# Sorption of \({}^{137}\)Cs and \({}^{90}\)Sr on Organic Sorbents
## 4 Discussion  
It follows from the results of the experiments that the number of moles sorbed per 1 m\({}^{2}\) of SSA of sorbent for all the studied organic sorbents is greater for Sr (II) than for Cs (I). In this case, the sorption of Sr (II) decreases in the order: high-moor peat > brown coal > shungite > hard coal. Cs (I) is sorbed in maximum amounts on brown coal and in smaller amounts on high-moor peat, shungite and hard coal. The high rates of cesium sorption on shungite in previous works [12] apparently indicate a strong influence of the admixture of clay minerals and the importance of a detailed study of the mineral composition of the studied rocks. Since impurities of clay minerals (illite and smectite), which have a high sorption capacity, are present in small amounts (Table 1), it can be assumed that their presence insignificantly affects the results of experiments, and the binding occurs mainly on organic surfaces. The maximum sorption of Cs (I) and Sr (II) cations on all studied sorbents was less than the number of functional groups on the surface of the sorbents (Tables 4 and 6). It can be assumed that not all functional groups are involved in sorption. Firstly, in different pH ranges, not all functional groups will be de-protonated and capable of sorption of cations, and secondly, the bond strength of the metal with the sorbent under different pH ranges can vary. Figure 5: Dependence of the amount of sorbed Sr (II), in % (**a**) and in mmol/m\({}^{2}\) (**b**), on the pH of the equilibrium solution. For (II) and other polyvalent cations have a higher affinity for organic matter than Cs (I) [22; 23; 24; 25]. Therefore, all the studied samples sorb more moles of Sr (II) than Cs (I) per 1 m\({}^{2}\) of surface, despite the fact that 1 mole of Sr (II) requires twice as many sorption sites as does 1 mole of Cs (I) (Table 6). Maxima on the diagrams of the buffering capacity of organic sorbents (Figure 6) can be used for the estimation of the number of functional groups and their ability to participate in protonation-deprotonation reactions in the studied range of pH values. The main functional groups on organic sorbents are carboxyl (-COOH), phenolic (-OH), methoxyl (-OCH3), amino groups, etc. Carboxyl groups and phenolic hydroxyls predominate in humic substances [20; 21; 26]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**mmol eq/m\({}^{2}\)**} \\ \cline{2-3}  & **Cs (I)** & **Sr (II)** \\ \hline Hard coal & 0.029 \(\pm\) 0.002 & 0.308 \(\pm\) 0.025 \\ \hline Brown coal & 0.435 \(\pm\) 0.078 & 2.544 \(\pm\) 0.458 \\ \hline Peat & 0.326 \(\pm\) 0.058 & 3.902 \(\pm\) 0.702 \\ \hline Shungite & 0.029 \(\pm\) 0.002 & 0.80 \(\pm\) 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Maximum sorption of metal ions. Figure 6: Buffering to acid and base according to pH intervals. Carboxyl functional groups are deprotonated in the pH range from 4 to 8, the pKa of which varies from 4 to 5. Phenolic hydroxyls are capable of deprotonation at pH > 8 and are characterized by pKa values of 10-11 [20; 27; 28; 29]. Substituted phenols are titrated at lower pH values. Hard coal has an insignificant buffering capacity for both acid and base (Table 4). On the diagram of the buffering capacity of this sample, maxima are distinguished in the pH intervals of 3.75-4.75 and 6-6.5, provided by the titration of carboxyl groups, and in the pH range of 7.5-8.75, in which phenolic hydroxyls are titrated (Figure 6). Large buffering values in the pH range of 7.5-8.75 in comparison with the acid range are in good agreement with the conclusions about the increase in the degree of aromaticity of organic matter, made on the basis of a change in the H: C atomic ratio (Table 3). Brown coal and high-moor peat are characterized by the highest buffering values among the studied sorbents (Table 4). The maximum buffering capacity of brown coal corresponds to the pH ranges 3.74-4.25, 6.25-7.25, 7.75-8.0 and 9.25-9.75 (Figure 6).