Linking 238U or 232Th mobility and soil-to-plant transfer with soil characteristics

SCK•CEN Mentor

Vanhoudt Nathalie, nvanhoud@sckcen.be, +32 (0)14 33 21 12

Expert group

Biosphere Impact Studies

Introduction

Naturally occurring radionuclides (NOR) such as 238U, 232Th and their decay-products are abundant in the environment. Increased environmental NOR levels can arise from various anthropogenic activities that exploit raw materials for commercial purposes (e.g. metal mining and smelting, the phosphate industry, etc.).

The ability to estimate the activity concentration of NOR in biosphere components is a key step in evaluating the long-term impacts of these contaminants on human health and the environment. Radiological assessment models have integrated modules to predict NOR accumulation in soil and vegetation. These modules are often formulated in terms of simple parameters such as the equilibrium solid-liquid distribution coefficient (Kd) and the soil-to-plant transfer factor (TF). Although simple to measure and to use in radiological impact assessments, the Kd and TF are highly variable. This variability can be attributed to several sources including soil and plant properties, radionuclide speciation, the time after contamination, land management practices and the experimental procedure used to determine these parameters. Several approaches have been proposed to reduce the variability in the Kd and TF data with varying degree of success. One approach is to group available data by soil texture (i.e. sand, silt and clay), organic matter content and crop type. Another approach is to relate the Kd to physicochemical parameters that can be used as surrogates for the underlying mechanisms that govern radionuclide retention in soils (e.g. pH, cation exchange capacity, organic matter content). This cofactors approach has shown a greater potential to reduce the variability in the Kd data for some radionuclides.

Objective

The objective of this project is to improve the experimental procedure to determine the Kd and TF for 238U or 232Th and to link them with soil characteristics, the 238U/232Th labile fraction or site history/mineralogy. The general hypothesis is that soil testing for labile (adsorbed) 238U/232Th, in combination with factors affecting the sorption (soil properties) will improve prediction of Kd and TF values as compared to the current approach in which soils are categorized in texture and organic matter classes. By replacing these parameters with functions of specific environmental properties, uncertainties associated with environmental impact assessment models will partially decrease.

As part of the bachelor thesis, a literature search will be conducted to identify the soil characteristics most likely to influence 238U or 232Th mobility and availability and to which extent certain parameters can already be used to predict mobility and transfer. In addition, the different experimental approaches to determine Kd and TF parameters will be identified and a work plan for the master thesis will be formulated. Based on the literature study some preliminary experiments can be set up to determine soil characteristics and test selective extraction methods to determine the radionuclide labile fraction in soils.

The minimum diploma level of the candidate needs to be

A2: Upper secondary school (+7th year BSO) , Professional bachelor

The candidate needs to have a background in

Biology , Chemistry