Is it possible to modify the CD-MUSIC model and use it as the Basic Stern Model?

[igergian]

Hi all,

I am very new to PHREEQC and maybe my question is answered somewhere else, but with a quick search in the forum I did not find anything.

So what I want to do is use PHREEQC to implement a 1-pK Basic Stern Model. I saw that PHREEQC does not provide support for this model but it is already implemented the CD-MUSIC. I read the proposed example (http://www.hydrochemistry.eu/phreeqc.chm.exe) for this model and even though I might be completely wrong, if in the CD-MUSIC model the capacitance of the beta (β) layer is set to zero and the ion complexation is done in the d-layer instead of the beta (β) then it resembles the Basic Stern Model.
Is this completely wrong?

Thank you

[dlparkhurst]

I don't think you are completely wrong. However, it has been so long since I looked at the basic equations, I can't say whether you are completely right either. My guess is that you can choose parameters to come close, but would  need to work through the equations from Hiemstra and Van Riemsdijk to be sure.

I hesitate to mention, there is also an undocumented, untested constant capacitance model, -ccm C, in the SURFACE keyword. I am not sure if that is the same as a basic Stern model or not.

[igergian]

Thank you very much for your answer.

Actually the capacitance model (CM) it is not the same as the Basic Stern Model since the CM does not consider any electrostatic interactions between surface and ions. The Basic Stern Model is more like the Triple Layer Model (or the CD MUSIC), but instead of splitting the Stern Layer in 0-plane, β-plane and d-plane it only considers 0-plane and d-plane (where d-plane is the start of the diffuse layer).

In my previous post I included a link to an example and inside there was this image that explains more or less the CD-MUSIC model (uploaded image http://imgur.com/kgOjlIJ). As you can see the CD-music has this intermediate β-plane and it need an extra capacitance to be given (C2). This is where I base my suggestion of putting this capacitance equal to zero ( -capacitance 0.85 0    # C1 and C2 in F/m2 ).

So to model an anion adsoprtion it would resemble something like this below
BoehmOH-0.5 + H+ + NO3- = BoehmOH2NO3-0.5; log_k 7.42
    -cd_music 1 0 -1       

What do you think?

[Charlie]

Although the H+ and anions might not be in direct competition for sorption sites, I think you could try to separate the surface hydrolysis from the sorption reactions of the anion,  which will have different log K data.   
The amphoteric nature of the surfaces is often modelled with a 1pKa model.  Hope this is helpful.

[SlippinJimmy]

Hi,

I'm not sure if you have already found a solution to this, but I was looking to do this myself a few months ago and found a paper by Heberling et al. http://dx.doi.org/10.1016/j.apgeochem.2014.03.006 where they use PhreeqC to calculate a Basic Stern SCM for surface charge development on calcite. The paper has an abbreviated PhreeqC input file as an appendix which shows the settings that they use (the paper is also on Research Gate if you don't have institutional access). I cobbled together a modified version of this where I simulate a high pH titration process by manipulating the saturation index for Portlandite in an equilibrium_phases block (see attachment). I can't speak to the accuracy of it though since it is an undocumented use of the cd music functionality.

[Sen]

Dear All,

I'm very interested in the content of this post, but the attachment has expired. It is hoped that the contents in the attachment can be re-uploaded.

Best regards
Sen