A novel green strategy for biorecovery of valuable elements along with enrichment of rare earth elements from activated spent automotive catalysts using fungal metabolites

Abstract

Metals recovery from spent automotive catalytic converters (SACCs) has gained great attention due to high metal content of SACCs and their potential to pollute the environment. This study presented a novel green strategy for treating SACCs using oxalic acid-enriched spent culture medium from Aspergillus niger cultivations. To enhance oxalic acid production, the Central Composite Design (CCD) was applied, which demonstrated that glucose (27.06 g/L), NaNO3 (0.9 g/L), disodium oxalate (7.7 g/L), MnSO4·H2O (0.28 g/L), and ethanol (0.65%(v/v)) were the optimum values leading to production of 15.3 g/L oxalic acid. The results of metals biorecovery with the fungal metabolites showed that pulp density of 15 g/L, temperature of 60 °C, and leaching time of 6 h resulted in the highest extraction of 99.1% Al, 99.3% Si, 82.2% Mn, 91.9% Zn, 17.6% Ba, 99.5% Fe, 92.2% Sr, 35.7% Ti, 60.9% Pt, and 73.7% Pd, as well as maximum enrichment of rare earth elements (REEs) in the residual powder. The EDX-mapping analysis indicated that the concentration of ∑REEs was nearly 8% in the initial waste powder, while it reached around 81% in the residual powder after bioleaching. The bioleaching mechanism was further analyzed by characterizing the bioleaching residues through XRD, FTIR, and FESEM analyses.