High Photocatalytic Efficiency of Al₂O₃-TiO₂ Coatings on 304 Stainless Steel for Methylene Blue and Wastewater Degradation

Abstract

This work explores the novelty of achieving high photocatalytic efficiency and remarkable bactericidal activity with Al₂O₃-TiO₂ coatings on perforated 304 stainless steel (SS) substrates, placed transversely along an airlift reactor of 0.980 L for wastewater treatment under visible light irradiation. The Al₂O₃-TiO₂ coatings achieved methylene blue and total organic carbon (TOC) concentration reductions of 97.3 and 96.51%, respectively, in a wastewater sample with heterogeneous photocatalsis. The Al₂O₃-TiO₂ coatings resulted in a 33.30% reduction in total and fecal coliforms and a remarkable 94.23% decrease in Salmonella spp. in the wastewater sample. XRD confirmed the TiO₂ anatase–rutile phases and Al₂O₃ α-γ phases in the coating. The particle size distribution ranges from 100 to 500 nm, and the coating surface was homogeneous without cracks confirmed using SEM and AFM, respectively. The roughness and thickness of the coatings were 85 ± 5 nm and 250 ± 50 nm, respectively.

1. Introduction

Every day, approximately 2 million tons of untreated wastewater and other effluents are discharged into rivers, lagoons, lakes, and seas, according to the United Nations [1,2]. Conventional wastewater treatment methods face challenges in eliminating emerging contaminants like dyes, pigments, antibiotics, and heavy metals [3,4,5]. Traditional techniques such as adsorbent resins, adsorption on activated carbon, ultrafiltration, reverse osmosis, ion exchange on synthetic materials, etc., remove dye pollutants from water, but these techniques only transfer organic compounds from water to another phase, promoting secondary pollution [6]. To address this issue, recent scientific and technological advancements have been focused on renewing and evolving water treatment methods [7,8]. Advanced oxidation processes (AOPs) have assisted or replaced some operations in wastewater treatment plants (WWTPs) [9]. AOPs can effectively and economically remove organic compounds and pathogenic microorganisms from wastewater [10]. One such AOP is heterogeneous photocatalysis, which employs a semiconductor material exhibiting photocatalytic properties in the ultraviolet or visible range [11,12]. When the energy is equal to or greater than its bandgap, highly oxidizing hydroxyl (OH^●) and superoxide (O₂^●−) radicals are generated. These species contribute to the total mineralization of the polluting organic compounds in water through successive redox reactions and to cellular lysis of pathogenic microorganisms due to oxidative stress [13,14].

There are significant differences between the use of photocatalysts in powder form versus immobilized form [15]. In the case of powders, dispersion of the photocatalyst in the solution requires stirring, which leads to increased processing time and cost due to the need for catalyst separation with filtration or centrifugation [16,17]. However, the use of photocatalyst coatings can eliminate this drawback by requiring high adherence between the substrate and coating [18,19]. Many methods for preparing ceramic coating have been employed, such as chemical vapor deposition, physical vapor deposition [20], sol–gel deposition [21], electroplating [22], spin-coating [23], dip-coating [24], chemical bath deposition [25], atomic layer deposition [26], and spray coating [27], and offer a diverse range of approaches. The dip-coating technique is exceptional because it provides outstanding coating uniformity and precise control over coating thickness. Furthermore, dip-coating is often cost-effective due to its simplicity and minimal equipment requirements. Another noteworthy aspect of dip-coating is its compatibility with various substrates, including plastics, metals, ceramics, and composites, further expanding its applicability across diverse materials and industries. One of the most important substrates of dip-coating is AISI 304 stainless steel (SS), a versatile and widely used material known for its affordability, excellent mechanical properties, ease of fabrication, and high resistance to corrosion and oxidation [28,29].

TiO₂ is one of the most widely used photocatalysts [30,31], known for its mesoporous network, high surface area, and crystallinity, which favor the accessibility and diffusion of target molecules in photocatalytic applications using either UV or visible light [32]. However, a major drawback of TiO₂ photocatalysts is that photo-generated charge carriers tend to recombine, thereby decreasing reaction efficiency [33]. Various methods, including heterojunction formation, ion doping, and nanosized crystals, have proven effective in improving the recombination process of photo-generated electrons in TiO₂ [34].

The Al₂O₃-TiO₂ material might reduce the recombination of photo-generated charge carriers and overcome the main shortcoming of TiO₂ photocatalysts. Numerous research studies have reported better photocatalytic activity of Al₂O₃-TiO₂ mixed oxides in suspension or as thin films under both UV and visible radiation when compared to pure TiO₂ [35,36,37]. Magnone and coworkers [38] reported that TiO₂-supported Al₂O₃ ceramic hollow fiber substrates exhibited a remarkable 91% removal efficiency of methylene blue MB (20 ppm) in one hour. These substrates displayed exceptional photocatalytic stability, retaining their high performance even after multiple cycles. For instance, 10 μm-thickness Al₂O₃-TiO₂ membranes synthesized using the sol–gel spin-coating method demonstrated 99.25% degradation of methylene blue (MB) under 30 W UV light, and notably after five cycles, they still exhibited high MB degradation (above 96%) [39]. Moreover, the catalytic performance of Al₂O₃-TiO₂ extends beyond MB degradation. In comparison to TiO₂, Al₂O₃-TiO₂ (75–90% of TiO₂) powders displayed twice the photocatalytic degradation rate when applied to a salicylic acid (SA) solution (0.144 mM) in 135 mL and 1 g/L of catalyst [40]. Additionally, Martinez-Gomez [41] prepared a γ-Al₂O₃-TiO₂ mixed oxide using hydrolysis, employing the boehmite method. The catalyst treated at 600 °C showed the highest activity since it degraded about 95% of phenol in 4 h in a glass reactor under UV light, using 200 mL of a phenol solution (400 ppm) and 100 mg of catalyst. These findings underscore the versatility and effectiveness of Al₂O₃-TiO₂ coatings.

Prior research focused on Al₂O₃-TiO₂ photocatalytic degradation primarily in low volumes with high-power UV lamps and lacked reactor design. Limited studies examined coatings using residential wastewater samples. To our knowledge, the photocatalytic and antibacterial activity of Al₂O₃-TiO₂ (with Al/Ti atomic ratio = 1.5) coating on 304 stainless steel using airlift rector of 0.918 L volume employing low-power (4 Watt) visible light have not been explored.

The present study evaluated the photocatalytic activity of Al₂O₃-TiO₂ coatings on perforated 304 SS substrates, which were placed transversely in a 0.980 L vertical glass reactor, employing visible light. The reduction in the concentration of MB as a model molecule was determined using UV-vis spectrophotometry. Furthermore, the concentration of total organic pollutants in a sample of residential wastewater was quantified as total organic carbon (TOC). Additionally, bactericidal activity was evaluated by quantifying the total and fecal coliforms.

2. Results and Discussion

2.1. Al₂O₃-TiO₂ Coating Characterization

2.1.1. X-ray Diffraction

Figure 1 displays prominent peaks that are ascribed to the (111), (200), and (220) planes of Fe-γ (JCPDS Card No. 33-0397, austenite) related to 304 SS [42]. The diffraction planes of the Al₂O₃-TiO₂ coating are shown at the bottom, corresponding to the magnified, red-framed section. The crystallization of the alumina α- and γ-phases (JCPDS Card Nos. 11-0425 and 10-0661) was induced by the synthesis route and thermal treatment, which promoted the transformation of AlOOH into α-Al₂O₃; this transformation normally takes place at low temperature (450 °C) [43], while the transformation into γ-Al₂O₃ occurs between 400–700 °C [44,45]. The presence of α-Al₂O₃ was evidenced by the diffraction peaks located at around 2θ = 25.3°, 33.2°, 35.6°, 37.9°, 43.3°, and 57.68°, which were assigned to the planes (102), (107), (104), (110), (113), and (116); the cubic phase was also detected, since the peaks at around 2θ = 45.6° and 66.51° corresponding to the (400) and (440) planes were found. The Al₂O₃-TiO₂ coating on the SS substrate treated at 700 °C for 5 h reveals diffraction peaks corresponding to the (101), (103), (200), and (204) planes of the anatase phase (JCPDS Card No. 21-1272) located at around 2θ = 25.28°, 37.54°, 24.46°, and 62.41°, respectively. Crystallographic planes (110), (101), (200), (111), (211), and (215) of the rutile phase (JCPDS Card No. 21-1276) were found at 2θ = 27.35°, 34.88°, 39.25°, 41.75°, 54.09°, and 76.07°. Sebastian Miszczak and Bożena Pietrzyk [46] reported that at 400 °C, the TiO₂ anatase phase was fully crystallized and low-intensity peaks of the rutile phase were observed at 700 °C, indicating the beginning of the transformation of anatase to rutile; this finding is in total agreement with the phases present in the Al₂O₃-TiO₂ coating.

The peaks located at approximately 29.8° and 35.5° correspond to the (220) and (311) planes, respectively, confirming the presence of magnetite. Additionally, a peak at (110) corresponding to ferrite (α-iron) is observed, which is typically formed during the cooling stage of 304 stainless steel. The presence of chromium oxide on the surface of 304 stainless steel is described as follows: During the heat treatment of 304 stainless steel, a protective chromium oxide layer forms on the surface, acting as a barrier against oxidation and corrosion. As the heat treatment progresses, chromium atoms from the oxide layer may diffuse away, creating voids or vacancies. Subsequently, iron atoms from the stainless steel matrix can migrate through these voids and react with oxygen from the atmosphere or the residual oxygen in the system. This reaction leads to the formation of magnetite. Therefore, the presence of magnetite indicates that the heat treatment has caused the transformation of the original chromium oxide layer.

The Rietveld refinement method was used to determine the phase composition of the Al₂O₃-TiO₂ coating on a 304 SS substrate [47]. The phases used in the refinement correspond to those indexed in Figure 1 and elements identified through EDS analysis. The Rietveld analysis was executed using Profex Software (version 4.3.6), yielding the following statistical values: R_wp = 40.47, R_exp = 3.77, X² = 1.57, and a Goodness of Fit (GoF) score of 1.25.

Table 1. Results of surface coating on 304 SS obtained with Rietveld method.

Phases	Quantity (%)	Lattice Constants (nm)
		a	b	c
α-Al2O3	7.4	0.4748	-	1.294
γ-Al2O3	3.0	0.8017	-	-
Anatase-TiO2	6.9	0.3801	-	0.9595
Rutile-TiO2	3.7	0.4620	-	0.2958
Fe Austenite	72	0.35901	-	-
Ferrite	4	0.558	1.465	0.542
Magnetite	1	0.8353	-	-
Cr2O3	2	-	0.5009	1.3734

listed the phase percentages and lattice constants of the Al₂O₃-TiO₂ coating on a 304 SS substrate obtained through the Rietveld refinement. Focusing exclusively on the Al₂O₃-TiO₂ coating, its composition comprises 49.5% alumina and 59.5% titania. In the alumina component, 35.2% exists in the alpha (α) phase, while 14.3% is in the gamma (γ) phase. Regarding titania, 32.9% is in the anatase phase, with 17.6% in the rutile phase.

2.1.2. Scanning Electronic Microscopy

The field emission scanning electron microscopy (FE-SEM) images in Figure 2a reveal that the Al₂O₃-TiO₂ coating is composed of particles with heterogeneous morphology due to their oxide nature. The tetrahedron form corresponds to Al₂O₃, while the rice-like morphology corresponds to TiO₂. The particle size distribution of the Al₂O₃-TiO₂ coating of 210 ± 90 nm considerably influences the catalysis due to its surface area [48]; it is worth noting that the particle size distribution agrees with the coating thickness estimated at 200 ± 50 nm with profilometry. The interfacial interaction between these two morphologies and their high adherence to the substrate form a heterojunction with the chromium oxide due to their differing electronic properties. This results in the Al₂O₃-TiO₂ coating exhibiting distinctive electronic properties [18].

Furthermore, the elemental analysis of the Al₂O₃-TiO₂ coating using the energy-dispersive X-ray spectroscopy (EDS) technique in Figure 2b confirms the presence of Fe and alloying elements in the substrate. 304 stainless steel is primarily composed of approximately 65–75% iron, 18–20% chromium, and 8–12% nickel, with trace amounts of manganese (<2%), silicon (<2%), phosphorus (<0.045%), and sulfur (<0.03%), while maintaining a low carbon content (<0.03%). Additionally, it may contain traces of phosphorus and sulfur [49]. The oxygen content of 21.46 wt.% is associated with the formation of Ti-O and Al-O bonds [50,51]. The EDS mapping reveals Ti, Al, and O, which is related to the Al₂O₃-TiO₂ coating.

2.1.3. Atomic Force Microscopy

Images obtained with atomic force microscopy (AFM), which allowed the study of the effect of immersion and thermal treatment parameters on the morphological characteristics of the Al₂O₃-TiO₂ coating with a scan size area of 2 μm, are shown in Figure 3. The surface shown in Figure 3a (height sensor) presents granular structures due to metal oxides on the 304 SS substrate, indicating efficient deposition conditions [52]. Figure 3b displays the topographic image of the Al₂O₃-TiO₂ coating, revealing a homogeneous surface with high uniformity; no cracks or other delamination phenomena are observed [53]. The topography image shows that the root mean square roughness (RMSR) was around 85± 5 nm, calculated from the valleys and peaks profile, Figure 3b. Since the particle size distribution (210 ± 90 nm) of the coating is very similar to its thickness (250 ± 50 nm), particles prefer surface interaction; as a result, a monolayer of particles is formed. Therefore, the particles tend to self-assemble into a compact monolayer on the surface of the substrate rather than randomly distributed throughout the bulk of the coating. This preference for surface interaction can be due to the balance between the van der Waals and electrostatic forces between the particles and the surface. A monolayer coating has significant implications for the properties and performance of the film, including its optical and mechanical characteristics. The SEM images also elucidate the monolayer since the SEM reveals that mostly all particles are in contact with the substrate (dark region); therefore, the interfacial interaction with chromium and nickel of Al₂O₃ and TiO₂ is high.

2.1.4. UV-Vis Spectroscopy

Figure 4a displays the UV–visible spectra of the Al₂O₃-TiO₂ coatings. Two distinct bands of maximum absorption in the UV range, approximately around 250 and 350 nm, attributed to TiO₂ particles, are observed. Martínez-Gómez and his research team [12] identified these bands as charge transfer processes from O²⁻ to Ti⁴⁺. These bands corresponds to the excitation of valence band electrons (O, 2p) to the conduction band (Ti, 3d), a characteristic feature of the anatase phase. The broadband from 400 to 650 nm reveals visible light absorption, which is shown in Figure 4a. TiO₂ nanoparticles doped with nonmetals, metallic cations, and metal oxides can alter the bandgap structure of TiO₂ by introducing energy levels within the bandgap and enhancing visible light absorption. This, in turn, extends the lifetime of charge carriers and reduces recombination phenomena in the Al₂O₃-TiO₂ coating [54,55]. The Tauc plot from the UV-vis spectrum of the Al₂O₃-TiO₂ coating is shown in Figure 4b. The Tauc calculation revealed bandgap values of 3.4 and 2.9 eV corresponding to the rutile and anatase phases of TiO₂, respectively. Values of 4.6 and 5.2 eV were observed for α-Al₂O₃ and γ-Al₂O₃, respectively. Those values are in the bandgap range for the anatase and rutile phases of TiO₂ and the alpha and gamma phases of Al₂O₃ [56,57].

The interfacial iteration between TiO₂ and Al₂O₃ and the combined factors, such as synthesis condition, substrate, thickness, and particle size distribution, promotes special spatial confinement. One of the main remarks in this work is the employment of visible light to activate photodegradation. It is worth mentioning that UV light usually activates TiO₂. The thermal treatment at 700 °C promotes the sensitization of the 304 SS substrate, enabling Cr-rich carbide in the grain boundaries, and the chromium in the carbide reacts with oxygen from the surrounding environment forming chromium oxide. Additionally, during sensitization, chromium from the grain boundaries can migrate to the surface due to the high 18.24% chromium content in the metallic material, when chromium reaches the surface, it can interact with oxygen from the atmosphere, forming a passive oxide layer, primarily chromium oxide (Cr₂O₃). This migration may result in interfacial interactions with the Al₂O₃ and TiO₂ phases in the coating, as both the coating structuration and crystallization originate from the same solution. It has been reported in the literature that heterojunctions with carbonaceous materials, metal nanoparticles, metal oxides, and phosphates enhance the photocatalytic activity, leading to better response in the visible spectrum and improvement of the TiO₂ antibacterial activity [58,59]. In this context, both the interfacial interaction and heterojunction (considering chromium oxide’s nature to be a semiconductor) may explain the high efficiency of the reactor.

It is well-known that coupling alumina with titanium dioxide can enhance the absorption of visible light and broaden its photocatalytic activity. The sol–gel method employed for synthesizing and depositing alumina and titania onto the 304 SS substrate ensures a controlled and uniform distribution, leading to improved interfacial alumina–titania phases and their interaction with chromium oxide (as a result of the sensitization from the heat treatment). Interfaces often introduce defect states or energy level mismatches, altering the electronic band structure and affecting the bandgap.

2.2. Photocatalytic Degradation

2.2.1. MB Degradation

Photocatalytic Al₂O₃-TiO₂ coating deposited on the six substrates inside the reactor reduced 97.3% of the MB concentration (Figure 5a). The high efficiency may be related to the high roughness that increases the retention of MB molecules; the coating thickness promotes a heterojunction with the chromium oxide and TiO₂. The airflow increases the dissolved oxygen in the aqueous solution, thus enhancing the oxygen mass transfer efficiency. As a result, a high number of oxygen species, singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (OH^●), and superoxide radical (O₂^●−) is formed by the Al₂O₃-TiO₂ coating interaction with the visible light. Some authors have shown that the horizontal alignment of the lamps improves the photocatalytic degradation, and the light source with higher Watts increases the number of irradiated photons, improving the photocatalytic efficiency of the photocatalyst [59]. Komaraiah et al. [60] reported 92.03% MB for a TiO₂ film after 4 h using a 200 W lamp and a volume of 70 mL, while Yu et al. [61] obtained up to 99.9% using a Cu₂O/TiO₂ thin film after 3 h, with a 500 W lamp and a total volume of 30 mL; in comparison, in this work, a total volume of 0.98 L was used, and four lamps of 4 W were employed.

For comparison, MB degradation was evaluated under photolysis (without substrates) and using non-heat-treated uncoated 304 SS substrates under the same experimental conditions (Figure 5a). Photolysis resulted in 2.6% of MB degradation, while the uncoated 304 SS substrate showed 4.9% of MB degradation. In contrast, the blue line in the same figure represents a significant increase in MB degradation, reaching 12.5%, when using the heat-treated 304 SS substrate (subjected to the same heat treatment for obtaining the Al₂O₃-TiO₂ coating). This remarkable increase is attributed to chromium oxide and magnetite formation promoted by the sensitization of stainless steel in the heat treatment, as evidenced in the XRD section.

Table 2. Comparison of the degradation efficiency of different photocatalyst systems with TiO₂.

Photocatalyst				Conditions of Photocatalysis					Degradation Efficiency (%)
Type	Doping	Presentation	Loading	Pollutant	Concentration	Light Source	Time (h)	Volume (L)		Ref.
TiO2	--	Spherical, 70 nm	Films (glass substrate) ---	MB	1 × 10−6 M	200 W Tungsten filament bulb. UV light (λ = 386 nm)	4	0.015	92.00	[62]
CuNTiO2	1:1:1 atomic ratio	Spherical, ~20 nm.	0.1 g	MB	3 × 10−5 M	500 W Xenon lamp (upon blocking UV light by a 400 nm glass filter)	6	0.5	91.00	[63]
CuO/TiO2	1% wt	nanorods	0.05 g	MB	1.8 to 18 mg L−1	386-LED or 450-LED light emitting diode	2	0.10	92.00	[60]
Ni_TiO2	0.5 Ni/Ti	Spherical, 50 nm	0.24 gL−1	MB	3.2 mg L−1	Osram 1000 W Xenon short arc display/optic lamp, XBO,	4	0.20	99.00	[64]
Ni_TiO2	0.75 Ni/Ti	Nanosheets 90–100 nm, thickness: 15–20 nm	0.03 g	MB	3 mg L−1	500 W power Xenon lamp (LSXS500) with a visible cut-off filter (l < 400 nm)	3	0.03	96.00	[65]
N_TiO2	1.6 N/Ti	Spherical, 10 nm	0.25 g	Phe	25 mg L−1	luminous intensity 15.54 mWcm−2 (400 < λ < 650 nm)	10	0.10	81.00	[66]
TiO2-Fe2O3	1:1	porous diameter < 1 µm	Coating (glass substrates)	MB	3 mg L−1	150 W, halogen lamp	10	0.003	67.00	[48]
TiO2-Fe2O3	5 Fe/Ti	Granular, 9–17 nm	0.4 g	MB	1 × 10−5 M	100 W Halogen Lamp, 100 PAR30/FL 240 V (350 < λ < 1050 nm) INTENSITY: 85 Mw cm−2	2	0.02	87.00	[67]
Al2O3-TiO2	0.1 Al/Ti	Granular, 10–20 nm	0.025 g	Rh(B)	2.0 × 10−5 M	500 W halogen lamp, cut-off filter (λ < 450 nm)	0.16	0.05	89.25	[68]
Al2O3-TiO2	1.5 Al/Ti	Tetrahedral, and rice-like, 210 ± 90 nm	Coating (304SS substrates)	MB	6 mg L−1	4 LED lamps (4.5 W, 400 Lumens, MR16 Philips) 410 < λ < 760 nm	6	0.98	97.43	*

MB: methylene blue dye, Rh(B): rhodamine B dye, Phe: phenol, and * this study.

presents a comprehensive review of various studies investigating the photocatalytic efficiency of TiO₂, both in its undoped form and when doped with metals and nonmetals, as well as its heterojunctions with other metal oxides. Notably, the doping of TiO₂ with metals [60,62,63,64,65,66] has shown significantly high photocatalytic efficiencies, comparable to the heterojunctions of TiO₂ with metal oxides [48,66,67,68] suspended in the medium.

The immobilization of the catalyst can reduce photocatalytic activity due to the loss of surface area. Despite this drawback, utilizing coatings on 304 SS substrates offers practicality since no recovery step for the catalyst is required. Moreover, the interfacial interaction between the coating and the substrate yields beneficial effects in photocatalysis. Al₂O₃-TiO₂ coatings on stainless steel substrates have demonstrated superior photocatalytic activity compared to the same powder system [48]. This enhanced photocatalytic activity of Al₂O₃-TiO₂ may be attributed to the presence of oxides of Cr and Fe, resulting from sensitization during heat treatment, which synergistically act with the photocatalyst. One of the main findings in this work is the high efficiency (97%) achieved by only using visible light provided by four lamps of 4 W, and an airlift reactor configuration of 0.98 L.

2.2.2. TOC Quantification

The Al₂O₃-TiO₂ coating with enhanced photocatalytic activity resulted in higher total organic degradation due to its ability to effectively produce reactive oxygen species, which can then degrade organic compounds. Since TOC monitoring is one of the most important parameters to be considered for drinking water and wastewater facilities, it is critical in helping to optimize treatment processes. TOC is a helpful technique for detecting organic contaminants such as petroleum products, organic acids, pesticides, pathogens, etc. TOC levels can be used to measure treatment efficacy and as an indicator of contamination [69]. For this reason, the Al₂O₃-TiO₂ coating in the airlift reactor was tested to degrade organic carbon. The degradation of organic contaminants in a sample from the local WWTP was 96.51%, measured as TOC, Figure 5b. The residual concentration of 3.49% represents 35.6 mg/L of TOC; the result is within the interval of the maximum permissible limits of the standards for the discharge of treated wastewater in national assets or can be discharged into the sewage system or for reuse in public service (SEMARNAT, 1996; 1997) according to ECOL-001, Standard 002 and 003 (SEMARNAT, 1996; 1997). In comparison, the WWTPs with a treatment cycle of 12 of 24 h obtain final concentrations from 64.9 to 134.5 mg/L of TOC [70,71]. The degradation of organic compounds followed a pseudo-first-order rate k_TOC = 0.0118 min⁻¹. The rate constant for MB degradation is comparable to Cu₂O/TiO₂ thin film [72], which diminished from 0.013 to 0.010 min⁻¹ with eight reuses, and the c rate constant of 0.0108 min⁻¹ for TiO₂ nanorod array thin films on fluorine-doped tin oxide (FTO) glass under 254 nm irradiation [73].

2.2.3. Bactericidal Activity

Several mechanisms have been proposed to explain the bactericidal activity of TiO₂ nanocomposites. Oxygen species formed by TiO₂ nanoparticles can enhance the peroxidation of the polyunsaturated phospholipids in the bacterial membrane and promote its disruption [74].

Furthermore, the disruption of both the nucleus and mitochondrial membranes can lead to DNA degradation and protein denaturation. Simultaneously, the holes formed in the valence band of TiO₂ nanoparticles have the capacity to capture electrons from coenzyme A (CoA), a vital molecule involved in various cellular processes. This electron capture results in CoA dimerization, where two CoA molecules combine [75]. When CoA loses an electron and forms a dimer, it disrupts the essential cellular respiration process, which is critical for bacterial survival. This disruption of cell respiration represents one of the mechanisms by which TiO₂ nanoparticles impact bacterial cells.

Before visible light exposure, 2400 MPN of total and fecal coliform bacteria and 1.56 × 10³ CFU of Salmonella spp. were in the wastewater sample. The bacterial disinfection by photocatalysis is different from organic pollutant degradation, Figure 5b; the bacterial population was drastically unchanged during the first hour, which could be due to the fact that bacteria have enzymatic protection and recovery mechanisms to overcome oxidative stress, catalyzing reactions that prevent reactive oxygen species’ accumulation. In the case of organic pollutants, the photocatalytic efficiency corresponds to their disappearance and sometimes to their mineralization [76]. In contrast, the total inactivation of a microorganism depends on the modification of the entire cellular structure due to partial decomposition of the outer membrane followed by the disordering of the cytoplasmic membrane, resulting in cell death promoted by the accumulation of reactive oxygen species such as O₂^●−, OH^●, and H₂O₂, Figure 6 [77]. After this period, the bactericidal effect can be ascribed to the inhibition caused by blocking nutrient uptake because the organic contaminants decreased by almost 40% measured as TOC, and this combined with the oxidative stress caused by OH^● and O₂^●− species, generated by photoactivation of the Al₂O₃-TiO₂ coating, promoted disruption of the cell membrane, DNA degradation, and protein denaturation [78].

The low efficiency of the Al₂O₃-TiO₂ coating against fecal and total coliforms is attributed to the presence of various bacteria with distinct characteristics. As a result, the Al₂O₃-TiO₂ coating only affects certain bacteria similar to Salmonella spp., while other bacteria remain unaffected. Since the bubbling and irradiation were maintained during the experiment, the mechanism and bacterial reduction rate should have remained constant. To bolster this claim, mathematical equations were defined and used.

As fecal and total coliforms have bacteria that can be affected or unaffected by the Al₂O₃-TiO₂ coating, X_DB is the bacteria fraction that the Al₂O₃-TiO₂ coating can reduce in fecal and total coliforms. The fraction reduction of Salmonella spp. at a certain time ($y_{s_t}$) is used in the computation as shown in Equation (1):

$$y_{s_t}=\frac{CF{U}_0-CF{U}_t}{CF{U}_0}$$

(1)

where $CF{U}_0$ stands for colony forming units at the initial time, and $CF{U}_t$ stands for colony forming units at time “t”. To study the behavior of the fecal and total coliform reduction, Equation (2) was employed and is as follows:

$$MP{N}_t=MP{N}_0-(MP{N}_0\times y_{s_t}\times X_{DB})$$

(2)

where $MP{N}_t$ is the most probable number at time “t”, and $MP{N}_0$ is the most probable number at the initial time. If all fecal and total coliforms were composed of Salmonella spp. or bacteria with similar features, then X_DB = 1; therefore, a similar equation of $y_{s_t}$ could be obtained. The reduction percentage of coliforms can be determined with Equation (3):

$${\%}_{D_c}=\frac{MN{P}_t}{MP{N}_0}\times 100$$

(3)

Equation (4) is obtained by combining Equations (2) and (3):

$${\%}_{D_c}{}_t=\left(1-1\times y_{s_t}\times X_{DB}\right)\times 100$$

(4)

By equating the calculated percentage reduction of total coliforms (${\%}_{D_c}{}_t$) with the experimental value at the end of the process, the equation enabled the calculation of $X_{DB}$. Surprisingly, the plotting of these data revealed a square correlation factor of 0.9765, which provides strong evidence for the high efficiency of the Al₂O₃-TiO₂ coating. The obtained X_DB value of 0.355 indicates that only 35.5% of the coliforms are suitable for reduction, further confirming the effectiveness of the coating.

3. Materials and Methods

3.1. Coating Preparation Using the Sol–Gel Route

First, 1 M NaOH (Meyer ≥ 97%) solution was added dropwise to 50 mL of 0.1 M Al₂(SO₄)₃·12H₂O (Kemira Al-C 2-5, 90%) until pH = 10 was reached (to favor the formation of aluminum hydroxides and oxo-hydroxides). After adjusting the pH, the suspension was kept under stirring at 70 °C for 5 h. Thereafter, the formed colloidal suspension was washed three times with a mixture of ethanol/water 50:50 v/v to remove excess SO₄²⁻ ions. The wet solid was re-dispersed in 20 mL of ethanol; this suspension was aged for 24 h.

In the second step, 20 mL of HNO₃ solution, 0.3 M (Meyer, 65%), was added dropwise to 20 mL of a 15% v/v titanium butoxide Ti[OC(CH₃)₃]₄ (Sigma Aldrich, St. Louis, MI, USA, 97%) in ethanol (Sigma Aldrich, 99.9). The suspension was stirred for 2 h to achieve the completion of the hydrolysis–condensation reaction. Finally, the aluminum oxo-hydroxides obtained in the previous stage were added to the forming [TiO(OH)₂]_n gel and kept under vigorous stirring until a milky colloidal suspension appeared (Al/Ti ratio of 1.5).

The circular substrates made of AISI 304 SS, with a diameter of 5 cm and orifice diameter of 2 mm, were first washed with detergent soap and then treated with acetone followed by ethanol using sonication at 400 kHz for 20 min. After this, the substrates were stored for 1 h in isopropyl alcohol. The coating was obtained by immersing and withdrawing the substrates at a speed of 20 mm/min. After each deposition, the substrates were dried at 80 °C for 1 h; and this deposition process was repeated five times. The substrate with five depositions was thermally treated at 180 °C for 1 h and annealed at 700 °C for 5 h.

3.2. Structural Characterization Measurements

The structural study of the Al₂O₃-TiO₂ coating was carried out first, with a Bruker D8 Advance Diffractometer (Bruker, Karlsruhe, Germany) using Cu Kα radiation (1.54184 Å) to determine the crystallinity and crystal phase of the produced coating. X-ray diffraction patterns were recorded at room temperature in the 2θ angle interval ranging from 20° to 80° with a step of 0.02 s⁻¹. The analysis of the morphology and size of the particles was performed by employing a field emission scanning electron microscope (FE-SEM) Hitachi SU5000 (Hitachi High-Tech, Tokyo, Japan); for the EDS analysis, a coupled Bruker’s Quantax XFlash 6/60 was used to investigate the compositional aspects of the coating. To characterize the homogeneity of the deposits and texture quality of Al₂O₃-TiO₂, an atomic force microscope NanoScope II (Veeco/Digital Instruments, Santa Barbara, CA, USA) was employed, and the coating thickness was measured by means of a Dektak3 profilometer (Veeco Instruments Inc., Plainview, NY, USA) coupled to the NanoScope II. XRD, SEM, FE-SEM, and AFM characterizations of the Al₂O₃-TiO₂ coating on the 304 SS substrate were conducted after all the tests in this study to demonstrate its stability. The optical characteristics of the Al₂O₃-TiO₂ coating were examined with a Perkin Elmer model Lambda 35 UV-vis spectrophotometer (PerkinElmer, Waltham, MA, USA) from 200 to 900 nm wavelength to register absorbance values of the coating scratch. The bandgap energy was obtained using the Tauc Equation (5), which relates the absorption coefficient to the energy of incident radiation and the bandgap. The intersection of the linear part of the curve ${\left(\alpha ·E\right)}^r$ vs. $E$ with the x-axis represents ${\left(\alpha ·E\right)}^r=0$, and therefore, E = Eg [79]:

$${\left(\alpha ·E\right)}^r=Ai\left(E-E_g\right)$$

(5)

where $\alpha$ is the absorbance, arbitrary units; $E$ is photon energy, eV; $Ai$ is a proportionality constant, eV; $E_g$ is the bandgap, eV; and $r$ is a coefficient that classifies indirect allowed transitions ($r$ = 2), direct allowed transitions ($r$ = 1/2), direct forbidden transitions ($r$ = 3/2), and indirect forbidden transitions ($r$ = 3).

$$E=h\nu =\frac{hc}{\lambda }$$

(6)

The energy of incident radiation $E$ was calculated from the incident beam wavelengths using Equation (6) where h is Planck’s constant = 4.136 × 10⁻¹⁵ eV s; c is the speed of light in a vacuum = 2.998 × 10¹⁷ nm/s; and $\lambda$ is the wavelength in nm.

3.3. Photocatalytic Degradation

A semicontinuous flow photocatalytic reactor was designed to measure all the degradation reactions with total volume of 0.980 L (internal diameter of 5 cm and height of 50 cm), Figure 7. The six substrates with Al₂O₃-TiO₂ coating were placed in a transverse position, separated from each other by 5 cm long tubes guided by a 50 cm long bar central to the reactor; the bar and tubes were made of 304 SS. An additional substrate, measuring 6 cm in diameter, is positioned at the upper edge of the reactor, serving as a guide stop. The linear ascent of bubbles at an average speed of 2.666 cm/s (Re = 1655) generated the convection currents that kept the system homogenized to favor the adsorption of the contaminant on the photocatalyst surface. The average temperature was 30 ± 0.5 °C. Four Phillips LED lamps (4 W, 400 lumens, MR16 with emission in 410–760 nm range and a maximum peak at around 600 nm) were used for visible light irradiation.

3.3.1. MB Degradation

In the reactor, a 6 ppm MB solution was kept under stirring for 30 min in the absence of light, using upward airflow of 5 L/min to allow adsorption–desorption equilibrium between the Al₂O₃-TiO₂ coating surface and contaminant. Six substrates (46.09 cm² of coating area per substrate) were placed in the reactor. Subsequently, 5 mL of sample was obtained to measure the initial MB concentration, monitoring the absorption band at 668 nm, corresponding to the maximal absorption peak of MB. Four Phillips LED lamps were turned on to start the photocatalytic degradation after collecting the sample. A 2 mL volume of samples was collected every hour to measure the MB concentration. Four experiments were performed under the same conditions.

3.3.2. TOC Quantification

An aliquot of 1 mL was collected every hour from the reactor containing 0.980 L of residual water from a WWTP. The employed residual water had a pH, total solids, and hardness of 7.5 ± 0.3, 550 ± 048 mg/L, and 109.0 ± 0.12 mg/L, respectively. After collecting the first 1 mL sample, the six LED lamps were turned on. The aliquot was filtered to remove suspended solids, and its pH was adjusted to 2 using a 20 v/v% H₂SO₄ solution to eliminate microorganism reactions and hence exhibited an accurate TOC measurement; a 50 µL volume of each aliquot was injected into a TOC-V CSN (Shimadzu Brand, Kyoto, Japan) with an automatic ASI-V counter at room temperature (25 °C). A calibration curve from 0 to 500 mg/L of potassium biphthalate (C₈H₅KO₄) was used. The procedure was performed according to ISO 8245:1999 [80], which guides the TOC determination in wastewater, applicable for water samples with organic carbon content ranging from 0.3 mg/L to 1000 mg/L. The efficiency of the degradation of contaminants was calculated with Equation (7):

$$\% degradation=\frac{C_o-C}{C_o}\times 100$$

(7)

where $C_o$ is the initial concentration of the contaminant before turning on the lamps, and C is the concentration of the contaminant after time t.

3.3.3. Bactericidal Activity

The bactericidal activity of the Al₂O₃-TiO₂ coating was measured by fecal and total coliform counts according to standard methods for wastewater testing using the most probable number (MPN) method, based on the ability of this microbial group to ferment lactose with acid and gas production when incubated at 35 °C ± 1 °C for 48 h, using a culture medium containing bile salts.

An aliquot of 50 mL was collected every hour from the 0.98 L reactor to inoculate five tubes containing 10.0 mL of sodium lauryl sulfate broth with 10.0 mL of sample each; gas formation (displacement of the medium in a Durham bell) after 48 h of incubation at 35 ± 1 °C was considered a presumptive positive test. Confirmation of the presence of total and fecal coliforms was performed by reseeding two drops taken with a Pasteur pipette from each tube that tested positive in the presumptive test in brilliant green bile broth; the appearance of turbidity (microbial growth) and gas production in the Durham hood following the 48 h incubation period at 35 ± 1 °C were recorded as positive tests. The MPN/100 mL was calculated according to NOM-210-SSA1-2014 [81], appendix H, with the MPN Index, 95% confidence, for the combination of positive and negative results when using 5 to 10 tubes with 10 mL of water sample.

Additionally, the presence of Salmonella spp. was analyzed by determining the plate count (Colony Forming Units, CFU). A 0.5 mL volume of each positive tube from the confirmatory coliform test was spread over the entire surface of plates containing Salmonella-Shigella (SS) agar and incubated for 48 h at 35 °C.

4. Conclusions

Al₂O₃-TiO₂ coatings were successfully deposited on perforated 304 SS substrates using the sol–gel dip-coating technique. Thermal treatments at 180 °C for 1 h and at 700 °C for 5 h, respectively, promoted the TiO₂ anatase–rutile phases and Al₂O₃ α-γ phases. An Al₂O₃-TiO₂ coating monolayer on the 304 SS substrate was elucidated due to the similarity between the particle size distribution (250 ± 50 nm) and the coating thickness (250 ± 50 nm). The Al₂O₃-TiO₂ coating formed a heterojunction with chromium oxide on the 304 stainless steel surface due to the difference in their electronic properties, resulting in better response in the visible spectrum to degrade organic pollutants and obtain improved antibacterial activity. The study demonstrated that five perforated substrates with Al₂O₃-TiO₂ coatings and four 4 Watt lamps in a 0.980 L airlift reactor efficiently reduced MB and total organic contaminants in a WWTP sample by 97.3% with UV-vis absorbance and by 96.51% with TOC determination, respectively. Root mean square roughness of the perforated Al₂O₃-TiO₂ coating on the 304 SS substrate of 85 ± 5 nm enhanced the retention of Salmonella ssp., promoting longer contact between the cell membrane and the Al₂O₃-TiO₂ coating. At the same time, the airflow favored the formation of reactive oxygen species in the aqueous solution, leading to high degradation of organic pollutants and improvement of the antibacterial activity; in this study, a reduction of 94.23% of Salmonella spp. was achieved in 5 h. The low efficiency of Al₂O₃-TiO₂ against total and fecal coliforms is attributed to differences in their bacterial compositions. Mathematical analysis revealed that only 35.5% of total and fecal coliforms have mechanism of action similar to that of Salmonella ssp., which was supported by a square correlation factor of 0.9919 between the calculated and experimental reduction percentage of total and fecal coliforms. As a perspective, future research may involve the utilization of ascorbic acid (OH^• scavenger), 2-propanol (O₂^●− scavenger), and ammonium oxalate (AO) (hole h⁺ scavenger) to elucidate the photocatalytic activity of Al₂O₃-TiO₂ coating in organic contaminant degradation and to identify the oxygen reactive species generated with light exposure.