Effect of Ionic Composition and Temperature on the Radiocaesium Fixation in Freshwater Sediments

The objective of this study was to verify in which way the ionic composition of the sediment and that of the overlying water column may have an effect on radiocaesium fixation, through possible structural modifications of the frayed edge sites (FES) pools of the sediments. Two experimental protocols have been considered: i) a condition in which sediments were homoionically saturated with either potassium, ammonium, calcium, magnesium or sodium ions. and ii) a mixed potassium-calcium scenario in the liquid phase. Nine freshwater sediments from four different locations were used in this study. For homoionic potassium and ammonium saturated sediments a nearly quantitative radiocaesium desorption (90–100%) was observed, whereas for calcium and magnesium the desorption yields scene about 20%. It appears that the action of strongly hydrated ions (Na⁺, Mg²⁺, Ca²⁺) leads to a pronounced enhancement of radiocaesium fixation in the solid phase, whereas poorly hydrated ions (K⁺, NH₄ ⁺) have the opposite effect and promote sorption reversibility. Another issue considered in this study concerns the effect of temperature and sediment drying on the radiocaesium fixation. Drying die sediments at 110°C leads to a significant increase in radiocaesium fixation levels, while drying the sediments at room temperature (25°C) has a very limited effect on radiocaesium fixation and appears to put a brake on the aging effects.