Siderite dissolution kinetics in acidic aqueous solutions from 25 to 100 °C and 0 to 50 atm pCO 2

Abstract

The rate of dissolution of polycrystalline siderite planes has been measured at 25, 60, 80 and 100 °C in 0.1 M (NaCl + HCl) solutions and pH from 1 to 4.6 at far from equilibrium conditions as a function of CO 2 partial pressure. Experiments were performed in batch titanium high pressure reactor under controlled hydrodynamic conditions using the rotating disk technique with stirring speed up to 1000 rpm. Few experiments were also carried out at 25 °C with siderite powder. Rates R were deduced from measurement of the amount of Fe released as a function of time using R = (d[Fe]/d t)/ S, where t designates the elapsed time, [Fe] stands for the number of iron moles released from the solid, and S is the siderite surface area. Linear dependencies between logarithm of dissolution rate and pH values (2 ≤ pH ≤ 4) were obtained, which allow us to determine the values of the kinetic constant and the order of the dependence on H + activity for siderite dissolution rate ( R = k +H + · a H + n ) for different temperatures: k +H + = 1.79 · 10 − 8 mol·cm − 2 ·s − 1 , n = 0.75 at 25 °C; k +H + = 6.20 · 10 − 7 mol·cm − 2 ·s − 1 , n = 0.96 at 60 °C; n = 0.94 at 80 °C and k +H + = 3.80 · 10 − 6 mol·cm − 2 ·s − 1 , n = 0.90 at 100 °C. Note that our results could not permit the determination of the rate constant at 80 °C with a reasonable accuracy. Apparent activation energy for siderite dissolution ( E app) varies from 61 kJ·mol − 1 at pH = 2.0 to 48 kJ·mol − 1 at pH = 4.0, in good agreement with values recently determined by Dufaud [Dufaud, F., 2006. Etude expérimentale des réactions de carbonatation minérale du CO 2 dans les roches basiques et ultrabasiques. Unpublished PhD thesis, IPG, Paris]. Finally, the effect of CO 2 partial pressure on siderite dissolution kinetics appears to be very weak, in agreement with previous works on calcite, dolomite and magnesite [Pokrovsky, O.S., Golubev, S.V., Schott, J., 2005. Dissolution kinetics of calcite, dolomite and magnesite at 25 °C and 0 to 50 atm pCO 2. Chem. Geol. 217, 239–255; Pokrovsky, O.S., Golubev, S.V., Schott, J., Castillo, A., 2009. Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 °C and 1 to 55 atm pCO 2: new constraint on CO 2 sequestration in sedimentary basins. Chem. Geol. (this issue)].