I'm doing an adsorption study of naproxen sodium. The previously published articles mentioned that naproxen sodium shows optimum adsorption at pH 4-6, and as the pH increases, the adsorption capacity decreases. But in my case, as the pH increases, the adsorption capacity increases. Why?
I guess that you talk about retention, quantifed through a solid solution partition coefficient, Rd (L/kg), and not "adsorption capacity" (which means Qmax)
Difficult to answer without further details.
It depends on your sorbent and of the composition of water (ions)
Naproxen is a Brönsted acid, with a pka ~ 4.
If its deprotonated form sorb (species A-), its retention (solid solution partition coefficient Rd) may increase with pH.
If the protonated form (species HA), retention should decrease with pH.
Finally depending on pzc of your sorbent, pH may also affects availability of sorbing sites.
Besides, Naproxen is also very lipophilic (log P ~2-3).
Hence, another retention mechanism occurs by partition into organic matter. If you sample is natural this may drive retention.
Finally, if other ions are in the media (Ca, K, Cl, ClO4, etc.), the whole speciation of naproxen should be calculated, to account for in retention model.
Ejaz Hussain Please give us more information about your experiment. Which sorbent material and type of experiment did you use. I think my group did did sorption experiments with naproxen and primary and secundary sludge a long time ago and the pH effect was not enough to be significant for sewage treatment plants.
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pH control depends on the charge of the molecule and the surface.
depending on the adsorbent, could a pH change induces a higher postive charged surface? does the pH change increases the surface area?