Solubility of uranium and thorium from a healing earth in synthetic gut fluids: A case study for use in dose assessments

The aim of this case study was to estimate the bioaccessibility of uranium (238U) and thorium (232Th) from a healing earth by analysing the solubility of these radionuclides in synthetic gastric and intestinal fluids. An easy applicable in vitro test system was used to investigate the fractional mobilization of the soil contaminants being potentially available for absorption under human in vivo conditions. These findings provided the basis for a prospective dose assessment. The solubility experiments were performed using two different in vitro digestion methods. The concentrations of 238U and 232Th in the solutions extracted from the soil were measured by inductively coupled plasma mass spectrometry (ICP-MS). The dissolved fractions in the synthetic gastrointestinal fluid ranged in average from 10.3% to 13.8% for 238U and from 0.3% to 1.6% for 232Th, respectively, depending on the digestion method. Subsequently, the committed effective doses from intake of 238U and 232Th after ingestion of the healing earth during 1 year were evaluated for adult persons. Thereby ingestion dose coefficients calculated as a function of bioaccessibility were used. The dose assessments ranged between 4.3 × 10−7–1.9 × 10−6 Sv y−1 for 238U and 5.6 × 10−7–3.3 × 10−6 Sv y−1 for 232Th, respectively. On the basis of the assumptions and estimations made, the present work indicates a relatively low radiation risk due to 238U and 232Th after internal exposure of the healing earth.