Site occupancies on exchanger phase

[Dave_S]

How do I set the occupancies on the (solid) exchanger phase when I'm using the ion exchange option in SpecE8 and React?

I can see how to set the total CEC and exchange constants, but not the site occupancies.

Any ideas?

[Tom Meuzelaar]

How do I set the occupancies on the (solid) exchanger phase when I'm using the ion exchange option in SpecE8 and React?

I can see how to set the total CEC and exchange constants, but not the site occupancies.

Any ideas?

Hi Dave:

Two ways to do this:

1. Set a non-reactive rock volume using the inert variable (Config - Variables menu). GWB will convert the volume to rock mass using a standard rock density of 2.65 g/cm3.

2. Define a reactive mineral mass either in your Basis setup, or make sure one precipitates during the course of a reaction path model. GWB will exchange mass between rock and fluid independent of mineral composition. That is, minerals that are not clays will exchange with the fluid if they are present in the system.

If you are using ionex.dat as a database template, GWB assumes that the sorbed cations are all Na+ cations. You can change this by re-writing the dataset. Refer to the instructions in ionex.dat for more detail.

Hope that helps,

Tom Meuzelaar

RockWare, Inc.

[Dave_S]

Hi Dave:

Two ways to do this:

1. Set a non-reactive rock volume using the inert variable (Config - Variables menu). GWB will convert the volume to rock mass using a standard rock density of 2.65 g/cm3.

2. Define a reactive mineral mass either in your Basis setup, or make sure one precipitates during the course of a reaction path model. GWB will exchange mass between rock and fluid independent of mineral composition. That is, minerals that are not clays will exchange with the fluid if they are present in the system.

If you are using ionex.dat as a database template, GWB assumes that the sorbed cations are all Na+ cations. You can change this by re-writing the dataset. Refer to the instructions in ionex.dat for more detail.

Hope that helps,

Tom Meuzelaar

RockWare, Inc.

Hi Tom

Many thanks for the pointers.

I think I understand what you mean, but it does seem a bit of an obtuse way of doing it!

Is this something that you might address in the next GWB update? I think a more transparent way of specifying the exchange composition would benefit a lot of users.

best regards

Dave

[Tom Meuzelaar]

Is this something that you might address in the next GWB update? I think a more transparent way of specifying the exchange composition would benefit a lot of users.

Hi Dave:

Thanks for the feedback. Do you have a recommendation for how the exchange composition should be specified?

Regards,

Tom

[Tom Meuzelaar]

Dave:

It was pointed out to me that your original question may have been referring to the equilibrium (or post-speciation) occupancy of cations on the clay. If this is the case, this is simply determined via equilibrium of the exchange sites with the fluid based on the selectivity coefficients for individual cations in the database, as well as the CEC and total rock mass.

Hope that clarifies things a bit more.

Regards,

Tom Meuzelaar

RockWare, Inc.

[Dave_S]

Hi Tom

Thanks for the additional information.

However, I'm still not entirely clear how to proceed with this.

For example, if I want to model exchange on smectite, which say has 4 exchange components (e.g. Na, K, Ca, Mg), do I need to have 4 versions of the IonEx database for each of the components, i.e. one each for Na, K, Ca and Mg?

It would be a lot easier to do this if the input requirements for GWB were like those in PHREEQC - presumably this would be easy to implement? Also, in PHREEQC you can calculate the exchanger composition in equilibrium with a given water composition, which is also a very useful feature.

Whilst I'm on the subject of GWB developments, I'm really surprised that there's no capability for modelling solid-solutions. Again, this capability is in PHREEQC, and as I recall, was available in EQ3/6 (albeit for ideal solid solutions) about 20 years ago. Is this something that you can include for the next version?

Many thanks for your help.

best regards

Dave

[Tom Meuzelaar]

Hi Dave:

No, you'll just need a single cation exchange database (use IonEx.dat as a template), and make sure that you have exchange reactions for Na <-> K, Na <-> Ca, and Na<->Mg etc. GWB will calculate the distribution of these exchanger sites based on equilibrium with the fluid.

As far as solid solutions are concerned, you can find a brief discussion and further references in this thread.

Hope that helps,

Tom