DETERMINATION OF RARE EARTH ELEMENTS IN ASPHALT BY INDUCTIVELY COUPLED PLASMA SPECTROMETRIES

The presence of rare earth elements (REE) in important fields, such as catalysis, automotive catalyst, petroleum industry and medicine, has attracted increasing attention of scientists to determine and develop new application for them1. However, the understanding of their toxicity and environmental impact is still an open challenge2. This study presents a novel analytical method to determine REE by inductively coupled plasma -mass and -optical emission spectrometries (ICP-MS and ICP OES), in asphalt sample. The method is composed of two steps: the first step identifies the organic or inorganic source of REE in asphalt sample through the extraction of the sample with organic solvent and subsequent acid leaching. The second step evaluates the availability of REE after exposition to acid leaching. The results show that REE are in the inorganic fraction of the asphalt pavement. However, leaching the sample with an acid rain simulation solution (5x10-6 mol L-1 HNO3 and 1x10-5 mol L-1 H2SO4)3 was not enough to extract REE in concentrations above the limits of quantification (LOQ) from the asphaltic pavement. For this reason, and in order to simulate a long-term effect of acid rain, a more aggressive leaching condition was tested (10 % v/v HNO3 or 10 % v/v H2SO4), both assisted by sonication. The analytical results were evaluated through the analysis of rock certified reference materials and through the comparison of the results obtained by ICP OES and by ICP-MS. For the total decomposition of reference materials, a mixture of HF, HNO3 and HClO4 was used4. The determinations were carried out with analytical solutions in the same acid media as the samples, with aqueous REE standards and Rh as internal standard. The obtained limits of detection ranged from, in µg L-1, 0.002 (Ho) to 0.010 (Nd) for the leaching with HNO3 and from 0.001 (Ho) to 0.014 (Tb) with H2SO4, by ICP-MS, and from 0.05 (Lu) to 2.5 (Pr) with HNO3 and from 0.06 (Lu) to 3.1 (Pr) with H2SO4, by ICP OES. The concentrations of REE in the asphalt sample ranged from 0.47 (Lu) to 65.6 (Ce) µg g-1, being the light rare earth elements (LREE) accounted for 83 % of total REE on the investigated asphalt sample. The order of average concentrations of REE in the asphalt was, in descending order: Ce > Nd > La > Pr > Sm > Eu > Gd > Dy > Er > Yb > Ho > Tm > Lu.