Kinetic reaction of CH2O

[Max1]

Hi all,

I'm trying to model the kinetics of a layer of organic matter (CH2O) that is used for reducing sulfate, S(6). But the results of modeling this layer with REACTION and KINETICS are very different. When using REACTION, almost all the S(6) is reduced to S(-2) whereas with KINETIC modeling a lot less of S(6) is reduced to S(-2) . I think I don't really understand the concept of the KINETICS modeling data block. This is what I got:

USE Solution 101
KINETICS 101-150
Organic_C
     -formula           CH2O    
     -m              0.001
     -m0              0.001
     -tol           1.e-8
     -steps            40 day in 50 steps
     -runge_kutta        6
     -cvode            false
     -bad_step_max        500
END

What is the reason only a small amount of S(6) is reduced?

Thanks, Max

PS, this is what I got for REACTION, otherwise:

USE Solution 101
REACTION 101-150                           
      CH2O 0.001      
END

[dlparkhurst]

With reaction, you definitely reacted 0.001 mol of organic matter. With KINETICS, it depends on the rate. My guess is that the reaction rate for kinetics is slow relative to the time you simulated. If you run the simulation longer do you get a greater amount of sulfate reduction? Provided the signs are right, the maximum amount of organic degradation by your KINETICS definition is 0.001 mol (as determined by M), but it may take a long time to use it all depending on the RATES definition.

[Max1]

I took the rate from the wateq database and modified so that it only reduces sulphate:

Organic_C
 -start
   1  rem      Additive Monod kinetics
   2  rem      Electron acceptors: SO4

   10 if (m <= 0) then goto 200
   20  mSO4 = tot("S(6)")
   30   rate = rate + 1.e-13*mSO4/(1.e-4 + mSO4)
   40  moles = rate * m * (m/m0) * time
   50 if (moles > m) then moles = m
   200 save moles
 -end

I also tried to change the timestep in my transport data block but this didn't give the results I was hoping for. I think I'm missing some information on the RATES data block. Is it perhaps because I put the data block in the wrong order?

[dlparkhurst]

As long as RATES is defined before or in the same simulation (before the same END), the order should not matter. Perhaps you should post a simple version of your file.

[Max1]

That is the case. I attached a simplified version of my file.

[dlparkhurst]

I think it is simply that your rate constant (1e-13) is very low. Your rate expression introduces about 3x10-9 moles of CH2O per year.

[Max1]

I get better results if I either: increase the amount of moles of CH2O, or increase the rate constant. But if I make the rate constant to large, it doesn't have any effect anymore. I could also change the time parameter, but how is this determined? By the time_steps in transport or the 'steps' in KINETICS?

[dlparkhurst]

You only have 0.001 mol to react (M in KINETICS). Once it is gone, the reaction stops.

The TRANSPORT time step will supersede the KINETICS time step.