Iron(III) solubility and speciation in aqueous solutions. experimental study and modelling: part 1. hematite solubility from 60 to 300°C in NaOH–NaCl solutions and thermodynamic properties of Fe(OH)4−(aq)

doi:10.1016/S0016-7037(99)00070-8

Geochimica et Cosmochimica Acta

Volume 63, Issue 15, August 1999, Pages 2247-2261

https://doi.org/10.1016/S0016-7037(99)00070-8 Get rights and content

Abstract

The solubility of natural and synthetic hematite (α-Fe₂O₃) was measured in NaOH–NaCl solutions (0.007 ≤ m(NaOH) ≤ 2.0) between 60 and 300°C at saturated water vapour pressure and under excess oxygen. Solubility constants determined in the present study and by Yishan et al. (1986) at 300°C were combined with the thermodynamic properties of hematite (Hemingway, 1990) and water (SUPCRT92, Johnson et al., 1992) to generate within the framework of the revised Helgeson–Kirkham–Flowers (HKF) model the standard partial molal thermodynamic properties at 25°C and 1 bar, and the revised HKF equations of state parameters of Fe(OH)_4⁻. The extrapolated value for the Gibbs energy of formation for Fe(OH)_4⁻ at 25°C is −201.97 kcal/mol. Thermodynamic calculations show that Fe(OH)_4⁻ exhibits a chemical behaviour different from that of Ga(OH)_4⁻ and Al(OH)_4⁻.

Introduction

The aqueous speciation of iron, which is the fourth most abundant element in the earth’s crust with a concentration of 4.32 wt.% (Wedepohl, 1995), affects the rates and the extent of many surface processes as important as weathering of silicate minerals (Schott and Berner, 1985), redox processes Brookins 1988, Macalady et al 1990, Alekseev 1994, ocean primary production and thus CO₂ consumption (Martin et al., 1994), and the behaviour of many trace elements that can adsorb onto the surface of freshly formed iron hydroxides Dzombak and Morel 1990, Charlet and Manceau 1993. The +2 and +3 oxidation states of iron are stable over broad regions of Eh and pH. Fe³⁺ complexes, however, dominate aqueous iron speciation in surface waters in equilibrium with the atmosphere (Brookins, 1988). Among those complexes, aqueous hydroxide species are most important in aqueous solutions with low organic matter concentrations at pH higher than 2.5 Byrne and Kester 1976, Millero et al 1995. Hydrolysis of Fe³⁺ starts at a pH of about 2 at 25°C (Baes and Mesmer, 1976). The hydroxide complexes Fe(OH)²⁺, Fe(OH)₂⁺, Fe(OH)^°₃, and Fe(OH)_4⁻ form successively as the pH increases. Polymeric hydroxide complexes form at acidic pHs and in concentrated iron solutions Baes and Mesmer 1976, Flynn 1984.

Analysis of a large body of literature data on Fe³⁺ hydroxide complexes shows that the standard thermodynamic properties of the Fe³⁺ ion and its first hydroxide complex are well constrained (Diakonov, 1995), but that large uncertainties exist in the very scarce data available on Fe(OH)₂⁺, Fe(OH)^°₃, and Fe(OH)_4⁻. These uncertainties lead to differences of up to 3 orders of magnitude in calculated iron mineral solubilities at 25°C in the pH range 5–9 Lengweiler et al 1961a, Byrne and Kester 1976, Diakonov 1995. Available thermodynamic data on these three species were mostly extracted from room temperature solubility measurements of iron hydroxides Biederman and Schindler 1957, Lengweiler et al 1961a, Lengweiler et al 1961b, Byrne and Kester 1976, Kuma et al 1993 and from a very few measurements of high-temperature hematite solubility Yishan et al 1986, Suleimenov 1988, Sergeyeva et al 1988. The low temperature solubility studies suffered from several handicaps including the formation of colloids and the very low solubilities of iron(III) hydroxide oxides at near neutral pHs (Byrne and Kester, 1976). On the other hand high temperature solubilities were too scarce and performed in too narrow a temperature interval to allow extraction of reliable thermodynamic data at 25°C.

The present study represents a part of a concerted effort to provide reliable thermodynamic data on the hydrolysis of iron(III). The thermodynamic properties of Fe(OH)_4⁻ are generated in this study from the measurements of pure well-crystallised hematite solubility in NaOH–NaCl solutions (NaOH 0.007–2.0 m) between 60 and 300°C at saturated water vapour pressure (P_SAT) and with added oxygen (3–13 bar at 25°C). Subsequent work (Diakonov and Tagirov, 1998; Tagirov et al., 1998a, 1998b) will focus on the thermodynamic properties for Fe³⁺, Fe(OH)²⁺, Fe(OH)₂⁺, and Fe(OH)^°₃ and mineral solubilities in the system Fe(III)–O–H–Cl.

Hematite (α-Fe₂O₃) was chosen in this solubility study because it is the Fe(III) oxide that is thermodynamically stable to 670°C, where it starts to transform to β-Fe₂O₃ (Hemingway, 1990). Even though it is not stable with respect to goethite (α-FeOOH) below 100°C (Diakonov et al., 1994), no cases of hematite to goethite transformation have been ever reported in the literature (Vorobyeva and Melnik, 1977). Thus, this oxide was also used in the experiments at 60°C.

Section snippets

Characterisation of the solid phase

Synthetic hematite (PROLABO, Rectapur, 99.9%) was used in most of the solubility experiments. X-ray diffraction (XRD) analysis of the powder in reflection mode using a Philips PW 1011 diffractometer with Mn-filtered Fe Kα radiation (32 kV–28 mA) showed it to be well-crystallised pure hematite (JCPOS card 33-664). Thermogravimetric analysis did not detect any trace of water. The specific surface area of this solid, as measured by nitrogen absorption using the Brunauer–Emmett–Teller (B.E.T.)

Preliminary solubility experiments at 300°C performed without added oxygen

Preliminary solubility experiments were conducted at 300°C in NaOH–NaCl solutions under P_SAT in Ti autoclaves. Duration of these experiments was up to 25 days. The composition of the reacting solutions and measured iron concentrations are listed in Table 2. Iron concentrations are very low (3 × 10⁻⁸–3 × 10⁻⁷ m) and are affected by a large scatter (2σ = 20%). X-ray diffraction analyses and Mössbauer spectra of the reacted powders revealed the presence of magnetite (Fe₃O₄) and ilmenite (FeTiO₃),

Standard partial molal heat capacity and volume of Fe(OH)_4⁻ at 25°C

In the absence of direct measurements, the standard partial molal heat capacity and volume of Fe(OH)_4⁻ were calculated using the correlations of Hovey (1988). The standard partial molal properties of an aqueous ion (ΔX°) can be expressed as (Helgeson et al., 1981): $ΔX°=Δ_{n} X°+Δ_{s} X°,$ where Δ_nX° and Δ_sX° represent the nonsolvation and solvation contribution to ΔX°, respectively. The solvation heat capacity and volume of an ion can be computed from the Born equations (Helgeson et al., 1981) given by:

Concluding remarks

Dissociation constants (pK_s14) of hematite (α-Fe₂O₃) were derived from solubility measurements performed at temperatures from 60 to 300°C at saturated water vapour pressure in NaOH–NaCl solutions and with added oxygen.

These data were combined with the thermodynamic properties of α-Fe₂O₃ (Hemingway, 1990) and water (supcrt92, Johnson et al., 1992) to calculate, within the framework of the revised HKF model, the standard partial molal properties and equations of state parameters for Fe(OH)_4⁻.

Acknowledgements

Financial support for this study was provided by the Institut National des Sciences de l’Univers (thème “Fluides dans la croûte” of the programme “Dynamique et Bilan de la Terre II”) and by CNRS/PIRSEM (ARC Metallogenie). We are indebted to C. Lourde and M. Thibault for technical assistance and hematite x ray, thermogravimetrical analyses, and surface area measurements. This work benefited from insightful discussions with J.-L. Dandurand, G. Pokrovsky, E. Oelkers, A. Zotov, and B. Tagirov.

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Original ArticlesIron(III) solubility and speciation in aqueous solutions. experimental study and modelling: part 1. hematite solubility from 60 to 300°C in NaOH–NaCl solutions and thermodynamic properties of Fe(OH)4−(aq)

Abstract

Introduction

Section snippets

Characterisation of the solid phase

Preliminary solubility experiments at 300°C performed without added oxygen

Standard partial molal heat capacity and volume of Fe(OH)4− at 25°C

Concluding remarks

Acknowledgements

Geochim. Cosmochim. Acta

Mar. Chem.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Comp. Geosci.

Wat. Res.

Mar. Chem.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Redox potential change rate in the natural solutions (in Russian)

Geokhimia

The Hydrolysis of Cations

The thermodynamics of cation hydrolysis

Am. J. Sci.

On the solubility product of precipitated iron (III) hydroxide

Acta Chem. Scand.

Eh-pH Diagrams for Geochemistry

Thermodynamic properties of iron oxides and hydroxides. II. Estimation of the surface and bulk thermodynamic properties of ordered and disordered maghemite (γ-Fe2O3)

Eur. J. Miner.

Thermodynamic properties of iron oxides and hydroxides. III. Surface and bulk thermodynamic properties of lepidocrocite (γ-FeOOH) to 500 K

Eur. J. Miner.

Thermodynamic properties of iron oxides and hydroxides. I. Surface and bulk thermodynamic properties of goethite (α-FeOOH) up to 500 K

Eur. J. Miner.

Surface Complexation ModellingHydrous Ferric Oxide

Hydrolysis of inorganic iron (III) salts

Chem. Rev.

Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressures and temperaturesIV. Calculation of activity coefficients, osmotic coefficients, and apparent molal and standard and relative partial molal properties to 600°C and 5 kbar

Am. J. Sci.

Thermodynamic properties for bunsenite, NiO, magnetite, Fe3O4, and hematite, Fe2O3, with comments on selected oxygen buffer reactions

Am. Miner.

Ion association of dilute aqueous sodium hydroxide solutions to 600°C and 300 MPa by conductance measurements

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Original Articles
Iron(III) solubility and speciation in aqueous solutions. experimental study and modelling: part 1. hematite solubility from 60 to 300°C in NaOH–NaCl solutions and thermodynamic properties of Fe(OH)_4⁻(aq)

Standard partial molal heat capacity and volume of Fe(OH)_4⁻ at 25°C

Thermodynamic properties of iron oxides and hydroxides. II. Estimation of the surface and bulk thermodynamic properties of ordered and disordered maghemite (γ-Fe₂O₃)

Thermodynamic properties for bunsenite, NiO, magnetite, Fe₃O₄, and hematite, Fe₂O₃, with comments on selected oxygen buffer reactions