TiO2 MOCVD coating for photocatalytic degradation of ciprofloxacin using 365 nm UV LEDs - kinetics and mechanisms

https://doi.org/10.1016/j.jece.2020.104544Get rights and content

Highlights

  • High efficiency was found by using MOCVD TiO2 coating at 365 nm.

  • The influence of a real water matrix on degradation efficiency was investigated.

  • Transformation products were identified by HPLC-MS investigation.

  • A potential degradation mechanisms of CIP degradation was proposed.

  • Kinetics degradation was performed by Langmuir-Hinshelwood model.

Abstract

This work presents a solution for the photocatalytic degradation of the antibiotic ciprofloxacin (CIP) in water, without using P25 TiO2 powder and thus getting rid of expensive separation steps. It consists in using a TiO2 coating that is directly deposited on the optical window of a photocatalytic micro-reactor and 365 nm UV LEDs as radiation source. P25 TiO2 powder was also studied as reference. HPLC-MS was used to determine the transformation products and the pathways reactions. CIP was slowly degraded by the photolysis reaction at 365 nm: (75 % removal after 8 h of UV irradiation). However, no significant decrease of the total organic carbon (TOC) was noticed, thus showing the presence of transformation products not degraded by the action of UV-light alone. For a low catalyst amount (i.e 0.12 g of TiO2, whatever the form, powder or coating, per liter of contaminated water,), excellent CIP degradation by photocatalysis was observed. Complete CIP degradation after 1 h of irradiation was required using P25 and 8 h using TiO2 coating. Different preferential reaction pathways were identified for both TiO2 catalysts. The Langmuir-Hinshelwood model showed a very good representation of the kinetics, unlike its simplified pseudo-first order model. Photocatalysis experiments did not show a complete mineralization (60–70 % of TOC removal), but most of the aromatic transformation products were degraded. The last transformation products were identified as small aliphatic acids. There is therefore a real interest in using MOCVD coating of TiO2 for sustainable wastewater treatment to avoid expensive catalyst separation. A study with a spiked real effluent from a wastewater treatment plant was performed and a satisfactory degradation was obtained. Slower kinetics were found due to the presence of additional organic products and scavenger compounds such as HCO3.

Introduction

Although water is an important resource on earth, its purity is essential for human consumption. In the past decades, many toxic molecules such as micropollutants (i.e. pharmaceuticals products: antibiotics, anti-inflammatories, analgesics) are present in waterways and underground leading to serious consequences for the environment. Even if the concentration of these micropollutants are very low (ranging from ng.L−1 to μg.L−1), anthropogenic activity induces regular increase of their occurrence in natural waters. Due to this increase, global and European environmental standards have been lowered to reduce environmental concerns [1]. Among all micropollutants, the most persistent are fluoroquinolones such as ciprofloxacin (CIP). These molecules are of great danger for the ecosystems due to bioaccumulation, which leads to high toxicity levels [2]. Classical wastewater treatment (i.e. filtration, decantation, biological processes…) cannot totally eliminate these molecules and additional treatments are therefore required. Separation techniques such as adsorption and stripping processes can be used but also degradation techniques such as Advanced Oxidation Processes (AOP) (chemical oxidation/reduction) [3]. AOP degrade the target molecules [4,5] into inorganic compounds (production of water and carbon dioxide) or harmless transformation products [6]. AOP are furthermore very efficient due to the production of strong chemical oxidants, mainly hydroxyl radicals (OHradical dot), which are strongly reactive due to their high oxidation potential [[7], [8], [9]]. Several AOP treatments have been studied and shown to be effective for the removal of fluoroquinolones. Individual processes (ozonation, photocatalysis, Fenton reaction…) and combined processes (O3/UV, O3/H2O2/UV and UV/H2O2) also provide interesting performances [[10], [11], [12]]. Amongst these AOP, photocatalysis is known to be an efficient technique to degrade organic compounds such as fluoroquinolones under both UV [[13], [14], [15]] and visible light [[16], [17], [18]]. This photochemical reaction occurs in presence of a metal oxide semiconductor catalyst. Titanium dioxide (TiO2) is the most commonly used catalyst. Indeed, TiO2 can absorb UV-light (λ < 385 nm) to create free electrons (e-) and free holes (h+) [19]. TiO2 is mostly used due to its low cost, photo-chemical stability. Furthermore, both its specific area and crystalline structures drive its photocatalytic efficiency [20]. The main drawback of using TiO2 is the particle size: it is usually a powder composed of nanoparticles, which are suspected to increase the risk of chronic intestinal inflammation and carcinogenesis [21]. Due to the very small size of the catalyst, an expensive filtration step is therefore required after treatment in order to separate the catalyst from the water. To avoid this separation step, an alternative technique consists of using supported catalysts [22] on classical supports such as glass [23], metal [24], activated carbon [25,26] or directly at the surface of a reactor [27]. To obtain a TiO2 coating on the surface of a support, several techniques are available including sol-gel deposition [28], Metal Organic Chemical Vapor Deposition (MOCVD) [29], impregnation and oxidation processes [30] such as anodization technology [31] or plasma electrolytic oxidation [32]. MOCVD is of great interest since it allows the control of the physical characteristics of the coating, namely the crystalline structure and the morphology. Moreover, this deposition technique is known to ensure adherent and conformal coverage of high aspect ratio surfaces, regardless of the nature of the substrate (metal, glass or silicon). Therefore, this technique has been selected for this work along with glass as the support.

For this study, the selected fluoroquinolone target molecule is the antibiotic CIP, which is representative of a persistent micropollutant after conventional wastewater treatment [33,34]. To degrade this molecule, photocatalysis was investigated using a TiO2 MOCVD coating on the upper glass window of a channel reactor and irradiated by a monochromatic LED lamp at 365 nm. The objective of this work is to study the performance of this catalyst under specific operating conditions where the quantity of catalyst used is 5–10 times lower than most of the work reported in the literature. The performance is then compared to the classical powder catalyst P25. In addition, a precise determination of the transformation products formed has been developed through the use of high-performance analytical techniques. Photocatalysis and photolysis experiments are compared with an objective of identifying the reaction pathways. This study also investigates the choice of an accurate model for the photocatalytic CIP degradation. The use of kinetics models can be an advantage for describing performances and to compare different catalysts. Two categories of models can be used: theoretical and empirical kinetics models [35]. In this study, theoretical kinetics model will be investigated. Most of the authors modelled the photocatalytic degradation with a first-order equation corresponding to a simplified Langmuir-Hinshelwood (Lsingle bondH) model or with a second-order model [36]. However, these models are not always adequate due to simplifying assumptions which can be discussed [37]. Therefore, the L-H model, studied in this study, appears to be a more suitable and very effective modeling method [35]. Some authors also propose modelling by coupling mass-balance equations (considering time, convection and diffusion terms), reaction mechanisms of each species (reaction rate, adsorption coefficient and kinetics constants) [38]. Finally, the influence of the liquid matrix is investigated by replacing distilled water with a real effluent from a wastewater treatment plant.

Section snippets

Chemicals

Analytical grade ciprofloxacin, C17H18FN3O3 (CIP, 98 % purity) with physico-chemical properties given in Table 1, TiO2 powder (DEGUSSA-P25), formic acid, CH2O2 (formic acid, >99 % purity) and Titanium tetraisopropoxide (TTIP, 99.999 %) were purchased from Sigma Aldrich.

A water sample collected from the outlet of a wastewater treatment plant (Castanet-Tolosan, France) was analysed by the Departmental Laboratory 31: Water – Veterinary – Air in Launaguet (France). The following measurements were

Characterization of TiO2 coatings

TiO2 coatings were columnar as shown in the SEM photos given in Fig. 2. Fig. 2(a) shows a top view of the coating indicating full coverage of the substrate and Fig. 2(b) shows a cross section of the coating used to estimate its thickness at 1.6 ± 0.2 μm. The quadratic roughness (31 ± 1 nm) was deduced from AFM measurements.

The crystalline structure was a pure anatase, as confirmed by the XRD analysis: no peaks from the rutile structure were detected according to the JCPDS cards depicted for

Conclusion

The TiO2 coating presented interesting photocatalytic properties even with smaller amount of TiO2 deposited compared to most studies. This study shows that antibiotics such as CIP can be degraded by a photocatalysis reaction using a wavelength present in solar radiation (365 nm) without doping the catalyst. Photocatalysis is confirmed as a non-selective process whereas photolysis is confirmed as a selective one. As expected, CIP can be degraded by photolysis, however most of the transformation

Credit author statement

Caroline Andriantsiferana and Claire Tendero, conceived, supervised and designed the experiments. Thibaut TRIQUET performed the experiments, HPLC and TOC analysis and did the calculation for each kinetics models. Laure Latapie did every HPLC-MS analysis. Claire Tendero did the TiO2 coating on glass by MOCVD with all characterizations of it (SEM, AFM, XRD). Thibaut TRIQUET, Claire Tendero, Caroline Andriantsiferana, Laure Latapie, Romain Richard and Marie-Hélène Manero wrote the paper.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to express kind regards to Pierre Albrand from Laboratoire de Génie Chimique (Université de Toulouse, CNRS, INPT, UPS, Toulouse, France) for his help to create the MATLAB code for kinetics coefficient determination. We would also like to express kind regards to the “Pole LCMS” from Laboratoire de Génie Chimique (Université de Toulouse, CNRS, INPT, UPS, Toulouse, France) for their help for the identification of transformation products by LCMS.

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