Desorption of rare earth elements (REEs) from schwertmannite under acid mine drainage (AMD) and AMD-seawater conditions

Abstract

Schwertmannite as a sink for rare earth elements (REEs) in environments affected by acid mine drainage (AMD) plays a significant role in the fate of these elements. The conditions to precipitate schwertmannite (i.e., sulfate-rich water and pH between 2.5 and 3.5) are not suitable for this Fe-oxyhydroxysulfate (Fe8O8(OH)6SO4) to adsorb REEs. In estuaries where AMD-impacted rivers meet (e.g. the Odiel and the Tinto rivers in the Ría de Huelva estuary in SW Spain), AMD mixes with seawater raising the pH between 4.5 and 8, thereby enabling REE adsorption on schwertmannite at circumneutral pH. However, the estuarine tidal dynamics exposes REE-enriched schwertmannite to more acidic water, inducing REE desorption, which has yet to be studied. In the present work, batch experiments were performed to study the REE desorption from a REE-enriched schwertmannite within the pH range 4.5–7 in the presence of sulfate at room temperature. Solution-chemistry data were used to obtain the REE desorption surface constants from different surface complexation. Desorption of a Lu-enriched schwertmannite at different pH was investigated with High Energy X-Ray Diffraction (HEXD) and Extended X-ray Adsorption Fine Structure (EXAFS) to characterize the changes in the surface complexes during desorption. The results indicate that (1) REEs desorb from schwertmannite at pH < 6 and desorption is pH dependent, (2) desorption of light REEs is higher than that of heavy REEs, (3) REE sorption onto schwertmannite surface is not a totally reversible reaction, and that (4) both monodentate and bidentate surface complexes are involved in the Lu-desorption reaction. These observations indicate that (1) REE-enriched schwertmannite remains stable in the areas of the estuary nearer the sea and that (2) tidal fluctuations displace schwertmannite colloids towards areas that are more affected by AMD, inducing REE desorption from schwertmannite.