U_Eh-pH diagram problem

[kelvin]

Dear Tom:

 

I use the Act2 program(GWB 8.0.8) to draw the pe-pH diagram of UO2++.

 

The basic information are as follows:

 

UO2++(total) = 1.000000E-06 (M)

|

HCO3-(total) = 5.304055E-04 (M)

 

T=25C & 80C

 

The results are shown as attachment.

 

From the results, we can see when pH=8 and pe=0,

 

the predominant species is UO2.25 at 25C, and the predominant species is UO2.3333(beta) at 80C.

 

But when I use the React program to perform the speciation modeling, it shows:

 

UO2.3333(beta) ppt at pH=8 and pe=0 at T=25C,

 

UO2.6667 ppt at pH=8 and pe=0 at T=80C.

 

The results from React and Act2 are different. Why?

 

Could you help me to confirm these results. Thanks a lot.

 

Kelvin

[Tom Meuzelaar]

Hi Kelvin:

 

Can you attach your React and Act2 scripts?

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

[kelvin]

Dear Tom:

 

The React and Act2 scripts are as attachments.

 

Regards,

 

Kelvin

U06C_25.ac2

U06C_80.ac2

U06C_25.rea

U06C_80.rea

[Tom Meuzelaar]

Dear Kelvin:

 

In your React script after initial speciation, your Eh and pH are allowed to vary while supersaturated phases are precipitating, whereas they are fixed on the activity diagram. If you add a fixed Eh and pH to your model (see attached), they put you in the correct stability field.

[kelvin]

Dear Tom:

 

Thanks a lot. May I ask more question about this case?

 

When I run React after Basis entry, it shows the Error message "Don't know how to set charge balance:species Cl- is not in the bsais".

 

After balance on HCO3-, it dose not show the Error message but the Run pane shows "Residual too large".

 

Then I balance off HCO3-, it runs normally.

 

Could you tell me how to run React correctly in this case?

 

Regards,

 

Kelvin

[Tom Meuzelaar]

Hi Kelvin:

 

I'm not sure what you changed in your model. Can you attach the new script that is causing the problem?

 

Regards,

 

Tom

[kelvin]

Dear Tom:

 

I will attach the script that is causing the problem to you.

 

Thanks.

 

Regards,

 

Kelvin

U_06C_25_Problem.rea

[Tom Meuzelaar]

Hi Kelvin:

 

When you originally load the script, React asks you how you would like charge balance set. If you choose to not balance your water, simply select the 'balance off' option under the unit selector to the right of the HCO3- entry. This means that you are running your script with a slightly unbalanced water. Whether or not you choose to charge balance is at your discretion.

 

Regards,

 

Tom

[kelvin]

Dear Tom:

 

Thank you very much.

 

But in U06C_80.rea script, I am not sure whether I need to select the "extrapolate" option as U06C_80.act script.

 

When I intend to run all programs in GWB above or below 25C, the "extrapolate" option must be turn on. Is it correct?

 

Regards,

 

Kelvin

[Tom Meuzelaar]

Hi Kelvin:

 

Turn on the extrapolate setting if there are aqueous species, minerals or gases of interest in your model that do not have log K values defined in the temperature range you are working in.

 

Hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

[kelvin]

Dear Tom:

 

1.How can I know the aqueous species, minerals or gases of interest in my model that do not have log K values defined in the temperature range?

 

2.Whether the log K values provided by all thermo data are defined in 25C ?

 

Regards,

 

Kelvin

[Tom Meuzelaar]

Hi Kelvin:

 

Just look in the thermodynamic database that loads when you load one of the modules- the default database is thermo.dat (the Lawrence Livermore National Lab database). To open the database, in any module, choose the Run - View - ..\thermo.dat.

 

Have a look at the Appendix entitled Thermo Databases in the GWB Reference Manual to learn more about how the solubility data are defined over the temperature interval of the database.

 

Hope that helps,

 

Tom Meuzelaar

RockWare, Inc.