A beginner level question

[vala khaledi]

Dear All, 

I'm afraid you may find my question so simple but I am just started learning about PHREEQC. 

I was wondering if anyone could help me realize if nitrification/denitrification in PHREEQC is calculated, and if so how? is there any example only for the nitrogen chemical equation, so that I could pursue to understand? 

Many thanks in advance, 

Kind regards, 

Valeh Khaledi

 

[Maxim P. Yutkin]

Dear Vala,

Could you be more specific about the process you wish to model? Specifying the reaction and the process nature/constraints would be a good starting point.

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[Valeh Khaledi]

Thanks for replying to my message, yes, of course, the process I meant is the process of transforming nitrogen in NH4 format to nitrate and nitrite 2NH4+ + 3O2  →  2NO2– + 4H+ + 2H2O and  2NO2–   + O2    →   2NO3– in the soil. Plus the denitrification:  2NO3− + 10e− + 12H+    →    N2 + 6H2O. 

I have data from a shallow groundwater level site, and I need to capture the organic matter decomposition as nitrogen content more specifically as I mentioned in the equation. 

Cheers, Valeh 

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[Maxim P. Yutkin]

Dear Valeh,

PHREEQC uses different databases to model various reactions, including red-ox reactions. The default database phreeqc.dat contains some red-ox reactions that involve N(-3) and N(+5). I suggest you review them in this file and decide if they are suitable for you. If so, the it is just the matter of specifying concentration of the components. If you wish to have other reactions, you could check other databases that come with phreeqc or introduce them yourself (provided there are thermodynamic data).

On Fri, Sep 9, 2022, 12:43 vala khaledi <vkld...@gmail.com> wrote:

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[Maxim P. Yutkin]

Further to your question. Below is an example of a comproportionation red-ox reaction with nitrogen. You can run it in phreeqc using the standard database and examine the output. You will see that both N(5) andN(-3) were consumed and the solution now has a new species N(0) (which is N2) which is the product of the red-ox reaction. The reaction that happens here is the difference of the following half reactions specified in phreeqc database:

1) 2 NO3- + 12 H+ + 10 e- = N2 + 6 H2O

2) NO3- + 10 H+ + 8 e- = NH4+ + 3 H2O

 1 - 2 = NO3- + 2H+ + 2e + NH4+ = N2 + 3H2O

The only thing I am worried about here is that the resulting pH of the solution is 1.5 (starting is 5) while the reaction is supposed to consume H+. I hope you will be able to figure it out.

SOLUTION 1
    temp      25
    pH        5
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg

Na 250 charge
N(5) 250
N(-3) 100

REACTION 1 # this is needed to start red-ox reaction
      

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[Valeh Khaledi]

Dear Maxim, 

I truly appreciate your insightful and thorough answer. The main and most important focus for me was on redox reactions.  I will go through your description and tips, if more question comes up, I'll write again. 

Many thanks, I wish you a nice day, 

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