Method variables affecting the bioaccessibility of arsenic in soil

Abstract

Arsenic bioaccessibility tests are now being commonly used in risk assessment. However, concerns remain about the reliability of such tests because the bioaccessibility of arsenic from soil may be susceptible to soil composition (including iron concentration), as well as method considerations such as varying liquid-to-solid ratios and the chosen buffer system. In this study, arsenic-contaminated tailings and soils were tested to compare two bioaccessibility methods: one that uses glycine as a buffer, and a second that is more physiologically based. With the glycine-buffered method, arsenic and iron bioaccessibility increased in the presence of a higher buffer concentration at higher liquid-to-solid ratios, whereas the results of physiologically-based tests were unaffected by variations in these parameters. In the glycine-buffered system, interactions between iron and glycine may influence the concentration of arsenic in solution, which may not be consistent with human gastrointestinal conditions. The choice of a physiologically-based method may be more appropriate to achieve representative arsenic bioaccessibility values toward estimating risks to human health.

Keywords:

• Bioaccessibility
• arsenic
• glycine
• contaminated soil
• tailings

Acknowledgments

The authors wish to thank A. Campbell, L. Easton and J. Harris for their help with sample analysis, and K. House for her insightful discussions. The authors gratefully acknowledge the support of the National Science and Engineering Research Council via the Metals in the Human Environment Strategic Network (see www.mithe-sn.org for a full list of sponsors) and a Discovery Grant (to KJR).

Notes

^a Abbreviations: N = number of replicates; L:S = liquid-to-solid ratio; P1 = gastric phase; P2 = gastric + intestinal phase; Method P = physiologically-based; Method G = glycine-buffered; SRM = standard reference material; RPD = relative percent difference, calculated as 100 (result of primary sample – result of duplicate sample) · average^{− 1}. All other abbreviations are described in the text.

^b Instrument detection limit is 1 μ g·L^{− 1} for arsenic and 10 μ g·L^{− 1} for iron. Results presented in this table represent limits of quantification.

^c Spike recovery of 100 ppb·L^{− 1} potassium arsenate (KH₂AsO₄, Fluka reagent grade) in blank PBET solution added immediately before analysis.

^d Calibration check solutions used in the ICP-MS analysis include both 50ppb and 750ppb multi-element solutions (PlasmaCAL), prepared from a different source solution than the ICP-MS calibration solutions.

^e All control limits for SRM are average ± three standard deviations for all laboratory results recorded (between 2001 and 2009 for arsenic; between 2006 and 2008 for iron), excluding the results of the present study.

^a Results are presented in order of decreasing molar iron to arsenic ratio. Numbers in bold indicate the higher of either P1 or P2 result where the relative percent difference, calculated as 100 · (P1 – P2) · average^{− 1} is greater than the acceptable laboratory repeatability (30%).

^b Abbreviations: L:S = liquid-to-solid ratio; P1 = gastric phase; P2 = gastric + intestinal phase; Method P = physiologically-based; Method G = glycine-buffered.

^c For Method G, the first two extractions used a 0.4 M glycine concentration, and a third extraction was performed using 0.02 M glycine.

^a Results are presented in order of decreasing molar iron to arsenic ratio. Numbers in bold indicate the higher of either P1 or P2 result where the relative percent difference, calculated as 100 · (P1 – P2) · average^{− 1} is greater than the acceptable laboratory repeatability (30%).

^b Abbreviations: L:S = liquid-to-solid ratio; P1 = gastric phase; P2 = gastric + intestinal phase.

^c Method P = physiologically-based; Method G = glycine-buffered.

^d Items marked n/a were not analyzed for iron concentrations.

^e For Method G, the first two extractions used a 0.4 M glycine concentration, and a third extraction was performed using 0.02 M glycine.