Geochemistry of rare earth elements in a neutral mine drainage environment, Anjir Tangeh, northern Iran

This study investigates the geochemical characteristics of rare earth elements and yttrium (REE + Y) in the neutral mine drainage (NMD) and river water in the vicinity of the Anjir Tangeh coal washing plant, Mazandaran province, northern Iran. The NMD water had a neutral pH along with high sulphate and bicarbonate concentrations. The total concentrations of REE in the NMD water were lower than in the river water. According to the North American Shale Composite (NASC) normalised concentrations, the patterns of NMD showed heavy REE (HREE) enrichment while the river water represented middle REE (MREE) enhancement. Both negative europium (Eu) and positive gadolinium (Gd) anomalies were observed in the drainage and river water within the study area. The sum of REE concentration increased gradually downstream of the discharging point of the plant reject water, in contrast to decreasing of Al, Fe, Mn and Cu + Pb + Zn concentrations. The river sediment with high Fe concentrations was enriched with lanthanum (La) and cerium (Ce). The overall REE pattern of the NMD approximates those of coal except for the depletion in light REE (LREE) and the enrichment of Eu. Dominant inorganic REE species in the river water included Ln (CO3)(2)(-) and LnCO(3)(+), while LnCO(3)(+), Ln (CO3)(2)(-) and Ln (SO4) (+) were the main complexes in the NMD. The saturation index indicated that Al and Fe were both oversaturated in the drainage and river water, which can control the fractionation of REE by the sorption process. According to both hierarchical cluster analysis (HCA) and principal component analysis (PCA), pH, Al, and P were the significant parameters in the REE distribution.