Dear Colleagues, I was trying to rationalize the fact that in the adsorption experiments, you always observe the adsorption capacity (q, mg/g) decreasing as you increase the adsorbent dose (g/L). How would this be explained at the molecular level?
The increase in the adsorbent dose might cause aggregation of adsorbent, and consequently, the avaibale adsorption sites might decrease as well due to the adsoprtion density.
Try to change the pH and investigate its effect.
Motasem Saidan is right upto an extent.
It occurs so because the adsorptive capacity of adsorbent available was not fully utilized at a higher adsorbent dosage in comparison to lower adsorbent dosage.Therefore, it might be possible that adsorption capacity decreases as adsorbent dosage increases. In such conditions it is fruitful to consider changing process parameters such as temp or ph. Since proces conditions may noy allow change in ph , experiments can be performed at different T and consequently the results may be observed.
And when I have very low concentration of pollutant? I have in solution (for example) dye with mmol concentration and I put some adsorbent into solution, I've observed zero efficiency of adsorption. In this case is dosage of adsorbent not important. Why?
Is this because this dye is into stronger interaction with water?
It seems there are two terms which create the confusion:
- Adsorption amount
- Adsorption capacity
Although units of both the above terms are same (e.g. mg/g), they are not always the same. The former could be variable but the latter remains constant.
Adsorption amount=((Initial Conc. - Final conc.)*Volume) /Adsorbent's mass
Adsorption amount can be calculated by the above equation while the adsorption capacity is estimated by some isotherm model (e.g. langmuir).
In conclusion, adsorption amount (not capacity) decreases by increasing the adsorbent dose simply because they have an inverse relationship as can be seen in the aforementioned equation.
The increase in the adsorbent dose may cause aggregation of adsorbent, and consequently, the available adsorption sites may decrease as well due to the adsorption density.
Thank you all for these comments!
The correct term in the question should have been "adsorption amount" as stated by Musharib Khan.
Working with a material that this effect is very evident, reducing the mass of the adsovente with raise the capacity of adsorption. Among the possibilities previously mentioned here, I add the possibility of being a material with heterogeneous distribution of active sites. As a consequence, increasing the mass does not give me an increase in the adsorption capacity.
My issue is related to removal efficiency, why does removal efficiency decreases with increase in adsorbent dosage at fixed adsorbate concentration in some cases?
because the adsorption capacity of adsorbent available was not fully utilized at a higher adsorbent dosage in comparison to lower adsorbent dosage .
how to calculate adsorption capacity?.
I have de Adsorption amount
*Estefany Thirsa Chavarria Quicaño
Maximum adsorption capacity, qm (mg/g) can be calculated by Langmuir Isotherm model. You can find the related research items by clicking the following link:
Mohammad Kashif Uddin
One of the factors that affect any chemical equilibrium is concentration. Therefore, the relationship between analyte concentration and sorption sites is also a chemical equilibrium phenomenon. There is a difference between sorption capacity (related to the maximum amount that an adsorbent adsorbs) and the amount adsorbed (determined by the equilibrium ratio to be achieved with the concentrations of the analytes and sorption sites available in the experiment). Therefore, when performing isothermal studies, I always work with a single concentration solution of the analyte and I conduct the experiment until the saturation of the adsorbent to determine the experimental sorption capacity (Labuto, Georgia; CARDONA, DS; DEBS, KB; Imamura, AR; Bezerra , KCH, CARRILHO, ENVM; FERREIRA, PSH Low-cost agroindustrial biomasses and ferromagnetic bionanocomposites to cleanup textile effluents (DESALINATION AND WATER TREATMENT (ONLINE), v. 12, pp. 80-89, 2018.) Thus, the theoretical model must reproduce the experimental sorption capacity experimentally obtained to be adequate to describe the experimental data.
The adsorption capacity increased after the dosage of the adsorbent is increased due to the greater accessibility of surface binding sites with the increased dosage of adsorbent. This can be explained by increasing of the available sites with increasing adsorbent dosage for the interaction with the target molecule in solution, and lead in increases in the removal efficiency of the target.
The amount of adsorption in equilibrium (qe), describes: the adsorbate mg / g of adsorbent, when you increase the amount of adsorbent it will logically decrease qe, because it is directly proportional to the concentration of adsorbate in equilibrium (Ce, which is the concentration of final adsorbate in solution after adsorption If you make a graph of the percentage of adsorption Vs the amount of adsorbent you will see that as the amount of adsorbent increases the percentage of adsorption increases.
@ Estefany Thirsa Chavarria Quicaño And why in some cases adsorption capacity remains constant after some point even if we increase adsorbent dose?
The amount adsorbed and the adsorption capacity are two different terms that can sometimes be used interchangeably but are not the same in meaning.
The amount adsorbed is variable while the adsorption capacity of any adsorbent material is constant.
I have observed that this is a common mistake for many researchers.
At the start of an adsorption process, there are several vacant adsorption sites on the adsorbent. But as adsorption progresses vacant adsorption sites are occupied progressively. Consequently, there are fewer vacant adsorption sites for adsorbates to get attached to.
DearTero Luukkonen
the equation qe = (Ci - Ce)*(V/m) gives the adsorption capacity at equilibrium for your experimental data. Now in your equation, the adsorption capacity at a particular mass decreases as the adsorbent mass increases. Has to do with the proportionality of the capacity and the mass of the adsorbent. This is because when more mass is added more adsorption sites are added and the amount of adsorbate remains constant.
Regards,
Themba Dominic Ntuli
For high degrees of adsorptive removal, the above arguments are correct. However, decreasing and increasing trends can also be found for removal degrees far below 100% in water solutions. If this is your case, my advice is: check the isoelectric point of your adsorbent/adsorptive system and the pH change with catalyst dosage. If adsorption is due to the electrostatic attraction between the adsorptive and the acid/base sites of the adsorbent sites, the concentration of these sites is proprotional to the catalyst dosage; increasing or decreasing the catalyst dosage will therefore alter the pH of the aqueous medium if the sites have a Brønsted character. The pH change will finally modify the acid/base sites distribution on the adsorbent surface. An example of both increasing and decreasing adsorption trends due to this effect with the same adsorbent (P25) can be found in:
Article Titanium dioxide: A heterogeneous catalyst for dark peroxida...
See below an extract of this work:
(3) Effect of the catalyst dosage on the surface site distribution. The decrease of MB adsorption capacity with the increase of catalyst dosage is usually ascribed to two main factors: (i) the agglomeration of the TiO2 particles [62] and (ii) the abundant active sites available for adsorbate sorption when the adsorbent concentration is increased while keeping the concentration of the adsorbate constant [63]. In our opinion none of these arguments has been adequately substantiated. A search through the bibliography has yielded some information on the MB adsorption capacity of a standard material such as P25 at different dosages [13,61,[64], [65], [66], [67], [68], [69]]. In conducting the review, special care has been taken in selecting the conditions for which the fraction of MB removed from solution (XMB) at the highest MB concentrations were low. Fig. S11 displays the P25 dosage – adsorption capacity data that show an evident decreasing trend. The low XMB values (indicated in the figure) thrust aside the active sites availability as the potential cause of the trend. Agglomeration might still be a factor for the trend depicted in Fig. S11, but we believe that the main cause is the decrease of the solution pH on increasing the catalyst dosage; the same cause that produces the reverse trend for H2O2 adsorption (see above). The acid MB dye is adsorbed by electrostatic attraction on negatively charged centers of the catalyst surface [60]. These centers are essentially hydroxide anions from water that stabilize Ti4+ cations acting as Brønsted acid sites. In conclusion, the increase of the catalyst dosage increases the number of sites available for H2O2 adsorption, producing a beneficial effect on the catalytic activity, and diminishes the number of sites available for MB adsorption, though with no apparent effect on the reaction rate since MB reacts from the liquid phase and the fraction of MB removed from solution at the conditions used in this work is negligible.
I mean adsorption percentage efficiency remains constant when ever increases adsorbent dose after reaching to saturation point.
The variation of molecular adsorption from the aqueous phase is also determined by the following parameters, at least in experimental scale:
-mechanism of adsorption, involving the arrangement of molecules adsorbed on the interface; molecules' packing, monolayer, multilayers.
-pH fluctuation, inferring the type and intensity of the intermolecular forces' developed.
-temperature change.
-equilibrium time.
-ionic strength of the aqueous phase, inferring the nature of chemical bonds' (such as that of Van der Waals- and hydrogen- ) developed.
-coexistence of molecules, like natural organic matter (NOM), causing competitive adsorption phenomena.
-heats of adsorption and adsorption kinetics.
There is no need to refer to the molecular level. It just comes from the definition of adsorption capacity, provided we mean "pure adsorption" with no side effects.
This can be attributed to aggregation of the adsorbents due to its increasing amount; and therefore decreasing the surface area of contact between the adsorbate and the adsorbent
As the adsorption capacity (experimental equation) say, mass is inversely proportional to qe, this mean when you increase the mass the qe (exp) shall decrease and vice versa
the decrease in adsorption capacity qe (mg/g) with increasing adsorbent dose is due to the split in the flux or the concentration gradient between solute concentration in the solution and the solute concentration in the surface of the adsorbent causing a decrease in equilibrium adsorption capacity
Is it possible that adsorption capacity (qe) decreases with increase in adsorbent dosage at fixed adsorbate conc, while the removal efficiency increases?
The experimental results are consistent with the mathematical hypothesis of the inverse relationship between the adsorption amount and the adsorption mass. qe=((Co-Ce) × V)/m
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