An X-ray absorption study of the dissolution of siderite at 300 bar between 50 °C and 100 °C

Dissolution of siderite has been monitored in a small size batch reactor at a constant pressure of 300 bar and temperatures of 50 °C, 75 °C, and 100 °C, in acidic conditions. The technique, based on X-ray absorption spectroscopy measurements, allows simultaneous measurements of aqueous iron concentration and speciation as a function of time under in situ high pressure and high temperature conditions. Far from equilibrium, dissolution can be described by r+ = kH+aH+ where aH+ stands for aqueous proton activity and kH+ is the dissolution rate constant normalized to geometric surface area. The values of the rate constant obtained in this study are 4.5 × 10− 9 mol cm− 2 s− 1, 4 × 10− 8 mol cm− 2 s− 1, and 1.7 × 10− 7 mol cm− 2 s− 1, at 50 °C, 75 °C, and 100 °C, respectively. The corresponding activation energy, ΔE = 73 kJ mol− 1, suggests a dissolution process controlled by chemical reaction, i.e., proton adsorption at the siderite surface. A NaCl molality of 1 mol kg− 1 is shown to strongly slow down the dissolution process. Speciation of Fe2+ in solution is dominated by hexahydrated octahedral species with some significant contribution (about 25%) of the chlorinated species only in 1 mol kg− 1 NaCl, in good agreement with recorded X-ray absorption spectra. No Fe3+ was detected. These results are found to be consistent with those obtained in previous siderite dissolution studies carried out under constant chemical affinity using the rotating disc technique. This agreement opens up interesting perspectives for retrieving, from batch micro-reactor experiments, kinetic parameters which are essential for the modelling of fluid-rock interactions in storage conditions.