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3D printed floating photocatalysts for wastewater treatment
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3D printed floating photocatalysts for wastewater treatment María J. Martín de Vidalesa, , Antonio Nieto-Márqueza, David Morcuendea, Evangelina Atanesa, Fernando Blayaa, Enrique Sorianob, Francisco Fernández-Martíneza ⁎
a b
Mechanical, Chemical and Industrial Design Engineering Department, ETSIDI, Universidad Politécnica de Madrid (UPM), Ronda de Valencia nº 3, 28012, Madrid, Spain Mechanical Engineering Department, Carlos III University, Avenida de la Universidad nº 30, 28911, Leganés, Madrid, Spain
ARTICLE INFO
ABSTRACT
Keywords: Fused deposition modelling Photocatalysis Polyethylene mesh Wastewater treatment 3D printing
Organic contaminants, specifically contaminants of emerging concern (CECs), have a great environmental impact, since the removal of these pollutants is of great difficulty by conventional treatments and the presence of these pollutants in the aquatic medium, even at low concentrations, is extremely hazardous to human health. Advanced oxidation processes and, specifically, TiO2-photocatalytic process is considered an option with positive results for an efficient treatment. However, the photocatalyst must be accessible to the UV radiation, for the activation of the TiO2. For this reason, it is recommendable to use a floating photocatalyst (with lower density than water) if the UV light comes from the solar radiation, because it will be on the water surface. In addition, this characteristic of the catalyst can entail an increase of the process efficiency if the pollutant is mainly located on the surface of water. In this context, the goal of this work is the preparation of floating photocatalysts for the removal of CECs from wastewater. TiO2 is deposited in low-density-polyethylene (LDPE), support with lower density than water and high stability and resistance to degradation. LDPE-TiO2 mixtures were prepared by different methods: mixing TiO2 and LDPE in a hot-cylinder-mixer or using o-xylene or an anionic surfactant as dispersing agent, in order to increase the dispersion of TiO2 before extrusion. Filaments obtained were printed as meshes in a Fused-Deposition-Modelling 3D-printer. The printed photocatalysts improved the activity in comparison with the plate obtained in the cylinder, used as benchmark. Thus, this study opens the doors to the in-situ treatment of CECs, using floating photocatalysts and solar radiation as the sole reagent, a very economical, efficient, easily implantable and environmentally compatible process.
1. Introduction In recent years, society has gained concern with environmental pollution, specifically with water pollution. A very important source of pollution comes from the pharmaceutical products. These compound are considered by many authors as contaminants of emerging concern (CECs) [1–4], because they exhibit a recalcitrant nature and cannot be easily removed by the conventional treatments [5–9]. Its presence in wastewater entails an issue of major concern not only for this reason, but also because of the acute consequences that may be associated to public health, such as disorders in endocrine and neurological systems, reproductive capabilities and hormonal control or various types of cancer (breast, ovary, prostate, testes, etc.) [10–12]. Because of this occurrence in environment, new treatment approaches are needed. Advanced oxidation processes (AOPs), based on the generation of hydroxyl radicals and other oxidizing agents (%Cl, % SO4,…) of organic matter, are considered by many authors an option with positive results for an efficient treatment of wastewater for the ⁎
removal of all types of organic pollutants [1,13–19]. Specifically, heterogeneous photocatalysis with supported TiO2 (semiconductor that can be activated by radiation with a wavelength in the range 300–380 nm), is considered as a robust technology whose efficiency has been widely demonstrated [20–25], being possible to completely degrade some organic pollutants. The main advantage of the process is its potential to incorporate solar energy in the form of photons, using sunlight as activator. With this background, photocatalysis with TiO2 can be applied for the removal of CECs from wastewater. It is important to take into account that having a floating catalyst (with lower density than water and therefore staying on the surface) leads to an added advantage, because TiO2 will be more accessible to the UV radiation, mainly if it comes from the solar radiation, and because the pollutants are frequently on the surface of water (especially in the case of oil pollutants). Some members of this research group has experience in the preparation of floating adsorbents/photocatalysts with cork as support [26], in order to obtain a high surface area bifunctional material obtained from waste
Corresponding author. E-mail address: [email protected] (M.J. Martín de Vidales).
https://doi.org/10.1016/j.cattod.2019.01.074 Received 26 July 2018; Received in revised form 12 December 2018; Accepted 29 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Vidales, M.J.M.d., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.074
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cork (granulated, caps…) with adsorbent (due to activated carbon) and photocatalytic (due to deposited TiO2) properties. However, cork can be detached during the wastewater treatment, decreasing the efficiency of the process. Thus, in order to obtain a more resistant and durable floating photocatalyst, low-density polyethylene (LDPE) stands as a potentially good support, because this polymer presents a lower density than water and a high stability and resistance to degradation [27,28]. Some authors have studied the wastewater treatment using floating photocatalysts (glass or polymeric) for the removal of different organic pollutants [29–31]. However, in most cases TiO2 is incorporated into commercial substrates of spherical geometry or other geometries with low surface/volume ratio, an important disadvantage for surface processes [32]. The incorporation of the active phase to commercial or labprepared substrates limits the amount of available geometries. This limitation can be solved with the application of 3D printing, technique that allows designing very different geometries that cannot be obtained by other methods of processing of plastics (molding, injection, etc.), looking for high surface/volume ratios. In this context, some studies have been carried out for the removal of organic pollutants using 3D printed photocatalysts, such as the conducted by Stefanov et al. [33], in gas-phase operation mode using cotton cloth impregnated with commercial titanium dioxide or by Hernández-Afonso et al. [34], and Skorski et al. [35] who use ceramic and acrylonitrile butadiene styrene supports, respectively, for the wastewater treatment, obtaining very satisfactory results for the degradation of different organic compounds with removal percentages higher than 80% in all cases. However, these supports are not floating, and this leads to thinking that the process efficiency can be improved thanks to the advantages that floating supports entail with the development of 3D printed floating catalysts. Taking this into account, the goal of this work is to study the removal of ofloxacin (C18H20FN3O4, antibiotic useful for the treatment of bacterial infections), as a model of CECs, from wastewater by photocatalysis with TiO2. Anatase phase is used in order to increase the catalytic activity, as Zhang et al. [36] explain in their research work. Different floating photocatalysts are prepared by mixing LDPE and TiO2, in a cylinder mixer or using o-xylene or a commercial anionic surfactant as dispersing agent before extrusion, looking for an increase of the TiO2 dispersion degree in LDPE. The content of TiO2-anatase and its dispersion degree in the filaments obtained by extrusion are analyzed by X-ray fluorescence and scanning electron microscopy, respectively, evaluating the influence of these parameters on the process efficiency. In addition, in order to improve the catalytic activity, meshes of LDPE/TiO2 were obtained by 3D printing, increasing the surface of the catalyst versus a conventional plate of LDPE/TiO2 (a higher surface/volume ratio is reached). Efficiency of the studied technology was initially evaluated using methylene blue, for a screening of operating conditions. Then, based on this previous evaluation, the removal of ofloxacin was conducted. To the best of author’s knowledge, this
work is the first study dealing with CECs removal from wastewater by photocatalysis with 3D printed floating supports, technology that allows carrying out the treatment at the spill point (industrial effluents, wastewater treatment plants, etc.) using solar radiation as the sole reagent. 2. Materials and methods 2.1. Chemicals and materials Ofloxacin (OFX), methylene blue (MB), o-xylene and TiO2 anatase (nanopowder, < 25 nm particle size) were analytical grade (> 99.0% purity) and supplied by Sigma-Aldrich Laborchemikalien GmbH (Steinheim, Germany). Commercial anionic surfactant was supplied by Henkel Ibérica, S.A. Granulated low-density polyethylene (LDPE Alcudia, PE-003) was supplied by Repsol-YPF. 2.2. Photocatalyst preparation Fig. 1 shows a scheme for the preparation of the different catalysts. Four LDPE-TiO2 photocatalysts were prepared: a plate of 180 × 77 × 2 mm and three different meshes of 25 × 25 × 0.4 mm, 1 mm mesh light ( ± 0.1 mm). The TiO2 loading was set at 1% w/w. The LDPE-TiO2 plate was prepared by in-situ mixture (granulated polymer and powdered anatase) using a hot cylinder mixer (CM) GUIX (cylinder temperature: 160 °C, front cylinder speed: 23.5 m min−1, rear cylinder speed: 19 m min−1, steam pressure cylinder heating: 73.5 N cm-2). This plate was evaluated as obtained, serving as activity benchmark. Then, for the preparation of the printed photocatalysts, the materials fed to the extruder were obtained from the chopping of the plate (LDPE-TiO2/CM) or by previous mixture of granulated LDPE with TiO2 in the presence of o-xylene or a commercial anionic surfactant, used as dispersing agents (LDPE-TiO2/O-xylene and LDPE-TiO2/Surfactant, respectively). The filaments extruded were 3D printed to obtain the final morphology. A square mesh was selected as an appropriate morphology, given its high surface/volume ratio with a mesh size of 1 mm ( ± 0.1 mm). The extruder was a Filafab PRO 350 EX (extrusion temperature: 235 °C, nozzle temperature: 194 °C, nozzle diameter: 1.75 mm, extrusion speed: 4 cm min−1). The 3D Prusa i3 printer uses a BQ hephestos v1.3 control system, Simplify3D software, with an extruder nozzle of diameter 0.4 mm. The design of the geometry was performed with CATIA v5. The FDM technology was used to perform the 3D printing, with a bed temperature set at 45 °C, extruder temperature of 200 °C and printing process speed of 1000 mm min−1. The printing bed was coated with LDPE film and lacquer to get the print stuck to the base during the process and then detached, respectively. No external cooling was needed.
Fig. 1. Scheme of preparation of the different photocatalysts.
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2.3. Analytical procedures
that catalysts were prepared with 1%w/w of TiO2, the use of an anionic surfactant as dispersing agent seems be the best option for a high loading of the oxide. SEM-EDS analysis of the filaments (Fig. 2) shows the surface morphology (first column, I), EDS mapping (second column, II) and EDS analysis (third column, III) of the filaments. In the three catalysts, both highly dispersed and agglomerated particles are observed, not allowing to quantitatively establishing a sequence in terms of dispersion. In fact, when an anionic surfactant or o-xylene were used as dispersing media, no apparent improvement is achieved and significant agglomerations can be observed (Figs. 2b and 2c, section II). EDS analysis (III) shows, in all cases, a significant presence of Ti where the SEM micrographs were taken, confirming that particles observed in SEM analysis correspond to TiO2. The presence of species such as Cl, Na, K, etc. corresponds to possible residues due to the use and/or handling of the catalyst.
Filaments of LDPE-TiO2 were analyzed by X-ray fluorescence (XRF) with a Bruker S2 Puma analyzer and by scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) with a JEOL JSM820 analyzer, in order to obtain the TiO2 loading and its dispersion degree, respectively. The OFX and MB concentrations of the samples taken during the wastewater treatment experiments was measured by UV spectrophotometry in a UVIKON 941 plus analyzer (λOFX = 288 nm, λMB = 664 nm). 2.4. Experimental procedures Bench-scale photocatalysis treatments were conducted under batchoperation mode. The initial concentration of the pollutant was 1–10 mg dm−3 and 1 mg dm−3 for MB and OFX, respectively (synthetic wastewater). Wastewater was stored in a glass tank with agitation where the catalyst was floating. All the experiments kept a constant gcatalyst/dm3solution ratio. An ultraviolet lamp with wavelength in the range 300–380 nm (near ultraviolet) is placed over the catalyst in order to activate the electron leap from the valence to the conduction band. The photon flow was determined by H2O2 (73.5 mM) actinometry [37], obtaining 6.7 × 10−8 einstein s-1, a very low value that demonstrate that results obtained in this study can be improved significantly applying more radiation power.
3.2. Wastewater treatment 3.2.1. Degradation of MB MB degradation experiments were carried out, taking as an initial benchmark the efficiency of LDPE-TiO2/Plate catalyst for the treatment of a wastewater polluted with 10 mg dm−3 of MB. The plate floated on the wastewater, also in contact with UV radiation. Fig. 3 shows the profile of MB obtained during this experiment. Results are compared with those obtained by photolysis and using dispersed (powder) TiO2. The concentration of MB is normalized for a better comparison. When TiO2 is used, the ratio g catalyst/dm3 solution remains constant. As observed, UV radiation entails a slight MB degradation, around 2%, due to the action of hydroxyl radical on the oxidation of organic matter [38]. However, the presence of TiO2-anatase as photocatalyst increases the process efficiency, obtaining a degradation of the pollutant of 8% in two hours. This compound is activated by UV radiation, increasing the formation of hydroxyl radicals and, consequently, the MB removal. Previous studies carried out by Zhang et al. [36], checked that this behaviour is observed to a greater extend when TiO2 is used in anatase phase, as in this study. They found that the supported anataseTiO2 on activated carbon can be excited leading to the production of more hydroxyl radicals than the supported rutile-TiO2 on activated carbon in aqueous solution. For an operation time of 30–80 min, process efficiency is higher when TiO2 is used in dispersed form than with LDPE-TiO2/Plate catalyst. This may be attributed to the TiO2 particles being more accessible to activation by UV radiation. However, after 80 min, similar degradation rate is observed, and both processes attain the same steady state (around 8% of degradation). Thus, a similar photocatalytic efficiency to that attained with dispersed TiO2 can be reached when TiO2 is inserted into a LDPE plate, even if a plate is considered a bad geometry in terms of surface/volume ratio. However, dispersed TiO2 is not a floating catalyst and a later separation stage is necessary, which greatly increases the process costs. In addition, a high amount of TiO2 is underutilized if not accessible to radiation (it is not on the surface of water). The catalytic activity can be improved by increasing the active surface of the photocatalyst by changing the geometry. In this context, meshes of LDPE-TiO2 are prepared by 3D printing, as explained before. Fig. 4 shows the MB concentration profiles for the experiments conducted with the different photocatalysts in the treatment of wastewater polluted with 1–5 mg dm−3 of MB. As observed, in the experiments conducted with LDPE-TiO2/CM mesh (Fig. 4a), for operation time below 30 min, profiles coincide and there is not any influence of the initial concentration of the pollutant on the process efficiency. This fact can be explained taking into account that for this time, an effective activation of the catalyst has not been performed, and only generation of hydroxyl radicals by UV radiation is occurring. However, when a 2% of MB is removed, the experiment
2.5. Enhancement factor The enhancement factor (%) in terms of maximal degradation reached is calculated to evaluate the effect of the use of photocatalyst meshes respect to the results obtained for the plate. It can be calculated according to Eq. 1.
Enhancement factor (%) =
Max . deg .mesh
Max . deg .plate
Max . deg .plate
100
(1)
3. Results and discussion 3.1. Catalyst characterization Characterization of LDPE-TiO2 filaments was carried out in order to correlate the behavior of the different catalysts to their physicochemical properties. Table 1 shows the XRF results. XRF was performed prior to printing for a better accommodation to the sample holder. It is assumed that TiO2 content is not modified upon printing. It is important to take into account that the filaments have not the specific geometry required for the equipment. Thus, the presence of heavy metals is related with the X-ray influence on the sample holder, and only the presence of Ti and LDPE must be considered. These results allow calculating, within the error of the technique, the % w/w of TiO2 in each catalyst: 0.70, 0.42 and 0.82% for LDPE-TiO2/CM, LDPE-TiO2/ O-xylene and LDPE-TiO2/Surfactant, respectively. Taking into account Table 1 X-ray fluorescence analysis obtained for the different filaments. Data shown in %w/w.
Titanium Matrix (LDPE) Heavy metals
LDPE- TiO2/ CM
LDPE- TiO2/ O-xylene
LDPE-TiO2/ Surfactant
0.42 99.50 0.08
0.25 99.57 0.18
0.49 98.85 0.66
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Fig. 2. SEM-EDS analysis of the filaments: SEM surface morphology (I), EDS mapping (II) and EDS analysis (III). Detail: x1500. a) LDPE-TiO2/CM, b) LDPE-TiO2/Oxylene, c) LDPE-TiO2/Surfactant.
carried out with an initial concentration of the pollutant of 1 mg dm−3, starts to increase its process efficiency significantly, reaching a final degradation of 14% in 120 min. Regarding the experiments carried out with initial concentration of the pollutant of 2.5 and 5 mg dm−3, only at the end of the studies, differences on the degradation rate were observed, being the conducted with the lower concentration the most efficient. Thus, the process efficiency decreases as the initial concentration of MB in the reaction medium increases. This behaviour was also observed when a dispersing agent is used in the preparation of the catalyst (Fig. 4b and c) and in the case of o-xylene, this influence seems to be higher when the concentration of the pollutant decreases from 2.5 to 1 mg dm−3. This can be explained on the basis that a higher pollutant concentration can adversely affect the effective arrival of the UV radiation to the catalyst surface, slowing down the activation reactions and therefore, the formation of the oxidizing agents of organic matter [39]. It is important to highlight that some plateau zones are observed, mainly for operation times between 30 and 80 min. This can be attributed to a competitive oxidation between MB and the reaction
Fig. 3. Profiles of MB concentration (normalized) obtained for the wastewater treatment by photolysis (▲), photocatalysis with dispersed TiO2 (□) and photocatalysis with LDPE-TiO2/Plate catalyst (●). [MB]0 = 10 mg dm−3. 4
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Fig. 5. Enhancement factors in terms of maximal degradation of MB reached (▲), and specific activity (○) for the treatment of wastewater polluted with MB by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. [MB]0 = 1 mg dm−3.
Fig. 6. Degradation of OFX by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. Concentration profiles in normalized form. (▲) LDPE-TiO2/CM mesh, (□) LDPE-TiO2/O-xylene mesh, (●) LDPE-TiO2/Surfactant mesh. [OFX]0 = 1 mg dm−3.
effect of TiO2 dispersion (independently of the total loading). Fig. 5 shows these results. As shown in the figure, when the catalyst is used as a mesh, the maximal degradation reached is significantly higher than with the plate geometry, with enhancement factors bigger than 550% in all the cases, attaining the maximal value with LDPE-TiO2/CM mesh: 1340%. The increase of the process efficiency is attributed to the higher active surface available to oxidize the pollutant. In addition, the extrusion and 3D-printing processes of the LDPE-TiO2 mixture can provide additional contact stages that can improve the polymer-TiO2 adhesion. LDPETiO2/Surfactant, with the highest oxide loading, developed the lowest activity improvement, suggesting a poor dispersion of anatase in the polymeric matrix. The maximal enhancement factor is attained using LDPE-TiO2/CM, with a slightly lower TiO2 content, what must be associated to a better dispersion of the active phase when it was incorporated by mixing in the cylinders. LDPE-TiO2/O-xylene catalyst developed a considerable improvement, despite its lower anatase loading. The specific activity results confirm the high dispersion attained when o-xylene is used as dispersing agent, which can be explained on the basis that o-xylene allows etching the LDPE surface and TiO2 is inserted into the polymer [43], favoring a higher dispersion. However, it is important to take into account that the high dispersion degree attained in this case can be related to the low TiO2 concentration
Fig. 4. Influence of the initial concentration of the pollutant on the treatment of wastewater polluted with MB by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. Concentration profiles in normalized form. a) LDPE-TiO2/CM mesh, b) LDPE-TiO2/O-xylene mesh, c) LDPE-TiO2/Surfactant mesh. (●) 5 mg dm−3, (□) 2.5 mg dm−3, (▲) 1 mg dm−3.
intermediates that can be formed during the processes. Thus, in these plateau zones, the oxidation of the intermediates would be prevailing instead of the MB. A similar behaviour was found by Martín de Vidales et al. [40], with a oxidative competition between triclosan and methanol. In this context, many studies in the literature demonstrate the formation of different intermediate compounds before the mineralization of MB. Thus, the oxidation of MB can entail the formation of different polycyclic aromatic hydrocarbons, aromatic and aliphatic hydrocarbons or carboxylic acids before carbon dioxide [41,42]. In order to compare the different photocatalysts, enhancement factor in terms of maximal degradation reached are calculated respect to the values obtained for the plate for studies with an initial concentration of the pollutant of 1 mg dm−3. Specific activity (moles of MB removed/moles of catalyst) is also calculated, in order to analyse the 5
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Fig. 7. SEM analysis of the LDPE-TiO2/CM mesh before (a) and after (b) use.
presents in the catalyst (Table 1). In fact, there seems to be a direct relationship between the concentration of TiO2 and the specific activity. Thus, the challenge lies in getting higher loads of TiO2 using o-xylene as dispersing agent.
Filaments obtained by extrusion were used to print meshes in a 3D printer. - Reproducibility of the process was evaluated and errors lower than 10% were observed, with a comparable activity of the catalysts after three runs. - This study opens the doors to the in-situ treatment of organic pollutants, such as CECs, using floating photocatalysts and sunlight, a promising technology due to its low cost, easy implantation, high efficiency and environmental compatibility.
3.2.2. Degradation of OFX Once the operating conditions of the studied process have been evaluated in terms of MB degradation, the technology is applied to the removal of ofloxacin, as model of CEC (contaminants of emerging concern), with an initial concentration of 1 mg dm−3 and using the different synthetized meshes. Fig. 6 shows a comparison of the results. Fig. 6 shows that, as with MB, the highest degradation of the pollutant is achieved when LDPE-TiO2/CM or LDPE-TiO2/O-xylene meshes are used. In this case, lower activity differences between them were observed. In order to check the stability of the photocatalysts, three runs were conducted with each system (consecutively, with washing with water between cycles and maintaining the same operating conditions), and the surface of the polymer was evaluated with SEM (Fig. 7), before and after use. The catalytic response, as shown with the error bars in Fig. 6, was comparable, with differences lower than 10% in all the cases. The SEM surface morphology of LDPE before and after use, for the LDPETiO2/CM as a representative photocatalyst, shows the surface cracks created on the polymer support after the reaction. Thus, despite the damage occurred on the surface of the polymer, the activity of the photocatalysts was stable after three runs. This is consistent with the reproducibility reported by Valencia et al., [44], Wang et al. [45], and Behnajady et al. [46], for photocatalytic processes using polyethylene supports for the removal of humic acids, methyl orange and tetracycline and p-nitrophenol, respectively, obtaining reproducibilities with errors lower than 12% in all cases.
Acknowledgements The authors acknowledge the collaboration of T. Aguinaco, J. Acosta, C. Alia and C. Moreno (Universidad Politécnica de Madrid) in the use of the cylinder mixer and the extruder. References [1] M. Salimi, A. Esrafili, M. Gholami, A. Jonidi Jafari, R. Rezaei Kalantary, M. Farzadkia, M. Kermani, H.R. Sobhi, Contaminants of emerging concern: a review of new approach in AOP technologies, Environ. Monit. Assess. 189 (2017) 1–22. [2] M.C. Campos-Mañas, P. Plaza-Bolaños, J.A. Sánchez-Pérez, S. Malato, A. Agüera, Fast determination of pesticides and other contaminants of emerging concern in treated wastewater using direct injection coupled to highly sensitive ultra-high performance liquid chromatography-tandem mass spectrometry, J. Chromatogr. A 1507 (2017) 84–94. [3] D.A. Alvarez, K.A. Maruya, N.G. Dodder, W. Lao, E.T. Furlong, K.L. Smalling, Occurrence of contaminants of emerging concern along the California coast (2009–10) using passive sampling devices, Mar. Pollut. Bull. 81 (2014) 347–354. [4] K.A. Maruya, N.G. Dodder, A. Sengupta, D.J. Smith, J.M. Lyons, A.T. Heil, J.E. Drewes, Multimedia screening of contaminants of emerging concern (CECS) in coastal urban watersheds in southern California (USA), Environ. Toxicol. Chem. 35 (2016) 1986–1994. [5] D.H. Quiñones, P.M. Álvarez, A. Rey, F.J. Beltrán, Removal of emerging contaminants from municipal WWTP secondary effluents by solar photocatalytic ozonation. A pilot-scale study, Sep. Purif. Technol. 149 (2015) 132–139. [6] F.C. Moreira, J. Soler, M.F. Alpendurada, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Tertiary treatment of a municipal wastewater toward pharmaceuticals removal by chemical and electrochemical advanced oxidation processes, Water Res. 105 (2016) 251–263. [7] N.M. Vieno, H. Härkki, T. Tuhkanen, L. Kronberg, Occurrence of pharmaceuticals in river water and their elimination in a pilot-scale drinking water treatment plant, Environ. Sci. Technol. 41 (2007) 5077–5084. [8] F. Souza, C. Saéz, M. Lanza, P. Cañizares, M.A. Rodrigo, Towards the scale-up of electrolysis with diamond anodes: effect of stacking on the electrochemical oxidation of 2,4 D, J. Chem. Technol. Biotechnol. 91 (2016) 742–747. [9] D.R. Manenti, P.A. Soares, A.N. Módenes, F.R. Espinoza-Quiñones, R.A.R. Boaventura, R. Bergamasco, V.J.P. Vilar, Insights into solar photo-Fenton process using iron(III)-organic ligand complexes applied to real textile wastewater treatment, Chem. Eng. J. 266 (2015) 203–212. [10] D. Pestana, G. Faria, C. Sá, V.C. Fernandes, D. Teixeira, S. Norberto, A. Faria, M. Meireles, C. Marques, L. Correia-Sá, A. Cunha, J.T. Guimarães, A. Taveira-
4. Conclusions From this work, the following conclusions can be drawn: - Photocatalysis with TiO2 has been studied in bench scale using floating catalysts that provide a better accessibility to the UV radiation. An efficient removal of ofloxacin, as model of CEC, from synthetic wastewater has been observed. - The catalytic activity is improved by increasing the active surface of the photocatalyst. To do this, meshes of LDPE-TiO2 were prepared by different methods: mixing TiO2 and LDPE in a cylinder mixer or using o-xylene or an anionic surfactant as dispersing agent. 6
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Catalysis Today journal homepage: www.elsevier.com/locate/cattod
3D printed floating photocatalysts for wastewater treatment María J. Martín de Vidalesa, , Antonio Nieto-Márqueza, David Morcuendea, Evangelina Atanesa, Fernando Blayaa, Enrique Sorianob, Francisco Fernández-Martíneza ⁎
a b
Mechanical, Chemical and Industrial Design Engineering Department, ETSIDI, Universidad Politécnica de Madrid (UPM), Ronda de Valencia nº 3, 28012, Madrid, Spain Mechanical Engineering Department, Carlos III University, Avenida de la Universidad nº 30, 28911, Leganés, Madrid, Spain
ARTICLE INFO
ABSTRACT
Keywords: Fused deposition modelling Photocatalysis Polyethylene mesh Wastewater treatment 3D printing
Organic contaminants, specifically contaminants of emerging concern (CECs), have a great environmental impact, since the removal of these pollutants is of great difficulty by conventional treatments and the presence of these pollutants in the aquatic medium, even at low concentrations, is extremely hazardous to human health. Advanced oxidation processes and, specifically, TiO2-photocatalytic process is considered an option with positive results for an efficient treatment. However, the photocatalyst must be accessible to the UV radiation, for the activation of the TiO2. For this reason, it is recommendable to use a floating photocatalyst (with lower density than water) if the UV light comes from the solar radiation, because it will be on the water surface. In addition, this characteristic of the catalyst can entail an increase of the process efficiency if the pollutant is mainly located on the surface of water. In this context, the goal of this work is the preparation of floating photocatalysts for the removal of CECs from wastewater. TiO2 is deposited in low-density-polyethylene (LDPE), support with lower density than water and high stability and resistance to degradation. LDPE-TiO2 mixtures were prepared by different methods: mixing TiO2 and LDPE in a hot-cylinder-mixer or using o-xylene or an anionic surfactant as dispersing agent, in order to increase the dispersion of TiO2 before extrusion. Filaments obtained were printed as meshes in a Fused-Deposition-Modelling 3D-printer. The printed photocatalysts improved the activity in comparison with the plate obtained in the cylinder, used as benchmark. Thus, this study opens the doors to the in-situ treatment of CECs, using floating photocatalysts and solar radiation as the sole reagent, a very economical, efficient, easily implantable and environmentally compatible process.
1. Introduction In recent years, society has gained concern with environmental pollution, specifically with water pollution. A very important source of pollution comes from the pharmaceutical products. These compound are considered by many authors as contaminants of emerging concern (CECs) [1–4], because they exhibit a recalcitrant nature and cannot be easily removed by the conventional treatments [5–9]. Its presence in wastewater entails an issue of major concern not only for this reason, but also because of the acute consequences that may be associated to public health, such as disorders in endocrine and neurological systems, reproductive capabilities and hormonal control or various types of cancer (breast, ovary, prostate, testes, etc.) [10–12]. Because of this occurrence in environment, new treatment approaches are needed. Advanced oxidation processes (AOPs), based on the generation of hydroxyl radicals and other oxidizing agents (%Cl, % SO4,…) of organic matter, are considered by many authors an option with positive results for an efficient treatment of wastewater for the ⁎
removal of all types of organic pollutants [1,13–19]. Specifically, heterogeneous photocatalysis with supported TiO2 (semiconductor that can be activated by radiation with a wavelength in the range 300–380 nm), is considered as a robust technology whose efficiency has been widely demonstrated [20–25], being possible to completely degrade some organic pollutants. The main advantage of the process is its potential to incorporate solar energy in the form of photons, using sunlight as activator. With this background, photocatalysis with TiO2 can be applied for the removal of CECs from wastewater. It is important to take into account that having a floating catalyst (with lower density than water and therefore staying on the surface) leads to an added advantage, because TiO2 will be more accessible to the UV radiation, mainly if it comes from the solar radiation, and because the pollutants are frequently on the surface of water (especially in the case of oil pollutants). Some members of this research group has experience in the preparation of floating adsorbents/photocatalysts with cork as support [26], in order to obtain a high surface area bifunctional material obtained from waste
Corresponding author. E-mail address: [email protected] (M.J. Martín de Vidales).
https://doi.org/10.1016/j.cattod.2019.01.074 Received 26 July 2018; Received in revised form 12 December 2018; Accepted 29 January 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Vidales, M.J.M.d., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.01.074
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cork (granulated, caps…) with adsorbent (due to activated carbon) and photocatalytic (due to deposited TiO2) properties. However, cork can be detached during the wastewater treatment, decreasing the efficiency of the process. Thus, in order to obtain a more resistant and durable floating photocatalyst, low-density polyethylene (LDPE) stands as a potentially good support, because this polymer presents a lower density than water and a high stability and resistance to degradation [27,28]. Some authors have studied the wastewater treatment using floating photocatalysts (glass or polymeric) for the removal of different organic pollutants [29–31]. However, in most cases TiO2 is incorporated into commercial substrates of spherical geometry or other geometries with low surface/volume ratio, an important disadvantage for surface processes [32]. The incorporation of the active phase to commercial or labprepared substrates limits the amount of available geometries. This limitation can be solved with the application of 3D printing, technique that allows designing very different geometries that cannot be obtained by other methods of processing of plastics (molding, injection, etc.), looking for high surface/volume ratios. In this context, some studies have been carried out for the removal of organic pollutants using 3D printed photocatalysts, such as the conducted by Stefanov et al. [33], in gas-phase operation mode using cotton cloth impregnated with commercial titanium dioxide or by Hernández-Afonso et al. [34], and Skorski et al. [35] who use ceramic and acrylonitrile butadiene styrene supports, respectively, for the wastewater treatment, obtaining very satisfactory results for the degradation of different organic compounds with removal percentages higher than 80% in all cases. However, these supports are not floating, and this leads to thinking that the process efficiency can be improved thanks to the advantages that floating supports entail with the development of 3D printed floating catalysts. Taking this into account, the goal of this work is to study the removal of ofloxacin (C18H20FN3O4, antibiotic useful for the treatment of bacterial infections), as a model of CECs, from wastewater by photocatalysis with TiO2. Anatase phase is used in order to increase the catalytic activity, as Zhang et al. [36] explain in their research work. Different floating photocatalysts are prepared by mixing LDPE and TiO2, in a cylinder mixer or using o-xylene or a commercial anionic surfactant as dispersing agent before extrusion, looking for an increase of the TiO2 dispersion degree in LDPE. The content of TiO2-anatase and its dispersion degree in the filaments obtained by extrusion are analyzed by X-ray fluorescence and scanning electron microscopy, respectively, evaluating the influence of these parameters on the process efficiency. In addition, in order to improve the catalytic activity, meshes of LDPE/TiO2 were obtained by 3D printing, increasing the surface of the catalyst versus a conventional plate of LDPE/TiO2 (a higher surface/volume ratio is reached). Efficiency of the studied technology was initially evaluated using methylene blue, for a screening of operating conditions. Then, based on this previous evaluation, the removal of ofloxacin was conducted. To the best of author’s knowledge, this
work is the first study dealing with CECs removal from wastewater by photocatalysis with 3D printed floating supports, technology that allows carrying out the treatment at the spill point (industrial effluents, wastewater treatment plants, etc.) using solar radiation as the sole reagent. 2. Materials and methods 2.1. Chemicals and materials Ofloxacin (OFX), methylene blue (MB), o-xylene and TiO2 anatase (nanopowder, < 25 nm particle size) were analytical grade (> 99.0% purity) and supplied by Sigma-Aldrich Laborchemikalien GmbH (Steinheim, Germany). Commercial anionic surfactant was supplied by Henkel Ibérica, S.A. Granulated low-density polyethylene (LDPE Alcudia, PE-003) was supplied by Repsol-YPF. 2.2. Photocatalyst preparation Fig. 1 shows a scheme for the preparation of the different catalysts. Four LDPE-TiO2 photocatalysts were prepared: a plate of 180 × 77 × 2 mm and three different meshes of 25 × 25 × 0.4 mm, 1 mm mesh light ( ± 0.1 mm). The TiO2 loading was set at 1% w/w. The LDPE-TiO2 plate was prepared by in-situ mixture (granulated polymer and powdered anatase) using a hot cylinder mixer (CM) GUIX (cylinder temperature: 160 °C, front cylinder speed: 23.5 m min−1, rear cylinder speed: 19 m min−1, steam pressure cylinder heating: 73.5 N cm-2). This plate was evaluated as obtained, serving as activity benchmark. Then, for the preparation of the printed photocatalysts, the materials fed to the extruder were obtained from the chopping of the plate (LDPE-TiO2/CM) or by previous mixture of granulated LDPE with TiO2 in the presence of o-xylene or a commercial anionic surfactant, used as dispersing agents (LDPE-TiO2/O-xylene and LDPE-TiO2/Surfactant, respectively). The filaments extruded were 3D printed to obtain the final morphology. A square mesh was selected as an appropriate morphology, given its high surface/volume ratio with a mesh size of 1 mm ( ± 0.1 mm). The extruder was a Filafab PRO 350 EX (extrusion temperature: 235 °C, nozzle temperature: 194 °C, nozzle diameter: 1.75 mm, extrusion speed: 4 cm min−1). The 3D Prusa i3 printer uses a BQ hephestos v1.3 control system, Simplify3D software, with an extruder nozzle of diameter 0.4 mm. The design of the geometry was performed with CATIA v5. The FDM technology was used to perform the 3D printing, with a bed temperature set at 45 °C, extruder temperature of 200 °C and printing process speed of 1000 mm min−1. The printing bed was coated with LDPE film and lacquer to get the print stuck to the base during the process and then detached, respectively. No external cooling was needed.
Fig. 1. Scheme of preparation of the different photocatalysts.
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2.3. Analytical procedures
that catalysts were prepared with 1%w/w of TiO2, the use of an anionic surfactant as dispersing agent seems be the best option for a high loading of the oxide. SEM-EDS analysis of the filaments (Fig. 2) shows the surface morphology (first column, I), EDS mapping (second column, II) and EDS analysis (third column, III) of the filaments. In the three catalysts, both highly dispersed and agglomerated particles are observed, not allowing to quantitatively establishing a sequence in terms of dispersion. In fact, when an anionic surfactant or o-xylene were used as dispersing media, no apparent improvement is achieved and significant agglomerations can be observed (Figs. 2b and 2c, section II). EDS analysis (III) shows, in all cases, a significant presence of Ti where the SEM micrographs were taken, confirming that particles observed in SEM analysis correspond to TiO2. The presence of species such as Cl, Na, K, etc. corresponds to possible residues due to the use and/or handling of the catalyst.
Filaments of LDPE-TiO2 were analyzed by X-ray fluorescence (XRF) with a Bruker S2 Puma analyzer and by scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) with a JEOL JSM820 analyzer, in order to obtain the TiO2 loading and its dispersion degree, respectively. The OFX and MB concentrations of the samples taken during the wastewater treatment experiments was measured by UV spectrophotometry in a UVIKON 941 plus analyzer (λOFX = 288 nm, λMB = 664 nm). 2.4. Experimental procedures Bench-scale photocatalysis treatments were conducted under batchoperation mode. The initial concentration of the pollutant was 1–10 mg dm−3 and 1 mg dm−3 for MB and OFX, respectively (synthetic wastewater). Wastewater was stored in a glass tank with agitation where the catalyst was floating. All the experiments kept a constant gcatalyst/dm3solution ratio. An ultraviolet lamp with wavelength in the range 300–380 nm (near ultraviolet) is placed over the catalyst in order to activate the electron leap from the valence to the conduction band. The photon flow was determined by H2O2 (73.5 mM) actinometry [37], obtaining 6.7 × 10−8 einstein s-1, a very low value that demonstrate that results obtained in this study can be improved significantly applying more radiation power.
3.2. Wastewater treatment 3.2.1. Degradation of MB MB degradation experiments were carried out, taking as an initial benchmark the efficiency of LDPE-TiO2/Plate catalyst for the treatment of a wastewater polluted with 10 mg dm−3 of MB. The plate floated on the wastewater, also in contact with UV radiation. Fig. 3 shows the profile of MB obtained during this experiment. Results are compared with those obtained by photolysis and using dispersed (powder) TiO2. The concentration of MB is normalized for a better comparison. When TiO2 is used, the ratio g catalyst/dm3 solution remains constant. As observed, UV radiation entails a slight MB degradation, around 2%, due to the action of hydroxyl radical on the oxidation of organic matter [38]. However, the presence of TiO2-anatase as photocatalyst increases the process efficiency, obtaining a degradation of the pollutant of 8% in two hours. This compound is activated by UV radiation, increasing the formation of hydroxyl radicals and, consequently, the MB removal. Previous studies carried out by Zhang et al. [36], checked that this behaviour is observed to a greater extend when TiO2 is used in anatase phase, as in this study. They found that the supported anataseTiO2 on activated carbon can be excited leading to the production of more hydroxyl radicals than the supported rutile-TiO2 on activated carbon in aqueous solution. For an operation time of 30–80 min, process efficiency is higher when TiO2 is used in dispersed form than with LDPE-TiO2/Plate catalyst. This may be attributed to the TiO2 particles being more accessible to activation by UV radiation. However, after 80 min, similar degradation rate is observed, and both processes attain the same steady state (around 8% of degradation). Thus, a similar photocatalytic efficiency to that attained with dispersed TiO2 can be reached when TiO2 is inserted into a LDPE plate, even if a plate is considered a bad geometry in terms of surface/volume ratio. However, dispersed TiO2 is not a floating catalyst and a later separation stage is necessary, which greatly increases the process costs. In addition, a high amount of TiO2 is underutilized if not accessible to radiation (it is not on the surface of water). The catalytic activity can be improved by increasing the active surface of the photocatalyst by changing the geometry. In this context, meshes of LDPE-TiO2 are prepared by 3D printing, as explained before. Fig. 4 shows the MB concentration profiles for the experiments conducted with the different photocatalysts in the treatment of wastewater polluted with 1–5 mg dm−3 of MB. As observed, in the experiments conducted with LDPE-TiO2/CM mesh (Fig. 4a), for operation time below 30 min, profiles coincide and there is not any influence of the initial concentration of the pollutant on the process efficiency. This fact can be explained taking into account that for this time, an effective activation of the catalyst has not been performed, and only generation of hydroxyl radicals by UV radiation is occurring. However, when a 2% of MB is removed, the experiment
2.5. Enhancement factor The enhancement factor (%) in terms of maximal degradation reached is calculated to evaluate the effect of the use of photocatalyst meshes respect to the results obtained for the plate. It can be calculated according to Eq. 1.
Enhancement factor (%) =
Max . deg .mesh
Max . deg .plate
Max . deg .plate
100
(1)
3. Results and discussion 3.1. Catalyst characterization Characterization of LDPE-TiO2 filaments was carried out in order to correlate the behavior of the different catalysts to their physicochemical properties. Table 1 shows the XRF results. XRF was performed prior to printing for a better accommodation to the sample holder. It is assumed that TiO2 content is not modified upon printing. It is important to take into account that the filaments have not the specific geometry required for the equipment. Thus, the presence of heavy metals is related with the X-ray influence on the sample holder, and only the presence of Ti and LDPE must be considered. These results allow calculating, within the error of the technique, the % w/w of TiO2 in each catalyst: 0.70, 0.42 and 0.82% for LDPE-TiO2/CM, LDPE-TiO2/ O-xylene and LDPE-TiO2/Surfactant, respectively. Taking into account Table 1 X-ray fluorescence analysis obtained for the different filaments. Data shown in %w/w.
Titanium Matrix (LDPE) Heavy metals
LDPE- TiO2/ CM
LDPE- TiO2/ O-xylene
LDPE-TiO2/ Surfactant
0.42 99.50 0.08
0.25 99.57 0.18
0.49 98.85 0.66
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Fig. 2. SEM-EDS analysis of the filaments: SEM surface morphology (I), EDS mapping (II) and EDS analysis (III). Detail: x1500. a) LDPE-TiO2/CM, b) LDPE-TiO2/Oxylene, c) LDPE-TiO2/Surfactant.
carried out with an initial concentration of the pollutant of 1 mg dm−3, starts to increase its process efficiency significantly, reaching a final degradation of 14% in 120 min. Regarding the experiments carried out with initial concentration of the pollutant of 2.5 and 5 mg dm−3, only at the end of the studies, differences on the degradation rate were observed, being the conducted with the lower concentration the most efficient. Thus, the process efficiency decreases as the initial concentration of MB in the reaction medium increases. This behaviour was also observed when a dispersing agent is used in the preparation of the catalyst (Fig. 4b and c) and in the case of o-xylene, this influence seems to be higher when the concentration of the pollutant decreases from 2.5 to 1 mg dm−3. This can be explained on the basis that a higher pollutant concentration can adversely affect the effective arrival of the UV radiation to the catalyst surface, slowing down the activation reactions and therefore, the formation of the oxidizing agents of organic matter [39]. It is important to highlight that some plateau zones are observed, mainly for operation times between 30 and 80 min. This can be attributed to a competitive oxidation between MB and the reaction
Fig. 3. Profiles of MB concentration (normalized) obtained for the wastewater treatment by photolysis (▲), photocatalysis with dispersed TiO2 (□) and photocatalysis with LDPE-TiO2/Plate catalyst (●). [MB]0 = 10 mg dm−3. 4
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Fig. 5. Enhancement factors in terms of maximal degradation of MB reached (▲), and specific activity (○) for the treatment of wastewater polluted with MB by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. [MB]0 = 1 mg dm−3.
Fig. 6. Degradation of OFX by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. Concentration profiles in normalized form. (▲) LDPE-TiO2/CM mesh, (□) LDPE-TiO2/O-xylene mesh, (●) LDPE-TiO2/Surfactant mesh. [OFX]0 = 1 mg dm−3.
effect of TiO2 dispersion (independently of the total loading). Fig. 5 shows these results. As shown in the figure, when the catalyst is used as a mesh, the maximal degradation reached is significantly higher than with the plate geometry, with enhancement factors bigger than 550% in all the cases, attaining the maximal value with LDPE-TiO2/CM mesh: 1340%. The increase of the process efficiency is attributed to the higher active surface available to oxidize the pollutant. In addition, the extrusion and 3D-printing processes of the LDPE-TiO2 mixture can provide additional contact stages that can improve the polymer-TiO2 adhesion. LDPETiO2/Surfactant, with the highest oxide loading, developed the lowest activity improvement, suggesting a poor dispersion of anatase in the polymeric matrix. The maximal enhancement factor is attained using LDPE-TiO2/CM, with a slightly lower TiO2 content, what must be associated to a better dispersion of the active phase when it was incorporated by mixing in the cylinders. LDPE-TiO2/O-xylene catalyst developed a considerable improvement, despite its lower anatase loading. The specific activity results confirm the high dispersion attained when o-xylene is used as dispersing agent, which can be explained on the basis that o-xylene allows etching the LDPE surface and TiO2 is inserted into the polymer [43], favoring a higher dispersion. However, it is important to take into account that the high dispersion degree attained in this case can be related to the low TiO2 concentration
Fig. 4. Influence of the initial concentration of the pollutant on the treatment of wastewater polluted with MB by heterogeneous photocatalysis with LDPE-TiO2 mesh catalysts. Concentration profiles in normalized form. a) LDPE-TiO2/CM mesh, b) LDPE-TiO2/O-xylene mesh, c) LDPE-TiO2/Surfactant mesh. (●) 5 mg dm−3, (□) 2.5 mg dm−3, (▲) 1 mg dm−3.
intermediates that can be formed during the processes. Thus, in these plateau zones, the oxidation of the intermediates would be prevailing instead of the MB. A similar behaviour was found by Martín de Vidales et al. [40], with a oxidative competition between triclosan and methanol. In this context, many studies in the literature demonstrate the formation of different intermediate compounds before the mineralization of MB. Thus, the oxidation of MB can entail the formation of different polycyclic aromatic hydrocarbons, aromatic and aliphatic hydrocarbons or carboxylic acids before carbon dioxide [41,42]. In order to compare the different photocatalysts, enhancement factor in terms of maximal degradation reached are calculated respect to the values obtained for the plate for studies with an initial concentration of the pollutant of 1 mg dm−3. Specific activity (moles of MB removed/moles of catalyst) is also calculated, in order to analyse the 5
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Fig. 7. SEM analysis of the LDPE-TiO2/CM mesh before (a) and after (b) use.
presents in the catalyst (Table 1). In fact, there seems to be a direct relationship between the concentration of TiO2 and the specific activity. Thus, the challenge lies in getting higher loads of TiO2 using o-xylene as dispersing agent.
Filaments obtained by extrusion were used to print meshes in a 3D printer. - Reproducibility of the process was evaluated and errors lower than 10% were observed, with a comparable activity of the catalysts after three runs. - This study opens the doors to the in-situ treatment of organic pollutants, such as CECs, using floating photocatalysts and sunlight, a promising technology due to its low cost, easy implantation, high efficiency and environmental compatibility.
3.2.2. Degradation of OFX Once the operating conditions of the studied process have been evaluated in terms of MB degradation, the technology is applied to the removal of ofloxacin, as model of CEC (contaminants of emerging concern), with an initial concentration of 1 mg dm−3 and using the different synthetized meshes. Fig. 6 shows a comparison of the results. Fig. 6 shows that, as with MB, the highest degradation of the pollutant is achieved when LDPE-TiO2/CM or LDPE-TiO2/O-xylene meshes are used. In this case, lower activity differences between them were observed. In order to check the stability of the photocatalysts, three runs were conducted with each system (consecutively, with washing with water between cycles and maintaining the same operating conditions), and the surface of the polymer was evaluated with SEM (Fig. 7), before and after use. The catalytic response, as shown with the error bars in Fig. 6, was comparable, with differences lower than 10% in all the cases. The SEM surface morphology of LDPE before and after use, for the LDPETiO2/CM as a representative photocatalyst, shows the surface cracks created on the polymer support after the reaction. Thus, despite the damage occurred on the surface of the polymer, the activity of the photocatalysts was stable after three runs. This is consistent with the reproducibility reported by Valencia et al., [44], Wang et al. [45], and Behnajady et al. [46], for photocatalytic processes using polyethylene supports for the removal of humic acids, methyl orange and tetracycline and p-nitrophenol, respectively, obtaining reproducibilities with errors lower than 12% in all cases.
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4. Conclusions From this work, the following conclusions can be drawn: - Photocatalysis with TiO2 has been studied in bench scale using floating catalysts that provide a better accessibility to the UV radiation. An efficient removal of ofloxacin, as model of CEC, from synthetic wastewater has been observed. - The catalytic activity is improved by increasing the active surface of the photocatalyst. To do this, meshes of LDPE-TiO2 were prepared by different methods: mixing TiO2 and LDPE in a cylinder mixer or using o-xylene or an anionic surfactant as dispersing agent. 6
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