July 1995

Document Type


Degree Name



Dept. of Environmental Science and Engineering


Oregon Graduate Institute of Science & Technology


An important consequence of the oxidation of Fe(II) in aquatic systems is the formation of colloid-sized particles. Colloids are particles with linear dimensions between 0.001 and 1.0 µm. The formation of colloidal Fe oxides is important to surface-reactive species, such as P, that interact with Fe. In general, colloid-associated elements or compounds will behave quite differently from species that are truly dissolved or associated with larger particles. This research investigated the extent, variability, and formation of colloidal iron phosphate (Fe-P) in surface waters of the Tualatin River Watershed of northwest Oregon. Phosphorus and iron were directly associated in colloid-sized particles but not in larger particles. The extent of colloidal Fe-P varied considerably with location and time of year. Results from a study of a small tributary, Bronson Creek, indicated that colloidal Fe-P was more prevalent during periods of low flow and low dissolved oxygen. This led to the hypothesis that colloidal Fe-P formed as groundwater- or sediment-released Fe(II) and P diffused to oxygen-containing surface waters, causing Fe(II) to oxidize and coprecipitate with P. Silicon and other elements were found with the colloidal Fe-P as well. Laboratory experiments were conducted to examine the formation and stability of colloidal Fe-P under conditions simulating natural systems. Silicon was found to be important in the formation process. At low concentrations of Si, Fe(II) oxidation resulted in the formation of unstable colloidal suspensions of lepidocrocite. At background concentrations of Si typical of the Tualatin and other systems, Fe(II) oxidation resulted in the formation of stable ferrihydrite colloids. The Si stabilized the ferrihydrite colloids by (1) increasing the negative surface charge and potential and (2) inhibiting the Ostwald ripening of the solid. The Si also inhibited the release of coprecipitated P from the Fe oxides, resulting in much higher P/Fe solid ratios than in the Fe-P coprecipitates formed in the absence of Si. The synthesized ferrihydrite colloids were very similar in appearance, composition, and structure to natural Fe-, Si-, and P-containing colloids from the surface waters of the Tualatin River.





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