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Over the last decade, AMD waters have gained more attention as a potential source of metals due to the emerging need to recover or recycle metals from secondary resources. Metals recovery supports sustainability and the development of a circular economy with benefits for resource conservation and the environment. In this study, five extractants (Acorga M5640, LIX 54, LIX 622, LIX 622 N, and LIX 864) diluted (15% (v/v)) in Shell GTL with 2.5% (v/v) octanol were compared and evaluated for Cu recovery from an extreme AMD sample (5.3 ± 0.3 g/L Cu) collected at the inactive São Domingos Mine in the Iberian Pyrite Belt of Portugal. Of the five extractants, Acorga M5640 showed the best selective efficiency. Further tests showed that 30% (v/v) of this extractant was able to selectively extract ≈ 96.0% of the Cu from the AMD in one extraction step and all of the remaining Cu (to below detection) in three steps. Among the different stripping agents tested, 2 M sulfuric acid was the most efficient, with ≈ 99% of the Cu stripped, and the recyclability of the organic phase was confirmed in five successive cycles of extraction and stripping. Furthermore, contact time tests revealed that the extraction kinetics allows the transfer of ≈ 97% of the Cu in 15 min, and aqueous to organic phase ratios tests demonstrated a maximum loading capacity of ≈ 16 g/L Cu in the organic phase. Raising the concentration of Cu in the stripping solution (2 M sulfuric acid) to ≈ 46 g/L through successive striping steps showed the potential to recover elemental Cu using traditional electrowinning. Finally, a biological approach for Cu recovery from the stripping solution was evaluated by adding the supernatant of a sulfate-reducing bacteria culture to make different molar ratios of biogenic sulfide to copper; ratios over 1.75 resulted in precipitation of more than 95% of the Cu as covellite nanoparticles. Graphical Abstract

This study investigates the potential of hydroalcoholic extracts of Cistus ladanifer L., Erica Andevalensis and Rubus idaeus L. as a green method for the recovery of platinum group metals (PGMs) from both synthetic unimetallic solutions and multimetallic solutions obtained from the leaching of two different spent automotive catalytic converters (SACC). Experiments with unimetallic solutions revealed that E. andevalensis and R. idaeus extracts could separate about 70% of Pd and less than 40% of other tested metals (Al, Ce, Fe and Pt) from the solutions. Then, application of the plant extracts to two different SACCs leachates showed that E. andevalensis and R. idaeus extracts can induce high precipitation (> 60%) of Pd and Pt with co-precipitation of less than 20% of other metals. UV–Visible spectra analysis confirmed the bio-reduction of Pd2+ ions into Pd0 nanoparticles by R. idaeus extract, and Fourier transform infrared spectroscopy (FTIR) analysis revealed the contribution of functional groups of the phytochemicals present in the extract (such as phenols, flavonoids and anthocyanins) in the Pd2+ bio-reduction and stabilization. Afterward, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX) analysis of the precipitate obtained from one leachate with R. idaeus extract demonstrated the presence of Pd particles along with organic compounds and particles containing other metals. Therefore, particles were subjected to a washing step with acetone for further purification. Finally, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDX) analysis showed the high purity of the final Pd particles and high-resolution STEM allowed to determine their size variation of 2.5 to 17 nm with an average Feret size of 6.1 nm and confirmed their crystalline structure with an interplanar lattice distance of ~ 0.22 nm. This green approach offers various benefits including simplicity of Pd separation from the leachates as valuable nanoparticles that makes the process more feasible from economic and environmental standpoints. A process cost of ~ 20 $/g of Pd particles recovered was estimated (excluding manpower). Graphical abstract

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