PhreeqcUsers Discussion Forum

Conceptual Models => Kinetics and rate controlling factors => Topic started by: justinninja1 on October 09, 2014, 01:09:46 PM

Title: SO2 Kinetics problem
Post by: justinninja1 on October 09, 2014, 01:09:46 PM
Hello,

I'm designing a model based on the kinetics of SO2 into water with various metal ions acting as a catalyst.

Since I am new to PhreeqC, I am starting off very basically and am just focusing on SO2 gas in pure water solution.

My aim is to plot the formation of Sulphate ions, SO42-, as a function of time. To do this, I understand I need to use the rates and kinetics keywords, and I have designed a statement which allows my SO2 to convert to SO3, but the problem is my kinetics stop the reaction there:

 RATES
Suox
 -start
 
   10 k = 1.4e5
   20 Sx = TOT("SuO3--")
   30 IF (Sx <= 0)then goto 200
   #30 k = 1.4e5
   40 rate = k*Sx
   100 moles = rate*TIME
   200 SAVE moles
 -end

 KINETICS
 Suox
        -formula Su      -1.0   O   -1.0    Sf +1.0
        -steps 100 400 #3100 10800 21600 5.04e4 8.64e4 1.728e5 1.728e5 1.728e5 1.728e5
        -step_divide 1e-4





Su is tetra-valent Sulphur (S+4) and Sf is hexa-valent Sulphur (S+6). I've removed the reactions which link S+4 with S+6 and am attempting to use the above statement to form my sulphate but the problem is that no SO4 will form:

Kinetics 1.   

   Time step: 100 seconds  (Incremented time: 100 seconds)

   Rate name       Delta Moles Total Moles   Reactant        Coefficient

   Suox             -0.000e+00   1.000e+00   O                        -1
                                             Sf                        1

This continues no matter what the time is, the SO3 ions are simply not being used up and I have no idea why this is no the case. I can see from the speciation at equilibrium that that have been formed with the reaction with water.

Any help would be appreciated, thank you.