3.2.4. Effect of pH
The influence of initial pH on adsorption amount was studied in
the range of 4–10. The relationship between the initial pH values
and the quantities of heavy metals adsorbed on MNHAP adsorbents
was presented in Fig. 7 which showed that the metal ions adsorbed
increased as pH increased. It was noticed that when the pH value
is higher than 8, the adsorption amount increased dramatically,
which was attributed to the fact that heavy metal ions started to
precipitate leading to the reduction of the metal ions in the aqueous
solution at higher pH value. Fig. S3 shows the XRD patterns of the
precipitation of metal complexes. It was observed that there were
two ingredients in the cadmium complexes including Cd(OH)2 and
CdCO3. Two other ingredients such as Zn(OH)2 and ZnO existed in
the precipitation of zinc complexes. Similar results were obtained
when divalent cations were removed by hydroxyapatite in the previous
literature [22]. From the data of the zeta potential (Fig. S4),we
knew that the charge sign on the surface of MNHAP adsorbents was
negative in the entire examined range which provided a strong theoretical
basis for the existence of electrostatic attraction between
the MNHAP surface and the metal ions. Also we could observe that
negative charge on the MNHAP surface reduced gradually when
pH value is greater than 5, while the adsorption amount increased
little by little. The results showed that some other sorption mechanism
should be in existence during the adsorption process except
for electrostatic attraction. The lowering of the Ca2+ peak intensity
of MNHAP adsorbents after loading with Cd2+ and Zn2+ in
the EDAX spectra (see Fig. 2) indicated the participation of certain
degree of interchange in the Cd2+ and Zn2+ adsorption. We
supposed that this phenomenon was due to the cationic exchange
mechanism in the adsorption of Cd2+ and Zn2+on the surface of
MNHAP adsorbents. To demonstrate this supposition, we carried
out some experiments to investigate whether Ca2+ released in the
solution after the completion of adsorption process. The results
showed that the concentration of Ca2+ in the solution increased
remarkably after heavy metal adsorption compared with that one
before heavy metal adsorption (from 0.3824 to 44.75 and 0.3835 to
31.29mgL−1 for Cd2+ and Zn2+ solutions respectively). Therefore,