3.3. Thermogravimetric analysis (TGA–DTG)
The thermal analysis (TG) and DTG curves of the pure SBA-15
silica sample, SBA15–0.2NH2 and SBA15–0.4NH2 samples are
shown in Fig. 5(a) and (b), respectively. The TG curves of the samples
indicate a significant weight loss, which starts immediately
with the increase of the temperature. Compared with 9.2% weigh
loss of the SBA15 sorbent recorded during heating from the room
temperature to 600 C, the SBA15–0.2NH2 and SBA15–0.4NH2
samples exhibit weigh losses of 12.3% and 17.0%, respectively.
The larger weigh loss in SBA15–0.4NH2 sample with respect to
SBA15–0.2NH2 sample means that more amino groups were introduced
into SBA-15. The TG curves of the SBA15–0.2NH2 and
SBA15–0.4NH2 samples (Fig. 5(a)) show the presence of the inflection
points at 45 and 250 C already observed in the pure SBA-15
silica sample. Those inflection points correspond to dehydration
and to the dehydroxylation of the SBA-15 silica material, respectively
[52]. Additionally, the samples grafted with amino groups
show the inflection points at approximately 298 and 317 C (Fig. 5(b)), which are related to the decomposition of the amino
and ethylene groups. Thus, the results of the thermogravimetric
analysis confirmed the presence of the functional groups on the
SBA-15 surface. Those groups have high thermal stability (above
250 C) suggesting that the SBA-15 silica sample has a stabilizing
effect on the temperature of decomposition of the surface species.
3.4. Determination of optimum amine group concentration in SBA-15
adsorbent
The behavior of amino group’s concentration in the retention of
Pb(II) uptake under natural pH, at 30 C for 1 h was investigated. As
showed in Fig. 6, their adsorption increases when the ratio TEOS/
APTES decreases, because of an increase of aminopropyl groups
present on the structure. The extent of adsorption reached a maximum
when the adsorbent molar ratio TEOS/APTES was 2.5
(SBA15–0.2NH2) and 3.3 (SBA15–0.3NH2). Further increase of the
amino group’s content had negligible effect on the adsorption of
Pb(II). Considering that the SBA15–0.2NH2 adsorbent shows similar
percentage of lead removal as the SBA15–0.3NH2 counterpart
(91.3% and 91.0%, respectively), the optimum adsorbent ratio
TEOS/APTES for adsorption of Pb(II) was found to be within the
range 2.5–3.3.