Objective
A disturbing feature of soil to plant transfer data is their variability, which is attributed to a variety of soil and plant properties depending on the radioelement studied. In order to reliably assess radionuclide migration in soils or their availability to crops, it is important that mechanisms of radionuclide complexation (immobilization and remobilization) and movement from soil to soil water and thence to plant root be as fully understood as possible. Soil characteristics indeed affect the availability of a nuclide, as well as the efficiency of the uptake process. In the case of caesium for example, the number of specific sites and the levels of K and NH4 regulate the caesium concentration in the soil solution, and the soil solution root transfer process is regulated by the K and NH4 levels in the solution. A separate analysis of these effects may allow a better prognosis to be made on the transfer expected and on the effectiveness of actions meant to reduce translocation.
The transfer of caesium along the soil to soil solution to plant pathway in phytotrons was studied. The transfer function for caesium from nutrient solution to plant varied between 2.2 and 19. Potanium concentration had a minor effect on caesium uptake. The transfer function decreased with increasing calcium concentration in the nutrient solution. Changes in pH did not affect the transfer function. The value of Kd for caesium varied between 250 and 550. The transfer function from soil solution to plant ranged from 1.1 to 34.8. The observed ratio of caesium to potassium for plant the soil solution ranged from 0.062 to 0.49.
It should be noted that the composition of the soil liquid sometimes changed pronouncedly in the course of an experiment, and that the transfer data have not yet been corrected for the change.
Soil water sampling equipment was set up in 4 plots in Cumbria for detailed investigation of radionuclide transfer along the soil to soil solution to plant pathway. Samples were analysed for caesium-137, strontium-90, plutonium-239, americium-241, potanium, calcium, ammonium, pH and conductivity. Results showed that most of the activity was held in the solid phase and was released into solution by desorption from the extensive cation exchange complex.
Hollow fibre ultrafiltration was used to determine the association of radionuclides is soil solution with different molecular size fraction, for soils artificially contaminated with caesium-137, strontium-90, plutonium-239 and americium-241. Up to 86% of the plutonium-239 and americium-241 was associated with high molecular weight material, probably not in bioavailable form. There was a suggestion that changes in chemical form of the isotopes were caused by freezing and thawing.
An investigation of the effects of various common agricultural treatments on the distribution of radionuclides has been carried out. Potassium and ammonium were found to increase the activity concentration of caesium-137 in the liquid phase by factors of about 2 and 4 respectively. Calcium had little effect; the soils under study already had a high calcium content. Application of sodium nitrate increased the desorption rate of strontium-90.
Samples of vegetation were collected from 8 sites in upland Cumbria and analysed for caesium-134 and caesium-137.
Experiments were carried out the determine whether plants differentiate between potassium and caesium in root uptake. A hydroponic system was used to grow tomatoes and to determine the relative amounts of potassium and caesium in the solution, shoots, fruits and calyces.
Various types of soil were analysed and characterized terms of specific radiocaesium interception potentials.
In order to assess the desorption potential of radiocaesium, a comparative study was carried out using a variety of displacing agents. Two groups were considered: firstly, bulky ions or complexes showing high selectivity for the nonspecific sites; secondly, common salts. For the first group of ions, desorption yields in the clay, loam and sandy loam were quite low, but the podzol data showed a different behaviour, with much larger desorption yields. The main conclusion of the study was that study was that the desorption of radiocasesium from specific sites might be drastically different om different soils.
A new procedure has been developed to determine the intrinsic availability of radiocaesium, overcoming the limitations of previous methods.
The ongoing study deals with each of the two aspects of the behaviour of radionuclides in soil and plant systems, that is the chemical speciation in soil systems and the efficiency of the uptake process.
Firstly speciation of caesium (and of some transuranics) in soil systems is studied in relation to major chemical characteristics of the soils (such as soil solution conductivity, K and NH4 levels). This speciation study concerns both naturally and artificially labelled soils, and accentuates the quantification of chemical forms in soil and soil solution. Recently developed methodology to characterize soils and to study speciation is improved and applied. A variety of specific methods are used.
Porous ceramic cups are used by NRPB to isolate the soil liquid phase in a number of UKAE's existing, well characterized sites in Cumbria, and in the lysimeters at NRPB. In laboratory experiments RIVM uses immiscible liquid displacement for the isolation of soil solutions. Ultrafiltration and chromatographic techniques will be used by NRPB and RIVM to investigate the association of radionuclides with different molecular size fractions in soil and soil solution and to monitor time dependent changes in radionuclide speciation. These data can then be related to Cumbrian studies on seasonal trends in radiocaesium (and some transuranic) levels in bulk vegetation being conducted by UKAEA and NRPB.
In order to produce meaningful distribution coefficients, NRPB conducts batch equilibrium experiments under conditions representative of the in situ situation. By studying soil solution and its associated radiolabelled soils as liquid and soil phases the effects of various treatments, and adsorption and desorption can be determined under laboratory conditions before pursuing expensive field experiments. KUL accentuates the quantitative characterization of a broad range of soils in terms of soil chemical parameters which regulate the specific interception potential and the K and NH4 levels, with particular attention being given to reversibility aspects and aging effects. As a result, it will be possible to estimate the effect of amendments on radiocaesium availability in problem soils.
Secondly, the research in which chemical form is studied is complimented by experiments in the field, and in phytotron on the bioaccumulation of caesium (and of some transuranics). The effects on transfer along the soil, soil solution, plant pathway are studied under controlled conditions in phytotron by RIVM and KUL, and validated with references to field observations made by UKAEA and NRPB on upland soils from Cumbria. RIVM, KUL and UKAEA further compare the uptake of plants from soils with the uptake from nutrient solutions in order to better distinguish between soil specific and plant specific phenomena.
The samples on which chemical analyses are performed, are divided from the soil systems used in these experiments. The radionuclide of main interest is Cs137, although some experiments on transuranics are planned as well. The time trend in the immobilization and remobilization of radiocontaminants is studied throughout the growing period in order to explain changes in transfer and to extend the information derived from the detailed soils studies.
The final goal will be a generalized description of the relationship between the concentration and species of a radionuclide present upon the solid phase of the soil, in both the liquid phase and in biota.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- natural sciences chemical sciences inorganic chemistry alkali metals
- natural sciences chemical sciences inorganic chemistry alkaline earth metals
- agricultural sciences agriculture, forestry, and fisheries agriculture horticulture fruit growing
- natural sciences chemical sciences nuclear chemistry radiation chemistry
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3720 BA BILTHOVEN
Netherlands
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