The cycling of iron in natural environments: Considerations based on laboratory studies of heterogeneous redox processes

Kinetics of microbial Fe(III) oxide reduction in freshwater wetland sediments

“…This formulation assumes that reactions at the Fe(III) oxide surface are surface controlled, i.e., that reactions at the surface are slow in comparison with other reaction steps, such as association of the ligand with the mineral surface to produce a surface species (Stumm and Morgan 1996). A well-known example of abiotic mineral dissolution is the reductive dissolution of crystalline hematite (␣ϪFe 2 O 3 ) by ascorbic acid (Sulzberger et al 1989;Suter et al 1991).…”

“…Detailed modeling of Fe(III) oxide reduction kinetics in the presence of such compounds would require information on the mechanism of their reaction with Fe(III) oxide surfaces, as well as their concentration and turnover rate-neither of which are available for TW or any other aquatic sediment. However, because such reactions are likely to be surface-controlled processes analogous to the well-studied interactions of synthetic chelators and soluble reductants with Fe(III) oxide minerals (Stumm 1992;Stumm and Morgan 1996), as a first approximation the effect of chelators and/or electron shuttling compounds can be assumed to be incorporated into the effective rate constant for Fe(III) oxide reduction, thereby permitting interpretation of Fe(III) oxide reduction kinetics in terms of the simple kinetic framework developed below. In support of this assertion, recent studies in our laboratory showed that the presence of electron shuttling humic compounds did not alter the basic first-order nature of enzymatic Fe(III) oxide by an acetate-oxidizing, Fe(III)-reducing enrichment culture obtained from TW surface sediments (Roden and Wetzel, unpubl.…”

“…Heterogeneity of Fe(III) oxide reactivity-Standard formulations for chemical oxide mineral dissolution assume that a single mineral of uniform specific surface area, reactive site density, and mineralogical stability is undergoing dissolution (Stumm 1992). In contrast, Fe(III) oxide assemblages in soils and sediments often possess a wide range of reactivity, owing to differences in mineralogy, crystallinity, grain size, and surface area (Postma 1993;Cornell and Schwertmann 1996).…”

Role of iron species in the photo-transformation of phenol in artificial and natural seawater

“…The understanding of what occurs in colloidal phase could be extrapolated from the equilibrium in solution, due to the analogies existing between colloidal and aqueous phases (Stumm and Sulzberger, 1992).…”

“…Fe(II) in seawater is usually present as complexed species, such as FeCO 3 or FeCl + and, under oxic conditions at the pH of seawater, Fe(II) species are rapidly oxidized to Fe(III) by O 2 and H 2 O 2 , as Fe(III) is the thermodynamically stable form of iron in seawater, freshwater and most aqueous systems containing dissolved oxygen. The transformation of dissolved Fe(II) species to particulate Fe(III)oxyhydroxides is central in the cycling of iron in aquatic environments (Stumm and Sulzberger, 1992;Rose and Waite 2002). Particulate Fe(III) oxyhydroxides are formed by oxidation of Fe(II) at the oxic/anoxic boundary in coastal marine waters (Yao andMillero, 1995, Kuma et al 1998).…”