Distribution of scandium in red mud and extraction using Gluconobacter oxydans

Abstract The aim was to study bioleaching of rare earth elements from red mud by a chemoorganotrophic microorganism, and to investigate the microbe-mineral interactions. Red mud is a promising resource for scandium, whose concentration was 92 and 54 mg/kg in two samples from Germany and India, respectively; however, both showed appreciably high concentration of zirconium. A gluconic acid-producing bacterium, Gluconobacter oxydans (DSMZ 46616), was used in the bioleaching experiments under parametric variation of inoculum concentration, pulp density, and culture adaptation. At a solid load of 10% (w/v) red mud with 10% (v/v) bacterial inoculum at 37 °C, a maximum solubilisation of 83% and 94% Sc was observed after 18–20 d of incubation for Indian and German red mud, respectively. The total amount of gluconic acid excreted by the bacteria increased with an increase in pulp density up to 10% (w/v). After bioleaching, SEM-EDAX analysis of the solid residue depicted enrichment of zirconium associated with the silicate matrix, and the bacterial cells were adhered to the major mineral matrix.