Errors encountered while modelling in "React" using species activities

[BC_IITB]

Hi,

I am modelling a hypothetical ground water system (an ideal system used in published literature to study carnotite stability) and I have generated carnotite stability diagram as attached (Carnotite_Eh_pH.png).

Now I am interested in modelling evolution of this system when it rises to the surface (i.e. evaporation, loss of CO2 etc). However the stability diagram was generated using species activities, while in the react program modelling is best performed using species concentrations.

I tried modelling using the species activities but the program gave errors like large residual errors for certain species or that initial solution is too saturated with carnotite or uraninite (I tried changing some parameters alternatively to correct for the error, but in vain). Moreover charge balance was also not successfully acheived. I suppose that an alternative would be to calculate concentrations from activities by using the formula a_i = y_im_i; where a_i = activity, y_i = activity coefficient and m_i = molality. However on referring to Bethke (2007) I understood that, in GWB, for dilute solutions "y_i" approaches to unity and hence "a_i" becomes equivalent to "m_i".

I am a bit skeptical to use the activity values as molarity values directly in React program. Hence kindly suggest a rational way of going about this. I have attached the .rea file for reference (Carnotite_React_GWB.rea). Please have a look at it.

My primary objective is to model carnotite precipitation from this system (see attached figure pH_vs_Mnrl_satrtn.png) but unfortunately carnotite mineral does not reach saturation or the error message that initial system in too saturated with carnotite appears.

Hoping to receive helpful comments and suggestions.

Many thanks,

BC

Carnotite_React_GWB.rea