Question about mass conservation in Phreeqc Calculation

[Christinali91]

Hi David,

I am working on a simple CO2-water case and use Phreeqc to calculate the CO2 solubility and species distribution under various pressure. The TOT_C is conserved pretty well as pressure changes. However, the TOT_O and TOT_H seem to slightly change as pressure increases, which I thought is not correct. In my understanding, the TOT(H) and TOT(O) should also be conserved in the aqueous phase since I don't have H2(g) and O2(g) in the gas phase. Do I miss something in the calculation? I've attached my excel sheet for the calculation and the Phreeqc code I use in the end.

I am also curious about (CO2)2. In some posts, you have explained this species as a correction of CO2 solubility. Is there a way to directly get total CO2 in the aqueous phase(CO2(aq)+(CO2)2)? Because I need that value in the aqueous for the transport equation.

Thanks so much for your help!

Best,
Christina

Code: [Select]

SOLUTION 1
-pressure  10  # atm
 temp      75.05
 
EQUILIBRIUM_PHASES 1
 CO2(g) 1  3 # 1000 atm
END


USE SOLUTION 1
USE EQUILIBRIUM_PHASES 1


SELECTED_OUTPUT
   -reset false
   -high_precision    true
   -molalities H+ OH- H2O CO2 (CO2)2 HCO3- CO3-2 H2 O2
   -file co2.dat
   USER_PUNCH
   -headings PRESSURE Fugacity PR_P_CO2 CO2_g TOT_water TOT_C
   10 PUNCH  PRESSURE
   20 PUNCH  10^SI("CO2(g)")
   30 PUNCH  PR_P("CO2(g)")
   40 PUNCH  EQUI("CO2(g)")
   50 PUNCH  TOT("water")
   60 PUNCH  [SYS("C(4)")]
END


[dlparkhurst]

TOT("H") is not conserved. TOT("H") is moles of H per kg water, and the mass of water may change (for example, CO2 + H2O = HCO3- + H+), even if moles of H in solution is constant.

Moles of H in the system is conserved. Because all of the hydrogen in your system is in the aqueous phase, moles of H is equal to TOTMOL("H") or TOT("H")*TOT("water"), where TOT("water") is kg water in the solution. You could also use SYS("H") to get the total number of moles of H in your system.

The number of moles of CO2 + 2*(CO2)2 is equal to

Code: [Select]

(MOL("CO2") + 2*MOL("(CO2)2"))*TOT("water")


[Christinali91]

Hi David,

Thanks so much for the timely reply.

I print out TOT("water") in SELETED_OUTPUT. It seems that the value of TOT("water") maintains the same for different pressure(shown in the excel sheet). Therefore, I think the value of TOT("water") should not affect the results of TOT("H").

Code: [Select]

SELECTED_OUTPUT
   -reset false
   -high_precision    true
   -molalities H+ OH- H2O CO2 (CO2)2 HCO3- CO3-2 H2 O2
   -file co2.dat
   USER_PUNCH
   -headings PRESSURE Fugacity PR_P_CO2 CO2_g TOT_water TOT_C
   10 PUNCH  PRESSURE
   20 PUNCH  10^SI("CO2(g)")
   30 PUNCH  PR_P("CO2(g)")
   40 PUNCH  EQUI("CO2(g)")
   50 PUNCH  TOT("water")
   60 PUNCH  [SYS("C(4)")]
END

Actually, my second question is about TOT("water"). I think the value of TOT("water") should change because of the reaction with CO2(e.g. CO2 + H2O = HCO3- + H+). However, the Phreeqc results show that the value of TOT("water") maintains the same for different pressure.

Is there something wrong with my Phreeqc case setup?

Thanks again!

Best,

[dlparkhurst]

Mass of water [TOT("water")] may be approximately conserved in your case, but it is not in general.

Here is an example of CO2 dissolution in a basic solution. As CO2 first dissolves, the reaction is CO2 + 2OH- = CO3-2 + H2O. If you look at the plots, mass of water [TOT("water")] increases, molality of H  [TOT("H")] decreases, but the moles of H [TOT("H")*TOT("water"), TOTMOL("H"), or SYS("H")] are constant.

Code: [Select]

SOLUTION
-units mol/kgw
pH 10 charge
Na 1
END

USER_GRAPH 1
    -headings               CO2 TOTMOL(H) TOT(H) CO2 (CO2)2 HCO3 CO3 TOT(water)
    -axis_titles            "CO2 dissolved, moles" "Moles" "Kg water"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y TOT("H")*TOT("water")
30 GRAPH_Y TOT("H")
40 GRAPH_sy TOT("water")
  -end
    -active                 true
USE solution 1
REACTION
CO2(g) 1
0.1 in 10
END