Biological methods for extracting metals
Using resources • Earth's resources and potable water
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Key concepts
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Definition of bioleaching
Bioleaching uses microorganisms, typically bacteria, to oxidise minerals and release metal ions into solution. Bacterial metabolism causes chemical changes: for example, sulfur-oxidising bacteria convert sulfide minerals to sulfuric acid, which dissolves metal sulfides and frees metal ions. Bioleaching causes metal mobilisation without high-temperature processing. The resulting metal-rich solutions undergo further chemical or electrochemical recovery. Limiting factors include temperature, pH, oxygen supply, presence of toxic compounds and the rate of bacterial growth.
Definition of phytomining
Phytomining uses hyperaccumulator plants that absorb and concentrate metals from soil into their tissues. Repeated harvest and incineration of plant biomass produce ash that contains concentrated metal compounds for extraction. Phytomining causes gradual removal of metals from low-grade ores or polluted soils with minimal disturbance. Limiting factors include plant growth rate, metal uptake capacity, soil chemistry, climate, and the time required to accumulate commercially useful metal quantities.
Mechanisms: how bioleaching works
Bioleaching mechanisms depend on microbial oxidation and acid production. Iron- and sulfur-oxidising bacteria convert metal sulfides into soluble sulfate and metal ions. For example, bacteria oxidise Fe2+ to Fe3+, and Fe3+ chemically oxidises metal sulfides, forming soluble metal sulfates. Cause: microbial metabolism produces oxidants and acids. Effect: metal sulfides dissolve and release metal ions into solution for recovery. Effective bioleaching requires controlled aeration, temperature and nutrient supply to sustain microbial activity.
Mechanisms: how phytomining works
Phytomining mechanisms require plants with natural or induced tolerance for metals. Roots absorb metal ions from soil solution; translocation pathways move metals to shoots where sequestration in vacuoles or binding to organic ligands limits toxicity. Harvesting removes the metal-laden biomass from the site. Cause: plant uptake and biochemical sequestration. Effect: metals concentrate in harvestable biomass, enabling subsequent recovery by incineration and processing of ash. Crop management, soil amendments and choice of species influence uptake efficiency.
Advantages compared with conventional methods
Biological methods lower energy consumption and reduce the need for high-temperature smelting. They allow extraction from low-grade ores and remediation of contaminated soils, turning waste into resource streams. Environmental disturbance is often lower because no large-scale excavation or smelting is required. Cause: biological processes operate at ambient conditions. Effect: reduced carbon footprint and potential recovery from sites unsuitable for conventional mining. Economic and time constraints may offset some advantages depending on metal value and scale.
Limitations and risks of bioleaching
Bioleaching rates are slower than pyrometallurgical methods and depend on microbial kinetics, which vary with temperature, pH and oxygen. Acid generation can cause acid mine drainage if uncontrolled, mobilising harmful metals into waterways. Some ores resist leaching, and recovery may require further chemical treatment. Cause: reliance on living organisms and on-site geochemistry. Effect: variable yields, longer processing times and potential environmental contamination if containment fails. Economic viability depends on metal grade and local conditions.
Limitations and risks of phytomining
Phytomining needs long cultivation periods and large land areas to produce significant metal yields. Plant growth is climate-dependent and susceptible to pests and disease. Incineration of biomass produces ash that requires handling and further processing; any residual contaminants demand safe disposal. Cause: biological growth constraints and low concentration per plant. Effect: slower, land-intensive recovery with logistical steps for harvesting, transport, incineration and metal extraction from ash.
Environmental and economic evaluation criteria
Evaluation criteria include metal yield per time, energy input, greenhouse gas emissions, land use, contamination risk, capital and operating costs, and social impact. Environmental trade-offs include reduced smelting emissions versus potential leachate or contaminated biomass issues. Cause: each method interacts with site-specific factors. Effect: decision-making requires quantitative information (metal concentrations, growth rates, microbial kinetics and remediation targets) for accurate comparison and selection.
Practical examples and applications
Bioleaching finds industrial use in copper extraction from low-grade ores and in in situ leaching operations. Phytomining research demonstrates nickel and gold uptake by certain plants in field trials and pilot projects. Both methods apply to remediation of contaminated sites and recovery from mine tailings. Cause: availability of specific microbes or hyperaccumulator species and appropriate economic drivers. Effect: targeted deployments where conventional methods are uneconomic or environmentally damaging.
Decision-making for HT evaluation questions
Evaluation requires identification of key variables: metal concentration, ore type, climate, timescale and environmental constraints. Comparative judgments state which method performs better for each variable, supported by data on yields, costs, and environmental outcomes. Cause: presence of contextual information. Effect: structured answers compare pros and cons, quantify trade-offs, and reach a justified conclusion about the preferred method for the given scenario.
Key notes
Important points to keep in mind