Nickel recovery through biohydrometallurgy presents a sustainable alternative to conventional pyrometallurgical and hydrometallurgical methods, leveraging biological systems for metal extraction. This approach encompasses microbial leaching, fungal bioaccumulation, and phytomining, each exploiting unique metabolic pathways to solubilize and recover nickel from ores, tailings, and industrial waste. The process reduces energy consumption and environmental impact while offering selectivity for nickel in complex matrices.
Microbial leaching employs acidophilic bacteria such as *Acidithiobacillus ferrooxidans* and *Acidithiobacillus thiooxidans* to oxidize sulfide minerals, releasing nickel into solution. These bacteria thrive in acidic environments, utilizing iron and sulfur as energy sources. The metabolic pathway involves the oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺), which acts as a chemical oxidant for sulfide minerals like pentlandite, a primary nickel sulfide ore. Simultaneously, sulfur-oxidizing bacteria convert reduced sulfur compounds to sulfuric acid, maintaining the low pH necessary for leaching. Genetic engineering has enhanced these pathways by overexpressing key enzymes such as rusticyanin in *A. ferrooxidans*, improving electron transfer efficiency and leaching rates. Modified strains with increased resistance to nickel toxicity have also been developed, enabling higher metal concentrations in leachates.
Fungal bioaccumulation utilizes species like *Aspergillus niger* and *Penicillium simplicissimum* to extract nickel through organic acid production. Fungi secrete citric, oxalic, and gluconic acids, which chelate nickel ions, forming soluble complexes. Unlike bacterial leaching, fungal systems operate at near-neutral pH, making them suitable for lateritic nickel ores where acid leaching is less effective. Fungal biomass can also passively adsorb nickel through cell wall chitin and extracellular polysaccharides. Metabolic engineering has focused on increasing organic acid yields by upregulating genes in the tricarboxylic acid cycle, with some strains achieving nickel recovery efficiencies exceeding 80% in lab-scale bioreactors.
Phytomining employs hyperaccumulator plants such as *Alyssum murale* and *Berkheya coddii*, which naturally concentrate nickel in their biomass at levels up to 5% of dry weight. These plants solubilize nickel through root exudates, including histidine and citrate, which mobilize metal ions in the rhizosphere. Once harvested, the biomass is ashed or processed to recover nickel. Genetic modifications targeting metal transporters, such as the overexpression of the *ZNT1* zinc-nickel transporter, have increased uptake capacities. Field trials demonstrate yields of 100 kg nickel per hectare annually, though scalability depends on soil conditions and plant growth rates.
Scalability remains a challenge across all biohydrometallurgical methods. Microbial leaching in heaps or stirred tanks requires long retention times, often exceeding 30 days, compared to chemical leaching completed in hours. Large-scale operations must optimize aeration, nutrient delivery, and temperature control to maintain microbial activity. Fungal systems face biomass handling issues, as large volumes of fungal culture necessitate containment and separation from leachates. Phytomining is limited by land use and seasonal growth cycles, though intercropping with energy crops could improve economics.
Waste biomass management is critical for environmental sustainability. Spent microbial and fungal biomass may contain residual metals, requiring stabilization before disposal. Composting and thermal treatment have been explored to reduce volume and recover energy. In phytomining, ash from incinerated biomass can be directly processed into nickel salts or alloys, minimizing waste.
Case studies illustrate the transition from lab to industrial application. A pilot-scale bioheap leaching operation in Finland achieved 70% nickel recovery from sulfide ore over 12 weeks, comparable to chemical methods but with lower acid consumption. In Canada, a fungal bioleaching process treated lateritic tailings, recovering 65% nickel in 15 days, though scaling required bioreactor modifications to maintain oxygen levels. Phytomining trials in Albania demonstrated economic viability where soil nickel concentrations exceeded 0.5%, with biomass yields of 10 tonnes per hectare.
Compared to chemical leaching, biohydrometallurgy offers lower operating costs and reduced emissions but suffers from slower kinetics and sensitivity to environmental conditions. Hybrid systems combining bioleaching with chemical steps, such as using biogenic ferric iron as a lixiviant, may bridge this gap. Advances in genetic engineering and process automation are expected to improve efficiency, making biological nickel recovery increasingly competitive with traditional methods.
The future of biohydrometallurgy lies in integrating these systems into circular economy models, where waste streams from mining and battery recycling become feedstocks for biological extraction. Continued research into strain optimization, reactor design, and biomass valorization will determine its role in sustainable nickel supply chains.