Kinetic characterization on reductive reactivity of iron(III) oxides in surface sediments of the East China Sea and the influence of repeated redox cycles: Implications for microbial iron reduction

Reductive reactivity of iron(III) [Fe(III)] oxides in surface sediments of the East China Sea (ECS) and its variations while the sediments were subjected to repeated redox cycles were characterized by three rate parameters: m0 (the initial amount of ascorbate-reducible Fe(III) oxides), k′ (the rate constant, s−1) and γ (a parameter describing heterogeneity of reactivity), obtained by applying a generalized dissolution rate law to time-dependent dissolution data in a buffered ascorbate solution (pH 7.5). The spatial pattern of the m0 values is generally controlled by that of the clay fraction, but the values cannot be quantitatively corresponding to any specific reactive Fe(III) pools operationally defined by the conventional chemical extractions. Unlike the m0, the spatial patterns of k′ and γ are more complex and may be controlled by more intricate factors relevant to Fe redox chemistry. At most sites studied, Fe(III) oxides have lower reactivity (k′) and higher heterogeneity (γ) than those of fresh ferrihydrite. Repeated redox cycles rendered m0 and γ apparently decreased mainly during the initial 2–3 cycles due to mineral ripening and neoformation of “purified” Fe(III) oxides, respectively. Although the initial 1–2 cycles resulted in a “temporary” increase in Fe(III)-oxide reactivity, prolonged cycles eventually rendered the reactivity decreasing relative to the initial value. An a priori assumption that long-term redox cycles in natural sediments could enhance reductive reactivity of Fe(III) oxides as well as heterogeneity of the reactivity is unwarranted.