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[OLD] pH Neutralization of high-CO3-- high-pH


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From: Henry Kerfoot

Subject: pH Neutralization of high-CO3-- high-pH

I am trying to use Geochemists Workbench to simulate pH neutralization of pH 12.9 groundwater using CO2(g) to evaluate the potential for effects such as precipitation. The groundwater has 12.7 mmolal CO3--. I cannot figure out how I can specify the initial pH and [CO3--] and then add CO2(g), since I have already swapped CO3-- for HCO3-. I tried to input data on the groundwater in React, enter "Go", then 'pickup' the fluid (liquid), and then react with CO2(g), but could not get that to work. Any ways to accomplish what I am trying to do, or is it just that I can't react a high-pH high-carbonate system with carbonic acid using GWB because of basis limitations?

 

From: Craig Bethke

Subject: Re: pH Neutralization of high-CO3-- high-pH

Of course you can do this calculation! But you don't provide an example of a script that doesn't work as expected, so it's hard to give you specific advice. In general, constrain your initial system at its alkaline pH, and then use either the "react" command to titrate CO2 into it, or the "slide" command to increase in the system the fugacity of CO2(g). (You can use the "dump" command to eliminate any mineral mass that precipitates when you equilibrate the initial system; this is a little easier than the "pickup" command.) There are several examples of similar runs in the User's Guide, as well as the green book (see Chapters 12 and 13).

 

From: D.C. "Bear" McPhail

 

G'day Henry and everyone else,

The problem is interesting, and intrigues me because I am having trouble with basis species with sulphur. I made a successful calculation for a simplified version of Henry's problem using the following input:

 

Cl- 1 molal

Na+ 1 molal

swap CO3-- for HCO3-

CO3- 12.7 mmolal

pH 12.9

Ca++ 1e-5

react 0.1 mol CO2(g)

 

I used the NaCl to help with charge balance and the calcium to include simple carbonate minerals. I didn't bother picking up the fluid, as I set the calcium concentration to be undersaturated with respect to minerals. The results made sense, with calcite precipitating roughly half way through the reaction with CO2(g) and the pH dropping to approximately 10 at the end of the run.

I don't know if that helps Henry, but my simple example worked OK.

 

Now to my problem. I am trying to simulate reactions with thiosulphate and other sulphur species. I'm using a database of thermodynamic properties where SO4-- is the basis species, there are several redox couples with other sulphur species (not thiosulphate, though) and thiosulphate and other sulphur species in the aqueous species block. I've been trying to plot thiosulphate, polythionate and other sulphur species on activity-activity diagrams, but they are metastable with respect to sulphate species, as I think it should be. I'd like to suppress SO4--, but Act2 won't let me because it's a basis species, even after I swap a redox couple species (e.g., S4O6--) for SO4--. Unless I've missed something obvious, it seems my best bet is to rewrite the database in terms of S2O3-- as the basis species for sulphur. I'm not keen to do this because I wish to use a large database and it would take longer than I have to rewrite it. My main purpose is to simulate thiosulphate reactions in react, but I like to see activity-activity diagrams first.

So here's my list of questions. Have others confronted similar problems with trying to suppress basis species, and found simple ways around them, using GWB? Have others had some experience in calculating equilibria with thiosulphate and other sulphur complexes? I would be very interested in incorporating kinetics of reactions involving aqueous (and mineral and gas) sulphur species, too, and would appreciate comments people might have on this.

I'm glad to see people using the group list for questions. It's quite valuable to see the comments and questions, and as long as we're not inundated with gazillions of e-mails, this will be a good venue/format for all of us!

 

Cheers, Bear

 

 

From: Craig Bethke

 

Hi Bear,

 

Regarding the sulfur diagram question, I think you are confusing two

concepts.

 

If you make a redox-pH diagram for S2O3-- by swapping it for SO4--, it will

look the same as the diagram for SO4--. The reason is that in both

instances you are pulling in all of the sulfur-bearing species that

can be formed by redox, (de)protonation, and (de)hydration reactions.

 

(Of course, you need to be using a thermo dataset such as thermo.com.v8.r6+.dat

that contains thiosulfate.)

 

To look at thiosulfate species alone, you want to decouple S2O3--. In this

case, you simply diagram S2O3-- on redox/pH axes, without specifying an

activity for SO4--.

 

To carry this a little further, you might want to make a diagram containing

all sulfur species (HS-, H2S, S2O3--, etc., etc.) except the sulfate species.

To do this, you would like to decouple SO4-- from the other sulfur species

and then omit it from the diagram.

 

The complication is that in the thermo dataset SO4-- is a basis species,

not a redox species. To get around this problem, simply reverse the entries

for SO4-- and HS-. HS- becomes a basis species, SO4-- becomes a redox species,

and the redox coupling reaction for SO4-- is simply the reverse of the

old coupling reaction for HS-.

 

Now you diagram HS-, decouple SO4--, and you're set.

 

Hope this helps,

Craig

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