i am using Langmuir and Freundlich isotherm for each of the following:
1. effect of pH
2. effect of contact time
3. effect of adsorbent dose
is it possible that one isotherm fit with some adsorption data and doesn't fit with other adsorption data for same characteristic (e.g contact time)? and what is the minimum acceptable correlation coefficient for Langmuir and Freundlich isotherm?
Kindly find the attached excel which shows the Langmuir isotherm for contact time effect.
This is true. Not all obtained data will fit to a given adsorption model. Depending on the linearity test of the obtained curves, its highly recommended to fit the data in other models to determine which fits best. Kindly refer to my article "Modeling adsorption data for chlorothalonil unto Nairobi river sediment"
Mr.James,
should i have one isotherm that best fits with overall adsorption data? or i can identify best isotherm for each characteristic like (pH, contact time and adsorbent dose)?
Typically, for a given model system, one isotherm fits all the data with reasonable accuracy. Fitting experimental results to existing models is a purely mathematical exercise though, and if your experimental conditions change drastically, you may see a different trend. It does not always means a change in adsorption mechanism, it only implies a deviation from the linear range. Since Langmuir and Freundlich usually can fit the same set of data with relatively small variation in regression coefficient. It is possible that your data may be going either way with changes in operating conditions.
I looked at your data, and they do not make sense. Did you do mass balance? Did you repeat the experiments in triplicates? Besides, (0,0) value can not be used for model fitting. Why take only 3 points for model fitting? How do you know if deviation is because of one bad data point? You should try to get more equilibrium data. Moreover, what is contact time for equilibrium? Effects of contact time is usually a plot of conc vs time, it does not mean you fit model to data, which are not at equilibrium.
Dear Al-Balushi,
Typically, Freundlich and Langmuir adsorpion isotherms are used to study the effect of solute concentration onto adsorbent. The same equation form would not hold good, if your interest is to see the effect of other variables. To study the effect of time different adsorption kinetic equations are available in the literature where you have to keep solute concentration same but vary the contact period and then fit your data to kinetic equations of zero, first order, second order equation and so on. Kinetic models are available to verify whether film diffusion or particle diffusion mechanism is applicable to a given case. To study the effect of pH, you may alter the pH by adding varying amount of acid and alkali and keep the total solution volume, solute concentration same. You may consult one of our recent publication on kinetics of fipronil sorption onto soils published in Bull Environ Contamination Toxicology (2014) 93: 758-763.
With best wishes!!
No sir. All data should fit to a given model. I mean, pH, contact time and adsorbent dose should fit to one model with high regression values. Say for example, if its Langmuir, it should fit for all your variables. Kindly refer to https://www.academia.edu/23362760/Modeling_of_Experimental_Adsorption_Isotherm_Data_for_chlorothalonil_by_Nairobi_river_sediment or http://erepository.uonbi.ac.ke/handle/11295/77776
Dear Kawthar,
I have checked your Excel file. Langmuir and Freundlich isotherm models are for equilibrium data. I mean you should determine the time to reach equilibrium firstly. You should measure the effect of time on the adsorbate concentration and see when the adsorption practically finishes (actually reaches equilibrium). You will see the concentration will not (practically) change at a time which is the equilibrium time. And then you can make your measurements for "the effect of concentration" at this time (so you will have the equilibrium data) and then apply the Langmuir/Freundlich isotherm models. I hope this will be helpful.
I fully agree with the suggestion Dr. Salihi. You must go for determining the necessary equilibrium time by carrying out the kinetic studies on adsorption and then use that period for studying the effect of concentration and pH.
These videos can answer your questions: https://www.youtube.com/channel/UC5MiS3nMoji95OZ7uE1V8qw/playlists?view=50&sort=dd&shelf_id=6