical compounds. Mefenamic acid [N-(2, 3-xylyl) anthracitic acid] is one of those pharmaceutical compounds that can cause serious problems to the environment if it is not being treated [4]. Different adsorbents have been utilized for adsorbing mefenamic acid (MFA) from water sources such as activated carbon, which showed a maximum removal efficiency of 60% after 120 minutes when combined with ultraviolet radiation [5]. According to Rodrigo’s team, 100% and 96% of MFA were removed from wastewater when activated carbon and red mud, respectively, were used as adsorbents before oxidation with chlorine [6]. In 2020, a novel polyurea formaldehyde with bentonite clay composite was utilized as an adsorbent for removing MFA. The results show that this adsorption is physical in nature and the adsorption potential is 16 mg\g at 47^oC and pH of 1.5. Recently, semiconductor nanostructures have been reported as good adsorbents for removing dyes and pharmaceutical compounds from water source [4, 7, 8]. Among these materials, Tin dioxide (SnO₂) has been studied extensively and used in various fields such as solid-state sensors, catalysts, transparent conductors, Li-batteries. SnO₂ is an n-type semiconductor that has a wide band gap (3.6 eV) which activated only UV irradiation [9, 10]. This activity is restricted by the recombination of the charge carriers (electrons and holes). Extending the absorption of SnO₂ to the visible region requires narrowing the band gap, which can achieve by doping with non-metallic elements. This project aims to synthesize and use SnO₂ nanoparticles, as an adsorbent, for the first time, to remove MFA from wastewater. The impact of SnO₂ dose, contact period, pH solution, and temperature was investigated. The adsorption and kinetic parameters were also studied to provide significant information about the adsorption process.

MATERIAL AND METHODS

Materials

Tin oxide pent hydrate (SnCl₄.5H₂O) was purchased from CDH and used without any purification. Concentrated ammonia was purchased from Charlie. De-ionized water was used in this project as a solvent.

Preparation of tin oxide nanoparticles (SnO₂ NPS)

To Prepare SnO₂ nanoparticles, 0.1752 g of tin (IV) chloride pentahydrate (SnCl₄.5H₂O) was dissolved in 50 mL of deionized water and stirred for 15 minutes. Thereafter, ammonia solution (25%) was added drop wise to the above aqueous solution under continuous stirring. This was carried out until the solution pH reached 8 and the white precipitate appeared. Deionized water was used to wash the precipitate (three times) to get rid of unwanted materials, in particular, chloride ions. To separate the precipitate, a 4000 rpm centrifuge was utilized for 15 minutes. The sample was dried in a drying oven for 24 hours at 100°C before annealing at 600°C.

Adsorption Studies

The stock solution of MFA was prepared by dissolving 1 g of MFA in 1000 mL of deionized water. This solution was then used to prepare standard solutions with different concentrations (30, 40, 50, 60, 90 mg/L). The best concentration of MFA was measured from the calibration curve and it was found to be 30 ppm.

Characterization

Many techniques were utilized to characterize the prepared SnO₂ nanoparticles. X-ray diffract meter (X pert pro-Analytical) was used to study the crystalline structure of the particles, in addition to the crystallite size. Ultraviolet–visible diffuse reflectance (UV-Vis DRS) spectrum was recorded in the range 200- 1000 nm using a 2550-UV spectrophotometer, Shimadzu-Japan. Fourier-transform infrared spectroscopy (FT-IR) (IRAffinity-1S instrument Shimadzu, Japan) was used to examine the functional group that presents on the SnO₂ particles surface. Field emission scanning electron microscope (Zeiss SIGMA VP-FESEM) and transmission electron microscope (JEOL JEM-2100 LaB6) were used to the characterization of the morphology of the particles and to measure the particle size. The presence of elements in the prepared particles was confirmed by Energy dispersive X-ray spectroscopy (EDS). The adsorption behavior of SnO₂ nanoparticles was studied using UV–Vis spectrophotometer (Double Beam-1800, Shimadzu-Japan).

RESULTS AND DISCUSSION

Optical properties

The probable mechanism for the formation of SnO₂ nanoparticles through the chemical precipitation method is that the addition of aqueous ammonia solution to the stannous salt (to make the solution pH 8) led to form hydroxide ions and subsequently form the precipitate of Sn(OH)₄. During the annealing process (600 ^oC), the precipitate was decomposed and SnO₂ nanoparticles formed [11].

UV-Vis Diffuse reflectance spectroscopy was used to study the optical properties of calcined SnO₂ nanoparticles. The SnO₂ nanoparticles’ spectrum, which is given in Fig. 1(a), shows a broad intense absorption edge rising at 320 nm. This broad peak is associated with the presence of a defect on the surface of the particles due to increasing the surface-to-volume area as the particle size approaches the Nano scale levels [12, 13].

The energy band gap of calcined SnO₂ nanoparticles can be estimated from the absorption spectrum using Tauc plot [14].

Here, n is the electronic transition which is equal to 0.5, [15] h is the Plank’s constant, C is a constant, α is the absorption coefficient, and ν is the frequency of light. By plotting (αhν)² vs photon energy (hν), one can measure the energy band gap by extending the linear part of the spectrum to the photon energy axis (x axis). Tauc plot gives a band gap of 3.7 eV (see Fig. 1(b)) which is larger than that of bulk SnO₂ (3.6 eV). This could be because the quantum confinement phenomenon [16]. It is well known that as the particle size decreases the band gap of semiconductor materials increased, the absorption edge is shifted toward lower wavelengths, and the quantum size effect becomes obvious. Therefore, this finding confirms the formation of SnO₂ at the nanoscale level.

FTIR analysis

FT-IR spectrum of SnO₂ nanoparticles was recorded in the range of 500-4000 cm^-1. The sharp peaks centered between 600 and 650 cm⁻¹ are assigned to asymmetric and symmetric Sn-O-Sn stretching vibrations which confirm the formation of SnO₂ nanoparticles [12, 17]. The peak centered at 585 cm^-1 representing the stretching mode of symmetric O-Sn-O (oxide-bridge functional group) [18]. The peak appears at 1441cm^-1, suggesting the presence of a Sn-OH bond [19]. The peaks around 2300 to 2400 cm-1 are probably attributed to the presence of CO₂ group It seems from the FTIR spectrum that there are no water molecules adsorb of the prepared particles surface due to the annealing process at 600 ^oC.

XRD analysis

The phase and the crystal structures of SnO₂ nanoparticles were studied using XRD. It is clear from Fig. 3 that the prepared SnO₂ nanoparticles have a polycrystalline structure and the diffraction peaks can be assigned to tetragonal rutile SnO₂ (based on JCPDS card No. 41-1445) [20] . This finding is quite similar to previous works [21, 22]. No Phase corresponding to any impurity can be detected in XRD pattern. The grain size of the prepared sample was calculated using Debye Scherer equation:

Where β is the complete width at half maximum and D is the average crystallite size “in radians”, K is the shape factor 0.9, λ is the wavelength of X-ray (0.154 nm), and theta is the Bragg’s angle “in radians”. The average crystallite size was found to be around 14.4 nm. The average distance between lattice planes was 0.18 nm.

Morphological and EDX Analysis

The surface morphology of SnO₂ NPS was examined using electron microscopy. Fig. 4 (a) shows that the as-prepared SnO₂ nanoparticles have spherical shapes and some of them are clustered together. The size of synthesized particles was measured using the image J software [23, 24] and the average size was found to be ~20 nm. The TEM image in Fig. 3(b) shows that SnO₂ nanoparticles are uniform and they almost aggregate with each other. Fig. 3(c) shows the elemental analysis of SnO₂ nanoparticles and it is revealed that the prepared particles have Sn (95.1%) and O (94.9%) elements in their composition. It is worth to mentions that the appearance of the Au element peak in the EDS graph is referred to the coverage process of the sample with gold atoms during the EDS preparation sample [25, 26].

Adsorption of MFA by SnO₂

The impact of adsorption time

To study the effect of adsorption time, five solutions of MFA were prepared with different concentrations (30, 40, 50, 60, and 90 mg/L) from the stock solution and 0.6 mg of adsorbents (SnO₂ nanoparticles) was added to each flask. The flasks were placed on a shaker for 80 min at 298 K. Samples were analyzed at different time intervals (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 minutes). The adsorbent particles were separated using centrifuge (8000 rpm) for 15 minutes after the predetermined amount of time. The maximum absorbance for mefenamic acid was observed at 284 nm using a UV-Vis spectrophotometer, the clear supernatant was examined both before and after adsorption. Calculations were made for the removal effectiveness and adsorption capacity (Q_e) using equations 3 and 4 [4, 27]:

It is clear from Fig. 5 the removal efficiency increased rapidly during the first 60 minutes, and it became stable at 80, 90, and 100 minutes. This could be because the active sites on the absorbent surfaces will saturate, indicating reaching the apparent equilibrium [28, 29]. Thus, the best efficiency was obtained at the contact time of eighty minutes.

The influence of adsorbent dose

The effect of SnO₂ nanoparticles weight on the adsorption process of MFA was investigated. Fig. 6 shows that when the amount of SnO₂ particles increase the adsorption of the MFA on their surfaces increases too. The removal percentage of MFA increased from 33% to 83.8when the amount of SnO2 particles increased from 0.004 to 0.6 g. The best explanation to this invreament is the presence of large number of active sites and the high surface-to-volume area [30].

The influence of PH

Due to the high surface area of as-synthesized SnO₂ nanoparticles, they consider an excellent candidate to remove pharmaceutical compounds and dyes from aqueous solutions. PH was varied from (3,6, and 10) by adding either 0.1 M of HCL or 0.1 M NaOH to the solution containing 0.6 g of SnO₂ and 20 mL MFA. The impact of pH on MAF adsorption effectiveness is presented in Fig. 7

The maximum removal efficiency (92.6%) was obtained at pH 3. Due to the positive charged of SnO₂ nanoparticles and negative charge of MFA, electrostatic attractive forces appear leading to enhance the adsorption at acidic pH. Competition between anionic MAF molecules and hydroxyl anions is what causes the reduction in the adsorption capability in an alkaline medium [31].

The effect of temperature on the adsorption process

To determine the temperature required for adsorption, different samples were prepared by mixing 30 ppm of MFA solution with 0.6 g of the prepared tin oxide particles at different temperatures (298, 308, and 318 K). These samples were stirred for 15 minutes at a speed of 140 tr/min and then filtered. The absorbance of the supernatants was then recorded by UV-Vis spectrophotometer. The optimal temperature for adsorbing MFA on the SnO₂ particles surface is 318 K (see Fig. 8(a)) indicating an endothermic adsorption process due to increasing the active adsorption sites and increasing the mobility of mefenamic acid molecules as the temperature increased [32].

Enthalpy change (∆H°), Gibb’s free energy change (∆G°), and entropy change (∆S°), which are thermodynamic parameters, were determined by Equations. (3) to (6) [33]. Fig. 8(b) illustrates the effect of temperature on the adsorption behavior of SnO₂ NPs using Van’t Hof plotting.

The following equations represent the thermodynamic parameter values:

Where, equilibrium adsorption constant is Keq, absolute temperature is K, and ideal gas constant is R. Table 1 reveals that the adsorption procedure is endothermic and spontaneous. The endothermic nature of the adsorption cycle is indicated by the fact that the adsorption capacity starts to rise as the temperature rises. Fig. 9 shows an increase in the MAF’s adsorption efficiency from 83.6 to 87% as the temperature increased from 298 to 318 K. In addition, the positive value of ΔS° suggests a high level of species randomization at the MAF adsorbent-adsorbate interactions. Last but not least, the values of H° show that the adsorption mefenamic acid is physisorption [34].

Adsorption isotherms

Isotherms are fundamental prerequisites to the adsorption system design because they allow for the correlation between the residual concentration of MAF in an aqueous solution and adsorption capacity. The Langmuir and Freundlich isotherm models were used to augment by making adjustments to the experimental findings on how temperature affects the adsorption process as shown in Table 2 and Fig. 10.

Langmuir isotherm

The monolayer adsorption described by Langmuir equation has a thickness of one molecule. To analyze the mefenamic acid adsorption process, the Langmuir equation was expressed in its linear form [35]:

a and b are the monolayer coverage-related maximum adsorption capacity (mg/g) and the Langmuir constant (mg/L), respectively. (Fig.10) represents the linear plot of Langmuir isotherm MFA.

Freundlich isotherm

Freundlich equation was used to demonstrate the multilayer adsorption. The following expression provides the Freundlich model [36].

Where n is a variable related to adsorption intensity and Kf (L/mg) is the Freundlich isotherm constant. Fig. 10 depicts the linear plot of the mefenamic acid Freun-Dlich equation (a and b).

CONCLUSION

The current study suggests a new route for the removal of mefenamic acid from aqueous solutions using metal-oxide nanoparticles. The prepared SnO₂ particles have a large optical band gap (3.7 eV) concerning bulk SnO₂ due to the quantum size effect. After research, it was found that 0.6 g of SnO₂ particles are able to adsorb mefenamic acid (MFA) from wastewater with a removal efficiency of ~ 92.6% at pH 3. Based on the thermodynamic functions, the adsorption process of mefenamic acid is endothermic and spontaneous.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.