Controls on rare-earth element transport in a river impacted by ion-adsorption rare-earth mining

Rare-earth elements (REEs) are known to be a group of emerging pollutants, but the geochemistry of REEs in river waters in ion-adsorption rare-earth mining areas has attracted little attention. In this study, samples of the <0.45 μm and 0.22-0.45 μm (large colloids) water fractions and acid-soluble particles (ASPs) were collected from a river impacted by ion-adsorption rare-earth mining activities. The roles of ligand complexation, colloid binding, and particle adsorption in REE transport and distribution were also investigated. Results showed higher concentrations of REEs in the <0.45 μm fraction of all sampling sites (3.30 × 10^-2-9.42 μM) compared with that in the control site (1.21 × 10^-3 μM); this fraction was also characterized by middle REE enrichment at upstream sites, where REEs are mainly controlled by the <0.22 μm fraction (55%-94% of the species found in the <0.45 μm fraction) and ligand complexation (REE³⁺, REE(SO₄)⁺, and REE(CO₃)⁺). At downstream sites, heavy REE enrichment was observed, which was largely determined by binding to large colloids (68%-83% of the species found in the <0.45 μm fraction) and adsorption to particles (>90% of the acidified bulk water). Furthermore, REE patterns indicated that the REE-associated large colloids were mineral or mixed mineral-organic matter (OM) at upstream sites and OM-dominated or functionalized at downstream sites. The particles were mainly coated by inorganic matter substances (e.g., Fe/Al oxyhydroxides). In summary, our results reveal that REE patterns provide a useful tool to study the fate of REEs in ion-adsorption rare-earth mining catchments.