how to keep CO2 from evolving in a column model

[Mark Himenz]

I have a column experiment that is run under high alkalinity with a pH range of ~6.5. The flow rate is fairly fast, fast enough that CO2 doesn't transfer into the gas phase at significant levels. However PHREEQC does produce significant amounts of CO2 gas, and as a consequence the pH in the model is higher by ~1-1.5 units compared to measured values.

Is there any way to tell Phreeqc to not produce CO2 gas during these simulations?

[dlparkhurst]

PHREEQC will only produce CO2(g) if you include CO2(g) in EQUILIBRIUM_PHASES or GAS_PHASE data blocks for one or more of the TRANSPORT cells. Production of CO2(g) will tend to increase the pH. If you do not include CO2(g) in either of these types of data blocks, you will not produce any CO2(g). It usually only makes sense to add CO2(g) to the top cell of a vertical column anyway (unless bubbles are present throughout).

I have a feeling that you are talking about CO2(aq), wihich is listed as CO2 in the "Distribution of species" section of the output. I don't think it is reasonable to suppress the formation of CO2(aq) in that it is a natural consequence of your reactions and aqueous equilibrium is relatively fast for the carbonate aqueous species.

[Mark Himenz]

When the model run finishes it gives me this message:  Estimated CO2 emission: 0.66 g. which I assumed to mean gas phase CO2 no?

I did not add CO2 into any EQUILIBRIUM_PHASES or GAS_PHASE block in the input file.

[dlparkhurst]

You must be using Phreeqc for Windows. Vincent Post used the message to tell you how many hydrocarbons were burned to power your computer to make the calculations. It is simply proportional to run time and has nothing to do with CO2 evolved in your column.

[Mark Himenz]

That is both hilarious and also very confusing...

Thank you for clearing up that confusion.