Potential capacity of coprecipitation of dissolved organic carbon (DOC) with iron(III) precipitates

Hypolimnetic anoxic water of Lake Onogawa was subjected to aeration experiments. When the samples were agitated by magnetic stirrers for 24 h, dissolved oxygen increased from 0 to more than 7.6 mg l−1, dissolved iron decreased from 98% to about 5% of the initial total iron, and from 32% to 48% of the dissolved organic carbon (DOC) disappeared. On the other hand, when the anoxic waters were left unstirred, dissolved oxygen increased from 0 to 2.2 mg l−1, dissolved iron decreased from 98% to 31%, and 20% of the DOC disappeared within 48 h. Further 24-h incubation had little effect on the DOC loss, although dissolved oxygen increased to 3.9 mg l−1 and dissolved iron decreased to 5%. These rates of DOC disappearance are too large to be explained by bacterial decomposition. It is quite conceivable that a part of the DOC is coprecipitated with iron(III) precipitates. When Fe(II) in the anoxic hypolimnion is oxidized by autumnal water mixing, probably anoxic water is mixed with aerobic water. The anoxic water must receive oxygen from the aerobic water during this mixing and be simultaneously diluted with the aerobic water. Because the present experimental conditions, especially the stirred one, significantly differ from in situ conditions, the present results are thought to be a potential capacity of DOC coprecipitation.