Selective Bioleaching of Base Metals and Rare Earth Elements from Gold-Rich Printed Circuit Board (PCB) Powder

• Sulphur-oxidising route outperformed iron-oxidising route in base metal bioleaching • thiooxidans leaches approximately 78-90% of key base metals within 4 days • Comparable recoveries are maintained when scaling bioleaching from 10 L to 80 L • Precious metals remain insoluble, enriching gold in the residue from 0.9% to 2.0% Selective extraction of base metals (BMs) and rare earth elements (REEs) from printed circuit boards with an exceptionally high Au concentration (993 ppm) was carried out using three chemoautotrophic microbial sets: Acidithiobacillus ferrooxidans (DSM 14882), Acidithiobacillus thiooxidans (DSM 9463), and a mixed culture of both. The bioleaching experiments were conducted at 30 °C and ambient pressure, with pre-cultivation periods optimised to maximise metal recovery while minimising overall processing time. Comparative analysis revealed distinct solubilisation profiles by iron-oxidising and sulphur-oxidising microorganisms, with A. thiooxidans achieving the highest recovery of BMs and REEs at a rate of 98% Co, 98% Cu, 90% Ni, 83% Sm, 78% Al, 78% Zn, 47% Nd, 47% Dy, 37% Cr, 37% Gd and 26% Pr within four days of bioleaching. Performance transferred from 10 L bench-scale to an 80 L pilot-scale bioreactor with broadly comparable extraction efficiencies. The extraction of precious metals (Ag, Pd, Pt, Au) was negligible, leading to an increase in Au concentration in the residual solid fraction from 0.9 to 2.0% (w/w). The removal of Co and Cr reduces the potentially hazardous metal inventory of the residue while producing a Co-enriched leachate for downstream recovery, highlighting a dual outcome of hazard reduction and resource recovery. These results provide a feed-specific basis for selecting sulphur- versus iron-driven bioleaching chemistries for Au-rich PCB fractions and demonstrate scalable, low-temperature selective BM and REE removal with reduced reliance on externally supplied mineral acids.