Saturation Index vs Partial presseure

[Jtageo]

Hello...

I am trying to calculate the saturation indices variation of some Mn oxides minerals of a groundwater sample by changing the partial pressure of O2(g) and CO2(g).
I made some progress with the REACTION keyword as follows.
###
REACTION 1
O2(g)    0.20   # mole ratio between CO2(g) and O2(g
#CO2(g)      0.000316
#0.0000001   0.000001   0.00001   0.0001   0.0002   0.00021   0.00022   0.00023   0.00024   0.00025
0.0004 moles in 1000 steps
INCREMENTAL_REACTIONS true
###

I would like to know if there is a more precise/ correct way of doing this directly with partial pressures or any other way.

Thank you.

[dlparkhurst]

You can set the log partial pressures of CO2(g) and O2(g) by using EQUILIBRIUM_PHASES. The following would set the log partial pressures to atmospheric values.

Code: [Select]

EQUILIBRIUM_PHASES 1
CO2(g) -3.4 10
O2(g)   -0.7 10
END


[Jtageo]

Dear sir,
Thank you for your comment.

I think I have not accurately explained my problem above.

My initial groundwater (more reduced) always undersaturated with respect to any Mn oxide phases. But once they are pumped to the surface we saw some brownish precipitation in our storage tank walls and pipe walls (hard to remove by washing). We analyzed the precipitation and understood they were easily reducible Mn oxides. I am trying to model the reason how it can be formed ( I am very new to PhreeqC).

I tried varying pressure, temperature, CO2(g) dissolution/ degassing, O2(g) dissolution, to see what processes make more sense.

I found out that the water needs very little O2 moles (1E-4) to precipitate some Mn Oxides.

Since I have so many samples, I want to run a simulation by increasing the pCO2 and/or pO2 values step by step, so I can find out when these Mn oxides start to precipitate.
I did it with the reaction Keyword (see the attachment) by adding step by step (1000 steps) O2 in moles to the system. That way I was able to create a graph that can explain how much O2(in moles) needed to be dissolved to start precipitation.

I am aware that I can use the Equilibrium phase and/ or gas phase keyword to calculate the SI.

If I want to increase the pO2(g) by 100 steps should I define all partial pressures and execute the Gas-phase / Equilibrium phase Keyword 100 times separately?
or is there an easy way of doing it to generate a graph similar to what I have attached?

I hope you can understand what I am trying to do?
Your valuable inputs are really appreciated.
Thank you in advance.