# Photo-Transformation of Effluent Organic Matter by ZnO-Based Sunlight Irradiation  
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Correspondence: [EMAIL_ADDRESS] (S.N.N. ); [EMAIL_ADDRESS] (J.O. )1  
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Footnote 1: email: {T.,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,  
by-products [1; 2; 3]. The characteristics and composition of EfOM are strongly dependent on the source of the wastewater and the operating conditions of the wastewater treatment facility. Compared to DOM, EfOM has a higher concentration of hydrophilic organic matter, a higher fluorescence index (FI), a higher proportion of protein-like substances, a greater organic nitrogen content, and a higher molecular weight [3; 4; 5]. The complex composition and chemical properties of EfOM negatively affect the quality of receiving waters and the efficiency of reclamation treatment processes [2; 3; 4]. Therefore, a better understanding of the characteristics of EfOM in receiving waters is important in evaluating and predicting the potential influences of EfOM on the sources of drinking water and in developing optimal and cost-effective treatment methods for drinking-water treatment plants. However, to the best of our knowledge, there have been few studies [3; 6; 7; 8] focused on the characteristics, fate, transport, and impact of EfOM in receiving waters. Recently, heterogeneous photocatalysis has become the most common and efficient method for the removal of organic matter from water and wastewater [9]. The method relies on the _in-situ_ production of highly reactive oxygen species (ROS) (e.g., hydroxyl and superoxide radicals) that decompose organic pollutants into CO\({}_{2}\), H\({}_{2}\)O, and other inorganic substances via the irradiation of a catalyst (e.g., TiO\({}_{2}\), or ZnO) under a light source (e.g., ultraviolet (UV), sunlight, or artificial sunlight) [10; 11; 12]. Previous studies have introduced various photocatalytic approaches [9; 11; 12; 13; 14], including (i) UV photocatalysis (e.g., UV/TiO\({}_{2}\), UV/ZnO), (ii) modified catalysts (e.g., Ag-doped TiO\({}_{2}\), N-doped ZnO, N-doped TiO\({}_{2}\)/graphene/(near) visible light), and (iii) simulated solar photocatalysis with Fe-doped TiO\({}_{2}\) or ZnO. In particular, solar photocatalysis using ZnO, a recyclable and chemically stable catalyst [10], has a high removal efficiency at a low operating cost [11]; thus, this process has been proposed as a cost-effective and sustainable technology for the degradation of organic matter in water and wastewater treatments. The mechanism of ZnO-assisted photodegradation of organic matter under artificial sunlight irradiation is based on the _in-situ_ generation of ROS [10; 11; 12]. When ZnO absorbs photons from artificial sunlight, pairs of valence-band hole \(\left(\text{h}_{\text{vb}}^{+}\right)\) and conduction-band electron \(\left(\text{e}_{\text{cb}}^{-}\right)\) are generated [11]. Subsequently, the valence band holes react with H\({}_{2}\)O and hydroxide ions in order to form hydroxyl radicals, while the conduction band electrons react with O\({}_{2}\) to form superoxide radicals [10; 11; 12]. The hydroxyl and superoxide radicals can oxidize and decompose molecules of organic matter into CO\({}_{2}\), H\({}_{2}\)O, and degradation products [9; 11; 12; 13; 14]. Consequently, the degradations of organic matter are based on three pathways, including oxidation by hydroxyl radicals, reduction by superoxide radicals, and adsorption by catalyst, although hydroxyl radicals are considered to be the dominant oxidant [9]. In this study, the photodegradation of the organic matter present in the receiving water within a ZnO-catalyzed artificial sunlight system is examined.