Enhanced REE Sorption to MnO ₂ with PO ₄ and P ₂ O ₇ Ligands and Selective Desorption and Enrichment of Heavy REEs

Acid mine drainage contains elevated levels of Mn(II) and critical minerals, including cobalt and rare earth elements (REEs). Even under acidic conditions, these metals can adsorb to the surface of MnO2, which can be formed in situ via Mn(II) oxidation by permanganate. To lower reagent costs, we investigated conditions in which only a fraction of Mn(II) was oxidized. Additionally, phosphorous-bearing ligands (PO4, P2O7) were added to alter the surface chemistry of MnO2, optimizing REE recovery. In stock solutions containing only REEs and MnSO4, with PO4 and P2O7 ligand concentrations as low as 3 mM, adsorption to MnO2 was significantly enhanced over that occurring in ligand-free solutions. MnO2 surface charge was substantially more negative when ligands were present. For a synthetic acid mine drainage (SAMD) solution, where only 30% of the Mn2+ was precipitated, adding these same ligands led to better or similar REE recovery as when all Mn2+ was precipitated with excess KMnO4. In scaled-up experiments, the amount of KMnO4 and NaOH used was decreased by up to 74 and 80%, respectively, while achieving similar or higher concentrations in the solid of both Co (24.4 mg of Co g–1 of solid) and REEs (7.07 mg of REE g–1 of solid). Furthermore, HREEs can be effectively and selectively desorbed from the surface of the MnO2 by adding oxalate solutions or by adjusting the pH to 9–9.5 in the case of trials where P2O7 was used to adsorb REEs to MnO2. This research promotes sustainable management of water resources and innovative recycling of waste, furthering the United Nation’s sustainable development goals, specifically regarding sustainable consumption, industry, and innovation.