Solubility > Dissolution/precipitation

Dissolution of Montmorillonite, only occuring 1 cell at a time


Hello, I am new to PHREEQc and been having trouble. I looked in the manual and the forum and couldn't find an answer to the problem I am having.

I am trying to model the affect of pH in a clay column in terms of the clay dissolving. What my problem is, my model is only dissolving the clay in cell 1, and won't start dissolving the clay in cell 2 until cell 1 has completely dissolved. The pH changes in the rest of the cells, but the amount of clay does not. I feel like its because I have not set up the code correctly. One thing that I can think of that is possibly happening is that the material dissolved in cell 1 is not allowing the rest of the cells to dissolve because of equilibrium balance.
My questions are if the dissolution should happen one cell at a time? If it is not, what can I do to fix it?

Thank you very much for your time.

TITLE pH Study
#This is a commit

SOLUTION 0  MgCl2 #solution to be flowed into the cells at cell 1
units mmol/kgw #molality 
# default --> 1 kg of solution
temp 25.0
pH 1.0     charge      # need this pH to be around 0, 1 or 2.
pe 12.5    O2(g)   -0.68
H(1) 0.016              #0.016 should be the molility of 37% HCl solution
# need to add H(1) to it for solution to reach pH target.
Cl 10

SOLUTION 1-10  Initial solution for column
units mmol/kgw #molality
temp 25.0
pH 10.0     charge
pe 12.5    O2(g)   -0.68
#Cl 20 # helps with the other cells, but they are still not disolving. AKA just starts lower

Ca-Montmorillonite 0 0.1

#[name of mineral] [saturation index] [amount]
-force_equality false

TRANSPORT  # 1D simulation
-cells 10 #designates the number of cells in 1D
-lengths 10*0.01
-flow_direction forward
-shifts 100 #number of times solution is shifted to next higher cell
-time_step 1 #seconds
-initial_time 0
-punch_cells 1-10 
-punch_frequency 1

-file pH_precipTest_1.sel    #name of file where results are written
-reset false    # sets some options to false, see user manual
-step       # prints out the advection shift number of transport calculations
-distance true
-totals H(1) Cl Ca Si Al S #prints out concentration in molatity of list
-water true #prints mass of water, perhaps in Kg
-equilibrium_phases Ca-Montmorillonite #prints out two lines, moles of each phase, and moles transferred
-pH true

#this is to print out other custom stuff to output
-heading  Pore_vol
  10 PUNCH (STEP_NO + .5)/10

END # end of the first simulation

At the first step, the water in each cell equilibrates with clay. Subsequently, the infilling water equilibrates in cell 1 and then is advected through the rest of the column, so, yes, the water reacts in cell 1, and not much if any in the remaining cells. It is the result of using EQUILIBRIUM_PHASES to require equilibrium in each cell. The alternative is to use KINETICS so that the clay does not immediately equilibrate.


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