Fix_e- coupled with Calcite in equilibrium phases

[paznativ]

Hi all
I am trying to understand if there is a way to prevent the pe drop while calcualting the CCPP of calcite on wastewater.
The solution has high ammonium conc, making the Eq. Phases reaction problematic as it reduce the CO2 into CH4, making the CCPP inaccurate.
i tried setting the Fix_e- with many redox species with no luck, it always changes the pH dramatically making the CCPP negative - which is inaccurate as the initial SI of calcite is positive.
any ideas?
solution speicification and the script is below

thank you all in advance

Paz

PHASES
Fix_e-
    e- = e-
    log_k     0

SOLUTION 1 Supernatant wastewater
    temp      25
    pH        7.6
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 2550
    Ca        51.6
    Cl        130
    F         20
    K         24
    Mg        45
    N(-3)     600
    Na        391.69
    P         311
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    calcite   0 10
    Fix_e-    -4 O2(g)       10

[dlparkhurst]

If you have that much ammonia in your solution, I don't think you should try to make the pe higher. By fixing the pe, you are doing more harm than good. The ammonium will oxidize to N2 or NO3-, depending on the pe that you chose, which causes changes in pe and CCPP.

If you are considering the solution in a reducing environment, then just skip the pe adjustment.

If you are considering the effects of bringing this water to an oxidizing environment, then you should probably oxidize the N(-3) to N(5). Nitrification processes generally go from N(-3) to N(5).

If you want to make N(-3) inert, you can define it as Amm and use the Amm.dat database.

Finally, make sure you use the right units. Phreeqc.dat has N(-3) as N by default, Amm.dat uses NH3. If the you use the wrong gfw, there is a moderate difference, so use the right gram formula weight. These definitions would override the defaults.

Code: [Select]

N(-3) 600 as NH3
or
Amm 600 as N


[paznativ]

Dear Dr. Parkhurst
Thank you once again for the fast and helpful reply.
this solved my problem

Best

Paz