ON THE INFLUENCE OF CARBONATE IN MINERAL DISSOLUTION .2. THE SOLUBILITY OF FECO3 (S) AT 25-DEGREES-C AND 1 ATM TOTAL PRESSURE

The solubility of siderite in acid, neutral, and alkaline medium was investigated. The experiments were performed as potentiometric titrations under two constant partial pressures of CO2 (g). This yielded the following equilibrium constants at 25-degrees-C in a 1.0 M NaClO4 medium (quoted errors through this work are given at 2-sigma confidence level). 1) The solubility constant of FeCO3 (s) FeCO3 (s) + 2H+ = Fe2+ + CO2 (g) + H2O, with log K(s0)* = 7.59 +/- 0.08. 2) The formation constants for the two determined Fe(II) carbonate complexes: Fe2+ + CO2 (g) + H2O = FeCO3 (aq) + 2H+, with log beta(11) = -12.9 +/- 0.12 Fe2+ + 2CO2 (g) + 2H2O = Fe(CO3)2(2-) + 4H+, with log beta(12) = -28.4 +/- 0.10. By extrapolation to infinite dilution, the following reaction equilibria are estimated for the infinite dilution standard state: FeCO3 (s) = Fe2+ + CO3(2-), with log K(sp) = -10.8 +/- 0.2 Fe2+ + CO3(2-) = FeCO3 (aq), with log beta(1) = 5.5 +/- 0.2 Fe2+ + 2CO3(2-) = Fe(CO3)2(2-), with log beta(2) = 7.1 +/- 0.2. The determined solubility constant is in fair agreement with previously derived solubility data. The identified complexes and their formation constants emphasize their importance in various anoxic waters. However, carbonate complex formation does not account for the supersaturation conditions with regard to siderite equilibrium observed in many other present-day environments.