Geochemical behavior of rare earth elements in acid drainages: Modeling achievements and limitations

Abstract

Rare earth elements (REE) concentrations in acid mine drainage (AMD) are several orders of magnitude higher than those in other surface waters and could be a future supplementary source of highly valuable REE. Therefore, knowing the mobility constraints of REE in natural waters, particularly in AMD, is of interest.Precipitation of schwertmannite and basaluminite occurs due to mixing of REE-enriched AMD and neutral streams. The REE fractionation between AMD and solid phases has been investigated in several localities in the Odiel-Tinto (SW Spain) and Arroyo del Val (N Spain) basins. The mobility of REE is related to the precipitation of Fe and Al phases that occurs at different pH values. The behavior of REE during the mixing is conservative under low pH conditions (pH <4 approximately), at which schwertmannite precipitates, whereas REE are scavenged under near-neutral pH conditions, at which basaluminite precipitates and what suggest that this phase could be responsible for REE retention. Indeed, when Fe and Al phases can be sampled separately, schwertmannite does not contain REE, which are entirely retained in basaluminite, confirming the REE removal by this later phase.The strengths and limitations of a recently proposed thermodynamic model of REE sorption on basaluminite have been assessed by comparing the model predictions with observations of samples recovered in the field. The model is able to predict the REE distribution pattern and confirms the preferential sorption of heavy REEs (HREEs) and Y over light REEs (LREEs) in all the samples. However, the lack of synchronicity between basaluminite precipitation and REE sorption during mixing in an open flow setting precludes an accurate prediction of the REE concentration in the solid phase.