Environ. Sci. Technol. 2003, 37, 3309-3315
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-Fe2O3), corundum (R-Al2O3), 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 6.8 is 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 (Fe3O4). The unique interactions of Fe(II) with iron(III) oxides would be environmentally important to understand the fate of redoxsensitive chemicals.
Introduction The sorbed iron(II)/iron(III) oxide redox couple often buffers the oxidation-reduction potential of anoxic systems (1), thus controlling the free energy and the activation energy for a variety of reactions. Sorbed Fe(II) is a stronger reducing agent * Corresponding author present address: Department of Biological Sciences, The University of Alabama, A122 Bevill Building 7th Ave., Tuscaloosa, AL 35487-0206; phone: (205)348-1803; fax: (205)3481403; e-mail:
[email protected]. 10.1021/es025900p CCC: $25.00 Published on Web 06/13/2003
2003 American Chemical Society
than dissolved Fe(II) (2-4), and sorbed Fe(II) on iron(III) oxides affects the rates of both abiotic and microbial reduction 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 of Fe(II) on iron(III) oxides (2, 22-27) because of the difficulty of maintaining strict anoxic conditions and the possible formation of secondary mineral phases such as magnetite or siderite (25, 28, 29). Some investigators have reported that the sorption of divalent 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 previously reported slow sorption of Fe(II) onto hematite (22). Most studies of Fe(II) sorption lack mass balance controls. Studies that included mass balances found that part of the initial Fe(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 Fe2O3, and conversion of Fe(II) to more stable mineral phases (22, 26). The primary goal of this 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-Fe2O3), of Fe(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 as master reactors). Syringes, glass bottles, 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 of Fe(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 bottles contained 0.90 mM Fe(II) and 23.3 mM Fe(III) as AFO. The pH was buffered at 8.1 with 0.1 M 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 halftime 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., 6.3 from adsorbed Fe(II) (59-61). Another example of heterogeneous IET is the oxidation of sorbed U(IV) by structural Pb(IV) in PbO2 (62). The sorption of U(IV) on lead(IV) oxide caused the oxidation of U(IV) producing dissolved UO22+ while structural Pb(IV) was reduced to Pb(II) and released into solution (62). In addition, Co(II) was oxidized to Co(III) at the surface of MnO2 and confirmed by XPS analysis (63, 64). Consistent with this alternative IET mechanism, the direct conversion of hematite and sorbed Fe(II) to magnetite is also proposed in Figure 4. MINTEQA2 (65) was used to evaluate the stability of various iron mineral phases in the presence of Fe(II). Figure 5 demonstrates that magnetite is the most stable phase for our experimental conditions [0.1250.25 mM Fe(II)] when the pH was above 5.75. In summary, the reactions of Fe(II) and Zn(II) with hematite and of Fe(II) with corundum and AFO were studied over several weeks. Fixation of Fe(II) to hematite occurred for many conditions. The results indicated that adsorption of Fe(II) on hematite resulted in secondary reactions to produce new phases. The secondary reactions could include IET; surface precipitation of AFO; and conversion of hematite, structural Fe(II), and AFO to magnetite when transformation is thermodynamically spontaneous. This hypothetical model is consistent with the fixation of Fe(II), the effect of low concentration of AFO in contact with hematite and Fe(II), the decreased pH that was observed when fixation occurred, and slow reaction kinetics associated with uptake of Fe(II) by hematite.
Acknowledgments Research support by the Natural and Accelerated Bioremediation Research Program (NABIR), Office of Biological and Environmental Research (OBER), Office of Energy Research, U.S. Department of Energy (DOE) Grants DE-FG02-98ER62691 and DE-FG02-01ER63180 is gratefully acknowledged.
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Received for review June 19, 2002. Revised manuscript received April 21, 2003. Accepted May 3, 2003. ES025900P
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