Modeling the behavior of the rare earth elements in acid drainages

Resumen

INTRODUCTION Lanthanide series (from La to Lu) plus scandium (Sc) and yttrium (Y) are referred to as Rare Earth Elements (REE) according to recommendation by the International Union of Pure and Applied Chemistry (IUPAC). REE are essential raw materials for modern technological applications. In mining environments, the dissolution of Fe sulfides generates acidic solutions with H2SO4, and dissolution of rocks are much more intense in Acid Mine Drainage (AMD) than in the rest of weathering profiles. Once in solution, REE form sulfate complexes, which inhibit their sorption in clays and stabilizes them in the solution. Consequently, REE concentrations in AMD are orders of magnitude higher than in the rest of natural waters. Since AMD is expected to run for hundreds of years, the total reserves are virtually unlimited, and then, AMD could become a small but continuous source of REE. Moreover, due to their common use in modern industry, some toxicological studies suggest that REE could have a significant pathogenic potential. Therefore, knowing the mobility constraints of REE in natural waters and particularly in AMD is a target of interest. REE geochemistry in AMD is strongly linked to pH, and therefore to AMD neutralization processes. Thus, when AMD effluents mix with the alkaline river water or are neutralized with limestone addition, its pH increases leading to schwertmannite (Schw) (Fe8O8(OH)6(SO4)2) and basaluminite (Bas) (Al4(SO4)(OH)10•5(H2O) formation. At pH higher than 4.5, however, REE concentrations in the water decrease below detection levels, indicating that these elements are trapped in these two solid phases.