Geochemical signatures of rare earth elements and yttrium in the vicinity of an ion-adsorption type deposit: roles of source sediment control

Abstract

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 (R 2 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