## 1 Introduction  
Colouring compounds in industrial wastewater pose a threat to the environment as they are not only very difficult to remove, but are also hazardous due to their toxic properties, low biodegradability and intense colour. Their presence in surface waters significantly affects the entire ecosystem by interfering with the process of photosynthesis, thus causing a direct, harmful effect on living organisms [1]. These compounds are also harmful to the human body and can cause various diseases. Cationic dyes can change the colour of water at concentrations as low as 0.1 mg/L, which impairs the transparency of water bodies [2]. Conventional biological wastewater treatment plants are not able to deal with this type of pollution with sufficient efficiency [3]. In addition, the degradation products of colourants are very toxic and mutagenic for fauna and flora. Therefore, new, more efficient systems for the removal of dyes from industrial wastewater are being sought. Various biological, chemical and physical methods for the treatment of wastewater containing dyes, such as chemical coagulation [4, 5], electrocoagulation [6, 7, 8, 9], ion exchange [10, 11], activated sludge [12, 13], biodegradation [14, 15, 16, 17], photocatalysis [18, 19, 20], advanced oxidation [21, 22, 23, 24], UV/H\({}_{2}\)O\({}_{2}\) process [25], membrane processes [26, 27, 28, 29], osmosis [30] or nanofiltration [31, 32, 33, 34], are widely described in the literature. The advantages of adsorption in the abovementioned methods of dye removal are its low cost, which is often limited to the cost of the sorbent or its transport; the effective removal of toxic substances; and the fact that no additional reagents are required. In addition, adsorption does not cause sludge formation or salinisation of the wastewater. After the adsorption process, the colourants can be easily and quickly separated from the purified solution together with the adsorbent. This technology is considered environmentally friendly. The efficiency of adsorption depends on the specific surface area of the sorbent, the pH value of the purified solution, the initial concentration of the solution, the process duration and the molar mass of the adsorbate. It is also important to choose the right adsorbent depending on the type of impurity. Sorbents can be waste materials that are often a by-product of industrial processes and do not have a high commercial value. Wastes from whichorbents with a high efficiency of pollutant removal can be obtained are usually sought in the food industry, agriculture and wood processing. These industries produce large quantities of seeds, husks, sawdust, bark and pomace that can be used as sorbents. These by-products consist largely of polymers--cellulose and lignin as well as hemicellulose--which contain many functional groups that can form bonds with colourants. Tree bark or sawdust is waste that is produced during wood processing. These products have recently become increasingly popular as sorbents due to their low cost, high availability and good sorption properties [35]. The cost of bark and sawdust from forestry waste is usually only related to the cost of transport from the place of storage to the place of use. The binding of pollutants by bark or sawdust can be achieved by physical or chemical adsorption [36,37]. The fundamental factor influencing the occurrence and efficiency of the sorption process is the specific chemical structure of the bark. The structure of the bark consists mainly of organic compounds, i.e., cellulose (about 45%), hemicellulose (about 30%) and lignin (approx. 20%), as well as proteins, starch and tannins [38]. Sawdust is characterised by the presence of functional hydroxyl (-OH) and aldehyde (-CHO) groups, which enable the binding of impurities (e.g., dyes) [39]. Data from the literature show that sawdust can be successfully used as a sorbent to remove dyes from industrial wastewater [40,41,42,43,44,45,46].