sorption of FeIII to cells

[flo]

Hi everyone,

 

I would like to model the sorption of Fe(III) species to a cell surfaces with different functional groups of which I know the adsorption constants. So I decided to start simple with a Kd model. Strangely enough I get quite obscure values that I know are wrong from experimental values.

 

The following questions arose:

Which species do I have to enter? Assume I use Fe+++ as sorbing species, one entry would look like

>Fe+++
    charge=  3.0       mole wt.=   55.8470
    1 species in reaction
      1.000 Fe+++
    Kd=   1.58E+6

Would I have to enter all other species relevant at the pH values as well, eg. FeOH++? Unfortunately I only have limited amounts of sorption Kds.

Thanks

 

Florian

[Tom Meuzelaar]

Hi everyone,

 

I would like to model the sorption of Fe(III) species to a cell surfaces with different functional groups of which I know the adsorption constants. So I decided to start simple with a Kd model. Strangely enough I get quite obscure values that I know are wrong from experimental values.

 

The following questions arose:

Which species do I have to enter? Assume I use Fe+++ as sorbing species, one entry would look like

>Fe+++
    charge=  3.0       mole wt.=   55.8470
    1 species in reaction
      1.000 Fe+++
    Kd=   1.58E+6

Would I have to enter all other species relevant at the pH values as well, eg. FeOH++? Unfortunately I only have limited amounts of sorption Kds.

Thanks

 

Florian

 

Hi Florian:

 

Indeed - one of the potential limitations of the Kd approach is the fact that one Kd value in the dataset represents a single aqueous species for a given chemical component. Look at the sorbed Sr++ concentration in the attached SpecE8 script. Next, change the pH to 13.5, run the script again, and compare sorbed Sr++ concentrations among the two output files. In the latter example, after speciation, much Sr is present as Sr(OH)+, and not available for partitioning.

 

I can think of two potential ways to approach your problem:

 

1. If you have the data, set up your Kd dataset with distribution coefficients for multiple Fe+++ aqueous species.
   
2. If you don't have the data, you could attempt to represent multiple aqueous species as one by Suppressing (Config menu) the species for which you don't have partition coefficients. This can obviously introduce great error as you are changing solution chemistry, but may be appropriate depending on how your distribution coefficient data was derived.
   

There are a few other issues to keep in mind with using the Kd modeling approach:

• GWB will partition solutes to any mineral mass present in your system, regardless of composition. If you do not define a mineral mass, no partitioning will occur. You can define a non-reactive mineral mass using the Config- Variables - Inert setting.
  
• The definition of the distribution coefficient in GWB is defined in terms of dissolved species activity, not concentration, and varies slightly from the general definition for Kd. For more detail, consult Craig Bethke's GBRM textbook, pp. 137-140.
  
• Be aware of the Exclude Sorbed Species setting, and its effect on your results. For more information, visit this forum thread.
  

 

I hope that helps,

 

Tom Meuzelaar

Rockware, Inc.