Fractionation of rare earth elements in water samples from the Paraíba do Sul River

Abstract

The Paraíba do Sul River Basin (PSRB), faces multiple pollution sources that may release rare earth elements (REE), yet data on their natural and anthropogenic levels remain scarce. This study provides the first spatiotemporal assessment of REE distribution in the PSRB, investigating total (TC), 0.45 µm filtered (FC), and labile (LC) concentrations in surface waters along seven distinct sites in São José dos Campos city during both wet and dry seasons. Labile concentrations were determined using passive samplers via the diffusive gradients in thin films (DGT) technique. Sum of TC (∑TCREE) ranged from 4.8 to 24 ng mL⁻¹, depending on the site and campaign. Sum of FC (∑FCREE) were higher than those reported for other Brazilian and pristine global rivers (ranged from 2.3 to 20 ng mL⁻¹) also depending on site and campaign. Maximum sum of LC (∑LCREE) was lower than 1.3 ng mL⁻¹. The high ∑FCREE/∑TCREE ratio observed (mean value of approximately 60% for both the wet and dry campaigns), combined with the low ∑LCREE/∑TCREE ratio (mean values of around 8% in the wet season and about 3% in the dry season), suggests that a substantial portion of REE is associated with particulate matter rather than being in labile complexes or truly dissolved form. While the FC showed limited seasonal variability, DGT results revealed a strong positive correlation between REE atomic number and lability during the dry season (r = 0.91). This trend was not observed in the wet season, likely due to increased hydrological variability, colloidal mobilisation, and complexation with dissolved organic carbon (DOC). The transport of REE in the PSRB was strongly influenced by colloids and the presence of Fe, Mn, and Si, particularly affecting light REE. The integration of DGT concentrations with equilibrium-based chemical speciation modelling enabled a more comprehensive assessment of REE speciation in the PSRB. Positive La, Gd, and Yb anomalies at polluted sites indicate anthropogenic inputs, with La and Gd as potential emerging contaminants. Combining DGT and conventional sampling effectively assessed REE speciation and complexes lability, supporting environmental monitoring and risk assessment.