The solubility of magnetite and the hydrolysis and oxidation of Fe2+ in water to 300°C

AbstractThe solubility of carefully characterized magnetite, Fe3O4, in dilute aqueous solutions saturated with H2 has been measured at temperatures from 100 to 300°C in a flow apparatus. Solution compositions included either HCl or NaOH molalities of up to 1 and 40 mmole-kg−1, respectively, and H2 molalities of 0.0779, 0.779, and 8.57 mmole-kg−1. The dependence of the equilibrium solubility on the pH and reduction potential were fitted to a scheme of soluble ferrous and ferric species consisting of Fe2+, FeOH+, Fe(OH)2, Fe(OH)3−, Fe(OH)3, and Fe(OH)4−. Solubility products from the fit, corresponding to the reactions $$\tfrac{1}{3}Fe_3 O_4 + (2 - b)H^ + + \tfrac{1}{3}H_2 \rightleftharpoons Fe(OH)_b^{2 - b} + (4/3 - b)H_2 O$$ and $$\tfrac{1}{3}Fe_3 O_4 + (3 - b)H^ + \rightleftharpoons Fe(OH)_b^{3 - b} + \tfrac{1}{6}H_2 + (4/3 - b)H_2 O$$ were used to derive thermodynamic constants for each species. The extrapolared value for the Gibbs energy of formation of Fe2+ at 25°C is −88.92±2.0 kJ-mole−1, consistent with standard reduction potentials in the range Eo(Fe2+)=−0.47±0.01 V. The temperature coefficient of the equilibrium Fe molality, (∂m(Fe, sat.)/∂T)m(H2).m(NaOH), changes from negative to positive as the NaOH molality is increased to the point where Fe(OH)3− and Fe(OH)4− predominate.

[1]  I. Kolthoff,et al.  The Solubility Product of Ferrous Hydroxide and the Ionization of the Aquo-Ferrous Ion , 1953 .

[2]  D. R. Stull,et al.  JANAF Thermochemical Tables. Second Edition , 1971 .

[3]  J. Smiltens.  The Growing of Single Crystals of Magnetite , 1952 .

[4]  H. Helgeson,et al.  Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressures and temperatures; I, Summary of the thermodynamic/electrostatic properties of the solvent , 1974 .

[5]  I. Chou,et al.  Solubility of magnetite in supercritical chloride solutions , 1977 .

[6]  P. Tremaine,et al.  A calculation of gibbs free energies for ferrous ions and the solubility of magnetite in H2O and D2O to 300°C , 1977 .

[7]  D. A. Young.  Thermodynamics of nuclear materials International atomic energy agency, Vienna, proceedings series, 808 pages, 1962, 66s, $ 11.00, NF 44, DM 38.50 , 1963 .

[8]  D. Crerar,et al.  Solubility of the buffer assemblage pyrite + pyrrhotite + magnetite in NaCl solutions from 200 to 350°C , 1978 .

[9]  N. McIntyre,et al.  X-ray photoelectron spectroscopic studies of iron oxides , 1977 .

[10]  D D Wagman,et al.  Selected values of chemical thermodynamic properties , 1952 .

[11]  E. D. Eastman,et al.  EQUILIBRIA INVOLVING THE OXIDES OF IRON , 1924 .

[12]  C. Baes,et al.  The hydrolysis of cations , 1986 .

[13]  S. Pinchas.  Infrared Absorption of Aldehydic C--H Group , 1955 .

[14]  Karl H. Gayer,et al.  The Solubility of Ferrous Hydroxide and Ferric Hydroxide in Acidic and Basic Media at 25 , 1956 .

[15]  D. Himmelblau.  Solubilities of Inert Gases in Water. 0° C. to Near the Critical Point of Water. , 1960 .

[16]  L. Morss,et al.  Partial molal heat capacity of aqueous ferrous chloride from measurements of integral heats of dilution , 1979 .

[17]  W. L. Marshall,et al.  Second Dissociation Constant of Sulfuric Acid from 25 to 350° Evaluated from Solubilities of Calcium Sulfate in Sulfuric Acid Solutions1,2 , 1966 .

[18]  P. Balakrishnan.  A radiochemical technique for the study of dissolution of corrosion products in high-temperature water , 1977 .

[19]  K. Goto,et al.  Effect of anions on the oxygenation of ferrous ion in neutral solutions , 1976 .

[20]  J. Lindgren,et al.  Electrochemical Behaviour of Iron. , 1960 .

[21]  K. Malén.  Thermodynamics of nuclear materials 1979: Proceedings of a Symposium, Jülich, 29 January–2 February 1979) (IAEA, Vienna, 1980). , 1981 .

[22]  W. L. Marshall.  Thermodynamic functions at saturation of several metal sulfates in aqueous sulfuric and deuterosulfuric acids at temperatures up to 350°C , 1975 .

[23]  L. Hepler,et al.  Electrode potentials and thermodynamic data for aqueous ions. Copper, zinc, cadmium, iron, cobalt, and nickel , 1968 .

[24]  L. Hepler,et al.  Thermodynamics of ionization of water over wide ranges of temperature and pressure , 1975 .

[25]  Robert E. Mesmer.  Hydrolysis of iron(2+) in dilute chloride at 25.deg. , 1971 .

[26]  C. Baes,et al.  The solubility of magnetite and hydrolysis of ferrous ion in aqueous solutions at elevated temperatures , 1970 .

[27]  K. Kelley,et al.  Entropies of the elements and inorganic compounds , 1961 .