Bioreduction of iron compounds as a possible clean environmental alternative for metal recovery

This work explores a biotechnological process for the treatment of iron oxides ores using Fe-reducing microorganisms as a way to reduce carbon dioxide emissions produced by the conventional steel making process.The experiments were performed with a mixed culture of anaerobic microorganisms using different sources of iron(III): one soluble (citrate) and two insoluble (an oxidized iron mineral and a synthetic amorphous solid).The iron reduction performance of the mixed culture using ferric citrate was better at pH 6 and 7 than at pH 5. At pH 7, vivianite (Fe3(PO4)2·8H2O) was formed from the ferrous ion produced microbiologically and the phosphate present in the nutrient medium.The influence of the solid substrate showed that, at pH 7, the rate of ferrous ion generation with the mixed culture was faster for the synthetic solid though the final amount of ferrous ion was similar to that obtained with the mineral. At pH 6, the differences were more marked and the concentration of ferrous ion reached was higher for the synthetic solid. The degree of crystallinity of the iron oxide seems to be the most influential variable in the microbiological attack.

Research highlights
► Low levels of reduction were obtained when using insoluble ferric sources.
► A limiting factor of the bacterial activity was the access to the solid.
► The attack of iron oxides is influenced by crystallinity.