In this context, Schmidt and Swiderska-Dabrowska
(2013) found that the largest rate of phenol removal by zeolite
was observed at pH values in the range 2e4, with the highest
phenol removal rate observed at a pH of ~2. Furthermore, phenol
adsorption decreases with increasing pH in the range 5e12. This is
due to phenol solubility increasing at higher pH values. Moreover,
Saravanakumar and Kumar (2013) observed that the uptake of
phenol by sodium zeolite was almost constant in the pH range 3e6.
On the other hand, adsorption decreased with increasing pH in the
range 7.0e9.0.