Hello,

I am working on heavy metals removal using adsorption technique and i want to know if physical adsorption or chemical adsorption occurred. From literature, I can determine the type of adsorption from four parameters ΔH°, activation energy Ea, the mean free energy E=√(1/2β) and effect of temperature. Until now, I worked on two adsorbents but I got the following results:

For the first adsorbent (Let’s refer to it by “X”):

(1)   The obtained ΔH° value was -68 and -43 kJ/mol for Cu and Cd respectively but generally, if the values of ΔH° are 0∼−20 kJ/mol so physical adsorption occurred and if ΔH° values −80∼−400 kJ/mol so chemical adsorption occurred. How to illustrate that? Physical or chemical adsorption?    

(2)   The obtained Ea value was 112 and 159 kJ/mol for Cu and Cd respectively. The activation energy of adsorption Ea for physical adsorption is 5∼40  kJ/mol, while the activation energy of adsorption Ea for chemical adsorption is 40∼800 kJ/mol. So it is supposed that “chemical adsorption occurred”.

(3)   The obtained mean free energy E from the relation E=√(1/2β) from  Dubinin–Radushkevich (D–R) isotherm was less than 8 kJ/ mol. If the magnitude of E is between 8 and 16 kJ/ mol, it is indicated that the adsorption process is chemical adsorption, while for value of E < 8 kJ/ mol; the adsorption process is physical in nature. So according to this point it is supposed that “physical adsorption occurred for both of Cu and Cd”.

(4)   The adsorption decreased upon rising temperature for Cu and Cd (This behavior indicates that “physical adsorption occurred”).   

Now, I cannot determine the nature of this reaction if it is chemical or physical adsorption! From these results, can I say that both of physical and chemical adsorption occurred?!

For the second adsorbent (Let’s refer to it by “Y”):

(1)   The obtained ΔH° KJ/mol value was -52 kJ/ mol for Cu and -108 for Cd but generally, if the values of ΔH° are 0∼−20 KJ/mol so physical adsorption occurred and if ΔH° values −80∼−400 KJ/mol so chemical adsorption occurred. So according to this point it is supposed that “chemical adsorption occurred for Cd” but what about Cu in this case? 

(2)   The obtained Ea value was 28 kJ/ mol for Cu and -58 kJ/ mol for Cd. The activation energy of adsorption Ea for physical adsorption is 5∼40  KJ/mol, while the activation energy of adsorption Ea for chemical adsorption is 40∼800 KJ/mol. So it is supposed that “physical adsorption occurred for Cu” but what about cadmium in this case?

(3)   The obtained mean free energy E from the relation E=√(1/2β) from  Dubinin–Radushkevich (D–R) isotherm for Cu and Cd was < 8 kJ/ mol. So according to this point it is supposed that “physical adsorption occurred”.

(4)   Upon rising the temperature the adsorption increased to a certain temperature then it decreased (This behavior indicates that chemical adsorption occurred) where with increase in temperature, chemisorption first increases as sufficient energy is being provided for the molecules to reach the activation energy. But after certain degree it decreases, as the further high temperature helps in breaking of the bond between the adsorbate and the adsorbed molecules. So according to this point it is supposed that for Cu and Cd “chemical adsorption occurred”.

In this case also, I cannot determine the nature of this reaction if it is chemical or physical adsorption! From these results, can I say that both of physical and chemical adsorption occurred?!

Kindly, help me to write a plausible interpretation for these confusing results. I really appreciate your help and your time to read my questions.

*   The attached file is a reference for points (1) and (2) so you can review it.

Thank you.

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