Insights into the recovery of rare earth elements from acid mine drainage: Fostering circular economy in mine water management.

Acid mine drainage (AMD) is an ongoing environmental challenge owning to its heterogeneous and complex chemical make-up, where a wide array of metals and minerals are found interwoven. Efforts for its sustainable management include its beneficiation, and, here, the recovery of rare earth elements (REEs) from coal mining AMD was explored through solvent extraction. Bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), Bis(2-ethylhexyl) phosphate (DEHPA), Tris(2-ethylhexyl) amine (TEHA), and tributyl phosphate (TBP) extractants were employed for REE recovery, with the first being the most effective (>94% for LREEs (i.e., Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd), Samarium (Sm), and Europium (Eu)) and >75% for HREEs (i.e., Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), and Lutetium (Lu)) and yttrium (Y)). Sulfuric acid (H2SO4) yielded the highest back-extraction (stripping) efficiency (59%), followed by hydrochloric acid (HCl) (49%) and nitric acid (HNO3) (45%). However, HNO3 was chosen as the most suitable stripping acid as it yielded the highest concentration of anionic species in the stripped (mother solution) solution, and therefore the optimum downstream secondary separation and purification. Sequential extraction revealed that REEs could efficiently be extracted using DEHPA. Results demonstrate that AMD could be used as a secondary source of minerals rather than being considered as a toxic, hazardous, and bio-recalcitrant wastewater matrix. REEs and metals removal and the effective treatment of the stripped solution can largely pretreat AMD, thus, in turn, averting numerous ecological challenges associated with its release to the receiving aquasphere. REEs extraction from AMD also improves its economic potential, hence making the mining process more sustainable and ecologically friendly by closing the production loop.