RARE EARTH ELEMENT AND YTTRIUM RESOURCES IN ACID MINE DRAINAGE IN THE ILLINOIS BASIN, USA

In the Illinois Basin, acid mine drainage (AMD) is often associated with abandoned coal mines and is characterized by low-pH and high concentrations of sulfate and metals. Our previous research showed that some AMD has anomalously high concentrations of lanthanide rare earth elements plus yttrium (REY), which are considered critical resources necessary for modern society.

We report here REY concentrations in AMD collected at 42 sites across the Illinois Basin. The AMD was characterized by a wide range of values for pH (i.e., 1.93-7.55), sulfate (i.e., 500-8,250 mg/L,) aluminum (i.e., 0.1-140 mg/L), and TDS (i.e., 430-6,525). Geochemical analyses indicate that the total concentration of REY in these samples varied between 7.2 and 9,880 μg/L with an average value of 1,058 μg/L. Most of the AMD showed enrichment in Y, Nd, Gd, Dy, Tb, and Ce as well as non-REY elements such as Ni, Co, Cu, Zn, and Cd. The REY concentrations were normalized to the North American Shale Composite (NASC), which represents a proxy for the rock with which mine waters interact, and showed that AMD in the Illinois Basin is generally enriched in the middle REY.

Our analysis of the data showed that even though low-pH discharges displayed higher REY concentrations than circumneutral discharges, overall, there was no statistical correlation between the pH and the total REY concentration. Notably, we have not observed any relationship between REY concentration in AMD and the spatial proximity to the Hicks Dome, which is considered a possible source of REY in the Illinois-Kentucky Fluorspar District. We hypothesize that the lack of spatial correlation between REY enrichment and Hicks Dome may be attributable to secondary hydrothermal REY enrichment of coal along major faults in the Illinois Basin.

Our results indicate that AMD in the Illinois Basin could serve as an important source of REY. Many of these drainages will continue to cause environmental problems for decades. Coupling recovery of REY with mediation of these waters could provide both economic and environmental benefits.