Characterisation of rare earth elements in natural and exhaust gas samples: SEM-microscopy and EDX-analysis for source identifications

Natural materials and exhaust gas emissions are sources of Rare Earth Elements (REEs) in the road environment. A methodology based on SEM-EDX analyses is proposed to: 1) provide a morphological and chemical characterisation of REEs particles in natural materials and exhaust gas; and 2) identify indicators that can distinguish the sources of REEs. The chemical composition of various washcoats was evaluated. Natural materials, exhaust gases, and ceramic monoliths from catalytic converters were described using a scanning electron microscope (SEM) and analysed with an Energy Dispersive X-ray analyser (EDX). The results indicated that REEs natural particles predominantly exhibited sharp corners in contrast to the spherical shapes of REEs particles within exhaust gases. Exhaust gas particles were smaller (0.07–1.22 μm) than those observed in natural materials (0.64–25.42 μm). REEs particles were associated with mineral carrier phase compounds (e.g., Al, Si, P) and, in some instances, with natural source fingerprints (e.g., Rb, Sr, Th). REEs particles in exhaust gases were embedded in organic combustion particles composed of C, Fe, S or in washcoat detected through Zr, Ti, Pd. La/Ce ratio of natural particles (0.20–0.63) is higher than that for exhaust gas particles (0–0.25). Hence, La/Ce ratio could be used as an indicator for exhaust gas particles in environmental samples. To go further, this study provides information on the physical and chemical speciation of REEs particles necessary to assess the transfer of particles from emission to the environment. • SEM-EDX approach to identify fingerprints for natural and anthropogenic sources of REEs. • REEs edgy corners particles in natural materials. • Spherical micro-to nanoparticles of REEs are emitted in the exhaust gas. • La/Ce ratio of 0.20–0.63 and 0–0.25 in natural materials and exhaust gas, respectively. • REEs associated with washcoat compounds are emitted in the exhaust gas.