Decreasing arsenic bioaccessibility/bioavailability in soils with iron amendments

Abstract

We investigated the use of various iron amendments (metallic Fe and soluble Fe(II)- and Fe(III)-halide salts) to reduce arsenic (As) bioaccessibility (as a surrogate for oral bioavailability) in contaminated soils. Soluble Fe(II)- and Fe(III)-salts were more effective than metallic Fe in reducing As bioaccessibility. Adding soluble Fe(III)-salts to soil reduces As bioaccessibility in two ways, by increasing the Fe(III) (hydr)oxide content and by lowering the soil pH. A detailed investigation into the effect of soil moisture when adding Fe(III) amendments indicated that the reaction can occur in situ if sufficient (≥ 30% moisture) is added. If the amendments are added to the soil without moisture, a reduction in bioaccessibility will occur in the extraction fluid itself (i.e., an experimental artifact not reflecting a true in situ reduction in bioaccessibility). Adding Fe (III)-salts to nine As-contaminated soils at a Fe:As molar ratio of 100:1 reduced the average bioaccessibility in the soils by approximately a factor of two. Greater reductions in As bioaccessibility can be achieved by increasing the Fe:As molar ratio. These results suggest decreasing As bioaccessibility and bioavailability in soil by adding Fe amendments may be an effective strategy to remediate As-contaminated soils.

Keywords:

• Bioaccessibility
• bioavailability
• arsenic
• risk
• soil
• amendments
• moisture content

Acknowledgements

This research was sponsored by the US Strategic Environmental Research and Development Program (SERDP) under the direction Dr. Andrea Leeson. We gratefully acknowledge Mike Ruby and Yvette Lowney of Exponent for supplying some of the As-contaminated soils and John Drexler of the University of Colorado for producing the photomicrographs. Oak Ridge National Laboratory is managed by the University of Tennessee–Battelle LLC, under contract DE–AC05–00OR22725 with the U.S. Department of Energy.

Notes

*—indicates not measured.

¹Averaged bioaccessibility values for all iron amendment sources.

²Initial values reported as the 0-day non-amended bioaccessibility for soil sample 1.

¹Dithionite-citrate-bicarbonate extractable iron oxides for initial iron content and calculated ferric chloride added to soil by amendment addition for final concentration.

²Calcium chloride determined soil pH before and after iron amendment addition.

³Initial As bioaccessibility measured in non-amended soil samples.

⁴Initial predicted As bioaccessibility assuming native Fe concentration and natural soil pH.

⁵Amended As bioaccessibility in soil samples amended with 100:1 Fe:As molar ratio with FeCl₃ amendment and 30% moisture content.

⁶Relative decrease calculated by Decrease % bioaccessibility Initial % bioaccessibility * 100