About the exchange_species error

[Jeonghwan Hwang]

Hello, this is Jeonghwan Hwang.
Firstly, thank you for reading.

I want to make the Cation exchange model between Ca-illite and Cs.
I made the code as below;

============================================
 EXCHANGE_MASTER_SPECIES
    Z   Z-
    D   D-
    X   X-
 
 EXCHANGE_SPECIES
    Z- = Z-
    log_k 0.0
    -davies
    Ca+2 + 2Z- = CaZ2
    log_k 0
    -davies
 
    D- = D-
    log_k 0.0
    -davies
    Ca+2 + 2D- = CaD2
    log_k 0
    -davies

 
    X- = X-
    log_k 0.0
    -davies
    Ca+2 + 2X- = CaX2
    log_k 0
    -davies

 
    2Cs+ + CaZ2 = 2CsZ + Ca+2
    log_k 15            
    -davies
    2Cs+ + CaD2 = 2CsD + Ca+2 
    log_k 5.4
    -davies
    2Cs+ + CaX2 = 2CsX + Ca+2
    log_k 5.4
    -davies
 end

 EXCHANGE 1
      CaZ2  0.000000145
      CaD2  0.000007245
      CaX2  0.000442
 end
============================================

Unfortunately, this model had an error as below;

============================================

ERROR: Coefficient of first species on rhs is not equal to 1.0.
ERROR: Equation for species CsD does not balance.
ERROR: Coefficient of first species on rhs is not equal to 1.0.
ERROR: Equation for species CsX does not balance.
ERROR: Coefficient of first species on rhs is not equal to 1.0.
ERROR: Equation for species CsZ does not balance.
ERROR: Calculations terminating due to input errors.

============================================

I think that the point of error is the sentences as below

============================================

    2Cs+ + CaZ2 = 2CsZ + Ca+2
    log_k 15            
    -davies
    2Cs+ + CaD2 = 2CsD + Ca+2 
    log_k 5.4
    -davies
    2Cs+ + CaX2 = 2CsX + Ca+2
    log_k 5.4
    -davies

============================================

However, I cannot understand why this error occurred...
I would appreciate it if anyone can answer this question.

Thank you.

Sincerely,

Jeonghwan Hwang

[dlparkhurst]

The first species on the right-hand-side (rhs) of the equation is the species that is defined by the reactions. PHREEQC is not smart enough to handle a coefficient for this species that is not 1.0. In the PHREEQC databases, the equation used would be

Cs+ + X- = CsX

with a log K for the exchange reaction Cs+ + NaX = CsX + Na+ and the definition that Na+ + X- = NaX has log K of zero.

You could rewrite the equation in other ways to have CsX  or Cs2X as the first species on the rhs, but you should consider your exchange convention. Please look at the following discussion from hydrochemistry.eu:

https://hydrochemistry.eu/pub/ap_pa02.pdf

[Jeonghwan Hwang]

Thank you.
I solved the problem with your help.
Have a nice day!

Sincerely,

Jeonghwan Hwang