Experimental Dissolution of Road Dust in Simulated Environmental and Biological Fluids

Road dust from six sites in Philadelphia, PA (USA) was characterized using a multi-analytical approach and subsequently subjected to dissolution in various types of fluids. Quartz and dolomite are the predominant minerals in the studied road-dust samples, with smaller amounts of calcite, feldspars, micas, and clay minerals also present. A comparison of the elemental bulk concentrations with those of a previous study conducted in the same locations documents temporal variability in the composition of the road dust. In the dissolution experiments using the EPA 3050B method as well as synthetic rainwater, simulated gastric fluid, and Gamble's solution (simulated lung fluid), it was demonstrated that in general, increasing acidity resulted in enhanced release of Al, Fe, V, Cr, Ni, Cu, Zn, and Pb. The results further showed that the finer road-dust fraction (<75 µm) released larger amounts of a given element than the coarser (<841 µm) material of the same sample. Moreover, the higher the initial bulk content of an element in the road dust is, the higher the amount extracted by the experimental solutions. In simulated gastric fluid, Al and Fe, as well as V and some other minor elements showed classic concentration vs. time diagrams with a steeper slope at the beginning and approaching steady state towards the end of the experiments (24 h). In synthetic rainwater, only a few elements at some sites display logarithmic concentration-time trends; rather, several trends exhibit a maximum before the endpoint of the experiments, which is probably due to re-precipitation of the elements as some secondary phase. The variability between the sites in regard to leaching behavior of the road-dust samples and the concentration-time trends of the released elements is most likely due to mineralogical differences between the sites. In particular, the distinct leaching behaviors of Al and V in Gamble's solution point to the presence of minor Al- and V-phases at some of the sites. The observed differences further 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.