## 1 Introduction  
Chromium (VI) is considered to be a bioavailable element due to its good solubility in water, strong oxidation properties and permeability through cell membranes. Unfortunately, chromium (VI) compounds cause toxic, mutagenic and carcinogenic effects on individual organisms [1]. It is more and more often observed that their permissible concentration in surface waters is exceeded several times, which is why it is extremely important to search for effective and economical methods of removing them. Sorption is one of the more effective and more frequently introduced methods [2; 3]. An additional advantage of this method is the possibility to burn a sorbent (with the addition of alkali) and further processing of the adsorbed chromium [4]. This process in a defined system (flow through or non-flow through) lasts until the equilibrium concentration is reached. The time of the equilibrium reaching depends, among others on the adsorbate particle size and its susceptibility to the sorption process. In the state of dynamic equilibrium, there is a specific distribution of the adsorbate between the solution and the adsorbent. This distribution is described by means of adsorption isotherms showing the relationship between the amount of substance adsorbed by the adsorbent mass unit and the equilibrium concentration of the adsorbate at a constant temperature [5]. Among the adsorption isotherms, the equations of Freundlich and Langmuir are particularly useful for the mathematical description of adsorption from dilute aqueous solutions. The Freundlich equation is commonly used, especially in relation to aqueous solutions [6], due to its simple construction, ease of mathematical solution and sufficient accuracy in the description of experimental results. In the sorption process, the most important element is the sorbent used [7; 8; 9; 10]. The most popular and effective sorbent for removal of heavy metals (including Cr(VI)) is activated carbon [11; 12]. The basic reason for the use of fossil carbons as raw materials for obtaining adsorbents is the relatively easy formation of a developed porous structure in them. In order to obtain activated carbons with a broad distribution of pore volume as a function of their radius, it is recommended to use carbons with a low degree of metamorphism and a high content of volatile parts [13]. Carbon for adsorption from solutions is obtained mainly from peat, wood, brown, hard coal and many other materials, e.g., from Peganum harmala seed (PPAC), sunflower seed, and waste biomass [14; 15; 16]. The basic technological operations used to obtain activated carbons are carbonization and activation. Some types of activated carbons, additionally after the activation process, are subjected to various refinement, modifying operations or their surfaces are covered with catalysts [17; 18; 19; 20; 21]. The efficiency of chromium removal in the adsorption process is very different and depends on the type of carbon used, the method of modification and many other factors. [PERSON] et al. [22] observed that the adsorption of Cr(VI) on various activated carbons is best at pH 2-4, and the time to reach the adsorption equilibrium does not exceed 6 h and the adsorption capacities are in the range of 60-75 mg/g. [PERSON] et al. [23] report that activated carbon from Leucaen leuccephala (Lam.) [PERSON] and commercial activated carbon-UCS modified with cationic surfactants (HDTMA and DDAB) used to remove chromium (VI) ion significantly increased the adsorption capacity compared to the adsorbent without modification. Activated carbon (UCS-DDAB) showed a maximum adsorption capacity of 3.46 mg/g. In turn, [PERSON] et al. [24] showed in their research that activated carbon prepared from sunflower head waste, modified with hydrochloric acid to increase porosity and additionally impregnated with Fe\({}_{3}\)O\({}_{4}\), achieved an adsorption capacity of 4.4 mg/g.