Degradation of Metronidazole from Aqueous Environment Using Hydrothermally Synthesized ZnO, N-Doped ZnO, and ZnO/ AC Nanoparticles

ZnO, ZnO (calcined at 400 ° C), nitrogen-doped ZnO nanoparticles, and activated carbon (AC) impregnated with ZnO (ZnO/AC) nanocomposites were synthesized by the hydrothermal method. Te structural, morphological, and optical properties of the synthesized complexes were studied by X-ray difraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformation infrared analysis (FTIR), Bru-nauer − Emmett − Teller (BET) analysis, and UV-visible spectroscopy analysis. Te degradation of the antibiotic metronidazole (MNZ) from aqueous solutions was examined by the photocatalytic process of those synthesized complexes. Among the four complexes, ZnO/AC was confrmed to be a capable prospective both as an efcient photocatalyst and as an adsorbent. Te optimal photodegradation condition obtained was 0.9g/L and pH=9. After 300minutes, 99% of MNZ was removed by ZnO/AC. Finally, gas chromatography-mass spectroscopy was conducted to identify the degradation intermediates.


Introduction
Bacteria are the source of disease as they eject toxins (e.g., botulism) [1,2] which are at large when the bacteria are split (e.g., typhoid) [3], or the sensitivity is increased to their antigenic properties (e.g., tuberculosis) [4]. An antibiotic is an antibacterial agent that is widely active in hostile bacterial infections for either killing or inhibiting their growth [5]. Te most common antibiotics that have been used in healthcare are penicillin, metronidazole, streptomycin, chloramphenicol, chlortetracycline, neomycin, azithromycin, and ciprofoxacin [6]. Te antibiotics which are not used or are metabolized end up in land, drains, surface water, or toilets [7]. When bacteria adapt to these antibiotics (which are designed to treat bacterial infections), antibiotic resistance develops. Terefore, antibiotics may not work properly in the human body [8,9]. Tere is still no comprehensive way out for stopping environmental pollution with these antibiotic elements [10]. Antibiotic and their metabolites were quantifed in the concentration of nanogram to milligram per liter from the aquatic environments of Poland, Europe [10], the United States [11], Bangladesh [12], and even Antarctic seawater [13] by separate study which is alarming for living organism such as human [14], animal [15], agriculture [16]. Among the antibiotics, some antibiotics such as penicillin are easily degradable, but most of them do not degrade naturally [17]. Metronidazole (C 6 H 9 N 3 O 3 ) (MNZ) which belongs to the nitroimidazole antibiotic group has a solubility of about 9.5 g/L in water and a molecular weight of 171.2 g/mol [18]. In majority of the time, it is employed to treat infectious disorders brought about by hostile bacteria and protozoa [6]. MNZ is easily accumulated in aqueous surroundings because it is very much soluble in water and are nonbiodegradable [19]. Tus, an efective method to eliminate MNZ is required.
Several approaches, including ultraviolet (UV) fltration [20], electrochemical degradation process [21], catalytic ozonation process [22], Sono−Fenton process [23], and photocatalytic degradation under UV and visible radiation with diferent composites, were reported to reduce the amount of MNZ present in drinkable and waste water from industries.
Te photocatalytic degradation under UV and visible radiation was conducted with various nanoparticles such as TiO 2 [24], ZnO [24], ZnO stabilized in stone surface [25], LP-ZnO stabilized by lemon peel aqueous extract [26], Ag-ZnO [19], CuO [27], Fe(OH)3 [28], and FeNi3/SiO2/CuS magnetic [29] nanocomposite, and many more were conducted. Te photocatalytic reactions are represented by the following equations [16,18]: Zinc oxide (ZnO) is the most widely used photocatalyst due to its stability, afordability, and lack of toxicity. Pure ZnO has a band gap energy of about 3.28 eV, which is 2.9 eV for a nitrogen-doped ZnO catalyst [30]. Te band gap of the specimens decreased when the Nd/Gd doping content ratio grew into ZnO according to the UV-vis data [31].
ZnO is usually used in wastewater management in the form of nanoparticles since it has an extremely huge surface area compared to its volume. Moreover, for centuries, activated carbon has been used as an excellent adsorption substrate because it has a high BET surface area and micropore volume. M. Sivachidambaram et al. showed the preparation of activated carbon (specifc surface area 633.43 m 2 /g) from borassus fabellifer fower under diferent activation temperatures by a chemical activation method applied as a precursor material for the production of low cost-high performance activated carbon electrode materials for electric double layer capacitors [32]. Yee et al. provided a roadmap for producing high-quality supercapacitors using carbon-based electrodes [33]. Shamima Begum and Ahmaruzzaman studied the synthesis of SnO 2 /activated carbon (from the stem of Corchorus olitorius) nanocomposites by the hydrothermal method that showed an invincible photocatalytic property in the degradation of naproxen (NPX) under direct sunlight [34]. Tran Ti and Lee fabricated composite materials of activated carbon fber (ACF) coated with zinc oxide (ZnO) using a commercial microwave oven which exhibited removal capacity (over 99%) and mineralization (90.7%) of tetracycline in aqueous solution within 1 h under UV irradiation [35]. Accordingly, it is found that metal oxide-fabricated AC exhibits a synergistic impact of AC on adsorption and metal oxide on the photodegradation of dangerous pollutants.
Te objective of this study is to the synthesize, evaluate, and compare the following: ZnO, ZnO (calcinated at 400°C), nitrogen-doped ZnO (N-ZnO), and activated carbon impregnated with ZnO (ZnO/AC) nanocomposites by the hydrothermal method, for their potentiality for metronidazole removal from wastewater.  2 . 2H 2 O. 66 mL of ethanol and 6 mL of reference solution A were combined in a beaker while being constantly stirred. Tis was ultrasonicated for 10 minutes and immediately transferred to a Tefon-lined autoclave with 100 ml capacity. Ten, it was placed in a furnace and the temperature was increased to 150°C and it was maintained there for 13 hours. Te autoclave was then given time to cool down to an ambient temperature. Te resulting sediment was thoroughly cleaned using distilled water and ethanol before being dried at 60°C. (400°C). ZnO (400°C) was prepared by calcining ZnO at 400°C for 4 hr and slowly cooled down.

Synthesis of N-ZnO
Nanoparticles. 66 mL of ethanol and 6 mL of reference solution A were combined in a beaker while being constantly stirred. After stirring for an hour, 200 mg of urea was obtained. Tis was ultrasonicated and collected in a Tefon-lined autoclave with a 100 ml capacity. In a furnace, the autoclave's temperature was increased to 150°C and it was maintained there for 13 hours. Te autoclave was then given time to cool down to ambient temperature. Te resulting sediment was thoroughly cleaned using distilled water and ethanol before being dried at 60°C.

Synthesis of ZnO/AC Nanocomposites.
Stems of the neem plant (Azadirachta indica) were gathered from the adjacent city of Dhaka and chemically activated to create activated carbon (AC). Neem stems were carefully chopped into pieces of a few centimetres, cleaned with distilled water, 2 Advances in Condensed Matter Physics and dried in a furnace for 24 hours at 100°C. Ten, 120 g of these neem pieces were placed in a mufe furnace and heated to 500°C at a rate 1°Cs −1 for 4.5 hours and cooled down. Te ashes were washed away, and only coals were taken. Ten, these were grinded/crushed into powder using a Panasonic home blander, and this was named as PCC (precarbonised carbon). Te weight ratio for neem before drying, after drying, and PCC is 10 : 5 : 1; that is, approximately 12 g of PCC is obtained from 120 g of neem stem. 5 g of PCC were impregnated with orthophosphoric acid (H 3 PO 4 ) solution (20 mL H 3 PO 4 + 5 mL DW) and heated on a hot plate for 4 hours with continuous stirring at 110°C for even distribution. For 2 hours, the mixture was heated to 700°C in a furnace. Tis was crushed and repeatedly cleaned with distilled water until the pH was neutral. Te product was then dried for an entire night at 120°C before being grinded into tiny homogeneous particles with a mortar and pestle. Again, 66 mL of pure alcohol, 6 mL of reference solution A, and 1.365 g of AC were combined in a beaker and continuously stirred for 30 minutes before being ultrasonicated for 10 minutes. Tis was collected in a Tefon-lined autoclave that was heated to 150°C and maintained at that temperature for 24 hours in a furnace. Te autoclave was then cooled down to an ambient temperature. Te resulting sediment was thoroughly cleaned using distilled water and ethanol before being dried at 60°C to produce ZnO/AC.

Characterization Instruments.
Carbon-coated samples were used for SEM and EDS studies, and the ZEISS EVO 18 Research SEM equipment was used to monitor them. For IR analysis, the IRPrestige-21 spectrometer was used. Te synthesized materials underwent XRD analysis by using a GNR Explorer difractometer from 2θ � 20°-70°with a scanning step of 0.01°. UV-vis spectroscopy was conducted using a HITACHI UH4150 UV-vis system. BET Sortometer (BET-201-A) was used to measure the surface area per unit volume. Talos f200x was used for TEM. UT 383 mini Light Meter was used to measure the light intensity of UV light at the surface of MNZ solution under degradation.

Metronidazole Removal Reactor Setup and Reaction
Analysis. Te mixture of photocatalysts and MNZ solution was kept under three UV lights that emit radiation at 254 nm wavelength in an open 200 mL hemispherical stainless glass vessel. Te UV lamps were placed at a distance of 12 cm from the surface of the solution where the efective power was about 20 W. Tis reaction container was covered with a transparent glass plate to avoid evaporation of metronidazole solution under continuous stirring (250 rpm). In each experiment, various known amounts of photocatalysts were suspended in 40 mL of model metronidazole aqueous solution of 80 mg/L concentration. Before illuminating the lamps, the solution was initially mixed in the dark for 30 minutes to allow for adsorption. After starting the UV light irradiation, 1.5 mL of samples were taken and placed into a collection tube at predetermined intervals. Tese underwent a 10-minute centrifugation at 1300 rpm while having been passed through 0.45 microns nylon 6,6 membranes. Te amount of MNZ in the solution was then measured by using a UV-vis spectrophotometer with a maximum absorption wavelength of 320 nm. Each experiment was carried out in identical conditions, and they were performed twice to make an average.
Te analysis of the kinetics of degradation was carried out according to the Langmuir−Hinshelwood model [19]. Te model is represented as Tis model relates the degradation rate r (mg·L −1 min −1 ) and the fractional surface coverage of the catalyst (θ s ) where C is the concentration of MNZ (mg·L −1 ), t is the reaction time (min), k is the rate constant (mg·L −1 min −1 ), and K L is the Langmuir adsorption constant (L·mg −1 ). Under the conditions of relatively low MNZ concentrations and/or weak adsorption, K L C will be much smaller than 1.
Terefore, the abovementioned equation is represented as where k app denotes the apparent frst-order rate constant. By integrating the abovementioned equation, it can be written as where C 0 is the initial concentration of MNZ. Te K app value is obtained from the slope of the plot of ln(C 0 /C) vs. t. Te lattice parameters are shown in Table 1. Te lengths of a and b are the same, while that of c is diferent in all the samples. Besides, the lattice parameters of all the samples are almost the same and hence also the cell volume. Among the samples, the cell volume of ZnO/AC is the smallest.

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Te average crystallite size was calculated by the Debye−Scherrer equation [36] written as follows: where D, λ, k, and β are crystallite size, X-ray wavelength, Scherrer constant (�0.9), and full-width half maxima, respectively. Table 2 shows the average crystallite size for the synthesized complexes. No signifcant change in the crystallite size was observed when ZnO was calcined for 4 hours at 400°C which does not match with the previous report [37]. Moreover, the crystallite size of ZnO on AC is signifcantly smaller than that of ZnO alone. Tough the ZnO/AC hydrothermal reaction was conducted for 24 hours long, the crystallite size is 16 nm. Tis implies that the environment for the crystallization of ZnO is not suitable when there is activated carbon in the solution.
No separate peak for nitrogen is observed. Figure 2 shows a minor peak shift after N-doping, moving towards lower values, indicating an increase in the interplanar space and a decrease in the lattice strain [36]. N-doping shows slight shrinking of the peak width and an increase in crystallinity as compared to pure ZnO. Tis represents that the crystal periodicity increased after N-doping, resulting in an increase in the crystallite size as shown in Table 2. Tese fndings demonstrate that the addition of N to the ZnO lattice was successful. Moreover, the peak width for ZnO/AC is wide and its crystallinity is low as compared to other complexes.

SEM Analysis. Figures 3(a)-3(d)
show the SEM images of ZnO, ZnO (calcined at 400°C), N-doped ZnO, and ZnO/AC nanocomposites, respectively. It is seen that all of the nanoparticles look like fowers and are loosely grouped into bush-like structures. Primary nanoparticles can form intricate assemblages known as aggregates or agglomerates in processes where particles originate by nucleation and growth. [38] Tese materials typically consist of spread of primary nanoscale (5-50 nm) aggregates that are chemically linked and typically range in size from 200 to 300 nm. Te larger structures that are created by aggregates are known as agglomerates, and they are kept together by weaker forces such as electrostatics, van der Waals, solvation, or capillary actions [39]. From Figures 3(a) and 3(d), it is seen that there was no signifcant change in morphology between ZnO and N-doped ZnO. In ZnO (400°C), higher grain diameter is observed compared to ZnO. In ZnO/AC, the fower-like grains of ZnO are observed to be attached on top of the activated carbon. Te morphology of the complexes was further studied by TEM.

EDS Analysis.
Te energy-dispersive X-ray spectroscopy (EDS) analysis determined the chemical composition of the samples as shown in Figure 4 and       [40,41], shown in Figure 5. Also, the peak at 670 cm −1 corresponds to the vibrational energies of Zn-O, which confrms the formation of the ZnO phase [42]. Te peak detected at ∼1020 cm −1 is due to the C-H bending vibrations [43]. Tose samples that display the transmittance band around 1600 cm −1 correspond to the bending vibrations of surface hydroxyl groups [44]. Te peak at around 2360 cm −1 shows the presence of CO 2 [42]. Te possible source of CO 2 is the atmosphere.

BET Analysis.
Te calculated total pore volume, BET surface area, and BJH pore size of ZnO/AC are 0.3257 cc/g, 558.14 m 2 /g, and 2.3 nm, respectively. Te mesoporous architecture and increased surface area are probably going to improve the adsorption performance [18]. Figure 6 shows the UV-visible spectra of the samples. Te absorption peaks of pure ZnO appear at 366 nm which has not changed signifcantly due to calcination or incorporation with activated carbon. All the electromagnetic radiations from 200 to 800 nm are absorbed by AC, which is also confrmed by previous studies [35]. However, slight red shifting of the    Advances in Condensed Matter Physics 5 absorption peak is observed due to N-doping, suggesting a decrease in the band gap compared to pure ZnO. Te band gap of the prepared samples has been estimated via the Tauc relation which is shown in the following equation:

UV-Visible Spectroscopy Analysis.
where α is the absorption coefcient (α � 2.303x A/t), A is absorbance, h is Planks' constant, ] is the frequency of the photon, B is the band tailing parameter, and Eg is the optical band gap. For direct and indirect transition, n is equal to 2 and ½, respectively. Te direct optical band gap (Eg) of the synthesized ZnO nanostructures was ascertained from Tauc's plot by extrapolating the linear region of the (αh]) 2 versus (h]) curve [36], as shown in Figure 7. From Tauc's plot (Figure 7), it is observed that the pure ZnO has a band gap of 2.8 eV. When compared to pure ZnO, ZnO (400°C), and N-ZnO, it showed a modest increase in the crystallite size, which led to a decrease in the band gap (2.7 eV) due to the quantum confnement efect [45]. However, the crystallite size decreased signifcantly in the case of ZnO/AC, and as a result, the band gap increased (4.1 eV) as shown in Table 4. Figure 8 show that ZnO, ZnO (400°C), N-doped ZnO, and ZnO grown on AC nanoparticles are nonuniform and rod-shaped with curved ends projected out. Figure 8(a) shows many intermediate primary crystallites; they are joined together to form larger crystals [46]. Te following action may take place in the autoclave as temperature and pressure rise under process conditions (150°C, 13 h):

TEM Analysis. Te TEM images in
Fine ZnO nuclei spontaneously develop in the solution when the proportion of Zn 2+ and OH − is above saturation. Te ZnO nuclei are combined together by interfacial energy, as they are unable to be combined with water solution. Also, it is possible to form crystals by growth in layers. Te typical crystal habit and growth form or the preferential growing plane of ZnO is wurtzite under hydrothermal conditions. Te growth plane was described by the "lowest-energy" theory [47]. Troughout the entire process, the rule is completely followed and single nanorods showed no signs of branching. Te solution combination undergoes an ultrasonic pretreatment that causes nucleus development to be irregular and reduces aggregation. Table 2 shows a comparison between crystallite sizes obtained from XRD and particle size from TEM. Te nanorods have diameters between 20 and 80 nm and a length between 100 and 200 nm. Te average diameters of ZnO and ZnO (400°C) are about 27 nm and 31 nm which are in better agreement with the size calculated by XRD. Te particle size of ZnO at ZnO/AC is 60 nm which is very high compared to the crystallite size calculated by XRD. Tis is possibly because the hydrothermal reaction time was 24 hours for ZnO/AC which made the particle size large compared to the crystallite size. Also, the average particle size of N-ZnO is 77 nm. Figures 9 and 10 show the selected area electron diffraction (SAED) pattern for all ZnO nanoparticle complexes. It shows that the particles are well crystallized. Te hexagonal wurtzite structure of ZnO nanoparticles is further demonstrated by the matching of the difraction rings on the (SAED) image with the peaks in the XRD pattern as shown in Figure 1 [48].

Removal of MNZ by ZnO Nanoparticles.
At photocatalytic doses of 0.9 g·L −1 and MNZ contents of 80 mg·L −1 , 83%, 89%, 66%, and 99% MNZ removal was observed in    ZnO, ZnO(400°C), N-ZnO, and ZnO/AC systems, respectively, in 300 minutes (Figure 11). At lesser MNZ levels, such as 10, 20, 30, 40, and 50 mg·L −1 , a similar MNZ elimination pattern was seen. According to the current investigation, the ZnO/AC system generated full MNZ elimination at a faster rate. Tis is due to the synergistic action of MNZ adsorbed on photocatalytic composites embedded with ZnO/AC, which combines adsorption with photocatalytic disintegration using hydroxyl radicals. MNZ adsorption on ZnO/AC was indeed the maximum and the fastest when compared to MNZ elimination by adsorption utilizing all others shown in Figure 11. Also, the degradation rate constant is compared for all the synthesized complexes in Figure 12. Since the rate constant is higher for ZnO/AC, the efect of catalytic dose, pH, and degradation intermediates were studied by using this. Figure 13 shows the adsorption in the dark when AC was used alone compared to ZnO/AC. It was found that AC decreased 60% of MNZ in the solution almost immediately. Since ZnO is a photocatalyst, ZnO along with AC worked better for eliminating MNZ completely. Te rapid fuctuation indicates that desorption also took place. Figure 14 shows the efect of catalyst doses from 0.3 g/L to 1.1 g/L for adsorption and photodegradation of ZnO/AC on MNZ. Te solution was stirred in the dark for 30 minutes for the adsorption to happen, and then the UV light was irradiated on the  Advances in Condensed Matter Physics solution. In the dark regions, the more the dose, the greater is the adsorption. 0.9 g/L is the optimum dose where degradation was the maximum. When the photocatalytic doses increased, the active site generated was larger which contributed to an increase in the degradation rate. However, when the dose is increased, UV light is blocked or scattered by the suspended photocatalysts.

Efect of pH on Degradation.
At pH of less than 4, MNZ is called protonated, and at pH greater than 12, it is called anionic [49]. Te photocatalytic degradation of MNZ over ZnO/AC at diferent pH values is shown in Figure 15. It is found that the rate of photodegradation is stable over pH from 5 to 9, but K app is low at low pH due to its electrostatic interaction with the catalyst. However, MNZ disappears almost immediately after it reaches a high pH value. So, pH � 9 is considered to be the optimal pH value. Very similar fndings were also obtained by other researchers [49].

Intermediates' Identifcation.
When UV light is absorbed by a ZnO photocatalyst, negative electron and positive hole pairs are thought to form. While ZnO's positive holes break up water molecules to form hydrogen ions and hydroxyl radicals, ZnO's negative electrons interact with oxygen molecules to form superoxide radicals. Superoxide and hydroxyl radicals are powerful oxidants of organic materials. Consequently, the photocatalysis of MNZ to   8 Advances in Condensed Matter Physics smaller byproducts is caused by the incidence of UV light at the catalyzed reaction. Te intermediates were discovered using LC-MS and GC-MS after metronidazole was removed using ZnO/AC. SHIMADZU LC-MS 8050 together with LabSolution software was used to perform LC-MS. Also, a gas chromatography (model no 8890) (Agilent Technologies, USA) system coupled with a mass spectroscopy (5977B) detector along with mass hunter analytical software was used to identify the intermediates. Te results of LC-MS and GC-MS analysis were combined together to fnd a pathway of degradation, as shown in Figure 16. Te oxidation of the methyl group may result in the formation of an intermediate of m/z 126, the loss of nitro groups, and water elimination. Te m/z 172 seems to be the rearrangement of MNZ, and m/z 158 is also the cleavage form of m/z 172. Tere were also other intermediates with reduced molecular weights. Tose were created when metronidazole was broken down, and they were then transformed into CO 2 , NO 2 , H 2 O, and NH 3 . Figure 16 shows a potential mechanism for disintegration.    Figure 17.

Stability and
Te test was therefore started from scratch. Although it took longer, 0.45 μm nylon 6,6 membranes were utilized this time to obtain the particles. Figure 18 shows how the ZnO/ AC composites exhibited high photocatalytic performance throughout the duration of three cycles of usage and regeneration while maintaining their stability. After the frst, second, and third cycles in the presence of ZnO/AC, the removal of 80 mg/L MNZ was greater than 99%, 98%, and 97%, respectively, after 300 minutes. Tis pattern was attributed to the gradual removal of ZnO from the AC surface during cleaning. Te elimination capability of the composite material was also shown to be unafected after three cycles by   Figure 16: Proposed degradation intermediates of MNZ by ZnO/AC photocatalyst under UV irradiation. the FTIR patterns shown in Figure 17 ZnO/AC(N). Tis established the suitability of the ZnO/AC composite material for application in industrial design and production for the eradication of MNZ.

Conclusion
In this study, ZnO, nitrogen-doped ZnO nanoparticles, and AC impregnated with ZnO (ZnO/AC) nanocomposites were synthesized by the hydrothermal method and applied for photocatalytic removal of metronidazole (MNZ) antibiotic from aqueous solutions under UV light. Te removal efciencies were compared in a way of how fast they can degrade MNZ from water. It takes around 660 minutes for N-ZnO and ZnO (400°C) to separately disintegrate MNZ. After calcination, the band gap of ZnO is lowered, but probably, the aggregation of ZnO (400°C) made it difcult to incorporate it with water. However, ZnO/AC was the fastest in degrading metronidazole and 60% of MNZ can be adsorbed by AC alone. Te highly porous AC captures MNZ molecules, but MNZ is not degraded by AC, and there is a chance of desorption. So, for splitting MNZ under UV light, ZnO was incorporated with AC. ZnO produces superoxide and hydroxyl radicals in an aqueous medium under UV light that oxidizes MNZ, which in turn produces smaller byproducts. Te trace of these residues in water will have various consequences and can pose a health hazard for humans and animals. Tese ZnO composites are stable in a wide range of pH and reusable for various cycles. Te reusability technique was examined keenly and explained. Reusability testing was shown to be successful when 0.45 m nylon 6,6 membranes were employed for fltering. Te result indicates that ZnO/AC can be a very promising nanocomposite over ZnO and N-ZnO for the degradation of MNZ from waste water.

Data Availability
Te data used to support the fndings of the study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.