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 investigatethe biologically induced leaching of some valuable elements from raw RM without preliminarybiomass enrichment and inoculum, simultaneously reducing RM polluting potential and extractingmetals for their subsequent recovery within a circular economy-based approach. In addition to themissing inoculum, such an approach is challenging since high RM alkalinity and pH, as well as theabsence 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). Bioleachingtests were performed at different temperatures (T = 22 C and 28 C; and also 15 C for RM-II) andsolid-to-liquid ratios (S/L = 2%, 5%). A sudden drop in pH from alkaline to constant neutral/acidicvalues was observed in almost all tests, and such results were attributed to biological activity sinceabiotic tests did not show any pH decrease. The best results in terms of extraction were achieved withRM-II, in particular for Al, Mg and Mn (17%, 42% and 47%, respectively). At 2% S/L, the highesttemperature allowed for a better metal release, while at 5% S/L, the highest extraction of Al, Mg andTi was observed at 22 C. As expected, iron was less available to leach at the achieved pH values, asit was mostly present as hematite in both RMs. Finally, the first microbiological characterisation ofthe autochthonous biomass selected during the bioleaching treatment of RM was provided.