| Abstract
| - We report a laboratory and field study that demonstratesthat soluble Fe(III) complexed to organic ligands existsin sediment porewaters. Synthetic solutions of Fe(III) withthe ligand Tris were used to simulate organic complexationof Fe(III) in natural waters. Size fractionation using gel filtrationchromatography shows that at least three differentmolecular mass species coexist. The molecular masses ofFeIIIx−Trisy complexes increase with time, suggestingthat aggregation occurs in solution. Soluble Fe(III) is detectedby voltammetry at a mercury drop electrode when it iscomplexed by an organic ligand. Depending on the age of Fe(III), two voltammetric waves can be observed: at ca.−0.3 V for “freshly” formed Fe(III) and at a more negativepotential for “aged” Fe(III). A mathematical model wassuccessfully developed to assess a mechanism for this agingprocess. This model involves oxidation and adsorption of Fe(II), aggregation, and precipitation of Fe(III) complexes.The species formed are extremely reactive. Upon additionof sulfide, reduction of Fe(III) instantaneously produces Fe(II) and FeS in solution, and a nonreductive breakdown of Fe(III) is promoted. The existence of soluble Fe(III) insediment porewaters has important implications on themineralization of organic matter. In addition to being highlyreactive, Fe(III) can diffuse in and out of sediments,supplying an electron acceptor at locations where hydrousiron oxides are already consumed, potentially shiftinglocal reactions. In addition, reduction of soluble Fe(III) bysulfide and successive formation of aqueous FeS shouldfacilitate the production of pyrite in natural sediments. Theuse of voltammetry to measure soluble Fe(III) can beapplied to any oceanographic or environmental system todetermine the fate of Fe(III) in the dissolved phase.
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