Fractionation and speciation of rare earth elements in legacy mining lakes using diffusive gradients in thin films (DGT) and single-particle ICP-MS (spICP-MS): from seasonal baselines to extreme atmospheric deposition events

This study investigates the geochemical dynamics and ecological risks of rare earth elements (REE) in abandoned clay mining lakes in the second world’s largest producer of ceramic tiles in Santa Gertrudes (CDSG), Brazil. Using a multi-technique approach, combining conventional filtration, diffusive gradients in thin films (DGT), single-particle inductively coupled mass spectrometry (spICP-MS), and geochemical modelling, a seasonal baseline was established and the influence of an atmospheric particulate deposition event associated with an unprecedent 2024 wildfire episode was evaluated. Results showed that seasonal variability produced relatively minor changes in REE behaviour, whereas the 2024 atmospheric deposition event promoted substantial shifts in REE partitioning. The median total concentration (∑REE T ) surged from 3,200 ng L -1 in 2022 to 25,800 ng L -1 during the 2024 event, with Cerium (Ce) reaching a peak of 12,500 ng L -1 . A critical decoupling between fractions was observed: while the dissolved fraction (<0.45 μm) accounted for up to 82% of REEs in 2022, it dropped drastically to <1% in 2024, with DGT-measured lability largely varying across all campaigns. The detection of Ce-containing nanoparticles via spICP-MS (8.4×10 9 to 1.7×10 10 Particles L -1 ), with a median size of ∼30 nm and an average mass of 0.13 fg, suggested relevant inorganic colloidal transport within the filtered fraction following the atmospheric deposition event. Geochemical signatures revealed persistent positive Cerium (Ce) Europium (Eu) anomalies of lithological origin. Although risk quotients (RQ) for the labile fraction decreased during the atmospheric deposition event due to low lability, the substantial increase in total REE burden suggests a transfer of ecological risk from the dissolved to the particulate phase. We conclude that conventional filtration overestimates REE bioavailability and that episodic atmospheric disturbances fundamentally alter the partitioning and fate of rare earth elements in freshwater systems.