Environ. Sci. Technol. 2004, 38, 5757-5765
Identification and Quantification of Mineral Precipitation in Fe0 Filings from a Column Study WIWAT KAMOLPORNWIJIT,† L I Y U A N L I A N G , * ,†,‡ GERILYNN R. MOLINE,† TODD HART,§ AND OLIVIA R. WEST† Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6036, Cardiff School of Engineering, Cardiff University, Queen’s Buildings, P.O. Box 925, Cardiff CF24 0YF, Wales, U.K., and Battelle Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352
Thermogravimetric analysis (TGA) combined with X-ray diffraction (XRD) was used to identify mineral phases and determine corrosion rates of granular iron samples from a 2-yr field column study. Similar to other studies, goethite, magnetite, aragonite, and calcite were found to be the major precipitated minerals, with Fe2(OH)2CO3 and green rust as minor phases. Based on TGA-mass spectrometry (MS) analysis, Fe0 corrodes at rates of 0.5-6.1 mmol kg-1 d-1 in the high NO3- (up to 13.5 mM) groundwater; this rate is significantly higher than previously reported. Porosity reduction was 40.6%-45.1% for the inlet sand/Fe0 interface and 7.4%-25.6% for effluent samples of two test columns. Normalized for treatment volumes, porosity loss values are consistent with studies that use high levels of SO42- but are higher than those using low levels of corrosive species. Aqueous mass balance calculations yield corrosion rates similar to the TGA-MS method, providing an alternative to coring and mineralogical analysis. A severely corroded iron sample from the column simulating a 17-yr treatment throughput showed >75% porosity loss. Extensive porosity loss due to high levels of corrosive species in groundwater will have significant impact on long-term performance of permeable reactive barriers.
Introduction Zerovalent iron (Fe0) has been widely used as a reactive medium in permeable reactive barriers (PRB) for treating organic (11) and inorganic contaminants (12). Knowledge of rates and products of Fe0 corrosion and other biogeochemical reactions associated with long-term water flow within the PRB is of critical importance for efficient application of this technology (3-5, 13). Precipitation of secondary minerals, defined as phases not involved in containment of the contaminant (1), can lead to loss of iron reactivity (13), reduced hydraulic conductivity (1, 14), and the development of preferential flow (15, 16). The effects of these on the longterm performance of a PRB will depend on the type of minerals precipitated as well as the mass of precipitation over time. A wide range of mineral precipitates has been * Corresponding author phone: +44(0)29 2087 6175; e-mail:
[email protected]. † Oak Ridge National Laboratory. ‡ Cardiff University. § Battelle Pacific Northwest National Laboratory. 10.1021/es035085t CCC: $27.50 Published on Web 09/29/2004
2004 American Chemical Society
identified in laboratory and field studies, including amorphous iron oxyhydroxide (ferrihydrite), aragonite, calcite, crystalline iron (oxyhydr)oxide (akaganeite, goethite, magnetite, hematite, lepidocrocite, etc.), green rust, siderite, and mackinawite (1-5, 8, 10). However, direct methods for quantifying mineral precipitates and the long-term impact of these corrosion products on Fe0-PRB longevity are not well established. Precipitates can be quantified using mass balance of pore water chemistry up- and down-gradient of a PRB, assuming that such precipitates (such as CaCO3) are the sink of aqueous species (9, 14, 17). Using mass balance approach to quantify iron-bearing precipitates, however, requires knowledge of iron corrosion rates because dissolved Fe from corrosion is concurrently removed by precipitation (9, 18, 19) within the PRB. Published values of Fe0 corrosion rate are sparse (6, 20) and are not applicable to all types of iron and groundwater. Direct XRD quantification of iron-containing precipitates in core samples suffers from overwhelming interference of Fe0 that cannot be completely separated from corrosion products. Sequential chemical extraction, a common method for soil mineral analysis, is not suitable for Fe0 samples because Fe0 dissolves in the extraction solution. An effective technique, therefore, must either permit the analysis of precipitates without separation from the Fe0 or be unaffected by the presence of Fe0. Thermogravimetric analysis (TGA) has been widely used in the study of phase transformations at different temperatures (21). In this technique, mineral phases are quantified by integrating weight changes during transformation. The use of mass spectrometry (MS) with TGA for off-gas analysis can confirm the presumed phase. Because Fe0 is thermodynamically stable under inert or reducing conditions, it does not influence TGA results. With knowledge of phase stability and transformation at given temperatures, identification and quantification of some important mineral phases can be made without having to extract or separate precipitates from the Fe0 media. In this study, quantification of precipitates and derivation of Fe0 corrosion rates were pursued using core samples from field columns subjected to ∼2-yr treatment of groundwater containing up to 13.5 mM NO3- (16). TGA-MS combined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to directly quantify precipitate compositions in iron core samples. Degrees of cementation and pore filling within Fe0 were estimated for samples treated under both accelerated- and normal-flow conditions. This method is compared with a mass balance approach which uses pore water composition to estimate Fe0 corrosion and precipitation. The impact of mineral precipitation on the performance of the PRB in terms of porosity reduction is assessed.
Experimental Methods Materials. Iron core samples were collected from two large field columns (92-cm long, 15-cm diameter, see Plate 1, Supporting Information) that were operated for ∼2 yrs at the Y-12 National Security Complex in Oak Ridge, Tennessee (16). The columns were oriented horizontally ∼70-ft upgradient from an existing PRB (Figure 1 in ref 22), using the same iron filings as the PRB. Two flow rates were used, yielding initial pore velocities of 9.4 m d-1 in Column I and 0.33 m d-1 in Column II. The slower flow was intended to simulate typical field conditions, while the faster flow was designed to simulate treatment of high throughput for a period of ∼17 years. Insufficient residence time may not VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Chemical changes along Column I (a and b) and Column II (c and d), showing pH (]), calcium (0), nitrate (4), and ferrous iron (O) in pore water along centerline. Steady state change was observed during early operation (a and c), exhibiting homogeneous flow. Heterogeneous flow appeared during later stage operation (b and d), where higher concentrations at down gradient location are due to mixing with preferentially transported influent (16). Dash lines show the interfaces of iron and sand, with Fe0 sandwiched between two layers of sands.
adequately simulate high volume treatment if biogeochemical reactions do not reach completion. Nonetheless, the columns do appear to have captured major corrosion and precipitation reactions (16, 17) as indicated by high effluent pH and progressive removal of Ca and NO3- in pore water (Figure 1a,c) during the first two-month operation. Site groundwater was directly pumped into the columns; the composition varied due to seasonal changes, exhibiting 6.6-7.4 pH, 3.76.4 mM alkalinity, 5.4-9.0 mM Ca, 0.8-13.5 mM NO3-, 0.60.9 mM SO42-, 1.0-2.5 mM Cl-, and ∼9 µM dissolved O2. Figure 1 shows several snapshots of chemical profiles along the columns: homogeneous flow condition during the early stage (Figure 1a,c) and heterogeneous at later stage (Figure 1b,d). Description of column set up, operation, and the geochemical and hydrologic monitoring results are presented in detail elsewhere (16, 17). Column I was disassembled after 680-d operation, having treated 11.5 m3 groundwater (1336 pore volume). Pore volumes were calculated based on an initial porosity measured at 60% (16). Dissection required use of a hole-saw for the collection of core plugs (2.9-cm diameter by 4.1-cm long) under atmospheric conditions. Exposure time was minimized by immediate transfer of samples to plastic bags, which were stored in a closed PVC tube flushed with Ar gas at 5-psi pressure. The column was dissected in ∼5-cm sections, typically with 5 cores from each section (Figure 2, also see Plate 2, Supporting Information). Column II was disassembled after 695 d, having treated 1.1 m3 groundwater (∼150 pore volume). Sample collection was done easily with a spatula. Samples were collected and stored in Ar as done for Column I samples. Sample Preparation and Analysis. Most of the iron core samples from Column I maintained the cylindrical shape of 5758
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the hole-saw. The surface of the core plug may be affected by drilling-induced heating and atmosphere exposure and was therefore removed prior to sampling for mineralogical analysis. Subsamples were used for XRD analysis and TGAMS and SEM characterization. The XRD samples were sonicated for 1/2 hour in acetone and then wet ground under Ar flow. Upon drying, samples were sieved through a #120 (125 µm) sieve and the finer fractions collected for XRD (Scintag XDS2000) analysis. Samples for TGA-MS and SEM analysis were dried anoxically before further preparation. TGA-MS analysis was performed on selected samples, including the following: (1) 0-4 cm from the inlet sand-Fe0 interface (CS-I-8, CS-II-1), (2) mid-span (CS-I-81, 37-41 cm into Column I; CS-II-3, 37 cm into Column II), and (3) 0-4 cm from the effluent sand-Fe0 interface (CS-I-49, CS-II-15). The term “CS” denotes “column sample”, roman numerals identify the columns, and integers identify samples (Figure 2). Stock iron filings were also analyzed. Samples (∼100 mg) were placed in pure alumina crucibles and heated to 900 °C at a rate of 10 °C min-1 under He gas flow at a rate of 100 mL min-1. Analyses were performed using a Netzsch STA 409 TGA/DSC and a Pfeiffer QMS300 MS. After reaching 900 °C, 2% H2 gas in He was passed through the furnace causing transformation of iron oxides to Fe0. A dry sample from Column I (CS-I-14) was prepared for SEM analysis. It was impregnated in Buehler Epoxide resin and hardener and subjected to vortexing for 30 s. The solidified sample was cut, polished, and dried anoxically before surface carbon coating (PELCO, CC-7A). The thin section was observed using Scanning Electron Microscope FEI XL30FEG, and elemental analysis was done using an energy dispersive detector (OXFORD ultrathin window).
FIGURE 2. Phase identification in numbered samples along and across Columns I and II. The mineral phases were listed in the decreasing order of XRD intensity. Calcite and goethite generally predominated over aragonite and magnetite near the influent and in regions adjacent to the column gap. The shaded dark gray sections show iron media, where cementation decreased from the influent to the effluent of the column, between two sand layers. Phase abbreviations are used as follows: G-goethite, C-calcite, A-aragonite, M-magnetite, FeFe2(OH)2CO3, Gr-green rust, Q-quartz.
Results and Discussion Visual Observation of Iron Corrosion. Column I. Iron corrosion and cementation were extensive at the inlet sandFe0 interface and reduced progressively in the column. Near the inlet, iron lost visible grains and showed heavy cementation (see Plate 2a, Supporting Information). Rusty stain was visible throughout the iron cross-section and on sand grains near the inlet. Cementation extended ∼60 cm into the column, accounting for ∼2/3 of the iron mass. Distinct iron grains were present ∼14 cm into the column, on otherwise solidly cemented samples. In the same section, iron was more easily broken up in the bottom part of the horizontal-oriented column, implying less extensive cementation. Loose filings mixed with cemented samples were observed at 22 cm, near the bottom of the column, while the upper part of this section was completely cemented. From 22 to 60 cm, however, the bottom regions show heavy cementation. Rust stain was also seen at these cross-sections with less intensity than at the inlet interface (see Plate 2b, Supporting Information). Starting at ∼60 cm, and extending to the effluent interface, loose but densely compacted dark gray to black filings predominated.
Column II. With one tenth of the volume treated in Column I, cementation and corrosion were substantially less in this column than in Column I. Cemented samples were obtained at the inlet sand-Fe0 interface, as in Column I. However, cementation extended only ∼2.5 cm into the Fe0 accounting for ∼1/12 of iron mass. In the remainder of the column, filings were loose with no observable cementation. Rusty stain was observed at the influent and effluent sandFe0 interfaces. For both columns, a 1-2 mm gap was observed between the media and the column casing across the entire length of the column, extending 6-7 cm in width along a ∼120 degree arc (see Plate 2, Supporting Information). The surfaces of the media facing this gap were completely coated with rusty deposits, the products of Fe0 oxidation by O2. This implies that the O2-rich influent was preferentially transported through the gap. It is unknown when the gap formed. It may have been initiated by settling and compaction of the iron grains during initial setup and exacerbated by Fe0 corrosion and grain size reduction. The gap itself, however, did not necessarily cause flow channeling. Changes in pore water VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Precipitate Mass and Porosity Loss in Column I Based on the Mass Balance of Pore Water Aqueous Species, Modified from Ref 16 porosity lossb corrosiona rate (mmol kg-1 d-1)
run time day
simulated time
pore volume
30
2.1
163
17
72
5.1
398
10.1
215
7.2
569
3.1
399
12.4
974
4.5
666
16.6
1310
2.5
phases
precipitate mass (mmol L-1)
entire columnc
first 50 cmd
CaCO3 Fe3O4/R-FeOOH total CaCO3 Fe3O4/R-FeOOH total CaCO3 Fe3O4/R-FeOOH total CaCO3 Fe3O4/R-FeOOH total CaCO3 Fe3O4/R-FeOOH total
3.6 5.4/16.3 9.0/19.9 1.8 1.9/5.6 12.7/27.3 0.9 1.4/4.1 15/32.3 1.7 1.5/4.6 18.2/38.6 1.0 1.0/2.9 20.2/42.5
2 2.1/5.1 4.1/7.1 2.4 1.7/4.2 8.3/13.8 1.8 1.8/4.5 12.0/20.1 5.7 3.5/8.6 12.9/20.5 4.3 3.0/7.3 20.1/32.1
3.3 3.5/8.5 6.8/11.8 4 2.9/7.0 13.7/22.9 3 3.0/7.4 19.8/33.3 9.4 5.8/14.2 35.1/56.9 7.1 5.0/12.1 47.1/76.1
a Assumed abiotic nitrate reduction prior to day 215, and biotic reduction afterward: abiotic reaction: 4Fe0 + NO - + 7H O f 4Fe2+ + NH + 3 2 4 + 10OH-; biotic reaction: 5Fe0 + 2NO3- + 6H2O f 5Fe2+ + N2 + 12OH-. b The porosity loss was calculated using 60% initial porosity. The porosity 0 loss due to Fe3O4 or R-FeOOH precipitation is separated by ‘/’ and has been corrected for any porosity gain from the loss of Fe mass due to corrosion. c Distributing mineral phases uniformly throughout the Fe0 medium. d Mineral phases allocated to the first 50 cm of the iron medium, where heavy cementation was observed.
chemistry over time (Figure 1a,b) suggest gradual development of preferential flow paths (16, 17). Additionally, hydraulic tracer tests in Column I showed homogeneous flow after 1.3-yr of simulated normal treatment but severe flow channeling after 3.9-yr simulation (16). Mass balance estimates indicate that 9-27.3 mM of precipitates formed in ∼3-yr of simulated treatment time (run time of 30 and 72 d in Table 1). We suggest that this precipitation at the inlet region caused hydraulic conductivity contrast between the gap and the bulk media and subsequently the diversion of influent through the gap. Following influent diversion, groundwater redistributed beyond the heavily cemented region, reacting with Fe0 and precipitating minerals nonuniformly into the column. The reduced groundwater flow through the lower part of the column may account for less cementation than in the top region. Thus, heterogeneous flow appears to be the main reason for the differing degrees of cementation across and along the columns (16). Mineral Phase Identification Using XRD. Quantitative use of XRD intensity normally requires proper standardization. However, change in intensity ratio of certain phase pairs implies change in mass ratio, provided samples have similar matrix and phase assemblage. In this study, changes in intensity ratios of two phase-pairs (calcite/aragonite and magnetite/goethite) were observed to elucidate the influence of pore waters on the phase formation. No attempt was made to quantify phases using XRD. Figure 2 summarizes the mineral phases identified in cores, showing characteristic XRD signal intensities in descending order. All samples showed a similar suite of minerals but with different intensity ratios. The major phases are goethite (R-FeOOH), magnetite (Fe3O4), calcite, and aragonite, with lesser amounts of Fe2(OH)2CO3 and green rust. These phases were identified under field and laboratory conditions (1-5), with some exceptions. Siderite and sulfide phases were not identified by XRD in this study but have been reported in the nearby PRB (5, 7) and others (3). The amorphous phases (iron oxide in particular), indicated by broad peaks or raised background in XRD diffractograms, were observed in field core samples (7) but not in this column study. Furukawa (4) reported that XRD failed to detect ferrihydrite, an amorphous phase, despite its ubiquitous 5760
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existence shown by selected area electron diffraction of field core samples. The presence of an amorphous phase in the columns may therefore be possible but to a much lesser extent than the crystalline phases. Distribution of predominant mineral phases in the columns is consistent with groundwater characteristics and known thermodynamic stability conditions for these phases (19, 21, 23). In Column I calcite and goethite generally predominated over aragonite and magnetite under conditions of lower pH and higher dissolved O2, NO3-, Ca, and alkalinity (i.e., near the influent region and adjacent to the column gap, Figure 2). Elsewhere in the column this predominance was reversed. In Column II goethite was detected only in samples at the sand-Fe0 interface and regions adjacent to the gap. The distribution of iron (oxyhydr)oxide is consistent with previous findings that goethite precipitates in waters containing dissolved O2 and lower pH, while magnetite is the predominant phase at alkaline pH and higher Fe2+ (23). Nonetheless, both goethite and magnetite may coexist when there is an oxidation potential gradient across the corrosion rind (5). During Fe0 oxidation, iron oxide layers of different oxidation state commonly formed (24). Chemical equilibrium modeling showed that influent solutions were undersaturated with respect to CaCO3 minerals, but that pore water solutions along the columns were supersaturated (Figure 2 in ref 17). Calcite and aragonite were both detected, but calcite appeared to be abundant only where dissolved Fe was relatively low (Figure 1b), reflecting the inhibitory effect of ferrous and ferric ions on calcite precipitation (25, 26). The fact that columns with different flow rates contain the same mineral assemblage confirms that the residence time in the fast flow column (Column I) was sufficient for major minerals to precipitate. Thus, use of accelerated flow of high groundwater throughput can simulate mass precipitation resulting from longer periods of “normal” flow, in this case 17 yrs within the 2-yr study period. However, certain minerals, such as siderite, although seen in field cores (3-5), were not detected in column studies here or elsewhere (2). This may be due to slow kinetics and competition with the high Ca in groundwater (21, 28). The absence of sulfide minerals in the present study reflects limited microbial activity (16). Sulfide formation mediated by sulfate-reducing bacteria requires a long inoculation time in the presence of
FIGURE 3. TGA-MS results of weight loss and off gas analysis for Column I core samples (a-c) and stock iron filings (d): (a) from inlet interface (CS-I-8), (b) from the center of the column (CS-I-81), and (c) from the effluent end of the column (CS-I-49). Note: for sample in (b), H2 was purged at 800 instead of 900 °C. high level SO42- (2). The presence of siderite and iron sulfide indicates that field PRBs encompass non-steady-state transport and reaction regimes (7, 10, 15), which may not be adequately simulated in column studies. Using TGA-MS in Phase Quantification. Weight loss patterns obtained by TGA-MS are very similar among the core samples (Figure 3a-c) and are significantly different from those of stock iron filings (Figure 3d). For all core samples, the normalized weight decreased gradually to ∼200 °C and then decreased sharply corresponding to the detection of H2O(g) and CO2(g). Weight loss continued at a slower rate to 600 °C and then dropped abruptly corresponding to a sharp increase in CO2. During the isothermal reduction period at 900 °C, a weight drop accompanied by H2O production was observed; the weight approached an asymptotic value (80-91% of initial weight) at the end of the period. For CSI-8, the inlet core sample, a spike of CO2 in addition to H2O was seen in the off gas (Figure 3a). The source of this CO2 peak is unclear but may be the remaining CO2 from CaCO3 transformation at 600 °C, which was flushed out by H2O. The TGA-MS results for the stock iron filing (Figure 3d) show a slight increase in weight at >400 °C, which is attributed to the oxidation of Fe0 with the surface-absorbed oxygen. The sample weight started to decrease at >600 °C, with the occurrence of CO2 in the off-gas. Because the stock iron contained a small amount of elemental carbon, heating could enhance C reacting with iron oxide to form Fe0 and CO2 (24). TGA-MS Analysis with XRD Identification. XRD analysis was performed on samples that had been heated to 200, 300,
400, 600, 800, and 900 °C (Figure 4). No obvious phase change occurred at 200 °C. The MS identified the presence of mainly atomic mass 18 and 17 in the off-gas, representing absorbed water (23) and its ionized product. A small quantity of CO2 was also detected. The goethite signal disappeared when samples were heated to 300 °C, but a small broad peak appeared at 2.7 Å, coinciding with the strong line of hematite and aragonite. We attribute it to hematite because it persisted at 600 °C. From 600 to 800 °C, CaCO3 transformed to CaO. The major phase at 800 °C was Fe3O4, with minor components of CaO‚Fe2O3, CaO, and Fe2O3 (Figure 4). At 900 °C, FeO, CaO‚Fe2O3, and CaO were identified. At the end of the isothermal reduction, the two major phases remaining were CaO and metallic iron. The phase changes seen in the XRD analyses are consistent with existing high-temperature phase transformation data for carbonate and iron-containing minerals. For example, aragonite undergoes polymorphic transition to calcite at about 450 °C (21). At ∼700 °C, calcite loses CO2 and transforms into CaO (29). Based on stoichiometry, the CO2 mass can be used to quantify the original mass of CaCO3 in the sample. The transformation of iron-containing phases is more complex than that of CaCO3. Goethite usually loses its structural OH between 250 and 400 °C (23), according to 2FeOOH f Fe2O3 + H2O. The transformation of Fe2(OH)2CO3 under Ar atmosphere at 500 °C is given by (30)
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FIGURE 4. Phase identification of samples (mixtures of CS-I-8, -81, and -49) treated at different temperatures. No obvious phase transformation between the original sample and the sample heated to 200 °C. Goethite signal disappeared between temperature of 200 and 300 °C. From 600 to 800 °C, CaCO3 transformed to CaO. The major phase at 800 °C was Fe3O4, and, at much lesser intensities, CaO‚Fe2O3, CaO, and Fe2O3. At 900 °C, FeO, CaO‚Fe2O3, and CaO were identified. After heating at 900 °C (with 2% H2 in helium), the two major phases remaining were CaO and Fe0, the later resulting from total reduction of iron oxide to metallic iron.
TABLE 2. Precipitate Mass and Porosity Loss Derived from TGA-MS Data (Figure 3) precipitates (mmol) samples stock iron column I CS-I-8 CS-I-81 CS-I-49 column II CS-II-1 CS-II-3 CS-II-15
as r-FeOOHb (200-300 °C)
as CaCO3 (600-800 °C)
as Fe3O4c (800-900 °C)
initial Fe0 d (mmol)
0.024
1.75
precipitate (%)a CaCO3 + Fe3O4 + r-FeOOH
corrosion ratese (mmol kg-1 d-1)
porosity lossf (%)
0.177 0.223 0.060
0.069 0.167 0.045
0.089 0.097 0.057
1.45 1.25 1.57
29.60 41.71 18.31
4.86 6.16 2.86
45.08 77.57 25.57
0.127 0.024 0.020
0.072 0.017 0.020
0.078 0.032 0.011
1.48 1.66 1.68
32.60 10.16 5.07
4.09 1.50 0.50
40.57 12.10 7.39
a Total mass percentage of all precipitates corrected for the initially oxidized mass based on data from the reduction of stock iron filings. b The mass of R-FeOOH calculated using mass of H2O during 200-300 °C heating assuming goethite is transformed to hematite (see Figure 4). c Mass of total oxidized iron transformed in this temperature range, calculated as Fe3O4. d Calculated from the final weight, less the mass of CaO. e Calculated from the mass ratio of total oxidized iron to the initial Fe0, averaging over the experimental run time (680 d for Column I, and 695 d for Column II). For Column I the run time is equivalent to ∼17-yr of simulated normal flow in Column II. f Porosity loss calculated based on 60% initial porosity, corrected for porosity gain from loss of iron due to corrosion using density values of 2.93 for CaCO3, 5.1 for Fe3O4, 3.8 for R-FeOOH, and 7.9 for Fe0.
No information is available on the thermo-induced phase transformation of amorphous iron oxide and green rust. Fe2O3 and Fe3O4 can be quantified from the loss of O atoms during the isothermal reduction at 900 °C by H2 as follows (24):
3Fe2O3 + H2 f 2Fe3O4 + H2O Fe3O4 + H2 f 3FeO + H2O FeO + H2 f Fe0 + H2O Since reduction of Fe3O4 to FeO occurs above 800 °C, oxidized iron should be calculated based on the mass difference at 800 °C and at the end of heating. Although losses from the oxidation of carbon in iron may occur, the percentage is small (
