Surface complexation model

[Ozan]

Hey,

I am trying to built a sorption database according to minerals and elements that I am interested in. I encounter so many surface complexation models such as; Constant Capacitance (CC), CD-MUSIC (CDM), Diffuse Double Layer (DDL), Extended Constant Capacitance (ECC), Ion Exchange (Iex), Non-electrostatic (NE), Triple Layer (TL), 1pK-Basic Stern (1pK-BS). I know that (from GWB Essentials) GWB can use two-layer surface complexation which includes constant capacitance, constant potential and Ion Exchange. My question is about DDL which is also double layer and I guess it is equal to constant potential. Is constant potential or DDL is a different model? Also, constant potential does not consider electrostatic interactions and NE also does not consider electrostatic interactions. So, is constant potential and NE are same models? I guess GWB does not model TL or CDM, could you please confirm that and give me more information if GWB can model any other surface complexation model. I am lost under many different names. I also checked out the book for these information but I couldn't find more. 

Here is the website I am using to find out the sorption data and these models (and explanation of models); https://www.hzdr.de/db/RES3T.login

Thanks!

Ozan

 

[Jia Wang]

Hello Ozan,

There are certainly a lot of different types of surface complexation models. Constant capacitance and constant potential are special cases of the two layer complexation (or diffuse double layer) models. If you set a constant value for the capacitance, either in the surface dataset header or in your input file, the programs use that to calculate surface potential directly from surface charge, rather than using the full two layer model. On the other hand, if you set a constant value for potential, the programs use this regardless of surface charge. In a further special case of the constant potential method, if you set potential to 0, you would be ignoring the electrostatic effects on sorption and thus the model is considered non-electrostatic.

You can refer to section 2.5 Two-Layer surface complexation model of the GWB Essentials Guide for more information on the surface complexation models available within GWB currently. Triple layer and CD-MUSIC model will be available in an upcoming release.

Hope this helps,

Jia Wang