Distribution, source and contamination of rare earth elements in sediments from lower reaches of the Xiangjiang River, China

Rare earth elements (REEs) are emerging micropollutants in aquatic environments. In this study, concentrations of REEs and major elements, and mineralogical compositions of sediments from lower reaches of the Xiangjiang River (China) were analyzed using ICP-MS technique. The results suggested that sediments were characterized by terrigenous compositions TiO2, SiO2, Al2O3, K2O, Na2O and P2O, and contained high concentrations of REEs with mean total REE concentrations (FREE) of 318.7 mg/kg. REEs were moderately enriched in upper river sediments, and slightly or less enriched in downriver sediments. The normalized REE distribution pattern for sediments was characterized by flat shalelike and Eu depleted V-shape REE patterns, which indicated REEs in sediments were lithologically contributed from sedimentary rocks and granites distributed in the watershed respectively. REEs in sediments were hosted mainly in Fe-Mn oxides, and sulfide and organic matters that were characterized by middle REEs (MREE) enrichments relative to light REEs (LREE) and heavy REEs (HREE), and the distribution and differentiation of REEs in sediments were controlled by clays, Fe-Mn oxides, organic matters and finer grains; and also by accessory minerals (e.g., zircon) from granite. The distribution features of REEs in sediments and BCR extraction results suggested that the sediment REE enrichment resulted from additional REE input from anthropogenic sources, including those in discharges from sulfide-ore smelting industries at Zhuzhou city and from phosphate fertilizer plants at Xiangtan city along the river. Thus, sediments were contaminated with REEs in moderate degree in upper river area, and REE contamination was then formed by superimposing anthropogenic REEs on lithological residues. Finally, concentrations of Ce > 100 mg/kg, Gd > 8.12 mg/kg, FREE >274.9 mg/kg, FLREE >252.3 mg/kg and FHREE >28.8 mg/kg here were recommended as the REE contamination levels that represented as REE indices for identifying and rating REE contamination in this mining impacted river.