Labile fraction-based assessment of rare earth elements in contaminated sediments

The distribution and bioavailability of lanthanides in metal-rich sediments have been studied performing a transect sampling across an estuary affected by acid mine drainage and the combination of 24 h passive sampler deployment (diffusive gradient in thin films; DGTs) and determination of labile fractions (i.e., porewaters, acid-extractable, associated to carbonate and ion-exchangeable). Relationship between concentrations in DGTs and the rest of labile fractions were no observable. Rare earth elements (REE) in DGTs ranged from 0.75 to 4.9 μg L^-1, 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 with river influence. REE and Y are preferentially associated to the carbonate-associated and acid-extractable fractions, although exhibiting a high variability (3.7-74 % for REE and 6.4-94 % for Y in the acid-extractable fraction and from 1.0 to 71 % for REE and 2.0-95 % for Y in the carbonate-associated fraction) but scarcely contained in the ion-exchangeable fraction. This variability seems to be controlled by the mineralogical assemblage, especially those REE-carrying minerals such as Al oxyhydrosulfates, phosphates, Fe oxyhydroxysulfates and aluminosilicates. REE and Y appears to be preferentially associated to Al and S in the acid-extractable fraction while in the carbonate-associated one these elements seem to be related to Fe and P. The application of NASC-normalized patterns to environmental compartments suggests REE and Y retained in the sediment not only come from labile species in porewaters but also from the passing of Al nanoparticles and colloids through the diffusion layers of the DGTs. This information would have important implications for the validation of these devices for monitoring REE and Y exposure in heavily metal-polluted sediments worldwide.