## 1 Introduction  
Dichlorane is one of the most frequently detected pharmaceuticals in the environment worldwide [1]. It is a nonsteroidal anti-inflammatory drug, which prevents inflammation by inhibiting prostaglandin synthesis and is used to reduce pain and to treat inflammatory diseases [2]. After consumption, diclofenac is excreted in a metabolized and parental form [3]. In Germany some 85 tons of diclofenac are disposed of per year [4]. The release of diclofenac into the environment leads to a high ecotoxicological risk [5; 6] and phototoxicity [7]. The predicted no-effect concentration for diclofenac was set to 0.1 \(\upmu\)g L\({}^{-1}\) by the European Commission in 2015 [8]. In Germany the yearly average concentration of diclofenac exceeded the defined predicted no-effect concentration with values up to 1 \(\upmu\)g L\({}^{-1}\) in 2015 at 21 out of 24 monitoring stations [4]. In 37 countries (out of 50) the reported diclofenac concentrations found in water exceed the predicted no-effect concentration of 0.1 \(\upmu\)g L\({}^{-1}\)[1]. Diclofenac evidently shows toxic effects at these concentrations present in the surface water [5; 6] and was the cause of a mass extinction of vultures in India and Pakistan [9; 10]. The main route of diclofenac into the environment is through wastewater treatment plants (WWTPs) [11]. Elimination of diclofenac in WWTPs is insufficient, as they were not designed to eliminate pharmaceuticals like diclofenac from water. The estimated removal efficacy for diclofenac using conventional WWTPs vary from 39% [12; 13] over 43% [14] to 70% [15]. In activated sludge only 30 to 35% of diclofenac is removed [16]. The current state to cope with diclofenac in WWTPs is to use an additional treatment stage. Since 2016 Switzerland regulates the concentration of certain micropollutants in water [17], thus scientists look closely at Switzerland's strategy and experience. Two main treatment methods are used in Switzerland [17]: ozonation [18] and activated carbon [19]. Both processes are accompanied by high operating expenses like pre-treatment and post-treatment [20; 21; 22]. Advanced oxidation processes (AOPs, like photocatalysis, photo-Fenton, ozonation or electrochemical oxidation) are highly discussed and investigated processes to eliminate micropollutants from water [23; 24; 25; 26]. Reactive oxygen species (hydroxyl radicals, hydrogen peroxide, ozone, and superoxide anion radicals) are generated and completely mineralize micropollutants with a low selectivity to carbon dioxide, water, and inorganic ions or acids. The main drawbacks of photocatalysis are the high band energy of the semiconductor material, which are mostly not visible light active and show poor activity, agglomeration of used nanoparticles, the difficulty to separate the catalyst after photocatalysis from the aqueous media and the reusability of the catalyst [27]. Ozonation or degradation of pollutants via hydrogen peroxide are always accompanied by the addition of toxic and expensive oxidants. Also, AOPs generate several oxidation by-products with a high toxic potential [28]. Conventional treatment methods have many problems and drawbacks. Therefore, further studies to evaluate new strategies to remove pollutants like diclofenac from water are highly needed. In the environment, diclofenac is degraded by sunlight in two different pathways. One way is the direct photolysis caused by absorption of solar radiation. The other way is the indirect degradation caused by strong oxidizing species like hydroxyl radicals or singlet oxygen formed due to solar radiation [29; 30; 31]. The reduction of diclofenac under irradiation is influenced by the absorption cross section of the UV light and the quantum yield [32]. The UV absorption cross section is expressed as a molar extinction coefficient, which measures how strongly a chemical specie absorbs light at a certain wavelength [33].