Bioleaching of Valuable Elements from Red Mud: A Study on the Potential of Non-Enriched Biomass

Red mud (RM) is the main residue produced by the alkaline extraction of alumina (Al2O3) from bauxite, and it contains valuable metals such as iron (Fe), aluminium (Al), titanium (Ti), magnesium (Mg), manganese (Mn), rare earth elements, etc. This research aimed to investigate the biologically induced leaching of some valuable elements from raw RM without preliminary biomass enrichment and inoculum, simultaneously reducing RM polluting potential and extracting metals for their subsequent recovery within a circular economy-based approach. In addition to the missing inoculum, such an approach is challenging since high RM alkalinity and pH, as well as the absence of any sulphides, constrain the use of the most common biohydrometallurgical techniques. Red Muds from two European locations were tested (RM-I and RM-II, respectively). Bioleaching tests were performed at different temperatures (T = 22 C and 28 C; and also 15 C for RM-II) and solid-to-liquid ratios (S/L = 2%, 5%). A sudden drop in pH from alkaline to constant neutral/acidic values was observed in almost all tests, and such results were attributed to biological activity since abiotic tests did not show any pH decrease. The best results in terms of extraction were achieved with RM-II, in particular for Al, Mg and Mn (17%, 42% and 47%, respectively). At 2% S/L, the highest temperature allowed for a better metal release, while at 5% S/L, the highest extraction of Al, Mg and Ti was observed at 22 C. As expected, iron was less available to leach at the achieved pH values, as it was mostly present as hematite in both RMs. Finally, the first microbiological characterisation of the autochthonous biomass selected during the bioleaching treatment of RM was provided.