JOURNAL OF COLLOID AND INTERFACE SCIENCE 194, 194–206 (1997) ARTICLE NO. CS975116 Reductive Dissolution of Fe(III) (Hydr)oxides by Cysteine: Kinetics and Mechanism Aria Amirbahman, 1 Laura Sigg, and Urs von Gunten Swiss Federal Institute of Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology, Zu ¨rich (ETH), Switzerland Received May 22, 1997; accepted August 7, 1997 Fe( III ) by anaerobic microorganisms. Iron ( III ) oxide surfaces Cysteine was used as a model reductant to gain further insight also provide important sites for adsorption of many inorganic into the kinetics of bacterially mediated iron reduction. Our experi- and organic micropollutants. Dissolution of these surfaces may mental data and modeling results indicated that the reductive result in the mobilization of the adsorbed compounds. dissolution of hydrous ferric oxide (HFO) takes place via surface Even though crystalline phases constitute most of the iron complex formation of cysteine and corresponds to the rate law oxides in natural environments, amorphous iron oxide, due d[Fe(II)]/ dt Å k 0 2 [ GFecys 0 ] / k 1 2 [ GFecys 0 ], where k 0 2 and to its large specific surface area and its high susceptibility k 1 2 are the corresponding rate constants for the cysteine surface to dissolution, is an important phase. Amorphous hydrous species GFecys 0 and GFecys 0 , respectively. The pH-dependent ferric oxide forms when alternating oxidizing and reducing dissolution behavior of HFO suggested that k 0 2 [ GFecys 0 ]  k 1 2 [ GFecys 0 ]. A value of 6.83 1 10 02 s 01 as the lower limit for conditions bring about rapid Fe(II)/Fe(III) turnover, or k 0 2 was obtained. These two surface species were related by the when crystallization of Fe(III) is inhibited by adsorbents following proton complexation equilibrium expression: GFecys 0 such as organics, phosphate, and silicate species that have high affinity for the iron oxide surfaces (2). / H / S k s 1 k s 01 GFecys 0 . A log K s int value of 7.5 was estimated for this The presence of Fe(II) species has been documented in several anaerobic groundwater systems (3–5). Heron and equilibrium relationship, indicating a reduction of 2.8 pH units in Christensen (6) have shown that solid Fe(II) and Fe(III) the acidity constant of cysteine’s amino group, following adsorp- tion onto HFO. The reductive dissolution rate of HFO exhibited species provide a significant redox buffer in subsurface envi- a maximum of 3.3 1 10 08 mol s 01 m 02 at pH 8.3, corresponding ronments. The Fe(II) species may hinder the oxidation of to the pH value where the concentration of GFecys 0 species was other reduced species. Iron(III) oxides, on the other hand, at maximum. Experiments in the presence of phosphate indicated may inhibit the development of a reducing environment by that at equilibrium concentrations as low as 50 mM, this ligand acting as electron acceptors, especially in zones where mi- brings about more than a sixfold reduction in the rate of dissolution crobial sulfate reduction is dominant. A common pathway of HFO by cysteine. Dissolution experiments with other iron oxide for the reduction of Fe( III ) oxides in anaerobic environments phases showed the following order for the reductive dissolution is the reaction with hydrogen sulfide, which is normally a rates: HFO ú lepidocrocite ú goethite.  1997 Academic Press product of microbial sulfate reduction (7). In the absence Key Words: reductive dissolution; Fe(III) (hydr)oxides; cys- of hydrogen sulfide, however, Fe(II) production may be teine; surface complexation modeling. attributed to the bacterially mediated ( dissimilatory ) iron reduction, where Fe(III) acts as the final electron acceptor in the oxidation of organic matter. INTRODUCTION Dissimilatory iron reduction may affect soil weathering Reductive dissolution of Fe(III) oxide species has many processes and have implications for groundwater quality. important geochemical implications. Production of soluble The presence of Fe(III)-reducing bacteria downstream of a Fe(II) enhances the abiotic reduction of several organic con- municipal landfill has been suggested by Albrechtsen et al. taminants such as halogenated organics and nitroaromatic (8), where Fe(II) is generated in the absence of significant compounds at surfaces (1). Oxidation of organic matter from sulfide production. Dissimilatory iron reduction has been different sources is coupled to the dissimilatory reduction of studied extensively in the laboratory environments for differ- ent bacterial cultures and iron oxide phases (9–12). All of these studies indicate that bacterial cell–mineral contact is 1 To whom correspondence should be addressed. Present address: Depart- a prerequisite for the reduction of iron and that a low redox ment of Civil and Environmental Engineering, 5706 Aubert Hall, University potential alone is not sufficient for the observed microbial of Maine, Orono, Maine 04469-5706, USA. Fax: (207) 581-3888. Tele- phone: ( 207 ) 581-1277. E-mail: aria@umit.umaine.edu. Fe(III) reduction rates. The electron transfer mechanism in 194 0021-9797/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.