Modeling incongruent dissolution of kaolinite

[taa18]

Hello,

I am trying to model incongruent/non stoichiometric dissolution of kaolinite where I have preferential release of AL over Si. The way I set up the model now with stoichiometric dissolution, I have a huge amount of Si in solution orders of magnitude higher than my experimental data but the model is capturing Al concentration in accordance with my experimental data. So I was wondering if there is any way to limit the Si dissolution or do a nonstoichiometric dissolution of kaolinite so that I can understand the phase assemblage of my experimental data. I tried adding gibbsite as a phase but it doesn't really help.

[dlparkhurst]

You could add quartz in EQUILIBRIUM_PHASES and adjust the target saturation index. Not sure how you would justify it. Maybe there are other secondary clay phases.

[dlparkhurst]

I'm not sure if there is an issue or not. It may simply be that your time increments are not capturing the reaction details. Here is a simulation with logarithmic time steps. There is fine detail for the early times as the reaction approaches equilibrium with gibbsite that is not evident at the final scale. You can restrict the time range to look at details.

Code: [Select]

RATES
    Kaolinite
-start
10 pk_H = 11.1249
20 pk_w = 13.9586
30 pk_OH = 10.8
40 rate = 10^-pk_H * ACT("H+")^0.51 + 10^-pk_w + 10^-pk_OH *ACT("OH-")^0.58
50 A0 = parm(1)
60 V = parm(2)
70 moles = (A0 / V) * (m/m0)^0.67 *rate* (1 -  SR("Kaolinite"))
80 save moles * time
-end
    Gibbsite
-start
1 A0 = parm(1)
2 V = parm(2)
3 rate = (A0 / V)*(m/m0)^0.67*10e-11*(1-SR("Gibbsite"))
4 save rate*time
-end
KNOBS
    -convergence_tolerance 1e-12
END
# Simulation 1
SOLUTION 1
KINETICS 1
Kaolinite
    -formula  Al2Si2O5(OH)4  1
    -m        0.25
    -m0       0.25
    -parms    1876.75 0.418
    -tol      1e-10
Gibbsite
    -formula  Al(OH)3  1
    -m        0.25
    -m0       0.25
    -parms    573 0.418
    -tol      1e-11
-steps       0.01 0.01 0.03 0.05 0.1 0.3 0.5 1 3 5 10 \
30 50 100 300 500 1000 3000 5000 10000 \
30000 50000 100000 300000 500000 1000000
-cvode true
INCREMENTAL_REACTIONS
USER_GRAPH 1
    -headings               Time Delta(Kaol) Delta(Gibb) SI(Kaol) SI(Gibb)
    -axis_titles            "Time" "Delta Moles" "SI"
    -chart_title            "M0(Gibbsite) = 0.25"
    -axis_scale x_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME
20 GRAPH_Y KIN_DELTA("Kaolinite"), KIN_DELTA("Gibbsite")
30 GRAPH_SY SI("Kaolinite"), SI("Gibbsite")
  -end
    -active                 true
END
USER_GRAPH 1
-detach
USER_GRAPH 2
-detach
END
# Simulation 2
SOLUTION 1
KINETICS 1
Kaolinite
    -formula  Al2Si2O5(OH)4  1
    -m        0.25
    -m0       0.25
    -parms    1876.75 0.418
    -tol      1e-10
Gibbsite
    -formula  Al(OH)3  1
    -m        1
    -m0       1
    -parms    573 0.418
    -tol      1e-11
-steps       0.01 0.01 0.03 0.05 0.1 0.3 0.5 1 3 5 10 \
30 50 100 300 500 1000 3000 5000 10000 \
30000 50000 100000 300000 500000 1000000
-cvode true
INCREMENTAL_REACTIONS
USER_GRAPH 1
    -headings               Time Delta(Kaol) Delta(Gibb) SI(Kaol) SI(Gibb)
    -axis_titles            "Time" "Delta Moles" "SI"
    -chart_title            "M0(Gibbsite) = 1.0"
    -axis_scale x_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME
20 GRAPH_Y KIN_DELTA("Kaolinite"), KIN_DELTA("Gibbsite")
30 GRAPH_SY SI("Kaolinite"), SI("Gibbsite")
  -end
    -active                 true
END


[taa18]

Okay, thank you SO much!