Mechanisms controlling the migration of radionuclides in the environment
The mechanisms controlling radiocaesium migration through the peat soil were investigated by considering physico-chemical & biological factors affecting radionuclide adsorption/desorption in the peat soil using the Batch & Column Methods as well as a "Direct Fumigation Extraction Method". The biological factors were investigated under different conditions by variation of the micro-organisms content in the peat soil, the soil solution and leaching conditions. Samples of peat soil were reacted with solutions containing caesium ions, and the effect on caesium-Kd determined. The caesium adsorption/desorption was found to be influenced by concentration of caesium and other cations (NH4+, K+ and Ca++) and by the pH of the solution. Below pH 3, in presence of calcium and potassium, desorption could occur as the exchangeable sites on microparticles were replaced by other cations. At pH>1, in presence of ammonium, adsorption could occur. The distribution coefficient, Kd value was strongly reduced following the addition of NH4+, K+ and Ca++ at a fixed pH. NH4+ was found to be more effective in the caesium desorption than the other cations. By increasing the concentration of radiocaesium (>0.2 MBq/100μl) the effect became much less leading eventually to an increase in caesium-Kd values. A decrease in the caesium-Kd was also observed at pH<3 in peat samples. It was found that the caesium adsorption approached to an equilibrium state at pH>3 depending upon caesium concentration in peat solution. Caesium adsorption took 180 days to reach equilibrium state. For the measurement of the release/uptake of radionuclides in the peat soils by the microbial biomass, homogenised wet peat samples were fumigated by chloroform. The influence of the microbial population present in the peat on the environmental mobility of caesium was clearly demonstrated.