Formation and stability of iron(II) oxidation products under natural concentrations of dissolved silica

Model solutions were used to evaluate the effect of dissolved silica on the mineralogy and stability of Fe(II) oxidation products. Mineralogy was evaluated with scanning electron microscopy (SEM) and X-ray diffraction. Stability was evaluated by measuring the decrease in turbidity of colloidal suspensions with time. At Si/Fe molar ratios of 0.1 or less, oxidation of Fe(II) produced lepidocrocite, a moderately crystalline oxide that settled rapidly out of solution. At Si/Fe molar ratios of 0.36 or higher, ferrihydrite formed from the oxidation of Fe(II). The ferrihydrite consisted of 0.1 μm spherical particles that were poorly crystalline and increasingly stable with higher Si/Fe molar ratios. The ferrihydrite was similar in structure and composition to Fe oxide colloids isolated from two natural samples: (1) a reduced groundwater sample that was allowed to oxidize in the laboratory and (2) the colloidal particles in the < 1.0 μm size class of surface water samples from the Tualatin River watershed in northwest Oregon. The similarity of the natural and synthetic colloids is evidence that Fe oxide colloids in natural waters can result from the oxidation of Fe(II) and that dissolved silica may contribute to the stability of the colloids.