Effect of the presence of bacterial surfaces during the synthesis of Fe oxides by oxidation of ferrous ions

Natural iron-oxides are often found in close association with bacterial cel ls in aquatic environments, but the effect of bacteria on their formation i s still under investigation. The present study was undertaken to assess the effect of two common bacteria, Bacillus subtilis and Escherichia coli, on the morphology and mineralogy of Fe oxides. All Fe oxides were synthesised by oxidation of Fe(II) (2 X 10(-4) M) at pH = 7. Three systems were studied , i.e., abiotic Fe oxides, Fe oxides formed in the presence of bacteria (wh ich we call "biogenic" Fe oxides) and abiotic Fe oxides mixed with bacteria l cells. Samples were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fe oxide particles in all systems showed a needl e-like morphology, with many needles seeming to be attached to a sheet, and were identified as lepidocrocite. However, the biogenic lepidocrocite crys tals were generally shorter than the abiotic ones, and the crystals were fo und in association with the bacterial cell-wall, especially with B. subtili s, a Gram-positive bacterium. Biogenic lepidocrocite crystals also displaye d an attenuation of the XRD 120 line, which is indicative of a low crystall inity, Growth limitation and poor crystalline order are then likely to affe ct the surface area of Fe oxides and indirectly, their sorptive capacity.