Influence of experimental parameters on the sorption behavior of Rare Earth Elements on manganese ferrite nanoparticles

Rare Earth Elements (REE) are nearly irreplaceable in many technologies, but their supply is limited by geopolitical factors. Additionally, their exploration and refinement bring serious environmental consequences. As such, alternative sourcing of these elements through methods such as sorption has recently been attempted. Nanoparticles are exceptional REE sorbents, however, difficulties in material separation post-sorption have fueled an interest in the use of magnetic nanoparticles, which can be easily separated from the solution by applying an external magnetic field. In this study, we explore the influence of sorbent dosage, pH and matrix complexity in the removal of trivalent cations of nine REE (Y, La, Ce, Pr, Nd, Eu, Gd, Tb and Dy) using manganese ferrite nanoparticles (MnFe2O4); herein the abbreviation REE is employed to indicate such elements including the trivalent cations. The REE removal increased for higher sorbent dosages and pH. More than 90% of REE in mono-elemental solutions were removed from water at pH 6 and 8. Removal below 10% was observed for pH < 4, due to the positive surface charge of the sorbent which did not favor interaction with cationic REE species. Increasing the complexity of the matrix, the removal of REE from water decreased considerably, which can be attributed an increase of total REE concentration and the limited number of sorption sites in the sorbent. Further sorption inhibition was reported in increased ionic strength scenarios (mineral and saline water), possibly due to competition with Na+, which may limit the application of this material in complex matrices.