3.2.2. Operational pH range
Mostly, the adsorption process is dependent upon solution pH
because variation in solution pH leads to variation in the surface
properties of hydrogels as well as the degree of ionization of the
adsorbate molecules (Nandi et al., 2009; Yagub et al., 2014). The pH
at which the adsorbent's surface charge is zero is called zero point
of charge (pHZPC) (Yagub et al., 2014). The effect of solution pH can
be described on the basis of its comparison with the pHZPC of the
adsorbent: When pH < pHZPC, the adsorbent acts as a positively
charged surface in the solution (Taleb et al., 2009) and when
pH > pHZPC, the adsorbent acts as a negatively charged surface.
Consequently, a cationic pollutant has higher affinity for the
adsorbent when solution pH > pHZPC and vice versa (Mezohegyi
et al., 2012). Numerous studies have investigated the effect of solution pH on the removal of pollutants using hydrogels (Table 1). In
most of the cases, the maximum adsorption occurs in a very narrow
pH range. It means that pH of the wastewater needs to be adjusted
within that specific range in order to attain high removal