The effect of residence time and fluid volume to soil mass (LS) ratio on in vitro arsenic bioaccessibility from poorly crystalline scorodite

Abstract

Percent arsenic bioaccessibility is occasionally dependent upon arsenic concentration; however, the mechanism(s) of this relationship has not yet been defined. To evaluate the mechanism of this relationship, the arsenic bioaccessibility from freshly synthesized poorly crystalline scorodite was measured in the stomach, small intestine, and colon stages of the Simulator of the Human Intestinal Microbial Ecosystem (SHIME). The shape of the arsenic dissolution isotherms were different between stages (stomach: linear; small intestine: exponential rise to maxima; colon: sigmoidal). These results indicate that arsenic bioaccessibility may be limited by either in vitro GI fluid saturation or in vitro GI model residence time, depending upon the chemical/microbiological conditions of the model. Gastrointestinal microorganisms increased arsenic bioaccessibility of scorodite up to two-fold in the SHIME colon; however, this was dependent upon the sample arsenic concentration. Up to 40% of the bioaccessible arsenic was reduced to arsenite; however this process was neither mediated by GI microorganisms nor associated with increased arsenic bioaccessibility.

Keywords:

• Arsenate
• arsenite
• scorodite
• SHIME
• intestinal microorganisms
• XANES
• Isotherm

Acknowledgment

The authors gratefully acknowledge the support of the NSERC MITHE Research Network. A full list of sponsors is available at www.mithe-rn.org. The research described in this paper was performed at the Canadian Light Source, which is supported by NSERC, NRC, CIHR, and the University of Saskatchewan. The authors also wish to acknowledge the technical support of Drs. Chang-Yong Kim and Ning Chen at the HXMA beamline (06ID-1). We would like to thank Kim Dekker for contributing to this laboratory research as part of her B.Sc. Honour's thesis in the Toxicology Program at the University of Saskatchewan. Brian D Laird was supported by a NSERC PGS-D and a Garfield Weston Award for Northern Research. Brian D Laird designed the study, performed the laboratory analysis, and composed the manuscript. Derek Peak provided technical expertise in the interpretation of the arsenic dissolution isotherms and performed the XANES analysis. Steven D Siciliano helped design the study, analyze the data and contributed to the writing of the manuscript. Additionally, we would like to thank Dr. Eric Lamb for assistance in the statistical analysis of the data.