Gunnars, A. Department of Structural Chemistry, annelig@struc.su.se

 
FORMATION OF IRON PHOSPHATE SPECIES AT REDOX BOUNDARIES IN FRESHWATER AND MARINE SYSTEMS
 
Comparative experiments with sediment cores shows that the fixation of dissolved phosphate at redox boundaries can be controlled by precipitating iron hydroxide, but that the scale of the process reveal major differences in freshwater compared to marine conditions. When shifting from anoxic to oxic conditions, Fe(II) is oxidized by oxygen. The subsequent simultaneous precipitation of iron and phosphate might be described as a process where phosphate ions are incorporated into to the structure of the growing iron hydroxide. Our results indicate that an adsorption model alone do not fully describe this process. The iron rich particles formed during the shift to oxic conditions seemed to be chemically homogeous, but were amorphous to X-rays. The Fe/P ratio in the particles was dependent on the initial Fe/P ratio in the anoxic dissolved phase. For high initital ratios, the relation approaches a linear function (Fe/P initial = Fe/P particles), whereas for low initial ratios the particulate Fe/P ratio have a lower limiting value of about 2. Within the pH-range 7-8, this empirical relation is independent of salinity. Consequently, at least two iron atoms are needed to precipitate a phosphate ion from the dissolved phase. Thus, when the relative concentration of iron is below 2, it will be in short supply to precipitate all phosphate. In this respect, the Fe/P ratio will control the extent to which phosphate can escape precipitation. A comparison of positive redox turnover in various sediment-water systems, demonstrates significant differences between freshwater and marine environments. After oxygenation, the levels of dissolved phosphate are higher in the marine systems, due to relative iron deficiency, i.e., low Fe/P ratios in the anoxic water.
 
Day: Thursday, Feb. 4
Time: Poster
Location: Sweeney Center
 
Code: CS63TH0514S