Redistribution and enhanced bioavailability of rare earth elements speciation induced by mining-driven transformation in ionic rare earth mining areas

Mining is an anthropogenic activity, that alters rare earth elements (REEs) distribution, changes the ecological environment of mining areas, and pollutes surrounding areas. However, the impacts of mining activities on REEs distribution and chemical morphological changes in mining soils are still poorly understood. In the present study, REEs fractionation and chemical speciation in both mining and unexploited soils were investigated through BCR sequential extraction. Most of the soil samples presented positive cerium (Ce) anomalies and negative europium (Eu) anomalies. The ratios of light rare earth elements (LREEs) to heavy rare earth elements (HREEs) and the total rare earth elements (TREEs) contents in mining soil were lower than those in unexploited soil. In unexploited soils, REEs were dominated by the residual fraction, and the exchangeable fraction was the lowest, accounting for 50.85-91.41% and 0.15-13.02% of TREEs, respectively. Mining activities modified REEs chemical speciations, which notably increased the exchangeable fraction (0.47-56.00%) and decreased the residual fraction (4.62-65.01%) in mining areas. Moreover, the reducible fraction also significantly increased in mining soils, accounting for 21.15-63.29% of TREEs. Compared with the unexploited areas, the relationships between soil properties and REEs distribution in mining areas were more pronounced. However, mining activities exerted little effect on the chemical speciation of Fe and Al, and the proportions of the residual fraction were dominant both before and after mining. These results demonstrated that mining activities decreased the contents of TREEs, redistributed REEs chemical morphological patterns, and increased their mobility and bioavailability, which are the principal anthropogenic sources of REEs entering environments and increase the risk of soil pollution.