Partition of Rare Earth Elements Between Sulfate Salts Formed by the Evaporation of Acid Mine Drainage

Acid mine drainage (AMD) contains rare earth element (REE) concentrations several orders of magnitude higher than those of the rest of natural waters and could be a secondary source of REEs. In arid to semiarid climates with a long dry season, the precipitation of efflorescent sulfates constitutes a transient storage of REEs. The REE partition among the Al-Fe-Mg-Ca sulfates formed by the evaporation to dryness of six different AMDs was investigated by statistical methods and by selective dissolution. The chemical composition of the evaporitic salts showed that only three principal components (PCs) could explain more than 80% of the variability in the six samples analyzed. PC1 was associated with Ca and light REEs and interpreted as gypsum, whereas PC2 was associated with Y and heavy REEs, which were not clearly associated with a major sulfate. Finally, PC3 included Mg, Fe and several transition metals (Cu, Ni, Co, Mn and Zn) and was interpreted as Fe(II)-Mg sulfates. Selective dissolution of the salt mixtures with solutions saturated in the major sulfates revealed that the REEs were only retained in gypsum and were practically absent from the rest of the sulfates. The incorporation of REEs into gypsum decreased from Pr-Nd to La and Lu and was strictly ruled by the differences in their atomic radii and that of Ca in eight-fold coordination. However, gypsum concentrated less than 20% of the REE inventory (< 1% for Sc); the rest probably formed one or more unidentified trace minerals. This indicates that gypsum may not be an efficient way to concentrate REEs from AMD.

Automated summary

Acid mine drainage (AMD) contains high concentrations of rare earth elements (REEs), but gypsum may not be an efficient way to concentrate REEs from AMD.