The oral bioaccessibility of heavy metals in soil in a contaminated area in Thailand

The current study investigated oral bioaccessibility of soil Cu, Zn, Cd and Pb in an agricultural community known for contamination of Cd due to mining activities nearby, using an in vitro test that simulates human gastric condition. Relatively high metal concentrations compared to Thai `background´ concentrations were observed and the %bioaccessibilites were found to be 28±13% for Cu, 42±18% for Cd, 54±21% for Pb and 85±29% for Zn. Statistical correlations were determined in order to study the nature of bioaccessibility of each metal. The results indicated that (i) bioaccessible concentrations might be predicted from the total concentrations (ii) portion of metals bound to Si could not be extracted under human gastric conditions, thus, non-bioaccessible (iii) Zn bound to Al, Fe and Mn oxides and Cd bound to Al oxides might be non-bioaccessible, though Cu and Pb might not form strong bonds with soil Al, Fe and Mn oxides (iv) Mg might compete well with all the metals, Ca with Zn and Pb, and K with Cu only (v) there were no antagonistic reactions among the metals in gastric environment. Other findings included that (i) no correlations with soil organic matter and bioaccessible metals inferring metals in topsoil might not form stable complexes with organic matter (ii) soil pH has no seeming effect on oral bioaccessibility; which may be contrary with the theory.