Processes > Oxidation and reduction equilibria

S0 oxidation and calcite dissolution

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Gaulty:
I am running a simulation whereby mill residue is added to a settling pond. The residue contains gypsum, calcite, and S0. We do not know the extent to which S0 oxidation will occur (and result in poor water quality) and I am trying to test this with Phreeqc by running some simulations with
a) no S0 (assumes no S0 oxidation) and
b) with S0 (S0 oxidation occurs).
Testing has suggested the residue will settle rapidly and we know oxidation will be limited by the water cover.

We have assumed a daily O2 flux density of 20 g/m2/day, and know the molar input of gypsum, calcite, and S0 on a daily basis. I have specified an initial water quality of the pond water.

Physico-chemical profiles in the pond suggest that there is some density stratification and that the pond may not be completely mixing, so I do not want this be a truly open system - I am supplying a finite amount of O2 (based on the daily flux density) instead of unlimited O2 (i.e. pO2 = -0.68 with 10 moles available).

With that in mind I'm wondering how to treat CO2. If I do not allow it to escape (i.e. not specifying it as an EQM phase) I have CO2 accumulate to high pressures due to calcite dissolution. I have tried setting pCO2 = -3.5 with 0 moles (i.e. to allow gas escape) but I don't think this actually allows any to dissolve since I have not supplied any gas (for the simulation with no S0 supplied in my phase assemblage CO2 delta is 0). Would 1 mole be more appropriate here? I read this in another example.

My runs are below:

SOLUTION 1 Pond water 2018 annual average chemistry
    temp      25
    pH        9.5
    pe        4
    redox     pe
    units     mg/l
    density   1
    C(4)      51
    Ca        263
    Cl        251
    Mg        33
    Na        1065
    S(6)      2560
    -water    1 # kg
END
USE SOLUTION 1      
EQUILIBRIUM_PHASES 1 Residue phases no S0 (daily amount added)
    CO2(g)    -3.4 0
    calcite   0 0.00013
    gypsum    0 0.028
REACTION 1 Daily O2 flux
    O2(g)      1
    0.00003079 moles in 1 steps
END
USE SOLUTION 1
EQUILIBRIUM_PHASES 1 Residue phases incl S0 (daily amount added)
    CO2(g)    -3.4 0
    Sulfur    0 0.066
    calcite   0 0.00013
    gypsum    0 0.028
REACTION 1 Daily O2 flux
    O2(g)      1
    0.00003079 moles in 1 steps
END

CO2 does not accumulate in the solution this way so I assume it is outgassing as required. Wondering if the amount should be 1 moles to allow some CO2 influx if needed?

Thanks,
Gaulty

dlparkhurst:
When you set equilibrium with native sulfur, you generate S(-2) and S(6), with or without the addition of oxygen. In fact, the addition of oxygen probably does not make much difference. So the first question is do you have sulfide in your waters? If so, then maybe your reaction set is plausible, but may also have organic carbon oxidation. If not, you may want the REACTION to be addition of SO3 (simulating a instantaneous oxidation of native sulfur to sulfate) in place of REACTION of O2 and Sulfur equilibrium.

As for CO2, the oxidation of native sulfur will produce acid , which will generate CO2(aq), so the flux of CO2 will be out of the water. You could use KINETICS to simulate a flux of CO2 out of the water that is dependent on the difference in partial pressure between the water and the atmosphere. If you have a stratified pond, perhaps the exchange of CO2 will not be significant and CO2 simply accumulates in solution, unless you actually form bubbles of gas that escape. I think you would need pCO2 greater than -1 (if dissolved nitrogen is present) and probably greater than 0 (if N2(aq) has been stripped by ebulition, and depending on the depth of the pond) to sustain bubble formation. You can look at the pCO2 in speciation calculations of your pond water to make a guess as to whether this is happening.

Gaulty:
Hi David,
Thanks for your prompt reply.

I did not consider organic carbon oxidation - good point. Thanks for mentioning that. The pond is currently hypothetical so I don't have data about this, or sulphide, but good to point out for my assumptions.

It does not appear that sufficient CO2 is present (added via acid production and CaCO3 dissolution and alkalinity consumption) to form bubbles. Speciation results give a log pCO2 of -3.4 (1.4e-5 moles CO2).  I assume this is constrained by the pCO2 I specified in my equilibrium phases and that any additional CO2 produced leaves the system - my total moles of CO2 (under phase assemblage) is an order of magnitude higher.

Under these conditions would you ever get bubbles forming since equilibrium of CO2 is set to pCO2 -3.4? Perhaps I am not interpreting this correctly (or telling the program to do the wrong thing?).

dlparkhurst:
If you include CO2(g) -3.4, you are assuming rapid interaction with the atmosphere and tacitly no bubble formation.

If your pond is stratified, and there is not much interaction with the atmosphere, it may make more sense to remove the CO2(g) from EQUILIBRIUM_PHASES and let CO2 accumulate in the water, potentially developing bubbles. Organic decomposition could also generate CO2(aq) and CH4(aq) (methanogenesis) contributing to bubbles, but probably not when there is a large reservoir of sulfate as an electron acceptor.

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