To get the Qmax, we have to identify which isotherm model it follows.
But before that, we have to have a set of kinetic study.
To have a set of kinetic study, we have to set an experiment with various concentrations of adsorbate.
So, is there any difference between Qmax calculated from a set of lower adsorbate concentrations and higher adsorbate concentration..?
Or will it finally come out with one constant value for a specific adsorbent that we used in the experiment..?
Thank you so much.
I suppose that you are talking about adsorption from liquid solution. In case of that there might be competition between solvent and solute (i.e benzene vs. Ethanol, please see http://pubs.rsc.org/en/Content/ArticleLanding/1952/JR/jr9520004123#!divAbstract, or Adsorption by Powders and Porous Solids, F. Rouquerol, et al., p. 133), but in case of i.e. dyes in H2O the competition should be minor.
On the other hand, to extrapolate the maximum adsorption capacity (by model fitting) you have to use set of various adsorbate concentrations, because amount adsorbed in equilibrium is controlled by concentration of adsorbate.
Yes, it's adsorption for liquid.
for example, if I use a set of adsorbate concentration ranging from 20 to 140 ppm, and another one is 1 to 5 ppm..? Will the Qmax affected?
The maximum Qx will be affected, subject to the degree of surface saturation. If a surface is already completely saturated with liquid molecules, the Qmax value will remain nearly unchanged. However, if vacant sites for adsorption on the surface is available, the adsorbate will be adsorbed more showing higher Qmax values.
If you study adsorption on activated carbon, ASTM standard may help you to design your experiment.
Standard Practice for Determination of Adsorptive Capacity of Activated Carbon
by Aqueous Phase Isotherm Technique
ftp://ftp.ecn.purdue.edu/olek/PTanikela/To%20Prof.%20Olek/ASTM%20standards/Adsorptive%20capacity%20of%20activated%20carbon%20D%203860.pdf
This may not the latest version...
You may not confuse between the adsorption capacity of the material (in general) and the adsorption capacity of your material in specific experimental conditions. If you did not reach saturation in low concentration, the calculated Qmax will be very speculative. The best way to calculate Qmax is to reach saturation by using appropriate amounts of adsorbates, that's why even if the used concentrations are often too high in comparison with environmental ones, numerous adsorption studies use "high" adsorbates amount.
Qmax is one of the two conditions to be satisfied for a given adsorbate/adsorbent system: SATURATION.
Saturation is achieved generally for high solute concentrations (or high partial pressures for gases). Qmax is not achieved in low solute concentrations. Qmax is a thermodynamic condition and is not affected by kinetics, i.e., is not dependent on time. A researcher can achieve it experimentally or estimate/calculate it through modelling (Langmuir, Tóth, Jensen-Seaton etc.). Different compositions will give you "Qeq" instead of Qmax. Qeq is the amount adsorbed in equilibrium with a given a composition.
The second condition is the Henry constant, achieved in the opposite way of Qmax: at very low concentrations.
Best regards!
This video indoduces how to compare experimental and calculated maximum adsorption capacity, qmax value in adsorption isotherm study easily! #aminulcheminnovation #AminulSir
https://youtu.be/GYCUOcpiUec
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