Anyone have seen this type of adsorption curve? How could that happen?
Any explanation? Besides adsorption, the adsorbent could induce crystallization even in a very low concentration?
Direct multilayer formation without going through the formation of the monolayer
Naouel Hezil
firming Multilayer is ok. Howeve, according to this curve, no matter how many adsorbate you put in the solution, it will end up with a similar Ce, how could that happen?
It looks like as an aggregation/precipitation of nanoparticles on the surface, when earlier attached particles provide surfaces for further growing. For better understanding multiple details are lacking: 1) are "the isotherms" proved to be at equilibrium. 2) are they not time-dependent? 3) how does "desorption isotherm" look like? 4) how is the suspension of nanoparticles stable per se...
from the paper, the data in the curve was collected at equilibrium and not time dependent. They did not present desorption isotherm. here is the link to the paper https://link.springer.com/content/pdf/10.1007%2Fs11356-019-05276-x.pdf
All is simple :-(.
(1) Foremost, the isotherms data are NOT for nanoparticles but for lead adsorption onto nanoparticles. The authors provide lead adsorption isotherms: Fig. 5 Non-linear Freundlich model for Fe3O4, SnO2,and TiO2 nanoparticles for Pb2+ sorption .
(2) Now, I suppose there is a mess in the data presentation. See Fig. 4: Fig. 4 Non-linear Langmuir model for Fe3O4 (a–c, 293–313 K), TiO2 (d–f, 293–313 K), and SnO2 (g–i, 293–313 K) nanoparticles for Pb2+ sorption. But the plots are shown as (!) solution concentrations (Y axis) vs sorbed concentrations (X axis).
I had never seen such a presentation albeit it could be yet possible. It is not a mistake. However, the curves show that sorbed concentrations increase infinitely when solution concentrations reach a certain threshold. Keep it in mind.
Fig. 5 inverted the axis assignment but shows exactly the same phenomenon as in Fig. 4.
(3) Now, the authors indicate that "The non-linear Langmuir model fitted well to adsorption data Fig. 4 ". For the data shown in Fig. 4 or 5, it is impossible to apply the Langmuir model. So, as far as I understand, the authors are dramatically confused with the axis assignment, the data or units. or ....with use of the Langmuir model.
Table 3 reports "well-fitted" Langmuir parameters to the data shown in Fig. 4 (???). How can it be? It is impossible. In the same time (!) The Freundlich model was reported to fit the data in Fig. 5 with the exponent exceeding 1, and it makes sense. However, the data in Figs. 4 and 5 shows the same trend!
The conclusion? Check me if I am wrong, but if I am not, I have a problem with this publication. Omitting all the story with the Langmuir model fitting, one may stay with "a cooperative trend" in lead uptake. But I have doubts in that and see no any reason for such an uptake of lead as well as for its "crystallization"
If you postulate an "autocatalytic adsorption", you find your isotherm at low concentrations. Calculated in phreeqc2:
Hfe_wOH = Hfe_wOH
log_K 0,00
Hfe_wOH + H+ = Hfe_wOH2+
log_K 7,29
Hfe_wOH = Hfe_wO- + H+
log_K -8,93
Hfe_wOH + Pb+2 = Hfe_wOPb+ + H+
log_K -2,70
Hfe_wOPb+ + 2Pb+2 + 2H2O = Hfe_wOPb3O2+ + 4H+
log_K -16,70
SURFACE 1
-equil solution 0 #sites[mol] specific_area[m2/g] mass [g]
Hfe_sOH 3,92215E-08 200,00 0,40 variabel
Hfe_wOH 3,91823E-05
Sorry, I complete:
Pb concentration in [mg/L]
I have calculated only some isotherms with variable start concentrations as an orientation.
Only in German:
https://www-docs.b-tu.de/fg-wassertechnik/public/Publikationen/Schriftenreihe/Heft22.pdf