Exploring Acid Mine Drainage (AMD) precipitates as a resource for critical metals: Integrating benchtop experiments, field studies, and modeling

Acid mine drainage (AMD) treatment generates precipitated solids rich in iron (HFO), aluminum (HAO), and manganese (HMO) oxides/hydroxides that are typically discarded, but can host critical metal concentrations comparable to low-grade ores.

To evaluate the biogeochemical conditions under which critical metals are enriched in AMD treatment systems, we conducted benchtop experiments, geochemical modelling, and year-long field investigations at two AMD treatment sites. These studies examined how microbial activity, sulfate concentration, pH, pre-existing sorbent, and other factors influence the mobility and precipitation of major element oxides and their ability to sequester critical metals during AMD treatment.

Benchtop experiments showed that biotic HMO and associated fungal biomass are highly effective sorbents, removing >99% of REE and 70% of Co, whereas abiotic HMO adsorbed substantially lower fractions. Ni adsorption was negligible by these sorbents. The relatively higher crystallinity and instability of abiotic HMO promoted mineral transformation and critical metal desorption. Sulfate enhanced REE adsorption by HAO at lower pH through increased sorbent availability and the formation of REE-SO4-ternary complexes, however, elevated sulfate concentrations reduced Al precipitation and REE adsorption at pH 4-5, due to the dominance of aqueous REE-sulfate and Al-sulfate complexes. Newly calculated binding constants for REE adsorption to amorphous Al(OH)₃ and basaluminite successfully modeled REE behavior observed in lab experiments. Field investigations showed that pH is the main parameter controlling Fe, Al, and Mn mobility, though flow rates, microbial activities, and presence of pre-existing sorbent also play important roles. We note significant Mn enrichment in precipitated solids, particularly under biotic conditions relative to concentrations in AMD. Mn-rich solids also preferentially sequester Co, Ni, Zn, and REE whereas Fe and Al phases shows low critical metal uptake. Geochemical models reproduced most observed trends in metal behaviors in the field setting. However, discrepancies between model and empirical data highlight the complexities of AMD systems and the role of microbially mediated processes which are potentially not adequately captured in the models.