I'm conducting experiments on phosphate adsorption by silicate-polymer composites. The isotherm data was fiited to Dubinin-Radushkevic, Langmuire-Freundlich combined models and got totally diffrenet qo values (for DR-17, for LF-51) even R2 of both models > 0.98. Could someone give an explanation for this.
Thank you.
Dear Hirunika,
Physical interpretation of Langmuir is that sorbate forms a monolayer on your sorbent and this is best for homogeneous surface. In your case, a silicate-polymer composite, I believe has a heterogeneous surface, making it more the Freundlich isotherm more useable than Langmuir. Freundlich isotherm, however, is emprical - not much physics behind it (as far as i know!).
Also, Dubinin-Radushkevich isotherm is applicable for sorption on the heterogenous surface and is more physical than Freundlich. I would suggest you to drop Langmuir part and just plot with Freundlich, and see if you get closer values to what you get with Dubinin-Radushkevich. If you get closer, i would recommend using Dubinin-Radushkevich values, since it has more physics to it.
My interpretation from this combination of evidence is that you have (1) more likely physisorption rather than chemisorption based on Dubinin-Radushkevich; and (2) you have a heterogeneous surface of adsorbent, that's why Langmuir doesn't work.
I am attaching to you a systematization and short discriptions of various isotherms I used when studying sorption of arsenates by zeolites (look under "1.6. Sorption Isotherms"). Hope this helps
Chapter Use of Synthetic and Natural Zeolites Tailored for As(V) Sorption
Cheers,
Andrey
Kalpani Hirunika Wijesinghe, you are welcome! I am glad if it helps
Best Regards,
Andrey
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