Innovative Photocatalytic Reactor for Sustainable Industrial Water Decontamination: Utilizing 3D-Printed Components and Silica-Titania Trilayer Coatings

: Industrial activities generate enormous quantities of polluted effluents, necessitating advanced methods of wastewater treatment to prevent potential environmental threats. Thus, the design of a novel photocatalytic reactor for industrial water decontamination, purification, and reuse is proposed as an efficient advanced oxidation technology. In this work, the development of the active reactor components is described, utilizing a two-step sol–gel technique to prepare a silica-titania trilayer


Introduction
The deterioration of water quality due to environmental pollution has become a significant concern [1,2], causing an urgent need for effective treatment [3][4][5][6], purification [7,8], and reclamation [9][10][11][12] of water and industrial wastewater.This has prompted the exploration of various innovative and sustainable methods for water treatment.Thus, to address these challenges and minimize pollution in both municipal and industrial effluents, strict regulations and technological advancements are being promoted.Stringent regulations and policies must be implemented to control the discharge of pollutants into water bodies, thereby alleviating the burden on water treatment systems.On the other hand, among the proposed solutions, Advanced Oxidation Technologies (AOTs) and Advanced Reduction Technologies (ARTs), as highlighted in the literature [13], offer promising avenues for combating water pollution.These technologies can be seamlessly integrated into existing infrastructures without the need for overly complex processes or the addition of harmful chemical reagents.Herein, photocatalysis, a process of utilizing light to accelerate a reaction, has emerged as a promising solution for various environmental, industrial, and energy applications.Titanium dioxide (TiO 2 ), also referred to as titania, is distinguished among photocatalysts due to its exceptional properties such as stability, non-toxicity, abundance, and cost-effectiveness [14][15][16].The photocatalytic activity of TiO 2 arises from its ability to generate electron-hole pairs when exposed to light irradiation, enabling them to participate in redox reactions with species adsorbed on its surface [14,16,17].
To date, laboratory-scale and pilot-scale reactors utilizing the photocatalytic properties of titania catalysts have exhibited considerable potential for future applications in wastewater management, as evidenced in the literature [18][19][20].Specifically, recent research has extensively explored the application of TiO 2 in both laboratory-scale and larger pilot-scale reactors [19,[21][22][23][24][25][26][27][28].Laboratory-scale reactors provide a controlled environment aiding in understanding the fundamental mechanisms of photocatalysis and optimizing parameters and catalyst properties.This also enables the exploration of innovative reactor designs with precise control over experimental parameters.Conversely, pilot-scale reactors serve to validate the scalability and feasibility of photocatalytic processes under more realistic conditions, including complex feed compositions, varying light intensities, and higher treated volumes.Developing effective pilot-scale reactors entails addressing challenges such as photon penetration depth due to the increase in light path length through the reactor [29], reactor geometry [30], limitations in mass transfer between reactants to the photocatalyst surface [25,31,32], stability of photocatalysts [33], economic feasibility [34], and strategies for immobilizing catalysts [35,36].Despite these challenges, the potential benefits of photocatalysis in addressing environmental and energy-related issues render it a promising technology for diverse environmental applications.
In line with these advancements, a novel approach involves the development of a 3D-printed photocatalytic filtration reactor (3D-PFR) tailored for wastewater purification and reuse, with a steel metal industry serving as a case study.The core innovation lies in the deposition of titanium dioxide photocatalysts onto 3D-printed flat polymeric filters.These bespoke structured photocatalysts are integrated into a continuous flow photocatalytic reactor, thus advancing a patented hybrid technology for water treatment that combines filtration and photocatalytic degradation processes.This versatile design enables cost-effective reactor manufacturing while facilitating parallel operational performance optimization and scalability.

Lattices Fabrication and Preparation of Trilayer Coatings
Initially, the polymeric flat filters in the form of lattices were fabricated utilizing Digital Light Processing (DLP) 3D-printing technology, employing the appropriate resins, software, and hardware equipment.The resulting 3D-printed lattices (Figure 1a) measured 21.4 mm in width, 4 mm in depth, and 21 mm in height, exhibiting a significant aspect ratio with volume and surface area values of 781.9 mm 3 and 6801 mm 2 , respectively.Additionally, scale-up rectangular lattices measuring 16.5 cm in width and height were produced, similarly boasting an enhanced aspect ratio.The deposition of the silica-titania trilayer coatings was carried out in three sequential steps using straightforward and facile sol-gel dip-coating techniques.
In this context, the deposition conditions of a silica-titania trilayer coating were precisely optimized and assessed for their efficacy in photocatalytic processes [37].In brief, initially, a silica (SiO 2 ) inert protective layer was applied to the membranes prior to the addition of TiO 2 .This step aimed to enhance robustness, prevent undesirable polymer degradation effects, and improve the catalyst's adhesion to the tiles (lattices).The photocatalytic activation process involved immersing the membranes in a TiO 2 precursor solution, followed by thermal treatment under ambient conditions [38].The final TiO 2 /P25 layer was formed on the modified lattices by dispersing the commercial Evonik Aeroxide TiO 2 P25 photocatalyst into the aforementioned titania precursor solution and repeating the dipping and annealing steps.maximum temperature limited by the thermal endurance of the polymeric lattice) was insufficient to convert the amorphous TiO2 coating into the photoactive, crystalline anatase phase.
Pristine, SiO2-, bilayer-, and trilayer-modified polymeric lattices underwent comprehensive characterization using microscopic and spectroscopic techniques to determine the morphological and structural characteristics of the photocatalyst.

Accelerated Aging Tests
The conduction of the accelerated aging tests encompassed chemical, irradiation, and thermal assessments.Specifically, regarding the chemical aging resistance of the 3Dprinted flat membranes, they underwent 24 h immersion in an acidic solution of ΗΝΟ3 More specifically, as aforementioned, initially, the silica layer was applied using the sol-gel dip-coating technique.The lattice was submerged in a SiO 2 precursor solution with precise control over both the dipping and withdrawal speeds employing a dip coater (Ossila Ltd., Sheffield, UK).Subsequently, the treated lattice was dried under mild conditions at room temperature and low humidity for a day.After several trials, the final optimized silica precursor solution comprised ethanol, tetraethyl orthosilicate (TEOS), water, and HCl, with a molar ratio of 1: 37:4:5.5 × 10 −2 [37].
The outer titania layer was also applied using the sol-gel dip-coating technique, with careful control over the dipping and withdrawal speeds.Following the preparation of the silica coating, the lattice was immersed in the TiO 2 precursor solution.The initial colloidal solution was prepared by dissolving titanium isopropoxide (TTIP) (15 mL) in ethanol (2.5 mL).Next, the peptizing agent was formulated by slowly adding 1 mL of perchloric acid (HClO 4 ) to 45 mL of ultrapure water (Milli-Q, 18 MΩ•cm), and was then gradually introduced into the mixed-metal alkoxide solution.The exothermic hydrolysis of TTIP produced an amorphous white precipitate.Following refluxing for 48 h at ~70 • C to promote crystallization, a stable primary sol was achieved [39].Once cooled down to room temperature (RT), the flat SiO 2 -coated lattices were immersed in the final solution to form the TiO 2 coating.Subsequently, after dip-coating, the samples were dried using a blow-dryer for approximately 30 s on each side from a distance of approximately 30 cm.
An additional layer with TiO 2 /P25 coating was prepared by dispersing the commercial TiO 2 photocatalyst Evonik Aeroxide P25 (1 g) into the aforementioned titania precursor solution (30 mL).Subsequently, the same dip-coating procedure was carried out (Figure 1b).Additionally, all the coated samples underwent post-annealing at 150 • C for 3 h.The final applied layer was implemented to ensure that the TiO 2 top layer would contain a significant proportion of the anatase phase, known for its enhanced photocatalytic activity.This was necessary because the post-annealing at 150 • C (the maximum temperature limited by the thermal endurance of the polymeric lattice) was insufficient to convert the amorphous TiO 2 coating into the photoactive, crystalline anatase phase.
Pristine, SiO 2 -, bilayer-, and trilayer-modified polymeric lattices underwent comprehensive characterization using microscopic and spectroscopic techniques to determine the morphological and structural characteristics of the photocatalyst.

Accelerated Aging Tests
The conduction of the accelerated aging tests encompassed chemical, irradiation, and thermal assessments.Specifically, regarding the chemical aging resistance of the 3D-printed flat membranes, they underwent 24 h immersion in an acidic solution of HNO 3 (pH ~5), followed by an equivalent duration in an alkaline solution of KOH (pH ~8).The irradiation tests comprised 24 h exposure to UV-A light, while the evaluation of the thermal endurance involved 3 h of soaking time at 150 • C.

Characterization Techniques
The structural characterization of the lattice and coatings was conducted using X-ray diffraction (XRD) analysis, employing a Siemens D-500 diffractometer (Siemens, Erlangen, Germany).This analysis utilized Cu Ka1 (λ = 1.5406Å) and Ka2 (λ = 1.5444Å) radiation.Additionally, vibrational Raman spectroscopy measurements were performed using a Renishaw inVia Reflex spectrometer (Renishaw, Wotton-under-Edge, UK) equipped with a solid-state laser emitting at 514.5 nm, along with a Leica DMLC microscope with a ×50 lens.The morphology of the lattice and coatings was examined using a FEI Quanta-Inspect scanning electron microscope (SEM) with a tungsten filament operating at 25 kV (FEI Company, Eindhoven, The Netherlands), coupled with an energy-dispersive X-ray spectrometer (EDS).The surface hydrophilicity of the lattices was evaluated using a contact angle meter (CAM 100, KSV Instruments Ltd., Helsinki, Finland).For static contact angle measurements, deionized water (5 µL) was used for the sessile drop type, and the affinity of the drop with the surface was assessed using a software fitting method.Ten measurements were collected from different spots on the lattice at room temperature (20 • C), and the mean contact angle and standard deviation were calculated.A TGA/DTA Thermogravimetric/Differential Thermal Analyzer (Setsys Evolution 18, Setaram Instrumentation, Caluire, France) and differential scanning calorimetry (DSC) were utilized to examine the thermal characteristics of the polymer lattice.

Photocatalytic Performance
Preliminary experiments involved comparing different nanoparticulate TiO 2 powders with commercial Evonic P25 to assess the performance of the synthesized powders.Subsequently, the photocatalytic efficiency of trilayer coatings on lattices was evaluated by studying the photodegradation of the organic non-ionic surfactant Triton X-100 (Scheme 1) at various concentrations.
(pH ~5), followed by an equivalent duration in an alkaline solution of ΚΟΗ (pH ~8).The irradiation tests comprised 24 h exposure to UV-A light, while the evaluation of the thermal endurance involved 3 h of soaking time at 150 °C.

Characterization Techniques
The structural characterization of the lattice and coatings was conducted using X-ray diffraction (XRD) analysis, employing a Siemens D-500 diffractometer (Siemens, Erlangen, Germany).This analysis utilized Cu Ka1 (λ = 1.5406Å) and Ka2 (λ = 1.5444Å) radiation.Additionally, vibrational Raman spectroscopy measurements were performed using a Renishaw inVia Reflex spectrometer (Renishaw, Wotton-under-Edge, UK) equipped with a solid-state laser emitting at 514.5 nm, along with a Leica DMLC microscope with a ×50 lens.The morphology of the lattice and coatings was examined using a FEI Quanta-Inspect scanning electron microscope (SEM) with a tungsten filament operating at 25 kV (FEI Company, Eindhoven, The Netherlands), coupled with an energy-dispersive X-ray spectrometer (EDS).The surface hydrophilicity of the lattices was evaluated using a contact angle meter (CAM 100, KSV Instruments Ltd., Helsinki, Finland).For static contact angle measurements, deionized water (5 μL) was used for the sessile drop type, and the affinity of the drop with the surface was assessed using a software fitting method.Ten measurements were collected from different spots on the lattice at room temperature (20 °C), and the mean contact angle and standard deviation were calculated.A TGA/DTA Thermogravimetric/Differential Thermal Analyzer (Setsys Evolution 18, Setaram Instrumentation, Caluire, France) and differential scanning calorimetry (DSC) were utilized to examine the thermal characteristics of the polymer lattice.

Photocatalytic Performance
Preliminary experiments involved comparing different nanoparticulate TiO2 powders with commercial Evonic P25 to assess the performance of the synthesized powders.Subsequently, the photocatalytic efficiency of trilayer coatings on lattices was evaluated by studying the photodegradation of the organic non-ionic surfactant Triton X-100 (Scheme 1) at various concentrations.Scheme 1.The molecular structure of Triton X-100 [Formula: C14H22O(C2H4O)n (n = 9-10)].
These lattices were placed into specially designed custom-made cells, and the photocatalytic experiments were conducted in a rectangular parallelepiped black box photoreactor (50 cm × 40 cm × 30 cm) under UV-A illumination, as previously described in detail [40].The cells were positioned 15 cm away from the illumination system.The reactor was equipped with four Sylvania GTE F15W/T8UV-A lamps, emitting light in the 350−390 nm range with an intensity of 0.5 mW•cm −2 , arranged in a parallel symmetric configuration atop the reactor.Photocatalytic kinetics were monitored by measuring the characteristic absorption peak of Triton X-100 at 224 nm using a Hitachi 3010 UV-Vis spectrophotometer), equipped with an integrating sphere and BaSO4 as the reference (Hitachi Ltd., Tokyo, Japan).Measurements were taken every 30 min over a total duration of 90 min.Before illumination, the solutions were allowed to equilibrate in the dark for 30 min to ensure adsorption-desorption equilibrium of the surfactant onto the photocatalyst.The photodegradation efficiency (%) was subsequently determined by the following equation: These lattices were placed into specially designed custom-made cells, and the photocatalytic experiments were conducted in a rectangular parallelepiped black box photoreactor (50 cm × 40 cm × 30 cm) under UV-A illumination, as previously described in detail [40].The cells were positioned 15 cm away from the illumination system.The reactor was equipped with four Sylvania GTE F15W/T8UV-A lamps, emitting light in the 350−390 nm range with an intensity of 0.5 mW•cm −2 , arranged in a parallel symmetric configuration atop the reactor.Photocatalytic kinetics were monitored by measuring the characteristic absorption peak of Triton X-100 at 224 nm using a Hitachi 3010 UV-Vis spectrophotometer), equipped with an integrating sphere and BaSO 4 as the reference (Hitachi Ltd., Tokyo, Japan).Measurements were taken every 30 min over a total duration of 90 min.Before illumination, the solutions were allowed to equilibrate in the dark for 30 min to ensure adsorptiondesorption equilibrium of the surfactant onto the photocatalyst.The photodegradation efficiency (%) was subsequently determined by the following equation: where C 0 is the pollutant concentration after the adsorption equilibrium (mg/L) and C represents the concentration at any time during the experiment (mg/L).
Similarly, to determine the extent of mineralization during the photocatalytic degradation process, total organic carbon (TOC) measurements were performed employing a TOC analyzer (BioTector B3500, Hach, Loveland, CO, USA), and the carbon removal efficiency (%) was calculated using the following equation: where TOC 0 is the pollutant's organic carbon concentration after the adsorption equilibrium (mg/L) and TOC denotes the organic carbon concentration at any time during the experiment (mg/L).

Accelerated Aging Results
As described in Section 2.2, the lattices underwent physical testing, being subjected to unusually high levels of chemical (Figure 2a), irradiation (Figure 2b), and thermal stress to accelerate natural aging effects.As depicted in Figure 2c, it was determined that neither chemical stress (at pH 5 and pH 8) nor irradiation (UVA 365 nm) stress significantly altered the morphology and chemistry of the polymeric lattices.However, a slight color change observed under UV irradiation prompted us to decide on depositing a protective SiO 2 layer beneath the photocatalytic TiO 2 layer.
Environments 2024, 11, x FOR PEER REVIEW 5 of 14 where C0 is the pollutant concentration after the adsorption equilibrium (mg/L) and C represents the concentration at any time during the experiment (mg/L).Similarly, to determine the extent of mineralization during the photocatalytic degradation process, total organic carbon (TOC) measurements were performed employing a TOC analyzer (BioTector B3500, Hach, Loveland, CO, USA), and the carbon removal efficiency (%) was calculated using the following equation: where TOC0 is the pollutant's organic carbon concentration after the adsorption equilibrium (mg/L) and TOC denotes the organic carbon concentration at any time during the experiment (mg/L).

Accelerated Aging Results
As described in Section 2.2, the lattices underwent physical testing, being subjected to unusually high levels of chemical (Figure 2a), irradiation (Figure 2b), and thermal stress to accelerate natural aging effects.As depicted in Figure 2c, it was determined that neither chemical stress (at pH 5 and pH 8) nor irradiation (UVA 365 nm) stress significantly altered the morphology and chemistry of the polymeric lattices.However, a slight color change observed under UV irradiation prompted us to decide on depositing a protective SiO2 layer beneath the photocatalytic TiO2 layer.Following thermal stress (annealing for 3 h at 150 °C), the samples' color changed from light blue to light yellow (Figure 2d).The Raman spectrum, as shown in Figure 3a, remained unchanged, indicating that the substrate was fabricated from Bisphenol A diglycidyl ether (DGEBA) epoxy resin, despite the emergence of a luminescent background.Moreover, exothermic events revealed in the TGA curve and DSC thermogram (Figure 3b) of the developed lattices show that the onset of thermal Following thermal stress (annealing for 3 h at 150 • C), the samples' color changed from light blue to light yellow (Figure 2d).The Raman spectrum, as shown in Figure 3a, remained unchanged, indicating that the substrate was fabricated from Bisphenol A diglycidyl ether (DGEBA) epoxy resin, despite the emergence of a luminescent background.Moreover, exothermic events revealed in the TGA curve and DSC thermogram (Figure 3b) of the developed lattices show that the onset of thermal decomposition of the DGEBA epoxy resin might start above 120 • C, potentially impacting the polymer.Furthermore, SEM was employed to observe the morphological features of lattices both before and after the stress tests.Consequently, minor alterations on polymer's surface were noticed following immersion in solutions of varying p However, it cannot be definitively stated that the material remains unaffected af undergoing the thermal stress test.Specifically, although the initially prepared 3D-prin lattices exhibited a highly uniform and smooth structure (with any random gra possibly arising from the Au sputtering), localized cracks emerged after 3 h of anneal at 150 °C (Figure 4).Furthermore, SEM was employed to observe the morphological features of the lattices both before and after the stress tests.Consequently, minor alterations on the polymer's surface were noticed following immersion in solutions of varying pH.However, it cannot be definitively stated that the material remains unaffected after undergoing the thermal stress test.Specifically, although the initially prepared 3D-printed lattices exhibited a highly uniform and smooth structure (with any random grains possibly arising from the Au sputtering), localized cracks emerged after 3 h of annealing at 150 • C (Figure 4).
decomposition of the DGEBA epoxy resin might start above 120 °C, potentially impacting the polymer.Furthermore, SEM was employed to observe the morphological features of the lattices both before and after the stress tests.Consequently, minor alterations on the polymer's surface were noticed following immersion in solutions of varying pH.However, it cannot be definitively stated that the material remains unaffected after undergoing the thermal stress test.Specifically, although the initially prepared 3D-printed lattices exhibited a highly uniform and smooth structure (with any random grains possibly arising from the Au sputtering), localized cracks emerged after 3 h of annealing at 150 °C (Figure 4).The general remarks from the stress tests conducted on the polymeric lattices, which serve as substrates for the development of the photocatalytic coatings, indicate that there are no discernible alterations following exposure to acidic and alkaline environments or UV irradiation.However, thermal stress indeed affected the membrane morphology.Consequently, thermal post-treatment under mild conditions may be considered subsequent to the deposition of SiO 2 and TiO 2 films.

Morphological Study of the Trilayer Coated Lattices
As aforementioned in Section 2.1 (Lattices fabrication and preparation of trilayer coatings), adjusting and optimizing the TEOS-to-HCl ratio resulted in a smooth and defectfree coating on the lattice surface with the elimination of any crack formation (Figure 5a).In addition, as indicated in the third column of Figure 5a, the quality of the coating was not affected by the duration of solution curing.
are no discernible alterations following exposure to acidic and alkaline environments or UV irradiation.However, thermal stress indeed affected the membrane morphology.Consequently, thermal post-treatment under mild conditions may be considered subsequent to the deposition of SiO2 and TiO2 films.

Morphological Study of the Trilayer Coated Lattices
As aforementioned in Section 2.1 (Lattices fabrication and preparation of trilayer coatings), adjusting and optimizing the TEOS-to-HCl ratio resulted in a smooth and defect-free coating on the lattice surface with the elimination of any crack formation (Figure 5a).In addition, as indicated in the third column of Figure 5a, the quality of the coating was not affected by the duration of solution curing.
Additionally, the SEM micrographs in Figure 5b indicated that the morphology of the SiO2 coating remained unchanged after post-annealing at 150 °C, smoothly covering the surface without inducing additional cracks.It should also be noted that despite the significant cracks in the polymer after annealing (Figure 5b, middle column), the SiO2 coating remained intact.This suggests that the SiO2 coating acts as a mending agent for cracks developed in the polymeric lattice under thermal stress at 150 °C.Subsequently, a comprehensive examination of the layer-by-layer coating morphology using SEM (Figure 6) confirmed the presence of a 3D honeycomb-like structure of the flat filters.Practically, the resolution capabilities of the 3D printer allow for the construction of the desired filter by printing micro-sized "bricks" (with dimensions of 16 μm × 16 μm × 34 μm) as the pattern for building blocks.As mentioned before, the subsequent application of the SiO2 layer resulted in a very uniform, homogenous, and smooth coverage, maintaining the initial surface roughness to a significant extent.Despite some minor imperfections that appeared on the surface, the high quality of the SiO2 layer ensures the efficient insulation of the polymeric substrate and improved adhesion of the top TiO2 coating.In addition, as can be observed, the substrate is completely covered by the thicker titania coating.The cracks in the TiO2 did not compromise the stability of the polymer, due to the insulation provided by SiO2.Upon adding Evonik P25 into the titania precursor solution, the coating morphology remains unchanged, except for the addition of white large grains incorporated onto the external surface.The P25 powder ultimately resulted in random grains ranging in size from 160 to 700 nm.Additionally, the SEM micrographs in Figure 5b indicated that the morphology of the SiO 2 coating remained unchanged after post-annealing at 150 • C, smoothly covering the surface without inducing additional cracks.It should also be noted that despite the significant cracks in the polymer after annealing (Figure 5b, middle column), the SiO 2 coating remained intact.This suggests that the SiO 2 coating acts as a mending agent for cracks developed in the polymeric lattice under thermal stress at 150 • C.
Subsequently, a comprehensive examination of the layer-by-layer coating morphology using SEM (Figure 6) confirmed the presence of a 3D honeycomb-like structure of the flat filters.Practically, the resolution capabilities of the 3D printer allow for the construction of the desired filter by printing micro-sized "bricks" (with dimensions of 16 µm × 16 µm × 34 µm) as the pattern for building blocks.As mentioned before, the subsequent application of the SiO 2 layer resulted in a very uniform, homogenous, and smooth coverage, maintaining the initial surface roughness to a significant extent.Despite some minor imperfections that appeared on the surface, the high quality of the SiO 2 layer ensures the efficient insulation of the polymeric substrate and improved adhesion of the top TiO 2 coating.In addition, as can be observed, the substrate is completely covered by the thicker titania coating.The cracks in the TiO 2 did not compromise the stability of the polymer, due to the insulation provided by SiO 2 .Upon adding Evonik P25 into the titania precursor solution, the coating morphology remains unchanged, except for the addition of white large grains incorporated onto the external surface.The P25 powder ultimately resulted in random grains ranging in size from 160 to 700 nm.The energy-dispersive X-ray spectroscopy (EDS) analysis indicates that upon application of the SiO2 coating, a pronounced peak from elemental Si was detected, as observed in Figure 7 (left).The addition of TiO2 leads to overlap with the SiO2 layer, as Ti coexists with the reduced Si peak (Ti signal low in Figure 7-middle).Following the application of the P25 coating, elemental Ti emerges as the predominant element on the lattice surface, as clearly depicted in Figure 6 (right).

XRD Analysis
The XRD analysis of the lattices indicated that the polymeric matrix solely exhibited a broad diffraction peak at approximately 20° (Figure 8), a common characteristic observed in many polymeric materials.The absence of any SiO2 or TiO2 peaks suggests their uniform and high dispersion of coatings on the membrane surface.Another remark is that the XRD pattern did not exhibit any Bragg peaks associated with crystalline TiO2, likely due to the low TiO2 content, which could not be detected on the polymeric substrate.This outcome is favorable, indicating the formation of an ultra-thin TiO2 film.An ultrathin film is advantageous as it maintains the high surface area of the photocatalytic filter and helps prevent an excessive pressure drop across its length.The energy-dispersive X-ray spectroscopy (EDS) analysis indicates that upon application of the SiO 2 coating, a pronounced peak from elemental Si was detected, as observed in Figure 7 (left).The addition of TiO 2 leads to overlap with the SiO 2 layer, as Ti coexists with the reduced Si peak (Ti signal low in Figure 7-middle).Following the application of the P25 coating, elemental Ti emerges as the predominant element on the lattice surface, as clearly depicted in Figure 7 (right).The energy-dispersive X-ray spectroscopy (EDS) analysis indicates that upon application of the SiO2 coating, a pronounced peak from elemental Si was detected, as observed in Figure 7 (left).The addition of TiO2 leads to overlap with the SiO2 layer, as Ti coexists with the reduced Si peak (Ti signal low in Figure 7-middle).Following the application of the P25 coating, elemental Ti emerges as the predominant element on the lattice surface, as clearly depicted in Figure 6 (right).

XRD Analysis
The XRD analysis of the lattices indicated that the polymeric matrix solely exhibited a broad diffraction peak at approximately 20° (Figure 8), a common characteristic observed in many polymeric materials.The absence of any SiO2 or TiO2 peaks suggests their uniform and high dispersion of coatings on the membrane surface.Another remark is that the XRD pattern did not exhibit any Bragg peaks associated with crystalline TiO2, likely due to the low TiO2 content, which could not be detected on the polymeric substrate.This outcome is favorable, indicating the formation of an ultra-thin TiO2 film.An ultrathin film is advantageous as it maintains the high surface area of the photocatalytic filter and helps prevent an excessive pressure drop across its length.

XRD Analysis
The XRD analysis of the lattices indicated that the polymeric matrix solely exhibited a broad diffraction peak at approximately 20 • (Figure 8), a common characteristic observed in many polymeric materials.The absence of any SiO 2 or TiO 2 peaks suggests their uniform and high dispersion of coatings on the membrane surface.Another remark is that the XRD pattern did not exhibit any Bragg peaks associated with crystalline TiO 2 , likely due to the low TiO 2 content, which could not be detected on the polymeric substrate.This outcome is favorable, indicating the formation of an ultra-thin TiO 2 film.An ultra-thin film is advantageous as it maintains the high surface area of the photocatalytic filter and helps prevent an excessive pressure drop across its length.

Raman Analysis
Subsequent examination of the vibrational properties via Raman spectroscopy revealed that the flat filters contained a Bisphenol-based epoxy resin (marked with an asterisk in Figure 9a), a commonly and widely used material in DLP 3D printing.However, additional modes were observed originating from additional monomers and/or the hardener present in the commercial resin solution [41,42].Moreover, the SiO2 gel obtained from the dried SiO2 precursor solution exhibited the characteristic spectrum of the amorphous phase [43].
Consistent with the XRD findings, the deposited silica and titania modes were still undetectable (SiO2 is in an amorphous phase).However, it should be mentioned that these spectra showed a less intense signal compared to the reference (uncoated) lattice, suggesting that the presence of the additional SiO2/TiO2 layers on the lattice surface decreases the intensity of the scattered laser beam.In contrast, the Raman spectra of the TiO2/P25 modified lattice exhibited a markedly different pattern, as depicted in Figure 9b; the anatase crystal phase of titania predominated [44], attributable to the high concentration of the P25 photocatalyst, while traces of the most intense epoxy resin modes remained detectable.

Raman Analysis
Subsequent examination of the vibrational properties via Raman spectroscopy revealed that the flat filters contained a Bisphenol-based epoxy resin (marked with an asterisk in Figure 9a), a commonly and widely used material in DLP 3D printing.However, additional modes were observed originating from additional monomers and/or the hardener present in the commercial resin solution [41,42].Moreover, the SiO 2 gel obtained from the dried SiO 2 precursor solution exhibited the characteristic spectrum of the amorphous phase [43].

Raman Analysis
Subsequent examination of the vibrational properties via Raman spectroscopy revealed that the flat filters contained a Bisphenol-based epoxy resin (marked with an asterisk in Figure 9a), a commonly and widely used material in DLP 3D printing.However, additional modes were observed originating from additional monomers and/or the hardener present in the commercial resin solution [41,42].Moreover, the SiO2 gel obtained from the dried SiO2 precursor solution exhibited the characteristic spectrum of the amorphous phase [43].
Consistent with the XRD findings, the deposited silica and titania modes were still undetectable (SiO2 is in an amorphous phase).However, it should be mentioned that these spectra showed a less intense signal compared to the reference (uncoated) lattice, suggesting that the presence of the additional SiO2/TiO2 layers on the lattice surface decreases the intensity of the scattered laser beam.In contrast, the Raman spectra of the TiO2/P25 modified lattice exhibited a markedly different pattern, as depicted in Figure 9b; the anatase crystal phase of titania predominated [44], attributable to the high concentration of the commercial P25 photocatalyst, while traces of the most intense epoxy resin modes remained detectable.Consistent with the XRD findings, the deposited silica and titania modes were still undetectable (SiO 2 is in an amorphous phase).However, it should be mentioned that these spectra showed a less intense signal compared to the reference (uncoated) lattice, suggesting that the presence of the additional SiO 2 /TiO 2 layers on the lattice surface decreases the intensity of the scattered laser beam.In contrast, the Raman spectra of the TiO 2 /P25 modified lattice exhibited a markedly different pattern, as depicted in Figure 9b; the anatase crystal phase of titania predominated [44], attributable to the high concentration of the commercial P25 photocatalyst, while traces of the most intense epoxy resin modes remained detectable.

Photocatalytic Performance
As detailed in Section 2.4, photocatalytic degradation tests were conducted against the Triton X-100 surfactant, a common chemical found in polluted wastewater from the steel industry.These tests were performed under UV light irradiation, and the degradation of the pollutant was determined by measuring its characteristic absorption peak at 224 nm.Initially, the UV photolysis of Triton X-100 was conducted using UV-A irradiation without a photocatalyst.After 1.5 h of UV exposure, the degradation of the surfactant was negligible.Subsequently, preliminary experiments employing different nanoparticulate TiO 2 powders under UV-A irradiation for 30 min revealed that the photodegradation of Triton X-100 is more effective with the commercial P25 catalyst, resulting in approximately 60% pollutant removal (Figure 10a).Consequently, it was integrated into an additional final coating on the lattice substrate.At this point, it could be pointed out that Triton X-100 exhibited negligible adsorption onto the surface of the photocatalysts, indicating that the removal effects were solely due to the photocatalytic process.

Photocatalytic Performance
As detailed in Section 2.4, photocatalytic degradation tests were conducted against the Triton X-100 surfactant, a common chemical found in polluted wastewater from the steel industry.These tests were performed under UV light irradiation, and the degradation of the pollutant was determined by measuring its characteristic absorption peak at 224 nm.Initially, the UV photolysis of Triton X-100 was conducted using UV-A irradiation without a photocatalyst.After 1.5 h of UV exposure, the degradation of the surfactant was negligible.Subsequently, preliminary experiments employing different nanoparticulate TiO2 powders under UV-A irradiation for 30 min revealed that the photodegradation of Triton X-100 is more effective with the commercial P25 catalyst, resulting in approximately 60% pollutant removal (Figure 10a).Consequently, it was integrated into an additional final coating on the lattice substrate.At this point, it could be pointed out that Triton X-100 exhibited negligible adsorption onto the surface of the photocatalysts, indicating that the removal effects were solely due to the photocatalytic process.Subsequently, photocatalytic experiments were conducted employing trilayer-coated lattice substrates at a concentration of 64 mg/L under UV-A irradiation for 90 min.As presented in Figure 10b, Triton X-100 can be effectively removed from the solution, as indicated by the decrease in absorbance peak at 224 nm.The results obtained (Figure 10c) demonstrate that the presence of TiO 2 /P25 deposited onto the lattice leads to the efficient photocatalytic removal of Triton X-100 pollutant, achieving up to 70% removal.
Simultaneously, the evolution of pH during the photocatalytic experiment was monitored, observing a decrease from 6.5 to 3.3 over time.This pH reduction during the photocatalytic degradation of Triton X-100 by the photocatalyst can be attributed to the generation of acidic species throughout the process.When the photocatalyst is irradiated with UV light, it generates electron-hole pairs (e − /h + ), which react with water and oxygen in the aqueous solution to produce reactive oxygen species such as hydroxyl radicals (•OH), superoxide anions (•O 2 − ), and hydrogen peroxide (H 2 O 2 ) [17].Triton X-100, a non-ionic surfactant, undergoes oxidation by these reactive oxygen species, resulting in the formation of intermediate products, including various organic acids (e.g., acetic acid, formic acid).These organic acids dissociate in water, releasing hydrogen ions (H + ), which contribute to the observed pH decrease (R-COOH → R-COO − + H + ).As the photocatalytic degradation proceeds, additional organic acids are generated and undergo dissociation, leading to an accumulation of H + ions in the solution.The continuous production and release of H + ions from these acids contribute to a gradual decline in the pH of the solution over time.Additionally, the mineralization of Triton X-100 involves the progressive oxidation of the organic pollutant into CO 2 .The dissolution of CO 2 in water results in the formation of carbonic acid (H 2 CO 3 ), which subsequently dissociates into bicarbonate (HCO 3 − ) and hydrogen ions (H + ) (CO 2 + H 2 O → H 2 CO 3 → HCO 3 − + H + ).This reaction introduces additional H + ions into the solution, thereby further decreasing the pH.The combined effect of the formation of acidic intermediates and the CO 2 dissolution leads to a significant decrease in pH during the photocatalytic process.
Moreover, as exhibited in Figure 10d, the TiO 2 /P25 film coating on the lattice substrate efficiently decomposed Triton X-100, irrespective of the initial concentration of the contaminant.It is worth mentioning that the concentration range investigated corresponds to actual levels of Triton X-100 found in the effluents from the steel industry.These results suggest that the immobilized quantity of TiO 2 /P25 on the membrane lattices should ideally be at a ratio of 1 g of catalyst per 1 L of wastewater effluent.
Furthermore, the total organic content (TOC) content during photocatalytic experiments was investigated and the results are presented in Figure 11a.As observed, the mineralization of Triton X-100 surfactant exhibits an almost linear relationship with time.Specifically, after 90 min of UV-A irradiation, the carbon content percentage decreases to 58%, indicating that 42% of Triton X-100 has been mineralized.However, the degradation of Triton X-100 was considerably lower, likely due to the formation of intermediate organic products during the process.These intermediates could be more resistant to further breakdown, thereby slowing down the overall mineralization process.While the initial breakdown may occur relatively quickly, the complete oxidation of intermediates to achieve mineralization is slower.Thus, the observed delay in the mineralization process relative to the degree of photocatalytic degradation is expected.The recalcitrant parent compound must first be converted into intermediate fragments, which then progressively transform into CO 2 and inorganic species.This eventually leads to the almost complete removal of the organic load (85%) after 5 h (Figure 11a).
Regarding the recycling study, the photocatalytic activity of the modified lattices was evaluated after they were regenerated by recovering, rinsing with ultrapure water, and drying.Five photocatalysis/regeneration cycles were conducted, and the results are illustrated in Figure 11b.It is apparent that the photocatalytic-modified lattices maintained their high activity, exhibiting only a slight performance decrease of approximately 5.6%.Regarding the recycling study, the photocatalytic activity of the modified lattices was evaluated after they were regenerated by recovering, rinsing with ultrapure water, and drying.Five photocatalysis/regeneration cycles were conducted, and the results are illustrated in Figure 11b.It is apparent that the photocatalytic-modified lattices maintained their high activity, exhibiting only a slight performance decrease of approximately 5.6%.
Overall, the outcomes showcase the advantages and innovation of the proposed 3Dprinted photocatalytic reactor technology for water purification.Further experiments are underway to optimize photocatalytic reaction conditions, including the presence of other organic contaminants [45].

Conclusions
In this study, flat filters in the form of lattices were fabricated using the 3D printing technique, demonstrating impressive surface area and volume characteristics, suitable for integration into innovative photocatalytic reactors.Through wet chemistry methods, highly uniform, homogeneous, and smooth SiO2 and TiO2 layers were prepared without any imperfections.Initially, a silica layer was deposited to insulate and protect the polymer lattice, followed by the application of an outer layer of the titania photocatalyst.Finally, the SiO2/TiO2 trilayer displayed exceptional homogeneity and smoothness, covering the entire surface seamlessly.
Both the filters and the trilayer-coated photocatalytic lattices were extensively characterized employing SEM, XRD, and Raman spectroscopy.The TiO2 photocatalytic activated lattices effectively degraded the Triton X-100 surfactant, a common water pollutant in the steel metal industry, reaching up to 70% removal, regardless of the initial concentration of the contaminant.The concentration range examined aligns with the actual levels of Triton X-100 found in effluents from the steel industry.Moreover, 85% of the organic load was eliminated after 5 h of UV-A irradiation.Overall, the photocatalyticmodified lattices sustained their high activity, exhibiting only a minor performance decrease of approximately 5.6% after five subsequent photocatalysis/regeneration cycles.These findings underscore the advantages of the proposed 3D-printed photocatalytic filtration reactor technology for water purification applications.
Author Contributions: conceptualization, D.I. and G.E.R.; methodology, G.V.T. and M.K.A.; validation, G.V.T. and M.K.A.; formal analysis, G.V.T. and M.K.A.; investigation, G.V.T. and M.K.A.; resources, G.E.R. and P.F.; writing-original draft preparation, G.V.T.; writing-review and editing, Overall, the outcomes showcase the advantages and innovation of the proposed 3Dprinted photocatalytic reactor technology for water purification.Further experiments are underway to optimize photocatalytic reaction conditions, including the presence of other organic contaminants [45].

Conclusions
In this study, flat filters in the form of lattices were fabricated using the 3D printing technique, demonstrating impressive surface area and volume characteristics, suitable for integration into innovative photocatalytic reactors.Through wet chemistry methods, highly uniform, homogeneous, and smooth SiO 2 and TiO 2 layers were prepared without any imperfections.Initially, a silica layer was deposited to insulate and protect the polymer lattice, followed by the application of an outer layer of the titania photocatalyst.Finally, the SiO 2 /TiO 2 trilayer displayed exceptional homogeneity and smoothness, covering the entire surface seamlessly.
Both the filters and the trilayer-coated photocatalytic lattices were extensively characterized employing SEM, XRD, and Raman spectroscopy.The TiO 2 photocatalytic activated lattices effectively degraded the Triton X-100 surfactant, a common water pollutant in the steel metal industry, reaching up to 70% removal, regardless of the initial concentration of the contaminant.The concentration range examined aligns with the actual levels of Triton X-100 found in effluents from the steel industry.Moreover, 85% of the organic load was eliminated after 5 h of UV-A irradiation.Overall, the photocatalytic-modified lattices sustained their high activity, exhibiting only a minor performance decrease of approximately 5.6% after five subsequent photocatalysis/regeneration cycles.These findings underscore the advantages of the proposed 3D-printed photocatalytic filtration reactor technology for water purification applications.

Figure 1 .
Figure 1.(a) 3D-printed flat filter of high surface area; (b) deposition of the final TiO2/P25 coating onto the filter.

Figure 1 .
Figure 1.(a) 3D-printed flat filter of high surface area; (b) deposition of the final TiO 2 /P25 coating onto the filter.

Figure 2 .
Figure 2. (a) Chemical and (b) irradiation stress testing in progress; (c) appearance of the small-scale lattices after chemical and irradiation stress testing; (d) appearance of the lattice after thermal annealing.

Figure 2 .
Figure 2. (a) Chemical and (b) irradiation stress testing in progress; (c) appearance of the small-scale lattices after chemical and irradiation stress testing; (d) appearance of the lattice after thermal annealing.
ironments 2024, 11, x FOR PEER REVIEW 6 o decomposition of the DGEBA epoxy resin might start above 120 °C, potentially impact the polymer.

Figure 3 .
Figure 3. (a) Comparison of the Raman spectra of the developed lattice and that after therm annealing at 150 °C; (b) TGA curve of polymer lattice; in inset, the DSC thermogram of polym lattice is depicted.

Figure 4 .
Figure 4. SEM images obtained at various magnifications for the untreated and thermally annea lattice at 150 °C.

Figure 3 .
Figure 3. (a) Comparison of the Raman spectra of the developed lattice and that after thermal annealing at 150 • C; (b) TGA curve of polymer lattice; in inset, the DSC thermogram of polymer lattice is depicted.

Figure 3 .
Figure 3. (a) Comparison of the Raman spectra of the developed lattice and that after thermal annealing at 150 °C; (b) TGA curve of polymer lattice; in inset, the DSC thermogram of polymer lattice is depicted.

Figure 4 .
Figure 4. SEM images obtained at various magnifications for the untreated and thermally annealed lattice at 150 °C.

Figure 4 .
Figure 4. SEM images obtained at various magnifications for the untreated and thermally annealed lattice at 150 • C.

Figure 5 .
Figure 5. (a) The morphology of the SiO2 layer on the lattice surface, prepared using the dip-coating technique, after one-and seven-days of solution curing; (b) SEM images of the untreated and heattreated lattice and the heat-treated SiO2-coated lattice.

Figure 5 .
Figure 5. (a) The morphology of the SiO 2 layer on the lattice surface, prepared using the dip-coating technique, after one-and seven-days of solution curing; (b) SEM images of the untreated and heat-treated lattice and the heat-treated SiO 2 -coated lattice.

Environments 2024 ,
11,  x FOR PEER REVIEW 8 of 14 of white large grains incorporated onto the external surface.The P25 powder ultimately resulted in random grains ranging in size from 160 to 700 nm.

Figure 8 .
Figure 8. XRD pattern of the modified lattice.

Figure 8 .
Figure 8. XRD pattern of the modified lattice.

Figure 8 .
Figure 8. XRD pattern of the modified lattice.

Figure 9 .
Figure 9. (a) Raman spectra of the developed lattice before and after SiO 2 deposition.In addition, the Raman spectrum of the SiO 2 powder is depicted, obtained after drying the precursor solution utilized for SiO 2 deposition; (b) Raman spectra of the developed lattice before and after the application of TiO 2 and TiO 2 /P25 layer coatings onto the lattice.

s 14 Figure 9 .
Figure9.(a) Raman spectra of the developed lattice before and after SiO2 deposition.In addition, the Raman spectrum of the SiO2 powder is depicted, obtained after drying the precursor solution utilized for SiO2 deposition; (b) Raman spectra of the developed lattice before and after the application of TiO2 and TiO2/P25 layer coatings onto the lattice.

Figure 10 .
Figure 10.(a) Removal of Triton X-100 at a concentration of 13 mg/L using nanoparticulate TiO2 powders (catalyst amount: 1 g/L) with UV-A exposure for 30 min (natural pH); (b) evolution of the Triton X-100 UV absorbance at a concentration of 64 mg/L in the supernatant aliquot; (c) study of the kinetics of Triton X-100 (64 mg/L) photocatalytic degradation with TiO2/P25 thin film on the lattice substrate (natural pH); (d) photocatalytic performance of trilayer-coated lattice as a function of Triton X-100 concentration under UV-A exposure for 90 min (natural pH).

Figure 10 .
Figure 10.(a) Removal of Triton X-100 at a concentration of 13 mg/L using nanoparticulate TiO 2 powders (catalyst amount: 1 g/L) with UV-A exposure for 30 min (natural pH); (b) evolution of the Triton X-100 UV absorbance at a concentration of 64 mg/L in the supernatant aliquot; (c) study of the kinetics of Triton X-100 (64 mg/L) photocatalytic degradation with TiO 2 /P25 thin film on the lattice substrate (natural pH); (d) photocatalytic performance of trilayer-coated lattice as a function of Triton X-100 concentration under UV-A exposure for 90 min (natural pH).

Author Contributions:
Conceptualization, D.I. and G.E.R.; methodology, G.V.T. and M.K.A.; validation, G.V.T. and M.K.A.; formal analysis, G.V.T. and M.K.A.; investigation, G.V.T. and M.K.A.; resources, G.E.R. and P.F.; writing-original draft preparation, G.V.T.; writing-review and editing, G.E.R.; visualization, G.V.T. and M.K.A.; supervision, P.F.; project administration, T.S.P.; funding acquisition, S.M. and D.I.All authors have read and agreed to the published version of the manuscript.Funding: This work received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 958274 within the framework of iWAYS-Innovative WAter recoverY Solutions project.