## 1 Introduction  
Industry development is mainly responsible for the introduction of a large volume of wastewater containing phenols into the environment. The phenolic compounds are one of the hugest groups of [1] toxic and corrosive [2] environmental pollutants released from various manufacturing industries, causing many environmental and toxicological problems [3]. The dihydroxybenzene isomers--i.e., catechol (CT), hydroquinone (HQ) and rescorinol (RS)--are common phenol-based compounds. These compounds are characterized by similar physicochemical properties and different locations of the two hydroxyl groups [4; 5]. The CT can be introduced into the environment from both anthropogenic--i.e., cosmetics, pharmaceutical or antioxidant industry--as well as natural sources--i.e., decomposition of humic and lignocellulosic substances [3; 6]. The RS (1,3-dihydroxybenzene) found many applications in dyes, plastic and synthetic fiber production, whereas HQ (1,4-dihydroxybenzene) is widely employed as an industrial solvent [6] and found application in photography and cosmetics for skin pathologies treatment [7]. The existence and co-existence of dihydroxybenzene isomers in nature is a serious hazard to biological systems and the ecological environment [8]. Dihydroxybenzene isomers can enter the human body through mucous membranes, skin [9] and the gastrointestinal tract [10] causing poisoning symptoms and sometimes also headache, pale, dizziness, tiredness or even liver and kidney function damage [9]. In addition, [PERSON] et al. [11] showed higher catechol toxicity compared to phenol because it damages erythrocytes functions at a 5-times lower concentration (50g/L for catechol, 250 g/L for phenol). A combination of their capacity accumulation in the environment [12]; low degradability level; and high toxicity for humans, animals and plants caused CT, HQ and RS to be considered as important environmental pollutants by the US Environmental Protection Agency (EPA) and European Union (EU) [3; 13]. Due to carcinogenicity of dihydroxybenzene isomers even at low concentrations, they are considered as priority pollutants [14]. Due to the complex and stable chemical structure of phenolic pollutants, for their removal from aqueous solution, many conventional (i.e., steam distillation, liquid-liquid extraction, adsorption, solid-phase extraction, wet air oxidation, catalytic wet air oxidation, biodegradation) as well as advanced techniques (i.e., electrochemical oxidation, photo-oxidation, ozonation, UV/H\({}_{2}\)O\({}_{2}\), Fenton reaction, membrane processes and enzymatic treatment) can be applied [15]. Among the conventional methods, adsorption of phenolic compounds [12] using different adsorbents--i.e., commercial and synthetic activated carbons [16; 17; 18; 19], synthetic resins [20; 21], zeolites, industrial waste/by-products i.e., coal [22], fly ash [23], red mud [14; 24] and waste sludge [25; 26]--is the most popular technique. The phenolic compound adsorption uses the fundamental interaction of aromatic rings [17] with adsorbent. However, its practical application has some disadvantages related to regeneration and high cost manufacturing of adsorbent [22]. Moreover, the use of adsorption also causes problems with the storage of the adsorbent containing adsorbed phenolic contaminants. Therefore, for several years, an increase in the use of Advanced Oxidation Processes (AOP\({}_{\text{s}}\)) in wastewater treatment containing phenols has been observed. [27]. Among AOP\({}_{\text{s}}\) is photocatalysis, which emerged in the 1980s. The definition of photocatalysis is still unclear, but usually this process is defined as a chemical reaction induced by photon absorption of a solid material (photocatalyst) which does not undergo chemical changes [28]. In the literature terms, photocatalyst and catalyst are used interchangeably because photocatalytic materials are sometimes used in catalysis. The clear difference between photocatalyst and catalyst terms is seen in terms of thermodynamics.