Environ. Sci. Technol. 2007, 41, 471-477
Fe(II) Sorption on Hematite: New Insights Based on Spectroscopic Measurements PHILIP LARESE-CASANOVA AND MICHELLE M. SCHERER* Department of Civil and Environmental Engineering, University of Iowa, 4105 Seamans Center, Iowa City, Iowa 52242-1527
We collected Mo¨ ssbauer spectra of 57Fe(II) interacting with 56hematite (R-Fe2O3) over a range of Fe(II) concentrations and pH values to explore whether a sorbed Fe(II) species would form. Several models of Fe(II) sorption (e.g., surface complexation models) assume that stable, sorbed Fe(II) species form on ligand binding sites of Fe(III) oxides and other minerals. Model predictions of changes in both speciation and concentration of sorbed Fe(II) species are often invoked to explain Fe(II) sorption patterns, as well as rates of contaminant reduction and microbial respiration of Fe(III) oxides. Here we demonstrate that, at low Fe(II) concentrations, sorbed Fe(II) species are transient and quickly undergo interfacial electron transfer with structural Fe(III) in hematite. At higher Fe(II) concentrations, however, we observe the formation of a stable, sorbed Fe(II) phase on hematite that we believe to be the first spectroscopic confirmation for a sorbed Fe(II) phase forming on an iron oxide. Low-temperature Mo¨ ssbauer spectra suggest that the sorbed Fe(II) phase contains varying degrees of Fe(II)-Fe(II) interaction and likely contains a mixture of adsorbed Fe(II) species and surface precipitated Fe(OH)2(s). The transition from Fe(II)-Fe(III) interfacial electron transfer to formation of a stable, sorbed Fe(II) phase coincides with the macroscopically observed change in isotherm slope, as well as the estimated surface site saturation suggesting that the finite capacity for interfacial electron transfer is influenced by surface properties. The spectroscopic demonstration of two distinctly different sorption endpoints, that is an Fe(III) coating formed from electron transfer or a stable, sorbed Fe(II) phase, challenges us to reconsider our traditional interpretations and modeling of Fe(II) sorption behavior (as well as, we would argue, of any other redox active sorbatesorbent couple).
Introduction The remarkable reactivity of iron oxides and the discovery of microbial iron respiration has led to intense interest in how Fe(II) interacts with the Fe mineral-water interface (17). Fe(II) sorbed at mineral surfaces has been implicated in the fate of contaminants in Fe reducing environments (8, 9), as well as inhibition of microbial respiration of Fe(III) oxides (10, 11). Several models of Fe(II) sorption (e.g., surface complexation models, SCMs) assume that stable, sorbed Fe* Corresponding author phone: (319) 335 5654; fax: (319) 335 5660; e-mail:
[email protected]. 10.1021/es0617035 CCC: $37.00 Published on Web 12/09/2006
2007 American Chemical Society
(II) species form on ligand binding sites of Fe(III) oxides and other minerals. Model predictions of changes in both speciation and concentration of sorbed Fe(II) species are often invoked to explain Fe(II) sorption patterns (12-15) as well as rates of contaminant reduction (13, 16). Although the SCM framework for describing uptake of cations has been spectroscopically confirmed for several metals (17, 18), spectroscopic validation of Fe(II) sorbed on an Fe(III) oxide has continued to elude us because of the challenges of distinguishing small amounts of sorbed Fe from the underlying bulk Fe oxide. We have previously attempted to characterize Fe(II) sorbed on Fe minerals using the isotope specificity of 57Fe Mo¨ssbauer spectroscopy (3), however, in that work, as well as the recent work of Silvester and co-workers (2005) (19), an Fe(II) contribution was never observed in the Mo¨ssbauer spectra because the sorbed Fe(II) was rapidly oxidized to Fe(III) by the underlying structural Fe(III). This data provided spectroscopic confirmation of Fe(II)-Fe(III) interfacial electron-transfer that had long been speculated to occur based on evidence from chemical extractions (e.g., (4, 20-24)), TEM (1, 25), AFM (26), Fe isotopes (6, 7), and Mo¨ssbauer spectroscopy (3, 19, 27). These techniques have been used to investigate not only the interaction of Fe(II) with oxide surfaces but also the subsequent Fe transformations. Interfacial electron transfer between sorbed Fe(II) and Fe(III) oxides is expected to play an important role in Fe transformations, including (i) reductive dissolution of structural Fe(III) in ferrihydrite, lepidocrocite, and goethite (7, 28), (ii) solid-state conversion or dissolution-reprecipitation of ferrihydrite to goethite, lepidocrocite, or magnetite (1, 19, 24, 29), and (iii) possible formation of mixed-valent near-surface precipitates such as magnetite on goethite (4) or hematite (21). The process of electron transfer has been explained based on semiconducting properties of minerals such as, in the case of hematite and mixed-valent iron oxides, an electron hopping mechanism between nearest neighbor iron atoms by way of overlapping d-orbitals (30, 31). With multiple lines of evidence from both macroscopic and spectroscopic measurements, as well as computational calculations indicating that Fe(II)-Fe(III) interfacial electron transfer occurs, we were compelled to question whether a stable, sorbed Fe(II) phase was simply a myth. To explore this, we extended our previous experiments to spectroscopically characterize 57Fe(II) reacted with hematite (R-Fe2O3) over an environmentally relevant range of Fe(II) concentrations and pH values (i.e., isotherms and pH edges). Our goal was to determine whether interfacial electron transfer occurred over this range of conditions or whether a sorbed Fe(II) phase would indeed form. We also hoped that the spectroscopic observations would help us interpret the macroscopic behavior observed in both the isotherm and Fe(II) desorption experiments.
Experimental Section 56Fe Enriched Hematite Synthesis and Characterization. Isotopically enriched Fe solutions were prepared as previously described (3). Synthesis details are provided in Supporting Information. X-ray diffraction patterns verified that 56Fehematite were pure, crystalline particles (Figure S1 in the Supporting Information), and BET analysis indicated a surface area of 30 m2 g-1. 57Fe(II)-56Hematite Sorption Experiments. All experiments were conducted inside an anoxic glove box (N2/H2 atmosphere) to avoid exposure to oxygen, which can rapidly
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TABLE 1. 57Fe(II) Sorption on 56Hematitea {Fe(II)}sorb µmol g-1
sample numberb
pH
[Fe(II)]Fe(OH)2(s)c mM
[Fe(II)]initial mM
[Fe(II)]final mM
[Fe(II)]sorbd mM
Fe(II)sorb %
1 2 3 4 5 6 7
7.5 7.4 7.4 7.4 7.5 7.4 7.5
11.4 18.1 18.1 18.1 11.4 18.1 11.4
0.14 0.46 0.97 1.95 4.71 8.53 10.96
Isotherm Data 0.07 0.36 0.79 1.71 4.43 8.19 10.58
0.07 0.10 0.18 0.24 0.28 0.34 0.38
47.6 21.4 15.8 12.6 6.0 4.0 3.5
33 50 90 123 142 170 190
N N N N Y Y Y
8 9 3 10
6.4 7.1 7.4 8.1
1800 72.0 18.1 0.72
0.99 1.03 0.97 0.95
pH Edge Data 0.96 0.90 0.79 0.71
0.03 0.13 0.18 0.24
3.4 12.4 15.8 24.3
17 64 90 120
N N N N
11 12 13 14 15
7.7 7.2 7.2 7.4 7.4
4.5 45.4 45.4 18.1 18.1
3.35 8.21 12.02 5.76 9.72
Other Conditions 3.06 7.95 11.63 5.45 9.37
0.29 0.26 0.39 0.31 0.34
8.8 3.2 3.2 5.4 3.5
147 130 192 156/121f 171/125f
Y Y Y Ng Ng
Fe(II) observed in spectra?e
a Experimental conditions: 30 mg 56hematite in 15 mL of 25 mM KBr (2 g/L 56hematite), 10 h equilibration time, buffers were PIPES for all pH except pH 8.1 which used TAPS. b Refers to labeled markers in Figure 1 and Mo¨ ssbauer spectra in Figure 2 and Figures S2 and S3 in the Supporting Information. c Dissolved Fe(II) concentration that would result in homogeneous precipitation of Fe(OH)2 (s) (log Ksp ) -15.1) (55) with ionic strength 75 mM. d Estimated from [Fe(II)]sorb ) [Fe(II)]initial - [Fe(II)]final. e N ) no, Y ) yes. Indicates whether an Fe(II) doublet was observed in the Mo¨ ssbauer spectra shown in Figure 2 and Figure S2 the in Supporting Information. f Data from desorption experiments in Figure 4. The amount of sorbed Fe(II) is reported for before and after resuspending solids, presented with the notation before/after. g Refers to spectra collected after resuspending solids in Fe(II)-free buffer.
oxidize Fe(II) in the presence of oxides at circum-neutral pH values (32). Care was taken to minimize the presence of oxygen within the glove box atmosphere as described in more detail in the Supporting Information. Sorption experiments were carried out in well-mixed batch reactors containing 25 mM KBr buffered with 25 mM Piperazine-1,4-bis(2-ethanesulfonic acid) (PIPES) for pH values 6.6-7.4 or with 25 mM N-Tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) for pH 8.1. The reactor solution, before addition of the oxide, was allowed to equilibrate after the addition of aqueous 57FeCl2 for a minimum of 12 h. The solution was filtered (0.2 µm) to remove precipitates that may have accumulated from the initial spike of 57FeCl2. The aqueous Fe(II) concentration was then measured and 30 mg of 56hematite was added. Each reactor was mixed end-overend on rotator for 10 h in the dark. After equilibration, the aqueous Fe(II) concentration was determined by filtering a portion of the oxide slurry. The loss of aqueous Fe(II) was determined by the difference between the amount of Fe(II) prior to the addition of the oxide phase compared to the amount after equilibration. The remainder of the oxide slurry was filtered onto 13 mm filter discs and mounted as a wet paste between two layers of Kapton tape for Mo¨ssbauer analysis (described in the Supporting Information). Chemical Analyses. Dissolved Fe(II) concentrations were determined on aliquots of filtered (0.2 µm) reactor solution that were diluted with deionized water to 1.0 mL and immediately acidified with 40 µL of 5 M HCl. Fe(II) was measured colorimetrically with the 1,10-phenanthroline method at 510 nm on a UV-VIS spectrophotometer. To minimize possible photochemical reduction of the Fe(III)phenanthroline complex (33), samples for Fe(II) analysis were kept in the dark except for a brief exposure to light (≈1 min) during the addition of the 1,10 phenanthroline and buffer agents. Recovery of Fe(II) was measured by complete dissolution of the hematite in 3.0 M HCl (usually about 6 days). To account for the background signal of Fe(III) from the hematite dissolution, we subtracted the absorbance measured in processed samples from reactors of dissolved hematite without Fe(II). 472
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Results and Discussion Fe(II) Sorption Isotherm and Edge. Macroscopic measurements of 57Fe(II) sorbed on 56hematite were made by monitoring loss of Fe(II) from solution after 10 h. Trends in sorbed Fe(II) concentrations follow isotherm and pH edge patterns typical for metal cations on oxide surfaces. More specifically, the amount of 57Fe(II) sorbed on 56hematite increases as the concentration of dissolved Fe(II) or solution pH increases (Table 1). The relationship between sorbed and dissolved Fe(II) at a constant pH of 7.4 (i.e., the sorption isotherm) displays a psuedo-Freundlich pattern with a steeper slope at low Fe(II) concentrations (
