Estimating the bioaccessibility of atmospheric trace elements within the Athabasca bituminous sands region using the acid soluble ash fraction of Sphagnum moss†

Abstract

Airborne trace elements (TEs) from the development of the Athabasca bituminous sands (ABS) in northern Alberta, occur in coarse and fine aerosols. Here, TEs in Sphagnum moss and acid soluble ash (ASA, obtained by leaching ash for 15 min using 2% HNO₃) are used to estimate the distribution of TEs between these two aerosol fractions. Total concentrations of all elements increase toward industry, but chemical reactivity of the ash varies. Most of the Al is acid soluble, but most of the Th is not; the former is assumed to reflect the abundance and reactivity of light minerals, and the latter is a surrogate for heavy minerals. In the ASA, the trends in Ni and V, the dominant metals in bitumen, resemble Al. In contrast, Mo (also enriched in bitumen), plus Pb, Sb and Tl, are more like Th in exhibiting limited reactivity. Trace element enrichments in both the total and ASA fractions, relative to crustal abundance, are restricted to plant micronutrients (e.g., Cu, Mn, Mo, Zn), or elements that are passively taken up by plants (e.g., Cd and Rb, but apparently also Ag and Re). The greatest enrichments of TEs occur at the reference site, even though it is located 264 km from the centre of industrial activities. The ash of moss collected nearest industry is dominated by quartz (67%) which explains the low concentrations of TEs, absence of enrichment relative to crustal abundance, and limited chemical reactivity of Pb, Sb and Tl. In this region, total concentrations of TEs in moss are a poor guide to their bioaccessibility in the environment.