Bioleaching of rare earth elements (REEs) from Indonesian red mud by the bacterium Bacillus nitratireducens strain SKC/L-2

Abstract

Red mud, a residue of the bauxite industry, represents a secondary source of rare earth elements (REEs) with substantial commercial value and untapped potential. Bioleaching, an innovative, cost-effective, and environmentally friendly method, offers a means of extracting valuable metals from mining wastes. This study explored the bioleaching of Indonesian red mud using Bacillus nitratireducens strain SKC/L-2 to recover REEs. The experiments were carried out over three days at 25 °C with different concentrations of red mud (1.5, 3, and 6 g/L) and a 10% v/v bacterial inoculum in a specific bioleaching medium. The findings indicated a slight reduction in REEs extraction by the bacterium as the red mud concentration increased from 1.5 to 6 g/L in the direct bioleaching process. In the experiment using 1.5 g/L red mud, 16 REEs were successfully extracted, with high extraction levels for specific elements such as Lu (92.0%), Tb (80.61%), and Gd (67.42%). However, when the red mud concentration was increased to 6 g/L, the survival potential of Bacillus nitratireducens strain SKC/L-2 decreased, leading to reduced recovery of elements such as Lu (76.80%), Tb (70.30%), and Gd (55.83%). The study reveals the behaviour of Bacillus nitratireducens in interacting with red mud and enduring high alkalinity, resulting in REEs extraction. These findings enhance the understanding of microbial interactions with red mud and provide insights into potential resource recovery applications.