making a CO2-charged solution

[AliMahaqi]

Hello,

I was wondering if you help me to make a CO2-saturated solution. I have a water sample and then calculate the amount of CO2 needed to saturated it based on Duan et al. 2006 (http://149.165.154.118/co2/co2calc2.php, as the T= 298.15 K, P= 1 atm, and m NaCl is 0.1). I just want to know what I am doing is correct, I put the needed CO2 for saturation of solution (0.0318m CO2(g)) in the reactants (as simple) and then react it with the water sample. For your convenience, attached the React file. I need this CO2-charged solution as a basic solution to do reaction path modeling as well as reactive transport modeling.

The next question is in the previous model, the pH of water sample is 8.2, but when the mixing of CO2 in water starts, the modeling shows the initial pH 7.4 (at step 0), why? 

Based on the procedure I told, the decreasing trend of pH should be from 8.2 to 5.2 (after mixing needed CO2 to make a CO2-saturated solution), but the modeling shows the range from 7.4 to 5.2.

Thanks for your explanation in advance. 

 

CO2_saturates_solution.rea

[Jia Wang]

Hello,

I think the issue here is not related to your reaction path but rather the initial state that of your system. React will solves for the equilibrium state of your initial fluid before any reaction path takes place. If any minerals are supersaturated, the program will by default allow minerals to precipitate. An easy way to see this is to perform a "go initial" run from the Command pane or go to Run -> Go Initial. If you open the resulting React_output.txt file (Results pane, View Results button), you can see that there are two step 0 blocks which reports slightly different results. The first block shows the results calculating speciation of the initial fluid and you can see that it is supersaturated with respect to a number of minerals in the Mineral Saturation states section. The second block shows the equilibrium state of your system after supersaturated minerals are allowed to precipitate. You can see that a small amount of quartz and dolomite is predicted to precipitate at equilibrium. The precipitation of these minerals will alter the original fluid composition and the pH in this particular case decreases. The second step 0 block is the result you see plotted in Gtplot.

If you disable precipitation, React will not allow any new supersaturated minerals to form. You will only see the first block of data outputted to your text file. This might make sense for certain reactions that are too slow to observe in the time span of your experiment.

For more information on setting up the initial system in React, please refer to section 2.3 in the GWB Reaction Modeling User's Guide.

Hope this helps,
Jia Wang

[AliMahaqi]

Thanks a bunch Jia, for your points. I got the point for different values of pH. (Just one thing remains, how we could find when and where to enable/disable the precipitation option in our modeling?)

And, I want to know if the first procedure that I explained is correct to make a CO2-saturated solution.

Much appreciated.

 

 

 

[Jia Wang]

Hello,

You're welcome. To disable precipitation in React, you can use the command "precipitation=off" or go to the Options... dialog under the Config menu and uncheck precipitation. You can see an example of this dialog in section 2.3 Initial Systems of the GWB Reaction Modeling User's Guide.

I looked at your titration path and it seems correct. Perhaps you would want to double check the conditions you initially set to calculate the solubility of CO2. Your initial fluid looks to have a much lower concentration of salts than 0.1 molal.

Best regards,
Jia