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Why does adsorption capacity decrease as adsorbent dose is increased?
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Why does adsorption capacity decrease as adsorbent dose is increased?
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.
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.
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:
(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.
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:
(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.
My issue is related to removal efficiency, why does removal efficiency decreases with increase in adsorbent dosage at fixed adsorbate concentration in some cases?
My issue is related to removal efficiency, why does removal efficiency decreases with increase in adsorbent dosage at fixed adsorbate concentration in some cases?
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
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
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.
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.
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
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
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
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
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.
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.
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
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
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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.
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.
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My issue is related to removal efficiency, why does removal efficiency decreases with increase in adsorbent dosage at fixed adsorbate concentration in some cases?
My issue is related to removal efficiency, why does removal efficiency decreases with increase in adsorbent dosage at fixed adsorbate concentration in some cases?
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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
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
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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.
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.
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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
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
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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
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
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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.
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.
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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.
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.
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I mean adsorption percentage efficiency remains constant when ever increases adsorbent dose after reaching to saturation point.
I mean adsorption percentage efficiency remains constant when ever increases adsorbent dose after reaching to saturation point.
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A complete guidance ..
https://youtu.be/wWIZyxrp04I
A complete guidance ..
https://youtu.be/wWIZyxrp04I
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It is a very useful idea containing this question, thx
It is a very useful idea containing this question, thx
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