Amount of water in the Diffuse Double Layer of WHAM organics?

[peterwadeuk]

Greetings good people.

A colleague drew my attention to a potential problem of interpretation of a component of the output when one runs the surface complexation model of the WHAM particulate organic model, e.g. in Example19b.

Input and output files are attached.

    The model is set up as Cd sorption on X, Hfo and OC in loamy soil.
    Using 1kgw and grams of soil given 0.5 saturation, 1.5g/cm3 bulk density

For each Reaction Step the output file yields the following information for the double layer of the organic phase (last step here):

"------------------------------Surface composition------------------------------

Diffuse Double Layer Surface-Complexation Model

H           
 4.475e-10  Surface + diffuse layer charge, eq
-4.519e-02  Surface charge, eq
-8.931e-03  sigma, C/m²
-5.815e-02  psi, V
 2.264e+00  -F*psi/RT
 9.617e+00  exp(-F*psi/RT)
 4.650e+04  specific area, m²/g
 4.882e+05  m² for   1.050e+01 g

Water in diffuse layer: 4.882e+00 kg, 99.8% of total DDL-water."

We are having problems with the "Water in diffuse layer: 4.882e+00 kg" because that implies that the water content of the DDL (4.882 litres) is much higher than in the entire  cell (1.000 litres).

Can anyone advise how to interpret this information?

All the best,
Peter

[John Mahoney]

I had this same effect drawn to my attention about five years ago for some uranium fits on humics based upon the WHAM model, of course after the report was completed. So there was little I could do or did do at the time but it did bother me.  SO sorry i do not have a solution.

[dlparkhurst]

You have defined a much greater surface area (4.8e5 m^2) than example 19b (1.7e2^2), thus the volume of the diffuse layer is greater (Area*thickness, default -donnan thickness is 1e-9), and the volume of water is much greater -- 4.8 kg versus 1.7e-3 kg.

It is a problem to have so much water in the diffuse layer. I think the surface area must be unreasonable for the volume of water (~1 L). Perhaps your specific area is per mol rather than per gram?

[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