Microbially induced corrosion and Ferric iron reduction: bacterial metabolism of insoluble iron species

Typically the iron bacteria are described as ferrous iron (Fe2+) oxidisers (with oxy¬gen) to ferric iron (Fe3+). Exposure of ferrous iron and sulphide containing ground¬water to atmospheric oxygen forms the well known phenomena of ferric iron and sulphur precipitation which typically, but not necessarily, involves iron and sulphide oxidising bacteria. In this process ferrous iron serves as the electron donor and mo¬lecular oxygen as the electron acceptor. The flow of electrons from ferrous iron to oxygen enables the respiration process of the bacteria and allows their growth. Also insoluble iron species allow an electron flow that bacteria can use for their respira¬tion and growth. Two such examples are covered in this paper: \n \n1. Bacterially influenced corrosion of steel. The iron in steel (Fe0) is even more reduced than ferrous iron and hence offers a more attractive electron donor for bacteria. Sulphate-Reducing bacteria can utilise the electrons from metallic iron using sulphate, sulphite or elemental sulphur as electron acceptor. Again, this elec¬tron flow from metallic iron to sulphate provides the bacteria with a suitable energy source for growth. And like usual this electron flow is used for respiration, here an anaerobic respiration. Sulphate-Reducing bacteria (SRB) play an important role in the corrosion of steel in anaerobic environments, such as oilfields, marine sediments, treatment tanks etc. This paper covers the principle and examples of steel corrosion by SRB. \n \n2. Bacterial reduction of ferric iron to ferrous iron. Also the most oxidized form of iron, ferric iron can be used by bacteria for growth. However, as it is in its most oxidised form it can not act as electron donor but as electron acceptor. Analogous to the oxygen respiration using O2 as electron acceptor for electrons derived from the oxidation of organic material, iron reducing bacteria can ʻrespireʼ ferric iron. This reaction occurs typically in anaerobic ecosystems with organic compounds (e.g. or¬ganic acids) as the electron donor. The role and economic potential of iron reducing bacteria in groundwater are discussed in this paper. \n \nBoth the above cases of iron metabolism involve insoluble species of iron. As bacte¬ria by definition can only metabolise soluble material, specific mechanisms need to be in place that enable the electron transfer. This paper discusses possibilities and implications of such electron flow by also covering other species relevant to ground¬water chemistry (e.g. elemental sulphur and chlorinated solvents).