For removal pollutants or adsorption process, FT-IR is not that much interested because you didn't have kind of synthesis of new compound. the key is applying Langmuir and Freundlich isotherm model and fit your data following by the values of thermodynamics was also confirmed the spontaneous of the adsorption process. 

Also check this, hope it help you.

Generally, adsorption is a complex multistep process and kinetic studies will provide valuable insights of the sorption mechanisms which involves mass transfer, diffusion and surface reaction phenomenon but kinetic models are not enough to describe the complex adsorption process. You may have to investigate the adsorption data using various isotherm models (e.g Dubinin-Radushkevich) and thermodynamic (magnitude of Free energy change and enthalpy change) evaluations in addition to the kinetic studies. FT-IR alone may not be sufficient to reach a conclusion, some other characterizations (SEM, BOEHM, XRD) may support what happened to your material after adsorption and isotherm-thermodynamic properties will complement your observation. Pseudo-second kinetics is a lump sum parameter including chemical reaction and physical diffusion process. 

I did all those studies too Mr. Akeem. The data fitted with Langmuir isotherm model and Intra-particle diffusion model. But for the characterization techniques, I analysed my samples with SEM and XRD only after activation, not after adsorption. 

You see, the accordance with Langmuir isotherm is explainable that is the adsorption occurred in monolayer; but I couldn't come up to a decision about this pseudo-second-order kinetics. 

I think I'll go with the 'Physical Diffusion' theory. 

Thanks.

Mr Sirajus, try to fit your data to Dubinin-Radushkevich isotherm (D-R), The values of Ea(mean energy of adsorption) of D-R model can give you information about the adsorption mechanism. When one mole of ions is transferred, its value in the range of 1-8kj/mol indicates physical adsorption and 8-16kj/mol indicates ion-exchange while its value in the range of 20-40kj/mol is indicative of chemical adsorption. 

Also fit in the data into thermodynamic parameters. Generally, the change in free energy (G) for physisorption is between -20 and 0kJ/mol, but chemisorption is the range of -80 to 400 kJ/mol. These will help you to conclude. 

Thank you very much for the superb explanation, Mr. Akeem. 

You are welcome, kindly let me know if you do need any other assistance!

Dear Sirajus,

The order of a kinetic process does not have to be directly related to the molecularity of the reaction. Likely it is a complex system as pointed Akeem. A  fractional-order model, ie. dc / dt = - k * c ^ n where 1 < n < 2, does not necessarily imply chemo-adsorption, but more components should involved in the reaction. In heterogeneous systems several  components in the system (eg.,  dissolved colloids)  could participate as intermediaries in the  adsorption giving a chain process:

solute + surface --> adsorbed

solute + colloid --> complex

 complex + surface  ---> adsorbed complex

A system  can  be described as a fractional-order .

The model is:

dq/dt=k (qe-q)2

where k is the rate constant of the pseudo-second-order adsorption model (g mg-1 s-1) and qe and q (mg g-1) denote the amounts of solute adsorbed at equilibrium and at time t (s), respectively.

Since (qe-q) is related to the empty adsorption sites, if we compare this process with a second-order chemical reaction and if we assume that the process is an elementary reaction, then the results suggest that each adsorbed molecule is attached to two active sites of the adsorbent, that the concentration of the solute is constant (because is enough large), and that the process is irreversible.

did you manage to study and conclude the adsorption mechanism?

I just needed to know the implication of pseudo-second order kinetics results, and I think I have some explanations. Why do you ask, Sir (Dr. Harry Chong)?

The question you were asked is how did you conclude it is chemisorption since there was no significant functional group attached to ur adsorbent surface? That is the more reason why chemisorption should be the major mechanism. This is because if a significant number of functional groups are present on the surface of ur adsorbent , it implies that adsorption is more likely to proceed through film diffusion where the metal ions more from the bulk of the solution to the abundant functional group on the surface of the adsorbent. this is due to the attraction between the functional groups and the positively charged cation. the adsorption is then more of a surface phenomenon and corresponds to physisorption, since lower amount of energy will be required for the desorption of the adsorbed metal ions from the surface of the adsorbent ( this is why physisorption has a lower energy than chemisorption). On the other hand you adsorbent showed no significant number of functional groups, which implies that when your metal ions get to the surface of the adsorbent due to the driving force obtained from the high concentration of metal in solution, it encounters fewer functional groups or active sites on the surface to be adsorbed. the high concentration of metal ions on the surface then pushes the adsorbed metal ions from the surface through the pores ( intraparticle diffusion) to the internal sites of the adsorbent. this implies that you process is more likely to conform to intraparticle diffusion mechanism. therefore more energy would definitely be required to remove the adsorbed metal ions from the interior of you adsorbent to the external surface and back into the solution, thereby conforming to chemisorption (higher energy than physisorption). This means you adsorbent must also have a porous structure from its SEM image for the metal ions to pass into the interior of ur adsorbent. Thank You.

Hi Sirajus,

If the adsorption floows the pseudo second order kinetics, then is is definitely chemisorption mechanism. Since there are electron rich functional groups on the surface of your adsorbent, then it is very likely that these provide attraction forces with your entity. FTIR will only show the presence of these surface functional group before and after adsorption, which is true. But they won't show the presence of the metal entities which are lossely held by attraction forces. Try SEM which might indicate the presence of these heavy metal entites. I hope this helps. 

I had tried using SEM-EDX to detect metal on adsorbent; however, if your adsorbent has low adsorption capacity, the task will be more difficult.

This indicates for weakly coordinating between functional group of adsorbent and metal ion. in this case FT-IR will not give you significant frequency change for the functional group before and after adsorption.

FTIR will only show the presence of these surface functional group before and after adsorption, but to quantify the functional groups you have to use Boehm Titration 

Can anybody here provide some good references telling that 1st Order adsorption model relates to physorption and 2nd Order adsorption model relates to chemisorption? I have read this type of statement all over here in research gate questions, however, I could not find yet any paper where it is clearly stated.

I appreciate all your help in advance. Thanks

Please see it the reference here about physorption and chemisorption.  P. Atkins and J. de Paula, Physical Chemistry. 9th ed., New York, Oxford University Press, (2010)

what if value we obtained higher from 40 in D-R isotherm?? (Like 150-200) 

@Bayram Poyraz, 

If you obtained above 40kj/mol, is a strong chemical adsorption. 

DEar akeem, is there any reference to refer about it( >40 kj/mol)?

 Dear Sirajus, you have to apply D-R isotherm to understend whether it has physico and chem, yet there has been some chemical shifts in FTIR and NMR as well as peak intensity alteration.

There could be references if one search. However, the major types of sorption are physical/ion-exchange and chemical sorptions. Since less than 20 falls to physical/ion-exchange then above 40 is definitely chemisorptive. 

Your right akeem, because our FTIR results support this idea but i have to refer at least with a paper.

Dear Dr Sirajus,

The applicability of  pseudo-second-order kinetic model increases the probability of the chemisorption, but doesn't confirm the chemisorption process.  . 

refer to "Adsorptive removal of tartrazine and methylene blue

from wastewater using melamine-formaldehyde-

tartaric acid resin (and a discussion about pseudo

second order model)"

Desalination and Water Treatment

doi: 10/5004/dwt.2012.2964

PSO model and chemical/physical postulation:

In some published papers in which PSO was sug-

gested to well represent the kinetic data [38–41], authors

directly concluded that adsorption occurs due to “chem-

ical sorption or chemisorption involving valency forces”

as Hoand McKay. presented [8], without specifi c identi-

fi cation of such chemical interaction. Some other papers

suggested PSO to well fi t the kinetic data without giving

the impact of this fi tness.

According to present work, PSO is believed to

represent both chemical and physical adsorption in

equal terms. Under the light of kinetics, isotherms,

Discussing theoretical models of sorption kinetics

(PSO, PFO and Elovich equations), Plazinski et al. stated

that “surface reaction” term may not necessarily mean

the actual chemical reaction occurring on the adsor-

bent surface involving the formation of chemical bonds.

Interactions of physical nature (van der Waals’ forces,

for instance) may also play a role [7]. In other words,

they suggest that physical and chemical processes can

be represented by PFO and PSO models.

Also, Azizian demonstrated mathematically that

observed adsorption rate constants of PFO and PSO are

not intrinsic. Instead, both are dependent on adsorp-

tion and desorption rate constants and are functions

of adsorbate initial concentration [48]. This indicates

reversibility character of the adsorption process which

is valid for both chemical and physical processes.

Determine the PH zero point charge of the absorbent before batch adsorption.Then the solution pH also.The pHzpc will let you know if there is existence of acidic functional groups in the adsorbent( PHzpc less than 7.00).The solution pH will let you know if OH ions or Hydrogen ions are readily avialable to combine with the functional groups on the adsorbent surface.This should give a good picture of the type of adsorption mechanism.

Suitable reference for D-R will be Adsorption of Cadmium using orange peels:Thermodynamic study by H.Tran 2016.Supplementary information

If your isotherm data are fitted to Langmuir isotherm model which primarily signifies chemisorption and monolayer adsorption and your kinetic data are well interpreted by pseudo second order model then it may be said that your the adsorption type is chemisorption. Regarding FTIR it can be said that the functional groups that you have mentioned are usually present on pristine sorbent and these groups participate in binding of heavy metal ions which can be noted from shifts in wavenumbers of FTIR spectrum of metal adsorbed sorbent.

I'm guessing that you mean either a differential equation in the form dqt/dt = k(qe-qt)2 or algebraic equations obtained after its solution with the initial condition qt(0) = 0 - in a linear form or non-linear one (hyperbolic function). Because the initial equation is an empirical equation, the equations obtained on its basis are also empirical. One can only say that qe is the upper limit of the qt, hence it can be treated as an equilibrium value. So the answer to your question is as follows - idicates nothing. Regards,

It suggests chemisorption mechanism between the adsorbate in the liquid phase and the solid surface of the adsorbent.

The compatibility of a few experimental points (one kinetic curve) with hyperbolic function allows one to conclude about the mechanism - in this case - chemisorption? I doubt it very much! Dear Kovo Akpomie - prove it. Regards,

Functional groups will be present in the sample, both before and after adsorption. What is significant is that you will observe change in wavenumber (responsible for a particular functional group) in FTIR spectra after adsorption if that functional group participates in metal binding.

Fitting of experimental data to pseudo second order model and Langmuir isotherm model usually signifies chemisorption.

Why? Could you explain it. The so-called PSOM is an empirical equation. Regards,

In addition, the so-called PSOM is in fact a hyperbolic equation that can be used to describe any experimental points whose graphic image is asymptotically rising curve without inflection points. This applies to many different processes. Regards,

Deae all

I have funny scenario where linear version (1) of pseudo second order fits but the non-linear version does not fit,,

Any advice to explain this

Pseudo second order kinetic model.a complete step by step process...

https://youtu.be/7Y7BdUeBzkA

It is quite normal. I recommend the following review: https://doi.org/10.1016/j.watres.2017.04.014