Biological and other reduction reactions

The products of oxidation are electrons and metabolites which upon complete oxidation go to CO2 and H2O. As described earlier, both the electrons and the protons thus produced are transferred to the oxide surface, thus causing formation and detachment of Fe. A bulk reaction equation can be written as. 

That the reduction takes place at the surface, receives support from an experiment in which the extent of reduction of various Fe oxides by Shewanella alga after 30 days was linearly correlated with the SAbet the exception was 2-line ferrihydrite for which a surface area of 600 m /g had to be assumed in order to fit the relationship (Roden Zachara, 1996). Although experimental (BET) surface areas of ferrihydrite are substantially lower than 600 m /g, calculated values based on particle size as well as those determined from ligand adsorption experiments (see Table 5.1) are in this range. Dissolved Fe was found to create a lag phase in the reduction process (in contrast to the behaviour in inorganic systems) because Fe is adsorbed at the cell surface (Liu et al. 2001). This effect can be overcome by complexing the Fe (e. g. 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

In natural environments, abiotic reduction may also be effected by a range of natural reductants including sulphide and methane. In the sulphide producing sediments in the Long Island Sound and the Mississippi Delta, for example, Fe oxides have been transformed to Fe sulphides (FeS and FeS2) (Canfield Berner, 1987 Boesen Postma, 1988). As a result, when there was a sufficient supply of reactive Fe oxides in the sediments, hardly any dissolved sulphide was found in the pore water. The reduci- 

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