1. Introduction
Sulfamethazine (SMZ) is a sulfonamide antibiotic used in human medicine, veterinary medication, and aquatic animals [1]. SMZ is widely applied in food processing and bacterial infections due to its inexpensive cost and ability to combat various genres of bacteria [2]. Due to the well-known water matrices and controlled volumes, removing SMZ from its sources, such as hospital wastes and, industrial effluent may be a more effective procedure. [3]. However, conventional biological wastewater treatment processes are ineffective in eliminating SMZ due to its toxicity and low biodegradability nature. Moreover, common physical and chemical processes such as filtration, flocculation, coagulation, and adsorption have been applied to remove SMZ [4]. Nevertheless, the crucial shortcomings of physiochemical technologies are low efficiency, excess consumption of chemicals, the production of large volumes of sludge, the inability to destroy, and high operational and capital cost [5].
To date, advanced oxidation processes (AOPs) can remove recalcitrant pharmaceuticals by generating insatiable reactive oxidant species (ROS), including superoxide and hydroxyl radicals, along with holes [6–8]. Photo-Fenton [9], Fenton and Fenton-like reactions [10], electrochemical oxidation (EO) [11], ozonation [12], and photocatalysis [13] are examples of successful SMZ degradation by AOPs. Over the past decades, heterogeneous photocatalysis has garnered tremendous interest in eradicating organic pollutants from ecosystems due to its high degradation and reuse efficiencies of photocatalysts. In photocatalysis, a semiconductor is irradiated by UV light with an energy higher than its bandgap to excite an electron from the valence band to the condition, leading to produce non-selective ROS. Thus, the parent pollutants are degraded into inferior intermediate and end products by the continuous generation of ROS [14, 15].
Many metal semiconductors including TiO2, ZnO, and WO3 have revealed some defects such as the rapid recombination rate of mainly generated species (e−/h+) and broad band gap [16]. Moreover, the practical application of visible or solar light is hampered by their low utilization efficiency. Hence, the research community is putting a lot of effort into discovering innovative catalysts. Despite the previous limitations, ZnO is one of the extensively researched semiconductors for photocatalytic applications, attributed to its low price, non-toxic nature, high catalytic action, availability, and stability that outweigh various drawbacks [17]. ZnO can be composited or doped with various compounds or elements, such as tin oxide (SnO2) [18], carbon (C) [19], and silver (Ag) [20], to enhance its photocatalytic efficacy. ZnO has been synthesized with hierarchical microstructures such as nanoflowers [21], nanowires [22], and nanorods [23]. Due to the higher surface area of such a hierarchical structure, which lessens charge hole recombination and increases the number of active sites on the catalyst surface, it is crucial for the photocatalytic reaction.
Metal-organic frameworks (MOFs), synthesized of metallic clusters joined by polydentate organic ligands, have aroused tremendous concern. Recently, MOFs have been employed as precursors to prepare ZnO with unique structures via a thermal process [17]. Tatekayev et al. synthesized ZnO/rGO composite using ZIF-8/rGO composite as a precursor [24]. The MOF-derived ZnO has several advantages, involving high surface area, the capacity of its pores to store a significant amount of oxygen, and the availability of active sites resulting in high oxidation performance [25]. In our previous work [26], Zn-free MOF(MIL-53Al) was used to fabricate ZnO with a unique structure. In a continuous and symmetric effort, MIL-53Fe was also chosen, among the diverse MOFs, to develop a novel visible-light-driven photocatalyst with a narrow band gap. Du et al. [27] reported that MIL-53Fe had effective visible-light photocatalytic activity for methylene blue decolorization. Moreover, it is noteworthy that the abundant iron-oxo clusters triggered the visible-light response of MIL-53Fe. MIL-53Fe, a three-dimensional porous solid consisting of infinite FeO4(OH)2 clusters joined by 1,4-benzenedicarboxylate (H2BDC) linkers [28], was applied for preparing MIL-53(Fe)/ZnO in the current study for its non-toxicity, stability, and visible light response.
Unfortunately, the fast recombination rate of the photogenerated electron-hole pairs limits the efficiency of the photocatalytic process [29]. Hence, numerous efforts have been performed to reduce the charge-hole recombination in the photocatalytic process. Hydrogen peroxide, persulfate, peroxydisulfate, or peroxymonosulfate has been introduced as an electron acceptor to the system to scavenge photoelectrons. Persulfate can be activated by electron-transfer reactions or energy. Consequently, various methods can be used, such as ultrasound, UV irradiation, biochar, electricity, heat, and transition metals [30]. Sulfate radicals are distinguished with high oxidative potential (2.5–3.1 V) and relatively longer half-life time (30–40 μs) than hydroxyl radicals (1.9–2.7 V, 20 ns) [31]. Gao et al. [32] incorporated persulfate as an electron acceptor into the MIL-53Fe/Vis system, affirming that introducing persulfate can dramatically accelerate the degradation of a wide range of organic pollutants.
Herein, this work targets to tackle some controversial limitations associated with the application of photocatalysis, such as the high e−/h+ recombination rate, semiconductors with wide-band energy resulting in low visible-light harvesting, and the cost. Thus, we prepared a novel nanocomposite MIL-53(Fe)/ZnO as a visible-light photocatalyst for the first time and used it to remove SMZ in the existence of PS ions. Optimization of operating conditions, as well as degradation mechanism and pathway, were thoroughly studied. Moreover, the amortization and operating costs for a large-scale photocatalytic reactor were estimated.
2. Materials and Methods
2.1. Materials
All reagents and solvents were utilized as received from commercial suppliers without additional purification. Sulfamethazine (SMZ), N,N-Dimethylformamide (DMF), Iron (III) chloride hexahydrate (FeCl3·6H2O), and 1,4-benzene dicarboxylic acid (H2BDC) were procured from Fisher Scientific. Potassium hydroxide, Zinc acetate, and Methanol were purchased from Sigma Aldrich. Benzoquinone (C6H4O2), Ammonium oxalate (NH4)2C2O4, Isopropanol (C3H8O), and Ethanol were obtained from Alfa Aesar (USA). Ferrous sulfate heptahydrate, Sodium bicarbonate, Potassium iodide, Sodium persulfate, Sodium sulfate, Hydrochloric acid, and Sodium hydroxide were supplied from El-Gomhouria, Egypt.
2.2. The Photocatalyst Synthesis
MIL-53Fe photocatalyst was synthesized according to the previous method informed by Horcajada et al. [33]. First, a 280:1:1 molar ratio mixture of DMF, H2BDC, and FeCl3·6H2O was moved to an autoclave and heated for 15 h at 150°C. The mixture was left to cool naturally to ambient temperature after heat treatment. The suspension was then centrifuged at 4000 rpm for 5 min to get rid of the DMF solvent. The attained powder was agitated in 200 ml of anhydrous methanol for three days to remove the residual impurities. Finally, the solution was centrifuged for 5 min and then vacuum dried at 100°C for 12 hr.
Afterward, the prepared MIL-53Fe was applied as a reactive template with a low amount to synthesize MIL-53(Fe)/ZnO by the same procedure of preparing MIL-53(Al)/ZnO under hydrothermal and calcination process, as demonstrated in our previous study [26].
2.3. Experimental Procedures
The photo-reactor comprised of 250 mL Pyrex vessel containing 250 mL of aqueous SMZ solution with an initial concentration of 10 mg/L. The photocatalyst was illuminated by a 150 W Xenon lamp as a light source with a 400 nm cut-off filter over the solution surface. The solution pH was adjusted to 3, 7, and 11 utilizing 1 M of HCl or 1 M of NaOH. After that, various doses of MIL-53(Fe)/ZnO (0.2, 0.6, 1.0 g/L) and persulfate (0.1, 0.3, 0.5 g/L) were added to study their effects on the SMZ degradation efficiency. To guarantee thorough mixing of the solution in the photocatalytic beaker, continuous stirring at 500 rpm was applied. The first 5 min of all experiments were run in the dark to reach the equilibrium state of adsorption-desorption. Afterward, the lamp was switched on to initiate the reaction.
Every 10 min, a sample aliquot of 2 ml was drawn and then filtered into the sample vials by a syringe filter of 0.22 μm. The SMZ concentrations before and after the photodegradation were measured by an HPLC system (Agilent 1200 series, USA) with a C18 reverse phase column (4.6 mm × 150 mm). AUV detector set to a wavelength of 255 nm was able to detect the signal peak of SMZ at a retention duration of around 2.9 min. The mobile phase was a mixture of ultra-pure water and methanol with (55%: 45% (V: V), respectively. Also, the flow rate was 1.0 mL/min at a temperature of 40°C. Shimadzu (2020) liquid chromatography-mass spectroscopy (LC-MS/MS) with a Shim-pack XR-ODS column was used to identify the transformation products produced during the SMZ photocatalytic degradation. Acetonitrile and 0.1 mM aqueous acetic acid solution were mixed to form the isocratic mobile phase at a flow rate of 0.1 mL/min. Moreover, a TOC analyzer (Shimadzu, Japan) was used to evaluate SMZ degradation by determining the total organic carbon (TOC).
Meanwhile, the main reactive species participating in the process were identified using free radical quenching agents such as Isopropanol (ISO) and Ethanol (EtOH). The recyclability and stability of MIL-53(Fe)/ZnO were assayed for five consecutive runs under the optimum operational conditions. Furthermore, an inductivity-coupled plasma spectrometer (NexION 2000B ICP Mass Spectrometer) was used to quantify the concentration of Zn+ ions leaching into the solution. Potassium iodide with a spectrophotometer (Shimadzu UV–visible, 1601 PC, Japan) was employed to measure the residual concentration of persulfate, as informed by Liang et al. [34].
2.4. Materials Characterization
Scanning electron microscopy (SEM, Hitachi FESEM-4800) was performed to investigate the surface morphology of the as-synthesized catalyst. To explore the detailed morphological properties of MIL-53(Fe)/ZnO, high-resolution images were acquired using transmission electron microscopy (TEM; JEOL, JEM-2100, Japan) coupled with selected area electron diffraction (SAED). Also, the primary chemical elements of the catalyst were specified by energy-dispersive X-ray spectroscopy (EDX; JEOL JSM-6510LV, Japan). An X-ray diffractometer (XRD 600 Shimadzu, Japan) with Cu K radiation (λ= 1.54 Å) was used to analyze the crystalline characteristics of the obtained catalyst. The functional groups of the catalysts were identified by Fourier-transform infrared (FTIR) spectra recorded on a Nicolet FTIR 6700 spectrometer. The specific surface area and pore size distribution were determined using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) techniques, respectively, applying N2 adsorption-desorption analysis by Belsorp-max automated apparatus (BEL Japan). Moreover, UV-Vis Diffuse Reflectance Spectroscopy (DRS) was carried out to determine the bandgap using the Kubelka-Munk function described elsewhere [35]. Photo-luminescence emission spectra (PL, Agilent Cary Eclipse Fluorescence Spectrophotometer) were acquired within an excitation wavelength range of 200–800 nm to evaluate the separation of electrons and holes. The point of zero charge (pzc) of MIL-53(Fe)/ZnO was also estimated by the solid addition method, as described in our previous work [26].
2.5. Design of Experiments by RSM
Optimal operating parameters were evaluated by the response surface method (RSM) with Box Behnken design (BBD). The chosen variables were the pH of the solution (X1), the catalyst dose (X2), and persulfate loading (X3). A set of 15 BBD-based experiments was implemented to investigate the interaction influence of the three independent parameters on SMZ degradation as a response variable. Table S1 depicts the experimental levels and ranges of independent variables. The BBD method has been applied as it is more efficient and has no point on the vertices of the cubic region formed by the lower and upper constraints of the parameters [36]. The relationship between independent variables and SMZ degradation efficiency was adapted to a quadratic polynomial empirical model, as expressed elsewhere [13]. Minitab statistical software version 19 was used for multivariate regression analysis and the optimization operation with the aid of RSM and the response optimizer tool in the software. The strength of the model was investigated by analysis of variance (ANOVA).
3. Results and Discussion
3.1. Characteristics of MIL-53(Fe)/ZnO
MIL-53Fe has been applied as an effective template to enhance the electronic structure of ZnO. Herein, we prepared ZnO-templated by MIL-53Fe to assess the efficacy of MIL-53(Fe)/ZnO in promoting SMZ photodegradation in aqueous solutions. TEM images in Fig. 1(a–c) demonstrate that the particle size oscillates between 10 and 100 nm. The characteristic lattice fringe of 0.26 nm was assigned to the (101) plane of MIL53(Al)/ZnO and MIL53(Fe)/ZnO. Compared to the MIL53Fe, the lattice fringes of the MIL53(Al)/ZnO and MIL53(Fe)/ZnO were narrowed, affirming the excellent interaction between ZnO and MIL-53Fe (hierarchal structure) in the composite. Moreover, the SAED patterns in Fig. 1 (d) suggested that the MIL53(Al)/ZnO and MIL53(Fe)/ZnO were polycrystalline materials containing (100), (002), (101), (102), (110) and (103), as was also found by XRD patterns.
As depicted in the SEM image Fig. 2(a), MIL-53(Fe)/ZnO appears with a star-like morphology with more ultrafine nanosheets. The SEM image of the modified ZnO displays a change in morphology as MIL-53Fe was used as a reactive template instead of MIL-53Al. MIL-53(Fe)/ZnO, star-like nanocrystals were present but not well defined as in the case of bare MIL-53(Fe) and MIL-53(Al)/ZnO, as displayed in Fig. 2(b, c), respectively. While MIL-53Fe appears ice cubes-like and MIL-53(Al)/ZnO appears flake-like, as previously mentioned [26].
The EDX-mapping analysis in Fig. S1 indicates the growth of Zn and O as the main chemical elements and the existence of a low amount of Fe besides the aforementioned elements in MIL-53(Fe)/ZnO. Furthermore, the weight ratios of the detected elements were determined and presented in the same graph, affirming that MIL-53Fe was also employed as a reactive template and not a processor to synthesize Nano ZnO in a hierarchical structure. Also, it can be noticed that the weight ratio of Fe is higher than the Al ratio in MIL-53(Al)/ZnO.
The XRD analysis was performed as a nondestructive test to analyze the crystallinity of the MIL-53(Fe)/ZnO. Fig. 3(a) displays the XRD patterns of ZnO prepared by MIL-53(Al) or MIL-53(Fe) as a reactive template and pure MIL-53Fe. The sharp and narrow diffraction peaks exhibited good crystallinity of the synthesized composite. The typical diffraction peaks at 2θ of 31.746°, 34.407°, 36.2462°, 47.5489°, 56.5663°, 62.8787°, 66.331°, 67.9447°, 69.0917°, 72.5716° and 77.0267°, which ascribed to (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystalline planes of ZnO (JCPDS Card ID 36-1451), respectively. Therefore, the XRD results are compatible with the hexagonal wurtzite structure of ZnO. Moreover, the two patterns of MIL-53(Al)/ZnO and MIL-53(Fe)/ZnO are similar and correspond to that of pure ZnO pattern (Fig. S2) according to our recent work [26]. Since no unique iron peaks were visible, this may be attributed to the low content of Fe in the as-prepared nanocomposite.
In the same idea, to verify the distinct functional groups associated with photocatalyst MIL-53(Fe)/ZnO, FT-IR spectroscopic measurements were recorded and compared with the spectra of MIL-53(Al)/ZnO and MIL-53Fe. As exhibited in Fig. 3(b), the typical absorbance peaks of MIL-53(Fe)/ZnO were obtained at 495, 538, 1630, and 3500 cm−1 [26]. The band at 459 cm−1 was attributed to the Zn-O stretching vibration. The peak at 538 cm−1 was ascribed to the Fe-O stretching vibration [37], revealing the production of a ferrite group along with ZnO. This outcome is consistent with that of EDX, as the weight ratio of iron ions is 3.4 %, which is higher than its rival MIL-53(Al)/ZnO. The H-O-H bending vibration of water molecules was assigned the band at 1630 cm−1 [38], whereas the stretching vibration bond of O-H was located at 3500 cm−1 [39]. The intense peaks of symmetric and asymmetric vibrations of carboxyl groups at 1390 and 1535 cm−1 were detected in the MIL-53Fe spectra and disappeared in the composites [28], confirming the application of MOFs only as reactive templates.
The isotherms of nitrogen adsorption-desorption are depicted in Fig. S3 (a) to illustrate the specific surface area and porous properties of MIL-53(Fe)/ZnO. The BET data indicated that the surface area of the prepared catalyst was about 20,081 m2/g; in addition, at P/P0 = 0.990, the total pore volume of the MIL-53(Fe)/ ZnO sample was found to be 0.0944 cm3/g. The resulting SBET for MIL-53(Fe)/ZnO was smaller than that for MIL-53(Al)/ZnO, attributed to the smaller surface area of MIL-53Fe (38.17 m2/g), while that of MIL-53Al (776.8 m2/g). The relative decrease in the SBET of MIL-53(Fe)/ZnO was due to the agglomeration of nanoparticles after the calcination process. Moreover, the pore size and pore volume of MIL-53(Fe)/ZnO were smaller than that of MIL-53(Al)/ZnO. One possible explanation was the pore size and pore volume of MIL-53Fe were also smaller than those of MIL-53Al.
In comparison to MIL-53Fe, the pore size of MIL-53(Fe)/ZnO was slightly larger, suggesting the loss of solvent in the pores or ascribing to a collapse of the crystal structure of MIL-53(Fe)/ZnO after calcination, resulting in forming larger mesopores. The detailed BET data for all catalysts are listed in Table S2. The MIL-53(Fe)/ZnO sample shows a type III adsorption curve according to the IUPAC classification and the hysteresis loop, suggesting an explicit characteristic of mesoporous material. The BJH analysis (Fig. S3(b)) implies that the as-synthesized catalyst is microporous, with pore sizes ranging mainly from 1 to 2 nm. The pore size between 2 and 50 nm is concerned with mesopores, and between 50 and 200 nm is foremost related to macropores. Furthermore, the wide pore size distribution of MIL-53(Fe)/ZnO confirmed the growth of the hierarchical structure. Moreover, the presence of micropores and mesopores mainly indicates enhanced photocatalytic performance.
In addition, the optical absorption characteristics of as-synthesized MIL-53(Fe)/ZnO composite investigated by UV–vis DRS spectra are exhibited in Fig. S4. The main intense light absorption band of MIL-53(Fe)/ZnO was in the range of 250–450 nm, implying its possible capability to capture visible light effectively. Thus, the derived optical band gap (Eg = 2.04 eV) of MIL-53(Fe)/ZnO was estimated by following the intercept of the tangent to a plot of (F(R) hυ)1/2 versus the photon energy (hυ) (Fig. 4(a)), where F(R), υ, and h represent the reflectance function, light frequency, and plank constant, respectively. The calculated band gap of MIL-53(Fe)/ZnO was remarkably narrower than the pristine MIL-53Fe (2.72 eV) and previous composite MIL-53(Al)/ZnO (2.85 eV) [26, 40]. The results indicated that MIL-53(Fe)/ZnO could produce more active sites than MIL-53(Al)/ZnO and MIL-53Fe for SMZ photodegradation by absorbing more visible light.
Fig. 4(b) elucidates the PL spectra of MIL-53(Al)/ZnO, MIL-53Fe, and MIL-53(Fe)/ZnO. The intensity of MIL-53(Fe)/ZnO is lower than MIL-53Fe and MIL-53(Al)/ZnO, revealing that MIL-53(Fe)/ZnO nanocomposite possesses a longer lifetime of photoinduced electron-hole pairs, resulting in an improvement of the photogenerated carrier transfer.
3.2. Optimization of the Operating Parameters
An initial concentration of 10 mg/L was chosen and applied in all experiments after investigating the degradation of various concentrations of SMZ in the range of 10–70 mg/L. As illustrated in Fig. 5(a), a long time was needed for complete removal at higher concentrations, but low concentrations of SMZ rapidly degraded due to abundant ROS.
Fig. 5(b) displays the change in SMZ concentration (C/C0) over time in different photocatalytic systems. Limited SMZ degradation was obtained in the absence of a photocatalyst after visible light irradiation for 30 min, indicating a quiet SMZ stability toward incident light. The control run was also carried out in the dark; about 48% removal of SMZ was achieved by adsorption, attributing to the MIL-53(Fe)/ZnO comparatively low surface area. The SMZ degradation by MIL-53(Fe)/ZnO, pristine MIL-53Fe, and MIL-53(Al) /ZnO in visible light were 81%, 52%, and 36% due to lower band gap of the prepared catalyst (2.04 eV) than of others (2.72, 2.85 eV). Intriguingly, the addition of PS increased the efficiency of SMZ degradation, and SMZ was completely removed after 30 min in the MIL-53(Fe)/ZnO/PS/Vis system. Moreover, the degradation of SMZ under UV light was decreased to lower than the DT (>0.036 mg/L) [41], confirming the activity of MIL-53(Fe)/ZnO under broad light spectra. Table S3 illustrates the specific conditions of 15 runs as well as the actual and predicted.
SMZ removal ratio after 30 min during the MIL-53(Fe)/ZnO/PS/ Vis process. The relationship between the SMZ removal % and the three operating parameters was expressed by a second-order polynomial equation Eq. (1):
where Y is the SMZ removal ratio after 30 min, X1 is the pH, X2 is the catalyst dose (g /L), and X3 is the initial persulfate loading (g/L). The correlation coefficient (R2) was 99.74%, and the adjusted R2 was 99.28%, affirming the effectiveness of that model for regressing the experimental data. As listed in Table S4, the relatively low P-values and high F-values guaranteed the relevance of the model. The operating parameters influence the degradation efficiency of SMZ in the following order, depending on the F-value: pH > PS loading > catalyst dose.
The contour lines in Fig. 6 portray the effect of the operating parameters on the SMZ degradation efficiency. The photocatalytic performance was enhanced in acidic conditions. SMZ was completely removed after 30 min at pH =3. Comparing the use of MIL-53(Fe)/ZnO photocatalyst to BNQDs/BPS-CN photocatalyst, the degradation occurred in a lower time equal to 30 min [42]. This phenomenon may be elucidated by the electrostatic interactions that occurred in an aqueous solution between the catalyst, PS, and SMZ. As depicted in Fig. S5, the point zero charge (pzc) of MIL-53 (Fe)/ZnO was ~4.1. The pKa value of SMZ is (2.28 – 7.42) [41]. At pH < pH (pzc), the MIL-53 (Fe)/ZnO surface is positively charged. The negatively charged S2O8 2− in solution was well suited for adsorption on the MIL-53(Fe)/ZnO surface. The photo-generated electrons were promptly captured by PS under visible light irradiation, improving the separation of electron and hole pairs and thereby enhancing the SMZ degradation efficiency. With a lower pH value, more S2O8 2− ions were adsorbed on the surface of MIL-53(Fe)/ZnO, facilitating SMZ degradation. The surface of MIL-53(Fe)/ZnO was negatively charged when the pH was above the pH (pzc). Furthermore, as the pH increased, less persulfate was adsorbed on the catalyst surface, impeding the SMZ degradation. At alkaline conditions, the degradation of SMZ was dramatically inhibited by the repulsive forces between the deprotonated form of SMZ and the catalyst [43].
Nevertheless, after 30 minutes of irradiation, the pH value was found to rise from 3.0 to 3.59, explained by the generation of alkaline intermediates from SMZs during the photocatalytic degradation. As a result, at a neutral pH, the treated real effluent must be rejected in the aquatic environment. Therefore, the pH adjustment may be required to post the photocatalytic process.
Increasing the catalyst dose may promote the competitiveness of the active sites on the catalyst surface and reduce the solution transparency, affecting SMZ degradation. In contrast, exceeding the optimum catalyst dose by more than 0.2 g/L lowered the degradation efficiency due to the deterioration of the light transmittance at the relatively high MIL-53(Fe)/ZnO dose. Furthermore, higher catalyst doses increase the agglomeration potential of nanoparticles, resulting in a reduced active surface area, according to previous reports [44].
The increase in the concentration of persulfate from 0.1 g/L to 0.3 g/L remarkably enhanced the SMZ degradation due to the higher photogenerated electron capture agent, thereby reducing the recombination of the photogenerated carriers and improving the photocatalytic performance. [43]. However, increasing the PS concentration to the optimum value reduced the SMZ degradation efficiency. It can be ascribed to the S2O8 2− molecules, acting as scavengers for OH−. radicals by transforming them to lower oxidants (S2O8 −), leading to a reduction in the degradation efficiency, according to Eq. (2) [6].
Based on the RSM model, Table 1 displays the optimal operational conditions and the actual and expected SMZ degradation efficiency after 30 min of the photocatalytic process. Moreover, TOC removal was monitored during the photocatalytic process (Fig. S6(a)). The results revealed a 78% reduction of the total organic carbon obtained after 30 minutes, indicating that part of the SMZ content was degraded to CO2 and H2O, while the remaining TOC was related to the oxidizing intermediates. These results demonstrated an effective reduction in total organic carbon due to the continuous degradation of the organic content.
3.3. Degradation Kinetics of SMZ
As depicted in Fig. S6(b), the Langmuir-Hinshelwood model was applied to calculate the rate of SMZ degradation. The model describes the relationship between the degradation rate of SMZ (Kr), initial concentration of SMZ (Co), and SMZ concentration at a time (t) (Ct), as expressed by Eq. (3) [45]:
The high degradation rate of SMZ was 0.089 min−1 with R2 of 0.9879 under the optimum conditions, affirming that SMZ degradation reaction followed the Langmuir-Hinshelwood model.
3.4. Degradation Mechanism
The reactive oxygen species involved in the MIL-53(Fe)/ZnO/ PS/Vis system were identified by conducting a radical quenching study. To investigate the participation of each ROS in SMZ photodegradation, benzoquinone (BQ); a well-known superoxide radical scavenger (Fouad, Alalm, et al. 2020), isopropanol (ISO); an inhibitor of hydroxyl radicals [41], ammonium oxalate (AO); a hole quencher (Mahanna and El-Bendary 2022), and ethanol (EtOH); a scavenger of sulfate radicals [46] were utilized (Fig. 7). After 30 min without any scavengers, the SMZ degradation efficiency was below the detection limit. Adding 1 mM ISO, AO, EtOH, and BQ reduced the degradation efficiency to 54.23%, 79.58%, 36.09 and 73.47%, respectively. The results confirm that superoxide, holes, sulfate, and hydroxyl radicals all contributed to the photodegradation of SMZ. Remarkably, the deactivation experiments revealed that sulfate radicals were the dominant ROS during the degradation process. This finding is similar to those found in the study of RB222 dye degradation (Mahanna and El-Bendary 2022). While in another study, it was noted that SMZ was degraded in lack of SO4 •− after 1 min [47]. As explained in Fig. S7, this finding is also assured by measuring the residual concentration of PS ions during the reaction, implying that the activation of PS was attributed to the electron donation from MIL-53(Fe)/ZnO. A plausible mechanism of SMZ degradation was proposed based on these results and depicted in Fig. S8.
The visible light irradiation and the catalyst were activators for PS in SMZ degradation as shown in Eqs.(4–9) [43]. The photo-generated holes (h+) and electrons (e−) were formed in the valence band (VB) and conduction band (CB) of MIL-53(Fe)/ZnO, respectively. The electrons in the CB can induce the dissolved oxygen (O2) into superoxide radicals (O2−) due to its sufficient reductive ability. Furthermore, the h+ in VB reacts with H2O or OH− ions, generating OH• radicals that attack any organic matter near or absorbed on the active surface of the semiconductor. Moreover, PS (an electron accepter) captures photogenerated electrons to generate SO4− • radicals, and reduce OH− ions into OH•, inhibiting the photogenerated e− and h+ pair recombination. Furthermore, these photogenerated radicals can oxidize SMZ to simpler intermediate products, H2O, and CO2 [42].
3.5. Performance of MIL-53(Fe)/ZnO Photocatalyst in Successive Cycles
The recycling effectiveness of the suspended photocatalyst was assessed using the same catalyst particles five consecutive times. Following each cycling run, the MIL-53(Fe)/ZnO was filtered from the aqueous suspension and washed five times with a 1 mol/L methanol aqueous solution and distilled water to eliminate the SMZ absorbed on the photocatalyst surface. The MIL-53(Fe)/ZnO was centrifuged for 5 min and then oven-dried for 30 min at 105°C.
As demonstrated in Fig. S9, the photocatalytic degradation efficiencies of MIL-53(Fe)/ZnO catalyst slightly decreased to 97, 92, and 90% after 3, 4, and 5 runs, respectively. The last cycles may need an extra reaction time to accomplish the degradation as the first cycles. So, MIL-53(Fe)/ZnO can maintain photocatalytic activity throughout the entire reaction with no appreciable decrease in performance after five cycles. Furthermore, an ICP optical emission spectrometer was executed to quantify the concentration of Zn+ ions leaching during the process. It can be noticed that hardly any Zn+ ions leached from the photocatalyst during the reaction (0.25 – 0.42 mg/L). The MIL-53(Fe)/ZnO exhibits high photocatalytic stability and reusability.
3.6. Transformation Products and Oxidation Pathway
LC-MS/MS analysis was employed to determine the intermediate products generated during the photodegradation of SMZ. The parental contaminant (SMZ) (m/z = 279) was exposed to sulfate and hydroxyl radicals, resulting in cleavages of the SMZ molecule. A product with an m/z of 214 was formed as a result of SO2 release from SMZ compounds [48]. The cleavage of the bonds connecting S–N induced by ROS attacks caused this release. Breaking the S-N bound bonds associates the cleavage of the intermediate sulfonamide group in the SMZ and generates this compound (m/z = 123) [49]. This product may also be generated by cleavage of the N-C bonds in the product with m/z = 2 14 [50]. In addition, SMZ oxidation can yield a transformation product with m/z of 294 [13]. A product with (m/z = 83) was detected in the last stages of oxidation due to the cleavage of the ring because of continuous ROS attacks [13, 51]. These by-products are less robust and can be degraded more easily by the constant generation of oxidant species. The end products can be amines, H2O, SO4 2−, CO2, and simple organic acids. Fig. S10 displays the proposed oxidation pathway according to the observed intermediate by-products.
3.7. Cost Estimation
The cost of photocatalytic treatment of one cubic meter of pharmaceutical wastewater is proportional to the amount of waste generated each day and the reactor volume. It is envisioned that in subsequent batches, a five cubic meters (CA) reactor will be employed as a cost guide for photocatalytic degradation. Because of the various matrices and competing pollutants, laboratory-treated industrial wastewater typically differs from the actual, but such investigations can assist in bridging the gap between practice and research of wastewater treatment approaches, particularly photocatalysis. The study was computed using the previously obtained optimum conditions. The reactor is anticipated to be a reinforced concrete structure with a 25-year life span (n). tc is one-cycle time, including 30 mins of reaction time for 1cycle of and 30 mins for reactor filling, draining, rinsing, and preparing. In addition, the daily work hours (tw) are 14 h. Also, 300 days are the working days (D) per year. The basin volume (CA) was estimated by Eq. (10):
The annual volume of treated wastewater is denoted by Vt. The capacity of the reactor (CA) was computed to be 4.76 m3 based on a Vt of 20,000 m3 but was roughly approximated to be 5.0 m3 to handle 3 % of flow fluctuations. The total cost of the process (TC) was calculated by taking both the operating cost (OC) and the amortization cost (AC) into account. That is, TC = OC + AC. The costs of the essential equipment and basic construction facilities, such as the mixer motor, the reinforced concrete body, and other proper accessories were summed up to calculate the AC cost. All the costs were equalized to one cubic meter of contaminated wastewater. Eqs. (11–13) can be utilized to evaluate the AC per m3 of the industrial wastewater [52]:
where Cp0 is the estimated cost of constructing a photocatalytic reactor and permanent facilities (3000 $). ACannual is the annual investment cost of Cp0, r is the annual interest rate (7%), Cp is the net cost of the reactor volume. The cost of the AC is assessed to be 0.10 $/m3 to treat 1 m3 of contaminated wastewater. The OC represents the costs of consumed energy, chemical components, and maintenance. To simplify the cost study, the cost of labor was not considered. Cch was computed, including the catalyst dose, pH adjustment if necessary, and sodium persulfate. Also, Clamps is the cost of the required lamps to irradiate the photocatalyst. The amount of energy (EC) consumed to operate the mixer, and lamps was calculated in $ /m3 according to Eq. (14):
where E refers to the consumed energy (kW), PE is the price of energy per unit (0.18 $/kWh) [52]. It was supposed that the maintenance cost would be 2% of AC [53]. Consequently, the total OC, such as maintenance, energy, and consumables, was determined using Eq. (15):
A comparison of the costs associated with wastewater treatment using the visible/MIL-53(Fe)/ZnO/PS and UV/MIL-53(Fe)/ZnO was processes conducted. The total cost of the MIL-53(Fe)/ZnO/PS/Vis and MIL-53(Fe)/ZnO/UV systems were calculated and listed in Table S5. Remarkably, Table S5 affirmed that applying MIL-53(Fe)/ ZnO/PS/Vis is more economical than using MIL-53(Fe)/ZnO/UV. Text S1 estimates and explains the detailed costs. Finally, it should be highlighted that these costs may vary by location.
4. Conclusions
MIL-53Fe was applied as a reactive template to synthesize a hierarchical MIL-53(Fe)/ZnO visible light photocatalyst under the hydrothermal method. This new approach of using MOFs in preparing metal oxides has a crucial impact on their morphological, structural, and optical characteristics. For the first time, the visible-driven-light photocatalyst (MIL-53(Fe)/ZnO) was used for removing SMZ. The optimum operating parameters for SMZ degradation were pH = 3, MIL-53(Fe)/ZnO dose = 0.2 g/L, PS dose = 0.39 g/L, and initial SMZ conc. = 10 mg/L, achieved 100% of SMZ degradation after a reaction time of 30 min. The pseudo-first-order kinetic rate constant (Kr) based on the Langmuir-Hinshelwood model for the photocatalytic degradation of SMZ by MIL-53(Fe)/ ZnO was 0.089 min−1 under the optimum conditions. The reusability of the suspended photocatalyst was verified in 5 successive runs (100% – 90.54%), while the concentration of Zn+2 ions leaching to the reaction solution was recorded to be 0.42 mg/L, assuring an extremely stable catalytic performance after a prolonged time of reaction and a prospect of extended reuse. The SO4 − radicals were the main significant oxidizing agents as maintained by quenching experiments. The SMZ degradation pathway was proposed via the analysis of generated intermediates during the photocatalytic reaction by LC/MS-MS tandem mass spectroscopy. Finally, the cost estimation study exhibited that the treatment of one m3 of similar contaminated water with SMZ decreased from 2.45 to 1.50 $/m3, this outcome indicated that the application of MIL-53(Fe)/ZnO/PS/Vis process extremely suppresses electrical and chemical costs.
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