A-Fe2o3) and Maghemite (Y-Fe2o3

Home , Maghemite

A AECL EACL

CA9800464 AECL-11668, COG-96-490-1

Comparison of the Solubilities of Synthetic Hematite (a-Fe2O3) and Maghemite (y-Fe2O3)

Comparaison des solubilites de l'hematite (Fe2O3 a) et de la maghemite (Fe2O3 y) synthetiques

Peter Taylor, Derrek G. Owen

29-43

February 1997 fevrier AECL EACL

COMPARISON OF THE SOLUBILITIES OF SYNTHETIC

HEMATITE (a-Fe2O3) AND MAGHEMITE (y-Fe2O.0

Peter Taylor and Derrek G. Owen

The work reported in this document was funded by the COG R&D Program: Working Party 41, WPIR No. 4103, under the participation of Ontario Hydro and AECL.

Whiteshell Laboratories Pinawa, Manitoba ROE 1L0 1997 AECL-11668 COG-96-490-I AECL EACL

COMPARISON OF THE SOLUBILITIES OF SYNTHETIC

HEMATITE (a-Fe^) AND MAGHEMITE (y-Fe2O3)

Peter Taylor and Derrek G. Owen

ABSTRACT

The solubilities of hematite (a-Fe2O3) and maghemite (y-FeaO.i), both of which were prepared by controlled oxidation of magnetite powder, were measured in dilute nitric acid (between 0.01 and 0.05 moldm'3). The results yielded an estimated Gibbs energy of transformation from maghemite to hematite of-13.0 ± 1.6 kJ-mol'1. The following thermodynamic quantities for maghemite are derived from our results and literature data:

AfG° = -731.3±2kJmol'; AfH° = -809±3kJmorV S° = +102.4 J-molK"1.

An anomalous dependence of the apparent solubility products, {Fe3+}{OH"}\ of maghemite, hematite and goethite (a-FeOOH) on ionic strength is discussed.

Whiteshell Laboratories Pinawa, Manitoba ROE 1L0 1997 AECL-11668 COG-96-490-I AEXX EACL

COMPARAISON DES SOLUBILITES DE L'HEMATITE (Fe2O3 a)

ET DE LA MAGHEMITE (Fe2O3 y) SYNTHETIQUES

par

Peter Taylor et Derrek G. Owen

RESUME

Les solubilites de 1'hematite (Fe2O3 a) et de la maghemite (Fe^ y), toutes deux obtenues par oxydation contr61ee de la poudre de magnetite, ont e"te mesurees dans de 1'acide nitrique dilu6 (a une concentration comprise entre 0,01 et 0,05 moldm'3). Les resultats obtenus ont donne une energie Gibbs estimative de transformation de la maghemite en hematite de -13,0 ± 1,6 kJmoi"1. Les donnees thermodynamiques suivantes pour la maghemite sont tirees de nos resultats et de la documentation scientiflque:

AfG° = -731,3±2kJ-mol'; : AfH° = -809±3kJmol ; S° = +102,4 JmolK1.

On y discute ladependance anormale des produits de solubilite apparents, {Fe3+}{OH}3, de la maghemite, de la hematite et de la goethite (FeOOH a) sur la force ionique.

Laboratoires de Whiteshell Pinawa (Manitoba) ROE 1L0 1997 AECL-11668 COG-96-490-I CONTENTS

1. INTRODUCTION 1

2. STARTING MATERIALS 2

2.1 MAGHEMITE PREPARATION 2 2.2 HEMATITE PREPARATION 3 2.3 SURFACE AREA MEASUREMENTS 3 2.4 SOLUTION PREPARATION 3

3. SOLUBILITY MEASUREMENTS 3

4. DATA ANALYSIS 5

5. DISCUSSION 6

5.1 RELATIONSHIP BETWEEN KSp AND / 6 5.2 THERMODYNAMICS OF MAGNETITE-HEMATITE TRANSFORMATION 7

ACKNOWLEDGEMENTS 8

REFERENCES 8

TABLES 11

FIGURES 15 1. INTRODUCTION

The solubility of iron(III) oxides and pxyhydroxides is important in natural systems and corrosion processes. Solubility products of these solids can be expressed by Equations (1) to (3):

+ 0.5(Fe2O3xH2O) (s) + (1.5 - 0.5x)H2O (/) = Fe> (aq) + 3OH(aq) (1)

3+ 3 Ksp = {Fe }{OH-} (2)

3+ = 3pKw-3pH-log{Fe } (3) where pY = -logioY, {Z} represents the activity of species Z\ and FeOOH and "Fe(OH)3" are represented by the empirical formulae Fe2OsH2O and Fe2O3-3H2O, i.e., JC = 1 and 3, respectively. Most reported values of pKsp for Fe2O3xH2O lie between 36 and 42, depending on the crystallographic form and degree of crystallinity of the solid [1-5]. There are only a few published measurements of the solubility and related thermodynamic properties of maghemite (y-Fe2O3). Langmuir [2] derived pKsp = 38.8 ± 0.5 for freshly precipitated maghemite, based on potential-pH measurements by Doyle [6], and suggested a value of about 40 for more crystalline material. Sadiq and Lindsay [7] obtained pKsp = 40.36 ± 0.07 for a natural maghemite extracted from soil. We are not aware of any recent solubility measurements on hematite (a-Fe2O.!) at low temperatures in dilute, non-complexing media, although the relative solubilities and stabilities of hematite and goethite (oc-FeOOH) have received much attention [2,3,8]. A pKsp value of 41.9 ± 0.4 is obtained [2,4,5,11] when current thermodynamic data for Fe3+(aq) and hematite are applied to equilibrium (1).

Maghemite is a persistent, metastable intermediate in the oxidative alteration of magnetite (Fe3O4) to hematite [9-13]:

4Fe3O4 + O2 -> 6y-Fe2O3 -> 6a-Fe2O3 .

Maghemite is stabilized by phosphate and silica(te), apparently because they form surface complexes that inhibit the formation of hematite by a dissolution/precipitation mechanism [11,12]. The solid-state transformation of maghemite to hematite can also be inhibited by cationic impurities [13]. Persistent occurrence of maghemite may lead to metastable redox conditions, defined by the maghemite/magnetite couple, that are less reducing than the hematite/magnetite couple [11]. This is of interest in the Canadian Nuclear Fuel Waste Management Program [14-16], because oxidation of magnetite may help to restore reducing conditions following closure of a geological fuel disposal vault in granite. Therefore, we have measured the solubilities of maghemite and hematite in dilute HNO3 and have used the solubility data to calculate some of the thermodynamic properties of maghemite. To minimize the effects of differences in particle size and morphology [3], both the maghemite and hematite were prepared by air oxidation of the same magnetite powder at moderate temperatures.

Elsewhere, we use [Z] to represent the concentration of species Z. -2-

2. STARTING MATERIALS

Magnetite is converted to maghemite by air oxidation at temperatures near 200°C, whereas maghemite is converted to hematite at higher temperatures [9,10]. It is not always possible to recover maghemite as a pure product, or to achieve conversion to hematite without significant sintering. For example, coarse magnetite particles may start to form hematite before maghemite formation is complete, whereas finely divided maghemite may resist thermal transformation at temperatures below 600°C [17]. We experimented with various magnetite powders, and found that "Mapico Black" (Columbian Chemicals Canada Ltd., Hamilton ON) gave the most satisfactory results, that is, it could be converted easily to both pure maghemite and pure hematite.

2.1 MAGHEMITE PREPARATION

Oxidation of Mapico Black magnetite powder in air at 175°C for 6 d or 250°C for 2 h yielded acceptable maghemite. X-ray powder diffraction (XRD) was performed with a Rigaku Rotaflex diffractometer, using a 15-kW rotating-anode CuKa X-ray source and a diffracted-beam monochromator. This revealed the expected spinel-based pattern with well-resolved non-spinel reflections corresponding to a primitive cubic cell (a = 8.343(3) A; 1 A = 0.1 nm), and one peak (d = 3.20 A) corresponding to the tetragonal a,a,3a superstructure; the XRD data are compiled in Table 1. The pattern agreed well with that reported (a = 8.3515(22) A) by Schulz and McCarthy [17], and the low-angle features corresponded closely to specimen G2 of the series of synthetic maghemites reported by Morales et al. [18]. For a more detailed discussion of the structure and composition of maghemite, see References 19 to 22.

It has been suggested that maghemite specimens prepared under dry and wet conditions may differ [12,19], and that the latter might be better described as HFe5Og (Fe2O30.2H2O) than Fe2O3. Because we were interested in the behaviour of maghemite formed from magnetite in aerated groundwaters, we also prepared a specimen by hydrothermal oxidation of Mapico Black in 3 aerated aqueous Na2HPO4 (0.01 mol-dm' ) at 175°C for 6 d. This addition of a phosphate salt was necessary to suppress recrystallization of the maghemite to hematite [11]. The XRD pattern of this specimen (Table 1) was virtually indistinguishable from that of the air-oxidized material, and the cubic cell parameter was 8.346(3) A. Since adsorbed phosphate species are likely to inhibit dissolution of the hydrothermally prepared material, the air-oxidized product (250°C, 2 h) was used in the solubility measurements described below.

Based on the cell parameter, a = 8.343 A, and discussion by Taylor [20] and Schulz and McCarthy [17], we estimate that our maghemite specimen may have contained between 0% and 6% residual Fe2+ (expressed as a percentage of total Fe). However, given the strongly oxidizing conditions for both the preparation and dissolution of the maghemite, it is highly unlikely that any Fe2+ persisted near the solid/solution interface. Even at ambient temperature, magnetite quickly forms a surface film of maghemite when exposed to air [23]. Therefore, the material is considered to be stoichiometric Fe2O.i for the purposes of our solubility measurements and calculations. -3-

2.2 HEMATITE PREPARATION

We wished to prepare hematite from the maghemite at the lowest practical temperature, to minimize sintering or other microstructural alteration of the powder. At 400°C, the maghemite was ca. 5% converted to hematite after 16 h, and 80% converted after 10 d; moreover, the hematite XRD peaks were rather broad. At 500°C, pure hematite was obtained after 10 d, as indicated by sharp XRD peaks in excellent agreement with the published diffraction pattern [24]. This material was used for the solubility measurements; the XRD data are presented in Table 2.

2.3 SURFACE AREA MEASUREMENTS

To confirm that the particle microstructure was not greatly modified by oxidation, surface-area measurements were obtained for the original Mapico Black, as well as the air-formed maghemite and hematite specimens. Measurements were provided by Particle Data Laboratories (Elmhurst, IL), and surface areas were based on the 3-point BET procedure using N2 gas. Reported surface areas for the three solids were Mapico Black, 8.8 ± 0.4 m2-g'; maghemite, 8.64 ± 0.09 m2-g"'; hematite, 6.0 ± 0.8 m2g"'. We conclude that there was no significant change in particle size during the oxidation to maghemite, and that only a small degree of sintering occurred during the subsequent transformation to hematite. The latter observation is consistent with the results of Sidhu [13], who observed two- to threefold reductions in particle size for hematite samples prepared by heating maghemite at 650°C for 3 h,

2.4 SOLUTION PREPARATION

Solutions were prepared from concentrated, reagent grade HNO3 and doubly distilled, de-ionized water. Stock solutions so prepared were calibrated by titration with freshly opened, commercially available standard solutions of NaOH.

3. SOLUBILITY MEASUREMENTS

Two solubility experiments were run. In the first, triplicate samples of both maghemite and hematite (0.2 g each) were added to aqueous HNO3 (100 cm3, nominally 0.03 moldm'3) in capped conical flasks, which were immersed in a water bath controlled at 25 ± 1°C. The flasks were agitated occasionally, and samples were retrieved at irregular intervals for analysis (Table 3). Solution samples (1.0 cm3) were passed through a filter (0.2-mm pore size), then diluted with aqueous HNO3 (5 or 25 cm3, 0.1 moldm3) to prevent reprecipitation, and analyzed for total iron concentration, [Fet0t], by inductively coupled plasma spectrophotometry.

These experimental conditions were selected for a variety of reasons: a. The equilibrium iron concentrations are sufficiently high to be analyzed easily. b. The pH is low enough that the iron speciation is dominated by Fe3+; thus the corrections 3+ 3+ needed to convert [Fetot] to [Fe ] are small, and only the first hydrolysis step of Fe has to be considered (see Section 4). -4-

c. The low surface-area-to-volume ratio (SA:V) should minimize the influence of active surface sites on solubility, that is, such sites should be eliminated by dissolution before saturation is reached.

d. The ionic strength, /, is low enough for satisfactory estimates of ionic activity coefficients [25] using the Davies Equation (4):

2 m -log,o/± = 0.5r[(/" / (I + l )) - 0.3/] , (4) where z is the ionic charge and {Z} =/± [Z].

e. The nitric acid is sufficiently dilute to be considered completely ionized [26] and chemically stable.

This set of conditions is met within a narrow range of acid concentrations, between about 0.005 and 0.1 mol-dm"3.

Unfortunately, it became apparent that the solutions were approaching equilibrium very slowly and that the iron concentrations continued to rise over a period of 8 months. Therefore, a second series of experiments was set up with higher SA:V (0.4 g solid in 10 cm3 HNO3) to accelerate approach to equilibrium (although possibly compromising point (c) above). These experiments covered a range of acid concentrations, nominally 0.01 to 0.05 mol-dm"', over which roughly two orders of magnitude variation in iron concentration could be expected, based on Equations (1) to (3), that is, {Fe3+} varies with the cube of {H+}.

In the first series of experiments, the solution pH was measured twice during the experiment and again when it was terminated. The 10-cm samples of solution used for the pH measurements were discarded, to avoid any contamination of the main body of solution by the pH electrode solution. In the second series of experiments, because of the smaller solution volume, pH was only measured at the end of the run. Relative pH values for the different solutions agreed well with calculations (see below), but the measured values tended to be slightly lower (by 0.05 to 0.1 units) than calculated. The starting solutions were calibrated by titration, and no base or buffer was added, therefore calculated pH values (corrected for H+ consumption during the oxide dissolution, and for ionic-strength effects) were used in the data analysis, in preference to the measured values.

At the end of the experiments, all solids were recovered and re-analyzed by XRD. There was no evidence of alteration of the maghemite to hematite or any other crystalline phase. All XRD features of the starting materials (Tables 1 and 2), including the non-spinel features (primitive cubic lattice) of the maghemite pattern, were retained. The colours of the solids (dull brown maghemite and brighter red-brown hematite, cf. Reference 27) also did not change in the course of the experiment, and there was no visible colloid formation. -5-

4. DATA ANALYSIS

The dissolved iron analyses are compiled in Table 3. All concentrations were measured and calculated in molar units; conversion to molal units would involve negligible corrections at the level of accuracy of the final calculated Ksp and AfG° values. Under the experimental conditions, the predominant dissolved iron species is Fe3+, with a significant contribution from Fe(OH)2+, whereas all other hydrolyzed forms of Fe(III) are insignificant, i.e., <2% of the [Fetot] value [28]. Milburn and Vosburgh [29] reported the following relationship2 for the hydrolysis of Fe3+ to Fe(OH)2+:

logioQi = -2.172 - [2.04 / m I (1 + 2.4 / m )] - 0.01 / (5) where Q, = [Fe(OH)2+][H+] / [Fe3+]

Total iron concentrations were converted to Ksp values by successive approximations, as illustrated by the following detailed calculation.

This example of the data analysis procedure is for the dissolution of maghemite in 3 0.0492 moldm" HNO3. This experiment yielded the highest iron concentration, and hence the largest deviations in pH and / from the initial solution values, and so the slowest convergence in the calculation procedure. Calculations were performed using Microsoft Excel 5.0 spreadsheet software.

The average of two measured iron concentrations (after 580 and 1488 h) is 0.003548 moldm"3. The initial [H+] value was 0.0492 moldm"3, and the initial value of /was the same. Equation (5) was used to obtain a first estimate of the concentration values, [Fe3+] and [Fe(OH)2+], using these starting values of pH and /. We thus obtained logioQi = -2.468, [Fe3+]/[Fe(OH)2+] = 0.069, and hence [Fe3+] = 0.00332 moldm"3 and [Fe(OH)2+] = 0.00023 moldm'3. These values were then used to recalculate [H+], according to the dissolution mass-balance equation:

+ + 3+ 2+ 3 [H ]final = [H ]initial - 3[Fe ] - 2[Fe(OH) ] = -0.03879 mol-dm' .

The values of [H+], [Fe3+], and [Fe(OH)2+] were then used to recalculate the ionic strength, /:

/ = 0.5 Ezi2Ci = 0.5([NOf ] + [H+] + 4[Fe(OH)2+] + 9[Fe3+]) = 0.05938 .

Using this value of/, we recalculated logioQi = -2.486, and hence [H+] = 0.03883 moldm"3, [Fe3+] = 0.003272 moldm"3, [Fe(OH)2+] = 0.000275 moldm'3, and hence/ = 0.05929. A further iteration of this calculation yielded no further changes in these values. The recalculated value of / was then used to estimate the ionic activity coefficients,/+, using the Davies equation (Equation (4)):

logi

This was then used to convert [H+] and [Fe3+] concentrations to the following activity values:

In Reference 29, Milburn and Vosburgh use the terms ki and |^, where we use Qi and /, consistent with current usage. -6-

+ log,0{H } = Iog,o(0.03883) - 0.089 = -1.500 (i.e., pH = 1.500);

3+ log,0{Fe } = log,0(0.003272) - (9 x 0.089) = -3.286;

whence

3+ pKsp = -log{Fe } + 42 -3pH = 40.79.

Similar calculations yielded the results summarized in Table 4. These are based on (a) estimated asymptotic values for the iron concentration in the first series of experiments at 3 [HNOS] = 0.0295 moldm" , and (b) the average of all iron analyses in the second series of experiments at higher SA:V and different nitric acid concentrations.

5. DISCUSSION

5.1 RELATIONSHIP BETWEEN K.P AND /

Our calculated pKsp values for the four acid concentrations are plotted against / in Figures 1 and 2. This compilation includes average values obtained from the second series of experiments ([HNO3] = 0.01, 0.02, and 0.05), and values based on the estimated saturation concentrations for the first series ([HNO3] = 0.03). Also included in these figures are reported pKsp values from a similar series of experiments reported by Hsu and Marion [30] with aged synthetic goethite that was allowed to equilibrate with aqueous perchloric acid media for 9 to 16 years. These perchlorate media had similar pH and / values to our nitric acid solutions. The data analysis procedure used by Hsu and Marion was essentially the same as ours. Figures 1 and 2 show a residual dependence of pKsp on /, after the usual corrections (i.e., the activity coefficient calculations described in the preceding section) were made.

Similar values of Qi, as a function of /, are obtained by using Equation (5) from Milburn and Vosburgh [29] and by using the Davies equation (Equation (4)), assuming a value logioQi = -2.172 at / = 0, (Table 5). Therefore, it is unlikely that the apparent dependence of pKsp on / is due to inaccurate calculation of ionic activity coefficients for Fe3+ and Fe(OH)2+. Moreover, since Fe3+ was the predominant iron species in all our experiments, our calculated pKsp values are rather insensitive to errors in Qi. Our pH values were not corrected for proton adsorption by the solid surfaces, but this correction would be small and, if anything, would tend to exaggerate the dependence of pKsp on /, that is, it would tend to reduce the calculated values of pKsp still further at low / and pH. We suggest, therefore, that the apparent dependence of pKsp on / is real, but we are unable to explain it. Alternative treatments involving nitrate association or specific ion interactions do not clarify the situation (R.J. Lemire, personal communication, 1995). Chemical contamination is unlikely, given the simplicity of the system, its insensitivity to air (i.e., the solubility equilibria involve only Fe(III) species), and the absence of complexing ligands. Further investigation of this apparent relationship between pKsp and / is desirable, because of the obvious implications for the reliability of oxide solubility calculations. -7-

5.2 THERMODYNAMICS OF MAGNETITE-HEMATITE TRANSFORMATION

The dependence of Ksp on /, discussed above, makes it difficult to obtain absolute values for Ksp and hence AfG° for the solids. The commonly recommended value of pKsp for hematite, as defined in Equation (2), is 41.9, which is very close to the values obtained here at higher / values, but this agreement may be fortuitous [2,4,5,11]. The parallel variations in pKsp between matched samples of maghemite and hematite in matched solutions (Figure 1) should give a more accurate measure of their relative, as opposed to absolute, stabilities. We obtain an average difference of 1.14 ± 0.14 between pKsp values for maghemite and hematite, which corresponds to the following Gibbs energy of transformation (per mol Fe2O3):

AG (transformation, y-Fe2O3 -» oc-Fe203) = -2RT[lnKsp(Y-Fe2O3) - lnKsp(a-Fe2O3)]

= -13.0 ± 1.6 kJ-mol"'.

If we assume the thermodynamic pKsp value for hematite is 41.9 ± 0.4 (see above), we obtain 1 pKsp = 40.8 ± 0.5 for maghemite. Similarly, if we assume AfG° = -744.27 + 1.25 kJmol" for hematite [31], we obtain AfG° = -731.3 ± 2.0 kJmol"1 for maghemite. This indicates that the magnetite/maghemite redox potential is about 200 mV more positive than magnetite/hematite [11].

Our value for pKsp of maghemite is not significantly different from the value of 40.36 ± 0.07 obtained by Sadiq and Lindsay [7] for soil maghemite at / = 0.1 and pH = 2.0 to 3.5. However, if the pKsp / / relationship, discussed above, holds for the soil maghemite, this would indicate that the soil mineral is slightly less stable (more soluble at given /) than the synthetic maghemite used in this work.

Sarda and Rousset [32] recently reported measurements of the enthalpy of the maghemite -» hematite transformation for a series of barium-doped Fe2Oi specimens, from which one can 1 estimate an enthalpy-of-transformation value of-17.5 ± 2 kJmol" for pure Fe2O3. Combining this with our Gibbs energy of transformation, -13.0 ± 1.6 kJmol"1, we obtain an entropy of transformation, AS = -15 ± 9 Jmol'-K"1. Further combining this with the recommended entropy value for hematite from Reference 31, we obtain the following thermodynamic quantities for O 1 maghemite: S 298 = 102.4 ± 10 Jmol'-K and AfH° = -809 ± 3 kJmol"'.

We conclude that the difference in stability between maghemite and hematite is primarily enthalpic in origin, with partial compensation by the entropy difference between these two polymorphs of Fe2O3. This follows the apparent pattern for other polymorphic series for which accurate thermodynamic data are available, e.g., SiO2 (quartz, cristobalite, tridymite, amorphous [33]) and A1OOH (diaspore, boehmite [34]).

The relative stability of maghemite, hematite and other Fe(III) oxides and oxyhydroxides is discussed in more detail in the context of nuclear fuel waste management elsewhere [16]. -8-

ACKNOWLEDGEMENTS

We thank K. Wazney for the iron analyses and A.M. Duclos for the X-ray diffraction data. J. Freire-Canosa, D.J. Jobe, R.J. Lemire and P. Pan provided helpful comments on the manuscript. This work was co-funded by Ontario Hydro, under the auspices of the CANDU Owners Group.

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34. Wefers, K. and C. Misra. 1987. Oxides and Hydroxides of Aluminum. Alcoa Laboratories, 21. -11-

TABLE 1

X-RAY DIFFRACTION DATA FOR MAGHEMITE

hkP d(obs)b I/Io" d(obsf I/Ioc d(pbsf I/Iod 110 5.918 5 5.9045 2 5.910 2 111 4.822 4 4.8178 2 4.818 3 210 3.740 6 3.7355 3 3.732 3 211 3.411 5 3.4062 3 3.408 3 [124f 3.2096 1 3.204 1 220 2.953 35 2.9515 33 2.949 33 221 2.7840 2 2.7878 1 2.7848 1 310 2.6435 2 2.6427 1 2.6374 1 311 2.5177 100 2.5168 100 2.5152 100 222 2.4119 3 2.4095 3 2.4080 3 320 2.3156 1 2.3177 1 400 2.0886 16 2.0867 20 2.0857 19 410 2.0255 1 421 1.8224 2 1.8220 1 1.8216 1 422 1.7045 10 1.7036 11 1.7029 11 430 1.6703 1 511 1.6073 24 1.6062 28 1.6056 29 520 1.5507 1 1.5492 1 1.5495 1 521 1.5248 2 1.5234 2 1.5242 1 440 1.4758 34 1.4751 41 1.4749 41 441 1.4537 1 530 1.4322 1 611 1.3547 1 1.3535 1 620 1.3204 3 1.3196 4 1.3192 4 540 1.3042 1 533 1.2730 5 1.2726 8 1.2724 7 622 1.2590 2 1.2581 2 1.2581 2 444 1.2053 1 1.2043 2 1.2041 2 710 1.1810 1 642 1.1159 2 1.1152 4 1.1151 3 731 1.0872 7 1.0864 9 1.0862 9

Based on cubic spinel unit cell; cell constants are indicated below. Data from Schulz and McCarthy [17]; a0 = 8.3515(22) A (1 A = 0.1 nm). Data for maghemite used in our solubility experiments, prepared by oxidation of Mapico Black magnetite powder in air at 250°C for 2 h; XRD file NO18124, 1992; a0 = 8.346(3) A. Data for maghemite prepared by oxidation of Mapico Black magnetite (0.5 g) in aqueous 3 3 NaH2PO4 (100 cm , 0.01 moldm' ) at 175°C for 6 d; XRD file NO18125, 1992; a0 = 8.343(3) A. Based on tetragonal a,a,3a unit cell, after Morales et al. [18]. - 12-

TABLE 2

X-RAY DIFFRACTION DATA FOR HEMATITE

hkt d(obsf I/Io" d(obsf I/Ioc 012 3.684 30 3.703 30 104 2.700 100 2.709 100 110 2.519 70 2.525 72 006 2.292 3 2.299 2 113 2.207 20 2.212 22 202 2.0779 3 2.083 2 024 1.8406 40 1.845 33 116 1.6941 45 1.698 42 211 1.6367 1 — — 122 1.6033 5 1.602 10 018 1.5992 10 — — 214 1.4859 30 1.488 27 300 1.4538 30 1.455 27 208 1.3497 3 1.351 2 1010 1.3115 10 1.313 10 119 1.3064 6 shc — 220 1.2592 8 1.260 6 306 1.2276 4 1.228 3 223 1.2141 2 wfoc — 128 1.1896 5 1.190 5 0210 1.1632 5 1.164 4 134 1.1411 7 1.141 7

Based on hexagonal unit cell, a = 5.0356(1) A, c = 13.7489(7) A; 1 A = 0.1 nm; data from Morris et al. [24]. Hematite prepared by heating Mapico Black magnetite powder in air for 2 h at 250°C, then 10dat500°C. XRDFileNo. JA08121 (1993). The d spacings are not corrected for minor systematic errors; the discrepancies between columns 2 and 4 correspond to a 26 calibration error of approximately 0.10°. sh = shoulder; wfo = weak feature observed. - 13-

TABLE 3

MEASURED IRON CONCENTRATIONS

3 3 Time (h) [Fejtotai, mmol-dm" (Hematite) [Fe]tolai, mmol-dm' (Maghemite) 3 3 (a) Experiments with 0.2 g of Fe2O3 in 100 cm of 0.0295 moldm" HNO3 49 .0149, .0163, .0372 .1113, .1099, .0722 138 .0154, .0177, .0223 .1122, .1201, .1215 211 .0354, .0200, .0237 .1471,.1490, .1429 816 .0423, .0386, .0376 .3548, .3436, .3445 985 .0439, .0496, .0723 .4507, .4120, .4255 1800 .0675, .0591, .0603 .6099, .6145, .6099 2712 .0823, .0771, .0828 .8054, .8240, .8101 5736 .1096, .0976, .1193, .1085, .0959, 1.1220, 1.1453, 1.1360, 1.0987, .1193 1.1127, 1.2011 °° (estimated) 0.14 1.35 3 3 (b) Experiments with 0.4 g of Fe2O3 in 10 cm of 0.0098 moldm HNO3 580 .0132 .1508 1488 .0259 .1420 4512 .0114, .0105 Data not obtained 3 3 (c) Experiments with 0.4 g of Fe2O3 in 10 cm of 0.0197 moldm" HNO3 580 .0501 .6844 1488 .0434 .7682 4512 .0413, .0467 Data not obtained 3 3 (d) Experiments with 0.4 g of Fe2O3 in 10 cm of 0.0492 moldm" HNO3 580 .4169 3.1285 1488 .5146 3.9666 4512 .5587, .5597 Data not obtained - 14-

TABLE 4

MEASURED AND CALCULATED CONCENTRATIONS, IONIC STRENGTHS. ACTIVITY COEFFICIENTS. PH VALUES. AND SOLUBILITY PRODUCTS

HEMATITE MAGHEMITE [HNO,] initial, moldm"3 .0295 .0098 .0197 .0492 .0295 .0098 .0197 .0492 [Fe], final, mmoldm"3 .14a .0153b .0454b .5125b 1.35a .1464C .726C 3.548C [Fe3+], calc, mmoldm"1 .124 .0104 .0375 .479 1.179 .0984 .591 3.272 /, calc .02989 .00984 .01982 .05067 .03321 .01014 .02161 .05929

-login/±(l:l) .0692 .0436 .0587 .0843 .0721 .0442 .0608 .0890 pH, calc 1.605 2.054 1.767 1.406 1.664 2.071 1.814 1.500 log{Fe3+}, calc 4.530 5.376 4.954 4.078 3.577 4.405 3.776 3.286

pKsp, calc 41.71 41.21 41.65 41.86 40.59 40.19 40.33 40.79

pKsn(Hem) - pK,,,(Mag) 1.13 1.02 1.32 1.07 (Average = 1.14)

Estimated asymptote of dissolution data (see Table 3) Average of four measurements (see Table 3) Average of two measurements (see Table 3)

TABLE 5

CALCULATED VALUES OF logmK, BASED ON EQUATIONS (4) AND (5)

Ionic strength, / Calculated values of logioKi Based on Eq. (4)n Based on Eq. (5)b 0 -2.172 -2.172 0.001 -2.233 -2.232 0.003 -2.274 -2.271 0.01 -2.348 -2.337 0.03 -2.449 -2.422 0.1 -2.593 -2.540

Calculated value of logjoQi, based on logioKi = -2.172 and activity coefficients estimated using equation (4) (Davies' equation).

Calculated value of logioQi, using Equation (5) (from Milburn and Vosburgh [29]). - 15-

42.5 Iron Oxide Solubilities 42 -

41.5 • Hematite • Maghemite • o Goethite (Ref. 30) 41 • • 40.5 • • 0 0 40 8 0 °oo 0 39.5 0.01 0.02 0.03 0.04 0.05 Ionic Strength

FIGURE 1. Apparent Ksp values for maghemite and hematite from our work, and for goethite from Hsu and Marion [30], as a function of ionic strength, /. All data from Reference 30 for / < 0.05 are shown. - 16-

42 Iron Oxide Solubilities 41.5

41 a a. 40.5

40 - Hematite - Maghemite -Goethite (Ref. 30) 39.5 0.05 0.1 0.15 0.2 0.25 Ionic Strength

FIGURE 2. Comparison of our maghemite and hematite data with average values for goethite from Reference 30 for data clustered in three narrow ranges of /. AECL-11668 COG-96-490-I

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