Because of the importance of Fe(III) in many geochemical processes there is a strong need for reliable thermodynamic data on Fe(III) aqueous species. However, despite the attention which has been already devoted to Fe(III) aqueous speciation, the thermodynamic properties of these species are not yet well known, particularly for the pH range of natural waters (5-8). For example, values proposed in the literature for the solubility of goethite at 25°C in this pH range vary by roughly three orders of magnitude (Diakonov, 1995).
In this study the equilibrium constant for the reaction:
0.5 Fe2O3 + 2.5 H2O = Fe(OH)4- + H+ (Ks14)
was determined from hematite solubility measurements between 110 and 300°C in NaOH solutions (0.02-0.1m) under oxygene pressure (3-12 bar at 25°C). LogKs14 values determined in this study at 110, 150, 200, 250 and 300°C are -17.75, -16.91, -16.33, -15.51 and -15.00 (±0.10), respectively. Fe(OH)4- standard thermodynamic properties at 25°C and revised HKF equations of state parameters (Shock et al., 1992) were extracted from these data using Hovey's (1988) correlations to approximate the heat capacity and volume of this species. The standard thermodynamic properties of Fe(OH)3° at 25°C were calculated from these data and from selected literature thermodynamic data for Fe3+, Fe(OH)2+, Fe(OH)2+, using the correlations proposed by Baes and Mesmer (1976), Miyano and Einaga (1981), and Hovey (1988).
The standard thermodynamic properties of Fe(III) aqueous species were used together with those for hematite (Hemingway, 1990), goethite (Diakonov et al., 1994), maghemite, lepidocrocite and amorphous iron hydroxide (Diakonov, 1995) to generate the solubility curves of these minerals at 25°C as a function of pH. The dependence of these solubilities on grain size and that of amorphous iron hydroxide on the aging time were also calculated.
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