Hello dear respected researchers!
I'm studying the adsorption of a certain drug onto both activated charcoal (AC) and onto kaolin.
For adsorption on AC: plotting Ce vs Qe yielded a type I- shaped curve, plotting Ce vs Ce/Qe yielded a straight line with r-square=0.99, plotting 1/Ce vs 1/Qe yielded a straight line with r-square=0.91 while the r-square of plotting logCe vs logQe was 0.94. I thought that from such results I could confidently assume that adsorption follows Langmuir's isotherm.
While for adsorption onto kaolin: plotting Ce vs Qe yielded a curve that seems like a type VI stepwise curve, plotting Ce vs Ce/Qe didn't fit to a straight line with r-square=0.2, although plotting 1/Ce vs 1/Qe yielded a straight line with r-square=0.99 higher than the r-square of plotting Freundlich (logCe vs logQe) which was 0.98. How should I decide which isotherm fits better in this case? Does the data have to fit both curves (Ce vs Ce/Qe and 1/Ce vs 1/Qe) to say it follows Langmuir?
I've attached some curves to clarify the question. Appreciate your time and consideration.
Linearization or changing the isotherms equation adds error to the result.
So, it's recommended to use the original form of the isotherms.
Dear Heba, From the adsorption of the drug on kaolin, could it be that the drug has multiple binding modes to kaolin, and hence different affinity constants. Or that kaolin being more complex than AC has different adsorption sites (having different affinities) for the drug. Is the drug an amphiphilic compound, does it have a pKa etc...,. The plot is indeed suggestive for multilayer adsorption, but it may also reflect the formation of dimers or aggregates of the drug. I see that on kaolin you are using higher drug concentrations when compared to AC. Can you correlate this with spectroscopic evidence? You can also analyze the data for kaolin taking the first plateau (20mg/g) as a maximum. Then you can apply the langmuir and freundlich isotherms and decide which one best describes the data. If none are satisfactory you can for example try the combined langmuir-freundlich isoterm, this may give you an idea if cooperativity (multiple interactions)is involved. Kind regards, Mike
Hello dear respectful research.
Dear Heba, for take decision between Langmuir and freundlich isotherm equation in your study, look at the value of R square. if R-square have the value proximate one, which isotherm is good tho describe the phenomena between adsorbant and adsobat. but use the original form of langmuir equation.
Dear Heba, just looking at the shape of your isotherms, the adsorption of the drug onto your kaolin does not follow the Langmuir model. I assume that you have used a non-cationic drug, the derived adsorption mechanisms cannot be cation exchange. Therefore, the Freundlich model (you isotherm is linear, and one of the main factor controlling the adsorption magnitude seems to be the solid/liquid partition) is the best suitable to fit your data onto kaolin. One of the best evidence is that your Ce/qe value remain approximately stable, indicating that in this concentration range, your adsorbed ratio remain identical. The R square can be useful, but just looking at the shape of the isotherms you have already enough information to make your choice.
Regards
Dear Michael and Thomas, thank you very much for providing your expertise and insights into my inquiry. I totally appreciate it!
Just wanted to add that the pKa of the drug is 8.81 and it's weakly basic. The study was performed in 0.1N HCl.
