Seaweed as a sustainable solution for rare earth elements removal from acid mine drainage

Acid mine drainage (AMD) is characterized by its acidic nature, persistence, and high levels of toxic elements, causing severe environmental damage. Traditional passive treatments focus on AMD neutralization and removing common toxic metals, often neglecting the recovery of valuable elements, such as rare earth elements (REEs). This study explores, for the first time, the use of seaweeds for REEs removal from real AMD, turning an environmental challenge into a potential valorisation scheme. Two seaweed species (Gracilaria sp. and Ulva sp.) in their dried (biosorption) and living (bioaccumulation) forms were studied to remove REEs. Direct application in original AMD pH was unfeasible (negligible removal-<= 10 %). Through chemical precipitation with NaOH, unwanted elements could be separated with no relevant precipitation of REEs. At pH 5.0, bioaccumulation was most effective in removing REEs, where Gracilaria and Ulva species showed high performances (60 and 54 % and FREEs 430 and 214 mu g/g in the seaweed biomass, respectively). In addition to pH, seaweed dosage was revealed to influence REEs removal efficiency, with Gracilaria sp. achieving a bioconcentration factor of up to 532 L/Kg. Seaweeds demonstrated removal efficiencies comparable to previously studied sorbents, synthetic and nature-based (with modifications), highlighting the significant potential of living seaweeds for REEs removal and recovery from real AMD with easy integration in passive systems.