DISTINGUISHING GEOGENIC FROM ANTHROPOGENIC PARTICLES IN DUST USING SYNCHROTRON MICROANALYSIS

A comprehensive understanding of the risk associated with metal-bearing dust includes identification of the solid phases hosting the metals. A near simultaneous combination of synchrotron-based X-ray fluorescence, diffraction and absorption experiments using a microfocused beam can provide information on elemental concentrations, crystal structure and oxidation state of individual particles. This approach can distinguish multiple metal-hosting minerals and industrial compounds in a single sample.

At abandoned gold mines in Nova Scotia, windblown and vehicle-raised dust was collected in seven aerodynamically fractionated size ranges (0.5 to 16 µm) using a cascade impactor deployed at three tailings fields. All three sites are used for recreational activities and off-road vehicles were racing on the tailings at two mines during sample collection. Measurements of absorption edge positions on mXANES spectra indicate that nearly all As occurs as As⁵⁺ in the airborne tailings with rare As^1- present, suggesting that there are some unoxidized sulfides, likely arsenopyrite. MicroXRD of individual particles was used to identify Fe-As⁵⁺ weathering products including scorodite (FeAsO₄•2H₂O) and amorphous hydrous ferric arsenate. Mineral identification was confirmed by Fe/As ratios obtained from microXRF analysis of individual particles.

Microanalysis was applied to house dust and garden soil from a single urban home in Ottawa, Canada with the purpose of distinguishing metal associated with soil particles from metal associated with indoor consumer products. Element correlations (via XRF mapping) and microXRD prove to be the most valuable tools. In the living room, Pb is associated with Mn and Fe hydroxide and phosphate minerals, similar to particles identified in the garden soil. However, in the bedrooms, Pb-based and non-Pb paint pigments as well as gypsum, bassanite and portland cement indicate in-home renovation. Zinc is present as zincite and wurtzite, constituents of the pigment lithophone, and also in metallic form.

A principal advantage of synchrotron-based microanalysis is that little or no sample preparation is needed and thus there is little risk of post-sampling modification. In addition, microXRD can provide unambiguous identification of microcrystalline metal-bearing particles.