Association of radionuclides with different molecular size fractions in soil solution: Implications for plant uptake

The feasibility of using hollow fibre ultrafiltration to determine the molecular size distribution of radionuclides in soil solution was investigated. As the soil solution is the intermediary phase in the soil-to-plant transfer of radionuclides, its physical and chemical composition plays a vital role in determining radionuclide uptake by plant roots. Soil solution samples were extracted from loam, peat and sand soils that had been artificially contaminated with ¹³⁷Cs, ⁹⁰Sr, ²³⁹Pu and ²⁴¹Am six years previously as part of a five-year lysimeter study on radionuclide uptake to crops. Ultrafiltration of soil solution was performed using hollow fibre cartridges with a nominal molecular weight cut off of 3 and 10 kD. The association of ¹³⁷Cs, ⁹⁰Sr, ²³⁹Pu and ²⁴¹Am with different molecular size fractions of the soil solution is discussed in terms of radionuclide bioavailability to cabbage grown in the same three soils. ¹³⁷Cs and ⁹⁰Sr were present in low molecular weight forms and as such were mobile in soil and potentially available for uptake by the cabbage. In contrast, a large proportion (61-87%) of the ²³⁹Pu and ²⁴¹Am were associated with colloidal and high molecular weight material and therefore less available for uptake by plant roots. The contribution from low molecular weight species of ²³⁹Pu and ²⁴¹Am to the total activity in soil solution decreased in the order loam > peat ≥ sand. Association of radionuclides with low molecular weight species of less than 3 kD did not, however, automatically imply availability to plants. In order to improve understanding of the relationship between activity concentrations of radionuclides in soil solution and uptake by plants, further characterisation of the low molecular weight fraction of soil solution is required.