Metal adsorption onto certain oxide surface using cd music

[zye4]

Hello, I was wondering how does PHREEQC recognize the different adsorbent surface. For example, in the code below, I am trying to model the adsorption of Na onto the MnO2 surface, I defined species and chemical reactions as well as K. So, how to let PHREEQC know the surface is MnO2? Thanks.

SURFACE_MASTER_SPECIES
   Surf_s Surf_sOH
SURFACE_SPECIES
   Surf_sOH = Surf_sOH
      log_k = 0
      -cd_music 0 0 0
       Surf_sOH = Surf_sO- + H+
      log_k  -7.5            
      -cd_music  -1 0 0
      Surf_sOH + Na+ = Surf_sONa + H+
            log_k  -1.9       
      -cd_music 0 1 0 # -1 +1 0

[dlparkhurst]

In the SURFACE definition you can relate the number of sites of surface to the amount of an EQUILIBRIUM_PHASE. If MnO2 is in the EQUILIBRIUM_PHASE, you can make the number of sites of Surf_s proportional to the moles of MnO2. So if MnO2 dissolves the amount of surface decreases.

[zye4]

Thank you for explaining to me regarding the sites.
I am also wondering how to relate the effect of ionic strength in the cd music. I was trying to run the simulation at different ionic strength (controlled by NaCl concentration). In the rage of 1e-3 mol/kg to 1 mol/kg I got the same set of surface parameters (sigma0, sigma1, sigma2, psi0, psi1, psi2) from the output. I believe the surface parameters supposed to change with the ionic strength.

[dlparkhurst]

The activities of the aqueous species must change with ionic strength, which must make some difference in the speciation and surface complexation. Depends on how much change you expect to see. An example is attached to run with wateq4f.dat.

[zye4]

Hello, it works well in the file you sent. But I couldn't find what is wrong with my transport code(attached). Could you help me with it?

Thank you.

[dlparkhurst]

You have 100 cells (If you are having problems, simplify.), 10 shifts, and punching (writing) results for cell 100. In 10 shifts, roughly the first 10 cells are affected, and cell 100 nearly not at all. If you look at results for cell 5, you will see a considerable change in surface properties. To see a change at cell 100, you need approximately 100 shifts or more.

[zye4]

I got it. Thank you so much.

[zye4]

Hello,

I was trying to output the sigma in the diffuse layer instead of "sigma2". But it gives me "0" if I use "sigmad". I was wondering what is the right language for "sigma, diffuse layer".

Thank you so much!

Sincerely,
Z

[dlparkhurst]

If you are using CD-MUSIC, there is charge at each of three planes, so "sigma", "sigma1", and "sigma2" give the charge densities at each of the planes. The sum would be the total charge density for the surface. For the diffuse double layer model, "sigma" would give the total charge density. The charge density in the diffuse layer would be equal and opposite.

You could also calculate the charge (and charge density) by using the function EDL_SPECIES and summing the moles of species times the charge of the species, which can be obtained with the function SPECIES_FORMULA. EDL_SPECIES can be used to get the area (and thickness) of the diffuse layer, so once the charge in the diffuse layer is calculated, you can determine the charge density.

[zye4]

Hello,

Thanks for the quick response. I'm using the CD-MUSIC, but why I always got "0" for sigma 2 in the output (showing below)?

Surface composition #section
Surf
         -1.646e-02  sigma, plane 0, C/m²
     5.620e-02  sigma, plane 1, C/m²
     0.000e+00  sigma, plane 2, C/m²
    -3.974e-02  sigma, diffuse layer, C/m²

Thanks,
Z

[Carlos Gallardo]

In the SURFACE definition you can relate the number of sites of surface to the amount of an EQUILIBRIUM_PHASE. If MnO2 is in the EQUILIBRIUM_PHASE, you can make the number of sites of Surf_s proportional to the moles of MnO2. So if MnO2 dissolves the amount of surface decreases.

Hi! How do you do that? I mean, make the number of sites of Surf_s proportional to the moles of MnO2?
Thank you!

[dlparkhurst]

See SURFACE in the user's manual. Note that there must be an EQUILIBRIUM_PHASES definition with the same user number as the SURFACE definition.

[Carlos Gallardo]

Thanks for the answer! But what happend when the solid phase appears for the mixing of two solutions? In my problem I know that goethite and Fe(OH)3 will appear, but if I use EQUILIBRIUM_PHASES for these solids my results change

[dlparkhurst]

You can define the initial moles of goethite present. Perhaps you want zero initially?

Code: [Select]

EQUILIBRIUM_PHASES 1
Goethite 0 0

The first zero represents the target saturation index, the second zero is the number of moles initially present.

[Carlos Gallardo]

Oh! I get it! That make sense. Thank you very much!!