Enhanced Sonophotocatalytic Degradation of Acid Red 14 Using Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) Based on Metal-Organic Framework

Abstract

Here, the magnetic Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) with a new core-shell structure was synthesized, and its sonophotocatalytic properties were evaluated for acid red 14 (AR14) degradation. Particle characterizations were determined by scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and vibrating-sample magnetometer (VSM), and the analysis results offered an excellent synthesis of mesoporous particles. Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe)/UV/US showed high degradation kinetics rate (0.0327 min⁻¹) compared to sonocatalytic processes (0.0181 min⁻¹), photocatalytic (0218 min⁻¹), sonolysis (0.008 min⁻¹), and photolysis (0.005 min⁻¹). Maximum removal efficiencies of AR14 (100%) and total organic carbon (69.96%) were obtained at pH of 5, catalyst mass of 0.5 g/L, initial AR14 concentration of 50 mg/L, and ultrasound power of 36 W. Evaluation of BOD₅/COD ratio during dye treatment confirmed that the sonophotocatalysis process can be useful for converting major contaminant molecules into biodegradable compounds. After recycling eight times, the prepared composite still has sonophotocatalytic degradation stability above 90% for AR14. Scavenging tests confirmed that holes (h⁺) and hydroxyl (^•OH) were the pivotal agents in the decomposition system. Based on the results, the synthesized sample can be suggested as an excellent and promising sonophotocatalyst for the degradation of AR14 dye and its conversion into biodegradable compounds.

1. Introduction

The textile industry is an industry with production of large amounts of wastewater containing organic compounds and metals, which are typically toxic and resistant to biodegradation. Azo dyes with azo (-N=N-), hydroxyl (^•OH), and -SO₃H groups are the most common dyes used in the textile industry. These dyes with recalcitrant properties and high solubility in aqueous solution are difficult to remove by the treatment process based on microorganism degradation [1,2]. According to studies, 15% of azo dye production is released into textile effluents and the entry of colored wastewater into the environment can lead to the extinction of active organisms in the ecosystem [1,3]. To remove persistent contaminants in wastewater, conventional physicochemical methods, e.g., reverse osmosis, coagulation-flocculation, adsorption, and filtration have recently been used. However, the present methods typically have problems such as the production of secondary pollutants, the high cost of post-treatment, and the non-decomposition of pollutants [4]. Compared to these removal techniques, advanced oxidation processes (AOPs) based on the production of free radicals such as ^•OH, ^•OOH, and O₂^•− can mineralize a wide range of contaminants without producing sludge and toxic compounds [5]. Among AOP, the photocatalysis system has recently attracted considerable attention from researchers for industrial effluent treatment because it has exhibited acceptable results in terms of its cost and efficiency and offered non-toxic and complete mineralization [6]. Photocatalytic systems, by absorbing light, generate electron/holes (e⁻/h⁺) pairs to degrade pollutants in the aquatic environment by producing reactive oxygen species (e.g., ^•OH and O₂^•−). In this method, TiO₂ and ZnO semiconductors are the most common materials for the production of free radicals. However, semiconductors in practical applications have disadvantages such as high cost, poor propensity for aromatic hydrocarbons, easy deactivation, and rapid recombination of the e⁻/h⁺ pair [7]. To overcome these disadvantages, the researchers suggested the creation of photocatalysts that are capable of high light absorption potential and proper electron transfer [8].

Metal-organic frameworks (MOFs) are a category of hybrid micropores composed of organic and inorganic linkers. These materials have been used in catalytic applications, molecular detection, separation, drug delivery system, and storage due to properties such as tunable composition, unsaturated metal sites, and porous structure [9,10,11]. In addition, the excellent specific area and sites of these materials in the adsorption of organic pollutants and charge transfer have recently led to their use as a promising semiconductor [10]. However, the photocatalytic performance of MOF in full-scale treatment is limited due to the recombination of photoproduced carriers. To overcome this problem, it is proposed to integrate magnetic nanoparticles with MOF due to their abundance, low toxicity, and suitable band gap. In the magnetic MOF complex, Fe-O clusters can be excited by transferring electrons from the superoxide to the ferric ion, and then its unsaturated centers lead to catalytic activity for the oxidation of the pollutant. In addition, the combination of iron and MOF can be beneficial due to easier separation by an external magnetic field [10,12,13]. Feng et al. synthesized MIL-53 based on magnetic iron for photocatalysis of methylene blue and obtained removal efficiencies above 85% and reported catalyst stability for three consecutive reaction cycles [14]. Li et al. performed photodegradation of organic compounds using MOF-Fe. Their results showed the complete removal of 460 ppm toluene and its mineralization to CO₂ under sunlight [15]. Despite these advantages, the composite still suffers from recombination, photodissolution, oxidation, tendency to aggregation, and failure to select complex matrices. To address these shortcomings, SiO₂ not only blocks the injection of electrons from the MOF surface into Fe₂O₃ but also can prevent the accumulation of particles over a wide pH range. In addition, SiO₂ particles make provision for a porous surface with a high surface area to volume for photocatalytic reactions [16,17]. Mandal et al. synthesized Fe₂O₃-SiO₂ nanospheres for photocatalytic degradation of methylene blue [18]. Lajevardi et al. [19] and Li et al. [20] synthesized SiO₂/Fe₂O₃ on MIL and ZIF for the treatment of aqueous solutions, respectively.

In recent years, for better treatment of strong wastewater as well as better photocatalyst activity, the combination of sonocatalysis and photocatalysis has been considered [21,22,23,24]. In the sonophotocatalytic process, growing microbubbles is achieved by absorbing energy from the ultrasonic field and leads to the formation of high values of ^•OH. Correspondingly, the photocatalyst is stimulated by light and ultrasonic radiation to form more active radicals [25,26,27,28]. The resulting cavitation not only cleans the active sites of the particles by removing sticky byproducts, but also develops the mechanisms of adsorption, mass transfer, and particle dispersion [29,30,31]. Despite these advantages, the use of sono-photocatalysis based on Fe₃O₄ SiO₂/PAEDTC@MIL-101 (Fe) is limited due to the lack of complete details. In addition, very little information is accessible on the properties and performance of the synthesized photocatalyst. Therefore, in the present study, the degradation of AR14 was evaluated by sonophotocatalytic process. Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) nanoparticles were prepared as a new catalyst and their properties were determined through various diagnostic analyses. The sonophotodegradation process efficiencies were compared with sonolysis, photolysis, sonocatalysis, and photocatalysis systems for the feasibility of a new photocatalyst for well-organized degradation of AR14. The evaluation of the present process was performed for practical purposes in the terms of (1) degree of mineralization, (2) biodegradability of the effluent, and (3) recyclability. Finally, the mechanism of dye degradation by the sonophotocatalytic process through the identification of reactive species was proposed.

2. Results and Discussion

2.1. Characterization of Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe)

Figure 1 shows the surface morphology and size of MIL-101 (Fe), Fe₃O₄@SiO₂/PAEDTC, and Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe). As can be seen, the MIL-101 (Fe) has an octagonal structure with a smooth surface. In addition, some of its crystals are split or have no angles, which according to previous studies [24], may be due to recrystallized H₂BDC (Figure 1a). Fe₃O₄@SiO₂/PAEDTC nanocomposite with a size of 20–40 nm has a uniform and homogeneous surface without agglomeration (Figure 1b). Figure 1c shows that Fe₃O₄@SiO₂/PAEDTC particles were uniformly loaded on MIL-101 (Fe) for photocatalytic activity. The existence of silica, iron, and oxygen in the mapping analysis confirms the proper formation of the Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) (Figure 1d).

TEM images of MIL-101, and Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) nanoparticles were represented in Figure 2. According to this figure, in addition to the formation of MIL-101 (Fe) with a smooth bed, Fe₃O₄@SiO₂/PAEDTC nanoparticles are in a core-shell microsphere structure (Figure 2a,b) on the main particle surface. This structure emphasizes that during TEOS hydrolysis, the magnetic nanoparticles were coated with the SiO₂ layer, as the evidence in Figure 2c,d confirms the findings. Silica-coated nanoparticles can be important to prevent the leaching of iron from the particle surface. Moreover, magnetic particles loaded on MIL-101 (Fe), can be useful for the adsorption of contaminants and photons in the sonophotocatalytic process. Comparable results were described by Hu et al., and Saboori [25,26] for the synthesis of MIL-100 (Fe)@SiO₂@Fe₃O₄ core-shell microspheres.

The crystalline nature of the nanocomposites was evaluated by XRD analysis and the results were shown in Figure 3a. Diffraction peaks below 30° in the XRD pattern of MIL-101 (Fe) indicate that the MIL-101 is well synthesized [27]. For Fe₃O₄@SiO₂/PAEDTC, the diffraction peaks at 30°, 35.5°, 43.2°, 52.25°, 57.1°, and 62.3° correspond to reflections of (220), (311), (400), (422), (511), and (440), respectively. These diagnostic peaks for Fe₃O₄@SiO₂@PAEDTC are agreed with the database of magnetite in JCPDS (JCPDS card: 19-629) [28]. The diffraction pattern of Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) contains reflections of Fe₃O₄@SiO₂/PAEDTC and a number of new peaks, which is possibly related to the MIL-101 (Fe) layers added to the magnetic structure. In order to evaluate the magnetic behavior of the samples, the particle properties at room temperature were measured by VSM, and the results of their hysteresis loops were brought in Figure 3b. According to this figure, the saturation magnetization of Fe₃O₄@SiO₂/PAEDTC and F Fe₃O₄@SiO₂/PAEDTC @MIL-101 (Fe) nanoparticles was 31 and 11 emu/g, respectively. The reduction in emu/g value during the synthesis process emphasizes that the magnetic nanoparticles are well coated on MIL-101. The N₂ adsorption-desorption isotherm was performed to study the porosity of MIL-101 (Fe) and Fe₃O₄@SiO₂/PAEDTC@ MIL-101 (Fe) nanoparticles. According Figure 3c,d and

Table 1. BET properties of synthesized samples.

	SSABET (m2 g−1)	Pore Volume (cm3 g−1)	Pore Size (nm)
MIL-101 (Fe)	2280	1.09	2.34
Fe3O4@SiO2/PAEDTC@MIL-101 (Fe)	992	0.65	1.35

, the pore volume and size for MIL-101 (Fe) were 1.09 cm³/g and 2.34 nm, respectively. In contrast, for Fe₃O₄@SiO₂/PAEDTC@ MIL-101 (Fe), the pore volume and size were 0.65 cm³/g and 1.35 nm, respectively. These results emphasize that both particles are synthesized with a mesoporous nature. According to IUPAC, porous materials are divided into three categories based on size, i.e., microporous (pore diameter: ≥50 nm), mesoporous (2–50 nm), and microporous (2≥) [29]. The BET surface areas for MIL-101 (Fe) and Fe₃O₄@SiO₂/PAEDTC@ MIL-101 (Fe) were found to be 2280 and 992 m²/g, respectively. Significant changes in surface area rate could be related to the integration of magnetic nanoparticles with MIL-101 (Fe).

It Is well known that the band-gap of the nanostructures materials plays a key role in utilizing photocatalytic applications. The diffuse reflectance spectroscopy (DRS) of Fe₃O₄@SiO₂, Fe₃O₄@SiO₂/PAEDTC and Fe₃O₄@SiO₂/PAEDTC@MIL-101 nanoparticles were illustrated in Figure 3e.

The band-gap of the samples was calculated using the following equation [32]:

(Ahυ)ⁿ = B(hυ − Eg)

(1)

where Eg is the optical band gap of the material, hυ is the photon energy, B is a material constant, A is the amount of the absorbance and n is constant that depends on the type of the electronic transition. The energy gap of the samples (Eg) was obtained by extrapolating the linear portion of the plots of (αhυ)² curve in return hυ to the energy axis. The band-gap (Eg) of Fe₃O₄@SiO₂, Fe₃O₄@SiO₂/PAEDTC and Fe₃O₄@SiO₂/PAEDTC@MIL-101 were 3.17, 2.66, and 2.38, respectively.

2.2. The Performance of Different Systems in AR14 Removal

To better understand the proficiency of the sonophotocatalytic system in degrading AR14, comparative experiments were performed for different systems under the same operating conditions. Based on Figure 4a, the removal efficiencies of 30.4% and 42.9% were obtained by photolysis and sonolysis systems, respectively. This remarkable efficiency indicates that UV and US have the potential to produce reactive species through the decomposition of water molecules. The adsorption process by Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) showed a removal efficiency of 55.1% at an irradiation time of 60 min. Improving adsorption efficiency might be described by the appropriate electrostatic interaction between particles and pollutants. The point of zero charge of the catalyst and the pollutant pKa were 5.8 and 6.5, respectively. Compared to the above processes, when Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) nanoparticles are added to the photolysis and sonolysis solution, the removal efficiency of AR14 during the 60 min reaction time increases to 75.2% and 70.9%. Similarly, the kinetic rate in photocatalytic (0.0218 min⁻¹) and sonocatalytic (0.0185 min⁻¹) processes was higher than that of photolysis (0.005 min⁻¹) and sonolysis (0.008 min⁻¹) alone. Developing efficiency might be linked to the combined effect of adsorption and oxidation occurring on the catalyst surface.

When the sonophotocatalytic process is used to remove AR14, a 13–18% increase in efficiency is observed compared to the sonocatalytic and photocatalytic processes. The kinetic rate shows a similar trend and is approximately 1.5 times greater than that of catalytic processes. This event can be related to (1) the effect of sonolysis on the ^•OH and HO₂^• production through the decomposition of water molecules, (2) the rapid mass transfer between the reaction solution and the photocatalyst surface to improve the decomposition rate, (3) the reduction of e⁻/h⁺ recombination through integrating MIL-101 with magnetic nanoparticles, and (4) the generation of more ^•OH through direct decomposition of water molecules by UV light and holes. Eshaq and El Metwally reported that the performance of the Bmim [OAc]-Cu₂O/g-C₃N₄/UV/US system was higher than that of the Bmim [OAc]-Cu₂O/g-C₃N₄-based photocatalytic and sonocatalytic systems [30]. In this study, the efficiency improvement was explained as follows: When the solution is exposed to ultrasonic radiation, a cavitation bubble leads to the production of high temperatures to increase the number of holes. In addition, integrating metals with the support material reduces e⁻/h⁺ recombination as well as improves electron excitation for producing extra reactive oxygen species [33,34].

To further prove the performance of the newly synthesized catalyst in our study, a comparison between different sonophotocatalysts in dye removal was made. As can be seen in

Table 2. Comparison of performance of different sonophotocatalysts in dye degradation.

Sonophotocatalyst	Dyes	Concentration of Dye (mg/L)	Time (min)	Efficiency (%)	Ref.
Mg:CS−SnO2	methylene blue	1	120	99	[32]
ZnO	methyl orange	50	100	54	[33]
Bi2O3	Basic brown 1	10	60	67	[34]
MgTi2O5	basic fuchsin	0.5 mM	75	100	[36]
ZnO\BiVO4\Co3O4	brilliant green	10	60	99.1	[37]
Ag/Mn3O4	Congo red	30	120	≈100	[38]
CuFe2F8(H2O)2	Rhodamine-B	6	35	82.22	[39]
ZnO/CNT	Methylene blue	20	90	99	[40]
Fe3O4@SiO2@MIL-101 (Fe)	AR14	50	60	100	This study

, all the particles prepared in the studies have a suitable efficiency in removing dyes depending on the operational conditions such as dye concentration, amount of particles, light source, radiation intensity, and ultrasound power, etc. Nevertheless, Fe₃O₄@SiO₂@MIL-101 (Fe) showed excellent performance in the degradation process of AR14 dye in our study. In this semiconductor, the presence of SiO₂ not only reduces the accumulation of magnetic particles, but also prevents the recombination of electron-hole between Fe₃O₄ and MIL-101 (Fe). This event can accelerate the production of active radicals in two surfaces for pollutant degradation in a short time. Another reason for the good sonophotocatalytic activity of nanoparticles is their high specific surface area and pore diameter, which provides more sites for the sonophotodegradation activity of AR14 dye. By evaluating the photocatalytic Fe₃O₄/SiO₂/CeO₂, Channei et al. found that particles containing SiO₂ have low electron-hole recombination. In addition, the formation of core-shell particles can provide a platform for more stability of the catalyst and create two surfaces for the activity of e⁻ and h⁺ in the production of active radicals [35].

2.3. Effect of Operational Parameters

The maximum efficiency of the adsorption and oxidation processes of contaminant molecules depends on the electrostatic interaction between the contaminant and the composite. To specify the appropriate position of the interaction, the effect of evaluated operating parameters was investigated in the current study. Figure 4b shows the link between initial pH and efficiency of the sonophotocatalytic process with a catalyst dosage of 0.4 g/L, AR14 concentration of 100 mg/L, reaction time of 60 min, radiation intensity of 36 W, and ultrasonic frequency of 35 kHz. According to this figure, enhancing the performance of studied system from 98.1% to 100% is detected with increasing pH from 3 to 5, while it significantly decreases to 53.4% in 60 min with a further increase in pH to very alkaline conditions (pH = 11). To explain the effect of pH, changes in photocatalyst charge and AR14 must be considered. It is reported that the pKa value of AR14 is 6.5. This means that for pH less than pKa, the surface charge of the pollutant is positive, while at pH above pKa, the surface charge will be negative by hydrolysis of the functional groups. On the other hand, the point of zero charge (PZC) for the photocatalyst was 5.8. Above this value, the surface area of the photocatalyst will be mostly negative, and at a pH lower than PZC, the particle surface will be protonated in the form of a positive charge. Based on these cases, a suitable interaction can occur between AR14 and Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) at a pH of 5 in the sonophotocatalytic process. Low degradation in acidic and alkaline environments can be due to the repulsive interaction between the photocatalyst surface and the contaminant. Additionally, the decrease in efficiency at acidic pH could be related to the use of ^•OH to produce H₂O molecules through excess hydrogen ions (Equation (2)). The decrease in efficiency under alkaline conditions may be due to the reduction in the ^•OH radical oxidation potential. According to studies [35], the ^•OH has an oxidation potential of less than 1.9 V at alkaline pH, while at acidic pH, the value will be 2.65–2.80 V. Gholami et al. informed comparable results for sonophotocatalytic degradation of sulfadiazine by MMH/g-C₃N₄@MFC₃ [22]. Rad et al., by examining the activity of FeCuMg and CrCuMg in the degradation of acid red 113, found that the maximum efficiency occurs at near-neutral pH. In this study, we perceived a diminution in efficiency at very alkaline pH; the observed finding might be explained according to the inhibition of various surface complexes of hydroxyl ions in the interaction of photocatalyst, ultrasonic, and visible light [21].

$${}^{•}\mathrm{OH}+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_2\mathrm{O}$$

(2)

Figure 4c represents the effect of catalyst dosage on the sonophotocatalytic degradation of AR14 in experimental conditions, e.g., pH of 5, AR14 concentration of 100 mg/L, radiation intensity of 15 W, and ultrasonic frequency of 35 kHz. As expected, with increasing photocatalyst dosage from 0.125 to 0.5 g/L, the degradation efficiency represented a significant development from 77.09% to 100% at an irradiation time of 60 min. The kinetic rate inside Figure 5c improved from 0.0231 to 0.988 min⁻¹ with an initial increase in dosage from 0.125 to 0.5 g/L. This may be related to the increase in the number of active sites for the uptake of the contaminant and its interaction with reactive species produced at the particle surface. In addition, increasing the number of active sites leads to the production of more ^●OH by improving the cavitation bubble production rate and the decomposition of H₂O₂ by iron loaded on the catalyst [41,42]. Despite these results, a further increase in the catalyst to 0.625 g/L reduces the process efficiency and the kinetic rate. When a large quantity of particles are added to the solution, the turbidity of the reaction solution increases, which leads to the scattering of ultrasound radiation and light close to the catalyst surface [43]. Moreover, high doses of photocatalysts, in addition to consuming reactive species through the high number of metal ions, increase the likelihood of accumulation in the liquid phase and loss of reaction sites. Isari et al. observed similar results for the effect of photocatalyst dosage on the sonophotocatalytic degradation of pollutants and reported that high catalyst values reduced the degradation efficiency through spontaneous decomposition of activated radicals [44]. Figure 4d shows the effect of ultrasound frequency on the degradation of AR14 by the sonophotocatalytic process. According to this figure, the pollutant degradation rate increases from 82.4% to 100% with increasing frequency from 20 to 50 kHz. The kinetic rate similarly improved from 0.0279 to 0.053 min⁻¹ with increasing ultrasound frequency. The development of efficiency might be related to an enhancement in bubble production and the consequent production rate of reactive oxygen species for the degradation of pollutants. In addition, improving the ultrasound frequency increases the turbulence rate for proper interaction of contaminants and particles. Further, increasing the frequency rate leads to reactivation and removal of the settled byproducts of the contaminant degradation from the catalyst surface. Sun et al. examined the TiO₂/Bi₂WO₆/N-Ti³⁺ based sonophotocatalysis process and found that the efficiency increased to over 95% by increasing the frequency from 35 to 53 kHz [45]. The effect of UV intensity on AR14 sonophotocatalytic degradation was evaluated (Figure 4e). As seen, by raising the radiation intensity from 8 to 36 V, the efficiency increases from 77.9% to 100% at 60 min. A similar trend was observed in the kinetic rate within Figure 4e. Our observed improvement can be explained by the increase in the amount of photons available to produce oxidative species.

The effect of the initial AR14 concentration on its removal by the Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe)/UV/US process is publicized in Figure 4f. It represents a decline in degradation efficiency at all reaction times as the initial AR14 concentration rises from 10 to 100 mg/L. The kinetic rate decreased similarly from 0.0988 to 0.0327 min⁻¹ with increasing pollutant concentration from 10 to 100 mg/L. This might be related to a decrease in the ratio of photocatalyst to AR14 molecules. Additionally, high AR14 concentrations can reduce the amount of UV light penetrating the catalyst surface, thus reducing the amount of active radicals produced at the catalyst surface and the reaction solution. Yang et al. observed a decrease in the removal efficiency of perfluorooctanoic acid through the sonophotocatalysis process. They explained the event based on the quenching effect of polluting molecules and their products on the production of reactive species [46].

2.4. Mineralization and Biodegradability

One of the important criteria for evaluating the performance of oxidation processes in eliminating organic matter is the amount of mineralization. This parameter is actually a tool used to show the ability of the oxidation process to degrade organic matter and break carbon bonds, especially benzene rings in the structure of organic matter [47]. In the present study, the TOC index under optimal conditions was used to evaluate the mineralization rate of AR14 removal in Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe)/UV/US. The TOC removal rate during AR14 degradation by sonophotocatalysis was demonstrated in Figure 5a. Considering this figure, with increasing the reaction time from 0 to 120 min, the efficiency increased from 0 to 69.96%. During this time, the reaction concentration of TOC decreased from 130.5 to 39.2 mg/L. This TOC removal rate was almost 30% lower than the AR14 removal rate, indicating the production of simple organic compounds during dye degradation.

In order to verify the results of TOC removal and to evaluate the applicability of the sonophotocatalysis process as a pretreatment process or an environmentally friendly process, a biodegradability test based on BOD₅/COD ratio was performed. According to studies, the BOD₅/COD ratio above 0.4 indicates the biodegradability of wastewater [48]. Considering the results in the inset of Figure 5a, it can be seen that with increasing the irradiation time from 0 to 120 min, the BOD₅/COD ratio increases from 0.23 to 0.88. These findings emphasize that the biodegradability of treated wastewater using the sonophotocatalytic process has been achieved from low irradiation time and that non-biodegradable compounds have been converted to simple biodegradable compounds. Employing carbon oxidation state (COS) and average oxidation state (AOS) techniques can be considered to be effective for further confirming the biodegradability of wastewater treated with oxidation processes [49,50]. Equations (3) and (4) show how to calculate the COS and AOS values,

$$\mathrm{COS}=4-1.5\left[\frac{\mathrm{COD}}{{\mathrm{TOC}}_{\mathrm{t}}}\right]$$

(3)

$$\mathrm{AOS}=4-1.5\left[\frac{\mathrm{COD}}{\mathrm{TOC}}\right]$$

(4)

where COD embodies the concentration of COD at time t (mg/L), TOC represents the initial concentration of TOC (mg/L), and TOC_t indicates the concentration of soluble TOC at time t (mg/L).

The COS and AOS values of the effluent treated by the sonophotocatalytic process were shown in Figure 5b. According to this figure, the AOS and COS values gradually improve from −0.73 and +0.46 to 1.50 and 3.44 after 120 min. These rate changes indicate that AMX molecules have been converted to degradable compounds through sonophotocatalysis. By evaluating the N-Cu co-doped TiO₂@CNTs based on the sonophotocalysis process, Isari et al. obtained ranges from −1 to +3 and +1 to +3 for AOS and COS, respectively and reported the excellent potential of the process to convert contaminant molecules into biodegradable compounds [44]. These changes might be imputed to the synergistic effect of ultrasound and UV radiation. When the photocatalysis process is subjected to ultrasound, forming the free radicals and bubble, transfer of pollutant mass to the particle surface and removal of byproducts from active photocatalyst sites are significantly increased.

2.5. Reusability and Consumption Energy

The high stability and recyclability of the catalyst are one of the important parameters in practical applications [51]. Therefore, in the present study, recycling experiments were fulfilled in eight consecutive reaction cycles (Figure 5c). As identified, from cycle no. 1 to no. 8, a decline in efficiency is detected from 100% to 90.7%. Nevertheless, the synthesized catalyst brought forward a recyclability and stability of over 90% for the entire sequential cycles. These findings emphasize that Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) is a stable, recyclable and effective photocatalyst for use in the degradation process. The observed result might be linked to saturating the active sites of photocatalyst, the destruction of particle pores, and releasing metal ions [52,53]. After eight series of catalyst recycling and reuse, the catalyst is still able to maintain its structure, which is confirmed by using the XRD image shown in Figure 5d.

Our observed result is approved by reported results in the studies, which used heterogeneous nanocatalysts [54,55]. In mentioned research, they reported the reduced degradation and described it according to the following reasons: a decrease in the catalyst amount in the recovery process, saturation and degradation of the active catalytic site, as well as releasing iron from the catalyst surface. In addition, intermediate products from pollutant degradation can poison active particle sites by competing with UV-induced photons. Al-Musawi et al. examined the diazinon degradation by GO– CoFe₂O₄/UV and found that the catalyst was capable of eight consecutive reaction cycles to degrade the pollutant with an efficiency above 90%. In this study, the reduction in efficiency was explained based on the release of cobalt and iron from the catalyst surface and the occupation of active catalyst sites with products produced from pollutant [56]. Lops et al. reported that increasing the reaction cycle from one to four reduced the performance of ZnO catalyst by approximately 10% [57]. Mansourian et al. gained same results for the ability of CeO₂/TiO₂/SiO₂ in the sonophotocatalytic system in four consecutive reaction cycles. In this study, the removal efficiency of chlorpyrifos was lessened from 80% to 75% [58].

Treatment cost, like particle stability, is another critical parameter for the practical application of the sonophotocatalysis process. Equation (5) was used to calculate energy [59,60], in which electrical energy is based on kWh/m³. In this equation, Pi signifies the power of ultrasound and UV lamp (kW), t implies the treatment time (h), V is the reaction volume (L), and C₀ and C_e are the initial and final concentrations of AR14. According to the calculations, the energy consumption for 25, 50, 75, and 100 mg/L of the pollutant at 30 min was 19.33, 26.66, 37.423, and 48.62 kWh/m³. Considering the results, it can be seen that low concentrations of AR14 require less energy than high concentrations of pollutants. In addition, the total energy consumption obtained for sonophotocatalytic treatment in the present study was lower than that of Yentür and Dükkancı for the degradation of carbamazepine by Ag/AgCl supported BiVO₄/UV/US [61]. Energy-efficient of the present system may be related to the activity of Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) in better electron transfer and production of more active species.

$${\mathrm{E}}_{\mathrm{E}\mathrm{O}}=\frac{{\mathrm{P}}_{\mathrm{i}}\times \mathrm{t}\times 1000}{\mathrm{V}\times 60\times \log \left(\frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{e}}}\right)}$$

(5)

2.6. Mechanism of Sonophotocatalytic Degradation

To pinpoint reactive active species in degrading AR14 by Fe₃O₄@SiO₂ /PAEDTC@MIL-101 (Fe)/UV/US process, experiments related to radical trapping were fulfilled with different scavengers. In this study, EDTA was used as the h⁺ scavenger, IPA as the ^•OH scavenger and BQ as the O₂^•− scavenger. The results in Figure 6 show that with the presence of BQ in the reaction solution, the efficiency decreases from 100% (reactor without scavenger) to 86.9%. This event shows that O₂ molecules were converted to O₂^•− through the excitation of electrons. When IPA and EDTA scavengers are added to the reaction solution, the efficiency decreases from 100% to 66.7% and 63.2%, respectively. These results emphasize that the ^•OH and h⁺ along with O₂^•− are produced in the reaction solution through the sonophotocatalytic process. Previous studies have confirmed the production of all three reactive radicals in degrading acid orange 7, ibuprofen and acid blue 113 using H₂O₂-US-UV/TiO₂ [62], Fe₃O₄@MIL-53 (Fe)/UV [63] and CrCuMg LDH/UV/US [21], respectively.

The possible degradation mechanism of the sonophotocatalytic process was proposed based on the findings of the trapping test and previous studies [64,65,66]. According to Figure 7, some contaminants can be removed by adsorption and photolysis. AR14 dye removal can also be achieved in solution and catalyst surface through sonolysis and sonocatalytic processes. During these processes, the hot spot event (occurred due to the cavitation effect) can decompose water molecules to produce ^•OH and H^• (Equation (6)). In the same pathway, US radiation through the sonoluminescence mechanism creates visible light radiation for forming electron/hole pairs in the valence and conduction bands of particles [67]. On the other hand, under UV radiation, Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) nanoparticles produce electron-hole pairs by absorbing light (Equation (7)). In the valence band, the holes, in addition to direct degrading the contaminant, can decompose H₂O molecules and hydroxyl ions (OH⁻) to produce ^•OH (Equations (8) and (9)). In the conduction band, electron leads to the production of O₂^•− through the oxidation of oxygen molecules (Equation (10)). Finally, all species produced in the solution and solid phases can convert pollutants to CO₂, H₂O, and biodegradable products (Equation (11)).

$${\mathrm{H}}_2\mathrm{O}+)))\to {}^{•}\mathrm{OH}+\mathrm{H}{}^{•}$$

(6)

$${\mathrm{Fe}}_3{\mathrm{O}}_4 {\mathrm{SiO}}_2@\mathrm{PAEDTC}/\mathrm{MIL}-101\left(\mathrm{Fe}\right)+ ))) \mathrm{or} \mathrm{UV}\to {\mathrm{h}}_{{\mathrm{VB}}^{+}}+{\mathrm{e}}_{{\mathrm{CB}}^{-}}$$

(7)

$${\mathrm{h}}_{{\mathrm{VB}}^{+}}+{\mathrm{H}}_2\mathrm{O}\to +{\mathrm{H}}^{+}+{}^{•}\mathrm{OH}$$

(8)

$${\mathrm{h}}_{{\mathrm{VB}}^{+}}+{\mathrm{OH}}^{-}\to {}^{•}\mathrm{OH}$$

(9)

$${\mathrm{e}}_{{\mathrm{CB}}^{-}}+ {\mathrm{O}}_2\to {\mathrm{O}}_2^{-•}$$

(10)

$$\mathrm{AR}14+\mathrm{active} \mathrm{radicals}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{byproducts}$$

(11)

3. Materials and Methods

3.1. Materials

The HCl, NaOH, tetraethyl orthosilicate (TEOS), methanol (MA), Ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ), isopropanol (IPA), 2-propanol, ammonium hydroxide (NH₄OH), acetone, carbon disulfide (CS₂), N-(2-Aminoethyl)-3-(aminopropyl) trimethoxysilane (AEAPTMS), 2-hydroxyterephthalic acid (H₂BDC), and dimethylformamide (DMF) were purchased from Merck and Sigma Aldrich company. Acid Red 14 dye (AR14, C₂₀H₁₂N₂Na₂O₇S₂, MW = 502.42 g/moL) was provided as the target contaminant by Merck Co., Germany. Magnetic nanoparticles (Fe₃O₄, purity = 98%, size = 20–30 nm) were prepared from US Research Nanomaterials Inc., Houston, TX, USA. Analytical grade of above mentioned chemicals without any purification were utilized in present work.

3.2. Synthesize of Fe₃O₄ SiO₂/PAEDTC

In Figure 8, the method for synthesizing the Fe₃O₄@SiO₂/PAEDTC nanoparticles has been represented. According to this figure, Fe₃O₄ (1.5 g) was added to the HCl solution (0.2 M, 20 mL) for modification. It was then rinsed with distilled water and dried in an oven at 60 °C for 24 h. Next, nanoparticles (1 g) were added to the H₂O/MA/NH₄OH/TEOS mixture (250, 75, 4, and 3 mL, respectively); this was done for formation of Fe₃O₄ SiO₂ particles. The mixture was separated by a magnet after 45 h of stirring at 45 °C, washed, and dried at room temperature. The synthesized brown particles were poured into a volumetric flask containing 100 mL of toluene and 1 mL of AEAPTMS and then stirred for 15 h. The impurities of the resulting product were eliminated by thoroughly washing it with acetone (15 mL)/methanol (20 mL). Dried Fe₃O₄ SiO₂ nanoparticles were then added to a volumetric flask containing 2-propanol (150 mL), sodium hydroxide (1.0 M, 3 mL), and CS₂ (2.4 mL) and stirred for 5 h. Finally, separating Fe₃O₄@SiO₂/PAEDTC nanoparticles from the mixture was performed by the magnetic field, exposure to the washing process with water/methanol, and the drying process, in an oven at 50 °C for 24 h.

3.3. Synthesize of Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe)

According to Figure 1, for the synthesis of our desired nanocomposite, solutions A and B need to be prepared. In solution A, H₂BDC with weight of 2 g was dissolved in 100 mL DMF, and in solution B, the Fe₃O₄@SiO₂/PAEDTC nanoparticles were dispersed in a volumetric flask, which comprised 80 mL DMF and 6 g FeCl₃·6H₂O. After this, solutions A and B were combined into a Teflon autoclave at 115 °C for 20 h. Lastly, catalyst were isolated using a magnet, and a rinsing process with water/ethanol mixture and drying process at room temperature was considered for them.

3.4. Characterization

Different characteristics of the prepared samples were examined through employing commonly used techniques. A list of these techniques is represented as follows: XRD analysis (Bruker Corporation, Karlsruhe, Germany) with Cu Kα radiation at 30 mA and 40 kV, SEM/EDX (JEOL, Tokyo, Japan), TEM (JEOL, Tokyo, Japan), vibrating sample magnetometer (VSM, MDK Kashan, Iran), BET, and Barrett—Joyner—Halenda (Micromeritics Instrument Corporation, Norcross, GE, USA).

3.5. Sonophotocatalytic Experiments

The AR14 degradation efficiency was evaluated by the sonophotocatalysis process in an ultrasonic bath in the presence of a UV lamp. A specific dose of our prepared catalyst (0.125–0.625 g/L) was poured to the 250 mL Erlenmeyer Pyrex containing studied concentrations of AR14 (25–100 mg/L) at a determined pH (3–11). The initial pH adjustment was performed by a pH meter with the addition of NaOH and H₂SO₄. The reactor was subjected to ultrasonic frequencies (20–50 kHz) and different UV light intensities (8–36 W) to apprehend the effect of the parameters better. During the different times (10–60 min), 1 mL sample was taken and the concentration was determined by UV-vis spectrophotometry at 550 nm. The degradation efficiency and reaction kinetics for AR14 degradation were calculated using Equations (12) and (13) [29,30,31]. In these equations, C₀ is the initial concentration of AR14, C_e is AR14 concentration at time t, k is constant kinetics (min⁻¹), and t is reaction time (min).

$$\mathrm{R}(\%)=\frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{\left({\mathrm{C}}_0\right)}\times 100$$

(12)

$$\ln \frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{e}}}=-\mathrm{kt}$$

(13)

The reactive species in the degradation of AR14 in sonophotocatalysis reactors were studied through EDTA, IPA, and BQ scavengers. The stability of the synthesized photocatalyst in the sequential reaction cycle was evaluated with the washing, drying, and reuse steps in the same condition. The amount of COD and BOD₅ were determined to estimate the biodegradability of the effluent by UV-vis spectrophotometry and BOD meter, respectively. The inlet and outlet TOC values of the reactor were determined by a TOC analyzer to appraise the mineralization rate provided by the evaluated process.

4. Conclusions

In the present research, the Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) was synthesized as a newfound sonophotocatalyst for degrading AR14 in aqueous solutions. Diagnostic analyses such as SEM, TEM, and XRD showed that particles with a surface area of 992 m²/g and pores of 2.05 nm were synthesized. Fe₃O₄@SiO₂/PAEDTC@MIL-101 (Fe) nanoparticles under UV light and ultrasound waves have a good performance against sonolysis, photolysis, adsorption, and photocatalytic processes. The removal efficiency of AR14 increases with increasing operating parameters such as nanoparticle content, ultrasound frequency, and radiation power, while it decreases with increasing the initial pH and initial concentration of AR14. Biodegradability (BOD₅/COD ratio) improved from 0.23 to 0.88 after 120 min of sonophotocatalytic treatment with COD removal of 84.15%. The maximum TOC removal was 69.96% under optimal conditions including pH of 5, catalyst dosage of 0.5 g/L, initial AR14 concentration of 100 mg/L, and US frequency of 35 kHz. The addition of different scavengers to evaluate the active species of the sonophotocatalytic process showed that all three species including O₂^•−, ^•OH, and h⁺ were produced during the process, and that h⁺ is the active species in the AR14 degradation into CO₂ and H₂O. Analysis of COS and AOS parameters and stability test showed that Fe₃O₄@SiO₂ /PAEDTC@MIL-101 (Fe) catalyst could be considered as a new sonophotocatalyst for AR14 mineralization. To complete this study in the future, the desired process can be investigated with the addition of hydrogen peroxide. It is also suggested to use the desired process in the real textile industry. In addition, in future studies, work function (or Fermi level) can be investigated to better understand the electron transfer between metal and semiconductors.