Processes > Dissolution and precipitation
H2O is zero in solubility of CO2
dlparkhurst:
Only define define solutions--no surfaces, equilibrium_phases, etc. Make sure the Phreeqc output file for the solutions matches your expectations.
If the calculation is successful, then you can add back reactants to find the problem.
If the calculation fails, remove first RunCells from the time stepping, and if it still fails, SetConcentrations, GetConcentrations from the time stepping. That will leave you with only transport.
Mohamadreza:
Thanks. How do I fixed mass or mole of water in geometry to maintain its amount in the geometry?
Do I need to explicitly inject water to maintain its amount in the geometry? Does PHREEQCRM (used for reactive transport) preserve the amount of water in the geometry?
dlparkhurst:
PhreeqcRM uses relies completely on the concentrations given in SetConcentrations and the volume--as determined by RV, porosity, saturation, and sometimes density (when using mass fraction)--to arrive at the number of moles of each element in each cell. If there are no reactants (equilibirum_phases, surfaces, etc), then Phreeqc will preserve the number of moles of each element in solution, including H and O (and charge balance).
Mohamadreza:
Thanks. Well, if we want to investigate the dissolution of carbon dioxide in a 2 molal CaCl₂ solution, the amount of water (in moles) will be consumed. And if a cell runs out of water, PHREEQCRM will encounter the same problem that I experienced.
dlparkhurst:
I think it might depend on your database. Phreeqc.dat and pitzer.dat define CO2(aq), not H2CO3(aq); so, dissolution of CO2 alone would not consume water. Activity of water will decrease as CO2 dissolves, basically as a mole fraction of the system. Phreeqc.dat uses 1 - 0.017*sum(m(i)) for activity, so there is a specific sum of molalities that will result in activity of water going to zero. pitzer.dat uses a formulation for activity of water based on the osmotic coefficient.
Depending on your system, water could be lost to a gas phase, to minerals, or perhaps to other aqueous species
Solubility of CO2 has been fit fairly carefully, although perhaps not as much in CaCl2 solutions. However, I think you should be able to model CO2 solubility in 2 molal CaCl2 within limits of say a few hundred atmospheres and 200 to 300 C. I think you said PHREEQC calculations were successful.
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