Behavior of rare earth elements in acid coal mine drainage in Shanxi Province, China

Rare earth element (REE) concentrations were determined in acid mine drainage (AMD), bedrock, pyrite, and coal samples from the Sitai coal mine and the Malan coal mine in Shanxi province, China. The AMD displayed high REE concentrations with typical convex shale-normalized patterns. The REE concentrations in the bedrock samples are one order of magnitude higher than those found in pyrite and coal samples. The high REE concentrations in AMD most likely come from the acidic solution leached out REE in bedrock. Results from laboratory and field experiments show that pH is the most important factor controlling the fractionation of REE; but Fe, Al, and Mn colloids and secondary minerals also affects their fractionation. As the pH increased from 4 to 6, the concentrations of total dissolved REE decreased from 520 to 0.875 µg L⁻¹. Fe and Al in AMD has less influence on the fractionation of dissolved REE than low concentrations of Mn. HREE were preferentially removed by secondary minerals and colloids, followed by MREE. Rare earth element’s speciation modeling indicates that sulfate complexes (LnSO₄ ⁺ and Ln(SO₄) ₂ ⁻ , 79–91%) and free-metal species (Ln³⁺, 8.8–21%) are the dominant REE species in the AMD, but the REE-sulfate complexation could not explain the MREE-enriched patterns.