Geochemical signatures and natural background values of rare earth elements in soils of Brazilian Amazon

Rare earth elements (REEs) are generally defined as a homogenous group of elements with similar physical-chemical properties, encompassing Y and Sc and the lanthanides elements series. Natural REEs contents in soils depend on the parent material, the soil genesis processes and can be gradually added to the soil by anthropogenic activities. The REEs have been considered emerging pollutants in several countries, so the establishment of regulatory guidelines is necessary to avoid environmental contamination. In Brazil, REE soils data are restricted to some regions, and knowledge about them in the Amazon soils is scarce, although this biome covers more than 40% of the Brazilian territory. Thus, the objectives of this study were to determine the REE content in soils of two hydrographic basins (Solimões and Rio Negro) of the Amazon biome, establish their Quality Reference Values (QRV) and to investigate the existence of enrichment of REEs in urban soils. The ΣREE(Y + Sc) content of Solimões surface samples was 109.28 mg kg−1 and the ΣREE(Y + Sc) content in the subsurface samples was 94.11 mg kg−1. In soils of Rio Negro basin, the ΣREE(Y + Sc) was 43.95 15 mg kg−1 surface samples and 38.40 mg kg−1 in subsurface samples. The ΣREE(Y + Sc) in urban topsoils samples was 38.62 mg kg−1. The REEs contents pattern in three studied areas are influenced in different amplitude by natural soil properties. The REEs content in urban topsoils were slightly higher than the Rio Negro pristine soils, but the ecological risk was low. QRVs recommend for Solimões soils ranged from 0.01 (Lu) to 145.6 mg kg−1 (Ce) and for Rio Negro soils ranged from 0.05 (Lu) to 15.8 mg kg−1 (Ce).