Factors influencing the in-vitro oral bioaccessibility in topsoil from industrial areas (Wallonia, Belgium)

Pereira, Benoît [UCL] Titeux, Hugues [UCL] Schneider, Arnaud [Université de Reims Champagne-Ardenne] Leclercq, Julie [SPAQuE] Sonnet, Philippe [UCL]

INTRODUCTION Oral bioaccessibility - the fraction of soil contaminant that dissolves in the human gastrointestinal tract - can be evaluated using various chemical extraction tests, some of which are designed to mimic biochemical conditions. As for any other partial extraction tests, bioaccessibility can be expected to be influenced not only by the soil parameters, but also by the characteristics of the contamination, such as the metal speciation, the contamination aging, etc. Here, we try to relate bioaccessibility measurements to factors that can only be obtained when the sampled soils are surveyed and mapped at a regional scale. The factors we consider are the type of industrial activity responsible for soil contamination in industrialized areas, the land-use, and the regional level of contaminant concentrations. The question which we address is whether a fixed bioaccessibility value can be proposed for risk-based approach of contaminated land or a variable bioaccessibility value must be considered in order to take into account regional differences. To substantiate our conclusions about the influence of these factors on bioaccessibility measurements, chemical sequential extraction tests were also performed on the same soil samples. MATERIALS AND METHODS In the course of the POLLUSOL 2 survey, 237 topsoil samples have been collected in urban parks, vegetable gardens and forests within several industrial areas of Wallonia (South Belgium). The in-vitro oral bioaccessibility test used in this study is Unified BARGE Method (UBM). We looked at the ‘stomach’ compartment of the test for the 237 samples, and at the ‘stomach and intestine’ compartment for a subset of 178 samples. Analysis of the extracts for eight trace metal elements (As, Cd, Co, Cr, Cu, Ni, Pb, Zn) was carried out by ICP-AES. Total metal contents were measured by aqua regia extraction and ICP-AES. The sequential extraction was performed on a subset of 103 samples out of the 237 using the modified three-step BCR extraction procedure, whose purpose is to evaluate the speciation of trace metals. In this method, the operationally-defined fractions are the exchangeable and weak-acid soluble fraction (step 1), the reductible fraction (step 2), and oxidisable fraction (step3). The BCR sequential extraction procedure was carried out for the eight trace metal elements. Statistical data analysis and regression curve fitting were carried out using robust statistics (StatDA package of the R software). RESULTS AND DISCUSSION We found very good correlation between the 'stomach' and the 'stomach and intestine' phases for As, Cd and Zn (r > 0.9). For Co, Cr, Ni and Pb, we found also a good correlation, although somewhat weaker (0.6-0.9). Because of this correlation, only 'stomach' values will be discussed in what follows. For As, Cd, Co, Cr, Cu, Ni and Pb, relative bioaccessibility was found to be constant across the entire range of total metal content. Zinc was the only metal that exhibited a relative bioaccessibility which varied according to the total metal content and the type of industrial activity. For the eight trace metals, the bioaccessible fraction was influenced to a variable degree by one or several soil parameters (pH, organic matter, Ca, Mg, Mn and Fe). A very good linear relationship was found between the sum of the two first steps of the BCR sequential extraction and the bioaccessible fraction (gastric) of UBM the test (e.g. R2> 97% for Zn). The bioaccessible fraction amounted to between 61% (for Co) and 137% (for Cu) of the exchangeable, acid-soluble and iron and manganese oxyhydroxide-bound fraction of the Walloon industrial soils obtained by the BCR extraction method. CONCLUSIONS Our data suggest that, for the eight trace metals studied in the industrial and urban zones of Wallonia, the bioaccessible fractions can be satisfactorily predicted basing on the total metal contents. This prediction can be improved by taking into account soil properties in the regression model. For Zn, a variable relative bioaccessibility should be considered, depending on the type of industrial activity and the level of total metal content. Our data also show that the sum of the two first steps of BCR sequential extraction closely predicts the ‘stomach’ phase measured by the BARGE method.