I did a point of zero charge experiment to my adsorbent (bael shell activated carbon ) and got 6.25,but I got maximum removal of Pb(II) at pH 4. Can anyone explain how we can interlink these results.
The point of zero charge is the pH at which the surface of your adsorbent is globally neutral, i.e., contains as much positively charged as negatively charged surface functions. Below this value, the surface is positively charged; beyond this value, it is negatively charged. So normally, it is always easier to adsorb a cation on a negatively charged surface, and an anion on a positively charged surface. However, other interactions may be stronger than purely electrostatic forces, making the effect of surface charge not so important. Additionally, a cation is often complexed with ligands, some of them being possibly negatively charged. Therefore, in such a case, the cation is in fact a negative complex, which may adsorb very well on a positively charged surface.
In summary, just saying that you have Pb(II) is not enough: what are the anions associated ? In which concentration ? For example, you may have PbCl42-, which is negative and may adsorb on a positively charged surface. This is maybe not best example for your case, however, as such complex only exists in highly concentrated chloride medium ...
Alain
I thank Mr.Alain Celzard, for a good answer. But in general laboratory scale experiments are conducted with distilled water, where other ions (such as Cl), may not be present. In such a case what could be the explanation?
The cation to be adsorbed is never alone, but with a counteranion. Cl- was an example, but it can be a nitrate or anything else giving a soluble salt.
Alain
The Pb can hydrolize the water molecule and form hydroxy complex with negative charges.
The adsorption mechanism is not always an electrostatic interaction between the adsorbate and the adsorbent, we use the pH of zero point charge (pHzpc) to explain either the adsorption mechanism follows an electrostatic or another mechanism. Generally, the results should agree with this concept but usually, other mechanism occurred during the adsorption. What is supposed to do is to measure the zeta potential of you adsorbent at a well-defined pH value in order to point that you have a negatively charged surface. Other mechanisms can also occur for example complexation, dissolution and precipitation and so on.
In your case chemical reaction is dominant comparef to electrostatic repulsion or attraction.
The surface functional species is H+ (from acid). The Pb2+ substitute the H+ similar to this reaction:
R-H + Pb SO4 -> R-Pb + H SO4
What should be the range of point of zero charge for adsorbents. what does high surface charge indicate? can anyone suggest?
This video can be helpful: https://www.youtube.com/watch?v=5mcrT6mO3b0
Potential of zero charge is related to the work function of metal and the nanostructuring. The nanostructuring influences the dipolar solvent packing density which further lowers the PZC. I will suggest to read J. Phys. Chem C, 2021, 2021, 125, 46, 25774-26783.
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