3.2.2. Effect of pH
The effect of pH on nitrite ion adsorption, onto ZnCl2/450-AC
and CAC, was investigated. Fig. 5 shows that within 60 min contact
time (before equilibrium), percentage removal of nitrite ion
increased with lower pH and decreased with higher pH values. At
each pH value, the AC prepared here was more efficient than the
CAC, in adsorbing the nitrite ions.
The effect of pH is understood by changing the nature of the
carbon surface with pH. At lower pH values, the solution contains
more protons, which convert the carbon surface into the H-form.
This favors adsorption of the negatively charged nitrite ions onto
the surface. At higher pH values, the carbon surface releases the H
ions and holds negative charges which retard the nitrite ions and
lower adsorption.
At lower solution pH values, the majority of nitrite ions are
converted into the weak nitrous acid in a way of an equilibrium
(Chang, 2010). Despite that, the nitrite ions/nitrous acid adsorb
more at lower pH values as observed from the Figure. This means
that the nitrite ions may be adsorbed in either free ionic form or in
nitrous acid form. The H-form of the carbon surface may thus
adsorb the nitrous acid via the oxygen atoms which carry partial
negative charges.
The high adsorption of the AC in acidic media and low adsorption
in basic media may impose limitation on using AC in nitrite
removal studies. On the other hand, this could be advantageous by
the fact that desorption and AC recovering strategies can be further
investigated by shifting acid-base conditions in the solution.