Bulk quantities and phase/activity diagrams

[Silvain Rafini]

Hi,

I'm taking my first steps with GWB, and I'd have some neophyte-questions, since I'm really not a specialist in geochemical modelling.

Bulk quantities. Could you just confirm that the quantities we input in React or other modules are bulk quantities as long as we don't specify "Free" in the left-button menu. In other words, it is the total amount of the element in water, all species included. This is the analytical concentration we would obtain from the lab. Thus, it shouldn't make any difference whether I enter a given element in any of its possible species, as long as I don't select "Free" (which would be rather appropriate if I know the species). Prior to any calculation, GWB will certainly equilibrate the speciation anyway... 

Other question (maybe even more neophyte-one) : it is still unclear to me what exactly is the difference between activity diagrams obtained with Act2 and phase diagrams obtained with Phase2. I guess this a matter of how it is built (pk approach versus modelling) but I still don't figure the precise implications.

Thank you very mich for your support.

Silvain

 

[Brian Farrell]

Hi Silvain,

As long as you don’t specify “free” or choose a unit that implies a free constraint (e.g. activity or pH), your constraint is for the bulk concentration of a component. To test this out, fire up SpecE8 and enter

HCO3- = .001 molal
pH = 6
Na+ = 1 molal
balance on Cl-
go

You can look in the text output file to see the concentrations of several carbon species: CO2(aq) has the highest concentration at .0005060 molal, then in decreasing abundance HCO3-, NaHCO3, CO3--, and NaCO3-. Add them up and you’ll find the molal concentrations will sum to .001 molal, which is the bulk constraint you supplied. Scroll down to “Original basis total moles” in the text file and you’ll see HCO3- = .001 moles, which matches your bulk constraint. You can of course look in the plot file as well. The “species concentration” variable type refers to all the individual aqueous species, and the “components in fluid” refers to bulk composition.  

You could alternatively use the input below and you’d get the same results. 

swap CO2(aq) for HCO3-
CO2(aq) = .001 molal
pH = 6
Na+ = 1 molal
balance on Cl-
go

Of course, if you use units like mg/kg you’ll have to account for the mole weight of whatever species is in your basis, or use the “as” setting to specify concentration in terms of mg Carbon, mg CO2, etc. 

Since we calculated the complete distribution of species, we can take our results from before and test out the “free” constraint setting:

swap CO2(aq) for HCO3-
CO2(aq) = .0005060 free molal
pH = 6
Na+ = 1 molal
balance on Cl-
go

Running this latest version should give equivalent results. 

For your second question:

Act2 calculates the simple type of diagram that geochemists have traditionally drawn by hand. By design, a number of simplifications make the calculation straightforward (but still laborious). For example, you can only have logarithmic axes (log activity, pH, pe or Eh, etc.). There’s no mass balance. You have to work in terms of activity, rather than concentration. The diagrams are in many cases fairly similar, though. If you want a general picture of how chemistry of a particular system works, a traditional calculation might be fine. Certain applications might demand a more rigorous solution. Beyond recreating activity or redox-pH diagrams, though, Phase2 calculates other diagrams that Act2 cannot even contemplate. You can include surface complexes in your calculations, for example. Or, you can diagram how various properties (solubilities, saturation indices, gas pressures, and so on) vary across the diagram using color maps or contours. You can account for isotope fractionation or kinetic reactions. You can plot assemblages (combinations) of stable minerals under different geochemical conditions. Basically, it has all the capabilities of React, so it's almost endlessly configurable.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC