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Eh-pH diagram for Pb


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Hi - I am looking to develop an Eh-pH diagram for lead speciation based on chloride, sulfate, and alkalinity (CaCO3). I have attached my GSS spreadsheet with the data for my site (as well as activities for the ionic species generated from SpecE8) and Act2 plotting file where I have been working to develop the speciation diagram. Our hypothesis is that based on the geochemistry of the site, our species should be mostly Pb2+ and anglesite, but so far I have only been able to get anglesite/galena OR Pb2+/galena on the plot (see the PDFs for these plots). I think that I need to adjust the activities of my ionic species (Pb and/or basis species) in some way, but I haven't been able to figure out that component of the model. If anyone has any thoughts or suggestions, I would really appreciate them. Thanks!






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I don't have experience with the plot overlay files, but I have a lot of experience with researching and doing plots for lead and copper solubility and mobility in potable water systems. The first two places I'd look at are that you need to assume a total lead(II) activity that's reaslistic for the diagram (log Pb++ = 0 in your .ac2 script) and also make sure the thermodynamic database has the right suite of aqueous and solid species in it for your system. We've had to make huge changes in the thermo data bases to get what we find are both thermodynamically and kinetically plausible phases for environmental systems, and the most realistic log K's. We're a lot more confident in the choices for the Pb-OH-CO3 system than the others. I've attached a PDF that shows a comparison of what I get with your script, and what I get for two useful types of diagrams with our own script from a somewhat similar geochemical system, though the chloride and sulfate levels in my example are probably a lot lower than in your system. I've also attached the thermodynamic database that we've used recently, and one of the scripts. Hope this helps....


--Mike Schock, USEPA, Water Supply & Water Resources Division, Cincinnati, OH.

thermo MS.dat

Lead diagram examples.pdf

Pb 10 DIC 18 Cl 35 SO4 96.ac2

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Hi GWB users,


Thanks for the reply Mike, you brought up a number of good points about Eh-pH diagrams and geochemical modeling in general. I'll expand just a little bit.


Keep in mind that activity diagrams like this assume chemical equilibrium. When a mineral is thermodynamically favored to form but kinetically limited, you can remove it from the database manually, or more easily (and less permanently) by suppressing it. Along those lines, thermodynamic databases are not perfect, and pertinent species or minerals may need to be added, equilibrium constants adjusted, etc.


An Eh-pH diagram is a simplified but powerful visual representation of a system. A speciation analysis can easily be done and should be viewed in conjunction with Eh-pH diagrams to better understand your system (concentration and activity of all species, not just the most predominant, as well as mineral saturation states). Using other diagrams, like the Pb++ solubility diagram Mike created, helps explain why certain minerals may or may not form under given conditions.


As for constraining the activity of your species, the best place to start is with the total analytical concentration in molal units. In dilute solutions the activity and molal concentration are similar, though this will not be true in higher ionic strength systems. After you get an idea of the important species in your system, you might choose to account for activity coefficient corrections.



Brian Farrell

Aqueous Solutions LLC

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