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Modeling carbon absence fluid rxn with atmosphere

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I am trying to modeling the reaction of a hyperalkaline Ca-rich rich fluid with atmospheric CO2 (log PCO2 = -3.5). I have tried several approaches using both cations or anions as balance species and swapping Ca++ out for a mineral (Portlandite), but each time, the simulations crater because of failure to converge. How do I get around this problem?




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Hi Bob,


I'm assuming the original f CO2 setting was arbitrarily chosen to give a very small amount of dissolved carbonate so that calcite starts off undersaturated. If that's the case, you probably want to go a fair amount lower considering the high pH. A fugacity of 10^-14 or lower worked well. You can check the concentration of the CO3-- component in the fluid at the start (before reaction with the atmosphere) to verify this makes sense. After that, I could do a few things to get the model to run with reasonable results. I could set Ca++ to the charge balancing species, since it's very abundant. Or, I could set a higher concentration background electrolyte, such as 10 mmolal Na-Cl, with Cl- as the balancing ion. Finally, I could simply turn charge balancing off for the calculation. In each case, Portlandite and then Calcite precipitate as the f CO2 increases and pH decreases.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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