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# Activity O2 (Aq)

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I'm a novice using this program, and realize this may be a simple question. I'm trying to create a very generalized redox-pH diagram that will help me understand the effect of changes in dissolved oxygen concentrations on the mobilization of chromium in groundwater. Using estimated concentrations of Cr6+ in groundwater at our site, I created the redox-pH diagram below. The current [O2]aq concentration in groundwater at the site is 4 mg/L, and is estimated to change to 8 mg/L due to the use of air sparging technology to remediate a different groundwater contaminant.

Unfortunately, I don't have more information regarding the concentrations of other dissolved constituents. So I've calculated the activity of O2(aq) for 4 mg/L assuming a very general activity coefficient of ~0.5. This gives me an O2(aq) activity of 6.3*10^-5, or roughly -4.2 on the y-axis of the attached diagram [log activity-O2(aq)]. My question is: are my assumptions correct? I'm confused because my understanding is that dissolved oxygen in groundwater generally ranges from 0 to 20 mg/L. How is it possible to obtain a value for log activity-02 aq greater than -30?

Please let me know if I've made an obvious blunder, or if there is a better way to approach this problem.

Thanks very much.

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Hi,

A few things. Your assumption of an activity coefficient close to 0.5 for O2(aq) is not the best, although in the grand scheme of where that plots on your diagram (on a log scale), it is insignificant. Activity coefficients for electrically neutral, nonpolar species like O2(aq) actually go above 1 with incresing salinity, according to the "B-dot" activity model used within GWB programs. You may be familiar with the "salting out" effect in which gas solubility decreases with increasing salinity.

As for the range of log a O2(aq) values, keep in mind that any redox reaction can be written in terms of O2, whether or not it is actually involved in the reaction, or even whether there is actually O2 present at all. It is simply a measure of the redox state of the system, useful in understanding equilibrium models of real systems, and not necessarily a measure of the true dissolved O2 concentration.

I assume by greater than -30 you mean more negative (more reducing, less oxygen, etc.). This is possible, either in terms of actual concentration being very low, or in a more theoretical thermodynamic context in which the environment is very reducing.

You should look into Chapter 8 of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text for information about calculation of activity coefficients, and any thermodynamics or geochemistry text for a discussion of oxygen activity/fugacity and redox state. Chapter 12 in Greg Anderson's Thermodynamics of Natural Systems, for example, has some explantations you may find useful. Let me know if I haven't answered your questions.

Hope this helps,

Brian Farrell

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