This situation can be related to the
surface charge of the adsorbent. DAH-bentonite has negatively
charged adsorption sites, but it is positively charged at low pH
values. Therefore the electrostatic interactions between negatively
charged –SO3
groups in the dye molecule and the
positively charged adsorbent increase. As a result, the amount of
dye molecules onto the modified bentonite increases at lower
pH values.