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Oxidation State and Size of Fe Controlled by Organic Matter in Natural Waters

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journal contribution
posted on 15.05.2008, 00:00 by John W. Gaffney, Keith N. White, Stephen Boult
In a well mixed-stream, in which the iron/organic carbon (OC) ratio varied from 0.333 to 0.05 with sampling point and discharge, 40–70% of the Fe load was found to be present as lightly bound Fe(II). In laboratory simulations of streamwater, after 24 h of aeration at pH 6.5, and with an Fe/OC concentration ratio of 0.417, 97% of Fe(II) was converted to Fe(III) (hydr)oxides, while at a ratio of 0.083, 87% of Fe(II) remained unoxidized. The particle size distribution of Fe contained <0.2 µm fractions only when OC was present, and comparison of Fe and OC size distributions suggested that there was more than one mechanism by which colloidal Fe was produced. At high Fe/OC ratios, <0.2 µm fractions may be predominantly Fe(III) (hydr)oxides stabilized by OC, but at low ratios, they must consist of otherwise soluble Fe(II) attached to <0.2 µm OC. The recognition in the field of the consequences of processes demonstrated in the laboratory suggests that OC may be a predominant control of both size and oxidation state of Fe in many natural waters.

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