# Photocatalytic Degradation of Selected Non-Opioid Analgesics Driven by Solar Light Exposure
## 1 Introduction  
Aquatic pollution caused by a great number of pharmaceutical compounds is an environmental problem that has become an important public health issue over the last years [1, 2]. Several laboratory studies have sought to identify the hazards and assess risks in the aquatic environment, and field studies have searched for targeted candidates and occurrence trends. The residues of pharmaceutical compounds are frequently detected in different environmental compartments (drinking water sources, water and sewage treatment plants, and aqua-fanna) due to their universal consumption, low human metabolic capability, and improper disposal [3, 4]. Many of these compounds are resistant to degradation and may remain in the environment over a long time, with the potential to cause adverse effects [5, 6]. Additionally, the excreted metabolites may become secondary pollutants and can be further modified in receiving waters. Various approaches have been conducted for the removal and degradation of pharmaceutical residues, such as membrane filtration [7], activated carbon adsorption [8, 9], bioremediation [10], and advanced oxidation processes (AOP) [11, 12, 13, 14]. AOP methods involve the in situ generation of hydroxyl radicals and include treatments such as ozonation, Fenton oxidation, and heterogeneous photocatalysis. Among them, semiconductor photocatalysis, based on the concept of achieving the degradation of pollutants by solar energy/UV lamps in the presence of a semiconductor photocatalyst, allows rapid transformation of the initial compound into harmless products. The mechanism of the phenomenon was well described by [PERSON] et al. [15]. This reagent-free methodology requires only a source of radiation (UV or sunlight) and a photocatalyst, and compared with other methods, the photocatalysts can be regenerated and recycled without the need for pretreatment. The used photocatalysts should offer the required combination of the properties (activity, selectivity, lifetime, and toxicity) and also be cost-effective [16]. Several semiconductors have been studied in photocatalytic oxidations, such as TiO2, Fe3O4, WO3, Fe2O3, and ZnO, as well as the mixtures of these oxides. Among them, titanium oxide is the most widely applied due to its high oxidation efficiency, non-toxicity, high photostability over abroad pH range, chemical inertness, and low cost [17]. However, despite these advantages, TiO\({}_{2}\) suffers from low photocatalytic efficiency under visible-light irradiation (only 3-5%) and a narrow light response range because of its intrinsic large band gap of 3.2 eV, which restricts its use under visible light. Therefore, solar systems that use only pure TiO\({}_{2}\) have limited efficiency in the degradation of pollutants. Several attempts have been proposed to reduce the band gap energy of TiO\({}_{2}\) and to shift its adsorption to visible light. Moreover, due to the surface limitation of TiO\({}_{2}\) and its non-porous structure, the studies are directed towards photocatalyst incorporation on porous support to obtain efficient absorption of target pollutants before their subsequent oxidation. Various noble and transition metals, carbon-derived nanoparticles, and minerals are doped with TiO\({}_{2}\) to enhance their photodegradation performance [12; 18; 19; 20]. In addition, a hybrid photocatalytic system integrating with membrane filtration has been proposed [11; 21; 22; 23]. The photocatalyst can be suspended in a sample solution or immobilized in/on a membrane [22]. The immobilized system could avoid catalyst aggregation and tedious recycling processes performing high-efficiency removal [22]. Photocatalytic membrane reactors have been applied in water and wastewater treatment to remove pharmaceutical residuals and personal care products. Doped TiO\({}_{2}\) photocatalysts were used for the degradation of some analgesics. With the presence of TiO\({}_{2}\)/KAl(SO\({}_{4}\))\({}_{2}\), 95% removal of 0.10 mM acetaminophen in 540 min irradiation time was achieved [24].