# Sorption of \({}^{137}\)Cs and \({}^{90}\)Sr on Organic Sorbents
## 4 Discussion
The diagrams of the buffering capacity of high-moor peat show maxima in the ranges of pH values of 4.25-5.25; 6.5-6.25; 6.5-6.75 and 7.75-10 (Figure 6). The first three ranges on the described buffering diagrams are provided by the deprotonation of carboxyl groups, and the last one--by the titration of phenolic hydroxyls of humic substances and lignin. Shungite has the lowest buffering capacity for both acid and base among the studied sorbents (Table 4, Figure 6). Based on the structure of shungite described above, it can be assumed that the shungite material contains the smallest number of functional groups capable of interacting with metals in comparison with the other studied sorbents. An increase in the sorption of Cs (I) ions with increasing pH is characteristic of all the studied sorbents and is explained, firstly, by the deprotonation of carboxyl and phenolic functional groups. A significant increase in the sorption of Sr (II) ions on high-moor peat occurs with an increase in pH values up to \(\approx\) 5 and is maintained at a constant level in the pH range of 5-6. Based on the titration results shown in Figure 6, it can be concluded that the main role in the fixation of Sr (II) ions on high-moor peat is played by carboxyl functional groups with pKa of \(\approx\) 4-7. The main role in the Sr (II) sorption on the surface of brown coal is played by functional groups of the same nature, but with higher pKa values of 6 to 7 (Figure 6). A decrease in the sorption of Sr (II) on high-moor peat at pH > 6 can probably be explained by a decrease in the bond strength of the cation with the sorbent. It is possible that phenolic hydroxyls do not play a significant part in the sorption of Sr (II) on the surface of the high-moor peat. Similar dependences on pH were obtained when studying the sorption of \({}^{110m}\)Ag, \({}^{60}\)Co and \({}^{65}\)Zn by humic substances extracted from peat [30]. Since the amount of Sr (II) sorbed on brown coal turned out to be much less than the number of functional groups potentially capable for sorption, the observed plateau on the sorption curve (Figure 4b) can be explained with the assumption that phenolic hydroxyls do not take part in sorption, and that the carboxyl groups available for interaction are completely filled. The sorption of Sr (II) on shungite and hard coal slightly increases with increasing pH (Figure 5). Considering that the organic matter in the composition of these sorbents has a high degree of aromaticity (Tables 3 and 4) and a relatively small total amount of carboxyl and phenolic functional groups, it is not possible to determine the role of carboxyl groups and phenolic hydroxyls at low sorption values. 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 [22; 31; 32]. Since experiments on the sorption of metals on organic sorbents were carried out with a wide range of pH values, it is obvious that the surface properties will change with a change in pH. Thus, under alkaline conditions, the leaching of humic substances increases. It has been shown that alkaline treatment of coals leads to an increase in the surface area and total pore volume [33]. In addition, humic substances undergo conformational and configurational changes during the pH change from 2 to 10 [20; 34]. Such changes can lead to a situation where some of the functional groups, capable of ionization at a given pH value, occur inside the molecule at a different pH level and thereby become inaccessible to the sorbate.