Summarizing, the hybrid silica samples were found to be efficient
sorbents for the removal of Pb(II) ions from aqueous solutions.
The abilities of the sorbents were found to be closely related with
the pore structure, pore density, and the accessibility of the grafted
amino groups. The amine-functionalized SBA-15 samples were
found to be thermally stable until temperature of 300 C. Although
the specific surface area decreased after incorporation of higher
amount of precursor APTS, the morphology of sorbent did not
change with respect to unmodified SBA-15 substrate. The aminefunctionalized
SBA-15 sample with the intermediate concentration
of –NH2 groups was found to be the most appropriate for its use as
sorbent because it still has very large specific surface area. The optimum
pH value for removal Pb (II) ions from aqueous solution ranged
from 5 to 6. The lead adsorption approached equilibrium state
in short time of about 60 min.
The binding energies of the N1s core-level spectra clearly show
the presence of both –NH3
+ and –NH2 groups on the sorbent surface
(Fig. 12(b) and (c), respectively). Since the concentration of
the former groups on the surface of the most effective sorbent
(SBA15–0.3NH2) increased after lead removal elimination from
aqueous solution having initial Pb(II) concentration of 200 ppm,
the mechanism of lead adsorption through ionic exchange of Pb(II)
ions with protons of –NH3
+ groups should be precluded. Contrary
to this work, it was proposed that the free amino groups could react
with Pb(II) to form stable metal complexes, in which the metal
ions are coordinated by ligating N atoms of aminosilanes [66,67].
Considering that amount of lead ions located on the sorbent surface
increased with an increase of Pb(II) concentration in solution
(Table 2) together with the simultaneous decrease of the sorption
capacity with an increase of Pb concentration in solution (Fig. 8),the mechanism of lead adsorption on the SBA15–0.3NH2 involves
van der Waals electrostatic interaction of Pb(II) ions with –NH2
groups present on the sorbent surface (Fig. 12(c)).