PhreeqcUsers Discussion Forum

Conceptual Models => Design of conceptual models => Topic started by: peterwadeuk on June 18, 2018, 02:29:39 PM

Title: Balancing dissolved carbon with atmospheric CO2
Post by: peterwadeuk on June 18, 2018, 02:29:39 PM
Greetings all.

I am working with a real leachate from a planted pot experiment which was irrigated with urea, the latter nitrogen species being oxidised into nitrate by microorganisms.

I need to use this "leachate" in modelling experiments involving columns - the "leachate" has to be the input solution and needs to be realistic.

I have attached the file with the chemical analysis, slightly modified.

Neither alkalinity nor dissolved inorganic carbon were determined in the samples.

The original chemical analysis revealed a very high nitrate concentration. Much higher than a charge balance would tolerate, in the complete absence of dissolved carbon.

I constrained a charge balance on the nitrate species.

I believe I need to simulate the fact that the solution is in contact with air, which has a pCO2 of -3.39 (the pCO2 was actually a bit higher in the growth chambers in which the plant pot experiments were effected).

It seems that I cannot simulate the above situation by creating a nitrate-charge-balanced solution and contacting it with atmospheric CO2 using the equilibrium_phases method, because the pH is pulled down from 8.240 to 6.455. It also seems that I can't simulate the situation by creating a solution with charge-balanced-nitrate, and specifying C(4) 1e-3 CO2(g) -3.39 because carbon is added to the system at the expense of the charge balance, which is modified to "Percent error, 100*(Cat-|An|)/(Cat+|An|)  = -23.63".

I thought I was dealing with a solution at pH 8.240 where all potential bicarbonate had been titrated away by nitric acid formed when microbes converted urea to nitrate.

Am I missing something geochemically fundamental? Is the situation of pH 8.240 of a water in contact with atmospheric CO2 impossible, given the cation and anion assemblage that has been analysed? Is the chemical analysis suspect in more than an error overestimating nitrate concentration?

Thank you for your consideration.
All the best, Peter.
Title: Re: Balancing dissolved carbon with atmospheric CO2
Post by: dlparkhurst on June 18, 2018, 04:37:34 PM

The following seems closest to your description. It adds C(4) to attain atmospheric CO2 and charge balances with nitrate. The pH is maintained at 8.24, so there is more than a millimole of alkalinity that is generated to produce the specified pCO2 (at 8.24, the ratio of HCO3- to CO2 is about 100:1). The negative charge of the alkalinity causes nitrate to be less than 10 mg/L at charge balance.

So pH 8.24 is high enough to maintain significant alkalinity at atmospheric CO2. Simplest solution is to get a new complete analysis.

Code: [Select]
SOLUTION 1 Effluent, initially Pore waters in presence of basalt
    temp      25
    pH        8.24
    pe        12.471
    redox     pe
    units     mg/l
    density   1
Ca 32.400
N(5) 29.286 charge
 C(4) 1.0 CO2(g)    -3.39
Cl 6.774
S(6) 6.249
Na 4.235
Si 3.337
#S 2.175
Mg 1.520
F 1.029
K 1.349
Ba 0.737
N(3) 0.824
Fe 0.335
Zn 0.165
Sr 0.085
Al 0.065
Ti 0.007
Li 0.003
Mn 0.003
    -water    1 # kg

Title: Re: Balancing dissolved carbon with atmospheric CO2
Post by: peterwadeuk on June 18, 2018, 04:47:06 PM
Thank you David.

I did suspect the nitrate to be in analytical error.

All the best, Peter