MB degradation under UV–Vis using ZnO-Fe2O3@SiO2 monolithic filters in a CPC reactor

MB degradation under UV–Vis using ZnO-Fe2O3@SiO2 monolithic filters in a CPC reactor | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article MB degradation under UV–Vis using ZnO-Fe2O3@SiO2 monolithic filters in a CPC reactor A. S. Galindo-Luna, I. Juárez-Ramírez, M. E. Zarazúa-Morín, D. Sánchez-Martínez, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9683860/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract It is reported the use of ZnO-Fe 2 O 3 @SiO 2 monolithic filters to the degradation of methylene blue (MB) under UV-Vis light irradiation in a CPC reactor. ZnO-Fe 2 O 3 @SiO 2 monolithic filters were obtained by deposition of ZnO and α-Fe 2 O 3 particles on monolithic silica filters through hydrothermal method. Structural analysis by X-ray diffraction (XRD) showed the presence of ZnO and SiO 2 phases, corroborated by Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS), where chemical composition also detected a low percentage of Fe. Likewise, by X-ray Photoelectron Spectroscopy (XPS) the oxidation state of the elements presented in all phases was detected. Results exhibited that ZnO and Fe 2 O 3 particles are homogeneously deposited and distributed on the silica surface to promote oxidation-reduction reactions during the degradation process. Photocatalytic activity of ZnO-Fe 2 O 3 @SiO 2 monolithic filters used in a CPC photocatalytic reactor for the degradation of methylene blue (MB) achieved 95% efficiency remaining intact after three reaction cycles, reaching also a high mineralization reaction (66%). The proposal mechanism indicates that the high generation of oxidizing species (peroxide and hydroxyl radicals), provoked by the presence of the deposited ZnO and α-Fe 2 O 3 semiconductors, causes the breaking of the bonds of the dye until mineralization. In conclusion, ZnO-Fe 2 O 3 @SiO 2 monolithic filters here reported are presented as an innovative alternative for their application to degrade methylene blue dye in a CPC photocatalytic reactor for water treatment, due to its easy preparation, practicality and recyclability. Silica filters ZnO-Fe2O3@SiO2 CPC reactor Photocatalysis Methylene blue Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. INTRODUCTION In recent decades there has been increased concern about the presence of emerging pollutants in wastewater due to possible health effects as endocrine disruptors, antiandrogens capable of creating microbiological resistivity or producing metabolic alterations in humans, while in aquatic organisms they may have a cumulative effect and sex change [ 1 – 3 ]. This problematic is because many of these organic compounds are highly stable, biodegradation-resistant, and water-soluble species. Synthetic dyes are widely used in the textile, paper, leather, plastic, printing, and cosmetic industries, so effluent with a small amount of these dyes can cause serious local environmental problems [ 4 ]. Heterogeneous photocatalysis is presented as a viable option for the degradation of organic pollutants due to its capacity to produce drastic changes in the structure of the contaminant due to the generation of oxidative species that achieve the mineralization of the molecule [ 5 – 7 ]. Photocatalysts are commonly used as powders suspended in solution for their practicability [ 8 ]. However, some problems are present during their use such as a low and inherent surface area, limited adsorption capacity, secondary contamination and difficulties in scaling up processes and recyclability [ 9 – 10 ]. To mitigate these disadvantages, different semiconductor deposition techniques have been used, among the most used are immobilized films on different substrates (FTO, ITO, metal sheets and polymers), coating of lamps or surfaces of a reactor and different types of sponges and monoliths [ 9 , 11 – 13 ]. However, the immobilization of the photocatalyst on substrates generally leads to a decrease in photocatalytic activity, reducing its efficiency by one tenth due to the low specific surface area and low dispersion of particles and light absorption [ 10 ]. Therefore, the deposition of semiconductors on absorbent monolithic materials has aroused great interest, providing a stable matrix that improves oxidation-reduction processes, increasing recyclability [ 9 – 11 ]. A few years ago, our group reported the use of silica monolithic filters which showed excellent behavior for the adsorption and photodegradation of methylene blue dye (MB) under UV–visible light irradiation [ 14 ]. In fact, SiO 2 has been used as ceramic support for the deposition of ZnO, α-Fe 2 O 3 metal oxides, which have energy bandgap values of 3.1 and 2.0 eV, respectively [ 15 – 17 ]. These catalysts are easily synthesized by soft chemistry methods, as well as being abundant and economically suitable, which suggests that they are appropriate for deposition on a monolithic filter. On the other hand, silica filters are materials used in different industries, due to its various physico-chemical properties, such as: high absorption capacity, being an excipient material, possessing pyroelectric and piezoelectric properties, and be the second most abundant material on the planet [ 18 – 19 ]. Also, it has been reported that SiO 2 template limits the agglomeration in the growth of nanoparticles, favors good dispersion and improves photonic absorption and charge transportation [ 20 ]. In addition, it favors the reduction of photoluminescence emission associated with the recombination of charge. Another application of silica is its use as an intermediate which improves the adsorption and stability of the compound due to the porous surface of the silica and the electron storage capacity of the semiconductor deposited on the surface with high oxidation capacity [ 21 – 22 ]. Therefore, this paper reports the preparation of ZnO-Fe 2 O 3 @SiO 2 monolithic filters exfoliated by hydrothermal method to promote the separation of photogenerated charges, as well as to improve the stability of this compound. In addition, these filters were evaluated in a CPC photocatalytic reactor, looking for better photocatalytic efficiency for the degradation of organic compounds such as methylene blue. Morphology, oxidation states of the metal oxides on the surface, adsorption capacity, oxidizing species identification and photocatalytic recyclability of the monolithic filters are discussed too in this paper. 2. EXPERIMENTAL 2.1 Materials and methods. SiO 2 commercial ceramic supports (BIOMAX) and ammonia solution (99%, DEQ, CAS: 1336-21-6) were used to adjust the pH for all experiments. Exfoliation of monolithic filter was performed by hydrothermal method [ 14 ]. 2.2 Synthesis of ZnO@SiO 2 . First, the monolithic filter was immersed in deionized water for 60 min to ensure maximum water absorption, then zinc acetate 0.489 M was added until completely dissolved. Immediately, NH 4 OH was used to adjust pH to 8, after that, polyvinyl pyrrolidone (PVP) (0.3 g) was added. The resulting solution and the monolithic filter were transferred to a Teflon-lined and heated at 180°C for 14 h, for the exfoliation process and the in-situ crystallization of ZnO. The recovered solid was washed with water and ethanol to neutralize the solution and finally dried to 80°C for 6 h. 2.3 Synthesis of Fe 2 O 3 @SiO 2 . The monolithic filter was immersed in deionized water for 60 min. 0.696 M of ferric nitrate was added. Subsequently, KOH (5 M) was added up to increase pH to 12. The resulting solution and the monolithic filter were transferred to a Teflon-lined and heated at 120°C for 8 h. 2.4 Synthesis of ZnO-Fe 2 O 3 @SiO 2 . The preparation of monolithic filter ZnO-Fe 2 O 3 @SiO 2 was carried out in two steps. First, the deposition of the zinc oxide particles is carried out, as outlined in section 2.2 . After that process, the iron oxide particles are deposited using the previously synthesized monolithic ZnO@SiO 2 filter, as detailed in previous section. Finally, the recovered monolithic filters were washed with distilled water and ethanol to neutralize the solution and finally dried at 80°C for 12 h. 2.5 Monolithic filters characterization. All monolithic filters were analyzed by X-ray diffraction (XRD) using a PANalytical Empyrean diffractometer with CuKα radiation (λ = 1.5418 Å) at a scanning range of 10–70 degrees and scan rate of 0.02 degrees per minute to analyze the phase formation and crystalline structure. Morphology of materials was analyzed by scanning electron microscope (SEM) JEOL JSM6490 LV operated at 20 kV and equipped with microanalysis dispersion X-rays (EDS) for chemical quantification. The oxidation states of the metal oxides on the surface of monolithic filter were determined by X-ray photoelectron spectroscopy (XPS) using a Thermofisher XPS Escalab 250Xi equipped with an AI Kα (1486.68 eV) monochromatic radiation. The surface area was determined by N 2 adsorption measurement at 77 K using a Mini Belsorp II (Bel Japan). All the samples were vacuum dried at 300°C for 1 h before the test. Dye degradation was followed by measuring the absorbance value at 663 nm in a UV-vis NIR spectrophotometer (Cary 5000) in the range of 200–800 nm. To determine the degree of MB mineralization, total organic carbon (TOC) analysis was conducted in a Shimadzu 5000 model equipment. Kinetic of the reaction was monitored by HPLC liquid chromatography using a Shimadzu equipment model LC-2030C with a C18 5 µm column (50 X 3.0 mm) and a PDA detector (λ = 264 nm), with a mobile phase of methanol, water (70:30), with a retention time of 5 min, and a flow rate of 0.5 µm, excited at a wavelength of 246 nm, while for DXT, the mobile phase was acetonitrile and water (50:50), with a retention time of 1.12 min, and a flux of 10 µm, excited at a wavelength of 254 nm. 2.6 Photocatalytic degradation. The experiments were carried out in a Pyrex glass reactor (150 ml), equipped with a propeller stirrer with a constant stirring rate of 150 rpm, illuminated with a xenon (Xe) lamp of 35 W (400–700 nm), 3200 lumens and 6000 K, maintaining a temperature around 30°C. The photocatalytic experiment was conducted with 100 ml of solution for each monolithic filter used. The solution was stirred in dark for 60 min before the light was switched on to ensure adsorption-desorption equilibrium. Aliquots of 3 ml were taken at a regular time interval and measured in the UV-Vis equipment and in the HPLC equipment. The degradation rate was calculated by Eq. 1 [ 7 ] \(\:d\left(\%\right)=\frac{{A}_{0}-{A}_{t}}{{A}_{0}}X\:100\) … (Eq. 1) Where d is degradation rate, A t is absorption after radiation and A 0 is absorption before radiation. To determine the oxidizing species used in the degradation process of the organic molecule during the reaction, different chemical agents were added to trap the species generated during the process. The procedure to determine the oxidizing species (h + , OH., H 2 O 2 and O 2 − ) was like the previously described for the photocatalytic evaluation. Table 1 showed the chemical reagents and the corresponding dye concentration. Table 1 Chemical agents for determination of oxidizing species. Chemical reagent Species Concentration (MB) KI h + 0.0006 g Isopropanol OH· 0.003 ml Catalase H 2 O 2 0.06 g Benzoquinone O 2 − 0.0004 g On the other hand, recyclability tests were carried out to determine the average lifetime of the monolithic filters. To determine the surface pore saturation and adsorption capacity, filters were dried at 80°C for 12 h, without additional treatment. The monolithic filter is evaluated in three consecutive cycles, without any further treatment of washes, in the degradation of the methylene blue dye. 3. RESULTS AND DISCUSSION 3.1 X-ray diffraction (XRD). Figure 1 shows the X-ray diffraction pattern of the monolithic filters with the incorporation of the metal oxide particles (ZnO and Fe 2 O 3 ) deposited. It is observed that the XRD pattern of the monolithic silica filter (a) chemically treated by the hydrothermal method at 180ºC for 14 hours showed the presence of the phases identified as α-cristobalite, β-quartz, Na 2 O and Al 4 O 4 C. During the exfoliation of the monolithic silica filter (b) it was observed that the increment in temperature during hydrothermal treatment and the pressure generated inside the reactor causes a variation in the length and angle in the silicon and oxygen bonds, favoring the crystallization of the α-cristobalite and the formation of silanoles Q 2 (Si-OH) and Q 3 (O 3 -Si-OH) which have been related to the adsorption capacity and stability of SiO 2 [ 14 ]. On the other hand, the monolithic filter with zinc oxide particles (ZnO@SiO 2 ) (c) showed the presence of highly crystalline particles of the ZnO, evidenced by the existence of the three representative peaks of the material at 31.73, 34.37, and 36.21º (JCPDS 01-070-8070). Likewise, XRD pattern of Fe 2 O 3 @SiO 2 (d) showed low crystallinity of Fe 2 O 3 (JCPDS 00-033-0664) peaks, which are overlapped by the SiO 2 peaks which normally present high crystallinity. Additionally, the diffractogram of the monolithic filter deposited with particles of zinc oxide and iron oxide (ZnO-Fe 2 O 3 @SiO 2 ) (e), showed mainly the presence of ZnO and SiO 2 peaks, while peaks of Fe 2 O 3 particles are not detected due to its low crystallinity. 3.2 X-ray photoelectron spectroscopy analysis (XPS). XPS analyses were performed to determine the oxidation states and elements present on the surface of the monolithic filters that were deposited with ZnO and Fe 2 O 3 particles. Figure 2 shows the XPS spectrum of the exfoliated filter, comparing the ZnO@SiO 2 , Fe 2 O 3 @SiO 2 , and ZnO-Fe 2 O 3 @SiO 2 filters. In the filter spectrum ZnO@SiO 2 , it is observed the signals from Zn 2p 3 to 1021.16 eV and Zn 2p 1 to 1043.90 eV, which demonstrate the existence of ZnO [ 23 ]. On the other hand, the filter Fe 2 O 3 @SiO 2 shows a spectrum similar to the exfoliated monolithic filter, with a slight emission of Fe 2p at 710.23 eV corresponding to the Fe 3+ cation of hematite (α-Fe 2 O 3 ) [ 24 ]. Finally, the spectrum corresponding to the filter ZnO-Fe 2 O 3 @SiO 2 is shown, which stands out for exhibiting more intense Fe emissions in 710.98 and 724.53 eV, which correspond to Fe 2p 3/2 and Fe 2p 1/2 , which agrees with the previously reported [ 24 – 25 ]. This result corroborates the presence of Fe 2 O 3 on the surface of the monolithic filter. Figure 2 b shows the decovolution of the elements present on the surface of the ZnO-Fe 2 O 3 @SiO 2 monolith. Subsection (a) shows the spectra of Si 2p which shows three components of SiO 2 to 101.43 eV, Si-O-Si to 102.36 eV and Si-OH to 103.33 eV [ 26 – 27 ]. In section (b) O 1s peaks are shown, the high peak (O L ) at 531.85 eV corresponds to the O 2 − ions frequently surrounded by Si ions in the SiO 2 structure. The O V peak at 531.03 eV is due to oxygen vacancies, and the O C peak at 529.7 eV is related to chemisorbed oxygens species. In section (c) shows three peaks of the adjusted Fe 2p spectrum, peaks at 710.8 and 713.1 eV were indexed to Fe 2p 1/2 , and peak 724.8 eV corresponds to Fe 2p 3/2 . While peaks at 719.1 and 732.7 eV were assigned to the Fe 3+ of the α-Fe 2 O 3 [ 25 , 28 ]. On the other hand, subsection (d) shows the spectrum Zn 2p divided into two peaks at 1044 and 1021.22 eV corresponding to Zn 2p 1/2 and Zn 2p 3/2 , respectively, which are assigned to the tetrahedral Zn with a 2 + oxidation state. Zn 2p 3/2 peak could originate in Zn-O bonds, in addition, it could be considered a weak peak at 1022.73 eV attributed to Zn-OH because hydrogen has a greater electronegativity than Zn, which leads to a higher density of positive charge in the atoms attached to the OH groups [ 29 – 32 ]. 3.3 Morphology of monolithic filters by scanning electron microscopy (SEM). A surface analysis was performed using the SEM technique on the internal and external surface of the monolithic filter. Figure 3 corresponds to the SEM images of the monolithic filter exfoliated by the hydrothermal method and the filters deposited with the metallic oxides (ZnO and Fe 2 O 3 ). Section a-b show a rough surface on the filters, which breaks into scales and falls off the surface, leading to the formation of small spherical particles of about 1 µm. Cracking and surface breakage is mostly influenced by reaction time, while the pH of the solution defines the particles cracked on the surface spherically, as reported by Xu Hai Yan et al., that the NH 4+ ion can modify the growth of crystals under hydrothermal conditions [ 14 , 33 ]. The surface modification of the silica filter caused by the hydrothermal process is due to a random rupture of the silicon-oxygen bonds. The different atomic arrangement and the various forms of bond break Si-O in the β-quartz and α-cristobalite crystalline structures gave rise to a different microstructure on the surface. Which could play a crucial role in adsorption processes and the deposition of semiconductors on the surface [ 34 ]. Section c-d shows the zinc oxide particles in the form of hexagonal prisms deposited on the monolithic filter. The internal surface shows a morphology of well-defined bars up to 5 µm in diameter that grow from a nucleus, and these are stretched to achieve the observed shape. While, on the outer surface, stands out the growth of ZnO bars on a rough surface, which were cultivated and grown on the surface of the silica filter and acts as a template for the continuous infiltration of Zn 2+ on the exfoliated surface, and some particles of Zn(OH) 2 that continue to crystallize as a hexagonal prism separating from the silica base. This crystallization mechanism is like that reported by other authors [ 12 , 35 – 36 ]. The morphological difference in zinc oxide particles on the external and internal surface of the filter is due to the absorption capacity of zinc ions on the surface of exfoliated silica during nucleation [ 37 ]. While, in the section e-f show the SEM images of the monolithic filter with particles of α-Fe 2 O 3 deposited on the surface. The SiO 2 matrix limits the growth of α-Fe 2 O 3 particles during the nucleation process. The particles deposited on the internal and external surface of the filter show a homogeneous size and less than 1 µm, with a morphology of leaves and needles [ 38 – 39 ]. On the other hand, in (g-h), the monolithic filter ZnO-Fe 2 O 3 @SiO 2 shows a completely cracked surface product of the hydrothermal process in the metal oxides deposit. In the paragraph (g) it is possible to perceive hexagonal prisms of ZnO on the surface, while in section (h) it shows a corrugated strip in the form of leaves, like that observed in (d) where the particles of ZnO are deposited and grow. Similar to that observed by Yanjun Liu et al., Fe 3+ ions adhere and distribute on the silica surface, some others adsorb on the ZnO surface to form ions of [Fe (OH) 4 ] − , during the hydrothermal process they lose OH − and H 2 O allowing the growth of α-Fe 2 O 3 particles [ 40 – 41 ]. Deposition of metal oxides on the surface of the silica filter depends on the adsorption capacity [ 12 ]. SiO 2 matrix prevents agglomeration of deposited particles [ 41 ]. Those particles that fail in inserting into the silica surface or adhering to previously deposited zinc particles did not crystallize. Similarly, the inner and outer part of the monolithic filter shows different adsorption capacity, product of surface exfoliation by hydrothermal method, which is important because surface exfoliation causes a greater pore generation and consequently a greater affinity for the growth of particles on the surface. EDS point analysis and mapping of the ZnO-Fe 2 O 3 @SiO 2 filter were performed to determine the chemical composition and study the distribution of the elements present. Figure 4 a shows two points on the surface of the monolithic filter. Spectrum 1 indicates the corrugated morphology in the form of leaves, the elements mostly present are the Si, Zn and O (showed in insert of spectrum 1). Demonstrating the growth of ZnO particles in this area, which corresponds to morphology results previously observed in Fig. 3 d, and in a smaller percentage, the presence of Fe particles deposited and distributed over the same area can be detected also. Spectrum 2 indicates a smooth morphology on the filter surface, which corresponds to SiO 2 according to elemental analysis (showed in insert of spectrum 2). Similarly, a low percentage of Fe is detected, confirming that Fe 2 O 3 is homogenously deposited and distributed on the silica surface and ZnO particles. The presence of each one of the elements detected by mapping is shown in Fig. 4 b (Si), 4c (Zn) and 4d (Fe). Table 2 shows the BET (Brunauer–Emmett–Teller) results for surface area diameter and average pore volume of monolithic filters. According to the results it can be mentioned that the silica monolithic filters have a low interaction between the adsorbent surface and the adsorbate, this is due to the high crystalline nature of the materials as observed by XRD, obtaining a surface area with values between 2–3 m 2 /g. After the hydrothermal exfoliation process, the silica filter slightly increases the surface area and pore volume with respect to the raw filter. The Fe 2 O 3 particle deposit does not represent an apparent change from the exfoliated filter, on the other hand, ZnO particle deposited shows an increase in the surface area and pore volume [ 9 , 42 ]. However, these results are not determinant because there is no significant difference between each sample and may not be related to the photocatalytic properties of the monolithic filter. Table 2 Surface area, volume, and pore diameter of monolithic filters. Sample Surface area (m 2 /g) Pore volume (cm 3 /g) Pore size (nm) Raw Filter 0.32 0.0742 106.52 Exfoliated filter 1.57 0.3621 24.74 ZnO@SiO 2 2.56 0.5897 24.30 Fe 2 O 3 @SiO 2 1.58 0.3638 31.74 ZnO-Fe 2 O 3 @SiO 2 2.34 0.5378 24.06 3.4 UV–visible diffuse reflectance spectroscopy. Figure 5 a shows the UV-Vis spectra to study the optical properties of monolithic silica filters synthesized with ZnO and Fe 2 O 3 particles. While in Fig. 5 b the energy band structure diagram of ZnO-Fe 2 O 3 @SiO 2 is showed. The bandgap energy of the samples was calculated using the Kubelka-Munk function. Raw and exfoliated filters show an intrinsic absorption in the ultraviolet region leading bandgap values of 3.7 and 3.8 eV, respectively. However, when ZnO was incorporated on the filters (ZnO@SiO 2 ), the E g value increases until 4.5 eV. While addition of Fe 2 O 3 provoked a considerable diminishing of the E g value on the filters (Fe 2 O 3 @SiO 2 ), which is around of 1.9 eV. This demonstrates that the presence of metal oxide on the filter surface has influence on the absorption in the UV-Visible spectrum. On the other hand, the ZnO-Fe 2 O 3 @SiO 2 heterojunction presented a bandgap value of 3.3 eV, which it can absorb more photons and generate a greater number of electrons and holes, which could be favoring a better photocatalytic activity under ultraviolet and visible region. 3.5 Methylene blue (MB) degradation by photocatalysis. The photocatalytic efficiency of the filters was evaluated in the degradation of MB dye at 10 ppm. Figure 6 a shows that the deposition of metal oxides has a direct influence on the surface adsorption capacity of the molecule, and subsequently on the degradation of the dye. In this case, activity of raw filter was improved 43 and 57% when ZnO and α-Fe 2 O 3 were deposited, respectively. This result was due to the adsorption capacity of the ZnO@SiO 2 and Fe 2 O 3 @SiO 2 filters, which is correlated with the bandgap and the particle size of the metal oxides deposited on the filter surface. Particularly, it is reported that nanometric particle size of α-Fe 2 O 3 allows greater generation of active sites to adhere the organic molecule and then react with the semiconductor [ 43 ]. Therefore, filters containing Fe 2 O 3 particles showed better absorption which enhances discoloration of the dye. According to the observed in Fig. 6 a, ZnO-Fe 2 O 3 @SiO 2 filter shows an adsorption capacity superior to the raw filter and ZnO@SiO 2 , and slightly lower than exfoliated filter and Fe 2 O 3 @SiO 2 . From the above results, it is concluded that deposition of semiconductors on the filter surface causes a loss in porosity and adsorption of the dye. However, the presence of metal oxides (ZnO and α-Fe 2 O 3 ) improves oxide-reduction throughout the reaction process, maintaining a constant kinetics and high degradation efficiency (94%) for ZnO-Fe 2 O 3 @SiO 2 . The changes in the UV-Vis absorption spectrum of MB during photocatalytic degradation corroborated the effect of using the ZnO-Fe 2 O 3 @SiO 2 filter as catalyst, Fig. 6 b. Specifically, the intensity of the spectrum when this semiconductor is used decreased steadily after the adsorption process. In addition, a peak at smaller wavelengths, between 200 and 250 nm, is observed, which is attributed to the generation of oxidative radicals and the insertion of OH into the organic molecule, which leads to a bathochromic effect at the peak of maximum absorbance at 663 nm. On the other hand, Fig. 6 c shows the reaction kinetics of monolithic filters, which demonstrated that ZnO-Fe 2 O 3 @SiO 2 filter shows a linear behavior with the highest reaction speed and lowest half-time life (25 times lower than raw filter), which indicates an exponential decay with values close to a first order reaction. In addition, recyclability tests were carried out on the ZnO-Fe 2 O 3 @SiO 2 filter where it can be observed that after three reaction cycles the efficiency keeps intact (94 ± 1% after each cycle), Fig. 6 d. This result indicates that the combination of ZnO and Fe 2 O 3 deposited on surface filters to obtain ZnO-Fe 2 O 3 @SiO 2 monolithic filters are a viable option that improve recyclability processes and keeps degradation efficiency, which could facilitate practical applications in scaling photocatalytic processes. Table 2 shows the percentage of degradation, and the half-time life of each monolithic filter evaluated during three consecutive cycles in MB degradation. The results indicate that the ZnO-Fe 2 O 3 @SiO 2 filter keeps the degradation percentage intact after three reaction cycles (94, 95, 94%, during first, second and third cycle, respectively). While efficiency of the exfoliated filter, Fe 2 O 3 @SiO 2 and ZnO@SiO 2 decreased its effectiveness 34, 30 and 13%, from the first, second and third cycle, respectively. Similarly, the ZnO-Fe 2 O 3 @SiO 2 filter maintains a lower half-time life in all reaction cycles, showing a reaction rate of k = 0.0075 and R 2 = 0.997, markedly higher than the other synthesized filters. Table 2 Percentage of MB degradation, reaction kinetics, half-time life ( t 1/2 ) over 3 reaction cycles. Sample Degradation cycle 1–3 t 1/2 (min) cycle 1–3 Reaction kinetics first cycle %D-1 %D-2 %D-3 C-1 C-2 C-3 R 2 k Raw filter 37 35 24 2310 2657 2763 0.893 0.0003 Exfoliated filter 94 70 60 105 330 385 0.959 0.0066 ZnO@SiO 2 79 70 66 165 231 239 0.973 0.0042 Fe 2 O 3 @SiO 2 96 94 66 224 150 239 0.701 0.0026 ZnO-Fe 2 O 3 @SiO 2 95 96 94 92 77 112 0.997 0.0075 TOC analysis of the reaction product was performed at 300 min reaction using the different monolithic filters to determine the percentage of mineralization of the molecule. Figure 7 a shows the TOC percentage during each reaction cycle, where it is observed that the ZnO-Fe 2 O 3 @SiO 2 filter was able to mineralize 66% of the MB dye during the first cycle and decreased only 10% after subsequently cycles. The efficiency of the ZnO-Fe 2 O 3 @SiO 2 filter increased the mineralization percentage by 48 and 51% compared to the α-Fe 2 O 3 @SiO 2 and ZnO@SiO 2 filters, respectively. These results demonstrate that the adsorption and degradation process of the exfoliated filter is high, but stability is only reached by depositing of the metal oxides ZnO and α-Fe 2 O 3 , enhancing the mineralization process too. In conclusion, the monolithic silica filter is an excellent material that acts as a support for the deposition of semiconductors to obtain effective materials for photocatalytic degradation. To understand the reaction mechanism, the reactive species responsible for the photocatalytic degradation process were identified using several scavengers. Figure 7 b shows the addition of potassium iodide as a hole scavenger (h + ), isopropanol for the capture of hydroxyl radicals (•OH), catalase as a scavenger of peroxide radicals (H 2 O 2 ) and benzoquinone as a scavenger of superoxide radicals (O 2 − ). It was determined that peroxide and hydroxyl radicals are those that have a greater interference to control the degradation reaction of the MB dye. While the holes and superoxide radicals influence photocatalytic activity to a lesser extent. 3.6 Infrared spectroscopy (IR). Additionally, a liquid-liquid extraction was performed on the reaction product of the degradation reaction of MB after the first cycle to follow up the molecule breakage using the FTIR technique. Figure 8 a shows the reaction product of the exfoliated filter, and the Fe 2 O 3 @SiO 2 , ZnO@SiO 2 and ZnO-Fe 2 O 3 @SiO 2 filters. It is observed that the reflections corresponding to alkane groups (CH) at 2971 cm -1 , secondary amines (CH-NH-CH) at 1463 cm -1 , aliphatic aldehydes (CH 2 -CHO) at 1369 cm -1 and aliphatic ethers (CH 2 -O-CH 2 ) at 954 cm -1 are the only reflections remaining after 300 min of reaction. The IR spectra of ZnO-Fe 2 O 3 @SiO 2 filter showed that bonds present low intensity reflections, indicating absence of the dye molecule after the degradation process, corroborating the efficiency of the monolithic ZnO-Fe 2 O 3 @SiO 2 silica filter for the degradation of MB dye, as it was observed in the UV-Vis, and TOC analyses. According to the above results it was possible to propose a mechanism of the breakage of the methylene blue dye molecule using the ZnO-Fe 2 O 3 @SiO 2 filter, which is showed in Fig. 8 b. It is assumed that the dye bond breaking starts in the central part of the molecule, destroying the double bond of the benzene ring. This assumption is supported by the bathochromic effect observed by UV-Vis. Thus, it causes a loss of color during the first minutes of reaction. The high generation of oxidizing species provoked by the presence of the deposited ZnO and α-Fe 2 O 3 semiconductors, causes the breaking of the bonds of the dye until mineralization, it means to convert the molecule to CO 2 and H 2 O [ 44 – 46 ]. CONCLUSIONS Deposition of ZnO and α-Fe 2 O 3 particles on monolithic silica filters was successfully achieved by hydrothermal method and the presence of these deposited elements was verified by EDS analysis. XRD, SEM-EDS and XPS analysis corroborates the presence of all phases, ZnO, SiO 2 and Fe 2 O 3 , exhibiting that ZnO and Fe 2 O 3 particles are deposited onto silica filters. ZnO-Fe 2 O 3 @SiO 2 filters showed to be highly effective when used in the CPC photocatalytic reactor because enhance the reaction stability in the degradation of methylene blue (MB) dye, with 95% efficiency remaining intact after three reaction cycles. In addition, the deposited semiconductors improved the reaction kinetics by mineralizing 66% of the dye in the first cycle and decreasing to 10% for the third reaction cycle. The presence of metal oxides (ZnO and α-Fe 2 O 3 ) promotes oxidation-reduction reaction during the degradation process, maintaining high reproducibility of degradation using MB after 3 cycles. In addition, the proposal mechanism indicates that the high generation of oxidizing species (peroxide and hydroxyl radicals), provoked by the presence of the deposited ZnO and α-Fe 2 O 3 semiconductors, causes the breaking of the bonds of the dye until mineralization. Therefore, ZnO-Fe 2 O 3 @SiO 2 monolithic filters prepared in this work are excellent materials that acts as a support for the deposition of semiconductor materials to obtain effective materials for photocatalytic degradation, which could be as an innovative alternative for water treatment, due to its easy preparation, practicality and recyclability, for its use in a CPC photocatalytic reactor. Declarations Author Contribution "I. Juárez-Ramírez. Intellectual contribution; draft of the paper; experimental design; interpretation of data; discussion of the results; responsible of project with financial support; revision and approval of the manuscript""A. S. Galindo-Luna. Intellectual contribution; draft of the paper; experimental design; collection, analysis and interpretation of data; discussion of the results; revision and approval of the manuscript""M. E. Zarazúa-Morín. Intellectual contribution; draft of the paper; experimental design; discussion of the results; revision and approval of the manuscript""D. Sánchez-Martínez. Draft of the paper; discussion of the results; revision and approval of the manuscript""V. J. Gallegos-Sánchez. Draft of the paper; discussion of the results; revision and approval of the manuscript" Acknowledgement Authors would like to thank SECIHTI. A. Sebastián Galindo-Luna thanks to SECIHTI for scholarship No. 783128. Thank you to UANL for the financial support through PROACTI 2024 (113-IDT-2024). We also thank the staff of Ecomateriales and Energy Department and Materials Construction Laboratory at Civil Engineering Institute of the Civil Engineering Faculty at UANL. In addition, thanks to Dr. Francisco Servando Aguirre Tostado from CIMAV-Mty for his support with the XPS analysis. References Miriam Janet Gil, Adriana María Soto, Jorge Iván Usma, O. D. G. (2012). 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9683860","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":638533753,"identity":"39dd14a4-71ce-43e4-9494-c42fa9644818","order_by":0,"name":"A. S. Galindo-Luna","email":"","orcid":"","institution":"Universidad Autónoma de Nuevo León (UANL)","correspondingAuthor":false,"prefix":"","firstName":"A.","middleName":"S.","lastName":"Galindo-Luna","suffix":""},{"id":638533755,"identity":"e4ae5ef1-09b9-4d7e-9046-fa1f3301e27d","order_by":1,"name":"I. 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Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e and ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e, b) XPS spectra deconvoluted a) Si 2p, b) O 1s, c) Fe 2p and d) Zn 2p of the monolithic filter ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/c4ce13906ed5abaf5664d86c.jpeg"},{"id":109170461,"identity":"0b9b81a2-a1d5-4994-a6d0-2d491f59c170","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":498151,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the filters (a, b) exfoliated SiO\u003csub\u003e2\u003c/sub\u003e; (c, d) ZnO@SiO\u003csub\u003e2\u003c/sub\u003e; (e, f) Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e; (g, h) ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/49a0f130b8310b39132a8bb0.jpeg"},{"id":109170462,"identity":"0dcceac7-f4c9-4747-992f-bf5e4f52729a","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":194590,"visible":true,"origin":"","legend":"\u003cp\u003e(a) EDS spectra with SEM micrograph of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e, (b–d) elemental mapping.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/a882ea1d8e07aade098e1b69.jpeg"},{"id":109170463,"identity":"5be7c6ee-de25-4ce6-bf74-9e54aac05eaf","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":250434,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Ultraviolet-visible absorption spectra of monolithic filters, (b) Energy band structure diagram of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/6071a195433c8e9d67ea2d5f.jpeg"},{"id":109170464,"identity":"80bbd665-ddcd-4539-b30f-edc9214bd17c","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":284357,"visible":true,"origin":"","legend":"\u003cp\u003ea) Photocatalytic degradation of MB; b) UV-Vis absorption spectra of MB using ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter; c) Pseudo-first-order model linear fitting of synthesized monolithic filters; d) Photodegradation efficiency of MB over ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e in different recycles.\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/2728602a832f16ca86208e12.jpeg"},{"id":109170460,"identity":"f7730732-be92-423f-bbfb-1922aa8bebc8","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":158210,"visible":true,"origin":"","legend":"\u003cp\u003ea) TOC analysis of monolithic filters in three reaction cycles; b) Scavenger effect on photocatalytic degradation of MB dye using ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/88229b5100846b45f7afee7f.jpeg"},{"id":109170465,"identity":"c2b5737c-8719-4e7f-8b52-72346caddaba","added_by":"auto","created_at":"2026-05-13 08:48:51","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":260258,"visible":true,"origin":"","legend":"\u003cp\u003eIR spectrum and reaction mechanism of methylene blue degradation by ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters.\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/c27a1553e8d27a90dc35576a.jpeg"},{"id":109209343,"identity":"fd7c78c9-788d-4fdc-bc9c-45c23f3f585e","added_by":"auto","created_at":"2026-05-13 15:28:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2669359,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9683860/v1/4637b8f5-daed-4368-a88b-f6cb69c08a73.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"MB degradation under UV–Vis using ZnO-Fe2O3@SiO2 monolithic filters in a CPC reactor","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eIn recent decades there has been increased concern about the presence of emerging pollutants in wastewater due to possible health effects as endocrine disruptors, antiandrogens capable of creating microbiological resistivity or producing metabolic alterations in humans, while in aquatic organisms they may have a cumulative effect and sex change [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. This problematic is because many of these organic compounds are highly stable, biodegradation-resistant, and water-soluble species. Synthetic dyes are widely used in the textile, paper, leather, plastic, printing, and cosmetic industries, so effluent with a small amount of these dyes can cause serious local environmental problems [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Heterogeneous photocatalysis is presented as a viable option for the degradation of organic pollutants due to its capacity to produce drastic changes in the structure of the contaminant due to the generation of oxidative species that achieve the mineralization of the molecule [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Photocatalysts are commonly used as powders suspended in solution for their practicability [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, some problems are present during their use such as a low and inherent surface area, limited adsorption capacity, secondary contamination and difficulties in scaling up processes and recyclability [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. To mitigate these disadvantages, different semiconductor deposition techniques have been used, among the most used are immobilized films on different substrates (FTO, ITO, metal sheets and polymers), coating of lamps or surfaces of a reactor and different types of sponges and monoliths [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. However, the immobilization of the photocatalyst on substrates generally leads to a decrease in photocatalytic activity, reducing its efficiency by one tenth due to the low specific surface area and low dispersion of particles and light absorption [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Therefore, the deposition of semiconductors on absorbent monolithic materials has aroused great interest, providing a stable matrix that improves oxidation-reduction processes, increasing recyclability [\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. A few years ago, our group reported the use of silica monolithic filters which showed excellent behavior for the adsorption and photodegradation of methylene blue dye (MB) under UV\u0026ndash;visible light irradiation [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. In fact, SiO\u003csub\u003e2\u003c/sub\u003e has been used as ceramic support for the deposition of ZnO, α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e metal oxides, which have energy bandgap values of 3.1 and 2.0 eV, respectively [\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. These catalysts are easily synthesized by soft chemistry methods, as well as being abundant and economically suitable, which suggests that they are appropriate for deposition on a monolithic filter. On the other hand, silica filters are materials used in different industries, due to its various physico-chemical properties, such as: high absorption capacity, being an excipient material, possessing pyroelectric and piezoelectric properties, and be the second most abundant material on the planet [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Also, it has been reported that SiO\u003csub\u003e2\u003c/sub\u003e template limits the agglomeration in the growth of nanoparticles, favors good dispersion and improves photonic absorption and charge transportation [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In addition, it favors the reduction of photoluminescence emission associated with the recombination of charge. Another application of silica is its use as an intermediate which improves the adsorption and stability of the compound due to the porous surface of the silica and the electron storage capacity of the semiconductor deposited on the surface with high oxidation capacity [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Therefore, this paper reports the preparation of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters exfoliated by hydrothermal method to promote the separation of photogenerated charges, as well as to improve the stability of this compound. In addition, these filters were evaluated in a CPC photocatalytic reactor, looking for better photocatalytic efficiency for the degradation of organic compounds such as methylene blue. Morphology, oxidation states of the metal oxides on the surface, adsorption capacity, oxidizing species identification and photocatalytic recyclability of the monolithic filters are discussed too in this paper.\u003c/p\u003e"},{"header":"2. EXPERIMENTAL","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials and methods.\u003c/h2\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e commercial ceramic supports (BIOMAX) and ammonia solution (99%, DEQ, CAS: 1336-21-6) were used to adjust the pH for all experiments. Exfoliation of monolithic filter was performed by hydrothermal method [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of ZnO@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/h2\u003e \u003cp\u003eFirst, the monolithic filter was immersed in deionized water for 60 min to ensure maximum water absorption, then zinc acetate 0.489 M was added until completely dissolved. Immediately, NH\u003csub\u003e4\u003c/sub\u003eOH was used to adjust pH to 8, after that, polyvinyl pyrrolidone (PVP) (0.3 g) was added. The resulting solution and the monolithic filter were transferred to a Teflon-lined and heated at 180\u0026deg;C for 14 h, for the exfoliation process and the \u003cem\u003ein-situ\u003c/em\u003e crystallization of ZnO. The recovered solid was washed with water and ethanol to neutralize the solution and finally dried to 80\u0026deg;C for 6 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Synthesis of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/h2\u003e \u003cp\u003eThe monolithic filter was immersed in deionized water for 60 min. 0.696 M of ferric nitrate was added. Subsequently, KOH (5 M) was added up to increase pH to 12. The resulting solution and the monolithic filter were transferred to a Teflon-lined and heated at 120\u0026deg;C for 8 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Synthesis of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/h2\u003e \u003cp\u003eThe preparation of monolithic filter ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e was carried out in two steps. First, the deposition of the zinc oxide particles is carried out, as outlined in section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e2.2\u003c/span\u003e. After that process, the iron oxide particles are deposited using the previously synthesized monolithic ZnO@SiO\u003csub\u003e2\u003c/sub\u003e filter, as detailed in previous section. Finally, the recovered monolithic filters were washed with distilled water and ethanol to neutralize the solution and finally dried at 80\u0026deg;C for 12 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Monolithic filters characterization.\u003c/h2\u003e \u003cp\u003eAll monolithic filters were analyzed by X-ray diffraction (XRD) using a PANalytical Empyrean diffractometer with CuKα radiation (λ\u0026thinsp;=\u0026thinsp;1.5418 \u0026Aring;) at a scanning range of 10\u0026ndash;70 degrees and scan rate of 0.02 degrees per minute to analyze the phase formation and crystalline structure. Morphology of materials was analyzed by scanning electron microscope (SEM) JEOL JSM6490 LV operated at 20 kV and equipped with microanalysis dispersion X-rays (EDS) for chemical quantification. The oxidation states of the metal oxides on the surface of monolithic filter were determined by X-ray photoelectron spectroscopy (XPS) using a Thermofisher XPS Escalab 250Xi equipped with an AI Kα (1486.68 eV) monochromatic radiation. The surface area was determined by N\u003csub\u003e2\u003c/sub\u003e adsorption measurement at 77 K using a Mini Belsorp II (Bel Japan). All the samples were vacuum dried at 300\u0026deg;C for 1 h before the test. Dye degradation was followed by measuring the absorbance value at 663 nm in a UV-vis NIR spectrophotometer (Cary 5000) in the range of 200\u0026ndash;800 nm. To determine the degree of MB mineralization, total organic carbon (TOC) analysis was conducted in a Shimadzu 5000 model equipment. Kinetic of the reaction was monitored by HPLC liquid chromatography using a Shimadzu equipment model LC-2030C with a C18 5 \u0026micro;m column (50 X 3.0 mm) and a PDA detector (λ\u0026thinsp;=\u0026thinsp;264 nm), with a mobile phase of methanol, water (70:30), with a retention time of 5 min, and a flow rate of 0.5 \u0026micro;m, excited at a wavelength of 246 nm, while for DXT, the mobile phase was acetonitrile and water (50:50), with a retention time of 1.12 min, and a flux of 10 \u0026micro;m, excited at a wavelength of 254 nm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Photocatalytic degradation.\u003c/h2\u003e \u003cp\u003eThe experiments were carried out in a Pyrex glass reactor (150 ml), equipped with a propeller stirrer with a constant stirring rate of 150 rpm, illuminated with a xenon (Xe) lamp of 35 W (400\u0026ndash;700 nm), 3200 lumens and 6000 K, maintaining a temperature around 30\u0026deg;C. The photocatalytic experiment was conducted with 100 ml of solution for each monolithic filter used. The solution was stirred in dark for 60 min before the light was switched on to ensure adsorption-desorption equilibrium. Aliquots of 3 ml were taken at a regular time interval and measured in the UV-Vis equipment and in the HPLC equipment.\u003c/p\u003e \u003cp\u003eThe degradation rate was calculated by Eq.\u0026nbsp;1 [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:d\\left(\\%\\right)=\\frac{{A}_{0}-{A}_{t}}{{A}_{0}}X\\:100\\)\u003c/span\u003e \u003c/span\u003e \u0026hellip; (Eq.\u0026nbsp;1)\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003ed\u003c/em\u003e is degradation rate, \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e is absorption after radiation and \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003e0\u003c/em\u003e\u003c/sub\u003e is absorption before radiation.\u003c/p\u003e \u003cp\u003eTo determine the oxidizing species used in the degradation process of the organic molecule during the reaction, different chemical agents were added to trap the species generated during the process. The procedure to determine the oxidizing species (h\u003csup\u003e+\u003c/sup\u003e, OH., H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) was like the previously described for the photocatalytic evaluation. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e showed the chemical reagents and the corresponding dye concentration.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical agents for determination of oxidizing species.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChemical reagent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSpecies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eConcentration (MB)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eh\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0006 g\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIsopropanol\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eOH\u0026middot;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.003 ml\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCatalase\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.06 g\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBenzoquinone\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0004 g\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eOn the other hand, recyclability tests were carried out to determine the average lifetime of the monolithic filters. To determine the surface pore saturation and adsorption capacity, filters were dried at 80\u0026deg;C for 12 h, without additional treatment. The monolithic filter is evaluated in three consecutive cycles, without any further treatment of washes, in the degradation of the methylene blue dye.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1 X-ray diffraction (XRD).\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the X-ray diffraction pattern of the monolithic filters with the incorporation of the metal oxide particles (ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) deposited. It is observed that the XRD pattern of the monolithic silica filter (a) chemically treated by the hydrothermal method at 180\u0026ordm;C for 14 hours showed the presence of the phases identified as α-cristobalite, β-quartz, Na\u003csub\u003e2\u003c/sub\u003eO and Al\u003csub\u003e4\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003eC. During the exfoliation of the monolithic silica filter (b) it was observed that the increment in temperature during hydrothermal treatment and the pressure generated inside the reactor causes a variation in the length and angle in the silicon and oxygen bonds, favoring the crystallization of the α-cristobalite and the formation of silanoles Q\u003csup\u003e2\u003c/sup\u003e (Si-OH) and Q\u003csup\u003e3\u003c/sup\u003e (O\u003csub\u003e3\u003c/sub\u003e-Si-OH) which have been related to the adsorption capacity and stability of SiO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOn the other hand, the monolithic filter with zinc oxide particles (ZnO@SiO\u003csub\u003e2\u003c/sub\u003e) (c) showed the presence of highly crystalline particles of the ZnO, evidenced by the existence of the three representative peaks of the material at 31.73, 34.37, and 36.21\u0026ordm; (JCPDS 01-070-8070). Likewise, XRD pattern of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e (d) showed low crystallinity of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (JCPDS 00-033-0664) peaks, which are overlapped by the SiO\u003csub\u003e2\u003c/sub\u003e peaks which normally present high crystallinity. Additionally, the diffractogram of the monolithic filter deposited with particles of zinc oxide and iron oxide (ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e) (e), showed mainly the presence of ZnO and SiO\u003csub\u003e2\u003c/sub\u003e peaks, while peaks of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles are not detected due to its low crystallinity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2 X-ray photoelectron spectroscopy analysis (XPS).\u003c/h2\u003e \u003cp\u003eXPS analyses were performed to determine the oxidation states and elements present on the surface of the monolithic filters that were deposited with ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the XPS spectrum of the exfoliated filter, comparing the ZnO@SiO\u003csub\u003e2\u003c/sub\u003e, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e, and ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filters. In the filter spectrum ZnO@SiO\u003csub\u003e2\u003c/sub\u003e, it is observed the signals from Zn 2p\u003csub\u003e3\u003c/sub\u003e to 1021.16 eV and Zn 2p\u003csub\u003e1\u003c/sub\u003e to 1043.90 eV, which demonstrate the existence of ZnO [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. On the other hand, the filter Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e shows a spectrum similar to the exfoliated monolithic filter, with a slight emission of Fe 2p at 710.23 eV corresponding to the Fe\u003csup\u003e3+\u003c/sup\u003e cation of hematite (α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Finally, the spectrum corresponding to the filter ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e is shown, which stands out for exhibiting more intense Fe emissions in 710.98 and 724.53 eV, which correspond to Fe 2p\u003csub\u003e3/2\u003c/sub\u003e and Fe 2p\u003csub\u003e1/2\u003c/sub\u003e, which agrees with the previously reported [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. This result corroborates the presence of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e on the surface of the monolithic filter.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb shows the decovolution of the elements present on the surface of the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolith. Subsection (a) shows the spectra of Si 2p which shows three components of SiO\u003csub\u003e2\u003c/sub\u003e to 101.43 eV, Si-O-Si to 102.36 eV and Si-OH to 103.33 eV [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. In section (b) O 1s peaks are shown, the high peak (O\u003csub\u003eL\u003c/sub\u003e) at 531.85 eV corresponds to the O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e ions frequently surrounded by Si ions in the SiO\u003csub\u003e2\u003c/sub\u003e structure. The O\u003csub\u003eV\u003c/sub\u003e peak at 531.03 eV is due to oxygen vacancies, and the O\u003csub\u003eC\u003c/sub\u003e peak at 529.7 eV is related to chemisorbed oxygens species. In section (c) shows three peaks of the adjusted Fe 2p spectrum, peaks at 710.8 and 713.1 eV were indexed to Fe 2p\u003csub\u003e1/2\u003c/sub\u003e, and peak 724.8 eV corresponds to Fe 2p\u003csub\u003e3/2\u003c/sub\u003e. While peaks at 719.1 and 732.7 eV were assigned to the Fe\u003csup\u003e3+\u003c/sup\u003e of the α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. On the other hand, subsection (d) shows the spectrum Zn 2p divided into two peaks at 1044 and 1021.22 eV corresponding to Zn 2p\u003csub\u003e1/2\u003c/sub\u003e and Zn 2p\u003csub\u003e3/2\u003c/sub\u003e, respectively, which are assigned to the tetrahedral Zn with a 2\u0026thinsp;+\u0026thinsp;oxidation state. Zn 2p\u003csub\u003e3/2\u003c/sub\u003e peak could originate in Zn-O bonds, in addition, it could be considered a weak peak at 1022.73 eV attributed to Zn-OH because hydrogen has a greater electronegativity than Zn, which leads to a higher density of positive charge in the atoms attached to the OH groups [\u003cspan additionalcitationids=\"CR30 CR31\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Morphology of monolithic filters by scanning electron microscopy (SEM).\u003c/h2\u003e \u003cp\u003eA surface analysis was performed using the SEM technique on the internal and external surface of the monolithic filter. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e corresponds to the SEM images of the monolithic filter exfoliated by the hydrothermal method and the filters deposited with the metallic oxides (ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e). Section a-b show a rough surface on the filters, which breaks into scales and falls off the surface, leading to the formation of small spherical particles of about 1 \u0026micro;m. Cracking and surface breakage is mostly influenced by reaction time, while the pH of the solution defines the particles cracked on the surface spherically, as reported by Xu Hai Yan et al., that the NH\u003csup\u003e4+\u003c/sup\u003e ion can modify the growth of crystals under hydrothermal conditions [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. The surface modification of the silica filter caused by the hydrothermal process is due to a random rupture of the silicon-oxygen bonds. The different atomic arrangement and the various forms of bond break Si-O in the β-quartz and α-cristobalite crystalline structures gave rise to a different microstructure on the surface. Which could play a crucial role in adsorption processes and the deposition of semiconductors on the surface [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSection c-d shows the zinc oxide particles in the form of hexagonal prisms deposited on the monolithic filter. The internal surface shows a morphology of well-defined bars up to 5 \u0026micro;m in diameter that grow from a nucleus, and these are stretched to achieve the observed shape. While, on the outer surface, stands out the growth of ZnO bars on a rough surface, which were cultivated and grown on the surface of the silica filter and acts as a template for the continuous infiltration of Zn\u003csup\u003e2+\u003c/sup\u003e on the exfoliated surface, and some particles of Zn(OH)\u003csub\u003e2\u003c/sub\u003e that continue to crystallize as a hexagonal prism separating from the silica base. This crystallization mechanism is like that reported by other authors [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The morphological difference in zinc oxide particles on the external and internal surface of the filter is due to the absorption capacity of zinc ions on the surface of exfoliated silica during nucleation [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhile, in the section e-f show the SEM images of the monolithic filter with particles of α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e deposited on the surface. The SiO\u003csub\u003e2\u003c/sub\u003e matrix limits the growth of α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles during the nucleation process. The particles deposited on the internal and external surface of the filter show a homogeneous size and less than 1 \u0026micro;m, with a morphology of leaves and needles [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOn the other hand, in (g-h), the monolithic filter ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e shows a completely cracked surface product of the hydrothermal process in the metal oxides deposit. In the paragraph (g) it is possible to perceive hexagonal prisms of ZnO on the surface, while in section (h) it shows a corrugated strip in the form of leaves, like that observed in (d) where the particles of ZnO are deposited and grow. Similar to that observed by Yanjun Liu et al., Fe\u003csup\u003e3+\u003c/sup\u003e ions adhere and distribute on the silica surface, some others adsorb on the ZnO surface to form ions of [Fe (OH)\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026minus;\u003c/sup\u003e, during the hydrothermal process they lose OH\u003csup\u003e\u0026minus;\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003eO allowing the growth of α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDeposition of metal oxides on the surface of the silica filter depends on the adsorption capacity [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. SiO\u003csub\u003e2\u003c/sub\u003e matrix prevents agglomeration of deposited particles [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Those particles that fail in inserting into the silica surface or adhering to previously deposited zinc particles did not crystallize. Similarly, the inner and outer part of the monolithic filter shows different adsorption capacity, product of surface exfoliation by hydrothermal method, which is important because surface exfoliation causes a greater pore generation and consequently a greater affinity for the growth of particles on the surface.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEDS point analysis and mapping of the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter were performed to determine the chemical composition and study the distribution of the elements present. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows two points on the surface of the monolithic filter. Spectrum 1 indicates the corrugated morphology in the form of leaves, the elements mostly present are the Si, Zn and O (showed in insert of spectrum 1). Demonstrating the growth of ZnO particles in this area, which corresponds to morphology results previously observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, and in a smaller percentage, the presence of Fe particles deposited and distributed over the same area can be detected also. Spectrum 2 indicates a smooth morphology on the filter surface, which corresponds to SiO\u003csub\u003e2\u003c/sub\u003e according to elemental analysis (showed in insert of spectrum 2). Similarly, a low percentage of Fe is detected, confirming that Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is homogenously deposited and distributed on the silica surface and ZnO particles. The presence of each one of the elements detected by mapping is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb (Si), 4c (Zn) and 4d (Fe).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the BET (Brunauer\u0026ndash;Emmett\u0026ndash;Teller) results for surface area diameter and average pore volume of monolithic filters. According to the results it can be mentioned that the silica monolithic filters have a low interaction between the adsorbent surface and the adsorbate, this is due to the high crystalline nature of the materials as observed by XRD, obtaining a surface area with values between 2\u0026ndash;3 m\u003csup\u003e2\u003c/sup\u003e/g. After the hydrothermal exfoliation process, the silica filter slightly increases the surface area and pore volume with respect to the raw filter.\u003c/p\u003e \u003cp\u003eThe Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particle deposit does not represent an apparent change from the exfoliated filter, on the other hand, ZnO particle deposited shows an increase in the surface area and pore volume [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. However, these results are not determinant because there is no significant difference between each sample and may not be related to the photocatalytic properties of the monolithic filter.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSurface area, volume, and pore diameter of monolithic filters.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSurface area (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePore volume (cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePore size (nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRaw Filter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.0742\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e106.52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExfoliated filter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.3621\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.5897\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.3638\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e31.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.5378\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.4 UV\u0026ndash;visible diffuse reflectance spectroscopy.\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the UV-Vis spectra to study the optical properties of monolithic silica filters synthesized with ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles. While in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb the energy band structure diagram of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e is showed. The bandgap energy of the samples was calculated using the Kubelka-Munk function. Raw and exfoliated filters show an intrinsic absorption in the ultraviolet region leading bandgap values of 3.7 and 3.8 eV, respectively. However, when ZnO was incorporated on the filters (ZnO@SiO\u003csub\u003e2\u003c/sub\u003e), the \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e value increases until 4.5 eV. While addition of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e provoked a considerable diminishing of the \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e value on the filters (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e), which is around of 1.9 eV. This demonstrates that the presence of metal oxide on the filter surface has influence on the absorption in the UV-Visible spectrum. On the other hand, the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e heterojunction presented a bandgap value of 3.3 eV, which it can absorb more photons and generate a greater number of electrons and holes, which could be favoring a better photocatalytic activity under ultraviolet and visible region.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Methylene blue (MB) degradation by photocatalysis.\u003c/h2\u003e \u003cp\u003eThe photocatalytic efficiency of the filters was evaluated in the degradation of MB dye at 10 ppm. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea shows that the deposition of metal oxides has a direct influence on the surface adsorption capacity of the molecule, and subsequently on the degradation of the dye. In this case, activity of raw filter was improved 43 and 57% when ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e were deposited, respectively. This result was due to the adsorption capacity of the ZnO@SiO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filters, which is correlated with the bandgap and the particle size of the metal oxides deposited on the filter surface. Particularly, it is reported that nanometric particle size of α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e allows greater generation of active sites to adhere the organic molecule and then react with the semiconductor [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Therefore, filters containing Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles showed better absorption which enhances discoloration of the dye. According to the observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter shows an adsorption capacity superior to the raw filter and ZnO@SiO\u003csub\u003e2\u003c/sub\u003e, and slightly lower than exfoliated filter and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e. From the above results, it is concluded that deposition of semiconductors on the filter surface causes a loss in porosity and adsorption of the dye. However, the presence of metal oxides (ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) improves oxide-reduction throughout the reaction process, maintaining a constant kinetics and high degradation efficiency (94%) for ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThe changes in the UV-Vis absorption spectrum of MB during photocatalytic degradation corroborated the effect of using the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter as catalyst, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb. Specifically, the intensity of the spectrum when this semiconductor is used decreased steadily after the adsorption process. In addition, a peak at smaller wavelengths, between 200 and 250 nm, is observed, which is attributed to the generation of oxidative radicals and the insertion of OH into the organic molecule, which leads to a bathochromic effect at the peak of maximum absorbance at 663 nm.\u003c/p\u003e \u003cp\u003eOn the other hand, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec shows the reaction kinetics of monolithic filters, which demonstrated that ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter shows a linear behavior with the highest reaction speed and lowest half-time life (25 times lower than raw filter), which indicates an exponential decay with values close to a first order reaction.\u003c/p\u003e \u003cp\u003eIn addition, recyclability tests were carried out on the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter where it can be observed that after three reaction cycles the efficiency keeps intact (94\u0026thinsp;\u0026plusmn;\u0026thinsp;1% after each cycle), Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed. This result indicates that the combination of ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e deposited on surface filters to obtain ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters are a viable option that improve recyclability processes and keeps degradation efficiency, which could facilitate practical applications in scaling photocatalytic processes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the percentage of degradation, and the half-time life of each monolithic filter evaluated during three consecutive cycles in MB degradation. The results indicate that the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter keeps the degradation percentage intact after three reaction cycles (94, 95, 94%, during first, second and third cycle, respectively). While efficiency of the exfoliated filter, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e and ZnO@SiO\u003csub\u003e2\u003c/sub\u003e decreased its effectiveness 34, 30 and 13%, from the first, second and third cycle, respectively. Similarly, the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter maintains a lower half-time life in all reaction cycles, showing a reaction rate of \u003cem\u003ek\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.0075 and R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.997, markedly higher than the other synthesized filters.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePercentage of MB degradation, reaction kinetics, half-time life (\u003cem\u003et\u003c/em\u003e\u003csub\u003e\u003cem\u003e1/2\u003c/em\u003e\u003c/sub\u003e) over 3 reaction cycles.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eDegradation\u003c/p\u003e \u003cp\u003ecycle 1\u0026ndash;3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003et\u003csub\u003e1/2\u003c/sub\u003e (min)\u003c/p\u003e \u003cp\u003ecycle 1\u0026ndash;3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c9\" namest=\"c8\"\u003e \u003cp\u003eReaction kinetics \u003c/p\u003e \u003cp\u003efirst cycle\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e%D-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e%D-2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e%D-3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eC-2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eC-3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003e\u003cem\u003ek\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRaw filter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2310\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2657\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e2763\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.893\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.0003\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExfoliated filter\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e330\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e385\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.959\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.0066\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e231\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e239\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.973\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.0042\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e224\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e239\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.701\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.0026\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e112\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.997\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.0075\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTOC analysis of the reaction product was performed at 300 min reaction using the different monolithic filters to determine the percentage of mineralization of the molecule. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea shows the TOC percentage during each reaction cycle, where it is observed that the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter was able to mineralize 66% of the MB dye during the first cycle and decreased only 10% after subsequently cycles. The efficiency of the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter increased the mineralization percentage by 48 and 51% compared to the α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e and ZnO@SiO\u003csub\u003e2\u003c/sub\u003e filters, respectively. These results demonstrate that the adsorption and degradation process of the exfoliated filter is high, but stability is only reached by depositing of the metal oxides ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, enhancing the mineralization process too. In conclusion, the monolithic silica filter is an excellent material that acts as a support for the deposition of semiconductors to obtain effective materials for photocatalytic degradation.\u003c/p\u003e \u003cp\u003eTo understand the reaction mechanism, the reactive species responsible for the photocatalytic degradation process were identified using several scavengers. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb shows the addition of potassium iodide as a hole scavenger (h\u003csup\u003e+\u003c/sup\u003e), isopropanol for the capture of hydroxyl radicals (\u0026bull;OH), catalase as a scavenger of peroxide radicals (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) and benzoquinone as a scavenger of superoxide radicals (O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e). It was determined that peroxide and hydroxyl radicals are those that have a greater interference to control the degradation reaction of the MB dye. While the holes and superoxide radicals influence photocatalytic activity to a lesser extent.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Infrared spectroscopy (IR).\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eAdditionally, a liquid-liquid extraction was performed on the reaction product of the degradation reaction of MB after the first cycle to follow up the molecule breakage using the FTIR technique. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea shows the reaction product of the exfoliated filter, and the Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e, ZnO@SiO\u003csub\u003e2\u003c/sub\u003e and ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filters. It is observed that the reflections corresponding to alkane groups (CH) at 2971 cm\u003csup\u003e-1\u003c/sup\u003e, secondary amines (CH-NH-CH) at 1463 cm\u003csup\u003e-1\u003c/sup\u003e, aliphatic aldehydes (CH\u003csub\u003e2\u003c/sub\u003e-CHO) at 1369 cm\u003csup\u003e-1\u003c/sup\u003e and aliphatic ethers (CH\u003csub\u003e2\u003c/sub\u003e-O-CH\u003csub\u003e2\u003c/sub\u003e) at 954 cm\u003csup\u003e-1\u003c/sup\u003e are the only reflections remaining after 300 min of reaction. The IR spectra of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter showed that bonds present low intensity reflections, indicating absence of the dye molecule after the degradation process, corroborating the efficiency of the monolithic ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e silica filter for the degradation of MB dye, as it was observed in the UV-Vis, and TOC analyses.\u003c/p\u003e \u003cp\u003eAccording to the above results it was possible to propose a mechanism of the breakage of the methylene blue dye molecule using the ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filter, which is showed in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb. It is assumed that the dye bond breaking starts in the central part of the molecule, destroying the double bond of the benzene ring. This assumption is supported by the bathochromic effect observed by UV-Vis. Thus, it causes a loss of color during the first minutes of reaction. The high generation of oxidizing species provoked by the presence of the deposited ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e semiconductors, causes the breaking of the bonds of the dye until mineralization, it means to convert the molecule to CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO [\u003cspan additionalcitationids=\"CR45\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eDeposition of ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles on monolithic silica filters was successfully achieved by hydrothermal method and the presence of these deposited elements was verified by EDS analysis. XRD, SEM-EDS and XPS analysis corroborates the presence of all phases, ZnO, SiO\u003csub\u003e2\u003c/sub\u003e and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, exhibiting that ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles are deposited onto silica filters. ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e filters showed to be highly effective when used in the CPC photocatalytic reactor because enhance the reaction stability in the degradation of methylene blue (MB) dye, with 95% efficiency remaining intact after three reaction cycles. In addition, the deposited semiconductors improved the reaction kinetics by mineralizing 66% of the dye in the first cycle and decreasing to 10% for the third reaction cycle. The presence of metal oxides (ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) promotes oxidation-reduction reaction during the degradation process, maintaining high reproducibility of degradation using MB after 3 cycles. In addition, the proposal mechanism indicates that the high generation of oxidizing species (peroxide and hydroxyl radicals), provoked by the presence of the deposited ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e semiconductors, causes the breaking of the bonds of the dye until mineralization. Therefore, ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters prepared in this work are excellent materials that acts as a support for the deposition of semiconductor materials to obtain effective materials for photocatalytic degradation, which could be as an innovative alternative for water treatment, due to its easy preparation, practicality and recyclability, for its use in a CPC photocatalytic reactor.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003e\"I. Ju\u0026aacute;rez-Ram\u0026iacute;rez. Intellectual contribution; draft of the paper; experimental design; interpretation of data; discussion of the results; responsible of project with financial support; revision and approval of the manuscript\"\"A. S. Galindo-Luna. Intellectual contribution; draft of the paper; experimental design; collection, analysis and interpretation of data; discussion of the results; revision and approval of the manuscript\"\"M. E. Zaraz\u0026uacute;a-Mor\u0026iacute;n. Intellectual contribution; draft of the paper; experimental design; discussion of the results; revision and approval of the manuscript\"\"D. S\u0026aacute;nchez-Mart\u0026iacute;nez. Draft of the paper; discussion of the results; revision and approval of the manuscript\"\"V. J. Gallegos-S\u0026aacute;nchez. Draft of the paper; discussion of the results; revision and approval of the manuscript\"\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eAuthors would like to thank SECIHTI. A. Sebasti\u0026aacute;n Galindo-Luna thanks to SECIHTI for scholarship No. 783128. Thank you to UANL for the financial support through PROACTI 2024 (113-IDT-2024). We also thank the staff of Ecomateriales and Energy Department and Materials Construction Laboratory at Civil Engineering Institute of the Civil Engineering Faculty at UANL. In addition, thanks to Dr. Francisco Servando Aguirre Tostado from CIMAV-Mty for his support with the XPS analysis.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMiriam Janet Gil, Adriana Mar\u0026iacute;a Soto, Jorge Iv\u0026aacute;n Usma, O. D. G. (2012). Contaminantes emergentes en aguas, efectos y posibles tratamientos. \u003cem\u003eProducci\u0026oacute;n + Limpia\u003c/em\u003e, \u003cem\u003e7\u003c/em\u003e, 52\u0026ndash;73.\u003c/li\u003e\n\u003cli\u003eBecerril, J. (2009). Contaminantes emergentes en el agua. \u003cem\u003eRevista Digital Universitaria\u003c/em\u003e, \u003cem\u003e10\u003c/em\u003e(8), 1\u0026ndash;7. http://www.revista.unam.mx/vol.10/num8/art54/int54.htm\u003c/li\u003e\n\u003cli\u003eThomas, E., Mendon\u0026ccedil;a, F., Xavier, T. P., Souza, D. R. 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Probing methylene blue photocatalytic degradation by adsorbed ethanol with in situ IR. \u003cem\u003eJournal of Physical Chemistry C\u003c/em\u003e, \u003cem\u003e111\u003c/em\u003e(37), 13813\u0026ndash;13820. https://doi.org/10.1021/jp0715474\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Silica filters, ZnO-Fe2O3@SiO2, CPC reactor, Photocatalysis, Methylene blue","lastPublishedDoi":"10.21203/rs.3.rs-9683860/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9683860/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIt is reported the use of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters to the degradation of methylene blue (MB) under UV-Vis light irradiation in a CPC reactor. ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters were obtained by deposition of ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles on monolithic silica filters through hydrothermal method. Structural analysis by X-ray diffraction (XRD) showed the presence of ZnO and SiO\u003csub\u003e2\u003c/sub\u003e phases, corroborated by Scanning Electron Microscopy (SEM) coupled with Energy-Dispersive X-ray Spectroscopy (EDS), where chemical composition also detected a low percentage of Fe. Likewise, by X-ray Photoelectron Spectroscopy (XPS) the oxidation state of the elements presented in all phases was detected. Results exhibited that ZnO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e particles are homogeneously deposited and distributed on the silica surface to promote oxidation-reduction reactions during the degradation process. Photocatalytic activity of ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters used in a CPC photocatalytic reactor for the degradation of methylene blue (MB) achieved 95% efficiency remaining intact after three reaction cycles, reaching also a high mineralization reaction (66%). The proposal mechanism indicates that the high generation of oxidizing species (peroxide and hydroxyl radicals), provoked by the presence of the deposited ZnO and α-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e semiconductors, causes the breaking of the bonds of the dye until mineralization. In conclusion, ZnO-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e@SiO\u003csub\u003e2\u003c/sub\u003e monolithic filters here reported are presented as an innovative alternative for their application to degrade methylene blue dye in a CPC photocatalytic reactor for water treatment, due to its easy preparation, practicality and recyclability.\u003c/p\u003e","manuscriptTitle":"MB degradation under UV–Vis using ZnO-Fe2O3@SiO2 monolithic filters in a CPC reactor","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-13 08:48:46","doi":"10.21203/rs.3.rs-9683860/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f491d12c-fb24-4f41-977d-3203b9ddce3a","owner":[],"postedDate":"May 13th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Rejected","date":"2026-05-13T14:15:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-13T14:14:21+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-05-13T09:40:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"Topics in Catalysis","date":"2026-05-11T20:02:45+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-05-13T15:22:36+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-13 08:48:46","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9683860","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9683860","identity":"rs-9683860","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}