Photocatalytic removal of naphthalene (C10H8) from aqueous environments using sulfur and nitrogen doped titanium dioxide (TiO2-N-S) coated on glass microbullets in presence of sunlight

Background and aims: Due to their toxicity and carcinogenic effects, polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (C10H8) are regarded as hazardous compounds for both humans and the environment, and it is essential to remove these contaminants from the environment. The present study aimed to remove naphthalene from a synthetic aqueous environment using sulfur and nitrogen doped titanium dioxide (TiO2-N-S) nanoparticles (NPs) immobilized on glass microbullets under sunlight. Methods: In this experimental study, TiO2-N-S NPs were synthesized using sol-gel process. The structure of NPs was investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray (EDX), and differential reflectance spectroscopy (DRS). In addition, using statistical analyses, the effects of parameters such as the initial concentration of naphthalene, pH, contact time, and the optimal conditions on naphthalene removal were investigated. Results: XRD patterns and SEM images of the samples confirmed the size of synthesized particles in nanometer. The EDX and DRS spectra analysis showed the presence of two elements (sulfur and nitrogen) and the optical photocatalytic activity in the visible region, respectively. The maximum level of naphthalene removal in the presence of sunlight was obtained to be about 93.55% using a concentration of 0.25 g of thiourea immobilized on glass microbullets at pH=5 and contact time of 90 minutes. Conclusion: The rate of naphthalene removal using the immobilized TiO2-N-S on glass microbullets was 93.55% in optimal conditions. Therefore, this method has an effective potential for naphthalene removal, and can be used to remove naphthalene from industrial wastewater.


Introduction
Water is a vital substance for human life and all people are aware of the importance of access to safe drinking water. One of the water pollutants is polycyclic aromatic hydrocarbons (PAHs). These compounds include a large group of organic compounds, with two or more aromatic rings that enter the environment through natural and industrial activities (1). Due to toxicity, mutagenicity, and carcinogenicity, these compounds have been ranked among the leading pollutants by the US Environmental Protection Agency (EPA), and the agency has declared a concentration of 0.5 mg/L of phenolic compounds in industrial effluent as permissible amount and even listed the lower concentrations of these compounds as permissible for aquatic and soil organisms (2). Naphthalene (C 10 H 8 ) is the first group of these hydrocarbons that has attracted the attention of many researchers in recent years as a common contaminant in water. Naphthalene is able to accumulate in water and soil for a long time (3). Exposure to naphthalene at high levels can cause hemolytic anemia, red blood cell breakdown (4), genetic mutations, fetal damage, congenital disorders, and kidney damage (5). Due to the extremely hazardous effects of naphthalene on human health, it is necessary to evaluate the likelihood of water pollution with this compound (6). Aerobic and anaerobic biological treatment, chemical oxidation, electrochemical oxidation, photochemical oxidation, coagulation, ion exchange filtration, adsorption onto activated carbon, buoyancy, biological degradation, and ozonation are important methods for the treatment of pollutants (7). Practically, there are limitations to the application of these methods, including the need for specific reactants, high cost, lack of removing small amounts of pollutants, production of by-products, and sludge production. Therefore, in recent decades, the use of new and more appropriate methods of wastewater treatment has been prioritized to maximize compliance with environmental regulations by researchers in this field. These methods also include those with chemical degradation, namely, advanced oxidation processes (AOPs) (8). AOPs include the effect of ultraviolet (UV) light on materials and the production of intermediates or active agents with oxidizing or reducing properties. The processes of AOPs themselves are subdivided into several large groups, the most important of which are photocatalytic oxidation processes.
Photocatalytic removal is carried out by UV radiation to the semiconductor surface such as ZnO or TiO 2 , and oxidation is accomplished based on the activity of hydroxyl radicals, which are highly reactive species (9). As anatase crystals, inhomogeneous photocatalysts, including TiO 2 , are the most popular photocatalysts due to their environmental friendliness, high optical activity, low cost, low toxicity, and high chemical and thermal stability (10). Over the past 30 years, TiO 2 nanoparticles (NPs) have been widely used as powders, but due to certain problems such as continuous mixing during work, high cost of filtration and centrifugation of powders for resuscitation, dispersion of NPs in solution and light blocking, efforts to stabilize photocatalysts on surfaces such as glass pellets, glass fibers (9,11), silica (12), activated carbon and zeolites and nanotubes are increasing. Optical degradation of pollutants as thin film fixation on a fixed bed has two major benefits, including the avoidance of high costs for catalyst separation (10,13,14) and lack of forming hazardous compounds due to advanced oxidation often used with other oxidants such as halogens (15). Besides, the broad TiO 2 gap has made it an efficient photocatalyst in the UV region. However, the sunlight naturally contains only 4% UV and the photocatalytic removal of naphthalene from aqueous solutions using UV increases the cost of purification and consumption, with practical limitations such as the low lifetime of the UV lamps and the consumption of electricity. UV lamps themselves are also considered a serious threat to the environment (10), so that methods such as doping with noble metals, metal ions and anions (C, N, S, F) are used to modify this inherent TiO 2 property and produce a new photocatalyst capable of retaining photocatalytic activity under visible sunlight (16,17). Muruganandham et al and Li et al doped the N and La on the TiO 2 lattice to reduce the energy gap and increase the TiO 2 optical activity in the visible light range (18,19). In this study, titanium dioxide (TiO 2 ) was first synthesized using the sol-gel method, which is an ideal method for preparing homogeneous products with high purity in the production of metal oxides. Moreover, the absorption of TiO 2 was transferred to the range of light by doping nitrogen (N) and sulfur (S). Furthermore, with the stabilization of TiO 2 -N-S on the bed of glass microbullets, the removal of naphthalene at concentrations of 5, 10, 15, 20, 25, and 40 ppm in the presence of sunlight was investigated considering the amount of total daily radiation and the optimal slope of pilot installation.

Photocatalyst synthesis
All chemicals were procured from Merck Co. (Germany), deionized (DI) distilled water from Zolal Company (Iran), and glass microbullets from Glass Seeds Company (Iran). In this study, TiO 2 -N-S NPs were first synthesized by solgel method (20). TiO 2 sol was prepared by hydrolysis of tetrabutyl orthotitanate (TBOT) in acidic solution. For this purpose, the mixture was first mixed with 2.5 ml of TBOT, 10 mL ethanol, and 2.5 mL acetyl acetone. After 30 minutes, a clear yellow solution was obtained. Then, 2 mL of deionized water was added to the solution and the resulting solution was stirred for 10 minutes on the strainer. Concentrated hydrochloric acid and sodium hydroxide were used to adjust the pH of the cell at approximately 1.8. Next, 0.25 g of thiourea, as a source of nitrogen and sulfur, was added to the synthesized sol. The addition of thiourea results in the transfer of TiO 2 photocatalytic activity to the visible region. After 2 hours, a stable yellow sol was obtained. In order to stabilize the prepared sol, 40 g of glass microbullets with diameters of 450 to 600 microns were used in each round of synthesis. The glass microbullets were first rinsed with chloroform solution and then washed several times with deionized water and dried in an oven at 105°C for 1 hour. The glass microbullets were then coated in the sol solution using immersion for 10 minutes on a magnetic stirrer. They were then left in an oven at 60°C for 4 hours to evaporate ethanol. Finally, the fixed film on the bullets was calcinated in the furnace at 500°C for 1 hour and left in an ultrasonic bath for 15 minutes to eliminate possible contamination. One of the important properties of heterogeneous photocatalysts is the selection of the appropriate bed for the stabilization of TiO 2 NPs. The important considerations for selecting an appropriate bed include high surface area, strong bonding between the bed and the photocatalyst, the stability of the bed over a long period of time, no change in catalyst activity during and after removal of contamination, as well as stability during irradiation and acquiring resistance to certain radicals produced (21).

Investigation of photocatalyst properties
The bonding gap of the samples was obtained using UVvisible spectroscopy (Avaspec 2048 Tech (AVANTES, USA) and according to the equation (1), where λ (nm) represents the wavelengths of adsorbed layers in the spectrum and Eg (eV) does the bonding gap.
Moreover, in the equation (2), S represents the average crystal dimension, k the crystal particle shape constant (0.89), θ the diffraction angle at the maximum peak, λ the x-ray wavelength, and β the peak width at half of its radius (22).
The surface morphology of the TiO 2 film was investigated by electron scanning microscope (SEM, Tescan MIRA3, Tescan Co., Czech Republic) equipped with X-ray diffraction (XRD) spectroscopy for elemental analysis.

Examination of photocatalytic optical activity
Optical degradation of naphthalene was investigated by synthesized photocatalyst under visible light and sunlight. The experiments were performed using 0.25 g of synthetic thiourea on 18 g of glass microbullets. The glass microbullets were distributed in four quartz glass tubes with a diameter and height of 8 and 200 mm, respectively, and placed on a flat mirror. Figure 1 illustrates the reactor used for photocatalytic experiments under sunlight. The volume of tested naphthalene solution in each test was adjusted at 500 mL and flow rate at 20 mL/min using a peristaltic pump in a closed system.
Photocatalytic experiments were carried out with initial concentrations of naphthalene 5, 10, 15, 20, 25, and 40 mg/L. The best naphthalene removal efficiency was obtained at pH 5. Residual concentrations of naphthalene were measured at a maximum amount of 276 λmax by a spectrophotometer (Perkin Elemer-Uv-Vis, USA), and naphthalene degradation efficiency was calculated using Equation 3. lambda where, C 0 represents the initial concentration and C t the residual naphthalene concentration in solution.

Statistical analysis
To perform data analysis, SPSS version 18 (SPSS Inc., Chicago, IL) was used. The statistical significance of intergroup differences was investigated by one-way ANOVA and to determine which times are different from other times, the Dunnett's post hoc test was used. Finally, Tukey's test was used to compare experimental groups. The results were expressed by mean ± standard deviation and P < 0.05 was considered as statistically significant.

Synthesis, morphology, and structural properties of photocatalysts
The compound structure of the TiO 2 -N-S NP phases prepared by XRD was determined. In order to investigate the formation of the anatase phase in the synthesized thin layers, the analysis was performed on the samples (23). The location of the peaks in the XRD of the anatase phase was measured according to the Joint Committee on Powder Diffraction Standards (JCPDS) of Card No. 0447-04 (23). Figure 2 illustrates the XRD of thin nanolayer of pure TiO 2 and thiourea-doped TiO 2 . Peak 2θ=25.4° shows the main peak of the anatase phase (24), while the main peaks of Rutile and Brookite are 27.4° and 30.8°, in none of which the layers are seen (25). Therefore, all layers contain the anatase phase and no Rutile and Brookite phases. In Figure 2d, the anatase phase of the TiO 2 -N-S powder sample was observed in peak 101 and 2θ=25.346°, and the particle size was calculated to be 14.5 nm.  In Figure 2c, the anatase phase of the TiO 2 -N-S powder sample was observed at peak 101 and 2θ=25.348°, and the particle size was calculated to be 11.5 nm. The size of the NPs coated on the glass microbullets according to Figure  2b was calculated to be the same as the powder sample and 11.5 nm.
An example of an uncoated glass microbullet for comparison in the formation of phases is illustrated in Figure 2a, on which no peaks appear, resulting in NPs fixed on the bed of glass microbullets with an anatase phase structure that has photocatalytic properties. Figure  3 illustrates the SEM images obtained from NPs of TiO 2 thin layer and TiO 2 -N-S coating on glass microbullets. The samples were examined in terms of appearance and morphology of the sample surface. Figures 3a and 3c illustrate that TiO 2 and TiO 2 -N-S NPs are distributed as nanoblocks with a diameter of 15.58 and 11.8 nm, respectively, on a uniform surface. Figures 3b and 3d illustrate the thickness of TiO 2 thin layer and TiO 2 -N-S coating on glass microbullets, respectively. Accordingly the thickness of the thin layer of pure TiO 2 and TiO 2 -N-S is 809.93 and 693.68 nm, respectively. Figure 4 illustrates the energy-dispersive X-ray (EDX) analysis of TiO 2 and TiO 2-N-S on glass microbullets. In this model, due to the presence of thiourea, the presence of two elements (sulfur and nitrogen) is observed in the mentioned film. The EDX analysis for the thin layer of coating TiO 2 with glass microbullets is illustrated in Figure  4a and the EDX analysis for TiO 2 -N-S thin layer coated with glass microbullets is illustrated in Figure 4b. Elemental and atomic analysis of a thin layer of TiO 2 coated on glass microbullets was created by adding thiourea (CH 4 N 2 S) to the orthotitanate sol solution. Thus, the percentage of sulfur and nitrogen elements was obtained as 6.58% and 0.57% of the weight of studied samples, respectively.
The analysis of diffuse reflectance spectroscopy (DRS) nano-UV-vis diffuse reflectance spectra of pure TiO 2 , TiO 2 doped with sulfur and nitrogen, and TiO 2 -N-S doped with sulfur and nitrogen is illustrated in Figure  5. As can be seen, the diffusion coefficient value of the TiO 2 -N-S spectrum in the visible region is stronger than pure TiO 2 . To estimate the optical band gap energy of nanostructures, the Tauc plot (known as Kubelka-Munk model) was used (26). The absorption value obtained for the TiO 2 thin layer was 378 nm and for the TiO 2 -N-S thin layer 416 nm, which was equivalent to the 2.3 eV energy gap of TiO 2 and 98.2 eV for TiO 2 -N-S thin layer, respectively.

Removal of naphthalene with synthetic photocatalyst
The effect of pH on naphthalene degradation The pH of the environment has different effects on the rate of oxidation reactions of materials. One of the most important determinants of removal efficiency is the electric charge level of the photocatalyst. The pH of the zero charge (pH pzc ) indicate the surface of catalyst has zero or neutral electric charge. (27). Figure 6 reports the zero point (PZC) pf the TiO 2 molecule at pH 6.25. Therefore, TiO 2 levels will have a positive charge in acidic state (a) and in weak alkaline conditions, (b), i.e. pH>6.25, the charge level of TiO 2 particles will be negative (18). Zhao et al reported that a positive charge on TiO 2 levels at pH less than 6 caused better migration of light-generated electrons and prevented the recombination of electrons and cavities, thus increasing the efficiency of the photocatalytic process (28). To determine the optimal pH, solutions with an initial constant concentration of 5 ppm of naphthalene were first made at pHs 3, 5, 7, 9, and 10, and then the efficacy of naphthalene removal efficiency was investigated in the presence of TiO 2 -N-S catalyst. Sampling was performed at intervals of 0, 30, 60, 90, 120, 150, 180, 210, and 240 minutes and according to the standard method at a wavelength of 276 nm. In addition, the amount of adsorption of samples was read with a spectrophotometer and the percentage of naphthalene removal using equation 3. According to the results shown in Table 1, the highest efficiency of naphthalene removal was obtained at pH 5 and 90 minutes, equivalent to 79.33±0.41, with a statistically significant difference with other pHs during this period (P < 0.05).

Investigating the Effect of Naphthalene Initial Concentration
After determining the optimal pH, naphthalene solutions with concentrations of 5, 10, 15, 20, 25, and 40 ppm were prepared at a constant pH of 5 and tested at each concentration using a TiO 2 -N-S photocatalyst under sunlight. Table 2 shows the naphthalene optical degradation with different initial concentrations in the presence of the TiO 2 -N-S photocatalyst under sunlight. Accordingly, with increasing concentration and contact time, the removal efficiency of naphthalene increased and up to 90 minutes of contact time, this increasing trend was observed in all concentrations and no significant changes in naphthalene removal efficiency were observed since 90 minutes.

Investigating the Effect of Solar Radiation on the Optical Removal of Naphthalene
In order to investigate the effect of total light (total solar radiation, TSR) on naphthalene removal, information about total and instantaneous radiation of Shahrekord meteorological synoptic station, which was located at a distance of 3640.77 m from the pilot installation, was   examined. Due to the fact that the optimal conditions for removing naphthalene were obtained within 90 minutes, the total and instantaneous amount of radiation during 90 minutes of contact of naphthalene solution with the photocatalyst was extracted from the statistics of Shahrekord meteorological solar radiation (Table  3). Accordingly, the efficiency of naphthalene removal increased with increasing radiation. According to the isotropic model (29), the best slope of installing panels containing mirrors and glass tubes against sunlight in spring, summer, autumn, and winter is equal to 2.66°, 11.33 o , 53.63 o , and 48.33 o angles. In general, the optimal annual installation angle is 29 degrees, which is almost close to the latitude of Shahrekord with a difference of less than 10% (30). In the study, the panel containing glass tubes was installed at an angle of 53.6 degrees to the horizon.

Intermediate compounds resulting from naphthalene decomposition
To evaluate the intermediate compounds obtained from the degradation process of naphthalene photolysis with TiO 2 -N-S photocatalyst, gas chromatography-mass spectrometry (GC-MS) device was used with Agilent Technologies 7890 A gas chromatography technique and Agilent Technologies 5975C mass spectrometer (USA). Based on the inhibition time factor as well as the library evaluation of the instrument regarding mass spectrum interpretation, the intermediate compounds include phthalic acid, 2formylcinnamaldehyde, 2-carboxy cinnamaldehyde, and ethanoic acid. The proposed mechanism for naphthalene removal is the oxidation process in the presence of OH and HO 2 radicals. It should be noted that previous studies have also suggested intermediate compounds resulting from the degradation of naphthalene photosynthesis and degradation mechanisms, which is in satisfactory agreement with the proposed results of this study (31,32). The results of GC-MS of the solution after irradiation at 120th minute indicate the removal of intermediates, and finally the mineralization of intermediate organic compounds and conversion to CO 2 gas and H 2 O.

Discussion
In this study, the TiO 2 -N-S photocatalyst was synthesized and fixed on glass microbullets using the sol-gel method to remove naphthalene from aqueous media. The morphological and structural characteristics of the photocatalyst using XRD, SEM, EDX, and DRS analyses showed that the photocatalyst was well fixed and synthesized on a fixed surface (glass microbullets). According to Figures 2c and 2b, the XRD analysis of the samples showed the anatase phase in the powder samples and the samples fixed on the glass microbullets bed, respectively, and the diameter of the NPs was obtained in both stabilized states on the surface of microbullets and NPs on very fine and homogeneous powder particles at 11.5 nm. Photocatalysts are used in the anatase phase under UV radiation and exhibit optical activity and catalytic properties (25).
According to the research by Zhuang et al, an increase The data is based on the standard deviation of the average removal efficiency. * In five pH groups and in each time group, there is a significant statistical difference (P < 0.05). ** There is a statistically significant difference between five pH groups and eight time groups (P < 0.05). The data is based on the standard deviation of the average deletion efficiency. Among the six concentration groups and 11 time groups, all groups had a statistically significant difference with control (P < 0.001). * There is a statistically significant difference between six concentration groups in each time group. ** There is a statistically significant difference between six concentration groups and 11 time groups (P < 0.05).
in thiourea leads to a higher accumulation of nanometer particles in some areas (33), which is clearly seen in the SEM images of samples (Figures 3a and 3c). In general, nanometer-sized photocatalyst particles tend to accumulate due to the van der Waals force between the surface of the particles (34). According to the studies by Brindha et al and Sathish et al (34 , 35), the diameter of TiO 2 -N-S NPs in the temperature range of 500°C was reported to be 15 nm, which is close to 14.1 nm of pure TiO 2 NPs. According to Figure 4b, EDX analysis, and elemental analysis of TiO 2 -N-S, using thiourea in the catalyst structure leads to the weight percentage of sulfur and nitrogen elements as 6.85% and 0.57%, respectively. Moreover, in comparison with the case where thiourea is not used (Figure 4a), which uses only pure TiO 2 , the presence of more titanium along with sulfur and nitrogen elements is observed. TiO 2 photocatalytic impurification with sulfur and nitrogen non-metals in accordance with the views of Zhuang et al and Hong et al has a significant effect on the removal of organic and mineral pollutants from aquatic environments (33,36).
Comparing the DRS spectrum of pure TiO 2 ( Figure  5a) and TiO 2 doped with sulfur and nitrogen (Figure 5b), the addition of two non-metallic elements of sulfur and nitrogen in the crystal structure of TiO 2 powder leads to a narrower energy gap and photocatalyst activity transfer of the TiO 2 to the visible area (37). According to researchers such as Zhang et al and Masoudipour et al, this energy gap is thinner than the mixture of P and non-metallic orbitals and 2P of TiO 2 oxygen (38,39). Shifu et al. used gas ammonia to dope nitrogen in a TiO 2 photocatalyst and stabilize it on the surface of hollow glass surfaces. They reported that the addition of nitrogen not only did not adversely affect the transfer of the TiO 2 phase from Rutile to anatase, but also caused the transfer of TiO 2 adsorption about 60 nm to the visible area (40). In the study, the absorption value of 38 nm was transferred to the visible area, which is in agreement with the results of Brindha et al and Rokhmat et al (34,41).
The results of this study showed that the rate of naphthalene removal is strongly dependent on the pH function and the contact time of the solution so that with decreasing pH, the efficiency of naphthalene removal increases. According to the results of Table 2, at a concentration of 5 ppm and the duration of 90 minutes of solution's contact with the photocatalyst, the removal efficiency increased from 7.28±2.69 at pH 10 to 79.31±0.41 at pH 5, indicating a statistically significant difference among five groups of pH and eight time groups with other pH and time groups (P < 0.05). Avisar et al investigated the effect of solution pH on the simultaneous purification and elimination of polymorphic sulfamethoxazole (SMX), dextracycline (OTC), and ciprofloxacin (CIP) compounds in the presence of UV radiation. Their results showed that when these compounds were examined separately, increasing the pH of the solution from 5 to 7 led to a decrease in SMX degradation rate and an increase in OTC and CIP degradation rates, and when used as a mixture, the best result was obtained at pH 5, and the rate of SMX degradation reached 99% and the rate of OTC and CIP degradation increased from 54% and 26% in pH 7 to 91% and 96% in pH 5, respectively, which is consistent with the results of our study (42). Therefore, the pH of the solution plays a key role in the removal of aromatic compounds in aquatic environment. Moreover, as Table  3 shows, at the initial pH of 5 and approximately 5 of naphthalene solutions, the pH value increased for 90 minutes of contact with the photocatalyst and reached a maximum of 5.765, which according to Figure 6, the naphthalene load in this range is positive and better electron migration has taken place on the surface of the photocatalyst, and the destructive power of naphthalene is increasing (18). The initial concentration of the solution was another important factor for the efficiency of naphthalene removal. According to Table 2  Statistical results showed that increasing the concentration of naphthalene solution is very effective in the process of photocatalytic removal of TiO 2 -N-S and the maximum efficiency of naphthalene removal was obtained in acidic conditions (pH=5) and at 40 ppm concentration of naphthalene solution at contact time of 90 minutes. Based on the results, the removal efficiency of naphthalene with TiO 2 -N-S NPs increased with increasing contact time and decreasing pH. Although naphthalene removal increases over time up to 90 minutes, the removal power of the photocatalyst decreases with increasing contact time. After 90 minutes, the amount of removal changes is very small. Although naphthalene degradation is based on oxidation, naphthalene must first come to the surface of the microglia for oxidation to occur, but the reason for the decrease in naphthalene removal efficiency with increasing contact time can be considered in saturation of absorption points on the NP surface (43).
Karimi et al in a study of advanced oxidation removal of naphthalene by H 2 O 2 /UV/TiO 2 process from aqueous media found that due to the use of 20 ppm H 2 O 2 at pH 3 and the radiation intensity of 5.6 (w/cm 2 ) at 254 nm wavelength, conditions for mineralization of naphthalene at a concentration of 15 ppm are provided after 100 minutes of contact time and the removal efficiency is reported to be 73% (44), while in the current study, the removal efficiency of naphthalene at the same concentration after 64.6 minutes of contact with photocatalyst reached 73%, which represents the high power of this photocatalyst in removing naphthalene without using disposable UV lamps and H 2 O 2 oxidizing compound. During a constant period, with increasing the concentration of the solution, the efficiency of naphthalene removal increases. Moreover, in case of the increase of the solution concentration from 5 to 40 ppm and the contact time of 90 minutes, the removal efficiency increased from 72.5% to 93.26%, respectively. This is due to the increased power of the concentration gradient with higher concentrations of naphthalene. At lower concentrations, the ratio of the initial number of naphthalene moles to the available adsorption sites is low, and as a result some of the initial adsorption will be independent of the initial concentration (45).
Muthukumar et al coated the Fe-ZnO NPs on the Amaranthus dubius plant and synthesized the catalyst (Fe-ZnO-NP) and used it under UV radiation to remove naphthalene. Optimal conditions for naphthalene removal were reported at initial concentration of 40 ppm and pH 4, using 60 mg/L Fe-ZnO-NP as being 92.33% at 240 minutes in exposure to a 16-watt UV lamp (46). However, the results of current study showed that the naphthalene solution at 40 ppm exhibited a higher removal efficiency at a lower contact time (90 min) and exposure of TiO 2 -N-S photocatalyst to sunlight.
Masoudipour et al used TiO 2 -N-S NPs stabilized on glass microbullets to remove cyanide from the aqueous medium in the presence of sunlight and reported removal efficiency of cyanide solution at optimal concentrations of 50 ppm, pH 11 and 240 minutes as being 100% (47). However, in our study, the optimal conditions for naphthalene removal occur in a shorter time and at a pH close to neutral pH, which is cost-effective in terms of time and energy. Moreover, the amount of sunlight has a positive effect on the removal of naphthalene.
According to the results of Table 3, in the initial constant concentration of 5 ppm of naphthalene solution and radiations of 528.42, 722.95, and 772.51 w/m 2 , the efficiency of naphthalene removal is equal to 50.59, 60.43, and 70.75, respectively. This shows that increasing the amount of irradiation increases the removal of naphthalene, and in close ranges of the amount of radiation and different concentrations of naphthalene, the concentration factor plays a decisive role in the efficiency of naphthalene removal (45), so that the efficiency of naphthalene removal in concentration of 20 ppm and the amount of radiation 522.17 w/m 2 was equal to 77.92% and in the concentration of 25 ppm and the amount of radiation 527.22 w/m 2 was equal to 78.36%; however, the amounts of radiation were not substantially different. As a result, the efficiency of naphthalene removal at higher concentrations increased, which is related to the number and amount of surface sites present at higher concentrations in naphthalene solution (21).
Xiaolong et al studied the highly efficient destruction of PAHs by Ag 3 PO 4 -doped graphene oxide under sunlight, and 7 minutes following exposure of the solution to the photocatalyst, the intermediate compounds 1-naphthol and 1,4-dinaphthol and 1,4-naphthoquinone and 1,2-benzenedicarboxylic acid and dialkyl ester and after 20 minutes of contact, two compounds 1,4-dinaphthol and 1,4-naphthoquinone were identified with GC-MS, which eventually converted to dialkyl ester and 1,2-benzenedicarboxylic acid (48). However, in the current study, the intermediate compounds obtained from naphthalene oxidation under sunlight were converted to water and carbon dioxide using the TiO 2 -N-S photocatalyst.

Conclusion
In this study, synthesis and stabilization of thin layer of TiO 2 -N-S by sol-gel method on a fixed bed (glass microbullets) was successfully performed. The composition of the thin layer along with its morphological and structural characteristics showed that the optical activity of the photocatalyst was directed towards visible light and could be used under the pure and endless energy of the sun to remove polyaromatic and resistant compounds and decompose them into harmless products such as water and carbon dioxide.