Haiyan Liu, Huaming Guo, Olivier Pourret , Maohan Liu, Zhen Wang, Weimin Zhang, Zebing Li, Bai Gao, Zhanxue Sun, Pierre Laine

The elevated concentrations of rare earth elements (REE) and yttrium (REE+Y) in acid mine drainage (AMD) constitute an opportunity for REE+Y recovery. However, the source and control of REE+Y signatures in AMD remains uncertain. Water, rock, sediment and sludge samples were collected from an ion-adsorption rare earth mining area to investigate REE+Y concentration and fractionation patterns in AMD. High concentrations of REE+Y occur in the pristine mine water (MW), and decrease progressively with MW passing through nitrification-denitrification (NDT) and coagulating-precipitation (CPT) treatment procedures in a water treatment plant. Concentrations of REE+Y are 1 to 3 orders of magnitude higher in MW, NDT and CPT samples than those in well water (WW), and are negatively correlated (R2 0.72) with pH (3.8 to 8.7), suggesting that an enhanced acidic dissolution of minerals contributes REE+Y to AMD from the source area. Speciation calculation indicates that REE+Y are mainly free ions and sulfate complexes in MW and NDT samples, while carbonato and dicarbonato complexes in CPT samples. Normalized REE+Y patterns of water samples show a coherent enrichment of heavy REE (HREE) over light REE (LREE) and negative Ce anomalies. HREE-enriched patterns and Ce anomalies are kept relatively constant in MW, NDT and CPT samples, despite their huge disparity in REE+Y concentrations, indicating a limited impact of preferential precipitation of LREE over HREE on REE+Y fractionations. The HREE-enriched patterns possessed by AMD are similar to those of sediment samples, while are distinct from whole rocks. These behaviors suggest a major role of sediment source in controlling REE+Y concentrations and patterns in AMD. Results of XRD show abundance of muscovite, kaolinite and feldspar in sediments. Combining to SEM-EDS examinations, minerals such as kaolinite, schwertmannite (Fe16O16(SO4)3(OH)6(H2O)10·10H2O) and ferrihydrite are suggested to be the main hosts of REE+Y in sediments. Hence, HREE-enriched patterns of AMD result from preferential leaching of HREE from sediments which accumulate REE+Y after being initially mobilized from rocks under the conditions prevailing in mine site. The free form and sulfate complexation preserves REE+Y patterns and facilitates REE+Y mobility in the AMD system. Based on the plant treatment capacity, the potentially recoverable LREE and HREE are calculated to range between 1116 g/day and 3373 g/day, and between 1288 g/day and 3764 g/day, respectively.