I have been using surface modified natural zeolites for the removal of diclofenac and ketoprofen.
As isotherm models, I have tried Langmuir, Sips and Toth isotherm model. In general, the last two have better goodness of fit, but in my opinion, obtained Qm is significantly higher than the values I have obtained experimentally. I don't have a perfect plateau, but a small increase in uptake for the higher concentrations. In some cases, Qm for Sips and Toth is 2-3 times higher comparing to Qm obtained with the Langmuir model. What should I take as more significant: goodness of fit (R2, AICc and BIC) or Qm prediction?
Sips and toth are not thermodynamically consistent as they do not reduce to henrys law at low concentrations. Since you have experimentally measured qmax, it would be good to fix that and fit the b value of the langmuir model.
Secondly try a dual site langmuir model that can be used for heterogeneity in adsorption.
Shreenath Krishnamurthy thank you for the answer and suggestions! I will try to use dual-site Langmuir, and afterwards I will share if it was successful.
I agree with Dr. Shreenath's answer that qmax would be good fit with the b value of the Langmuir model.