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CO2 gas-water partitioning


Tanya
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I try to calculate CO2 partitioning between gas and liquid phases using GWB. I enter total carbon either as CO2g or HCO3 and I want to determine how this amount distributes between gas and liquid (Total carbon = CO2g + CO2aq+HCO3- + CO32-). However, react always uses the total carbon I enter as a total carbonate in solution (in case of HCO3) or in gas (in case of CO2g) and calculates the corresponding amount in equilibrium with it. Any advise on how to do the partitioning calculations with GWB?

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Hello Tanya,

The GWB accounts for the effect of gas on an aqueous system by gas partial pressure or fugacity in equilibrium with the system. I am not sure what your system is like but you can set up the Basis pane with the initial bulk HCO3- concentration in the fluid and have the GWB calculate the corresponding CO2(g) fugacity. If your system have a super CO2-charged fluid for example, you could use a sliding fugacity path to simulate degassing to the point of equilibrium with some external reservoir (e.g. atmosphere). Under Reactant properties in Gtplot plot, you can find the amount of CO2 gas exsolved (negative value means gas is leaving the system). You can also track the distribution of species under the variable species concentration. 
 
For more information on sliding fugacity paths, please refer to section 3.6 of the GWB Reaction Modeling User Guide. 
 
Hope this helps,
Jia Wang
Aqueous Solutions LLC
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Thank you very much for your reply. If I understand correctly, GWB should have a known bulk HCO3- in the fluid or fugacity to calculate the corresponding fugacity or dissolved carbonates, respectively. The software cannot calculate carbon distribution between gas and liquid phases based on a total carbon equal to a sum of CO2 in the gas phase and aqueous carbonate species, right?

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Hello Tanya,

Yes, the bulk concentration for HCO3- is specified for the sum of aqueous species and carbonate minerals (if supersaturated). The speciation calculation does not distribute mass to the gas phase but instead will calculate fugacity of that gas in equilibrium with the fluid.

Best regards,
Jia

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