Kinetic of Calcite Precipitation

[Hamed]

Dear all,

I am working on a model that considers calcite precipitation through a kinetic approach. In the model, Na2CO3 and CaCl2 solutions are added to a brine equilibrated with calcite, and the amount of Ca+2 precipitation and pH is modeled. In the experiments in the lab, some treatment on the surface of chalk can lead to more precipitation of calcite due to available nucleation sites. However, in my model, I will always reach a constant Ca precipitation at equilibrium, which is almost 3 mg. I wonder what I can change in the model to be able to play with the final amount of Ca precipitation to match my model with the experimental results. I tried to change the equilibrium constant but could not get a good result (maybe my model has a problem). Any help from your side would be greatly appreciated. The database I use is Pitzer or Phreeqc.

Code: [Select]

RATES
Calcite
-start
10 SR_mineral=SR("Calcite")
20 if (M<0) then goto 200
30 if (M=0 and SR_mineral<1) then goto 200
40 if (M0<=0) then SA=PARM(1) else SA=PARM(1)*(M/M0)^0.67
50 if (SA<=0) then SA=1
60 k_acid = 0.5*EXP(-14400/8.314*(1.0/TK-1.0/298.15))
70 k_neut = 1.55e-6*EXP(-23500/8.314*(1.0/TK-1.0/298.15))
80 k_base = 3.31e-4*EXP(-35400/8.314*(1.0/TK-1.0/298.15))
90 k_rateconst = k_acid*act("H+") + k_neut + k_base*act("H+")
100 r = k_rateconst * SA * (1-SR("Calcite"))*PARM(2)
150 moles = r * TIME
200 SAVE moles
-end

END

SOLUTION 1
-temperature 25
-pH  7  charge
-units   mol/kgw

Na   0.104
Cl   0.108
Ca   0.004
C    0.004 as HCO3

EQUILIBRIUM_PHASES 1
Calcite 0 0
SAVE solution 1
END

USE solution 1

REACTION 1
Na2CO3 1
CaCl2 1
75e-5 moles

KINETICS 1

Calcite
    -m        0.2
    -m0       0.2
    -parms    26.4  0.046  # correction factor
    -tol      1e-09

-steps 600 in 60 steps  #seconds
-runge_kutta  6

USER_GRAPH 1
    -headings               time pH
    -axis_titles            "Time(s)" "pH(-)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X (30 + TOTAL_TIME/60)
20 GRAPH_Y -La("H+")
  -end
    -active                 true

USER_GRAPH 2
    -headings               time ca
    -axis_titles            "Time(s)" "Ca Precipitated(mg)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X (30 + TOTAL_TIME/60)
20 GRAPH_Y KIN_DELTA("Calcite")*40*1000/10   #for 100 cc stock solution
  -end
    -active                 true
END

[dlparkhurst]

You should consider what is happening with CO2(g). If you change the P(CO2) of your solution, you will have a different end point.

For example, add the following.

Code: [Select]

EQUILIBRIUM_PHASES
CO2(g) -3.4 10