Distribution and bioavailability of rare earth elements (REE) in metal-rich marshland soils using passive samplers

Rare earth elements (REE) are a group of chemically similar metallic elements (i.e. lanthanide which are becoming increasingly important in the transition to a green, low carbon economy due to their key role in permanent magnets, lamp phosphors, batteries, catalysts and other applications [1]. The growing use of REE has led to an inherent increase of REE emissions into the environment and may pose a risk to living organisms, therefore their distribution and bioavailability should be studied across different environments. The distribution and bioavailability of REE in metal-rich marshland soils was studied by performing a transect sampling across the marshland and the combination of 24h passive sampler deployment (diffusive gradient in thin films; DGTs) and determination of labile fractions (i.e., porewaters, ion-exchangeable, acid-extractable and associated to carbonate). REE in DGTs ranged from 0.75 to 4.9 μg/L, while in porewaters most samples exhibited values below the detection limit of the equipment, which highlights the suitability of these devices to monitor trace pollutants at low concentrations in estuarine sediments. A spatial trend in REE and Y absorption by DGT was observed, with increasing values in sediment depth and with river influence. REE and Y are scarcely contained in the ion-exchangeable fraction, and preferentially associated to the carbonate-associated and acid-extractable fractions, although exhibiting a high variability. This variability seems to be controlled by the mineralogical assemblage, especially those REE carrying minerals (e.g., Al oxyhydroxysulfates, phosphates, poorly-sorbed onto Fe oxyhydroxysulfates and aluminosilicates). A comparison between REE and Y concentration in labile fractions (porewaters, DGTs, sequential extractions) showed no relationship between concentrations in DGTs and the rest of labile fractions. The application of NASC-normalized patterns to environmental compartment studied suggest that the REE and Y retained in the sediment does not only come from labile species in porewaters but also from the passing of Al nanoparticles and colloids contained in the sediments through the membranes of the DGTs.

Acknowledgements

This work was supported by the Spanish Ministry of Economy and Competitiveness under the research project TRAMPA (MINECO; PID2020-119196RB-C21).

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