PhreeqcUsers Discussion Forum
Processes => Oxidation and reduction equilibria => Topic started by: isaoxer on July 05, 2021, 12:31:28 PM

Dear reader,
I am doing some experimentation with PHREEQC and Freundlich isotherm for sorption purposes.
I used the provided example of Cadmium to build up my script.
My question is that I have been "playing" with the log k value in order to fit my lab results with the PHREEQC results. When I get the desired graph on PHREEQC, I calculate the n (here given n=0,722) from the log k equation in the Cd example [ log(KFreundlich, PHREEQC) = log(0.421) + (0.722  1) × log(112.4×10 6 )  log(10100) ].
My reasoning behind it is that the heterogeneity of my soil/sand might affect the equilibrium constant, but I do no know if this is acceptable, by this I mean chaging the log K values until the results of PHREEQC are in the same interval of my lab results.
Thank you for any insight you may share.
Best regards,
Isa

I don't have a problem with a sitespecific Freundlich isotherm. The quantity and sorptivity of organic matter and other surfaces can certainly vary from site to site.
The transfer value of your results is probably not high until there are enough similar studies to try to identify the underlying mechanisms for sorption.

Thank you for the quick answer. I understand better now.
I had another question, what makes the computed results different from the ones obtained in the lab ? and is it possible to still present in this case the results obtained in phreeqc ?
thank you

I thought you would be fitting an isotherm to the laboratory results, so I am not sure what you are asking. The isotherm would be a model for what is happening in the field. If it does not fit the field data, then you should consider why.

I am sorry, I might not have explained that well.
What I tried to do was to compute the sorption of Mn using the same conditions I had when I was doing my lab experiments.
the results I got from PHREEQC showed the same trend of increasing sorption. However, the concentration I obtained were 1 to 2 magnitude smaller than my lab concentrations.
Consequently, I tried change my LogK value that I used in phreeqc to see where my computed values fits my lab results. I was not very sure if doing this was correct or I should just use my computed values.

You know your objectives better than I do, but I would fit an isotherm (or surface complexation) to the lab data.