Diopside dissolution kinetics as a function of pH, CO 2 , temperature, and time

Details Diopside dissolution kinetics as a function of pH, CO 2 , temperature, and time Diopside dissolution kinetics as a function of pH, CO 2 , temperature, and time

Jan 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57..285k&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 2, pp.285-294

Physics

15

Scientific paper

We have measured the dissolution rate of diopside in dilute solutions (far from equilibrium) at 25, 50, and 70°C from pH 2 through pH 12 using a flow-through reactor. Reducing the CO 2 concentration tenfold produced little, if any, effect on dissolution rate at alkaline pH (pH 8 through pH 12) at 25 and 70°C. Linear dissolution kinetics (i.e., time-invariant rates) were eventually observed in all runs. The overall trend with increasing pH is decreasing diopside dissolution rate based on the release rate of all constituents. We fit these rates by regression to a general rate law of the form r = Ak [ a H +] n , where A is the surface area, k is the rate constant, and n is the order with respect to hydrogen ion activity. At 70°C over the range pH 2 through pH 10 in solutions equilibrated with atmospheric CO 2 , the rate of diopside dissolution based on Si release is rate (mol/cm 2 -s) = 2.45 (±0.96) × 10 -13 a H + 0.18±0.03 . At 50°C the rate based on Si release is rate (mol/cm 2 -s) = 1.10(±0.61) × 10 -13 a H + 0.21 ± 0.04 . At 25°C the rate based on Si release is rate ( mol / cm 2 - s ) = 2.88(±1.33) × 10 -14 a H + 0.18 ± 0.03 . Based on a regression of the rate constants, over the temperature interval 25 to 70°C, the activation energy for the dissolution of diopside is 9.7 ± 0.4 kcal/mol. This energy is indicative of a surface-reaction controlled dissolution process, as is the observation of crystallographically controlled etch pits.

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