Photocatalytic degradation of paracetamol on immobilized TiO2 in a low-tech reactor by solar light for water treatment

Open Ceramics 18 (2024) 100599 Available online 6 May 2024 2666-5395/© 2024 The Authors. Published by Elsevier Ltd on behalf of European Ceramic Society. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Photocatalytic degradation of paracetamol on immobilized TiO 2 in a low-tech reactor by solar light for water treatment Lukas Dufner a , * , Lluc Arest´ e-Sal´ o b , Mois` es Graells b , Montserrat P´ erez-Moya b , Frank Kern a , Wolfgang Rheinheimer a a Institute for Manufacturing Technologies of Ceramic Components and Composites, University of Stuttgart, Allmandring 7b, 70569, Stuttgart, Germany b Chemical Engineering Department, Universitat Polit` ecnica de Catalunya, Escola D’Enginyeria de Barcelona Est (EEBE), Av. Eduard Maristany, 16, 08019, Barcelona, Spain ARTICLE INFO Handling Editor: Dr Catherine Elissalde Keywords: Photocatalysis TiO 2 Solar reactor Geopolymer coating Paracetamol Degradation Water treatment ABSTRACT Photocatalytic water treatment is an appealing concept to mineralize organic contaminants in contaminated water. In this study, the UV-resistant drug paracetamol (PCT) in aqueous solution is degraded by using a solar irradiated loop-reactor equipped with a TiO 2 (P25) photocatalyst. The catalyst powder was dispersed in a geopolymer surface coating applied to aluminum-U-profiles connected by filament-printed polymer elements. Different assays with variations in initial PCT concentration were performed under natural solar radiation to determine the degradation kinetics of the PCT and correlate it to applied solar radiation energy density. Depending on experimental conditions, degradation efficiencies of 37.0–75.1 % were achieved within 3 h. Control samples without catalyst show no degradation. No degradation of the catalyst activity was observed within 42 h of operation time. The study shows the feasibility of this simple and scalable reactor design and coating technology for water purification under off-grid conditions provided sufficient solar radiation is available. 1. Introduction Access to clean and safe drinking water is a fundamental requirement for human health and well-being. However, more than 2.2 billion people worldwide still lack access to adequate water treatment facilities, particularly in rural areas and developing regions [1,2]. The absence of reliable infrastructure poses significant challenges to achieving widespread water purification. Consequently, there is a growing need for innovative and sustainable low-tech solutions that can effectively address this pressing issue [3–5]. The SODIS (solar disinfection) method is a simple, low-cost, and easily implementable technique for water treatment, based on exposing water in transparent PET bottles to direct sunlight. The UV radiation of solar light can inactivate some bacteria, viruses, and other pathogens in the water. The simple method is, however, not capable to provide large amounts of purified water. To increase the activity of SODIS, it is possible to add an oxidation step using a photocatalyst [6]. Photocatalysis offers a promising approach for water purification. Especially TiO 2 photocatalysts are able to mineralize various organic contaminants and disinfect water by generation of highly reactive hydroxyl radicals [7–9]. In recent years, the potential of TiO 2 photocatalysis as an alternative technology for water purification was investigated [10]. The use of natural sunlight as an energy source provides a sustainable and readily available option for catalyst activation, eliminating the need for costly artificial UV light sources [6,11]. The properties of titania, including biocompatibility, low cost, non-toxicity, and photostability, contribute to its widespread utilization [8,12–14]. Among various different titania powders for photocatalysis, the mixed anatase/rutile powder Evonik P25 is the most frequently studied. It demonstrates excellent performance in water treatment applications [15,16]. The band gap of the TiO 2 (anatase phase) is 3.3 eV [17]. This corresponds radiation in the UV-A range. Focusing solar radiation to substitute expensive artificial UV-A light for catalyst activation saves energy and resources. In the field of water treatment, the TiO 2 powders are either introduced directly as a suspension into the water or immobilized on a substrate using a coating process [7]. Generally, dispersed powders exhibit a higher activity than the immobilized coatings due to their higher * Corresponding author. E-mail address: [email protected] (L. Dufner). Contents lists available at ScienceDirect Open Ceramics journal homepage: www.sciencedirect.com/journal/open-ceramics https://doi.org/10.1016/j.oceram.2024.100599 Received 19 February 2024; Received in revised form 19 April 2024; Accepted 1 May 2024 Open Ceramics 18 (2024) 100599 2 specific surface area, higher catalyst concentrations with respect to water throughput and a more uniform particle distribution [18–20]. However, a technical process (e.g. membrane filtration) is required, which is energy-intensive and slow, to separate the catalyst particles form the water [7,21], and is therefore out of question for low-tech applications. The literature describes many processes for the deposition of TiO 2 particles direct or dispersed onto a substrate [22]. Since the metastable anatase phase of TiO 2 is important for high photocatalytic activity [23], all thermal processes (T >600 ◦C) like thermal spraying methods [24–26] or processes requiring a post thermal treatment like sol-gel methods [27,28] bear the risk of converting anatase to the thermodynamically stable but less active rutile phase. Regarding the low-tech approach, all methods requiring complicated, expensive or energy-intensive processes such as thermal spraying, vapor/solution deposition [29–32] or electrochemical methods [33] are not suitable. The photocatalytic activity in the mentioned studies above is hard to compare due to the variations in experimental setup. However, a simple and inexpensive coating process for immobilizing TiO 2 particles was chosen, which has already been published [34]. Numerous studies have demonstrated the effectiveness of TiO 2 photocatalysis in degrading a wide range of organic pollutants, including pesticides, pharmaceuticals, and dyes, as well as inactivating bacteria and viruses [11,19,35–40]. The photocatalytic process mineralizes these contaminants to harmless products, such as CO 2 and H 2 O, securing the water for consumption [9,41]. The analgesic medicine paracetamol (PCT) is one of the widest spread drugs worldwide and belongs to the compounds of emerging concerns (CEC) [42]. Since PCT is UV-stable it was chosen as a model substance to test photocatalytic purification in this study [43]. PCT consumed by humans is secreted and agglomerates in the aquatic ecosystem [44–47]. Some studies are clearly showing that TiO 2 particles suspended in water and illuminated by UV-A light degrade PCT [42,48–51], even for solar light a degradation was observed [52]. Earlier studies often used loose and dispersed titania powder. Recently, titania was banned as a food additive (E171 EU) [53, 54] as teratogenic effects could not be excluded. Therefore an immobilized catalyst (coating) was chosen for a low-tech reactor, hence, no dispersed titania powder has to be removed by a costly post-processing procedure [9,21]. In this study, photocatalytic titania particles were incorporated into a potassium silicate geopolymer coating onto alumina substrate. The first objective is the design and construction of a solar autarkic photocatalytic reactor for water purification in remote areas as low-tech approach matching the following requirements: material simplicity, global availability, component durability, low maintenance, off-grid capability, no artificial illumination, solar radiation only, costeffectiveness and scalability. The second and main objective is to quantify the photocatalytic performance of such coatings in combination with the low-tech reactor concept. A measurable influence of the catalyst coating in the designed low-tech reactor setup compared to the disinfection of the UV radiation should be shown and characterized. Therefore, UV-stable paracetamol solution is used as model substance and irradiated only by direct and diffuse solar light in the reactor. The third objective is assessing the performance and stability of the reactor concept under different weather conditions. The results will be used to assess whether the reactor concept in combination with the photocatalyst coating is generally suitable for solar autarkic water purification and whether the concept should be explored further. 2. Experimental procedures 2.1. Reactor design Open aluminum U-profiles (Rapa GmbH, Selb, Germany) were chosen as water-bearing parts, connected to each other with 3D-printed elements (printed by Original Prusa i3 MK3S, Prusa Research a. s., Prague, Czech Republic) made of PLA filaments (niceshops GmbH, Saaz, Austria) to follow the requirements formulated in the introduction. Both materials are cheap and largely available. The aluminum-U-profiles (650 mm long, 60 mm width, 15 mm high, with a thickness of 1 mm) and 3D-printed curved connecting elements are designed as modular system to scale up the reactor easily. It can be extended to any length with additional curved elements and a corresponding support structure. The contaminated water can be filled from a reservoir (e.g. a plastic bucket) with a dosing unit at the upper end in reactor and is recollected at the lower end after passing through. No complicated technical parts are needed. The reactor can be used off grid since the catalyst coating gets activated by the sun and does not require any complicated maintenance. For the present study, a building block consisting of two coated aluminum chutes (Fig. 1) is used as the catalytic component of a loop reactor. The contaminated water contained in a stirred tank is recycled with a peristaltic pump at constant feed rate over the external loop. The U-profiles were coated with the TiO 2 catalyst coating as described in section 2.2. Control experiments were carried out without applied catalyst coating. The aluminum profiles were laid out on a level structure with a height difference of 16 mm per profile (1–4, Fig. 2) to keep a constant flow of the contaminated water through the reactor. The profiles themselves are not inclined but horizontal. A constant water flow is enabled by the slope of the curved connection element. A wall dam of 4 mm height (2–1, Fig. 2) backlogs the water to wet the entire profile. A water level of approximately 8 mm occurs in each profile due to the surface tension which leads to a total volume of 0,31 dm 3 per profile. The curved elements were glued to the aluminum profiles with an UVresistant commercially available silicone glue to prevent water leakage (Sanit¨ arsilikon, Probau Eco). The CAD drawings for 3D printing of the PLA parts can be found in the supplementary information. The data is accessible with any freeware slicer software and can be customized for the corresponding 3D printer. 2.2. Preparation of TiO 2 catalyst coating The application of the TiO 2 catalyst coatings is kept very simple without using any complex technical devices or laboratory equipment. The exact procedure is published elsewhere [34]. Briefly, TiO 2 (P25 – Evonik AG, Essen, Germany) catalyst powder dispersed in a potassium silicate solution which consists of a modified aluminosilicate and modified alkali silicates (so called “water glass”) was applied directly to aluminum profiles by a roller. To immobilize 1 g TiO 2 catalyst powder, it was mixed with 5 g potassium silicate (Geosil14517, Woellner GmbH, Ludwigshafen, Germany) and 2 g demineralized water. After adding 12.5 g hardener (Stabisil 0, Woellner GmbH, Ludwigshafen, Germany) the suspension is stirred for 10 min and applied directly to the substrate (aluminum U-profiles) using a paint roller and left to dry on air for 24 h. Subsequently a second layer of coating is applied by the identical procedure to reach a coating thickness about 100 μ m. 2.3. Experimental site and setup The experimental site is located at the Campus Diagonal Bes` os at the Universitat Polit` ecnica de Catalunya, Barcelona Tech, in Barcelona (latitude 41◦N, longitude 2◦E), Spain. Experiments were conducted under natural solar light as irradiation source during fall season and took place between 11 a.m. and 5 p.m. to receive the highest solar dose of the day. Solar radiation data were acquired by a PMA2200 single input radiometer from Solar Light Company LLC, Glenside, USA. The sensor was placed right next to the experiment to ensure that the exact local solar radiation values are measured. The device records the intensity of the UV-A-spectra (320–400 nm wavelength) in mW/cm 2 . The amount of solar energy within experimental time can be exactly determined by integration of the irradiation. L. Dufner et al. Open Ceramics 18 (2024) 100599 3 Water parameters such as temperature and pH were controlled but not discussed further as they were within a typical range for potable water (pH 6–7). The experiments were carried out with water temperatures between 16 and 24 ◦C. A loop reactor process with a test volume of 1 L paracetamol solution with varying concentration (6–14 mg/L) was used to determine the photocatalytic activity of the reactor, shown in Fig. 3. The reactor’s outflow was collected in a storage tank, which was connected to the reactor’s inlet via a peristaltic pump. The storage tank was stirred with an agitator (>100 rpm) for homogenization. The fluid was circulated at a varying flow rate between 100 and 200 mL/min. They were estimated for a larger reactor with ~1 m 2 size (size of a Euro pallet) which should provide the drinking water of a family for one day, the flow rate was then recalculated to the model rectors accordingly. The sun radiation measured by an UV-A sensor (320–400 nm) was recorded every second for 180 min. Every 15 min a sample (V =0.5 mL) was taken from the storage tank and kept for analyzation by high pressure liquid chromatography (HPLC). A control reactor without catalyst coating was set next to the prototype reactor and run at the same time under the same conditions to prove that the photocatalyst is responsible for the PCT degradation. PCT concentration was determined via HPLC, using an Agilent 1200 series with UV-DAD array detector. Stationary phase was an Akady 5 μ m C-18 150 ×4.6 mm column, maintained at 25 ◦C and the diode array detector was set at 243 nm. The mobile phase was a mixture of methanol and water (25:75) flowing at 0.4 mL/min 20 μ L samples were injected by a manual injector. Under these conditions, retention time was 8.6 min. A nine-level calibration curve (range 0–50 mg/L) was used for contaminant quantification. 2.4. Photocatalytical degradation experiments 2.4.1. Preliminary experiments Three preliminary experiments were carried out to investigate the influence of different flow rates and the possibility of PCT degradation only by UV-light. The experiments are divided into assays with catalyst named “A” (for assay) and the control experiments without catalyst named “C” (for control) plus the corresponding flowrate, as shown in Table 1. The typical experimental duration was 180 min. 2.4.2. Design of experiment All experiments for the main experiment are summarized in Tables 2 and 3 and named the following format (C_N) where C is the initial concentration of PCT [mg/L] and N is the number of repetitions of that experiment. For example, the third experiment with an initial concentration of 10 mg/L would be named “10_3”. The experimental plan consists of seven experiments on three different concentration levels: low (6 mg/L), intermediate (10 mg/L) and high (14 mg/L). The central triplicate experiment with 10 mg/L is carried out three times, the higher and lower ones two times, as shown in Table 2 in ascending order. For the triplicate, the data form the preliminary section is used and renamed. The experiments were carried out in a random order. For each experiment a control experiment without catalyst was carried out at the same time. Afterwards, the triplicate experiments with 10 mg/L were repeated Fig. 1. A building block consist of two aluminum U-profiles coated with TiO 2 photocatalyst and shown with inlet and outlet as reactor setup used in this study. Fig. 2. Conception of the curved element. The total height difference which is overcome is 16 mm from point 1 to 4. Fig. 3. Experimental setup to determine the photocatalytic activity of the prototype reactor. Table 1 Flowrate and initial PCT concentration for the preliminary experiments. Experiment PCT concentration [mg/L] Flowrate [mL/ min] Catalyst coating A100 10 100 Yes C100 10 100 No A120 10 120 Yes C120 10 120 No A200 10 200 Yes C200 10 200 No L. Dufner et al. Open Ceramics 18 (2024) 100599 4 to track a possible change of the activity of the catalyst coating over time, shown in Table 3. 3. Results The process performance is assessed through the reduction of the concentration of PCT dissolved in demineralized water as measured via HPLC. The observed parameter is the fraction of PCT relative to the initial concentration (C/C 0 ). In order to ensure that the PCT degradation occurred by photocatalysis and not only due to UV radiation, control experiments using the same reactor but without catalyst coating were performed. The variables affecting the process performance taken into account are the flowrate, the initial concentration of PCT, and the solar radiation received during the experiment. 3.1. Impact of the flow rate The preliminary experiments were carried out to prove that PCT is an UV stable contaminant, and the prototype reactor is able to degrade it in contrast to the control reactor without catalyst. The influence of the flowrate is investigated as well. The PCT concentration was plotted against the cumulative solar radiation, which explains the different length of the degradation curves, shown in Fig. 4. The data provide clear evidence that the PCT concentration for the control reactor experiments increased slightly. This effect is caused by evaporation of water during the experiment from the open chutes. No degradation of PCT took place, which means that PCT is a suitable model substance for the investigation of the photocatalytic activity of the prototype reactor. However, all PCT degradation curves for the reactors with catalyst (A100, A120, A200) show comparable degradation rates. The absence of a distinct correlation between high and low flow rates indicates that the flow rate and hereby mass transfer limitation has no influence on the degradation kinetics in the tested range (100–200 mL/min). 3.2. Impact of the initial concentration Experiments were carried out as shown in Table 2. Fig. 5 compares the data measured using the prototype reactor with photocatalyst coating. The measured concentrations are plotted against the cumulative solar radiation during the experimental duration (between 135 min and 210 min). Different symbols indicate different initial concentrations. The different number of data points in the individual series is due to the different number of repetitions of experiments carried out for the same initial concentration. It is clearly shown that a degradation of PCT in the reactor with photocatalytic coating occurred for all experiments. The triplicate (initial concentration of 10 mg/L) and the two repetitions of 6 and 14 mg/L result in traces of low scattering. The results of the control experiments are not shown here, since the PCT degradation behavior without catalyst did not deviate from the preliminary experiments. The experimental data for all assays performed is summarized in Table 4. The planned duration of the experiments was 180 min and was achieved in all experiments except 14_1, as it started to rain after 135 min. For two experiments (10_3 and 14_2) data collection was continued until 210 min. The degradation varying between 37.0 % (experiment 10_1) and 75.1 % (experiment 10_3). The solar radiation (UV-A dose) showed a significant influence of the PCT concentration. Higher amounts of solar radiation lead to significant higher PCT degradation. For all experiments the degradation/energy ratio was within the same range between 0.41 and 0.45 %/(kJ/m 2 ), except for the experiments with the lowest initial concentration (6 mg/L), there it was higher. Based on this data, the degradation behavior of PCT seems to appear linear and correlate directly with the solar radiation and the initial concentration level. There might be an influence of different weather conditions, such as wind and humidity, which could affect the evaporation and explain why the experiments 06_01 and 06_02 performed better than expected. Covering the open chutes in order to avoid evaporation was not considered as it may impair the results by water condensation and thereby alter the transmission of radiation. Fig. 6 shows the degradation of PCT on a mainly sunny day (A) with Table 2 Experimental plan sorted by the initial concentration of PCT solution. Experiment PCT concentration [mg/L] 06_1 6 06_2 6 10_1 10 10_2 10 10_3 10 14_1 14 14_2 14 Table 3 Experimental plan of the additional triplicate experiments. Experiment PCT concentration [mg/L] 10_4 10 10_5 10 10_6 10 Fig. 4. PCT degradation with initial concentration of 10 mg/L in the reactor with catalyst and the control reactor without catalyst variating the flow rate. The experimental duration was 180 min. Fig. 5. Degradation of PCT for different initial concentrations 6 mg/L (06_1, 06_2), 10 mg/L (10_1, 10_2, 10_3) and 14 mg/L (14_1, 14_2). The experimental duration varies from 135 min to 210 min. L. Dufner et al. Open Ceramics 18 (2024) 100599 5 high concentration (14_1) and on a mainly cloudy (B) with low concentration (06_1). The illustration suggest that high solar radiation leads to high degradation rates, but it is dependent on the initial concentration. The degradation curve of the experiment with lower concentration decreases more slowly compared to the one with higher concentration, for a comparable amount of solar energy, see Table 4. 3.3. Reuse of catalyst – behavior of inactivation The “triplicate” experiment (10_1; 10_2; 10_3) was repeated to obtain information on the change in activity of the catalyst coating over the experimental time. Since there was a random order of the experiments in the previous section, the second triplicate experiments (10_4; 10_5; 10_6) were carried out sequentially. The results are shown in Table 5. Before the second triplicate experiment, the catalyst coating was operated for 33 h, including some pre-experiments. Additional 9 h experimental time for these experiments was added, totaling the operational time of the catalyst coating to 42 h. A comparison of the normalized degradation rates in Fig. 7 shows that the degradation of the second triplicate (purple) is faster than the first triplicate (yellow). Surprisingly, the catalyst did not lose activity (which would be typical for some catalyst degradation phenomena such as poisoning or deposition of material); the activity increased by approximately 20–25 %. Presumably, it takes time for the catalyst to activate and develop its full capacity. This is a very interesting result for a photocatalytic catalyst coating, as this poses a major research question for further investigations. Furthermore, the normalization shows a linear dependency of the degradation and the sun irradiation for all experiments, as already assumed in the previous section. Comparing the degradation energy ratio to the first triplicate in Table 4, the values are stable (0.53–0.55 %/(kJ/m 2 ) and within the same range. This is very interesting and will be addressed to a future work about investigating the degradation behavior by a model. Table 4 Overview of the PCT degradation for the experiments introduced in Table 2. Exp. Time [min] Conc. [mg/L] Degradation [%] Total solar energy [J/m 2 ] Ratio: Degradation/ Energy [%/(kJ/ m 2 )] Initial Final 06_1 180 6.09 2.96 51.4 72.69 0.71 06_2 180 6.01 2.23 62.9 94.50 0.67 10_1 180 10.13 6.38 37.0 82.19 0.45 10_2 180 10.26 4.54 55.8 133.23 0.41 10_3 210 10.10 2.51 75.1 171.48 0.44 14_1 135 14.86 9.10 38.8 86.35 0.45 14_2 210 14.86 6.66 55.2 133.53 0.41 Fig. 6. PCT degradation (blue) and intensity of the solar radiation (black line) over time illustrated in one figure for experiment 14_01 (A) and 06_01 (B). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Table 5 Results of the additional triplicate experiments introduced in Table 3. The data of the experiments 10_1, 10_2 and 10_3 is shown in Table 4. Experi ment Time [min] Conc. [mg/L] Degradation [%] Total solar energy [kJ/m 2 ] Ratio: Degradation/ Energy [%/(kJ/m 2 )] Initial Final 10_4 180 10.92 4.07 62.7 113.5 0.55 10_5 180 10.71 3.29 69.3 130.8 0.53 10_6 180 11.42 5.75 49.6 93.45 0.53 Fig. 7. Normalized PCT degradation of the previous experiments with 10 mg/L (10_1, 10_2, 10_3) compared to additional triplicate experiments with 10 mg/L (10_4, 10_5, 10_6) plotted over the received solar radiation. The standard deviation estimated roughly from the scattering of the linear plots ranges from ± 5–10 % for the triplicates. L. Dufner et al. Open Ceramics 18 (2024) 100599 6 4. Discussion The reactor presented in this study belongs to the group of nonconcentrating solar reactors [7]. It distinguishes itself from previously introduced reactors in its simplicity and operational principle, which operates entirely without external energy [55–58]. The energy source for the catalyst activation is the UV radiation of the sun. Another innovation is the integration of solar disinfection and photocatalysis through a simple and robust geopolymer-based coating directly applied to the aluminum U-profiles, exhibiting strong adhesion. The coating technique is low-tech as well and has been previously published elsewhere [34]. Paracetamol, known as substance accumulated in the environment [42], was employed, as model substance to characterize the photocatalytic activity. The typical PCT dose in tablet is in the range between 100 and 500 mg. In water it is strongly diluted, e.g. in ground water PCT concentrations are up to 2 μ g/L, in surface water up to 16 μ g/L and in industrial wastewater may reach up to 100 μ g/L [59]. The chosen concentration in the experiments is far higher, as the study aimed at determining if PCT is degradable at all, therefore the concentration was chosen to be accurately measurable with the HPLC apparatus available in order to determine the degradation kinetics. It could be confirmed that PCT solved in water was not degradable by solar light only. An oxidation catalyst is required. The degradation rates of PCT in the experiments with the given setup ranged from 37 % to 75.1 %. This result is extending previous experiments, where the degradation of methylene blue under solar light was observed, which, unlike PCT, is not UV-stable. Therefore, in the previous studies, the degradation could not be clearly attributed to the catalyst or UV radiation. Comparing PCT degradation with literature is challenging due to variations in experimental setups, reactor designs and the combination of degradation techniques in comparable publications [48,49,51,60]. Setups with dispersed catalysts generally lead to better degradation rates but cannot be directly compared, most of them operate under lab conditions with artificial illumination [7]. Comparable studies with immobilized catalysts are unavailable. Another important aspect is the change in catalyst activity over time often reported in publications with very short operational lifetimes [61, 62], which are faced to overcome with different strategies like doping [63,64] or regeneration methods [65,66]. Throughout the entire experimental series, including preliminary tests, a total runtime of 42 h was achieved, and the catalyst’s performance did not significantly decline; in fact, it may have slightly improved based on the data. The authors speculate that the coating may require some time for activation. If and how fast the catalyst activity will decline at prolonged operational time is an open question. Detailed information on deactivation time would be required to know when to replace the catalyst-coated chutes. By joining many of the tested modular building blocks the reactor can simply be scaled up to achieve a higher disinfection performance, changing also from stirred tank/loop reactor to a continuous flow concept in which the degradation occurs in a single pass Without recirculation no peristaltic pump is needed and the reactor can be operated without energy using a gravity-fed reservoir provided that the reactor length is sufficient to guarantee a sufficient degradation. The profiles themselves can be made wider or longer, but the height of 15 mm cannot be changed, as the water column above the catalyst coating has a direct effect on the photocatalytic performance [67]. The final length of the profiles or number of stages depends on the degradation efficiency to be achieved. Depending on the application, preliminary tests are absolutely necessary to determine the correct dimension of the reactor. In the area within the sun belt for which the reactor was designed, the solar radiation is even higher, and the disinfection rates should be stronger compared to the experimental site (sun belt ±35◦[68], experimental site: Barcelona 41◦). Besides the removal of drugs (such as PCT) or their metabolites, the reactor concept seems primarily attractive to remove viruses and bacteria in drinking water. Considering the unexpectedly high degradation rates, the authors are confident that this photocatalysis technology can be applied for removal of trace contaminants from wastewater or surface water and significantly save energy by utilization of solar radiation. Future experiments with bacteria and viruses, contaminated drinking water or industrial waste water will be necessary to prove these assumptions. The use for municipal wastewater will not be realistic due to the high turbidity and the high degree of contamination lowering the penetration depth of solar radiation extremely [69]. 5. Conclusion This study presents three main achievements. •A solar-autonomous prototype reactor is demonstrated as a low-tech approach for water treatment. Aluminum U-profiles coated with a TiO 2 photocatalyst and 3D-printed connecting elements were used as materials, which are widely available. The reactor parts are easily producible, manageable, and replaceable. It combines the purification processes of the solar disinfection with the oxidation by the photocatalyst. Designed as a modular system, the scalability allows for precise adaptation to the required demand. The study has demonstrated the functionality of the 2-profile prototype reactor. •The study demonstrates the capability of the reactor to degrade paracetamol (PCT) as a UV-stable model substance dissolved in water. The degradation occurs only through the oxidation by the catalyst with efficiencies ranging from 37.0 % to 75.1 % under the utilized experimental setup depending on the received solar radiation for each experiment. The degradation can be further increased by extending the residence time or illuminated area. It is assumed that higher solar radiation levels, e.g. in summer, increase the degradation as well. •The characterization of process performance and stability under different (weather) conditions supports the feasibility of employing this solar reactor for sustainable water treatment. The catalyst coating operated continuously for 42 h without exhibiting any signs of inactivation. Even under challenging conditions such as cloudy skies, the catalyst was activated by diffuse radiation, resulting in the degradation of PCT. In the future, achieving a more comprehensive understanding of the reactor and its properties several steps are needed. Employing a model based on solar radiation is essential for a better understanding of the parameters influencing the degradation. Additionally, scaling up the prototype reactor and treating real wastewater will be crucial to assess its performance in practical applications. CRediT authorship contribution statement Lukas Dufner: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – original draft. Lluc Arest´ e-Sal´ o: Data curation, Formal analysis, Investigation, Software, Writing – original draft, Visualization. Mois` es Graells: Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review & editing. Montserrat P´ erez-Moya: Conceptualization, Data curation, Formal analysis, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing – review & editing. Frank Kern: Funding acquisition, Supervision, Writing – review & editing. Wolfgang Rheinheimer: Supervision, Writing – review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence L. Dufner et al. Open Ceramics 18 (2024) 100599 7 the work reported in this paper. Acknowledgements The authors are grateful to the JECS Trust for funding the visit of Lukas Dufner to CEPIMA, Universitat Polit` ecnica Catalunya (Contract No. 2023344) and the Boysen Stiftung (BOY-185) for the financial support of his PhD. Grant PID2020-116051RB-I00 (CEPI) funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe” is fully acknowledged. The authors are also grateful to the Generalitat de Catalunya (2021 SGR 01061-ENCORE) for the financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.oceram.2024.100599. References [1] World Health Organization (WHO). Drinking-water: Key facts. Available from:: <https://www.who.int/news-room/fact-sheets/detail/drinking-water>. [2] World Health Organization (WHO). Sanitation: Key facts. Available from: <https ://www.who.int/news-room/fact-sheets/detail/sanitation>. [3] Unidted Nations (UN). The 17 Goals. [4] S. Loeb, R. Hofmann, J.-H. Kim, Beyond the pipeline: assessing the efficiency limits of advanced technologies for solar water disinfection, Environ. Sci. Technol. Lett. 3 (3) (2016) 73–80, https://doi.org/10.1021/acs.estlett.6b00023. [5] P.R. Hunter, Household water treatment in developing countries: comparing different intervention types using meta-regression, Environ. Sci. Technol. 43 (23) (2009) 8991–8997, https://doi.org/10.1021/es9028217. [6] K.G. McGuigan, R.M. Conroy, H.-J. Mosler, M. Du Preez, E. Ubomba-Jaswa, P. Fernandez-Iba˜ nez, Solar water disinfection (SODIS): a review from bench-top to roof-top, J. Hazard Mater. 235–236 (2012) 29–46, https://doi.org/10.1016/j. jhazmat.2012.07.053. [7] D.D. Dionysiou, D. Bahnemann, G.L. Puma, J. Ye, J. Schneider (Eds.), Photocatalysis: Applications, Royal Society of Chemistry, Cambridge, 2016. [8] P. Pichat (Ed.), Photocatalysis and Water Purification: from Fundamentals to Recent Applications, Wiley-VCH-Verl, Weinheim, 2013. [9] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1) (1993) 341–357, https://doi.org/10.1021/cr00017a016. [10] N. Pichel, M. Vivar, M. Fuentes, The problem of drinking water access: a review of disinfection technologies with an emphasis on solar treatment methods, Chemosphere 218 (2019) 1014–1030, https://doi.org/10.1016/j. chemosphere.2018.11.205. [11] S. Malato, P. Fern´ andez-Ib´ a˜ nez, M.I. Maldonado, J. Blanco, W. Gernjak, Decontamination and disinfection of water by solar photocatalysis: recent overview and trends, Catal. Today 147 (1) (2009) 1–59, https://doi.org/10.1016/j. cattod.2009.06.018. [12] M. Ismael, A review and recent advances in solar-to-hydrogen energy conversion based on photocatalytic water splitting over doped-TiO2 nanoparticles, Sol. Energy 211 (2020) 522–546, https://doi.org/10.1016/j.solener.2020.09.073. [13] A. Omar, M.S. Ali, N. Abd Rahim, Electron transport properties analysis of titanium dioxide dye-sensitized solar cells (TiO2-DSSCs) based natural dyes using electrochemical impedance spectroscopy concept: a review, Sol. Energy 207 (2020) 1088–1121, https://doi.org/10.1016/j.solener.2020.07.028. [14] T. Ochiai, A. Fujishima, Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification, J. Photochem. Photobiol. C Photochem. Rev. 13 (4) (2012) 247–262, https://doi.org/10.1016/j. jphotochemrev.2012.07.001. [15] B. Ohtani, O.O. Prieto-Mahaney, D. Li, R. Abe, What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test, J. Photochem. Photobiol. Chem. 216 (2–3) (2010) 179–182, https://doi.org/10.1016/j.jphotochem.2010.07.024. [16] D.M. Tobaldi, R.C. Pullar, M.P. Seabra, J.A. Labrincha, Fully quantitative X-ray characterisation of Evonik aeroxide TiO2 P25, Mater. Lett. 122 (2014) 345–347, https://doi.org/10.1016/j.matlet.2014.02.055. [17] Y. Bai, I. Mora-Ser´ o, F de Angelis, J. Bisquert, P. Wang, Titanium dioxide nanomaterials for photovoltaic applications, Chem. Rev. 114 (19) (2014) 10095–10130, https://doi.org/10.1021/cr400606n. [18] A. Rachel, M. Subrahmanyam, P. Boule, Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilised form for the photocatalytic degradation of nitrobenzenesulfonic acids, Appl. Catal. B Environ. 37 (4) (2002) 301–308, https:// doi.org/10.1016/S0926-3373(02)00007-3. [19] V. Cohen-Yaniv, N. Narkis, R. Armon, Photocatalytic inactivation of Flavobacterium and E. coli in water by a continuous stirred tank reactor (CSTR) fed with suspended/immobilised TiO2 medium, Water science and technology a journal of the International Association on Water Pollution Research 58 (1) (2008) 247–252, https://doi.org/10.2166/wst.2008.664. [20] R.L. Pozzo, M.A. Baltan´ as, A.E. Cassano, Supported titanium oxide as photocatalyst in water decontamination: state of the art, Catal. Today 39 (3) (1997) 219–231, https://doi.org/10.1016/S0920-5861(97)00103-X. [21] J.C. Espíndola, R.O. Crist´ ov˜ ao, A. Mendes, R.A. Boaventura, V.J. Vilar, Photocatalytic membrane reactor performance towards oxytetracycline removal from synthetic and real matrices: suspended vs immobilized TiO2-P25, Chem. Eng. J. 378 (2019) 122114, https://doi.org/10.1016/j.cej.2019.122114. [22] M. Dell’Edera, C. Lo Porto, I de Pasquale, F. Petronella, M.L. Curri, A. Agostiano, et al., Photocatalytic TiO2-based coatings for environmental applications, Catal. Today 380 (2021) 62–83, https://doi.org/10.1016/j.cattod.2021.04.023. [23] A. Bojinova, R. Kralchevska, I. Poulios, C. Dushkin, Anatase/rutile TiO2 composites: influence of the mixing ratio on the photocatalytic degradation of Malachite Green and Orange II in slurry, Mater. Chem. Phys. 106 (2–3) (2007) 187–192, https://doi.org/10.1016/j.matchemphys.2007.05.035. [24] F.-L. Toma, G. Bertrand, D. Klein, C. Meunier, S. Begin, Development of photocatalytic active TiO 2 surfaces by thermal spraying of nanopowders, J. Nanomater. 2008 (2008) 1–8, https://doi.org/10.1155/2008/384171. [25] F.-L. Toma, G. Bertrand, S. Begin, C. Meunier, O. Barres, D. Klein, et al., Microstructure and environmental functionalities of TiO2-supported photocatalysts obtained by suspension plasma spraying, Appl. Catal. B Environ. 68 (1–2) (2006) 74–84, https://doi.org/10.1016/j.apcatb.2006.07.009. [26] F.-L. Toma, G. Bertrand, D. Klein, C. Coddet, C. Meunier, Nanostructured photocatalytic titania coatings formed by suspension plasma spraying, J. Therm. Spray Technol. 15 (4) (2006) 587–592, https://doi.org/10.1361/ 105996306X147234. [27] U.G. Akpan, B.H. Hameed, The advancements in sol–gel method of doped-TiO2 photocatalysts, Appl. Catal. Gen. 375 (1) (2010) 1–11, https://doi.org/10.1016/j. apcata.2009.12.023. [28] N. Arconada, A. Dur´ an, S. Su´ arez, R. Portela, J.M. Coronado, B. S´ anchez, et al., Synthesis and photocatalytic properties of dense and porous TiO2-anatase thin films prepared by sol–gel, Appl. Catal. B Environ. 86 (1–2) (2009) 1–7, https://doi. org/10.1016/j.apcatb.2008.07.021. [29] X. Zhang, M. Zhou, L. Lei, Preparation of anatase TiO2 supported on alumina by different metal organic chemical vapor deposition methods, Appl. Catal. Gen. 282 (1–2) (2005) 285–293, https://doi.org/10.1016/j.apcata.2004.12.022. [30] B.-H. Kim, J.-Y. Lee, Y.-H. Choa, M. Higuchi, N. Mizutani, Preparation of TiO2 thin film by liquid sprayed mist CVD method, Mater. Sci. Eng., B 107 (3) (2004) 289–294, https://doi.org/10.1016/j.mseb.2003.12.010. [31] T.K. Ghorai, D. Dhak, S.K. Biswas, S. Dalai, P. Pramanik, Photocatalytic oxidation of organic dyes by nano-sized metal molybdate incorporated titanium dioxide (MxMoxTi1−xO6) (M=Ni, Cu, Zn) photocatalysts, J. Mol. Catal. Chem. 273 (1–2) (2007) 224–229, https://doi.org/10.1016/j.molcata.2007.03.075. [32] P. Babelon, A. Dequiedt, H. Most´ efa-Sba, S. Bourgeois, P. Sibillot, M. Sacilotti, SEM and XPS studies of titanium dioxide thin films grown by MOCVD, Thin Solid Films 322 (1–2) (1998) 63–67, https://doi.org/10.1016/S0040-6090(97)00958-9. [33] S. Karuppuchamy, N. Suzuki, S. Ito, T. Endo, A novel one-step electrochemical method to obtain crystalline titanium dioxide films at low temperature, Curr. Appl. Phys. 9 (1) (2009) 243–248, https://doi.org/10.1016/j.cap.2008.02.004. [34] L. Dufner, F. Ott, N. Otto, T. Lembcke, F. Kern, Immobilization of TiO2 photocatalysts for water treatment in geopolymer based coatings, Catalysts 13 (5) (2023) 898, https://doi.org/10.3390/catal13050898. [35] M. Fouad, M. Gar Alalm, H.K. El-Etriby, D.C. Boffito, S. Ookawara, T. Ohno, et al., Visible-light-driven photocatalytic disinfection of raw surface waters (300-5000 CFU/mL) using reusable coated Ru/WO3/ZrO2, J. Hazard Mater. 402 (2021) 123514, https://doi.org/10.1016/j.jhazmat.2020.123514. [36] A. Joss, E. Keller, A.C. Alder, A. G¨ obel, C.S. McArdell, T. Ternes, et al., Removal of pharmaceuticals and fragrances in biological wastewater treatment, Water Res. 39 (14) (2005) 3139–3152, https://doi.org/10.1016/j.watres.2005.05.031. [37] T. Schnabel, N. Jautzus, S. Mehling, C. Springer, J. Londong, Photocatalytic degradation of hydrocarbons and methylene blue using floatable titanium dioxide catalysts in contaminated water, Journal of Water Reuse and Desalination 11 (2) (2021) 224–235, https://doi.org/10.2166/wrd.2021.118. [38] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Mari˜ nas, A. M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (7185) (2008) 301–310, https://doi.org/10.1038/nature06599. [39] M.I. Din, R. Khalid, J. Najeeb, Z. Hussain, Fundamentals and photocatalysis of methylene blue dye using various nanocatalytic assembliesa critical review, J. Clean. Prod. 298 (2021) 126567, https://doi.org/10.1016/j. jclepro.2021.126567. [40] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, et al., Photocatalytic degradation of various types of dyes (alizarin S, crocein orange G, methyl red, Congo red, methylene blue) in water by UV-irradiated titania, Appl. Catal. B Environ. 39 (1) (2002) 75–90, https://doi.org/10.1016/S0926-3373(02) 00078-4. [41] J. Schneider, D. Bahnemann, J. Ye, Puma G. Li, D.D. Dionysiou (Eds.), Photocatalysis, Royal Society of Chemistry, Cambridge, 2016. [42] N. Serpone, Y.M. Artemev, V.K. Ryabchuk, A.V. Emeline, S. Horikoshi, Light-driven advanced oxidation processes in the disposal of emerging pharmaceutical contaminants in aqueous media: a brief review, Curr. Opin. Green Sustainable Chem. 6 (2017) 18–33, https://doi.org/10.1016/j.cogsc.2017.05.003. [43] J. Peuravuori, Aquatic photochemistry of paracetamol in the presence of dissolved organic chromophoric material and nitrate, Environ. Sci. Pollut. Res. Int. 19 (6) (2012) 2259–2270, https://doi.org/10.1007/s11356-011-0730-y. [44] ˆ A. Almeida, M. Sol´ e, A.M.V.M. Soares, R. Freitas, Anti-inflammatory drugs in the marine environment: bioconcentration, metabolism and sub-lethal effects in L. Dufner et al. Open Ceramics 18 (2024) 100599 8 marine bivalves, Environ. Pollut. 263 (Pt A) (2020) 114442, https://doi.org/ 10.1016/j.envpol.2020.114442. [45] O.S. Folarin, A.A. Otitoloju, N.H. Amaeze, Comparative ecotoxicological assessment of acetaminophen and diclofenac using freshwater african catfish Clarias gariepinus (burchell 1822), J. Appl. Sci. Environ. Manag. 22 (9) (2018) 1519, https://doi.org/10.4314/jasem.v22i9.26. [46] M. Patel, R. Kumar, K. Kishor, T. Mlsna, C.U. Pittman, D. Mohan, Pharmaceuticals of emerging concern in aquatic systems: chemistry, occurrence, effects, and removal methods, Chem. Rev. 119 (6) (2019) 3510–3673, https://doi.org/ 10.1021/acs.chemrev.8b00299. [47] E.J. Rosi-Marshall, T.V. Royer, Pharmaceutical compounds and ecosystem function: an emerging research challenge for aquatic ecologists, Ecosystems 15 (6) (2012) 867–880, https://doi.org/10.1007/s10021-012-9553-z. [48] V. Vaiano, O. Sacco, M. Matarangolo, Photocatalytic degradation of paracetamol under UV irradiation using TiO2-graphite composites, Catal. Today 315 (2018) 230–236, https://doi.org/10.1016/j.cattod.2018.02.002. [49] R. Trujillano, V. Rives, I. García, Photocatalytic degradation of paracetamol in aqueous medium using TiO2 prepared by the sol-gel method, Molecules 27 (9) (2022), https://doi.org/10.3390/molecules27092904. [50] L. Rimoldi, D. Meroni, E. Falletta, A.M. Ferretti, A. Gervasini, G. Cappelletti, et al., The role played by different TiO2 features on the photocatalytic degradation of paracetamol, Appl. Surf. Sci. 424 (2017) 198–205, https://doi.org/10.1016/j. apsusc.2017.03.033. [51] L. Yang, L.E. Yu, M.B. Ray, Degradation of paracetamol in aqueous solutions by TiO2 photocatalysis, Water Res. 42 (13) (2008) 3480–3488, https://doi.org/ 10.1016/j.watres.2008.04.023. [52] N. Jallouli, K. Elghniji, H. Trabelsi, M. Ksibi, Photocatalytic degradation of paracetamol on TiO2 nanoparticles and TiO2/cellulosic fiber under UV and sunlight irradiation, Arab. J. Chem. 10 (2017) S3640–S3645, https://doi.org/ 10.1016/j.arabjc.2014.03.014. [53] Komission Europ¨ aische, Verordnung (EU) 2022/63 zur ¨ Anderung der (EG) Nr.1333/2008 über Lebensmittelzusatzstoffe, 2022. [54] Sabine Juelicher, Director of Food and deed safety. Goodbye E171: The EU bans titanium dioxide as a food additive. Available from:: <https://ec.europa.eu/ne wsroom/sante/items/732079/en>. [55] M. Martín-S´ omer, J. Moreno-SanSegundo, C. ´ Alvarez-Fern´ andez, R. van Grieken, J. Marug´ an, High-performance low-cost solar collectors for water treatment fabricated with recycled materials, open-source hardware and 3d-printing technologies, Sci. Total Environ. 784 (2021) 147119, https://doi.org/10.1016/j. scitotenv.2021.147119. [56] S.J. Khan, R.H. Reed, M.G. Rasul, Thin-film fixed-bed reactor for solar photocatalytic inactivation of Aeromonas hydrophila: influence of water quality, BMC Microbiol. 12 (2012) 285, https://doi.org/10.1186/1471-2180-12-285. [57] M.S. Mehos, C.S. Turchi, Field testing solar photocatalytic detoxification on TCEcontaminated groundwater, Environ. Prog. 12 (3) (1993) 194–199, https://doi. org/10.1002/ep.670120308. [58] J. Aguado, R. van Grieken, M. L´ opez-Mu˜ noz, J. Marug´ an, Removal of cyanides in wastewater by supported TiO2-based photocatalysts, Catal. Today 75 (1–4) (2002) 95–102, https://doi.org/10.1016/S0920-5861(02)00049-4. [59] A. Edrees Wh, A.G. Al-Kaf, K.M. Naji, Q.Y. Mohammed Abdullah, Occurrence of paracetamol in aquatic environments and transformation by microorgan-isms: a review, Chronicles of pharmaceutical science 2017 (1) (2017) 341–355. [60] GEd Nascimento, M.A. Soares Oliveira, R.M. Da Rocha Santana, B.G. Ribeiro, D. C. Silva Sales, J.M. Rodríguez-Díaz, et al., Investigation of paracetamol degradation using LED and UV-C photo-reactors, Water science and technology a journal of the International Association on Water Pollution Research 81 (12) (2020) 2545–2558, https://doi.org/10.2166/wst.2020.310. [61] S.-Z. Chen, P.-Y. Zhang, W.-P. Zhu, Le Chen, S.-M. Xu, Deactivation of TiO2 photocatalytic films loaded on aluminium: XPS and AFM analyses, Appl. Surf. Sci. 252 (20) (2006) 7532–7538, https://doi.org/10.1016/j.apsusc.2005.09.023. [62] H.U. Farouk, A.A.A. Raman, W.M.A.W. Daud, TiO2 catalyst deactivation in textile wastewater treatment: current challenges and future advances, J. Ind. Eng. Chem. 33 (2016) 11–21, https://doi.org/10.1016/j.jiec.2015.10.022. [63] D.V. Dao, M. van den Bremt, Z. Koeller, T.K. Le, Effect of metal ion doping on the optical properties and the deactivation of photocatalytic activity of ZnO nanopowder for application in sunscreens, Powder Technol. 288 (2016) 366–370, https://doi.org/10.1016/j.powtec.2015.11.030. [64] W. Zhang, G. Li, H. Liu, J. Chen, S. Ma, M. Wen, et al., Photocatalytic degradation mechanism of gaseous styrene over Au/TiO2@CNTs: relevance of superficial state with deactivation mechanism, Appl. Catal. B Environ. 272 (2020) 118969, https:// doi.org/10.1016/j.apcatb.2020.118969. [65] M.-G. Jeong, E.J. Park, H.O. Seo, K.-D. Kim, Y.D. Kim, D.C. Lim, Humidity effect on photocatalytic activity of TiO2 and regeneration of deactivated photocatalysts, Appl. Surf. Sci. 271 (2013) 164–170, https://doi.org/10.1016/j. apsusc.2013.01.155. [66] L. Cao, Z. Gao, S.L. Suib, T.N. Obee, S.O. Hay, J.D. Freihaut, Photocatalytic oxidation of toluene on nanoscale TiO2 catalysts: studies of deactivation and regeneration, J. Catal. 196 (2) (2000) 253–261, https://doi.org/10.1006/ jcat.2000.3050. [67] A.M. Dattilo, F. Decembrini, L. Bracchini, S. Focardi, S. Mazzuoli, C. Rossi, Penetration of solar radiation into the waters of Messina Strait (Italy), Ann. Chim. 95 (3–4) (2005) 177–184, https://doi.org/10.1002/adic.200590020. [68] J. Moreno-SanSegundo, S. Giannakis, S. Samoili, G. Farinelli, K.G. McGuigan, C. Pulgarín, et al., SODIS potential: a novel parameter to assess the suitability of solar water disinfection worldwide, Chem. Eng. J. 419 (2021) 129889, https://doi. org/10.1016/j.cej.2021.129889. [69] J. Wang, J. Seyed-Yagoobi, Effects of water turbidity and salt concentration levels on penetration of solar radiation under water, Sol. Energy 52 (5) (1994) 429–438, https://doi.org/10.1016/0038-092X(94)90120-Q. L. Dufner et al.