Carbonate simulation

[Y.Y]

Hi,

I am working on simulating the speciation of a digest from anaerobic digestion with known ionic concentrations and the presence of CO2 and CH4, and then titrating Ca(OH)2 to observe the changes in calcite precipitation and PCO2 using REACT.

I am not very familiar with this software and here are the issues I encountered and my questions:

Basis Species Swap: I swapped CO2(g) for HCO3- in the basis species and entered the PCO2 value in atm. However, when I ran the simulation, the initial PCO2 value was always higher than the value I set. I assume this is because CO2(g) as a component includes all the carbonate species, and the system performs equilibrium calculations among different carbonate species. Is the PCO2 value I entered being calculated and converted to the total carbon concentration? If so, what calculation method does the system use for this conversion?

Initial Equilibrium Process: The result panel shows that CO2 is swapped out of the basis vector instead of HCO3-. I was wondering why this happens and what it means in the context of the simulation?

Setting Initial PCO2: Is there a way to directly set the real PCO2 value in the initial data entry so that it remains as specified?

Another question is about Gtplot, is the variable type "species concentration" include only aqueous species? (e.g. CaCO3 here is the dissolved concentration)

I have attached my REACT file for reference.

Thank you very much for your assistance!

Best regards,
Y.Y

React_Ca(OH)2.rea

[Brian Farrell]

Hi Y.Y,

Settings for gas partial pressure serve as free constraints, not as total (bulk) values. If you look in the text output file, you’ll see that there are two initial steps (“Xi = 0”) in the output. The first honors your PCO2 value and other settings, but while the fluid is in an internal state of equilibrium, it is supersaturated with respect to several minerals. The program brings the fluid to equilibrium with respect to minerals by finding the stable mineral assemblage. During this procedure, several minerals precipitate, changing the pH, PCO2, dissolved inorganic carbon, etc. You can suppress minerals that might be thermodynamically favored to precipitate but in your judgement aren’t expected to form due to kinetic limitations. However, I believe that even calcite will precipitate from this initial system (before any Ca(OH)2 addition). So, you might need to check your input, your conceptual model, the thermodynamic data, etc. For more information, please see section 2.3, Initial system, in the GWB Reaction Modeling Guide, as well as the Suppress command in the GWB Command Reference.

The program sets a closed system before searching for the stable phase assemblage by eliminating gas buffers (in this case, it swaps out CO2(g) for HCO3-). For more information, see 6.1.4, Stable Phase Assemblage, in the Geochemical and Biogeochemical Reaction Modeling text.

If you wish to maintain an open system in which CO2(g) is buffered at its original value throughout the simulation, then you can “fix” its fugacity, as described in 3.5, Buffered paths, in the GWB Reaction Modeling Guide. 

In the current version of Gtplot, “Species concentration” refers only to aqueous species (or surface species, if included). Minerals that actually exist are found under a different heading – “Minerals” – in Gtplot. So, the species CaCO3 you see in Species concentration is simply an ion pair, much like CaHCO3+, except that it is uncharged. Under Minerals, however, you’ll find the abundance of the mineral Calcite.

Hope this helps,
Brian Farrell
Aqueous Solutions