Background: Red mud is the residual mass resultant from NaOH digestion of bauxite ore by following the Bayer process for aluminum extraction. The global stockpile of red mud is estimated to be about 3 billion tons that either sent to marine disposal or as the dry stack in open space. In both cases, it poses a great environmental threat due to the high alkalinity and the presence of several heavy metals. On contrary, red mud contains a significant amount of rare earth elements (REEs). The increasing demands of REEs with fast depleting mineral reserves are therefore presenting red mud a potential secondary reservoir for REEs’ exploitation. In recent times, numerous studies are conducted using the pyro- and hydro-metallurgical routes albeit the disadvantages like high energy consumption, low recovery, and generation of secondary wastes have found to be associated with the disclosed processes. Due to the inherent benefits of low environmental stress, the less energy-intensive process using the microbial activity, bioleaching is increasingly attractive to the metallurgical operations, especially from the secondary resources.

Results: Henceforth, a feasibility study on the bioleaching of red mud has been investigated using Penicillium chrysogenum strain KBS3 (accession number GQ228447) with glucose, sawdust, and molasses as the substrate material. Three different modes of bioleaching (one-step, two-step, and spent medium) were examined with different capacity of metabolic production of acids. One-step bioleaching involving 12 mM citric acid, 2.5 mM oxalic acid, 1.8 mM tartaric acid, and 1162 mM gluconic acid. Whereas, the respective biogenic acid production was observed to be 15 mM, 1 mM, 0.5 mM, and 152 mM in two-step bioleaching, which were 63 mM, 29 mM, 23 mM, and 3 mM in the spent medium bioleaching while using glucose as the substrate and pulp density at 3%. Concomitant bio-mobilization was analyzed to be 79% Y, 28% La, and 28% Ce in a single-step bioleaching system. In the spent medium bioleaching 63% Y, 28% La, and 28% Ce could be mobilized into biogenic lixiviant, which was 67% Y, 20% La, and 15% Ce in a two-step leaching mode. Using molasses as the substrate, citric acid (4.21 mM, 3.57 mM, and 4.85 mM), oxalic acid (1.55 mM, 1.0 mM, and 0.09 mM), tartaric acid (1.18 mM, 0.95 mM, and 2.17 mM), and gluconic acid (210.19 mM, 52.5 mM, and 0.09 mM) were involved in one-step, two-step, and spent-medium bioleaching, respectively at the same pulp density of 3%. The resultant bio-mobilization was analyzed to be 57% Y, 13.5% La, and 12.77% Ce in one-step; 57% Y, 14% La, and 12% Ce in a two-step, and 49% Y, 6.3% La, and 2.9% Ce in the spent-medium bioleaching system. The insignificant mobilization of REEs (1-5%) was observed in the case of abiotic controls.

Conclusions: One-step bioleaching has shown good potential for the bio-mobilization of REEs from red mud using Penicillium chrysogenum strain KBS3 and glucose substrate. Future studies in this context may result in a more efficient process in terms of higher-yielding along with its environmentally-friendly properties.