Experimental dissolution of road dust in simulated environmental and biological fluids

Road dust from six sites in Philadelphia (PA, USA) was subjected to time-series dissolution experiments using the EPA 3050B method, synthetic rainwater, simulated gastric fluid, and Gamble's solution (simulated lung fluid) in order to study environmental availability and bioaccessibility of selected metals (Al, Fe, V, Cr, Ni, Cu, Zn, Pb). In general, the results showed that increasing acidity led to enhanced metal release, and that larger amounts of a given element were leached from the fine (< 75 µm) than the coarse (< 841 µm) size fraction of the same sample. Moreover, the higher the initial bulk metal content in the road dust, the higher the amount extracted. In simulated gastric fluid, many elements showed classic logarithmic concentration vs. time trends. In synthetic rainwater, only a few elements at some sites displayed such a pattern; rather, several metals exhibited a maximum concentration before the endpoint of the experiments, which is probably due to re-precipitation. In Gamble's solution, some samples released Al, V, Cu, and Zn, whereby distinct leaching behaviors of Al and V point to the presence of minor Al- and V-phases at some of the sites. The wide variability in concentration-time trends of the released metals as well as in their potential environmental availability, their environmental availability, and their bioaccessibility is most likely due to mineralogical differences between the sites. These differences document that it is problematic to draw general conclusions about possible environmental and health impacts of road dust, unless the speciation of potentially toxic elements is known. Our study further suggests that the maximum element-extraction rates may help in evaluating the effects of speciation on leaching behavior and duration as well as on potentially associated health risks subsequent to unintentional ingestion or inhalation of road-dust particles.