Kinetics and Mechanisms for Reactions of Fe(II) with Iron(III) Oxides BYONG-HUN JEON,* BRIAN A. DEMPSEY, AND WILLIAM D. BURGOS Department of Civil and Environmental Engineering, The Pennsylvania State University, 212 Sackett Building, University Park, Pennsylvania 16802-2450 Uptake of Fe(II) onto hematite (R-Fe 2 O 3 ), corundum (R-Al 2 O 3 ), amorphous ferric oxide (AFO), and a mixture of hematite and AFO was measured. Uptake was operationally divided into adsorption (extractable by 0.5 N HCl within 20 h) and fixation (extractable by 3.0 N HCl within 7 d). For 0.25 mM Fe(II) onto 25 mM iron(III) hematite at pH 6.8: (i) 10% of Fe(II) was adsorbed within 1 min; (ii) 20% of Fe(II) was adsorbed within 1 d; (iii) uptake slowly increased to 24% of Fe(II) during the next 24 d, almost all adsorbed; (iv) at 30 d, the uptake increased to 28% of Fe(II) with 6% of total Fe(II) fixed; and (v) uptake slowly increased to 30% of Fe(II) by 45 d with 10% of total Fe(II) fixed. Similar results were observed for 0.125 mM Fe(II) onto 25 mM iron(III) hematite, except that percent of adsorption and fixation were increased. There was adsorption but no fixation for 0.25 mM Fe(II) onto corundum [196.2 mM Al(III)] at pH 6.8, for 0.125 mM Fe(II) onto 25 mM iron(III) hematite at pH 4.5, and for 0.25 mM Zn(II) onto 25 mM iron- (III) hematite at pH 6.8. A small addition of AFO to the hematite suspension increased Fe(II) fixation when 0.25 mM Fe(II) was reacted with 25 mM iron(III) hematite and 0.025 mM Fe(III) AFO at pH 6.8. Reaction of 0.125 mM Fe(II) with 2.5 mM Fe(III) AFO resulted in rapid adsorption of 30% of added Fe(II), followed by conversion of AFO to goethite and a decrease in adsorption without Fe(II) fixation.The fixation of Fe(II) by hematite at pH 6.8is consistent with interfacial electron transfer and the formation of new mineral phases. We propose that electron transfer from adsorbed Fe(II) to structural Fe(III) in hematite results in oxidation of Fe(II) to AFO on the surface of hematite and that solid-phase contact among hematite, AFO, and structural Fe(II) produces magnetite (Fe 3 O 4 ). The unique interactions of Fe(II) with iron(III) oxides would be environmentally important to understand the fate of redox- sensitive chemicals. Introduction The sorbed iron(II)/iron(III)oxide redoxcouple often buffers theoxidation -reduction potentialofanoxicsystems(1),thus controlling the free energy and the activation energy for a varietyofreactions.Sorbed Fe(II)is a stronger reducingagent than dissolved Fe(II) (2-4), and sorbed Fe(II) on iron(III) oxidesaffectsthe ratesofboth abioticand microbialreduction of inorganic species (2, 5-14) and organic chemicals (15, 16). The adsorption of divalent metal ions onto ferric oxides has been well-documented (e.g., refs 17-21). There are few reports describing adsorption ofFe(II) on iron(III) oxides (2, 22-27) because ofthe difficulty ofmaintaining strict anoxic conditions and the possible formation ofsecondarymineral phases such as magnetite or siderite (25, 28, 29). Some investigatorshave reported that the sorption ofdivalent metal ions onto metal oxides is fast, and equilibrium was reported within seconds for some systems (30, 31).Other investigators have reported that sorption of divalent metals was slower (32-35).We previouslyreported slow sorption ofFe(II)onto hematite (22). Most studies of Fe(II) sorption lack mass balance controls.Studiesthat included massbalancesfound that part ofthe initialFe(II)was not recovered (22, 26).Several hypotheses have been suggested to explain the incomplete recovery of Fe(II) including slow diffusion of Fe(II) through micropores, electron transfer from adsorbed Fe(II) to bulk Fe 2O3,and conversion ofFe(II)to more stable mineralphases (22, 26). The primarygoalofthis research was to describe the slow reactions of Fe(II) with ferric oxides. The specific objectives were to (i) determine long-term (up to 45 d) uptake of Fe(II) and Zn(II)onto hematite (R-Fe 2O3),ofFe(II)onto amorphous ferric oxide (AFO), of Fe(II) onto a mixture of hematite and AFO, and of Fe(II) onto corundum (R-Al2O3); (ii) determine the effects of AFO on adsorption and fixation of Fe(II) by hematite; and (iii) develop a mechanistic model for the reactions of Fe(II) with hematite and with AFO. Materials and Methods Experiments were conducted in 1-L Pyrex glass reaction bottles(referred to asmaster reactors).Syringes,glassbottles, and plastic vials were used for sample processing. All glassware and plastic bottles were acid-washed with 20% nitric acid, rinsed several times with distilled and deionized water (DDW), and purged with O2-free N2/H2 before use. All chemicals were reagent grade or better, unless otherwise described. All work was performed at 20-25 °C in a 97% N2/ 3% H2 atmosphere inside an anaerobic chamber (Coy Laboratory Products, Inc.) that was equipped with a palladium catalyst to remove trace O2. Despite these precautions, it was discovered that the chamber contained up to 5 × 10 -6 atm O2 (36). Since this partial pressure could result in significant oxidation ofFe(II) at pH 6.8, all experiments with Fe(II) were conducted using a low-temperature oxygen trap that is described elsewhere (36). Briefly, the O2 trap bottlescontained 0.90 mM Fe(II) and 23.3 mM Fe(III) as AFO. The pH was buffered at 8.1with 0.1M tris(hydroxymethyl)aminomethane (Tris). The half-time for reduction of O2 in the suspension phase of the oxygen trap was less than 0.5 s, and the half- time for transfer of O2 from the gas phase within the traps to the water phase was 6 min. The oxygen trap removed O2 to strict anoxic conditions (i.e., <10 -8 atm O2)(36). DDW was purged with O2-free N2 overnight and stored in the chamber for preparation of all solutions and suspen- sions. Stock solutions of 0.25 M Fe(II) and 0.15 M Zn(II) were prepared in the chamber from chloride salts at pH <1 and were stored in amber glass bottles that were wrapped in aluminum foil to exclude light. Fe(II) stock solutions were calibrated by titrating against primary standard K2Cr 2O7 to a ferroin end point (37), and the Zn(II) stock solution was *Correspondingauthorpresent address: Department ofBiological Sciences, The University of Alabama, A122 Bevill Building 7th Ave., Tuscaloosa, AL 35487-0206; phone: (205)348-1803; fax: (205)348- 1403; e-mail: bxj114@bama.ua.edu. Environ. Sci. Technol. 2003, 37, 3309-3315 10.1021/es025900p CCC: $25.00  2003 American Chemical Society VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3309 Published on Web 06/13/2003