Basic Stern Model in PHREEQC

[nickjones]

Hi all,

I'm currently attempting to fit some acid-base titration, and batch sorption data of cobalt onto mixed spinel oxides using PHREEQC and PHREEPLOT.

I noticed another thread here that inquired about the implementation of a BSM model into PHREEQC by manipulation of the CD-MUSIC model. However, it was not resolved as to whether this would be mathematically correct, and I have a couple of questions.

To my understanding, the BSM is similar to the modified triple layer model (Hayes and Leckie), where specific sorption is located at the 0-plane, and outer-sphere complexation is located at the beta-plane, the difference being that the C2 capacitance is neglected and the d-plane potential is set equal to that of the beta plane in the BSM. There is therefore no specific adsorption at the d-plane, but it marks the closest approach of diffuse layer ions, similar to the generalized two-layer model.

A link to a paper in the previous thread was given where Heberling et al (http://dx.doi.org/10.1016/j.apgeochem.2014.03.006) used a modified version of the CD-MUSIC model to produce a BSM in PHREEQC, by setting the second capacitance very high, at 999.99.

My question arises from the fact that the TLM and BSM assume charged particles are point charges, and located on the specific planes - however in the CD-MUSIC, the charge of specifically adsorbed ions is divided between the surface and intermediate planes. Does PHREEQC place the charge of ions in this way? and if so, would that make the conversion of the CD-MUSIC to a BSM invalid?

Apologies if this is an obvious question, but I'm relatively new to the area.

Kind regards

Nick

[dlparkhurst]

I am not going to get into this too deeply because I no longer know the answers. However, the definitions in SOLUTION_SPECIES for CD_MUSIC species allow you to identify the location of the charge among the planes. You can place the charge completely in one plane. But I don't know if that is sufficient to reproduce a triple layer model.

[nickjones]

Thank you for your reply, I think that answers my question and should be sufficient for my model.

Kind regards,

Nick