DISSOLUTION AND SOLUBILITY OF NATURAL MAGNESIAN SIDERITE [(Fe0.8Mg0.2)CO3] AT 25°C

摘要

Natural magnesian siderite [(Fe0.8Mg0.2)CO3] was used to investigate its dissolution in different waters at +/- 25 degrees C for 5520 h. For the dissolution in pure water, air-saturated water, CO2-saturated water and solution of initial pH 9, the aqueous Fe concentrations increased slowly to a steady state in 2160 h with the final concentrations of 0.0070 similar to 0.0072, 0.0060 similar to 0.0079, 0.0058 similar to 0.0067 and 0.0061 mmol/L, respectively. For the dissolution in natural waters after 4080h, the final aqueous Fe concentrations existed in the sequence of sea water (0.030 similar to 0.032 mmol/L) > river water and groundwaters (0.0058 similar to 0.0069 mmol/L). The potential formation of magnesite, calcite, aragonite and dolomite could hinder the dissolution of siderite and limited the Fc release into water. The average ion activity product (IAP) at the steady state (approximate to K-sp) was computed to be 10(-11.4)(1 +/- 0.)(28) with the Delta G(f)degrees value of -756.57 +/- 1.62 kJ/mol for the natural magnesian siderite [(Fe0.8Mg0.2)CO3], which dissolved congruently at the early stage, and then incongruently and moved progressively from right to upper-left of the solutus curve, the saturation curve for FeCO3 and the minimum stoichiometric saturation curve for (Fe0.8Mg0.2)CO3 in the Lippmann diagram. The aqueous Fe-poorer and Mg-richer solution was finally in equilibrium with the FeCO3-richer and MgCO3-poorer solid.

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