Under alkaline condition It gives best results . but pH above 9 efiiciency of adsorption process may decreases. Actually threre are so many factor , but it is aceptable range.

Dear Mohammad,

At higher pH the negatively charged adsorbent surface will not favor deprotonated congo red adsorption due to electrostatic repulsion. Hence it depends on the nature of your adsorbent too. I would suggest you to conduct 5-8 pH range. You can refer doi:10.1016/j.jhazmat.2009.09.066

Following

In the absorption process there is not an optimal pH since the adsorption depends on the surface charge of the adsorbent and the adsorbed molecule charge i.e. the zero charge point (PZC) of the adsorbent and the pKa of the molecule (dissociated form or molecular form) must be known. • If pH PZC then the charge is negative • If pH pka the dissociated form if the surface of the adsorbent and the adsorbed molecule have the same charge so there is a repulsion if the surface of the adsorbent and the adsorbed molecule have different charges so there is an attraction between the molecule and the surface and the adsorption can take place

THe choice of pH value is determined by surface natures of material commonly based on isoelectric point (IP). When pH is higher than IP, the negative charge of surface could absorb positive ions.

There is no constant optimum pH for adsorption.

optimum pH is dependent on the interaction between the adsorbent and adsorbate mainly determine by adsorbate pKa and the functional groups of the adsorbent.

You can the determine optimum pH by varying the pH of the adsorbate for example 1-12 with constant adsorbent dosage then the one with highest adsorption efficiency or capacity becomes the optimum pH with can be used throughout your adsorption studies.

But in this process do not forget to determine the pH point of zero charge or zeta potential of your adsorbent in order to be able to explain the surface chemistry of your adsorption.