CHARACTERISTICS OF LACUSTRINE DIAGENETIC IRON OXYHYDROXIDES

Diagenetic iron oxyhydroxides formed by the oxidation of iron(II) were allowed to deposit onto inert collectors vertically inserted into the sediments of eight lakes chosen to represent a range of lake pH values. Particles comprising the iron-rich deposits have been characterized at macroscopic (composition; X-ray diffraction) and microscopic (transmission electron microscopy; energy dispersive spectroscopy; electron diffraction) levels. The only crystalline forms of Fe(III) oxyhydroxides identified were poorly ordered ferrihydrite and lepidocrocite. Morphologies of lepidocrocite (lath-type) and presumably of ferrihydrite (spherical and ellipsoidal particles) are similar to those reported previously as being formed in the water column of seasonally anoxic lakes. Deposition of iron oxyhydroxides in the lake sediments also appears to occur on bacterial cells and on their exopolymers. Silicon, sulfate. chloride, phosphate, manganese, calcium, and aluminum represented minor components of the diagenetic Fe(III) oxyhydroxides, whereas organic carbon was a more abundant component. Calculations with a diffuse layer version of the surface complexation model using the ambient dissolved concentrations, and comparison of the predicted and measured compositions of the iron-rich particles suggest that sulfate, phosphate, and probably calcium were sorbed on the iron oxyhydroxides, whereas silicon was probably more firmly bound at the surface.