Rare earth element (La, Ce, Y, Sc, and Gd) removal, biomass accumulation, and phyto-ash enrichment in floating treatment wetlands planted with Juncus inflexus.

Anthropogenic use of rare earth elements (REEs) in modern technologies has increased their occurrence in wastewater streams and aquatic environments. This study evaluated floating treatment wetlands (FTWs) planted with Juncus inflexus for removal, biomass accumulation, and phyto-ash enrichment of selected REEs (La, Ce, Y, Sc, Gd) from synthetic high-strength wastewater. Three REE systems were tested: MIX1 (La + Ce), MIX2 (Y + Sc), and MIX3 (Gd), at 1-20 mg L-1 per element. Removal efficiency, plant responses, uptake kinetics, mass distribution, short-term remobilization, and phyto-ash enrichment were assessed. After 14 days, total REE removal exceeded 54.9% across all treatments, reaching up to 95.5% for La/Ce and >91% for Gd. Bioconcentration factors exceeded 100 at lower exposure levels, consistent with relatively high plant-associated accumulation. Time-course measurements showed a rapid initial decrease in aqueous REE concentrations within the first 1-3 days, followed by slower stabilization. Mass-balance analysis at 5 mg L-1 showed that 40.6-68.1% of the initial REE mass was quantified in plant tissues after 14 days, predominantly in roots. Release experiments showed limited short-term REE remobilization from harvested biomass, while plant responses included descriptive variations in root anatomy and mainly transient changes in chlorophyll fluorescence. Root-derived ash showed clear REE enrichment, with AEF values of 16-22 for La/Ce and calculated REO-equivalent concentrations of 0.19-2.63 wt%. Overall, FTWs planted with Juncus inflexus reduced aqueous REE concentrations and concentrated REEs in harvestable biomass, supporting further evaluation of this system as a potentially low-input polishing and pre-concentration step for selected REE-containing wastewater streams.