Lactobacillus pentosus enabled bioleaching of red mud at high pulp density and simultaneous production of lactic acid without supplementation of neutralizers

Abstract

Heterogenous bioleaching by microorganisms producing organic acids is deemed as a compelling pathway for the recovery of valuable metals from red mud, but it is often performed at low pulp density, making this process not economically competitive. Here, Lactobacillus pentosus was employed to bioleach red mud at high pulp density and simultaneously produce lactic acid. The bioleaching performance of L. pentosus was first compared with Aspergillus niger and Gluconobacter oxydans, two most commonly used bioleaching strains. At 20 wt% pulp density and with pure glucose as a carbon source, L. pentosus could leach 31 % Al from red mud, while the leaching efficiency did not exceed 1 % for other two strains. Increasing the pulp density to even 30 wt% did not compromise the leaching efficiency of L. pentosus. When the enzymatic hydrolysate of rice straw was used to replace pure glucose as a carbon source, a comparable leaching efficiency of 30 % for Al was achieved, demonstrating that rice straw could be a cheap and renewable feedstock for bioleaching. Moreover, the alkali contained in red mud eliminated the addition of costly neutralizer as did in traditional lactic acid fermentation, enabling the simultaneous production of approximately 80 g/L lactic acid. Overall, this study offered a potentially economical and effective pathway for the bioleaching of red mud at high pulp density, and opened a new avenue for the production of lactic acid from lignocellulosic biomass.