Ferric iron

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Also designated Iron(III), Fe3+. Ferric iron can be reduced to ferrous iron if the conditions are right.


Ferric iron is the mineral solvent in the bioleaching reaction.

A lack of iron in bioleaching can be remediated in most cases by adding iron in some form, preferably as pyrite because by oxidation of pyrite not only iron ions are formed but also hydrogen ions.

In ponds on the top of dumps operated by the pond system, ferric iron compounds often precipitate. This hinders the infiltration of the liquid by plugging the upper layer of the ore dump. From time to time the precipitates have to be scraped off.

Ferric to ferrous iron ratio

Microbes have different affinities for metal-ion substrates like Fe(II). They also have different tolerances to metal-ions like Fe(III)). Therefore the ferric to ferrous iron ratio (redox potential) is a factor which may affect the relative amounts of species in certain microbial communities.

Iron(II) and Iron(III) matters in competition

Concerning the competition between 'Leptospirillum' and T. ferrooxidans:

"The main factor in determining which bacteria are likely to dominate commercial bioleaching or biooxidation processes is the ferric to ferrous iron ratio (redox potential). The redox potential will be affected by whether treatment of an ore is carried out using a batch culture or continuous culture type of process. Under batch culture conditions the ferric to ferrous iron concentration (redox potential) is low at the start of oxidation, but high once all the ferrous iron has been oxidized to ferric.

When grown in batch culture in a liquid ferrous iron or pyrite medium, T. ferrooxidans will initially outgrow its iron-oxidizing competitors and dominate the population. This is largely because during the initial stages of batch culture the redox potential is low and T. ferrooxidans has a faster growth rate than `Leptospirillum' (Fig. 1, below 690 mV). Under such conditions T. ferrooxidans is able to build up large numbers of cells before conditions become more favourable for `L. ferrooxidans'. However, because the leptospirilli have a greater affinity for ferrous iron and are less sensitive to inhibition by ferric iron on prolonged aeration `L. ferrooxidans' is likely to dominate (Norris et al., 1988). "

Source: “Reasons why 'Leptospirillum'-like species rather than Thiobacillus ferrooxidans are the dominant iron-oxidizing bacteria in many commercial processes for the biooxidation of pyrite and related ores.” [1]

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