We know that, Qe and Ce measured from adsorption experiments are fitted in several adsorption isotherms (Like Langmuir, Freundlich, Temkin, Hill, Dubinin Radushkevich etc) to understand the adsorption mechanism. My question is, in which condition the Qe and Ce values should be fitted to the isotherm? For adsorption experiment, one set has to be done keeping initial concentration of adsorbate fixed and varying the solid adsorbent dose. An optimal adsorbent dose can be observed in this series. Parallel to this, another set has to be done where the initial concentration of adsorbate is varied, to find out which concentration of adsorbate can result in maximum adsorption. In both of the sets, equilibrium is reached at a point. Now, which one is to be followed for isotherm fitting?
In my experiment, the first set shows best fit with Dubinin Radushkevich and second one fits the best with Temkin, although regression analysis differs very little with each other. I'm not being able to decide which one is to be taken. Different publications follow different and in fact, in most of the journals, it is not mentioned that which set they have considered
Adsorption isotherm nothing but varying the C0 (initial concentration) fixed time interval nothing but equilibrium time. Fixed temperature. Also fixed dosage
Both are correct, varying initial adsorbate concentration or adsorbent concentration. Both are accepted. Most often varying adsorbate concentrstion is seen in research papers. Difference in best fit isotherm with change in condition is obvious.
Adsorption isotherm experiment is done to find out maximum adsorption capacity of an unit mass of sorbent towards a particular sorbate i.e. qmax value(mg/g). So the experiment is done by varying initial concentration of the sorbate keeping the quantity of the sorbent fixed. Of course for convenience sorbent dose is optimized before isotherm experimentation. The procedure is mentioned in any standard literature.
In principle when you have a monomolecular pure sorbet and a purified sorbent the way to measure the sorption should not affect the result. The adsorption model used does not tell you anything about the cause of your problem, and is therefore irrelevant.
When may you expect a difference.
1. When you vary the amount of sorbent that is not completely purified
you also vary the amount of impurities that are carried (bound) by the sorbent and the impurities may affect the sorption of your prime component.
When your vary the amount of adsorbate and the adsorbate is not completely pure
you also vary the impurity level; also this has consequences for the measured sorbed amount.
The area to volume ratio is therefore important when either sorbent or adsorbate are not pure
2. A special sample characteristic might also affect the result. When your adsorbate is in fact a mixture of similar molecules with a polymer sample, preferential adsorption may occur of say the high molar mass fraction. In this case varying the sorbent concentration changes the area to volume ratio: the more area available the the stronger the fractionation. A similar situation occurs with an adsorbate that is a mixture of amphipolar molecules.; also in this case the area to volume ratio is important.
3. With porous solids the kinetics are also important and the sorption may become a function of the equilibration time. With more sorbent the concentration or pressure reduces quicker , but a low concentration or pressure slows down the rate of adsorption. Therefore, with different sorbent concentrations different equilibration times may apply.
I hope that this gives you a clue to your problem.
Luuk Koopal
You can start by varying the initial concentration while every other parameters is fixed, taking note of the optimum concentration. You can as well resume by varying adsorbent dosage too.
Dear Sayantan for fitting any model equation observe the model equation and find out which factor is independent variable and assign it to x-axis and the one which is dependent variable has to be taken to the y-axis. In your case initial concentration is independent variable so vary initial conc and fix the amount of adsorbent dose at constant temperature and develop equilibrium relation.
- Badges
- Science topic
- Similar topics
- Correction