Digestive fluid components affect speciation and bioaccessibility and the subsequent exposure risk of soil chromium from stomach to intestinal phase in in-vitro gastrointestinal digestion.

ABSTRACT

The simulated in-vitro gastrointestinal method provides a simple way to evaluate the health risk of human body exposed to soil contaminants. Several in-vitro methods have been successfully established for soil As, Pb, and Cd. However, the method development for soil Cr failed up to now, which could be resulted from alteration in the species of Cr (e.g., Cr(VI)/Cr(III)) caused by the gastrointestinal digestion components, ultimately affecting the accessibility of Cr. This study explored the transformation and bioaccessibility of Cr in two Cr-contaminated soils during the physiologically based extraction test. The water-soluble and exchangeable Cr in soil was dissolved in gastrointestinal tract, accompanied with reduction of Cr(VI) into Cr(III), and the reduction occurred after the chemical extraction in two soils rather than during the extraction. Pepsin and organic acids in gastric phase could reduce Cr(VI) into Cr(III) and reduction efficiency were 20.4%- 53.0%, while in intestinal phase, pancreatin and bile salt had little effect on the Cr(VI) reduction, instead, more Cr(VI) was released from soil. In the gastric solution, Cr(VI) was mainly present as HCrO4- and Cr(III) as free Cr3+ ion. In the intestinal phase, Cr(VI) mainly occurred as CrO42- and Cr(III) as Cr(OH)3 (aq). Cr in the soil solid phase was dominated as the precipitates of Cr-Fe oxide, which was hardly extracted. Bioaccessibility of Cr in gastric phase increased as extraction duration increased and decreased in the intestinal phase, the contrary trend was observed for the hazard quotient of Cr in two phases due to Cr(VI)/Cr(III) transformation. This study indicates that the gastrointestinal components could influence the Cr transformation and subsequently affect the Cr bioaccessibility, which would help for a successful establishment of in vitro determination method for soil Cr bioaccessibility.