Bioaccessibility of Arsenic in Mining-Impacted Circumneutral
River Floodplain Soils
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Abstract
Floodplain
soils are frequently contaminated with metal(loid)s
due to present or historic mining, but data on the bioaccessibility
(BA) of contaminants in these periodically flooded soils are scarce.
Therefore, we studied the speciation of As and Fe in eight As-contaminated
circumneutral floodplain soils (≤21600 mg As/kg) and their
size fractions using X-ray absorption spectroscopy (XAS) and examined
the BA of As in the solids by in-vitro gastrointestinal (IVG) extractions.
Arsenopyrite and As(V)-adsorbed ferrihydrite were identified by XAS
as the predominant As species. The latter was the major source for
bioaccessible As, which accounted for 5–35% of the total As.
The amount of bioaccessible As increased with decreasing particle
size and was controlled by the slow dissolution kinetics of ferrihydrite
in the gastric environment (pH 1.8). The relative BA of As (% of total)
decreased with decreasing particle size only in a highly As-contaminated
soil − which supported by Fe XAS − suggests the formation
of As-rich hydrous ferric oxides in the gastric extracts. Multiple
linear regression analyses identified Al, total As, C<sub>org</sub>, and P as main predictors for the absolute BA of As (adjusted <i>R</i><sup>2</sup> ≤ 0.977). Health risk assessments for
residential adults showed that (i) nearly half of the bulk soils may
cause adverse health effects and (ii) particles <5 μm pose
the highest absolute health threat upon incidental soil ingestion.
Owing to their low abundance, however, health risks were primarily
associated with particles in the 5–50 and 100–200 μm
size ranges. These particles are easily mobilized from riverbanks
during flooding events and dispersed within the floodplain or transported
downstream