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Kinetics of Fe(III) Reduction in a Marine Sediment

The importance of reduction of Fe-oxides in marine sediments, as electron accepting process coupled to organic matter oxidation, has received increasing recognition in recent years. The findings of Canfield et al. (1993) indicate that up to 50 % of organic carbon oxidation in a marine sediment may proceed by reduction of Fe(III). Thamdrup et aL (1994) found in addition reduction of Fe(III) to be an important pathway for reoxidation of dissolved sulphides in sediments. The source of Fe(III) in sediments is generally heterogeneous: Fe(III) may be present as different Fe-oxides of variable reactivity, as well as in clays in different coordination. Frequently only part of the Fe-oxides in sediments become reduced (Canfield et al., 1992), indicating a kinetic control on the reduction of Fe-oxides. Recently, Postma (1993) presented a kinetic approach to describe the reactivity of synthetic Fe-oxides and of heterogeneous mixtures of Fe-oxides in sediments. The reactivi ty of polydisperse synthetic Fe-oxides was described by using the r a t e e q u a t i o n f o r c r y s t a l d i s s o l u t i o n (Christoffersen, 1980).