Precipitation before kinetics

[luicho]

I pretend that after performing a kinetics reaction,  I'm trying to know how much of amount of a phase it is forms as a result of the redox reaction after some t time.


My model is like this:

     DATABASE C:\phreeqc\database\sit.dat
     SOLUTION 1
         temp      18
         pH        4
         pe        4
         redox     pe
         units     umol/kgw
         density   0.998
         Cr(6)     100
         Na        0.001 charge
          -water    1 # kg
   
      SAVE solution 1
      END
      USE solution 1

     RATES
        Cromato    #name of the rate
     -start
     10 k = parm(1)
     20 A0 = parm(2)
     30 C = TOT("Cr(6)")
     40 rate= k * (C^0.5) * (act("H+")^0.5) * (A0)
     50 moles =  rate*time
     60 save moles
      -end

    KINETICS 1
       Cromato    #name of the rate defined in RATERS
        -formula   Fe  1
        -m        1
        -m0       1
        -parms    9.083e-007 100
        -tol      1e-008
        -steps       21600 43200 # seconds
        -step_divide 1
       -runge_kutta 3
       -bad_step_max 500
     INCREMENTAL_REACTIONS True

     SAVE Solution 2
     END

    USE Solution 2
    EQUILIBRIUM_PHASES 1
       Ferryhydrite 0 0 #Fe(OH)3
       Ferrihydrite(am) 0 0 #Fe(OH)3 (am)
       Chromite  0 0


Is my model is correct?

Luis

[dlparkhurst]

That will work, but you could also include EQUILIBRIUM_PHASES before the END so that the minerals will precipitate during the KINETIC reaction.

[luicho]

Thanks Mr. Parkhurst,

That's will work for knowing the final amount of precipitation just after the kinetics had finished, right?

If I change the steps for a continuous time, for instance 86400 seconds, and I want to know how it is the formation of the precipitations during that time, how I can do that?

Luis

[dlparkhurst]

What you had will do the reactions sequentially. If you move the equilibrium phases before the end, the equilibrium with minerals will happen simultaneously with the kinetic reaction. The second way could affect the rate of the kinetic reaction, so the results will not necessarily be the same. Which is right depends on how you think the reactions occur, are they simultaneous or sequential?

[luicho]

Yes, I don't know what I was doing wrong but last night I changed the order and seemed to run fine. Now I see this comment and I assume that it finally works.

I changed the order as in the example in Exercise 6 of the manual.

thanks Mr. Parkhurst.