Rare Earth Elements’ particles in road dust: A mineralogical perspective for source identification

Roadside dust contains many mineral grains and, among them, some of REEs (Rare Earth Elements) which have an uncertain origin. This paper characterizes and illustrates for the first time the occurrence of such REEs' particles from four locations near Barcelona (Spain), by means of mineralogical studies, with the aim of identifying their source and understanding their accumulation. The observed REEs' particles contain Ce and low amounts of La and Nd and are spheroidal, rounded, elongated, or angularly shaped (<5 μm), often forming nanoparticles' aggregates. The identical chemical composition and morphology of the REEs' particles (CeO₂), independently from the sampling location and its geological background, points to a sole non-natural anthropogenic origin. Moreover, the absence of U and Th in the studied CeO₂ grains is an additional robust indicator of their anthropogenic source, as the naturally formed CeO₂ (the mineral cerianite) is characterized by containing those radioactive elements. Therefore, the identified grains consist of ceria particles, the non-natural cerium oxide. Ceria must originate in autocatalytic converters, where it is broadly used in solid solution with ZrO₂ (zirconia) in the form of CeZrO_x. The roadside dust ceria particles do not contain Zr, but zirconia crystals are also found in the dust, with no Ce traces. This might indicate that they were released as separated neo-formed ceria and zirconia phases, which is consistent with a metastable CeZrO_x segregation at the autocatalyzer under vehicles’ engine operating conditions. The small size, and thus inhalable, ceria particles emitted by vehicles are a significant environmental health hazard, revealing the need for further assessment of CeO₂ concentrations generated by automobiles in urban areas worldwide.