Studies of Radiocesium Release from Plant Mass Biodegradation
A consequence of the contamination of plants with radiocesium after a nuclear accident or a radiological event with a radioactive source is that the biodegradation of plant tissues accumulated in soil will release the radionuclide, inducing its recirculation in the environment. The relevance and the mechanisms involved in this phenomenon are the questions to be answered in this research. An experimental protocol was arranged in order to distinguish the 137Cs behaviour during biodegradation when plants are contaminated through atmospheric deposition (i.e. external contamination) or soil to root upatake (i.e. internal contamination) and to describe the role of humic acids in the 137Cs transport (Figure 1). Small scale laboratory experiments consisted of: a) preparation of contaminated biomass ; b) biodegradation in containers with regular sampling of compost eluates; c) determination of 137Cs by gamma spectrometry, and d) analyses of humic acids by chromatograpy.
Figure 1. Illustration of 137Cs cycle resulting from plant biomass degradation in soil.
Achievements in 2016
Almost all radiocesium is released from the biomass with water eluates. Small differences are found between the 137Cs concentrations in the residual detritus from the both experimental trials. The lower concentration is in the detritus from internal contamination. The role of humic acids in the 137Cs transport is very important: more than 90% of 137Cs is associated with humic acids.
- Oral presentation - “Behaviour of Cs-137 during biodegradation of contaminated biomass. J. Mihalík, J.A. Corisco and M.J. Madruga”, 2nd International Conference on Radioecological Concentration Processes ( 50 years later), Seville (Spain), November 6th – 9th, 2016“.
Achievements in 2017
When Cs 137 associated with fresh humic acids from the compost reaches the soil, it is redistributed among various organic compounds that are present in the soil system. Almost all Cs 137 is adsorbed on clay or clay coated with humic acids. To estimate its later bioavailability for plants, an extraction with 1 M ammonium acetate was applied. In course of extraction 25 – 45% of Cs 137 was released from the clay.
- Oral presentation - “The role of humic acids in 137Cs mobility. J. Mihalík, J.A. Corisco and M.J. Madruga”, ENVIRA 2017 – 4th International Conference on Environmental Radioactivity, May 29th – June 2th, Vilnius, Lithuania.
An experiment on the analysis of uranium speciation in plant xylem is still ongoing. The experiment demands precise collection of the xylem sap from decapitated sunflower stem. The speciation analyses are provided by J. Marçalo (ESI MS) and C. Fernandes (HPLC) in the scope of a collaborative effort within in the C2TN Thematic Strand “EARTH SYSTEMS, RADIOACTIVITY and CULTURAL HERITAGE / ENVIRONMENTAL SCIENCES “.
Contact persons: Maria José Madruga, José Corisco e Ján Mihalik