Calculating pH of rain water from pCO2

[Jaxon Dii Horne]

Hi,

I want to know if I have this set up correctly. I am using GWB10 Professional. I want to take a hypothetical solution that will react with a komatite-type rock and change the initial pCO2 concentration to see the initial pH in the rainwater. How I have approached this by trying to use a sliding fugacity of pCO2 (e.g., 0.003 pCO2 to 0.003 pCO2) to solve for the initial pH of the rainwater before it reacts with the rock. I might be thinking about this wrong, but I have attached a script. Hope this helps.

Cheer,

Jaxon

0.003_EW.rea

[Jia Wang]

Hello Jaxon,

If you swap in a fugacity value for a component, the program will use that constraint as a calculation for the initial pH concentration. From your description above, it sounds like you would like to use the sliding fugacity path to evolve the initial fluid to equilibrate at a final end point CO2 fugacity. In this case, you need to set the initial concentration of your fluid in the basis, not swap in a CO2(g) fugacity. If you have all the components needed to calculate the CO2(g) fugacity (i.e., HCO3-, H+, H2O), the program will calculate the CO2(g) fugacity as part of its equilibrium calculation. You can set up a sliding fugacity path for CO2(g) to arrive at the end result of a fluid in equilibrium with 0.003. You can pick up the end result as the starting point of a new run in React. The pickup function allows you to pick up just the fluid or the entire system, including precipitated minerals. For more information on pickup, please see the GWB Reference Manual.

Aside from that, I would consider choosing another ion for charge balancing, since your dissolved carbonate component would change according to the CO2 fugacity. Typically, you would want to choose the ion that has a high abundance and low uncertainty. For example, React is set up to use Cl- as the default ion for charge balancing, given its typically abundant presence and the ability to calculate it from charge balance based on other species concentrations measured in chemical analyses. You can also turn off the charge balance and run your calculation to see if your solution is in excess of cations or anions.

If you would like further help with troubleshooting, please provide more details regarding the assumption that you are making in the initial input. For example, you are fixing the activity of SO4-- and I am not sure why.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Jaxon Dii Horne]

Hi Jia,

Thank you for the clarification. I was attempting to review the examples in the Bethke book and the essential guide, but I thought I might have set it up incorrectly. I appreciate your suggestion and will try it out. 

Regarding the constant fugacity of SO4--, it was previously used in a script and model to analyze evaporation/weathering cycles that occurred 4-3.5 billion years ago as a proxy for the quantity of volcanic gases released into the atmosphere. However, this may not be the appropriate approach as balancing on the HCO3- or SO4-- changes the resulting pH. Hence, I need to reconsider this approach. 

I'm curious about what you meant by turning off the charge balance and determining if the solution is in excess of cations or anions. How can I identify if it's in excess based on the calculation output? Can I then use this information to balance the basis and ensure it's comparable to the published dataset I'm trying to model? 

I hope this makes sense. I have also attached a new script trying to address what you suggested, I think. 

Cheers,

Jaxon

0.003_EW_JDH.rea

[Jia Wang]

Hello Jaxon,

You can turn off charge balancing, either by selecting this option from the units in the GUI or in the command line using "balance off". You can perform a "go initial" run (Run menu -> Go initial) and look under Chemical Parameters in Gtplot or in the header block of the text output file for charge imbalance. The charge imbalance will inform you whether your solution is in excess of positive or negative charge. Please see chapter on Using Gtplot in the GWB Reaction Modeling User Guide if you are having issues configuring plot results.

Also note that the fugacity of SO4-- is not fixed, SO4-- is basis species and not a gas. You have swapped the HS- component with H2S(g) in your basis and then fixed the fugacity of H2S(g) in the Reactants pane.

In general, you should try to simplify your input file for troubleshooting. When I performed a "go initial" run, your input file will not converge. This typically occurs when you have specified constraints or composition that is not possible for the equilibrium calculation. I would suggest that you check the initial composition of fluid first. In my previous post, I had suggested that you input the composition of your fluid, not with CO2(g) swapped in, and then use the sliding fugacity path to arrive at an end CO2(g) fugacity desired. Do you have a pH value for your hypothetical fluid? Check that the temperature parameter and the decoupling of the HS-/SO4-- couple makes sense for your fluid.

Hope this helps,
Jia

[Jaxon Dii Horne]

Hi Jia,

Thanks for the clarification. I misunderstood what you said, and a simpler model is better than a complex one. I appreciate your suggestions and will look more into the resources you suggested. This way, I will be able to understand the finer points of GWB.

 

Cheers

Jaxon Dii Horne