Preferential reduction of amorphous to crystalline iron oxides by bacterial activity

Reduction of pedogenic iron oxides by the activity of Clostridium butyricum was studied in model experiments under controlled conditions of pH, particle size (63 to 125 μm), glucose (2 percent), and anaerobic conditions. Three horizons—from a gley, pseudogley, and red clay soil, respectively—with different Fe_o:Fe_dratios were selected as test material. Except for Fe(II) formation, pH and Eh were measured at regular intervals. In all soils the total amount of free iron oxides (Fe_d), as well as the crystalline Fe (Fe_d- Fe_o), decreased while Fe(II) increased. The amount of amorphous Fe (Fe_o) remained relatively constant in those soils low in Fe_o:Fe_d(0.1 to 0.19), but diminished in the sample with a relatively high amount of amorphous Fe (Fe_o:Fe_d= 0.78). This suggests that crystalline rather than amorphous Fe forms are dissolved preferentially by acting as terminal electron acceptors. Nevertheless, the reduction of crystalline⁵⁹Fe-labeled oxides (hematite and goethite) mixed with Fe-amorphous gley soil material (Fe_o:Fe_d= 0.78) clearly showed that oxalate-soluble Fe was attacked in preference to⁵⁹Fe-labeled crystalline Fe. The processes of Fe(III) reduction in a microsite are discussed in relation to the mechanism of Fe(II) formation in flooded soils.