## 1 Introduction  
The rapid development of various industrial sectors around the world has recently led to an increase in the diversity of pollutants that reach natural water bodies. Many of these pollutants cannot be removed naturally from these waters. Therefore, they have a long residence time and can have a serious long-term negative effect on these natural systems [1, 2]. A significant category of these persistent pollutants is represented by refractory organic compounds, which include chlorophenols. They end up in wastewater resulting from the manufacture of pesticides, paints, polymers, cosmetics, etc., and because they cannot be removed by conventional methods in wastewater treatment plants, they arrive in natural water bodies [3, 4]. 2,4-dichlorophenol is one of the most toxic chlorophenols with a major negative impact on aquatic life and human health and, therefore, is classified as a priority pollutant in the United States of America (USA) and European Union (EU) [5]. It is also persistent and bioaccumulative in the environment, being suspected of producing carcinogenic and mutagenic effects on living organisms [6]. The World Health Organization (WHO) has set a maximum allowable concentration of 0.1 ug/L in drinking water and 1 mg/L in treated wastewater discharged into natural water bodies [7]. Photochemical advanced oxidation processes are the most widely used methods for wastewater treatment containing refractory organic compounds. Of these, the most popular seems to be the photocatalytic oxidation [8, 9]. Titanium dioxide is the most used photocatalyst due to its advantages such as high stability, low toxicity, availability,low cost, and low environmental adversity [10]. The photocatalyst can be immobilized in the photocatalytic reactor (either by depositing it on a membrane, by adding it to a membrane, or by making a stand-alone photocatalyst membrane), or it can be dispersed in the aqueous solution to be treated. It has been shown that each option to configure the photocatalytic reactor has both advantages and disadvantages depending on the specific application [11]. For example, photocatalytic reactors with dispersed photocatalysts usually have the main advantage of a larger active surface area which generally gives them a higher substrate degradation efficiency than photocatalytic reactors with immobilized photocatalysts. Instead, this type of photocatalytic reactor has the major disadvantage of the need to separate the photocatalyst from the suspension and reuse it [12; 13; 14; 15]. At the same time, photocatalytic reactors with immobilized photocatalyst have, among other things, the great advantage of the possibility of being operated continuously, at least for the time interval in which the membrane has sufficient photocatalytic activity. Regeneration of photocatalytic membranes is usually much easier to achieve compared to dispersed photocatalysts [16; 17; 18; 19]. However, regardless of the reactor configuration, the photocatalytic activity of the system largely depends on its operating parameters, such as the initial organic substrate concentration, photocatalyst loading, photocatalytic reactor and photocatalytic membrane geometry, the physico-chemical characteristics of the aqueous solution subjected to the treatment, light wavelength and its intensity, etc. Despite the many studies conducted in this field that have led to significant achievements, there are still many problems that need to be solved, especially for specific applications and photocatalytic systems designed to work on an industrial scale [20]. Usually, the efficiency of removing the refractory organic material from aqueous solutions depends on the particularities of the photocatalytic system used [21]. For example, studies on TiO\({}_{2}\)-based UV photocatalytic degradation of 2,4-dichlorophenol have shown that the efficiency of removing refractory organic material from aqueous solutions as a function of its initial pH differs from one photocatalytic system to another.