Valorisation of acid mine drainage
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2024-12-06
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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.The present results highlight the potential of living seaweeds (Gracilaria sp. and Ulva sp.) in removing rare earth elements from raw acid mine waters, indicating the technology's potential in addressing a global threat to the aquatic environment and resource valorisation. Through the bioaccumulation process using Gracilaria sp., removal efficiencies up to 75 % were achieved within 24 h both in the original physicochemical condition and pH-adjusted condition of AMD. Seaweed biomass could recover the REEs from the AMD waters up to 1400-fold more concentrated. To the best of our knowledge, these are the first results indicating the use of living seaweed as the basis for a technology to remove and recover REEs from true AMD waters. Although pH adjustment is deemed necessary to remove non-interest elements, the proposed methodology could easily be integrated into current AMD treatments such as wetlands. Furthermore, the valorisation and beneficiation of AMD water is a promising opportunity for a circular economy. The proposed process could be an alternative to conventional methods of extracting REEs from ores and lead to AMD being perceived as a resource rather than a waste material. This will ensure that the sustainable development process is achieved, and AMD environmental footprints are minimized.
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