Prozac® photodegradation mediated by Mn-doped TiO₂ nanoparticles: Evaluation of by-products and mechanisms proposal

Abstract

TiO₂ nanoparticles were efficiently doped with Mn applying microwave-assisted synthesis method. Anatase phase was confirmed by XRD pattern, which did not exhibit the peaks associated with MnOx contamination. Raman spectroscopy showed a displacement of up to 5 cm⁻¹ in the O-Ti-O vibrational mode, suggesting that Ti was replaced by Mn. Oxidation state (Mn³⁺) was confirmed via high-resolution XPS spectrum of Mn 2p energy level. Incorporation of Mn into the TiO₂ network resulted in increased surface area (64.6 m² g⁻¹), average particle size up to 16 nm, and bandgap reduction of up to 43 %. The photocatalytic properties of the materials were investigated through Prozac® degradation and improvement of up to 14 % (Ti-0.73 Mn) and 26 % (Ti-0.38 Mn) was seen on comparing doped TiO₂ versus pure TiO₂ during photocatalysis. Furthermore, after Prozac® photodegradation, TFMP was identified as the main by-product while MAEB and PPMA (other by-products) were seen to compete with each other in the degradation mechanism. In addition to the quantitative analysis done for Prozac®, TFMP, MAEB, and PPMA, LC MS-Q-TOF analysis was performed, confirming that in the presence of Mn-doped TiO₂, specific by-products were formed, probably due to more oxidizing species being present in this reaction system. The present study has shown unprecedented evidence that the application of TiO₂ nanoparticles efficiently doped with Mn promotes Prozac® degradation by specific mechanisms.

Introduction

Charge transport has been one of the most researched properties of nanomaterials in recent years due to its importance to different applications such as electrochemical sensors [1], fuel cells [2], photovoltaic devices [3], and photocatalysts for environmental remediation [4]. In light-irradiated materials, photon absorption promotes the valence electron into the conduction band, giving rise to oxidation (h⁺) and reduction (e⁻) sites in their structures [5,6]. However, electron excitation is dependent upon many factors, such as the crystalline structure of the material, bandgap energy, incident photon energy, transport mechanism, and recombination of photogenerated charge carriers [7,8]. Considering that most solar radiation is in the visible region, semiconductors that are able to absorb a wider range of electromagnetic radiation have broader applications [9,10].

TiO₂ is a semiconductor that absorbs UV radiation to promote electronic excitation [11]. When it was modified with dopant elements, its charge transference efficiency and visible light absorption were optimized [9,[12], [13], [14]]. When ZnO, TiO_2, or ZrO₂ semiconductors were doped with Mn and applied in the photocatalytic degradation of dyes, bandgap reduction was achieved, thereby improving semiconductor performance [[15], [16], [17]]. This is because Mn can be incorporated into the TiO₂ network at different oxidation states (2+, 3+, or 4+), distorting its crystalline structure [13,[18], [19], [20]] and optimizing its optical properties. A search of the literature on TiO₂ doped with Mn^{1+ to 6+} confirmed that the Mn^{4 +} dopant had the highest stability [21]. Moreover, Mn^x+ doping in any TiO₂ oxidation state exhibited enhanced photocatalytic properties [21,22], showing that obtaining materials with reduced bandgap energy may prove to be a potential solution in the control of environmental pollution [23,24]. According to the literature, Mn-doped TiO₂ has been efficiently applied to reduce Cr⁶⁺ to Cr³⁺ [25], in chemical speciation of metals/non-metals in environmental and food samples [26,27], and in photocatalytic oxidation of emerging contaminants such as pharmaceuticals [28].

To obtain these materials with optimized electronic optical properties, different synthetic routes have been proposed for doping ceramic materials, particularly the microwave-assisted hydrothermal method (MHM) [29,30]. Compared to other synthesis methods, MHM enabled shorter synthesis times, lower processing temperatures and less waste generation [31,32]. Yet, the current methods of choice for obtaining Mn-doped TiO₂ are still the sol-gel method and the hydrothermal method in autoclave [[33], [34], [35], [36]]. Therefore, considering the advantages of MHM and the few reports in the literature for obtaining Mn-doped TiO₂ [37], investigation into this method of synthesis is shown to be promising.

Photoactive materials have been widely applied in the chemical oxidation of environmentally-persistent organic compounds, also denominated emerging contaminants (EC). One of these ECs is fluoxetine, commonly known as Prozac® and used as an antidepressant. Due to the persistence of Prozac® in the environment [38,39], it is an emerging contaminant that has attracted widespread interest in the field of environmental protection and, consequently, has been investigated in advanced oxidation processes [40,41]. Its high consumption has led to the presence of residues in aquatic ecosystems [42] since Prozac® is photochemically stable under visible radiation [43]. However, Prozac® can be oxidized to different intermediates on application of different advanced oxidation processes [41,44]. Literature data have so far only described qualitatively the formation of these intermediates [41,45], limiting progress in the characterization of Prozac®-contaminated ecosystems. One of the Transformation Products (TPs) reported during Prozac® degradation was 4-(Trifluoromethyl)phenol (TFMP) by the following reaction:

Reaction: TFMP formation mechanism after Prozac® degradation [45,46].

Considering that many of these TPs may be formed in different ecosystems, a quantitative approach to this degradation mechanism of Prozac® is deemed urgent and mandatory. In view of the harmful effects Prozac® exerts on aquatic organisms [38,39], quantitative studies of the main TPs formed are scientifically and environmentally relevant. Also, this approach involving both mechanistic and quantitative aspects is of great importance to understanding the influence that different degradation processes have on the Prozac® degradation mechanism and its relationship to the TPs. This work therefore consists of a quantitative investigation of the photodegradation mechanism of Prozac® using Mn-doped TiO₂ photocatalysts obtained by MHM. This degradation mechanism has been quantitatively investigated using high-performance liquid chromatography (HPLC UV) and LCMS-Q-TOF techniques. It must be underlined that three of the by-products formed were defined as major in the degradation mechanisms studied since such delineation allows for a better understanding of how Prozac® behaves in the environment. Finally, our results showed that the approach to heterogeneous photocatalysis cannot be restricted to the physical-chemical nature of the material or degradation of the main compound since the by-products formed are also scientifically and environmentally relevant.