Using SpecE8

[Eden Chang]

Hi everyone,

 

I am very new to SpecE8, and I am trying to predict the speciation given a defined environment. You can see my parameters in the attached screenshot. My O2(g) is zero because I have an anoxic environment, but these input seem to not return any speciation, would anyone take a look what was wrong with my set up?

 

Thanks!

 

[Jia Wang]

Hello Eden,

The software solves a set of numerical equations by iteration to arrive at a stable solution for the equilibrium state. For solving problems numerically, the software does not like zeros for constraints. If the quantity of the constraint is very small, you can typically set a negligible value.

The constraint for O2(aq) is very important because it sets the oxidation state of your system. It would be best if you can set it using some information you can reasonably assume or measure directly. Do you have any information that you can use such as a Eh or pE measurement? If so, you can swap in the 'e-' for O2(aq) and provide the value to the corresponding unit. The software also allows you to use the concentrations of a redox pair to set the oxidation state for the whole system. For example, if you have Fe++ and Fe+++ concentrations you can add the basis species, Fe++, and swap the other species, Fe+++, for O2(aq).

You might already be aware of this but redox species are by default set in equilibrium in the GWB, swapping species of a different oxidation state into the basis does not set the specific concentration for that valence state. Concentrations are set for the bulk component. If you would like to specify concentrations of separate species in different oxidation states, you will need to decouple it first. Once you have done so, you can add the redox species to the basis. Please refer to section 7.3 Redox disequilibrium in the GWB Essentials Guide. More information regarding bulk concentrations vs. free concentrations, please see the example in section 7.2 Equilibrium models in the same guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC