STUDIES ON THE TRANSFORMATIONS OF IRON IN NATURE

Abstract

Equations expressing equilibrium conditions in Fe solutions under the influence of atmospheric O and CO2 were developed and used to interpret observations made when various types of microorganisms were grown in presence of metallic Fe, Fe oxides and soluble Fe salts. The various forms of Fe were added to nutrient media inoculated with either pure or mixed cultures of heterotrophic bacteria, as well as with mixed cultures of Fe bacteria. Representative cultures were incubated under both aerobic and anaerobic conditions. After growth had taken place, the composition of the solution was studied by analyzing for the various forms of Fe. Heterotrophic microorganisms may dissolve metallic Fe, dissolve and reduce ferric hydroxide, or may bring about precipitation of ferrous carbonate. Activity of Fe bacteria appears to occur only under environmental conditions favorable to spontaneous oxidation of ferrous Fe by chemical agencies. The Fe content of natural water in Fe springs is not reduced by the Fe bacteria to a concentration below which it will go by pure chemical reaction. Solution and precipitation of Fe in nature are seen to be associated with equilibrium conditions which depend on 0 and CO2 tensions, acidity, and presence of organic compounds. These conditions may be modified very extensively by bacterial activity. The studies indicate that the heterotrophic bacteria influence transformations of Fe in nature to a much greater extent than do the true Fe bacteria. The importance of the latter has apparently been over-emphasized, whereas the importance of the former in the transformations of Fe in nature has not been fully appreciated.