Experimental determination of the effect of dissolved CO2 on the dissolution kinetics of Mg and Ca silicates at 25 °C

Dissolution rates of diopside, forsterite (Fo100 and Fo92), wollastonite, and hornblende were measured at 25 °C in NaCl solutions as a function of pH (1≤pH≤12), pCO2 (0 and 1 atm) and activity of HCO3− and CO32− (10−5≤∑CO2≤0.1 M). Dissolution rates of diopside, forsterite, and horneblende are not affected, within the experimental uncertainty, by 1 atm pCO2 at pH∼4 and by the presence of bicarbonate (up to 0.1 M) and carbonate (up to 0.01 M) ions at pH of 8 and 11-12, respectively. Although wollastonite dissolution rates are unaffected by pCO2 at pH∼4, its rates are increased by the presence of HCO3− at 0.01≤[HCO3−]≤0.1 M and pH=7-8 and decreased slightly by CO32− at [CO32−]>0.0001 M and pH∼12. These results should be useful for modeling the effect of silicate rock weathering on the global biogeochemical cycle of carbon. For the first time, we were able to distinguish the net effect of pCO2 versus that of pH. It is observed, in general accord with previous results, that the direct effect of CO2 on basic silicates dissolution rates is very weak. As a result, the major parameter governing basic silicates dissolution is pH of soil solutions in contact with minerals.