[OLD] Carbonate vs Eh

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From: Brian Gibney

Subject: carbonate vs Eh

I am modeling solubility of carbonate in a micro-aerobic to anaerobic groundwater system with a sliding Eh to 1.0. Groudwater conditions are as follows.

 

add "H2O"

1 free kg "H2O"

swap "e-" "O2(aq)"

-.5 Eh "e-"

add "H+"

pH 6.8

add "SO4--"

2.38 mg/kg "SO4--"

add "Fe++"

031 mg/kg "Fe++"

add "F-"

09 mg/kg "F-"

add "Na+"

5.95 mg/kg "Na+"

add "K+"

1.57 mg/kg "K+"

add "Ca++"

10.4 mg/kg "Ca++"

add "Al+++"

018 mg/kg "Al+++"

add "Mg++"

1.7 mg/kg "Mg++"

add "SiO2(aq)"

25 mg/kg "SiO2(aq)"

add "Cl-"

86 mg/kg "Cl-"

balance on "Cl-"

add "HCO3-"

54.6 mg/kg "HCO3-"

The output is as follows

Solving for initial system.

 

Loaded: 130 aqueous species,

212 minerals,

7 gases,

0 surface species,

13 elements,

11 oxides.

 

N-R converged in 602 its., resmax = 6.85e-012, Xi = 0.0000

Charge balance: Cl- molality adjusted from 2.426e-5 to .0009466

27 supersaturated phases, most = Muscovite

 

Following reaction path.

 

Swapping Al(OH)4- in for Al+++

Swapping CH4(aq) in for HCO3-

Swapping H2S(aq) in for H+

Swapping Al(OH)2+ in for SO4--

Caution: molality of Cl- component forced negative by charge balance: -2.349e+008

*N-R didn't converge after 400 its., resmax = 1.6e+131, Xi = 0.0100

 

Cutting step size to find solution

Swapping SO4-- in for Al(OH)4-

Swapping HCO3- in for CH4(aq)

Swapping Al+++ in for H2S(aq)

Swapping H+ in for Al(OH)2+

Swapping H2S(aq) in for SO4--

Swapping CH4(aq) in for HCO3-

Swapping Al(OH)4- in for Al+++

Residuals too large, 667-th interation

-- Didn't wake up, abandoning path

 

From: Craig Bethke

Subject: Re: carbonate vs Eh

There are several issues with this run:

The endpoint Eh values you specify are far outside the stability limits of water, which makes it unlikely the program will converge. If you start at the reducing end, you need to set an appropriate basis (i.e., CH4(aq) instead of HCO3-, HS- instead of SO4--). It is probably easiest to start at the oxidized endpoint.

I expect that you want pH to be held constant, rather than drift freely. If so, set a fixed pH reactant on the Reactant pane. Because your solution is very dilute, there aren't enough counterions to achieve charge balance over the entire range of possible Ehs. You will probably need to turn off charge balance to do this run.