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Accessing the Risk of Arsenic Ingestion with Mineralogy

Canadian researchers working at NSLS created a method for determining how much of the arsenic in soil tailings — byproducts of the mining industry — will enter the bloodstream if ingested.

In order to ensure that harmful mining waste is properly dealt with, it is important to determine how much of an ingested chemical is absorbed by the body rather than passed harmlessly through the digestive system. Scientists can approximate this as the chemical’s bioaccessibility. Instead of feeding samples to a test subject, a bioaccessibility test runs the sample through conditions that simulate a digestive system, complete with mock gastrointestinal fluids. The percentage of arsenic that dissolves during this process reveals the sample’s bioaccessibility.

But instead of running these tests on numerous soil samples from throughout a mining site, the researchers sought to describe the relationship between a sample’s bioaccessibility and the specific minerals in it — its mineralogy. The researchers, from the Royal Military College of Canada, Queen’s University, the British Geological Survey, and Geological Survey of Canada, examined soil samples taken in the gold mine districts of Nova Scotia, Canada, to try to predict, given the mineralogy of a sample, whether it’s highly bioaccessible.

The research team tested the samples’ mineralogy using micro-x-ray fluorescence and diffraction imaging techniques at NSLS and x-ray absorption near-edge structure analysis at Argonne National Laboratory’s Advanced Photon Source.

Next, researchers compared the mineralogical results from NSLS with the samples’ bioaccessibility, and found some interesting connections.

Single arsenic minerals, such as arsenopyrite, had lower bioaccessibility — over 90 percent of the arsenic in the sample would pass through the digestive system without being absorbed by the body. This is due to arsenopyrite’s low solubility, or ability to dissolve in fluid. A more soluble compound, arsenic bearing iron(oxy)hydroxides, had 10 times the bioaccessibility of arsenopyrite.

But their findings don’t mean that the old bioaccessibility tests should be eliminated. For example, certain compounds can increase the bioaccessibility of the entire soil sample surrounding it. Despite knowing the bioaccessibility of the minerals in the sample, researchers still need a bioaccessibility test of the entire sample to reveal its higher-than-expected toxicity.

L. Meunier, S.R. Walker, J. Wragg, M.B. Parsons, I. Koch, H.E. Jamieson, K.J. Reimer, “Effects of Soil Composition and Mineralogy on the Bioaccessibility of Arsenic from Tailings and Soil in Gold Mine Districts of Nova Scotia,” Environ. Sci. Technol., 44, 2667 (2010).

Top: Mineralogy, percent arsenic bioaccessibility and total arsenic concentration of samples from Nova Scotia mine tailings. Detailed mineralogical analyses of individual samples revealed up to seven arsenic species in individual samples (six shown here as major arsenic phases). Results of a physiologically based extraction test are for the < 150 µm particle size fraction. A weak correlation is observed between total and bioaccessible arsenic concentrations. The percent arsenic bioaccessibility is most influenced by the presence of a more soluble arsenic species, even in low concentrations. Lower percent bioaccessibility in the majority of samples is associated with sparingly soluble arsenopyrite and scorodite. Higher percent bioaccessibility in some samples is attributed to the presence of calcium-iron arsenates and arsenic-bearing iron oxides. The star denotes the presence of a minor calcium-iron arsenate phase.

Bottom: Lead researcher Kenneth Reimer, director of the environmental sciences group at the Royal Military College of Canada

Figure 2 Figure 1