Speaker
Description
Phytoremediation is an attractive method for decontamination of soils because of its versatility, eco friendliness and sustainability. It has the potential to reduce the volume of low-level contaminated soil significantly at comparatively low costs, which is of great interest, e.g., regarding the shutdown of German nuclear power plants in 2023. It is long known, that plants can take-up radionuclides from the soil to a certain degree, however in order to use phytoremediation economically the efficiency has to be increased. Therefore, a better understanding of the migration and accumulation of radionuclides from the soil into the plants and within the plants, i.e. from the roots to the upper parts, is essential. Here we present the results of pot experiments using excavated contaminated soils from two sites of the first nuclear reactors built in the former German Democratic Republic. The reactors were in operation for more than 20 years and shut down in the early 1990s, meaning that the soils were contaminated over decades and radioactive and chemical equilibria have been settled.
The soils from both sites are characterized by a high fraction of sand (90 – 99%) and contain significant amounts of activation and fission products as well as transuranic radionuclides, such as Co-60, Eu-152, Eu-154, Eu-155, Sr-90, Cs-137, Am-241 and Pu. The activity concentration in the clay and silt fraction is 10 times higher than in the sand fraction because of the high sorption capacity of clays.
Pot experiments were carried out, growing lucerne (medicago sativa) and sunflowers (helianthus annuus) on the contaminated soils, in a climate chamber with optimized growth parameters (i.e., daylight, temperature and humidity). After three months the plants were harvested, dried, ashed and digested in order to determine the activity of γ-emitting radionuclides by HPGe-detectors. Combining these results with the activities determined in the soil samples gives transfer factors (TFs) for lucerne of 0.24, and 0.31 for Co-60 and Cs-137, respectively. For sunflower transfer factors of 0.35 for Co-60 and 0.15 for Cs-137 were determined.
Furthermore, sequential extraction has shown that only ~50 % of the Cs-137 in the soils is bioavailable for the plants. In order to increase the bioavailability and potentially the transfer of radionuclides into the plants, biodegradable phosphonates are added to the soils, expected to significantly enhance the mobility of some radionuclides.
The focus of future experiments will lie on determining TFs for the transuranic elements (Pu and Am) and on increasing the bioavailability of radionuclides by adding phosphonates and fungi to the soils, to increase the efficiency of phytoremediation.