###### Abstract
Due to the increased interest in the problem of sorption and concentration of \({}^{137}\)Cs from liquid media of complicated composition, all over the world, there is a continuous search, development, and improvement of sorption materials with characteristics such as high selectivity to the radionuclide, sorption efficiency, mechanical and chemical strength, as well as simplicity of production and low cost. Thus, for \({}^{137}\)Cs sorption, a selective resorcinol-formaldehyde resin (RFR) was proposed [5], the advantage of which is the possibility of its repeated use after the stages of radionuclide elution and regeneration. A composite material based on RFR with different contents of barium silicate was created, tests of which showed that an increase in the concentration of barium silicate leads to a decrease in cesium distribution coefficients [6]. High values of cesium distribution coefficients, when recovered from acidic, neutral, and alkaline media, were also obtained on sorbents based on sodium titanosilicate [7; 8]. The most widely used sorbents for the \({}^{137}\)Cs recovery are those based on ammonium phosphomolybdate (AMP) [9; 10] and transition metal ferrocyanides [4; 11]. Various c supports are impregnated with phosphomolybdate: PAN fiber (AMP-PAN) [9], silica gel (AMP-SiO\({}_{2}\)) [10], and others. However, it should be noted that sorbents based on AMP are prone to adsorption of \({}^{40}\)K, which leads to an overestimation of the result of measuring \({}^{137}\)Cs activity; therefore, sorbents based on transition metal ferrocyanides are preferable [4]. Transition metal ferrocyanides can be used to extract \({}^{137}\)Cs in pure form [12] or deposited on a support: PAN fiber (KNiFC-PAN) [4; 13; 14], silica gel (SiO\({}_{2}\)-KMCHCF [15], FSS [16], FC-Cu-EDA-SAMMS [17]), aluminosilicates (Niket-k) [18; 19], chitin-containing materials (FFCG-SO\({}_{4}\)-FC [20], Mikoton-Cs [21], CFC Zn-K, CFC Cu-K, CPC Ni-K [22]), hydrated zirconium dioxide (Thermoxide 35) [16; 23; 24], activated carbon (C/NiHCFe, C/InHCFe, C/CuHCFe) [25], cellulose support (Anfezh [16; 19; 26], Uniket [18], NKF-C [27; 28], FN and FN-50 [19; 29]), natural minerals (diatomite (FND) [19; 24], tripolite [30], clinoptiolite (NPF-CL) and quartz-glauconite concentrate (NPF-GL) [23; 31; 32]) and others. Zeolite-imidazole frameworks [33], mesoporous silica gels [34], aerogels [35], and hydrogels [36], which showed high sorption characteristics in cesium recovery, should be mentioned as new promising supports. However, their widespread use is hampered by commercial unavailability. In addition, under dynamic conditions, these sorbents have high hydrodynamic resistance and do not allow for achieving high transmission rates of the test solution. The use of natural raw materials, including renewable ones, as a support can significantly reduce the cost of sorbents in industrial production. Most often, natural biopolymers are used in the form of cellulose raw materials. However, it should also be noted that the sorbents obtained from natural cellulose initially contain \({}^{137}\)Cs radionuclides up to 36.3 mBq/g, which complicates their use in the concentration and determination of \({}^{137}\)Cs in environmental objects, including natural waters [31]. The physicochemical properties of the support (matrix), in particular the presence of functional exchange groups, determine the peculiarity of the transition metal ferrocyanide fixation on an inert support. Fixation of the ferrocyanide active phase on supports that do not have ion exchange properties occurs due to adhesion or mechanical fixation of transition metal ferrocyanide crystals. If the support contains functional groups, additional electrostatic interaction of ferrocyanide particles with them takes place [37; 38; 39]. Modification of ion exchange resins is one of the first methods for the synthesis of composite sorbents [40]. A feature of such material synthesis is the need for sequential processing of the ion exchange resin with solutions of transition metal salts and potassium ferrocyanide.