# Sorption of \({}^{137}\)Cs and \({}^{90}\)Sr on Organic Sorbents
## 2 Materials and Methods  
For the research, the following samples of natural sorbents were selected: peat from the \"Staroselsky Mokh\" high-moor bog (Tver region), brown coal from the Pavlovsky deposit (Primorsky Territory), hard coal from the Chernogorsky deposit (Republic of Khakassia) and shungite from the Zazhoginsky deposit (Republic of Karelia). All samples were dried at 90 \({}^{\circ}\)C and milled to a fraction of particle size smaller than 69 \(\upmu\)m.  
X-ray diffraction patterns were obtained with an X-ray diffractometer Ultima-IV (Rigaku, Tokyo, Japan) acquired with the funding of the Moscow State University Development Program (Cu-K\(\alpha\) radiation, semiconductor 1D detector D/Tex-Ultra, scan range 3-65\({}^{\circ}\)20, scan speed 3\({}^{\circ}\)20/min and step--0.02\({}^{\circ}\)20). Non-textured powder specimens were prepared by sieving the sample powder into a sample holder and cutting up the excess material with a razor. The mineral composition was analyzed according to the method of [13], the quantitative composition was estimated using the Rietveld method [14] with the Profex software (Version 3.14.3, [PERSON], Solothurn, Switzerland, 2019) [15]. Chemical analysis was performed using the X-ray fluorescence method in accordance with standard procedure using the spectrometer Axios mAX (PANalytical, Almelo, The Netherlands) at IGEM RAS (Moscow). The samples were dried at 110 \({}^{\circ}\)C and prepared by fusion with lithium borate at 1200 \({}^{\circ}\)C. The iron content was determined only as total Fe2O3, regardless of the actual valence state of the Fe. The effective cation exchange capacity (CEC) was determined as the sum of exchangeable Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\) and exchangeable acidity. Exchangeable Ca\({}^{2+}\), Mg\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\) were displaced by an ammonium ion from an unbuffered 1M NH4 Cl solution [16]. The displaced cations were determined by inductively-coupled plasma optical emission spectrometry (Agilent 5110). Exchangeable acidity was determined in 1M KCl solution (the ratio of the solid:liquid phase was 1:25) by titration using a Mettler Toledo DL 58 autofluorescence. The microstructure of the sorbents was studied with a scanning electron microscope (SEM) LEO1450 VP (Carl Zeiss, Oberkochen, Germany). Samples for SEM were prepared in the form of individual particles and aggregates. A sample in powder form was deposited on double-sided electrically conductive adhesive tape. Then the excess sample particleswere removed using compressed air. As a result, a monolayer of individual particles and aggregates on an adhesive tape was obtained. The studied surface was coated with a thin gold film, 5-10 nm thick, under vacuum. A conductive coating is required to avoid the electrical charging of the sample during analysis. The total buffering capacity of sorbents to acid and to base was determined by potentiometric titration from the initial titration point (pH\({}_{\text{tip}}\)) to pH 3.25 and to pH 10 by 0.02 N HCl and 0.02 N NaOH solutions, respectively. 5 mL of distilled water was added to 0.5 g of sorbent and the resulting suspension was titrated in a conditioned atmosphere (atmospheric air devoid of CO\({}_{2}\)), with constant mixing. An additional portion of titrate was added after 2 min. The volume of titrant portion used during titration of NaOH and HCl for high-moor peat and brown coal was 0.2 mL and 0.05 mL, respectively. For the titration of the other sorbents, 0.05 mL of acid or alkali was added to the samples. The buffer capacity of the sorbent with respect to acid or base, \(\beta\), at each titration point was estimated by the following formula:  
\[\beta=\Delta Ci/\Delta pH, \tag{1}\]  
where \(\Delta Ci\) is the change in the concentration of H+ or OH\(-\) during titration with acid and base, respectively, and \(\Delta pH\) is the change in pH after adding each portion of the titrant.