3.2.3. Effect of pH
The influence of pH on the sorption process of Cd2+ ions on
nHAp was studied in the pH range from 3 to 11 (see Fig. 8). The pH
can influence the adsorption kinetics and the sorption capacity of
metal ions on solid surfaces by changing the surface charges of the
adsorbent and Cd species present at different pH as in its speciation
diagram [34]. It can also affect the number of active sites available
for this adsorption process [35]. In this investigation, the pH of the
aqueous solution of Cd2+ ions could affect its uptake by nHAp, and
the uptake increased at higher pH values. This can likely be
attributed to a competition between H+ and Cd2+ ions to be
adsorbed on nHAp, and at lower pH values, H+ ions are able to
exclude a significant number of adsorption sites at nHAp from the
Cd2+ adsorption process [36,37]. At high pH values, the H+ ions
competition disappears and the positively charged Cd2+ and
Cd(OH)+ ions can easily attach to the free binding sites, increasing
the Cd2+ ions uptake [38]. It is known that precipitation plays a
major role in removing of Cd2+ ions in alkaline media [39]. The
dominant Cd2+ species at pH higher than 8.0 is Cd(OH)2. Therefore,
it can assumed that Cd2+ removal by nHAp was dominantly
controlled by adsorption at pH values between 6.0–8.0, but it could
be slightly enhanced by cadmium hydroxide precipitation at pH
higher than 8.0 [40].