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