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how to setup correct ph for soultions

Posted: Sun Mar 15, 2020 5:28 pm
by CAss
I am currently working on a project using Hydrus 1D and HP1 to model bioretention column during a rainfall event. I am currently having some issues regarding solute transport.
- pH value is not the same as I assigned. I coded 'pH 5.99 charge' for initial condition and it shows 11 in the output gunplot. There is KH2PO4 in the initial solution and I can only assign P in the initial solution. I think this where it went wrong, but I have no idea how to fix it.

- Chloride concentration is basically the same between inflow and outflow in the lab experiment. However it demonstrates a significant reduction in my model. I am not sure if I should add exchange phases into the code.

Re: how to setup correct ph for soultions

Posted: Sun Mar 15, 2020 5:50 pm
by Jirka
You need to use the keyword "charge", which will indicate the code, which species to adjust to get charge balance. J.

SOLUTION 1
pH 7 charge
Cl 0.01
Na 1

SOLUTION 2
Cl 1.3 charge
Na 2

In the first example, pH of this solution will be adapted to obtain charge balance => pH = 10.98. In the second example, Cl concentration will be adapted t to obtain charge balance at a pH of 7, => [Cl] = 2 10-3 mol/kgw

Re: how to setup correct ph for soultions

Posted: Tue Mar 17, 2020 5:32 am
by diederik
From the input files you have send, I noted that you use three valance states of N in your initial solution. The initial calculation shows that they are is no redox equilibrum which is kept during the initial calculation.
From the moment the transport simulations start, solution speciation is always calculated to attain redox equilibrium as well (phreeqc settings) and the pH is adapted to attain charge balance.
You can test what the pH would be for redox equilibrium by the statement:
MIX 1001
1001 1
end
The result of this batch calculation is shown in the phreeqc.out file

If you want to attain the disequilibrium, you should decouple the three redox states (an example is available for Fe in the phreeqc manual, example 9). For example, you define the master species Amm (already in the phreeqc.dat database, but disabled), Nitri, Nitra and corresponding solution species.
If you want to have the oxidation-reduction processes in you model, you should define rate equations (see also example 9).