Comparative analysis of hydrometallurgy and biosorption techniques for recovering rare earth elements from acid mine drainage at an abandoned coal mine

Abstract

Rare earth elements (REEs) are essential components in many modern technologies. Acid mine drainage (AMD) is a low-grade REE feedstock that could serve as a potential secondary source of REEs, though sustainable REE extraction techniques remain under development. This study compared a biosorptive approach with conventional hydrometallurgical recovery of REEs from AMD generated at an abandoned coal mine. In the hydrometallurgical method, solvent extraction with di-(2-ethylhexyl) phosphoric acid (D2EHPA) under natural AMD pH (∼2.5) recovers nearly all REEs from the aqueous phase, maintaining performance over three consecutive cycles. Stripping with 6 mol/L HNO₃ achieves high recovery of light (96.61% ± 4.32%), heavy (98.91% ± 4.51%), and total REEs (98.92% ± 4.52%) across three cycles. Subsequent oxalic acid precipitation yields total rare earth element (TREE) recoveries of 46.69% ± 0.73% and 56.17% ± 1.81% at pH 1.0 and pH 2.0, respectively. In comparison, biosorption using a Serratia spp. consortium achieves >99% extraction of light, heavy, and total REEs within 90 min at pH 4.0 with a reduced biomass dosage. The biosorbent exhibits an irregular, rough, and porous surface containing numerous cavities and folds that serve as potential binding sites for metal ions. The experimental data fit well to the Freundlich isotherm, suggesting multilayer adsorption on a heterogeneous surface. Kinetic analysis indicates that the adsorption follows a pseudo-second-order model, with an equilibrium TREE adsorption capacity of 0.62 μg/mg biomass. Nitric acid stripping of REEs from the biomass preferentially recovers REEs over non-REE impurities. In contrast, oxalic acid precipitation also exhibits pH-dependent recovery but shows much lower selectivity, with higher pH promoting REE precipitation while simultaneously increasing co-precipitation of non-REE impurities. Both approaches demonstrate high efficiency, achieving comparable TREE recoveries through the sequential steps of mixing, stripping, and precipitation. However, transition-metal interference, particularly from Fe at elevated pH, affects the purity of the biosorption-derived stripped solution, even though similar amounts of REEs are recovered. This study demonstrates the potential of biosorption as a complementary approach to conventional methods, offering comparable REE recovery with lower reagent use and reduced secondary waste generation. Future work should include comprehensive environmental evaluations, such as life-cycle assessment and energy use analysis, to substantiate these benefits and guide scale-up for field applications.