Rare earth elements uptake in bivalves and assessment of DGT as a proxy for bioavailable fractions

Rare earth elements (REEs: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu) are increasingly recognized as emerging contaminants in marine environments, yet their bioavailability and bioaccumulation processes remain insufficiently understood. Sediment-bound REEs may be bioaccumulated by benthic organisms, with deposit- and filter-feeders at greatest risk. This study investigates the bioaccumulation of sediment-released REEs in the bivalve Ruditapes philippinarum under controlled laboratory conditions, with a focus on tissue-specific accumulation patterns and the evaluation of Diffusive Gradients in Thin Films (DGT) as a proxy for bioavailable REEs. Sediments were collected from two contrasting coastal environments in SW Spain: the Río San Pedro, a relatively unpolluted site, and the Guadiana estuary, characterized by elevated metal content. Sediment REE concentrations in the Río San Pedro are relatively low (84.8 mg kg^-1), whereas the Guadiana estuary exhibits elevated levels (215.8 mg kg^-1), potentially influenced by wastewater discharges and the proximity of a hospital. Clams and DGT exposure experiments were conducted in triplicates. Water-type DGTs with Chelex resin were deployed at the sediment-water interface to integrate labile REE fluxes over time. From each tank, three clams and one DGT device were sampled after 0 h, 24 h, 48 h, and 7 days. REE concentrations, along with selected metals (Co, Cu, Zn, Ni, Pb) are determined in different clam tissues (gills, digestive gland, and remaining soft tissues) to assess tissue-specific uptake and internal partitioning, while DGTs are used to characterize time-resolved accumulation dynamics of labile REEs. By comparing REE accumulation in clam tissues with DGT uptake kinetics the aim of the study is to critically assess whether DGT-measured concentrations are representative of biologically available REEs and to assess the suitability of passive sampling techniques for monitoring REE contamination in coastal environments. The results also provide a preliminary assessment of potential exposure of benthic organisms to REEs and other metals, contributing to an initial evaluation of ecological risk in sediment-influenced systems and contribute to the current understanding of REE bioavailability.