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Input parameters of DDL model for Kd values calculation

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Dear Jiang Wang,

I would like to know the relationship of input parameters of DDL (dielectric double layer) of SCM (surface complex model) betwen Phreeqc and GWB. As shown in the arrachment (File nmae: Phreeqc DDL model input parameters), only Debye length is needed to input. However, if the constant capacitance is slected in the sorbing surface of GWB, I  don't know the relationship between Debye length (default value: 1.0E-08 m, for ionic strength=0.001) and capacitance. The potential data can be obtained from Phreeqc output (Phreeqc output_potential) and if the data was input in the GWB, the Kd values were almost the same between GWB and Phreeqc. I am curious about it and your response will be highly appreciated.


Polly Tsai


Phreeqc DDL model input parameters.jpg

Phreeqc output _potential.jpg

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Hi Polly,
I took a quick look at the PhreeqC approach to modeling surface complexation and it seems like the debye lengths input is used in the Donnan approach to calculate the composition of the diffuse layer. GWB does not use the Donnan approach. The program automatically assigns composition to the diffused layer. The surface complexation model ensures that reactions in the surface datasets are honored in the calculation.
For any given calculation, you can fix the surface potential in a two layer model in a calculation. If you do that with a surface potential value retrieved from the PhreeqC model, then you might get similar results from both models. Though, I would not recommend doing that as other inputs required between the two models may vary. I would recommend running a full two layer model without setting a constant potential. 
Best regards,
Jia Wang
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