## 1 Introduction  
The development of biodegradable polymer composition materials which can decompose under the action of the environment significantly reduces the negative influence of petrol-based polymers on ecosystems [1; 2; 3]. Among the various numerous fields of their application, the biobased polymer systems can be successfully used for the creation of adsorbents for the purification of an aquatic environment from industrial waste includingtoxic heavy metals, particularly iron, which negatively influences human health. According to the requirements of the European Union, the maximum total content of Fe\({}^{3+}\) should not exceed 1.0-1.5 mg/L [4]. At the same time, such methods of water purification of metals including precipitation or flocculation are the low efficiency and demand for the substitution of new, ecologically safe, and efficient sorbents with high productivity. In this connection, the development of sorbents based on the eco-friendly polymers of different classes is a new approach to the solution of this problem. Biodegradable polysaccharide chitosan, obtained by the deacetylation of natural polysaccharide chitin and due to presence of amino groups, possesses a good sorption ability that explains its successful application as a sorbent of heavy metals, including iron [5; 6], but its significant swelling in aqueous media leads to a decrease in the mechanical characteristics [7; 8]. On the other hand, the polyesters polylactide (PLA) and poly(3-hydroxybutyrate) (PHB) derived from natural raw materials as well as their compositions are widely used in various fields [9; 10; 11; 12; 13; 14]. According to European Bioplastics [15], the total global production capacity of bioplastics in 2021 was 2.42 million tons. Out of this, the production of PLA was 18.9%. However, the applications of PLA and PHB are limited by their fragility, which is due to their high crystallinity, low thermal stability, and the low differences between their melting and thermal decomposition temperatures [16; 17; 18]. In this connection, the use of chitosan in compositions with PLA and PHB allows one to obtain biodegradable materials that combine good mechanical characteristics along with a high sorption capacity that determines the prospects of their use as the sorbents of heavy metals from wastewater. The main advantage of such compositions as compared with compositions based on synthetic polymers is their ability to remain biodegradable after the end of their exploitation period, which determines their use as environmentally safe materials. Additionally, as is well known, food packing films used for containing, protecting and prolonging the life of food, which are prepared from petroleum-based raw polymers, cause serious pollution in oceans, lakes, rivers, and agricultural lands [19]. Moreover, this requires determining how the synthetic polymers prepared from petroleum-based raw polymers can be completely replaced by natural polymers, which is also currently a very demanding task. [20; 21]. Earlier, we carried out a comparative study of the thermal properties of binary polyester compositions PLA-PHB [22], as well as the sorption capacity of the ternary compositions PLA-PHB-chitosan with respect to Fe\({}^{3+}\) ions [23], prepared from petroleum-based raw polymers, which cause serious pollution in oceans, lakes, rivers, and agricultural lands. The choice of such compositions was connected with the fact that the polyesters PLA and PHB, which perform reinforcing functions, provide stability to the compositions with chitosan in aqueous media. At the same time, PLA is a cheaper polymer than PHB, while PHB demonstrates an increased ability to biodgrade its base articles after the end of its service life.