Reactivity of Freshly Formed Fe(III) in Synthetic Solutions and (Pore)Waters: Voltammetric Evidence of an Aging Process M. TAILLEFERT,* A. B. BONO, AND G. W. LUTHER, III College of Marine Studies, University of Delaware, Lewes, Delaware 19958 We report a laboratory and field study that demonstrates that soluble Fe(III) complexed to organic ligands exists in sediment porewaters. Synthetic solutions of Fe(III) with the ligand Tris were used to simulate organic complexation of Fe(III) in natural waters.Size fractionation using gel filtration chromatography shows that at least three different molecular mass species coexist. The molecular masses of FeIII x -Tris y complexes increase with time, suggesting that aggregation occurs in solution. Soluble Fe(III) is detected by voltammetry at a mercury drop electrode when it is complexed 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 negative potential for “aged” Fe(III). A mathematical model was successfully developed to assess a mechanism for this aging process.This model involves oxidation and adsorption of Fe- (II), aggregation, and precipitation of Fe(III) complexes. The species formed are extremely reactive. Upon addition of 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) in sediment porewaters has important implications on the mineralization of organic matter. In addition to being highly reactive, Fe(III) can diffuse in and out of sediments, supplying an electron acceptor at locations where hydrous iron oxides are already consumed, potentially shifting local reactions. In addition, reduction of soluble Fe(III) by sulfide and successive formation of aqueous FeS should facilitate the production of pyrite in natural sediments. The use of voltammetry to measure soluble Fe(III) can be applied to any oceanographic or environmental system to determine the fate of Fe(III) in the dissolved phase. Introduction It is widely accepted that ferric iron plays a significant role in the mineralization of natural organic matter in marine and freshwater sediments. It can be utilized as a terminal electron acceptor by bacteria (1, 2), reduced by sulfides (3- 5), or reduced by both (6, 7). In addition, ferric iron can be reduced or nonreductivelydissolved byorganic compounds in the absence ofmicroorganisms(8-10),or it can be reduced by dissolved ferrous iron (11). The reactivity of ferric iron has been assessed on the premise that this species is in its precipitated or colloidal form and that the reduction step proceeds at the surface of the particle (4, 5, 10, 11).Kinetic studies have shown that the rate of reduction depends (i) on the reactivity of the Fe(III) phase,an amorphous phase beingmore reactive than a well- crystallized species, and (ii) on the strength ofthe reductant (4, 5, 10). Among reductants present in sediments, dissolved inorganic sulfide may reduce Fe(III) (4, 5, 7), with the rate- limiting step being apparently the rate of dissolution of the surface ferrous hydroxide (4). However, Luther et al. (9) demonstrated that dissolution offerrihydrite in the presence of an organic ligand is possible and that the soluble Fe(III) complexformed may be reduced by pyrite much faster than in the absence of organic ligand. Later, Liang et al. (12) and Luther et al. (13) demonstrated the presence of dissolved Fe(III) in groundwater and naturalporewaters, respectively. Recently, voltammetric measurements of redox species O2, ∑H2S, S2O3 2- , Fe(II), and Mn(II) in sediment porewaters have been performed with Au/Hg microelectrodes on a submillimeter scale (14).Avoltammetric signalattributed to Fe(III) complexed by an organic ligand was reported using these microelectrodes in sediments (15) and at the surface of biofilms (16). Such signals have also been found in freshwater environments (17) and in laboratory studies in acidic (18) and circumneutral conditions (19). Von Gunten and Schneider (19) used Tris [tris(hydroxymethyl)ami- nomethane]to complexFe(III)in lowionicstrength solutions. High concentrations of Tris stabilized Fe(III) in solution for months in the form ofa polynuclear species (hydrodynamic radius ∼1nm),but voltammetricsignalsshifted toward more negative potentials with time. These authors explained this voltammetric behavior by the concomitant transformation ofhydroxybridgesoffreshlyformed Fe(OH)3 into oxobridges at the surface of Fe(III) upon aging of the solution (19). The objectivesofthisstudywere (i)to showwith laboratory solutions that the species observed by voltammetric mea- surements in the field with Au/Hgmicroelectrodes are indeed Fe(III) complexes; (ii) to recreate in the laboratory the aging process observed in sediment porewaters; and (iii) to study its reactivity with a strong reductant such as sulfide. An example of these processes in marine sediments is then provided. Materials and Methods Laboratory Experiments. Unless mentioned, all chemicals used were ACS grade and from Sigma. Fe(III) used in the laboratory experiments was produced by oxidation ofFe(II) from 0.01 M stock solutions ofFe(NH4)2(SO4)2‚6H2O (Fisher) in the appropriate oxygenated medium.The media consisted of either MilliQ water, 0.54 M NaCl, or filtered seawater to which an appropriate mass of Tris buffer (MW ) 121.1) was added to reach a concentration of 0.5 M. Solutions were not stirred to avoid precipitation of iron oxide phases. Aliquots were not collected earlier than 30 min after the start of the experiments to ensure that any Fe(III) formed had diffused uniformlythroughout the solution.The pH wasadjusted with hydrochloric acid (Fisher, trace metal grade) and measured usinga Sensorexcombination electrode and pH meter (Fisher Accumet, model 815 MP). Fe(III) was measured by volta- mmetry and separated into size fractions by gel filtration chromatography (Sephadex G-25). In addition, Fe(III) re- activity was investigated by reduction in the presence of sulfide.Forthese experiments,an excess(475 µM)ofan 8-day- old Fe(III)-Tris solution in NaCl (0.54 M) was reacted with bisulfide (21 µM) at pH 7.5. Fe(III), Fe(II), FeS, and ∑H2S were monitored by voltammetry with time. The FeS volta- mmetric peak is due to the reduction ofFe(II) from FeS(20). *Correspondingauthor phone: (302)645-4284;fax: (302)645-4007; e-mail: mtaillef@udel.edu. Environ. Sci. Technol. 2000, 34, 2169-2177 10.1021/es990120a CCC: $19.00  2000 American Chemical Society VOL. 34, NO. 11, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2169 Published on Web 04/22/2000