Bioaccessibility of lead in high carbonate soils

Metal bioaccessibility is not currently considered in the French human health risk assessment procedure. For contaminants such as Pb this parameter could have a significant effect on the risk calculation. From the literature, it seems that Pb bioaccessibility is strongly controlled by the occurrence of cerrussite in soils: with a high cerussite content showing a positive correlation with high Pb bioaccessibility. Assuming this hypothesis, Pb contained in carbonated soils could show strong human bioaccessibility. This study aims to examine the link between high carbonate soil minerals, focussing particularly on cerrussite and the Pb bioaccessibility. Four soils were sampled in a high carbonate area in France contaminated with Pb due to mining activities. Pb bioaccessibility was measured using the in vitro RIVM protocol. In parallel, the solid phase Pb distribution was determined using chemical sequential extraction for each sample and physical analysis on the more contaminated sample. Results showed that Pb bioaccessibility was significantly different among soil samples. As shown in literature, cerussite (PbCO3) was a highly bioaccessible phase and 40% of Pb in the waste material was present in this form. However, cerussite alone did not account for all of the Pb which was also present in highly stable minerals containing sulphur. Pb associated to these minerals is also likely to be significantly bioaccessible. Indeed, sequential extraction showed that most of the Pb was associated with the sulphur-compartment. This study shows that Pb bioaccessibility in high carbonate soils can be low (down to 20% of the total soil Pb content) and is not correlated with cerussite soil contents even if the concentration of this mineral is relatively high. Consequently, risk management of Pb contamination should not focus only on high carbonate soils but also on Pb contained in other minerals which is also likely to be significantly bioaccessible.