Question about including secondary species in ion exchange modeling with PFLOTRAN

[chohyun park]

Hi all, I am currently working on modeling sorption reactions between minerals and radionuclides using PFLOTRAN. I am attempting to simulate sorption by utilizing both the ion exchange and surface complexation models within PFLOTRAN.

I have a question regarding the inclusion of strontium in the ion exchange model. Specifically, I would like to include both Sr²⁺ and SrOH⁺ as ion exchange species. Based on previous references, I understand that the ion exchange reactions may be expressed as follows:

• XNa + Sr²⁺ ⇌ X₂Sr + Na⁺

• XNa + SrOH⁺ ⇌ XSrOH⁺ + Na⁺

However, when I attempt to define selectivity coefficients for both Sr²⁺ and SrOH⁺ at a single exchange site, the model fails to run because SrOH⁺ is not recognized as a primary species.

In this case, is there a recommended way to include SrOH⁺ in the ion exchange model within PFLOTRAN?
Is it possible to account for this reaction, and if so, how should it be implemented?

I would be very grateful for any advice or guidance you could provide.

Thank you for your time and consideration.

Best regards,

Chohyun Park

[Hammond, Glenn E]

Chuhyun,

The cations in the the ion exchange reaction must be primary species. That would preclude you modeling SrOH+ as a complex. Please send the PRIMARY_SPECIES and SECONDARY_SPECIES blocks of your input file to confirm.

Glenn

From: pflotra...@googlegroups.com <pflotra...@googlegroups.com> on behalf of chohyun park <chgu...@gmail.com>
Date: Wednesday, July 16, 2025 at 10:44 PM
To: pflotran-users <pflotra...@googlegroups.com>
Subject: [pflotran-users: 8499] Question about including secondary species in ion exchange modeling with PFLOTRAN

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[chohyun park]

 Here is the list of primary and secondary species. As you can see, I defined Sr²⁺ as a primary species and Sr(OH)⁺ as a secondary species. Since I tried to include both ions in the ion exchange model, it seems that this is not allowed.  

CHEMISTRY
  PRIMARY_SPECIES
    H+
    Mg++
    K+
    Na+
    Al+++
    H4(SiO4)
    NO3-
    Cl-
    Ca++
    Fe++
    O2(aq)
    Sr++
  /
  SECONDARY_SPECIES  

    OH-
    Mg(OH)+ ! remove
    Na(OH) ! remove
    Al(OH)++ ! remove
    Al(OH)2+ ! remove
    Al(OH)3 ! remove
    Al(OH)4- ! remove
    H2(SiO4)-- ! remove
    H3(SiO4)- ! remove
    Si2O2(OH)5- ! remove
    Si2O3(OH)4-- ! remove
    Si3O5(OH)5--- ! remove
    Si3O6(OH)3--- ! remove
    Si4O6(OH)6-- ! remove
    Si4O7(OH)6---- ! remove
    Si4O8(OH)4---- ! remove
    Ca(H3SiO4)+ ! remove
    Ca(OH)+ ! remove
    Fe+++
    Fe(H3SiO4)++ ! remove
    Fe(OH)+ ! remove
    Fe(OH)++ ! remove
    Fe(OH)2 ! remove
    Fe(OH)2+ ! remove
    Fe(OH)3 ! remove
    Fe(OH)3- ! remove
    Fe(OH)4- ! remove
    Fe(OH)4-- ! remove
    Fe2(OH)2++++ ! remove
    Fe3(OH)4+++++ ! remove
    H2(aq) ! remove 
    Sr(OH)+
/

2025년 7월 19일 토요일 오전 2시 39분 48초 UTC+9에 Hammond, Glenn E님이 작성:

[Hammond, Glenn E]

Chohyun,

 

Thank you for the lists of species. You are correct that ion exchange does not support species listed as secondary aqueous complexes (SECONDARY_SPECIES). Only aqueous species listed under PRIMARY_SPECIES may participate in ion exchange reactions.

 

Glenn

 

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