Valorisation of acid mine drainage: Studying biosorption and bioaccumulation of rare earth elements by seaweeds

Acid mine drainage (AMD) nature, persistence and the considerable amount of toxic elements cause significant environmental damage. Traditional passive treatment systems typically focus on neutralizing AMD using limestone and removing common toxic metal(loid)s, and often overlook the recovery of economic and strategic elements (e.g., rare earth elements (REEs)). This study is aimed at assessing for the first time the use of seaweeds to remove REEs from AMD, transforming an environmental problem into a resource. The ability of three seaweed species (Gracilaria sp., Ulva sp., and Fucus sp.) to remove REEs was studied in their dried (biosorption) and living (bioaccumulation) forms. Bioaccumulation was the most efficient process, with Gracilaria and Ulva species showing better performances (75 and 44 %, respectively), also removing over 60 % of Fe. Adjusting the pH of AMD with NaOH successfully separated unwanted elements with minimal REEs loss. After pH adjustment, REEs removal did not improve for either species, except for Dy removal. Seaweed dosage was crucial for a higher REEs removal, with Gracilaria sp. showing a higher bioconcentration factor (up to 1470). FTIR and SEM-EDS analysis identified sulphonate, carboxyl, and alkyne groups as key in binding elements to Gracilaria sp. biomass. Overall, the results demonstrate that seaweed-based biotechnologies are a promising alternative for treating AMD and recovering valuable elements, which can be easily incorporated into the current passive treatment systems.