Ferric iron in sediments as a novel CO2 mineral trap:: CO2-SO2 reaction with hematite

Thermodynamic simulations of reactions among SO2-bearing Co-2-dominated gas, water and mineral phases predict that Fe-III in sediments should be converted almost entirely to dissolved Fell and siderite (FeCO3), and that SO2 should simultaneously be oxidized to dissolved sulfate. The reactions are however, subject to kinetic constraints which may result in deviation from equilibrium and the precipitation of other metastable mineral phases. To test the prediction, a laboratory experiment was carried out in a well stirred hydrothermal reactor at 150 degrees C and 300 bar with hematite, 1.0 m NaCl, 0.5 m NaOH, SO2 in quantity sufficient to reduce much of the iron, and excess CO2. The experiment produced stable siderite and metastable pyrite and elemental S. Changes in total dissolved Fe are consistent with nucleation of pyrite at similar to 17 h, and nucleation of siderite at similar to 600 h. Dissolution features present on elemental S at the conclusion of the experiment suggest nucleation early in the experiment. The experiment did not reach equilibrium after similar to 1400 h, as indicated by coexistence of hematite with metastable pyrite and elemental sulfur. However, the results confirm that Fe-III can be used to trap CO2 in siderite if partly oxidized S, as SO2, is present to reduce the Fe with CO2 in the gas phase. (c) 2005 Elsevier Ltd. All rights reserved.