Anatase-phase TiO 2 -tubular support as photocatalyst for the photocatalytic degradation of RB5 dye under sunlight

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Morales Ortiz, Richard S. Ruiz Martinez, Elizabeth Rojas Garcia This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9226072/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 20 You are reading this latest preprint version Abstract Porous tubular supports coated with a thin film of anatase phase TiO₂ were synthesized using the sol–gel method. Rutile phase TiO₂-tubular support exhibited photocatalytic activity for the degradation of RB5, achieving a degradation efficiency of 44.5% at a feed flow rate of 0.116 L s⁻¹ and an apparent rate constant (k app′ ) of 4.64 L·kJ − 1 . The incorporation of more than one rutile phase TiO₂-tubular support in the photocatalytic reactor increased the degradation of the NR5 dye under the same reaction conditions, from 28.1% (one membrane placed at the center) to 39.9% (two membranes arranged horizontally inside the quartz tube). Furthermore, the incorporation of a thin film of anatase phase TiO₂ enhanced the photocatalytic activity compared with the unimpregnated support, reaching up to 60.6% degradation and increasing the apparent reaction constant to 11.8 L·kJ − 1 . Porous tubular supports coated with a thin film of anatase-phase TiO₂ represent a promising option as photocatalysts for the degradation of contaminants in wastewater. In this configuration, the TiO₂ film remains firmly attached to the support, preventing catalyst detachment and eliminating the need for a separation step that is typically required when TiO₂ is used in suspension in the reaction medium. RB5 dye anatase phase TiO2-tubular support sunlight photocatalysis photocatalytic degradation solar reactor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Water is one of the most essential natural resources on the planet; therefore, its contamination by chemical substances derived from industrial activities has become a global environmental concern. Among the different industrial sectors, the textile industry has been recognized as one of the largest consumers of water and one of the main contributors to water pollution [1]. Among the wide variety of dyes used in industry, azo dyes represent the largest group of chromophores and account for approximately 70% of global dye production due to their ease of synthesis and versatility. However, they are also among the most hazardous dyes because they can generate aromatic amines that are potentially carcinogenic [2]. Solar photocatalysis has been widely investigated as an effective treatment method for dye-containing effluents. Numerous studies have demonstrated that this technology can efficiently remove color and significantly reduce both chemical oxygen demand (COD) and total organic carbon (TOC) in wastewater. In addition, photocatalysis promotes the degradation of pollutants into smaller, less harmful molecules with low or negligible toxicity [3]. Photocatalysts play a crucial role in the photocatalytic process. Among the wide range of photocatalytic materials, titanium dioxide (TiO₂) is the most widely studied and applied due to its advantageous properties, such as chemical stability, environmental compatibility, low toxicity, resistance to poisoning, and relatively low cost. In aqueous media, its surface exhibits amphoteric behavior, meaning that it can acquire positive, negative, or neutral charges depending on the pH of the solution [4]. TiO₂ occurs in three main crystalline phases: rutile, anatase, and brookite. Several studies have reported that the anatase phase exhibits higher photocatalytic activity, with a band gap of approximately 3.2 eV, whereas the rutile phase is thermodynamically more stable and has a band gap of about 3.0 eV [5]. In most experimental systems, photocatalysts are used in suspended form in the reaction medium because this configuration provides a larger surface area available for reaction, which enhances the mineralization rate of contaminants [6,7,8]. However, despite the excellent properties of TiO₂, its large-scale application faces significant challenges. One of the main limitations is the high cost associated with separating fine catalyst particles from the treated water after the reaction process, as well as the reduced light penetration caused by the opacity of catalyst suspensions. To overcome these limitations, one promising strategy is the immobilization of the photocatalyst onto a suitable substrate (membrane) or support, which allows catalyst reuse and simplifies separation. Nevertheless, several studies have reported that immobilization may reduce the effective surface area available for photocatalytic reactions [8,9,10,11]. Therefore, TiO₂-based photocatalytic systems continue to be extensively investigated to develop new strategies aimed at improving their efficiency. Different reactor configurations have been reported for immobilized photocatalytic systems, regarding the geometry and material of the substrate employed. The geometries most commonly reported for the supports consists of flat surfaces and cylindrical rods, and a variety of materials have been used as supports such as polymeric membranes (e.g. polysulfone (Grzechulska et al. (2009); polyvinylidene (fluoride Akter (2019)), as well as glass, alumina, titania, and ceramic supports, among other materials, have been reported (Marín et al. (2007); Jiménez and Gelover (2007); Zhang et al. (2006); Espíndola et al. (2019); Rosero et. al. (2022); Alventosa De lara, E. (2015)). In many of these works the active phase consisted of TiO 2 and dip-coating and sol-gel were often employed as the deposition method. The above-mentioned studies report different degrees of degradation of a contaminant molecule, however information on the effect of support material and deposition method on both reactor performance and immobilization stability is often missing. In this study, the performance of tubular TiO₂ supports in the rutile phase, as well as those coated with a thin film of anatase-phase TiO₂, was evaluated for the degradation of Reactive Black 5 (RB5), which is a widely used textile dye because it allows the production of deep black and navy-blue shades. In addition, the effects of several operational parameters were investigated, including the feed flow rate, the number of tubular supports placed inside the reactor, and the thickness of the anatase-phase TiO₂ film deposited onto the rutile-phase TiO₂ support. 2. Experimental section 2.1 Impregnation of tubular support The rutile-phase TiO 2 tubular support was obtained from TAMI Industries, and Filtanium model. This support was coated with a thin layer of anatase-phase TiO 2 . To prepare the TiO 2 precursor solution, the sol-gel method was used. The methodology for preparing the sol-gel has been previously reported in the literature by Jiménez & Gelover [12]. The synthesis of the solution required controlled conditions, such as the use of a fume hood, constant stirring, and regulated temperature. Initially, the acid hydrolysis medium was prepared (solution A), consisting of 38 mL of absolute ethanol (Sigma-Aldrich, ACS reagent, ≥ 99.5%, 0.65 mol), 2.3 mL of deionized water (0.13 mol), and 0.27 mL of hydrochloric acid (Sigma-Aldrich, 37%, ACS reagent, 0.008 mol). This solution was prepared under constant stirring in an ice bath. Once the previous solution was prepared, it was stored under refrigeration until later use. Subsequently, the solution containing the metallic precursor was prepared using 38.5 mL of titanium isopropoxide (Sigma-Aldrich, ACS reagent, ≥ 97.0%, 0.13 mol) and 38 mL of absolute ethanol. The absolute ethanol was added slowly and carefully dropwise down the side of the round-bottom flask, which was kept in an ice bath under constant stirring (solution B). Finally, solution A was added slowly and continuously to solution B under an ice bath and constant stirring for a period of six hours. A cooling system was also incorporated to minimize alcohol evaporation and ensure the stability of the reaction. As a result, a translucent white solution was obtained. The final solution was stored in an airtight container under refrigeration, conditions under which it can remain stable for at least 45 days without premature gelation [13]. For impregnation of the support, a dip-coating system was constructed, the characteristics of which are described in detail in Fig. 1 . This system allowed for control over the immersion and extraction speeds. The photocatalyst immobilization process began by depositing 256 ml of the sol-gel precursor solution into the deposition tube. In this work, the constructed system allowed the tubular support to be immersed in the precursor solution at 1.15 cm s⁻¹ and extracted at a speed of 2.38 cm s⁻¹, remaining submerged in the solution for 10 min. Once extracted from the sol-gel solution, the tubular support was dried for 30 minutes at room temperature. Subsequently, the supports underwent was calcined using a heating rate of 10°C min⁻¹ at 400°C by 2 hours. 2.2 Characterization of the material Powder X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer equipped with a copper anode (CuK α radiation, λ = 0.15406 nm) over a 2θ range of 10–70°. Raman spectra were acquired with a Renishaw InVia Raman microscope in the range of 100–800 cm⁻¹ using a green laser (λ = 532 nm) in order to obtain chemical and structural information about the support. The optical band gap was determined from UV–Vis absorption measurements performed with an Agilent Technologies Cary 100 spectrophotometer equipped with an integrating sphere for solid samples. The spectra were recorded in the appropriate wavelength range, and the band gap energy was calculated using the following equation: \(\:{\left(F\left(R\right)*hv\right)}^{1/n}=B(hv-{E}_{g}\) ) (1) SEM analysis allows visualization of the morphology of the outer surface of the tubular supports. The SEM images were obtained using a Zeiss Supra 55VP instrument, with secondary and backscattered electrons and a voltage of 50 kV. 2.1 Photocatalytic activity The photocatalytic reaction system consisted of three quartz tubes with an inner diameter (ID) of 2.6 cm, an outer diameter (OD) of 3.0 cm, and a length of 60 cm. These tubes are fixed to an aluminum box, which can be adjusted to different angles to align with the optimal angle of incidence of solar radiation. This box measures 100 cm long by 55 cm wide and is supported on an aluminum base. To improve the efficiency of photocatalyst activation, compound parabolic collectors (CPCs) were added. These collectors, with a geometry of two parabolic segments, have a concentration factor equivalent to 1 sun and are made of aluminum with a reflectivity index of approximately 99.7% [14]. The transport of the solution with the dye through the tubes was carried out by means of ½ HP stainless steel JET pumps, AQUA PAK brand, model ALY05/1230, 230 V, and three high resistance industrial hoses, with a diameter of 1 in and a length of 1.5 m. For the photocatalytic tests, the unimpregnated and impregnated supports were placed inside the quartz tubes (reactor). 12 L of a NR5 solution at 5 ppm were used in each reactor. The solution was then pumped through of the reactor for 30 minutes in darkness to allow to reach adsorption/desorption equilibrium. After this time, the experimental phase began under sunlight, aliquots were taken every half hour with a reaction time of 8 hours and analyzed in an UV-Vis spectrophotometer (HACH DR6000). Dye decolorization was monitored by measuring the absorption peak at 595 nm. Irradiance was measured with an Extech LT 366 luxmeter. Since the meter displays radiation in kLux, a conversion factor of 1 W/m² = 122 lux was applied [15]. 3. Results and discussions 3.1 Characterization of materials Powder X-ray diffraction patterns provide information that allows the identification of the phases present in the unimpregnated and impregnated TiO 2 supports. Figure 2 shows the XRD diffraction patterns of the unimpregnated and impregnated supports. As can be seen, the X-ray diffraction pattern of the unimpregnated support shows peaks at 2θ = 26.6, 35.3, 38.5, 40.4, 43.4, 53.6, and 55.9°, characteristic of the rutile phase of TiO 2 with a tetragonal structure and consistent with reference pattern number JCPDS 4-551[16]. No additional peaks attributable to the presence of TiO 2 in the anatase or brookite phases, or to impurities, were detected, and the observed signals are well-defined, indicating high crystallinity. While the impregnated sample shows the same peaks as the unimpregnated sample, indicating the presence of the rutile phase of TiO 2 , peaks corresponding to the anatase phase of TiO 2 are not observed, probably due to the small amount of this phase present in the sample in powder form. Figure 3 shows the Raman spectra of the unimpregnated and impregnated supports. Raman spectrum of unimpregnated support shows four active modes at 610 cm⁻¹ (A 1g ), 443 cm⁻¹ (E g ), 239 cm⁻¹ (Multi-phonon process), and 143 cm⁻¹ (B 1g ), characteristic of the rutile phase of TiO₂, results similar were observed in the XRD analyses [8]. In the impregnated sample, in addition to the aforementioned bands, other bands are observed at 143 cm⁻¹ (E g ) and 512 cm⁻¹ (A 1g ), indicated the presence of the anatase phase of TiO₂ [8]. This indicates the presence of the anatase phase on the support surface, which was the objective of the impregnation. Adding a thin film of anatase-phase TiO₂ to the support aims to increase photocatalytic activity, as this is the most active phase in photocatalysis [17]. Diffuse reflectance UV–Vis spectroscopy analysis, performed using the Kubelka–Munk function in combination with the Tauc equation, allowed the determination of the optical band gap of the samples [16]. The unimpregnated sample exhibited a band gap of 2.74 eV (452.5 nm), characteristic of rutile-phase TiO₂ [18]. After impregnation of the support with an anatase-phase TiO₂ thin film, the band gap slightly increased to 2.80 eV (442.8 nm), suggesting the presence of a small amount of anatase-phase TiO₂. Stepanova et al. [18] reported that an increase in the anatase phase fraction in TiO₂ results in a larger band gap and improved photocatalytic degradation of Rhodamine B under solar irradiation. In our analysis, a small portion was taken from one end of the support and ground into a fine powder prior to measurement. Figure 4 presents SEM micrographs of the samples under study. Figure 4 a shows the morphology of the external surface of the unimpregnated support, where pits of varying diameters formed during the synthesis process of support can be observed. In contrast, Fig. 4 b displays the external surface of the impregnated support, where the pits appear partially filled with anatase-phase TiO₂ particles. Finally, Fig. 4 c shows the SEM image of a cross-sectional profile of the support, where the film formed on the external surface of the support can be observed, indicating the homogeneity of the coating. 3.2 Photocatalytic evaluation In the photocatalytic evaluation were analyzed several parameter that affect the photocatalytic activity of unimpregnated and impregnated support. 3.2.1 Effect of recirculation flow rate To evaluate the effect of the recirculation flow rate, the unimpregnated support was used and two flow rates were tested: 0.116 and 0.538 L s⁻¹ (Fig. 4 ). Figure 4 a presents the results obtained from the photocatalytic evaluation of the unimpregnated support at a flow rate of 0.538 L s⁻¹ under solar irradiation, together with the monitoring of the irradiance reaching the reactor. As observed, only a small amount of RB5 dye was adsorbed onto the surface of the support under dark conditions. Once the reaction system was exposed to sunlight, the degradation of the dye began, reaching 25.4% of degradation after 8 hours of reaction. When the recirculation flow rate was reduced to 0.116 L s⁻¹ (Fig. 4 b), a significant increase in the RB5 degradation percentage was observed, reaching 44.5%. This improvement can be attributed to the increase in residence time within the reactor, which allows greater interaction between the reactant and the catalytic surface. Furthermore, the lower flow rate reduces mass transfer limitations, promoting better contact between the dye molecules and the catalyst surface, thereby facilitating diffusion and enhancing the photocatalytic reaction. Other author has shown this same behavior [19, 20, 21, 22]. 3.2.2 Effect of the number of membranes arranged inside the reactor Subsequently, two configurations were simultaneously evaluated under the same solar irradiation conditions: a single membrane placed at the center of the tube (reactor), and two membranes arranged horizontally inside another quartz tube reactor. Figure 6 shows the effect of the photocatalytic activity as a function of the number of unimpregnated photocatalytic supports inside the reactor. As the number of supports inside the reactor increased, an increase in the degradation percentage was observed, from 28.1% (one support) to 39.9% (two supports). This result indicates that increasing the number of supports inside the reactor leads to an increase in the effective surface area, which could explain the higher activity observed in the degradation of the RB5 dye. It has been reported in the literature that an increase in the effective surface area is generally associated with an increase in the degradation percentage [13,23,24], due to the higher number of active sites where the chemical reaction can occur. 3.2.3 Efecto del espesor de la película en la superficie externa del soporte Figure 7 shows the photocatalytic activity as a function of time and film thickness for the impregnated supports with anatase-phase TiO₂. As observed, increasing the thickness of the film leads to an increase in photocatalytic activity. The sample with a film thickness of 256 µm exhibited a degradation percentage of 60.6% after 8 hours of reaction. Additionally, an increase in the BET surface area was observed as the film thickness increased, which may explain the enhanced degradation of RB5. This improvement can be attributed to the higher number of active sites where charge carriers (electron–hole pairs) are generated, which are responsible for the formation of oxidizing species involved in the degradation of the RB5 dye. 3.2 Kinetic model The Langmuir-Hinshelwood (L-H) kinetic model has been widely used in the literature to describe the photocatalytic degradation of dyes [4,7,8,11] which is mathematically expressed as: $$\:r=-\frac{d{C}_{RB5}}{dt}={k}_{r}\frac{K{C}_{RB5}}{1+K{C}_{RB5}}$$ 2 where r is the degradation rate (mg·L − 1 ·min − 1 ); C RB5 is the RB5 concentration at time t (mg·L − 1 ); k r is the reaction rate constant (min − 1 ); and K is the apparent adsorption coefficient of the pollutant on the catalyst surface (L·mg − 1 ). Under certain experimental conditions, such as relatively low concentrations for which 1 > > KC , can be simplified to a pseudo-first order kinetic equation: $$\:-r=-\frac{d{C}_{RB5}}{dt}={k}_{r}K{C}_{RB5}={k}_{app}{C}_{RB5}$$ 3 F urthermore, the apparent k app reaction rate for a photocatalytic process is affected by the amount and intensity of the irradiation absorbed by the catalyst [25]; and for a solar reactor in terms of the UV irradiation reaching the reactor surface, E ac , the reaction rate can be determinated by the following equation: $$\:-r={k}_{app}{´E}_{ac}{C}_{RB5}$$ 4 where, the UV energy absorbed in the reactor is calculated by E ac = W A a / V T , where W is the solar UV irradiation rate (kJ s − 1 m − 2 ), A a is the reactor absorbing area as obtained by solar ray analysis (m 2 ), and V T is the total volume of water treated (L). For the determination of apparent rate constant, k app ´, the procedure describing by E. Rojas et.al [14] was used, obtained the following equation: $$\:{k}_{app}´=-\frac{{v}_{o}}{{V}_{R}{\:E}_{ac}}ln\left(1-m\:{V}_{SV}/{v}_{o}\right)$$ 5 where, \(\:{v}_{o}\) is the liquid volumetric flow rate, V R is the reactor volume, \(\:{V}_{SV}\) is the stirred vessel volume, and m is the slope of the graph of \(\:ln\left(\frac{{C}_{RB\text{5,0}}}{{C}_{RB\text{5,1}}}\right)\:\) vs time, where C RB5,0 is the initial dye concentration (t = 0) in the mixing vessel and \(\:{C}_{NR\text{5,1}}\) is the dye concentration at the reactor entrance which varies with respect to time. The above equation was derived from the mass balance relationship between the reactor and the mixer, since the aliquots collected at different times are taken from the mixer, which does not directly represent the concentration of RB5 dye at the reactor outlet [14]. Figure 8 presents the pseudo-first-order kinetic plots for the photocatalytic tests performed on both unimpregnated and impregnated samples. The experimental data were fitted using linear regression to determine the slope. From the slope and Eq. ( 5 ), the apparent rate constant (k app′ ) was calculated. The k app′ values for tests using unimpregnated supports are summarized in Table 1 . As shown, decreasing the feed flow rate by placing an unimpregnated support in the reactor led to an increase in k app′ , indicating that a lower feed flow rate improved mass transport condition (Fig. 8 a). Furthermore, doubling the number of unimpregnated supports in the reactor resulted in an approximately twofold increase in k app′ , which is consistent with the larger effective surface area provided by the additional supports (Fig. 8 b). This values of k app′ are most higher that the shown in the photolysis reaction, indicated the effect of rutile-phase TiO 2 tubular support. Table 2 presents the k app′ values obtained when varying the film thickness of the impregnated samples (Fig. 8 c), along with the percentage of degradation after 8 hours of reaction and the BET surface area. It can be observed that increasing the film thickness led to an increase in the apparent rate constant, as well as in the BET surface area and the percentage of RB5 degradation. This behavior is summarized in Fig. 8 d, where increasing the film wall thickness results in an increase in the apparent rate constant. Table 1 Percentage of degradation of RB5, and apparent kinetic constant of photolysis and unimpregnated support under conditions different. Membrane number Feed flow (L·s − 1 ) % degradation* Slope (h -1 ) R 2 k app ´ (L·kJ − 1 ) Photolysis 0.116 23.3 0.0323 0.9565 2.13 1 0.538 25.4 0.0445 0.9961 3.51 1 0.116 44.5 0.0730 0.9977 4.64 1 0.116 28.1 0.0323 0.9850 2.24 2 0.116 39.9 0.0603 0.9906 4.15 *reaction time: 5h Table 2 Apparent kinetic constant, percentage of degradation of RB5, and BET surface area at different film thickness of impregnated support. Film thickness (µm) Feed flow (L·s -1 ) % degradation* Slope (h -1 ) R 2 k app ´ (L·kJ -1 ) BET surface area (m 2 ·g -1 ) 93 0.116 47.0 0.0614 0.9796 5.76 0.5 148 0.116 47.3 0,0829 0.9776 5.97 0.6 256 0.116 60.6 0.1223 0.9949 11.8 1 *reaction time: 5h Conclusions Azo dyes such as Reactive Black 5 (RB5) are recalcitrant pollutants that are difficult to degrade using conventional treatment methods, particularly biological processes. In contrast, solar photocatalysis represents an effective alternative for the removal of these contaminants through the combined action of solar radiation and a photocatalyst. Among the various photocatalysts, TiO₂ is one of the most widely used due to its excellent physicochemical properties. However, when TiO₂ is dispersed in the reaction medium as a powder, it tends to deposit on different components of the reaction system (e.g., hoses, reactors, and pumps), which makes its recovery difficult and requires an additional separation step. The use of TiO₂ supports provides an effective strategy to immobilize the catalyst and thus avoid these operational drawbacks. In this study, rutile-phase TiO₂ tubular supports exhibited photocatalytic activity in the degradation of NR5, achieving a degradation efficiency of 44.5% at a feed flow rate of 0.116 L·s⁻¹ and an apparent reaction rate constant (kapp′) of 4.64 L·kJ-1. Furthermore, the incorporation of a thin film of anatase-phase TiO₂ on the support via the sol–gel method significantly improved the photocatalytic performance, increasing the degradation efficiency up to 60.9% and kapp′ of 11.8 L·kJ-1, which can be attributed to an increase in the effective surface area of the photocatalyst. Although the use of photocatalytic reactors with supported catalysts overcome some drawbacks associated with operating with a dispersed catalyst, in comparative terms, the latter tends to have the advantage of a higher catalyst surface area. Therefore, the design of a reactor with fixed catalysts should optimize the total absorbed solar energy on the supports, which will depend on factors such as the shape, size, number, configuration, and distribution of supports within the reactor, and hence it would be important to conduct further studies that contemplate such design variables on reactor performance. Declarations Author Contribution SGMO – Conducted the experiments and analyzed and discussed the results obtained.RSRM – Wrote the main text and discussed the results obtained, and and provided the funding for carrying out the experiments.ERG – Wrote the main text, prepared all the figures, discussed the results obtained, and provided the funding for carrying out the experiments. Acknowledgment We would like to thank the Basic Sciences and Engineering Division of the Universidad Autónoma Metropolitana–Iztapalapa for approving the project and founds. We also would like to thank to Deyanira Ángeles-Beltran and for performing the SEM analyses on the samples. References Vaiano, V.; Sacco, O.; Pisano, D.; Sannino, D.; Ciambelli P. (2015). From the design to the development of a continuous fixed bed photoreactor for photocatalytic degradation of organic pollutants in wastewater. Chem. Eng. Sci. Vol. 137, pp. 152–160. 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(2007) Photocatalytic degradation of an azo dye in a tubular continuous-flow photoreactor with immobilized TiO 2 on glass plates. Chem. Eng. J. , Vol. 127, No. 1–3, pp 167–176. DOI: https://doi.org/10.1016/j.cej.2006.09.013 . Damodar, R.A.; Swaminathan, T. (2008) Performance evaluation of a continuous flow immobilized rotating tube photocatalytic reactor (IRTPR) immobilized with TiO 2 catalyst for azo dye degradation. Chem. Eng. J. , Vol. 144, No. 1, pp. 59–66. DOI: https://doi.org/10.1016/j.cej.2008.01.014 Kassahun S. K., (2026) Preparation, characterization and performance assessment of immobilized visible light-active boron-doped TiO 2 with inclined photoreactor on the degradation of diclofenac in aqueous solution, Results Chem ., Vol. 19, pp.102961. DOI: https://doi.org/10.1016/j.rechem.2025.102961 Wang, W.Y.; Irawan A.; Ku Y. (2008) Photocatalytic degradation of Acid Red 4 using a titanium dioxide membrane supported on a porous ceramic tube, Water Research , Vol. 42, No. 19, pp. 4725–4732. DOI: https://doi.org/10.1016/j.watres.2008.08.021 Yoko, T.; Hu,L.; Kozuka, H. & Sakka, S. (1996). Photoelectrochemical properties of TiO2 coating films prepared using different solvents by the sol-gel method. Thin Solid Films , Vol. 283, No. 1–2, pp. 188–195. DOI: https://doi.org/10.1016/0040-6090(95)08222-0 Zhang, H.; Quan, X.; Chen, S.; Zhao, H. & Zhao, Y. (2006). Fabrication of photocatalytic membrane and evaluation its efficiency in removal of organic pollutants from water. Sep. Purif. Technol. Vol. 50, No. 2, pp. 147–155. DOI: https://doi.org/10.1016/j.seppur.2005.11.018 Hummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. , Vol. 80, pp. 1339, 1958. DOI: https://doi.org/10.1021/ja01539a017 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 18 May, 2026 Reviews received at journal 14 May, 2026 Reviews received at journal 14 May, 2026 Reviews received at journal 12 May, 2026 Reviews received at journal 12 May, 2026 Reviewers agreed at journal 12 May, 2026 Reviews received at journal 11 May, 2026 Reviews received at journal 09 May, 2026 Reviewers agreed at journal 08 May, 2026 Reviewers agreed at journal 08 May, 2026 Reviewers agreed at journal 07 May, 2026 Reviews received at journal 07 May, 2026 Reviewers agreed at journal 07 May, 2026 Reviewers agreed at journal 06 May, 2026 Reviewers agreed at journal 06 May, 2026 Reviewers agreed at journal 06 May, 2026 Reviewers invited by journal 06 May, 2026 Editor assigned by journal 02 Apr, 2026 Submission checks completed at journal 02 Apr, 2026 First submitted to journal 25 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-9226072","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":639390096,"identity":"3dbd16de-0082-4a7a-8d9e-226d707de757","order_by":0,"name":"Selene G. Morales Ortiz","email":"","orcid":"","institution":"Universidad Autónoma Metropolitana","correspondingAuthor":false,"prefix":"","firstName":"Selene","middleName":"G. Morales","lastName":"Ortiz","suffix":""},{"id":639390097,"identity":"5bc68eef-56cd-41d7-a682-576dc489ff77","order_by":1,"name":"Richard S. Ruiz Martinez","email":"","orcid":"","institution":"Universidad Autónoma Metropolitana","correspondingAuthor":false,"prefix":"","firstName":"Richard","middleName":"S. Ruiz","lastName":"Martinez","suffix":""},{"id":639390098,"identity":"57734a73-7b0b-426a-83f0-1104c34193cb","order_by":2,"name":"Elizabeth Rojas Garcia","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtklEQVRIiWNgGAWjYHACxgcka2E2IFkLmwRp6vmlj1+r5qm5k7i2gcfw0w2GbYkNhLRI9uWU3Zxx7FnitgM8xtI5DLeNCdpicIYn7cYHtsMgLQYgLXIEtdgDtRQk/ANrMf4N1MJD2BYe9mMMH9vAWsyIs0XiDA+z5My+w8bbDrOVWecYEOEX/h72h595vh2W3Xa8efPtnIrbhEOMgYEHGpPMYHcSVg8E7A+IUjYKRsEoGAUjGAAAb848Feo0S4MAAAAASUVORK5CYII=","orcid":"","institution":"Universidad Autónoma Metropolitana","correspondingAuthor":true,"prefix":"","firstName":"Elizabeth","middleName":"Rojas","lastName":"Garcia","suffix":""}],"badges":[],"createdAt":"2026-03-25 17:39:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9226072/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9226072/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109257948,"identity":"eac10fcd-5eb7-4121-a2b4-6e90948ac034","added_by":"auto","created_at":"2026-05-14 10:32:12","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":316280,"visible":true,"origin":"","legend":"\u003cp\u003eProcedure for the impregnation of support.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/d8dd8d636130bb2e7b8b9b72.png"},{"id":109296565,"identity":"592315d3-77c0-42c3-8c22-9484917d17e3","added_by":"auto","created_at":"2026-05-15 08:48:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":141338,"visible":true,"origin":"","legend":"\u003cp\u003ePower XRD patterns of the materials, a) unimpregnated support, and b) impregnated support.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/7d9216f97ce2ef0932743ed3.png"},{"id":109257950,"identity":"b4fec49b-ad3f-4efb-8de8-4327cf8cf653","added_by":"auto","created_at":"2026-05-14 10:32:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":159095,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of the materials, a) unimpregnated support, and b) impregnated support.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/1a1c103e75dcb76c72f2277c.png"},{"id":109257951,"identity":"96566410-8c70-4a4f-9a01-56e6cd467717","added_by":"auto","created_at":"2026-05-14 10:32:12","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":706608,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of a) unimpregnated support, b) and c) impregnated support.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/86352cb7a3431842227ce508.png"},{"id":109296054,"identity":"74fda311-3579-4836-b1b2-e29f7f2db305","added_by":"auto","created_at":"2026-05-15 08:44:57","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":68533,"visible":true,"origin":"","legend":"\u003cp\u003ePhotocatalytic evaluation of the unimpregnated support using a feed flow of A) 0.538 L/s, and B) 0.116 L/s, as well as monitoring the irradiance with respect to reaction time.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/7c3e99ba09e12c027b871318.png"},{"id":109257952,"identity":"11f7f43e-803f-418d-9eb2-0ac4193ec618","added_by":"auto","created_at":"2026-05-14 10:32:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":47049,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of photocatalytic activity as a function of the number of impregnated supports in the reactor.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/560b1ad2ad09b38a8fc86f48.png"},{"id":109296228,"identity":"9714722c-996e-4d17-955f-d97f74c24e01","added_by":"auto","created_at":"2026-05-15 08:46:17","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":141276,"visible":true,"origin":"","legend":"\u003cp\u003ePhotocatalytic activity as a function of film thickness using impregnated supports.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/ff2c32b84c704228b1489d0e.png"},{"id":109257955,"identity":"aabbf5d9-a31a-4561-92d7-afec74d373fc","added_by":"auto","created_at":"2026-05-14 10:32:12","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":133705,"visible":true,"origin":"","legend":"\u003cp\u003eKinetic study of the photocatalytic degradation of RB5 dye under conditions different, a) low (0.116 L·s\u003csup\u003e-1\u003c/sup\u003e) and high flow (0.538 L·s\u003csup\u003e-1\u003c/sup\u003e), b) one and two unimpregnated support, c) \u0026nbsp;\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/3c38e8e57eacac7af69cbe88.png"},{"id":109296505,"identity":"5fafedbd-ccb5-49e4-8a65-14e70e59a41b","added_by":"auto","created_at":"2026-05-15 08:47:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1697918,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9226072/v1/707eeb98-582e-49b2-bcf0-b0d4fd979946.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Anatase-phase TiO 2 -tubular support as photocatalyst for the photocatalytic degradation of RB5 dye under sunlight","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWater is one of the most essential natural resources on the planet; therefore, its contamination by chemical substances derived from industrial activities has become a global environmental concern. Among the different industrial sectors, the textile industry has been recognized as one of the largest consumers of water and one of the main contributors to water pollution [1]. Among the wide variety of dyes used in industry, azo dyes represent the largest group of chromophores and account for approximately 70% of global dye production due to their ease of synthesis and versatility. However, they are also among the most hazardous dyes because they can generate aromatic amines that are potentially carcinogenic [2].\u003c/p\u003e \u003cp\u003eSolar photocatalysis has been widely investigated as an effective treatment method for dye-containing effluents. Numerous studies have demonstrated that this technology can efficiently remove color and significantly reduce both chemical oxygen demand (COD) and total organic carbon (TOC) in wastewater. In addition, photocatalysis promotes the degradation of pollutants into smaller, less harmful molecules with low or negligible toxicity [3]. Photocatalysts play a crucial role in the photocatalytic process. Among the wide range of photocatalytic materials, titanium dioxide (TiO₂) is the most widely studied and applied due to its advantageous properties, such as chemical stability, environmental compatibility, low toxicity, resistance to poisoning, and relatively low cost. In aqueous media, its surface exhibits amphoteric behavior, meaning that it can acquire positive, negative, or neutral charges depending on the pH of the solution [4]. TiO₂ occurs in three main crystalline phases: rutile, anatase, and brookite. Several studies have reported that the anatase phase exhibits higher photocatalytic activity, with a band gap of approximately 3.2 eV, whereas the rutile phase is thermodynamically more stable and has a band gap of about 3.0 eV [5].\u003c/p\u003e \u003cp\u003eIn most experimental systems, photocatalysts are used in suspended form in the reaction medium because this configuration provides a larger surface area available for reaction, which enhances the mineralization rate of contaminants [6,7,8]. However, despite the excellent properties of TiO₂, its large-scale application faces significant challenges. One of the main limitations is the high cost associated with separating fine catalyst particles from the treated water after the reaction process, as well as the reduced light penetration caused by the opacity of catalyst suspensions.\u003c/p\u003e \u003cp\u003eTo overcome these limitations, one promising strategy is the immobilization of the photocatalyst onto a suitable substrate (membrane) or support, which allows catalyst reuse and simplifies separation. Nevertheless, several studies have reported that immobilization may reduce the effective surface area available for photocatalytic reactions [8,9,10,11]. Therefore, TiO₂-based photocatalytic systems continue to be extensively investigated to develop new strategies aimed at improving their efficiency.\u003c/p\u003e \u003cp\u003eDifferent reactor configurations have been reported for immobilized photocatalytic systems, regarding the geometry and material of the substrate employed. The geometries most commonly reported for the supports consists of flat surfaces and cylindrical rods, and a variety of materials have been used as supports such as polymeric membranes (e.g. polysulfone (Grzechulska et al. (2009); polyvinylidene (fluoride Akter (2019)), as well as glass, alumina, titania, and ceramic supports, among other materials, have been reported (Mar\u0026iacute;n et al. (2007); Jim\u0026eacute;nez and Gelover (2007); Zhang et al. (2006); Esp\u0026iacute;ndola et al. (2019); Rosero et. al. (2022); Alventosa De lara, E. (2015)). In many of these works the active phase consisted of TiO\u003csub\u003e2\u003c/sub\u003e and dip-coating and sol-gel were often employed as the deposition method. The above-mentioned studies report different degrees of degradation of a contaminant molecule, however information on the effect of support material and deposition method on both reactor performance and immobilization stability is often missing.\u003c/p\u003e \u003cp\u003eIn this study, the performance of tubular TiO₂ supports in the rutile phase, as well as those coated with a thin film of anatase-phase TiO₂, was evaluated for the degradation of Reactive Black 5 (RB5), which is a widely used textile dye because it allows the production of deep black and navy-blue shades. In addition, the effects of several operational parameters were investigated, including the feed flow rate, the number of tubular supports placed inside the reactor, and the thickness of the anatase-phase TiO₂ film deposited onto the rutile-phase TiO₂ support.\u003c/p\u003e"},{"header":"2. Experimental section","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Impregnation of tubular support\u003c/h2\u003e \u003cp\u003eThe rutile-phase TiO\u003csub\u003e2\u003c/sub\u003e tubular support was obtained from TAMI Industries, and Filtanium model. This support was coated with a thin layer of anatase-phase TiO\u003csub\u003e2\u003c/sub\u003e. To prepare the TiO\u003csub\u003e2\u003c/sub\u003e precursor solution, the sol-gel method was used. The methodology for preparing the sol-gel has been previously reported in the literature by Jim\u0026eacute;nez \u0026amp; Gelover [12]. The synthesis of the solution required controlled conditions, such as the use of a fume hood, constant stirring, and regulated temperature. Initially, the acid hydrolysis medium was prepared (solution A), consisting of 38 mL of absolute ethanol (Sigma-Aldrich, ACS reagent, \u0026ge;\u0026thinsp;99.5%, 0.65 mol), 2.3 mL of deionized water (0.13 mol), and 0.27 mL of hydrochloric acid (Sigma-Aldrich, 37%, ACS reagent, 0.008 mol). This solution was prepared under constant stirring in an ice bath. Once the previous solution was prepared, it was stored under refrigeration until later use. Subsequently, the solution containing the metallic precursor was prepared using 38.5 mL of titanium isopropoxide (Sigma-Aldrich, ACS reagent, \u0026ge;\u0026thinsp;97.0%, 0.13 mol) and 38 mL of absolute ethanol. The absolute ethanol was added slowly and carefully dropwise down the side of the round-bottom flask, which was kept in an ice bath under constant stirring (solution B). Finally, solution A was added slowly and continuously to solution B under an ice bath and constant stirring for a period of six hours. A cooling system was also incorporated to minimize alcohol evaporation and ensure the stability of the reaction. As a result, a translucent white solution was obtained. The final solution was stored in an airtight container under refrigeration, conditions under which it can remain stable for at least 45 days without premature gelation [13].\u003c/p\u003e \u003cp\u003eFor impregnation of the support, a dip-coating system was constructed, the characteristics of which are described in detail in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. This system allowed for control over the immersion and extraction speeds. The photocatalyst immobilization process began by depositing 256 ml of the sol-gel precursor solution into the deposition tube. In this work, the constructed system allowed the tubular support to be immersed in the precursor solution at 1.15 cm s⁻\u0026sup1; and extracted at a speed of 2.38 cm s⁻\u0026sup1;, remaining submerged in the solution for 10 min. Once extracted from the sol-gel solution, the tubular support was dried for 30 minutes at room temperature. Subsequently, the supports underwent was calcined using a heating rate of 10\u0026deg;C min⁻\u0026sup1; at 400\u0026deg;C by 2 hours.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization of the material\u003c/h2\u003e \u003cp\u003ePowder X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer equipped with a copper anode (CuK\u003csub\u003eα\u003c/sub\u003e radiation, λ\u0026thinsp;=\u0026thinsp;0.15406 nm) over a 2θ range of 10\u0026ndash;70\u0026deg;. Raman spectra were acquired with a Renishaw InVia Raman microscope in the range of 100\u0026ndash;800 cm⁻\u0026sup1; using a green laser (λ\u0026thinsp;=\u0026thinsp;532 nm) in order to obtain chemical and structural information about the support. The optical band gap was determined from UV\u0026ndash;Vis absorption measurements performed with an Agilent Technologies Cary 100 spectrophotometer equipped with an integrating sphere for solid samples. The spectra were recorded in the appropriate wavelength range, and the band gap energy was calculated using the following equation:\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(F\\left(R\\right)*hv\\right)}^{1/n}=B(hv-{E}_{g}\\)\u003c/span\u003e \u003c/span\u003e) (1)\u003c/p\u003e \u003cp\u003eSEM analysis allows visualization of the morphology of the outer surface of the tubular supports. The SEM images were obtained using a Zeiss Supra 55VP instrument, with secondary and backscattered electrons and a voltage of 50 kV.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Photocatalytic activity\u003c/h2\u003e \u003cp\u003eThe photocatalytic reaction system consisted of three quartz tubes with an inner diameter (ID) of 2.6 cm, an outer diameter (OD) of 3.0 cm, and a length of 60 cm. These tubes are fixed to an aluminum box, which can be adjusted to different angles to align with the optimal angle of incidence of solar radiation. This box measures 100 cm long by 55 cm wide and is supported on an aluminum base. To improve the efficiency of photocatalyst activation, compound parabolic collectors (CPCs) were added. These collectors, with a geometry of two parabolic segments, have a concentration factor equivalent to 1 sun and are made of aluminum with a reflectivity index of approximately 99.7% [14]. The transport of the solution with the dye through the tubes was carried out by means of \u0026frac12; HP stainless steel JET pumps, AQUA PAK brand, model ALY05/1230, 230 V, and three high resistance industrial hoses, with a diameter of 1 in and a length of 1.5 m. For the photocatalytic tests, the unimpregnated and impregnated supports were placed inside the quartz tubes (reactor). 12 L of a NR5 solution at 5 ppm were used in each reactor. The solution was then pumped through of the reactor for 30 minutes in darkness to allow to reach adsorption/desorption equilibrium. After this time, the experimental phase began under sunlight, aliquots were taken every half hour with a reaction time of 8 hours and analyzed in an UV-Vis spectrophotometer (HACH DR6000). Dye decolorization was monitored by measuring the absorption peak at 595 nm. Irradiance was measured with an Extech LT 366 luxmeter. Since the meter displays radiation in kLux, a conversion factor of 1 W/m\u0026sup2; = 122 lux was applied [15].\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussions","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterization of materials\u003c/h2\u003e \u003cp\u003ePowder X-ray diffraction patterns provide information that allows the identification of the phases present in the unimpregnated and impregnated TiO\u003csub\u003e2\u003c/sub\u003e supports. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the XRD diffraction patterns of the unimpregnated and impregnated supports. As can be seen, the X-ray diffraction pattern of the unimpregnated support shows peaks at 2θ\u0026thinsp;=\u0026thinsp;26.6, 35.3, 38.5, 40.4, 43.4, 53.6, and 55.9\u0026deg;, characteristic of the rutile phase of TiO\u003csub\u003e2\u003c/sub\u003e with a tetragonal structure and consistent with reference pattern number JCPDS 4-551[16]. No additional peaks attributable to the presence of TiO\u003csub\u003e2\u003c/sub\u003e in the anatase or brookite phases, or to impurities, were detected, and the observed signals are well-defined, indicating high crystallinity. While the impregnated sample shows the same peaks as the unimpregnated sample, indicating the presence of the rutile phase of TiO\u003csub\u003e2\u003c/sub\u003e, peaks corresponding to the anatase phase of TiO\u003csub\u003e2\u003c/sub\u003e are not observed, probably due to the small amount of this phase present in the sample in powder form.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the Raman spectra of the unimpregnated and impregnated supports. Raman spectrum of unimpregnated support shows four active modes at 610 cm⁻\u0026sup1; (A\u003csub\u003e1g\u003c/sub\u003e), 443 cm⁻\u0026sup1; (E\u003csub\u003eg\u003c/sub\u003e), 239 cm⁻\u0026sup1; (Multi-phonon process), and 143 cm⁻\u0026sup1; (B\u003csub\u003e1g\u003c/sub\u003e), characteristic of the rutile phase of TiO₂, results similar were observed in the XRD analyses [8]. In the impregnated sample, in addition to the aforementioned bands, other bands are observed at 143 cm⁻\u0026sup1; (E\u003csub\u003eg\u003c/sub\u003e) and 512 cm⁻\u0026sup1; (A\u003csub\u003e1g\u003c/sub\u003e), indicated the presence of the anatase phase of TiO₂ [8]. This indicates the presence of the anatase phase on the support surface, which was the objective of the impregnation. Adding a thin film of anatase-phase TiO₂ to the support aims to increase photocatalytic activity, as this is the most active phase in photocatalysis [17].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDiffuse reflectance UV\u0026ndash;Vis spectroscopy analysis, performed using the Kubelka\u0026ndash;Munk function in combination with the Tauc equation, allowed the determination of the optical band gap of the samples [16]. The unimpregnated sample exhibited a band gap of 2.74 eV (452.5 nm), characteristic of rutile-phase TiO₂ [18]. After impregnation of the support with an anatase-phase TiO₂ thin film, the band gap slightly increased to 2.80 eV (442.8 nm), suggesting the presence of a small amount of anatase-phase TiO₂. Stepanova et al. [18] reported that an increase in the anatase phase fraction in TiO₂ results in a larger band gap and improved photocatalytic degradation of Rhodamine B under solar irradiation. In our analysis, a small portion was taken from one end of the support and ground into a fine powder prior to measurement.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents SEM micrographs of the samples under study. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea shows the morphology of the external surface of the unimpregnated support, where pits of varying diameters formed during the synthesis process of support can be observed. In contrast, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb displays the external surface of the impregnated support, where the pits appear partially filled with anatase-phase TiO₂ particles. Finally, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec shows the SEM image of a cross-sectional profile of the support, where the film formed on the external surface of the support can be observed, indicating the homogeneity of the coating.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Photocatalytic evaluation\u003c/h2\u003e \u003cp\u003eIn the photocatalytic evaluation were analyzed several parameter that affect the photocatalytic activity of unimpregnated and impregnated support.\u003c/p\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Effect of recirculation flow rate\u003c/h2\u003e \u003cp\u003eTo evaluate the effect of the recirculation flow rate, the unimpregnated support was used and two flow rates were tested: 0.116 and 0.538 L s⁻\u0026sup1; (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea presents the results obtained from the photocatalytic evaluation of the unimpregnated support at a flow rate of 0.538 L s⁻\u0026sup1; under solar irradiation, together with the monitoring of the irradiance reaching the reactor. As observed, only a small amount of RB5 dye was adsorbed onto the surface of the support under dark conditions. Once the reaction system was exposed to sunlight, the degradation of the dye began, reaching 25.4% of degradation after 8 hours of reaction.\u003c/p\u003e \u003cp\u003eWhen the recirculation flow rate was reduced to 0.116 L s⁻\u0026sup1; (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), a significant increase in the RB5 degradation percentage was observed, reaching 44.5%. This improvement can be attributed to the increase in residence time within the reactor, which allows greater interaction between the reactant and the catalytic surface. Furthermore, the lower flow rate reduces mass transfer limitations, promoting better contact between the dye molecules and the catalyst surface, thereby facilitating diffusion and enhancing the photocatalytic reaction. Other author has shown this same behavior [19, 20, 21, 22].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.2.2 Effect of the number of membranes arranged inside the reactor\u003c/h2\u003e \u003cp\u003eSubsequently, two configurations were simultaneously evaluated under the same solar irradiation conditions: a single membrane placed at the center of the tube (reactor), and two membranes arranged horizontally inside another quartz tube reactor. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the effect of the photocatalytic activity as a function of the number of unimpregnated photocatalytic supports inside the reactor. As the number of supports inside the reactor increased, an increase in the degradation percentage was observed, from 28.1% (one support) to 39.9% (two supports). This result indicates that increasing the number of supports inside the reactor leads to an increase in the effective surface area, which could explain the higher activity observed in the degradation of the RB5 dye.\u003c/p\u003e \u003cp\u003eIt has been reported in the literature that an increase in the effective surface area is generally associated with an increase in the degradation percentage [13,23,24], due to the higher number of active sites where the chemical reaction can occur.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Efecto del espesor de la pel\u0026iacute;cula en la superficie externa del soporte\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the photocatalytic activity as a function of time and film thickness for the impregnated supports with anatase-phase TiO₂. As observed, increasing the thickness of the film leads to an increase in photocatalytic activity. The sample with a film thickness of 256 \u0026micro;m exhibited a degradation percentage of 60.6% after 8 hours of reaction. Additionally, an increase in the BET surface area was observed as the film thickness increased, which may explain the enhanced degradation of RB5. This improvement can be attributed to the higher number of active sites where charge carriers (electron\u0026ndash;hole pairs) are generated, which are responsible for the formation of oxidizing species involved in the degradation of the RB5 dye.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Kinetic model\u003c/h2\u003e \u003cp\u003eThe Langmuir-Hinshelwood (L-H) kinetic model has been widely used in the literature to describe the photocatalytic degradation of dyes [4,7,8,11] which is mathematically expressed as:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:r=-\\frac{d{C}_{RB5}}{dt}={k}_{r}\\frac{K{C}_{RB5}}{1+K{C}_{RB5}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003er\u003c/em\u003e is the degradation rate (mg\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e); \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eRB5\u003c/em\u003e\u003c/sub\u003e is the RB5 concentration at time \u003cem\u003et\u003c/em\u003e (mg\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e); \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e is the reaction rate constant (min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e); and \u003cem\u003eK\u003c/em\u003e is the apparent adsorption coefficient of the pollutant on the catalyst surface (L\u0026middot;mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Under certain experimental conditions, such as relatively low concentrations for which 1\u0026thinsp;\u0026gt;\u0026thinsp;\u0026gt;\u0026thinsp;\u003cem\u003eKC\u003c/em\u003e, can be simplified to a pseudo-first order kinetic equation:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:-r=-\\frac{d{C}_{RB5}}{dt}={k}_{r}K{C}_{RB5}={k}_{app}{C}_{RB5}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eF\u003c/b\u003eurthermore, the apparent \u003cem\u003ek\u003c/em\u003e\u003csub\u003eapp\u003c/sub\u003e reaction rate for a photocatalytic process is affected by the amount and intensity of the irradiation absorbed by the catalyst [25]; and for a solar reactor in terms of the UV irradiation reaching the reactor surface, \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eac\u003c/em\u003e\u003c/sub\u003e, the reaction rate can be determinated by the following equation:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:-r={k}_{app}{\u0026acute;E}_{ac}{C}_{RB5}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere, the UV energy absorbed in the reactor is calculated by \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003eac\u003c/em\u003e\u003c/sub\u003e = \u003cem\u003eW A\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e /\u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eT\u003c/em\u003e\u003c/sub\u003e, where \u003cem\u003eW\u003c/em\u003e is the solar UV irradiation rate (kJ s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sub\u003e is the reactor absorbing area as obtained by solar ray analysis (m\u003csup\u003e2\u003c/sup\u003e), and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eT\u003c/em\u003e\u003c/sub\u003e is the total volume of water treated (L).\u003c/p\u003e \u003cp\u003eFor the determination of apparent rate constant, \u003cem\u003ek\u003c/em\u003e\u003csub\u003e\u003cem\u003eapp\u003c/em\u003e\u003c/sub\u003e\u0026acute;, the procedure describing by E. Rojas et.al [14] was used, obtained the following equation:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:{k}_{app}\u0026acute;=-\\frac{{v}_{o}}{{V}_{R}{\\:E}_{ac}}ln\\left(1-m\\:{V}_{SV}/{v}_{o}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{o}\\)\u003c/span\u003e\u003c/span\u003e is the liquid volumetric flow rate, \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eR\u003c/em\u003e\u003c/sub\u003e is the reactor volume, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{SV}\\)\u003c/span\u003e\u003c/span\u003e is the stirred vessel volume, and m is the slope of the graph of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:ln\\left(\\frac{{C}_{RB\\text{5,0}}}{{C}_{RB\\text{5,1}}}\\right)\\:\\)\u003c/span\u003e\u003c/span\u003evs time, where \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eRB5,0\u003c/em\u003e\u003c/sub\u003e is the initial dye concentration (t\u0026thinsp;=\u0026thinsp;0) in the mixing vessel and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{NR\\text{5,1}}\\)\u003c/span\u003e\u003c/span\u003e is the dye concentration at the reactor entrance which varies with respect to time. The above equation was derived from the mass balance relationship between the reactor and the mixer, since the aliquots collected at different times are taken from the mixer, which does not directly represent the concentration of RB5 dye at the reactor outlet [14].\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the pseudo-first-order kinetic plots for the photocatalytic tests performed on both unimpregnated and impregnated samples. The experimental data were fitted using linear regression to determine the slope. From the slope and Eq.\u0026nbsp;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e5\u003c/span\u003e), the apparent rate constant (k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e) was calculated. The k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e values for tests using unimpregnated supports are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. As shown, decreasing the feed flow rate by placing an unimpregnated support in the reactor led to an increase in k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e, indicating that a lower feed flow rate improved mass transport condition (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). Furthermore, doubling the number of unimpregnated supports in the reactor resulted in an approximately twofold increase in k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e, which is consistent with the larger effective surface area provided by the additional supports (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). This values of k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e are most higher that the shown in the photolysis reaction, indicated the effect of rutile-phase TiO\u003csub\u003e2\u003c/sub\u003e tubular support.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e values obtained when varying the film thickness of the impregnated samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), along with the percentage of degradation after 8 hours of reaction and the BET surface area. It can be observed that increasing the film thickness led to an increase in the apparent rate constant, as well as in the BET surface area and the percentage of RB5 degradation. This behavior is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed, where increasing the film wall thickness results in an increase in the apparent rate constant.\u003c/p\u003e \u003cp\u003e \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\u003ePercentage of degradation of RB5, and apparent kinetic constant of photolysis and unimpregnated support under conditions different.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMembrane\u003c/p\u003e \u003cp\u003enumber\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFeed flow\u003c/p\u003e \u003cp\u003e(L\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e% degradation*\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSlope\u003c/p\u003e \u003cp\u003e(h\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ek\u003csub\u003eapp\u003c/sub\u003e\u0026acute;\u003c/p\u003e \u003cp\u003e(L\u0026middot;kJ\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhotolysis\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e23.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0323\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9565\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.538\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e25.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0445\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9961\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.51\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e44.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0730\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9977\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e28.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0323\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9850\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.24\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e39.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.0603\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9906\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4.15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"6\"\u003e*reaction time: 5h\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \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\u003eApparent kinetic constant, percentage of degradation of RB5, and BET surface area at different film thickness of impregnated support.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\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=\"left\" 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=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFilm thickness (\u0026micro;m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFeed flow\u003c/p\u003e \u003cp\u003e(L\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e% degradation*\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSlope\u003c/p\u003e \u003cp\u003e(h\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003ek\u003csub\u003eapp\u003c/sub\u003e\u0026acute;\u003c/p\u003e \u003cp\u003e(L\u0026middot;kJ\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eBET surface area (m\u003csup\u003e2\u003c/sup\u003e\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e47.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.0614\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9796\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e148\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e47.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0,0829\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9776\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e256\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.1223\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.9949\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e11.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003e*reaction time: 5h\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eAzo dyes such as Reactive Black 5 (RB5) are recalcitrant pollutants that are difficult to degrade using conventional treatment methods, particularly biological processes. In contrast, solar photocatalysis represents an effective alternative for the removal of these contaminants through the combined action of solar radiation and a photocatalyst. Among the various photocatalysts, TiO₂ is one of the most widely used due to its excellent physicochemical properties. However, when TiO₂ is dispersed in the reaction medium as a powder, it tends to deposit on different components of the reaction system (e.g., hoses, reactors, and pumps), which makes its recovery difficult and requires an additional separation step. The use of TiO₂ supports provides an effective strategy to immobilize the catalyst and thus avoid these operational drawbacks. In this study, rutile-phase TiO₂ tubular supports exhibited photocatalytic activity in the degradation of NR5, achieving a degradation efficiency of 44.5% at a feed flow rate of 0.116 L\u0026middot;s⁻\u0026sup1; and an apparent reaction rate constant (kapp\u0026prime;) of 4.64 L\u0026middot;kJ-1. Furthermore, the incorporation of a thin film of anatase-phase TiO₂ on the support via the sol\u0026ndash;gel method significantly improved the photocatalytic performance, increasing the degradation efficiency up to 60.9% and kapp\u0026prime; of 11.8 L\u0026middot;kJ-1, which can be attributed to an increase in the effective surface area of the photocatalyst.\u003c/p\u003e \u003cp\u003eAlthough the use of photocatalytic reactors with supported catalysts overcome some drawbacks associated with operating with a dispersed catalyst, in comparative terms, the latter tends to have the advantage of a higher catalyst surface area. Therefore, the design of a reactor with fixed catalysts should optimize the total absorbed solar energy on the supports, which will depend on factors such as the shape, size, number, configuration, and distribution of supports within the reactor, and hence it would be important to conduct further studies that contemplate such design variables on reactor performance.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eSGMO \u0026ndash; Conducted the experiments and analyzed and discussed the results obtained.RSRM \u0026ndash; Wrote the main text and discussed the results obtained, and and provided the funding for carrying out the experiments.ERG \u0026ndash; Wrote the main text, prepared all the figures, discussed the results obtained, and provided the funding for carrying out the experiments.\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eWe would like to thank the Basic Sciences and Engineering Division of the Universidad Aut\u0026oacute;noma Metropolitana\u0026ndash;Iztapalapa for approving the project and founds. We also would like to thank to Deyanira \u0026Aacute;ngeles-Beltran and for performing the SEM analyses on the samples.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eVaiano, V.; Sacco, O.; Pisano, D.; Sannino, D.; Ciambelli P. (2015). From the design to the development of a continuous fixed bed photoreactor for photocatalytic degradation of organic pollutants in wastewater. Chem. Eng. Sci. Vol. 137, pp. 152\u0026ndash;160. DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://dx.doi.org/10.1016/j.ces.2015.06.023\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eOlivo-Alan\u0026iacute;s, D.; Garc\u0026iacute;a-Gonz\u0026aacute;lez, A.; Mueses, M.A.;\u0026iexcl; Garc\u0026iacute;a-Reyes, R.B. (2022). Generalized kinetic model for the photocatalytic degradation processes: Validation for dye wastewater treatment in a visible-LED tubular reactor. App. Catal. B, Vol. 317, pp. 121804. 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Mater.\u003c/em\u003e Vol. 150 No. 2 pp. 250\u0026ndash;256. DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhazmat.2007.04.086\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eParrino, F., \u0026amp; Palmisano, L. (2021). Titanium Dioxide (TiO\u003csub\u003e2\u003c/sub\u003e) and its applications. Elsevier. ISBN: 978-0-12-819960-2 DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/C2019-0-01050-3\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eSong, L.; Zhu, B.; Jegatheesan, V.; Gray, S.; Duke, M. \u0026amp; Muthukumaran, S. (2018). Treatment of secondary effluent by sequential combination of photocatalytic oxidation with ceramic membrane filtration. \u003cem\u003eEnviron. Sci. Pollut. Res.\u003c/em\u003e Vol. 25 pp. 5191\u0026ndash;5202. 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Effect of two types of reflective surfaces with different geometry in a solar pilot plant for the photocatalytic degradation of RB5 dye using a TiO\u003csub\u003e2\u003c/sub\u003e/rGO nanocomposite.\u0026nbsp;\u003cem\u003eInt. J. Chem. Reactor Eng.\u003c/em\u003e DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1515/ijcre-2025-0122\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eMichael, P. R.; Johnston, D. E.; Moreno, W. (2020) A conversion guide: solar irradiance and lux illuminance, \u003cem\u003eJ. Measurements in Engineering\u003c/em\u003e, Vol. 8, No. 4, pp. 153\u0026ndash;166. 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DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0040-6090(95)08222-0\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eZhang, H.; Quan, X.; Chen, S.; Zhao, H. \u0026amp; Zhao, Y. (2006). Fabrication of photocatalytic membrane and evaluation its efficiency in removal of organic pollutants from water. \u003cem\u003eSep. Purif. Technol.\u003c/em\u003e Vol. 50, No. 2, pp. 147\u0026ndash;155. DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.seppur.2005.11.018\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003cli\u003eHummers, W. S.; Offeman, R. E. Preparation of graphitic oxide. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e, Vol. 80, pp. 1339, 1958. DOI: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1021/ja01539a017\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"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":"discover-nano","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"narl","sideBox":"Learn more about [Discover Nano](https://www.springer.com/journal/11671)","snPcode":"11671","submissionUrl":"https://submission.nature.com/new-submission/11671/3","title":"Discover Nano","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"RB5 dye, anatase phase TiO2-tubular support, sunlight, photocatalysis, photocatalytic degradation, solar reactor","lastPublishedDoi":"10.21203/rs.3.rs-9226072/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9226072/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePorous tubular supports coated with a thin film of anatase phase TiO₂ were synthesized using the sol\u0026ndash;gel method. Rutile phase TiO₂-tubular support exhibited photocatalytic activity for the degradation of RB5, achieving a degradation efficiency of 44.5% at a feed flow rate of 0.116 L s⁻\u0026sup1; and an apparent rate constant (k\u003csub\u003eapp\u0026prime;\u003c/sub\u003e) of 4.64 L\u0026middot;kJ\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The incorporation of more than one rutile phase TiO₂-tubular support in the photocatalytic reactor increased the degradation of the NR5 dye under the same reaction conditions, from 28.1% (one membrane placed at the center) to 39.9% (two membranes arranged horizontally inside the quartz tube). Furthermore, the incorporation of a thin film of anatase phase TiO₂ enhanced the photocatalytic activity compared with the unimpregnated support, reaching up to 60.6% degradation and increasing the apparent reaction constant to 11.8 L\u0026middot;kJ\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Porous tubular supports coated with a thin film of anatase-phase TiO₂ represent a promising option as photocatalysts for the degradation of contaminants in wastewater. In this configuration, the TiO₂ film remains firmly attached to the support, preventing catalyst detachment and eliminating the need for a separation step that is typically required when TiO₂ is used in suspension in the reaction medium.\u003c/p\u003e","manuscriptTitle":"Anatase-phase TiO 2 -tubular support as photocatalyst for the photocatalytic degradation of RB5 dye under sunlight","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-14 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