Oxidative conditions

[Annr]

Hello!
I performed a sorption experiment under oxidizing conditions with NaClO added. Now I want to model my experimental data. To do this, I am trying to fix the pH and Eh.

SOLUTION 1
    temp      25
    pH        7
    pe        10.4
    redox     pe
    units     mol/l
    density   1
    Cl        0.1
    Na        0.1 charge
    -water    1 # kg

SURFACE 4
    -equilibrate with solution 1
    -sites DENSITY
    Get_s      0.045   161       0.2
    Get_w      2.265
SAVE surface 2
END
SOLUTION 2
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/l
    density   1
    Cl        0.1
    Na        0.1 charge
    Pu         1.5e-06
    -water    1 # kg

END

PHASES                                  # pH = constant
       Fix_H+
       H+ = H+
       log_k  0.0
      
       Fix_pe
         e- = e-
       log_k     0
   
USER_PUNCH
-headings
-start
...
-end

SELECTED_OUTPUT
    -file                ///.txt
    -reset                false
    -ph                   true
    -pe                   true

USE solution 2; USE surface 4; EQUILIBRIUM_PHASES 1;  Fix_H+ -7.0   HCl 10.0;  Fix_pe  -18.1 HClO  1; End
USE solution 2; USE surface 4; EQUILIBRIUM_PHASES 1; Fix_H+ -6.0      HCl 10.0;  Fix_pe -19.2 HClO  1; End

But it doesn't work...

Can you please tell me what I need to do?

[dlparkhurst]

First, you are not giving a complete input file. There are no definitions for Get_s and Get_w.

But the real problem is that you are setting pH and pe conditions that are outside the stability field of water.  For pH 6 and 7, the maximum pe should be about 14.8 and 13.8, respectively. Higher values make O2(aq) more stable than water.

In general, it is better to set a partial pressure of O2(g) than to try to specify a pe because of stepping outside the water stability field.

Code: [Select]


PHASES                                  # pH = constant
       Fix_H+
       H+ = H+
       log_k  0.0
     
       Fix_pe
         e- = e-
       log_k     0
END
SOLUTION 1
    temp      25
    pH        7
    pe        10.4
    redox     pe
    units     mol/l
    density   1
    Cl        0.1
    Na        0.1 charge
    -water    1 # kg
END
SOLUTION 2
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mol/l
    density   1
    Cl        0.1
    Na        0.1 charge
    Pu         1.5e-06
    -water    1 # kg

END
USE solution 2; 
EQUILIBRIUM_PHASES 1; 
Fix_H+ -7.0   HCl 10.0; 
#Fix_pe  -18.1 HClO  10;
O2(g) 0 10
End
USE solution 2;
EQUILIBRIUM_PHASES 1;
Fix_H+ -6.0      HCl 10.0; 
#Fix_pe -19.2 HClO  1;
O2(g) 0 10
End


[Annr]

Thank you for your reply!

I know that my experimental conditionis beyond the stability of water, but it is a real experimental data.
And in fact experiments under such conditions have been and are being done by other authors too.

So PHREEQCE cannot work outside of water stability?

[dlparkhurst]

By default, PHREEQC calculates redox equilibrium. In that ideal situation, your HClO will decompose to Cl- and O2. If you force HClO to be stable at a pH and pe, then all of the water will decompose.

You are operating in a metastable environment, where HClO is not reacting to thermodynamic equilibrium. PHREEQC can handle this situation, but you would have to define a non reactive HClO, something like Hclo in SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES. Then you could either leave Hclo unreactive with the rest of the O2 and Cl system, or slowly react it with KINETICS.

However, I doubt you need to do that. You just need to have conditions that keep Pu in the oxidized redox state for your SURFACE calculations. I think that should be within the stability field of water, so it is not critical to have an even more oxidizing state because it won't change the valence of the plutonium.

[Annr]

Thank you!