Mobility and speciation of rare earth elements in acid minesoils and geochemical implications for river waters in the southwestern Iberian margin

This paper discusses the abundance, fractionation and chemical speciation of the rare earth elements (REE) released during two contrasting water–soil interactions, using acid sulfate and circumneutral soil samples from an abandoned mine land watershed in SW Spain. Our simulative experiment shows that the REE mobility is strongly dependent on the soil solution acidity. The acid solution extracts (pH = 3.1–3.7) extracted from the minesoils display overall REE concentrations much higher (up to three orders of magnitude) than those of the near-neutral soil extracts. The middle REE (MREE) were preferentially removed when compared to light REE (LaN/SmN = 0.07) and heavy REE (GdN/YbN = 2.49). The origin of this MREE-enriched signature is attributed to dissolution and/or solid–liquid exchange reactions involving iron oxy-hydroxides particles and surface coatings. The REE were found to behave conservatively under the strong acid conditions of the soil solutions, with sulfate complexes (mainly LnSO4+) being the largely dominant aqueous species (75–80%) despite the competitive effects of aluminium. Free metal ions (Ln3+) accounted for most other species in solution (12–16%). All other ligands analysed (phosphate, nitrate, chloride, fluoride and bicarbonate) were not predicted to be significant complexers of REE, according to speciation calculations. The results of this study have implications for the REE geochemistry of the rivers draining the mine sites of the Iberian Pyrite Belt. The acid sulfate soils represent a major readily leachable MREE reservoir for the Tinto–Odiel fluvial system, which reflect the geochemical signature of the soil–water interaction in the mine lands watersheds.