Environ. Sci. Technol. 2005, 39, 6494-6500
Heterogeneous Oxidation of Fe(II) on Ferric Oxide at Neutral pH and a Low Partial Pressure of O2† BYUNGTAE PARK AND BRIAN A. DEMPSEY* Department of Civil and Environmental Engineering, Pennsylvania State University, 212 Sackett Building, University Park, Pennsylvania 16802
The objective of this study was to identify the rate and mechanism of abiotic oxidation of ferrous iron at the waterferric oxide interface (heterogeneous oxidation) at neutral pH. Oxidation was conducted at a low partial pressure of O2 to slow the reactions and to represent very low dissolved oxygen (DO) conditions that can occur at oxic/ anoxic fronts. Hydrous ferric oxide (HFO) was partially converted to goethite after 24 h of anoxic contact with Fe(II), consistent with previous results. This resulted in a significant decrease in sorption of Fe(II). No conversion to goethite was observed after 25 min of anoxic contact between HFO and Fe(II). O2 was then introduced into the chamber and sparged (transfer half-time of 1.6 min) into the previously anoxic suspension, and the rate of oxidation of Fe(II) and the distribution between sorbed and dissolved Fe(II) were measured with time. The concentration of sorbed Fe(II) remained steady during each experiment, despite removal of all measurable dissolved Fe(II) in some experiments. The rate of oxidation of Fe(II) was proportional to the concentration of DO and both sorbed and dissolved Fe(II) up to a surface density of 0.02 mol Fe(II) per mol Fe(III), i.e., ∼0.2 Fe(II) per nm2 of ferric oxide surface area. This result differs from previous studies of heterogeneous oxidation, which found that the rate was proportional to sorbed Fe(II) and DO but did not find a dependence on dissolved Fe(II). Most previous experiments were autocatalytic; i.e., the initial concentration of ferric oxide was low or none, and sorbed Fe(II) was not measured. The results were consistent with an anode/cathode mechanism, with O2 reduced at electron-deficient sites with strongly sorbed Fe(II) and Fe(II) oxidized at electron-rich sites without sorbed Fe(II). The pseudo-first-order rate constants for oxidation of dissolved Fe(II) were about 10 times faster than those previously predicted for heterogeneous oxidation of Fe(II).
Introduction Ferric oxides are important sorbents for polar compounds (1). The Fe(II)/Fe(III) redox couple is relatively labile (2, 3) and iron is the fourth most abundant element in the earth’s crust. As a result, the concentration, speciation, and reactivity of ferric oxides and Fe(II) can have significant influences on the speciation, toxicity, and mobility of a variety of con†
This paper is part of the Charles O’Melia tribute issue. * Corresponding author phone: (814)865-1226; fax: (814)863-7304; e-mail:
[email protected]. 6494
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 17, 2005
taminants, such as uranium (4, 5), chromium (6-8), arsenic (9), nutrients (10), and other trace metals (11). Iron salts are also used during water and wastewater treatment (12). Fe(II) is a major constituent of many acidic industrial and mine discharges; treatment strategies may require rapid oxidation sometimes accompanied by phase changes to minimize the volume of residuals (13). This paper deals with the oxidation of Fe(II) by O2 under conditions where heterogeneous oxidation is expected to dominate. Sorbed Fe(II) is a stronger reducing agent than dissolved Fe(II) (4, 14-20). Previous studies reported that the rate of oxidation of Fe(II) by O2 was first-order with respect to the concentration of sorbed Fe(II) and have implied or stated that sorbed Fe(II) was oxidized, but there has not been any direct evidence that this implication is true. Williams and Scherer (21) recently reported that reduction of nitrobenzene required both sorbed Fe(II) and dissolved Fe(II); sorbed Fe(II) without dissolved Fe(II) was insufficient for reduction of nitrobenzene, casting doubt on the presumption that sorbed Fe(II) was oxidized during the reaction. The possibility that sorbed Fe(II) is insufficient for the reduction of nitrobenzene and other pollutants has important implications for remediation programs. It is possible that a reservoir of sorbed Fe(II), in the absence of dissolved Fe(II), could be impotent regarding reductive immobilization of contaminants. This paper also deals with transformation of hydrous ferric oxide (HFO) into more stable phases and the effects of these transformations on the rate of heterogeneous oxidation of Fe(II) by O2. It is interesting that a critical compilation of the sorption of metals and ligands onto hydrous ferric oxide (1) did not include data about the sorption of Fe(II). The lack of data for sorption of Fe(II) may have been due to experimental difficulties that have been addressed in more recent publications, such as changes in solid phases and mechanisms due to subtle changes in pH or concentrations of other components (22-27). Cornell and Giovanoli (26) reported that HFO tends to transform to hematite by internal reorganization and transforms into goethite by dissolution and recrystallization. Phase changes of ferric oxide can result in changes in specific surface area, which is expected to result in decreased sorption (28) and decreased rates of oxidation/ reduction reactions (29, 30). Phase changes can also change the bulk density of ferric oxides, affecting the permeability of groundwater environments. The main objective was to determine the rate and mechanism of the heterogeneous oxidation of Fe(II) in the presence of HFO at neutral pH and a low partial pressure of O2 (PO2). The application of a low PO2 was motivated by the possibility that such conditions might occur in groundwater environments and by the experimental need to slow the rate of oxidation of Fe(II) at neutral pH. Secondary objectives were to study phase changes and the effects of phase changes on the sorption of Fe(II). Although O2 was used as the oxidizing agent in these experiments, it is anticipated that similar mechanisms might occur in the presence of uranyl (UO22+), arsenate (AsO43-), or other chemicals that are oxidizing relative to the Fe(II)/Fe(III) couple.
Materials and Methods Reagents. Chemicals were reagent grade or better. All glassware and plastic bottles were acid washed with 10% HNO3, rinsed several times with distilled and deionized (DI) water, and air-dried prior to use. O2-free DI water was prepared by bubbling with N2 and then stored inside an anaerobic chamber for at least 3 days prior to preparation 10.1021/es0501058 CCC: $30.25
2005 American Chemical Society Published on Web 07/22/2005
TABLE 1. Experimental Conditionsa experiment no.
Fe(III) (mM)
total Fe(II) (mM)
preequilibration time (h)
pH
PO2 (atm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 0.5 0.5 0.5
0.121 0.122 0.133 0.066 0.136 0.191 0.147 0.635 0.263 1.264 1.356 0.126 0.132 0.102 0.049 1.270
24 24 24 0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42 24 0.42 0.42 0.42 24
6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 6.8 7.0 7.0 6.8 7.0 7.0
0.00214 0.00312 0.00960 0.00960 0.00960 0.00960 0.00067 0.00960 0.00960 0.00960 0.00960 0.00960 0.00960 0.00960 0.00960 0.00960
FIGURE 1. Experimental procedure. of reagents. Amorphous HFO was prepared by dissolving 0.676 g of FeCl3‚6H2O in 500 mL of DI followed by precipitation at neutral pH by slow addition of NaOH for 4 h. Then, 500 mL of 0.02 M sodium piperazine N,N′-bis(2-ethanesulfonic acid) (sodium PIPES) was added to produce a suspension of 2.5 or 0.5 mM Fe(III). For some experiments, HFO was made by instantaneous mixing of equal volumes of FeCl3‚6H2O and pH-adjusted 0.02 M sodium PIPES solutions so as to produce a constant pH of 6.8 without subsequent addition of NaOH solution. A stock solution of 250 mM Fe(II) was prepared by dissolving 12.4 g of FeCl2‚ 4H2O in 250 mL of acidified (pH < 1.0) DI water, calibrated by titration of the primary standard K2Cr2O7 to the ferroin end point (31), and stored in a dark bottle. Anoxic Environment. All reagent preparation and experiments except for final Fe(II) measurements were conducted in an inert atmosphere (97% N2 and 3% H2) at room temperature inside the anaerobic chamber (Coy Laboratory Products, Inc.), which was equipped with a palladium catalyst to remove oxygen traces. The palladium catalyst was replaced regularly, and the temperature in the chamber was controlled at 25 °C by the heating unit. Desiccants (Drierite, VWR) and/ or a mini-dehumidifier were used to prevent moisture accumulation in the chamber. An additional oxygen trap (32) was used to maintain a strict O2-free atmosphere. Analytical Methods. Fe(II) was measured by a modification of the 1,10-phenanthroline method (33). Dissolved Fe(II) was measured after filtration using a 0.2-µm filter. Total Fe(II) was measured after extraction in 0.5 N HCl for 20 h, followed by filtration (24). All color-developing reagents were added inside the chamber. Then the solutions were removed from the chamber for measurement of absorbance (510 nm, Shimadzu UV-1601). Standard curves were developed after acid extraction in the presence of ferric oxides, and this demonstrated complete recovery of Fe(II) and elimination of interference due to dissolved Fe(III). Adsorbed Fe(II) was the measured difference between total Fe(II) and dissolved Fe(II). Procedure. The experimental protocol is outlined in Figure 1. The HFO sample was prepared inside the anaerobic chamber and pH-adjusted, and then the suspension was connected to an additional oxygen trap (32) and aged 24 h. After 1 day, Fe(II) was introduced into the reactor by syringe, and the suspension was continually mixed with a magnetic stirrer. After 25 min or 24 h of preequilibration of the HFO with Fe(II), samples of the suspension were withdrawn and were analyzed for dissolved and total Fe(II). Subsequently, the palladium catalysis was taken out of the chamber, and the suspension was transferred to a 1 L beaker. A known volume of air was released into the chamber to achieve PO2 between 0.00067 and 0.00960 atm for various experiments, with rapid stirring of the suspension and fine-bubble diffusion of gas from the chamber. Samples were taken at time increments for analysis of dissolved and total Fe(II). Triplicate samples were analyzed for each analyte and for each sampling time. The experimental conditions for all of the experiments are reported in Table 1.
a
All experiments contained 0.02 M sodium PIPES.
Several batch aeration tests were conducted to determine the oxygen transfer rate for the mixing conditions that were described above, indicating an oxygen mass transfer coefficient (kLa) of 0.424 min-1. This value was used to improve the estimate for the oxidation rate constants for Fe(II), since the O2 transfer and the Fe(II) oxidation half-times were similar in some experiments. After each test, solid samples from the Fe(II)-HFO suspension were prepared by filtration inside the chamber, and transmission 57Fe Mo¨ssbauer spectroscopy was used to monitor the phase changes. The analytical techniques and methods for identification of ferric oxide phases have been previously described (25).
Results and Discussion Reactions between Fe(II) and HFO. Some examples of experimental results are shown in Figure 2. The solid lines represent dissolved Fe(II), and the dashed lines represent total Fe(II). Parts a-c represent experiments with 25 min of preequilibration between Fe(II) and HFO prior to the introduction of O2. Parts d-f represent experiments with 24 h of preequilibration. All of the results in Figure 2 are for experiments with 2.5 mM Fe(III), total Fe(II) was between 0.121 and 0.263 mM, and pH was 6.8 or 7 (Table 1). There was more than an order of magnitude range in PO2 in the six experiments shown in Figure 2. All of the experiments were undersaturated with respect to solid phases except magnetite, which could contain Fe(II). Mo¨ssbauer spectroscopy indicated that magnetite did not form in any of our experiments, using techniques that were described by Jang et al. (25), sensitive to
