Thats depends on the adsorbate, For instance, heavy metals needs microporosity and negative charge on surface. So, organics with high molecular size cant be adsorbed on micropority and difusion is vera important. You can read the work of Rodríguez-Reinoso et.al.
Thank you Carlos for clarification and providing a relevant reference.
Activated carbons can be prepared with desired properties by means of modification of surface functional groups and introduction of acidic/basic properties in its skeleton which can be performed by many tretament procedures some are mentioned as:
Use partial oxygen gasification, nitric acid treatment, urea impregnation followed by pyrolysis and pyrolysis in a urea saturated stream. The surface properties of Activated carbons can be estimated by multibasic titration method of stated by Boehm and by CO/CO2 gas evolution profiles, while pore structure development can be measured by N2 & CO2 gas adsorption isotherms.
1. The oxygen gasification resulted activated carbon's skeleton with surface area slightly lower that raw activated carbon; introduction of surface functional groups
depended upon severity of treatment: carbonylic and adsorbate nature, The
temperatures & contact times enhanced basic character of Activated carbons. But, nitric acid treatment for preparation of Activated carbons introduce
high nitrogen amounts in its skeleton and reduce surface area and develop surface containing acidic groups with negligible HCl neutralization capacity.
2. Treatment of activated carbon by urea supported formation of basic groups and
carbonyls.
3. Presence of such surface functional groups affected the adsorption capacity of activated carbons for removal of specific pollutants like phenols, acidic dyes etc.
4. Urea treated Activated carbons yields a basic character and high nitrogen content results in highest phenol removal capacity; nevertheless nitric acid
treated Activated carbons and oxygen gasified Activated carbons yields an acidic surface functionality to exhibit a low phenol adsorption capacity.
5. The beneficial role of nitrogen on phenol adsorption was attributed to adsorbate–adsorbent interactions i.e. physicochemical philicity towards active sites of activated carbons.
Thank you Rajendra for very comprehensive and elaborative response. Certainly, these points will help in taking future course of line.
The answer depends on the phase in which the adsoprtion is carried out: gas or liquid phase ? In the gas phase, there is a consensus that porosity and surface area controls the adsorption. At similar porous texture, the surface functions may improve or not the phenomenon. For example, an acid gas will be more favourably adsorbed onto a basic surface, but this effect does not prevail. In contrast, in the liquid phase, the effect of moieties is at least as important as that of porosity and surface area.
Alain
it depends. surface area is the main characteristic for activated carbon so I would say it's the number 1 issue even though very high surface area is something we don't really need in life.
However, if, just if, you have to work with gas adsorption, then oxygen sometimes if not many, is one of the key factor to higher adsorption rate. But one note: it varies from one lab to other. So do your experiments and you'll always find something new interesting data to begin to work with. Good luck
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