Bioremediation
Biomining, phytoremediation and tailings valorisation
Indirect sulfide oxidation, chalcopyrite passivation and the biomass-times-concentration limit that decides which metals can be farmed out of a tailings dam and which cannot.
Two biological approaches meet on a tailings dam, and they work by unrelated mechanisms. Bioleaching dissolves metal out of sulfide minerals. Phytoextraction moves metal that is already mobile into plant tissue. Both are constrained more sharply than they first appear.
Bacteria do not attack the mineral
The central point about bioleaching is that the microorganisms are catalysts of a chemical cycle rather than direct agents. Ferric iron is the oxidant that attacks the sulfide lattice; the reaction reduces it to ferrous iron, and Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum and their relatives re-oxidise it with atmospheric oxygen, at rates orders of magnitude above the abiotic reaction. A second group, including A. thiooxidans, oxidises the sulfur intermediates to sulfuric acid and keeps the pH low enough — typically below 2 — for ferric iron to stay in solution at all. Attached cells contribute by holding the ferric iron in an exopolysaccharide layer against the mineral surface, which is why cell attachment matters even though the chemistry is indirect.
Mineralogy then sets what can be done. Acid-soluble sulfides such as sphalerite and chalcopyrite dissolve by a polysulfide route; acid-insoluble ones such as pyrite and molybdenite go through thiosulfate. Chalcopyrite is the industry’s standing problem: at ambient temperature and normal leach potentials its surface passivates behind a layer of polysulfide and jarosite, and extraction stalls at low recovery. Secondary copper sulfides leach well in heaps, and thermophilic archaea operating near 70 °C attack chalcopyrite far better, but a stirred thermophilic tank is a different capital proposition from a heap. Biooxidation’s other established use inverts the goal: oxidising the sulfide matrix of refractory gold ore to expose gold for cyanidation, where the metal itself is never leached.
Phytoextraction is limited by arithmetic
Hyperaccumulators are plants that transport metal to the shoot instead of excluding it at the root — over-expressed membrane transporters, active xylem loading, and vacuolar sequestration with ligands such as histidine and nicotianamine. The conventional threshold is about 1,000 mg/kg of nickel in dry shoot tissue, or 10,000 mg/kg for zinc and manganese.
The ceiling follows directly. Annual removal equals harvestable biomass times shoot concentration. Ten tonnes per hectare of a nickel hyperaccumulator at one per cent nickel removes on the order of 100 kg per hectare per year — enough to make nickel agromining genuinely economic, since the ash from burning that biomass is richer in nickel than laterite ore. For lead, arsenic or a mixed tailings inventory measured in tonnes per hectare, no combination of biomass and concentration gets the timescale below centuries. Uptake is also confined to the root zone and to the fraction that is chemically available, which in a cemented, acidic, nutrient-free tailings is small.
That is why most tailings work is phytostabilisation rather than extraction: establishing a cover that limits dust and water infiltration and immobilises metals in the root zone. Its own limit is that residual pyrite continues to generate acid, so the cover has to survive an acidity it does not stop.