Pautas de distribución de especies de las Tierras Raras en soluciones ácidas naturales

Abstract

A speciation analysis of Rare Earth elements (REE) in acidic waters of four natural systems in Brasil, Spain, Canada and USA is carried out. The geochemical diversity of the samples span a pH range of 3.3-5.2, a fairly broad range of SO⁼₄ F^- and P0^3-₄ concentrations and a diference in measured REE concentrations of four orders of magnitude. The shale-normalized REE patterns show a systematic, almost universal, IREE enrichment in all the acidic waters studied.
From REE speciation calculations using the PHREEQE code, it can be concluded that the dominant forms of REEs are sulfate complexes (mainly LnSO⁺₄) with minor free ionic species (Ln³⁺). These free ionic species only became dominant when sulfate concentrations in waters are very low.
Other ligands present in solution (P0^3-₄ and F^- ) do not compete with sulfate in REE speciation in the concentration range spanned by the samples. At least two reasons can be put forward to explain this result: (i) P0^3-₄ concentrations are too low to affect REE speciation and (ii) F^- which is present in concentrations high enough to interfere with lanthanide speciation, actually associates with aluminum.
Aluminum, usually in high concentrations in acidic waters, is then the responsible of inhibiting REE fluoride complexation and hence, of inhibiting the important effect that this ligand has in REE speciation. Therefore, analytical determination of aluminum must be performed in order to properly calculate REE speciation in acidic waters.