Biohydrometallurgical approaches represent an emerging and environmentally sustainable method for recycling critical metals from spent batteries. Unlike conventional hydrometallurgical techniques that rely on strong acids or solvents, biohydrometallurgy employs microorganisms to extract metals through natural biochemical processes. This method is particularly relevant for recovering cobalt, nickel, lithium, and manganese from lithium-ion batteries (LIBs), offering a lower-energy, less toxic alternative to traditional methods.

One of the most studied bacterial strains in biohydrometallurgy is *Acidithiobacillus ferrooxidans*, a chemolithoautotrophic bacterium that thrives in acidic environments. This microorganism oxidizes ferrous iron (Fe²⁺) to ferric iron (Fe³⁺), which acts as a potent oxidizing agent for dissolving metals from battery cathodes. Another key species, *Acidithiobacillus thiooxidans*, metabolizes sulfur compounds to produce sulfuric acid, further aiding metal solubilization. Heterotrophic bacteria such as *Pseudomonas* and *Bacillus* species have also been explored for their ability to secrete organic acids like citric and gluconic acid, which chelate metals from battery waste.

The bioleaching mechanism involves two primary pathways: direct and indirect leaching. In direct bioleaching, bacteria adhere to the surface of battery materials and enzymatically oxidize metal sulfides or other compounds, releasing metal ions into solution. Indirect bioleaching relies on microbially generated ferric iron or sulfuric acid to chemically dissolve metals without direct microbial contact. For LIB recycling, indirect leaching is often more efficient due to the complex oxide composition of cathode materials (e.g., LiCoO₂, LiNiMnCoO₂). The process typically operates at ambient temperatures (20–40°C) and near-neutral to moderately acidic pH (1.5–3.0), significantly reducing energy consumption compared to pyrometallurgical smelting.

Pilot-scale applications of biohydrometallurgy for battery recycling have demonstrated promising results. A study conducted at the Technical University of Braunschweig achieved over 90% cobalt and lithium recovery from spent LIBs using a mixed culture of *Acidithiobacillus* species in a continuous stirred-tank reactor. Similarly, a pilot plant in Finland utilized *At. ferrooxidans* to process black mass (crushed battery waste), recovering 85% nickel and 78% cobalt within 10 days. These systems often incorporate pre-treatment steps such as mechanical crushing and sieving to enhance microbial accessibility to metal-bearing phases.

Contrasting biohydrometallurgy with chemical leaching reveals distinct advantages and limitations. Traditional chemical leaching employs inorganic acids (e.g., HCl, H₂SO₄) or reducing agents (e.g., Na₂S₂O₅) at high concentrations (1–4 M) to achieve rapid metal dissolution. While chemical methods offer faster kinetics (hours vs. days for bioleaching), they generate hazardous waste streams requiring neutralization and disposal. Biohydrometallurgy, though slower, produces minimal secondary pollution and can be integrated with downstream processes like biomineralization, where bacteria facilitate selective metal precipitation as sulfides or hydroxides.

Operational challenges remain in scaling biohydrometallurgical processes. Bacterial activity is sensitive to heavy metal toxicity, necessitating adaptive strains or genetic engineering to enhance tolerance. Process optimization is also critical; parameters such as pulp density (typically 1–10% w/v), aeration, and nutrient supply must be carefully controlled to maintain microbial viability. Recent advances include the use of extremophiles like *Sulfolobus* species, which tolerate higher temperatures (up to 80°C) and metal concentrations, expanding the range of processable waste types.

Economic analyses suggest that biohydrometallurgy could reduce operating costs by 30–50% compared to conventional methods, primarily due to lower reagent and energy inputs. However, capital costs for bioreactor systems and longer processing times may offset these savings in high-throughput industrial settings. Hybrid approaches, combining brief chemical leaching with subsequent biological metal recovery, are being investigated to balance efficiency and sustainability.

Regulatory and environmental considerations further support biohydrometallurgical adoption. The process aligns with circular economy principles by minimizing carbon emissions and avoiding toxic lixiviants. Life cycle assessments (LCAs) indicate a 40–60% reduction in global warming potential compared to pyro- and hydrometallurgical routes. As battery recycling mandates tighten globally (e.g., EU Battery Regulation 2023), biohydrometallurgy presents a compliant and scalable solution for sustainable metal recovery.

Future research directions include engineering consortia of bacteria and fungi to target multiple metal species simultaneously and developing immobilized cell systems to improve process stability. Advances in metagenomics and proteomics are enabling the identification of novel microbial strains with enhanced leaching capabilities, while bioreactor designs are evolving to accommodate larger particle sizes and heterogeneous waste streams.

In summary, biohydrometallurgical techniques offer a viable, eco-friendly alternative for battery recycling, leveraging microbial activity to recover valuable metals with minimal environmental impact. While challenges in scalability and kinetics persist, ongoing innovations in strain optimization and process engineering are steadily advancing this technology toward commercial viability. The integration of biohydrometallurgy into existing recycling frameworks could play a pivotal role in achieving sustainable battery life cycles and reducing reliance on primary mineral resources.