# Sorption of \({}^{137}\)Cs and \({}^{90}\)Sr on Organic Sorbents
## 5 Conclusions  
The sorption properties of the natural organic sorbents in relation to the \({}^{137}\)Cs and \({}^{90}\)Sr radionuclides depend on various degree of lithification, pH, the quantity and properties of available oxygen-containing functional groups. As a result of the conducted research, it can be concluded that Sr (II) shows a higher sorption than Cs (I) in the studied range of concentrations and pH values. The maximum sorption of Sr (II) reaches 95-100% on high-moor peat and brown coal, and lower values for shungite and hard coal. Sorption of Sr (II) significantly increases with increasing pH and reaches a maximum at pH 5 to 7 and decreases at pH values higher than 7. The maximum sorption of Sr (II) decreases in the order: high-moor peat > brown coal > shungite > hard coal. Sorption of Cs (I) increases with increasing pH, and the highest sorption capacity is observed in samples of brown coal (up to 40%) and high-moor peat (up to 24%). The maximum sorption of Cs (I) decreases in the raw: brown coal > high-moor peat > shungite > hard coal. Sorption values, normalized per surface unit, showed a difference compared with percentage per unit mass. The amount of Cs (I) sorbed on 1 m\({}^{2}\) of the surface of brown coal and high-moor peat turned out to be significantly higher than that on hard coal and shungite. The maximum amount of Sr (II) per 1 m\({}^{2}\) is sorbed on high-moor peat, while brown coal sorbs half as much Sr (II). Shungite sorbs much less Sr (II) than both high-moor peat and brown coal, but almost two times more than hard coal. From the data obtained, it can be concluded that the main mechanism for fixing metal ions is their interaction with negatively charged functional (deprotonated) groups of organic matter. Cs (I) and Sr (II) can be fixed on the surface of organic sorbents in an exchange form. Sr (II) can form stable complexes with oxygen-containing functional groups of organic matter. It is suggested that Cs (I) can be fixed on carboxyl functional groups and, possibly, in insignificant amounts on phenolic hydroxyls of all four studied organic sorbents. The main role in the fixation of Sr (II) on high-moor peat and brown coal would then be played by carboxyl functional groups with pKa values of \(\approx\) 4-7 and \(\approx\) 6-7, respectively. The organic matter in the composition of hard coal and shungite has a high degree of aromaticity and a relatively small total amount of carboxyl and phenolic functional groups, which determines the low sorption capacity of these sorbents. It is therefore difficult to distinguish, with certainty, the role of carboxyl groups and phenolic hydroxyls at low sorption values of Cs (I) and Sr (II) on these sorbents. [PERSON]: prepared material, conceived design of the experiments and wrote the paper; [PERSON] and A.R. : carried out sorption experiments with radionuclides; [PERSON]: carried out X-ray diffraction analysis; [PERSON]: analyzed specific surface area and porosity; [PERSON] and Y.I. : carried out CEC determination, buffering capacity to acid and base and interpretation of the results; [PERSON]: collected the data for mineral analysis and interpretation. All authors have read and agreed to the published version of the manuscript. Sorption experiments and cation exchange capacity of the studied sorbents were conducted with the support of the Russian Science Foundation, project No 18-77-00015. Mineral composition and buffering properties of the sorbents were studied with the support of the Russian Foundation for Basic Research, project No. 18-29-12115. Experimental studies were partially performed using the equipment acquired with the funding of Moscow State University Development Program (X-ray Diffractometer Ultima-IV, Rigaku and Scanning Electron Microscope LEO 1450 VP, Carl Zeiss). This research was performed according to the development program of the Interdisciplinary Scientificand Educational School of M.V. Lomonosov Moscow State University \"future planet and global environmental change\" and RF President stipendium to ASS (SP-4892.2021.2).