About kinetics on aqueous components

[Pak]

So I would like to reproduce the numerical benchmark of wissmeier & barry (2011), FEFLOW piChem Plug-in - DHI (2016), COMSOL(2008) or  packages SWMS-2D (Simunek et al., 1994) and HYDRUS-2D (Simunek et al., 1999).

There is which Aldicarb infiltrates into the domain.

Aldicarb suffers the following chain of reactions that I would like to reproduce in Phreeqc. All these reactions are first-order dc/dt =  k c

Aldicarb (A) --> Aldicarb Oxime (Ao)
Aldicarb (A) --> Aldicarb Sulfoxide (Asx)
Aldicarb Sulfoxide (Asx) --> Aldicarb Sulfoxide Oxime (Asxo)
Aldicarb Sulfoxide (Asx) --> Aldicarb Sulfone (Asn)
Aldicarb Sulfone (Asn) --> Aldicarb Sulfone Oxime (Asno)

I attach the database where the reactions rate can be found. (link to the source of the database --> https://infoscience.epfl.ch/record/143469)


So my problem is that it is not clear to me how to do kinetics with the values of the solution datablock.

For instance if I use the following script:

INCREMENTAL_REACTIONS true
solution   #solnum
#water
units   mol/L
pH 7 charge
A 0
Ao 0
Asx 0
Asxo 0
Asn 0
Asno 0

kinetics #solnum
R_1
-formula A -1 Asx 1
-tol 1e-10
R_2
-formula Asx -1  Asn 1
-tol 1e-10
R_3
-formula A -1  Ao 1
-tol 1e-10
R_4
-formula Asx -1 Asxo 1
-tol 1e-10
R_5
-formula Asn -1 Asno 1
-tol 1e-10

-steps 100 day in 100 steps

The output at the last time step will look like:

Kinetics 1.   

   Time step: 1 seconds  (Incremented time: 100 seconds)

   Rate name       Delta Moles Total Moles   Reactant        Coefficient

   R_1              -0.000e+00   1.000e+00   A                        -1
                                                              Asx                       1
   R_2              -0.000e+00   1.000e+00   Asn                       1
                                                              Asx                      -1
   R_3              -0.000e+00   1.000e+00   A                        -1
                                                              Ao                        1
   R_4              -0.000e+00   1.000e+00   Asx                      -1
                                                             Asxo                      1
   R_5              -0.000e+00   1.000e+00   Asn                      -1
                                                             Asno                      1

-----------------------------Solution composition------------------------------

   Elements           Molality       Moles

   Pure water     
 


So, I wonder why there is 1 mol. I know that if I do not give -m0 or -m in kinetics by default the value is one but I expected PHREEQC to take it from the solution, namely 0.

But for instance if I input:

INCREMENTAL_REACTIONS true
solution   #solnum
#water
units   mol/L
pH 7 charge
A 1
Ao 0
Asx 0
Asxo 0
Asn 0
Asno 0

kinetics #solnum
R_1
-formula A -1 Asx 1
-tol 1e-10
R_2
-formula Asx -1  Asn 1
-tol 1e-10
R_3
-formula A -1  Ao 1
-tol 1e-10
R_4
-formula Asx -1 Asxo 1
-tol 1e-10
R_5
-formula Asn -1 Asno 1
-tol 1e-10

-steps 100 day in 100 steps

and output the values of the solution elements and seem to be ok.


I wonder:

1) If I want to do for instance A --> B reaction with elements in solution and use the values of the solution how should I do it? Is it right what I applied here (i.e. second case)?

Thank you

[Pak]

Sorry for the long post I think the question can be reduced to how I can do a first-order ("decay") reaction where A and B are to components in the solution, and initially there is only A in the solution.

[dlparkhurst]

If you do not define M or M0 in the KINETICS data block, the default is 1 mol for each. The KINETICS data block is independent of any other definitions, including SOLUTION.

Your rate expressions use the function TOT(), which provides the total concentration of an element in solution. So, if you define all concentrations in solution as zero, you will get no kinetic reaction. If you set the concentration of A, then you will see the sequence of degradation products. So, I  think you have defined things correctly, provided you have a nonzero concentration of aldicarb in solution.

If you want to make the kinetic rate a function of the amount of kinetic reactant, then you would use the value of M rather than TOT(). This would make sense if you had a solid that was reacting with the solution.

[Pak]

Ok, thanks.