NUKLEONIKA 2005, 50(Supplement 1):S29-S37

Caesium bioaccumulation experiments were carried out at 4 to 60°C using natural samples of the lichen Hypogymnia physodes. Thalli were incubated in 2.5 mmol l^-1 CsCl solutions labelled with ¹³⁷CsCl for up to 24 h at pH values from 2 to 10. Bioaccumulation of Cs+ ions in the first phase of the lichen-CsCl solution interaction is rapid, neither pH, nor temperature dependent within the range 4 to 60°C and observed also with the lichen biomass thermally inactivated at 60°C or chemically by formaldehyde. The second phase of ¹³⁷Cs bioaccumulation is time, temperature and pH dependent and is inhibited by formaldehyde and thermal inactivation. The process at the initial concentration C₀ = 2.5 mmol l^-1 CsCl and 20°C reached equilibrium within 12 hours. It can be described by the first order reaction kinetics equation: log [C_t] = 1.89 – 0.00153 t, R = -0.950. Maximal values of Cs-bioaccumulation were observed at 20°C with minimum at 4°C and 40°C and at pH 4-5 with minimum at pH 2 and pH 6. Low caesium efflux values from lichen thalli by water and 0.1 mol l^-1 neutral salts at 20°C and 24 h equilibrium were observed. Efflux characterized by distribution coefficients D = [Cs]_solution/[Cs]_biomass at biomass/solution ratio 1:25 (w/v, wet wt.), decreases in the order: Li⁺– 78 ´ 10^-3 > NH⁴⁺ = K⁺- 15 ´ 10^-3 > Cs⁺ = Na⁺ - 11 ´ 10^-3. Low extractability of caesium from lichen by water and salt solutions can explain long persistent times of radiocaesium contamination sorbed by lichens, observed by many authors in caesium-contaminated forest and mountain regions. Hypothesis of the role of the lichen secondary metabolites as caesium binders is discussed.