An Evaluation of Bio-ethanol Byproducts for Biogenic Cyanide Production and Its Application for Sustainable Platinum Group Metals Recovery from Spent Automotive Catalysts

Biohydrometallurgy is a sustainable alternative for precious metal recovery but is often limited by the high cost of nutrient media for cyanide-producing bacteria. This study evaluated condensed distillers solubles, a low-cost byproduct of the bioethanol industry, as a substitute for yeast–peptone medium for Chromobacterium violaceum. Amino acid profiling, elemental analysis, and Fourier transform infrared spectroscopy confirmed that condensed distillers solubles is nutritionally comparable to yeast–peptone medium while being about 29-fold cheaper ($10.6/m³ versus $310/m³). Optimization of cultivation parameters identified 6% condensed distillers solubles, 5 g/L glycine, and pH 7 as optimal conditions, producing 57.8 ± 1.5 mg/L cyanide, approximately twice that obtained in yeast–peptone medium. The optimized culture was applied to bioleach Pd and Pt from spent automotive catalysts. Using a two-step bioleaching process, recoveries of 43.6 ± 3.5% Pd and 21.2 ± 0.4% Pt were achieved at 5% pulp density, outperforming one-step bioleaching and chemical cyanide controls. Spent automotive catalyst structural analyses revealed diffusion barriers from cordierite carriers and γ-Al₂O₃ coatings, explaining modest yields and underscoring the need for pretreatment. These findings provide a proof-of-concept for leveraging industrial byproducts to achieve cost-effective and scalable cyanide production for biohydrometallurgy. Condensed distillers solubles offers a sustainable pathway for precious metal recycling, advancing circular economy initiatives in urban mining.