From waste to value: agromining of nickel, cobalt, and selenium from mine waste amended with sewage sludge

Background and aims: The global demand for critical elements such as nickel, cobalt, and selenium is projected to increase significantly, driven by the transition to renewable energy. Mine wastes and sewage sludge, often enriched with these elements, represent untapped secondary resources. Agromining, an emerging phytotechnology that employs hyperaccumulator plants to extract metals from contaminated or enriched soils, offers a sustainable recovery method. The aim of this study is to provide a crucial proof-of-concept for the recovery of the critical elements nickel, cobalt, and selenium from these alternative resources through agromining. Methods: Mine wastes from three Italian mining sites were amended with sewage sludge at rates of 0, 15, and 30 kg m⁻². Three hyperaccumulator species targeting cobalt, (Berkheya coddii), selenium (Astragalus bisulcatus) and nickel (Odontarrhena chalcidica) were grown on these wastes. Biogeochemical characterization of the substrate was performed before and after agromining. In addition, plant biomass and metal accumulation were monitored. Results: Sewage sludge increased biomass across all species, especially in A. bisulcatus, but metal accumulation decreased by 33–60% for nickel, 0–41% for cobalt, and 67–74% for selenium. Furthermore, sewage sludge and agromining enhanced mine waste fertility and microbial diversity. Conclusions: Agromining of sewage sludge-amended mine wastes shows potential for sustainable recovery of selenium and cobalt. However, high sewage sludge application increased plant biomass, but concurrently reduced plant tissue metal concentrations. This study underscores the importance of optimizing sewage sludge / mine waste ratios for successful agromining.