Visible light active photocatalysts for the degradation of drug residues in polluted waters

Abstract

Removal of pharmaceuticals and their residues from water and wastewater streams is an important environmental issue which has gained a huge amount of attention over the past decades. Photocatalysis is one of the remediation approaches to be considered in this regard in the field of water and wastewater treatment. Hence, solar- or visible light- driven photocatalytic processes and visible light-absorbing materials have recently been the focus of many scientific studies. In the current work, two members of the phenothiazine group of antipsychotic pharmaceuticals, namely, methylene blue and chlorpromazine are considered as model pollutants. Within the scopes of this doctoral thesis, initially, the photocatalytic conversion of methylene blue in the presence of seven self-prepared zinc ferrite semiconducting powders is evaluated. Herein, the zinc ferrite samples are characterized and the impact of different synthetic methods on their properties including the crystalline size and purity, the surface area, the flat band potential and the bandgap energy is determined. Moreover, the reported visible light-activity of zinc ferrite as well as the usefulness of methylene blue to determine the activity of this semiconducting material are investigated through a series of detailed wavelength dependent measurements for the photocatalytic conversion of methylene blue. The experimental results reveal that the observed photobleaching of methylene blue under visible light irradiation in the presence of zinc ferrite is rather contributed to the electron transfer from the photo-excited methylene blue molecules adsorbed at the surface of zinc ferrite into the latter’s conduction band. In addition, the photocatalytic conversion of chlorpromazine, another very important member of the phenothiazine group of antipsychotics is studied. Since very little is known about the possibility of the photocatalytic conversion of chlorpromazine and its metabolites under visible light irradiation, this issue is assessed in detail in the current study. Accordingly, the photocatalytic transformation of chlorpromazine in the presence of KRONOClean 7000 (K-7000) under different experimental conditions is evaluated. K-7000 is a visible light-absorbing semiconducting material, more specifically, a carbon-based modified titanium dioxide photocatalyst. It is shown that despite the high stability of chlorpromazine under visible light irradiation, in the presence of K-7000 a complete conversion of this organic compound is achievable. Moreover, a detailed comparison between the photocatalytic and the photolytic conversion pathways of chlorpromazine and the reaction products/intermediates formed upon UV and visible light irradiation under both anaerobic and aerobic conditions is presented in the current study. The intermediates analysis employing the high performance liquid chromatography (HPLC-MS) technique reveals that the main reaction intermediate of the photocatalytic conversion of chlorpromazine is its sulfoxide metabolite. Chlorpromazine sulfoxide is highly persistent upon visible light irradiation even in the presence of K-7000. This observation evinces that under different circumstances, the ability of K-7000 for the photocatalytic conversion of some organic materials might be inadequate. Thus, deeper investigations are carried out concerning the origin of the visible light activity of K-7000 by means of diffuse reflectance transient absorption spectroscopy. The dynamics of the charge carriers alternatively induced by UV or visible light laser irradiation under inert or reactive atmospheres are studied by considering K-7000 as TiO2 sensitized by a carbon-based layer. The experimental results reveal that the ability of K-7000 for the oxidation of methanol under visible light irradiation is extremely limited, while, this photocatalyst is able to successfully oxidize chlorpromazine. These observations obtained from the transient absorption spectroscopy measurements are also supported by a series of photocatalytic experiments performed to convert chlorpromazine and methanol under identical conditions. In this regard, the possible photocatalytic reactions with chlorpromazine and methanol after UV and visible light excitation in K-7000 are determined and compared with each other. Hence, by considering the one-electron redox potentials of the corresponding reactions, a mechanism is proposed from which the varying ability of K-7000 for the photocatalytic conversion of different organic compounds, (i.e., chlorpromazine, methanol and chlorpromazine sulfoxide), is explained. In total, this doctoral dissertation provides a closer insight into the principles of the visible-light driven photocatalytic conversion of organic molecules, specifically the group of phenothiazine antipsychotic pharmaceuticals. This study confirms that the photogenerated charge carriers formed in visible light active semiconductors are likely to have a limited ability to drive oxidation reactions. This property can, however, be used to improve the selectivity of the oxidation half-reaction in photocatalytic processes, so that an undesired pollutant can be oxidized selectively and be transformed into a valuable product.