CO2 degassing

[mcossio23]

Hi guys,

I am new to the forum and also relatively new to PHREEQC. I am in process of modelling Calcite precipitation in high salinity waters. These waters have been aerated to promote pH increase and calcite precipitation via CO2 degassing. So far, I have solution data and equlibrium_phases data. I am aware that you could gradually add O2(g) to your solution;however, I am not entirely sure how/if CO2(aq) to CO2(g) via aeration can be modeled. It would be greatly appreciated if any of you guys could provide me with some hints. Thank you so much

Regards,
Manny

[dlparkhurst]

You can either remove CO2 with REACTION (or KINETICS)

REACTION
CO2  -1
1 mmol in 2 steps

Or you can set the partial pressure of CO2, logically to atmospheric CO2 if you are using air to strip the CO2

EQUILIBRIUM_PHASES
CO2(g) -3.4 10

[pia said]

If i want to use kinetics in CO2 dissolution. Where can i find information about the rate constants of CO2? Is recommendable to use kinetics? Or with equilibrium phases will I have the similar results?

[dlparkhurst]

You will have to decide. For starters, equilibrium is much simpler to model, and gas/water exchange is relatively fast.

[mcossio23]

Hello,

We are looking at calcite precipitation from CO2 degassing from seepage from a waste rock dump. In summary, intrinsic pyrite is being oxidized dissolving dolomite, increasing the internal pCO2 to ~ - 3. Seepage data shows an increasing pH and observable evidence of calcite precipitation suggest that CO2 is degassing, increasing the pH and promoting calcite precipitation. There is not any evidence of calcite precipitation inside the dump with elevated pCO2.

Our goal here is to model this phenomena using PHREEQC with a two step equilibrium phase. First, our starting solution will be equilibrated with pCO2 at -3 followed by a second equilibration with pCO2 at -3.5 (atmospheric) and calcite 0.

1) Does this approach make sense? (see the code in the attached text file)
2) if we want to force calcite precipitation at SI 1 we would then need to change the code to Calcite 1, correct?

Thank you for your help!

[dlparkhurst]

Your initial solution has a PCO2 of 10^-1.45. Your second step lowers the PCO2 to 10^-2, at which point calcite is slightly undersaturated, and your final step lowers the PCO2 10^-3.5 and precipitates calcite to a supersaturation of SI=1. Seems like the file matches more or less what you described.