Carbon dioxide ejection

[tianshiyu]

I'm a master student from china. Recently, I'm studying in Carbon dioxide ejection in urine. Urine usually contains a lot of carbonate. I'm trying to stripping CO2 by using H2SO4. My simulation is as follow:

SOLUTION 1
    temp      19
    pH        9.2
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    C(4)      250
    Cl        170
    K         46
    Na        83
    P         8.97
    S(6)      10
    N(-3)     550
    -water    1 # kg

SAVE solution 1
END

USE solution 1
REACTION 1
    H2SO4      1
    360 millimoles in 1000 steps

EQUILIBRIUM_PHASES 1
    CO2(g)    0 0
    Amm(g)    0 0
USER_GRAPH 1
    -headings               H2O C
    -axis_titles            "pH" "Concentration of C" ""
    -chart_title            CO2 stripping
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x -LA("H+")
20 GRAPH_Y TOT("C")
  -end
    -active                 true

END

The result show that when pH decrease below 6, the solution will contains very little total carbon, but this is different with what i did in my experimental. In the phenomenon of my experimental, though the pH decrease below 6 there still little bubble spilled out of the solution. I'm confused what cause the difference.
And I didn't see the equilibrium equation of H2CO3 in Amm. database.

Thank you for your continued help.

[dlparkhurst]

With Amm.dat, you should use Amm rather than N(-3) in the SOLUTION definition.

I think you intend the solution to equilibrate with atmospheric CO2(g), which means the EQUILIBRIUM_PHASES definition should be

CO2(g) -3.4 0

Similarly, the target saturation index for Amm(g) should not be 1 atm (log P(Amm(g) = 0). At high ph, there may be some loss of Amm(g) from solution, whereas at lower pH, Amm(aq) would be quite small.

The titration will convert CO3-2 to HCO3- to CO2(aq). The aqueous reactions should be quite fast. CO2 in the description of solution represents both CO2(aq) and H2CO3(aq); CO2(aq) is predominant. The loss of CO2 to the atmosphere may be a slower process that depends on stirring, surface area, temperature, etc. Bubbles do not necessarily form.

[tianshiyu]

Thanks a lot. I thank it is right that the experiment didn't reach to the equilibrium. and I never thought the gas phases equilibrium, I will Think about it.
And I have another question, can Phreeqc add organic matter in to reaction. Thank you very much.

[dlparkhurst]

Frequently, the formula CH2O is used to represent organic matter in REACTION or KINETICS.

When adding this reaction to solution, the carbon ends up as C(IV) if there are thermodynamically favorable oxidants in the solution (O2, NO3, SO4, and others), or it ends up as a combination of C(IV) and C(-IV) (methanogenesis).