Bioleaching of critical raw materials from bauxite residue with Aspergillus sp.

Bauxite residue (red mud) represents a promising secondary resource for critical raw materials (CRMs). Biohydrometallurgy offers an eco-friendly route for CRM recovery. In this study, an Aspergillus sp. was used to bioleach CRMs from bauxite residue (BR) at a relatively low temperature (22 °C), a short contact time (5 days), and high pulp densities (up to 20 w/w%). During one-step bioleaching, fungal activity lowered the pH to 3.1‒5.3, depending on the BR concentration. The highest recovery efficiencies were achieved at 1 % BR, reaching 77 % Al, 47 % Ti, 50 % V, 42 % Ga, 61 % Y, 65 % Zr, 20 % La, 18 % Ce, and 35 % Nd. Increasing the BR concentration led to higher CRM concentrations in the leachate but reduced overall recovery efficiency. Vanadium extraction was maximised at 20 w/w% BR, yielding 48 mg/L in the leachate, corresponding to an 18 % leaching efficiency. Ecotoxicological assessment revealed increased toxicity of the leaching residue obtained at 20 % BR compared with the original material. Washing further increased residue toxicity and generated a toxic washing effluent, indicating that alternative post-treatment strategies are required. Overall, the results demonstrate that low-temperature, short-term fungal bioleaching at lower pulp densities can provide a promising approach for CRM recovery from bauxite residue.