4.3 Adsorption mechanism

SEM, FTIR, and XPS analysis have been extensively used to identify the possible metal cation-adsorbent interactions. In order to examine the morphology structure of the adsorbents, SEM micrographs were taken after metal ion adsorption (Figure 13).

These micrographs indicated clearly the deformation and the presence of many shiny small particles over the surface of both supports M1 and M2 after the adsorption process. Moreover, there was also a decrease in the pore sizes after metal adsorption. This observation evidenced the surface coverage of adsorbents by metal ions.

To gain further insights into the mechanism involved in the metal ions uptake process, the FTIR spectra were analyzed, and the band positions for each adsorbent exposed to metal ions are listed in Table 8.

In the M1 xerogel IR spectrum, the strong band that occurred at around 3325 cm<sup>1</sup> attributed to NH and NH2 stretching vibration was shifted and becomes weaker after metal adsorption. This is likely due to the chelation between amino groups and metal ions. Besides, the peak at about 1614 cm<sup>1</sup> ascribed to NH2 and NH groups disappeared suggesting that the adsorption process is mainly dominated by the coordination of nitrogen with metal cations. However, the characteristic peak at 2680 cm<sup>1</sup> assigned to the stretching vibration of sulfhydryl group (S-H) was disappeared. This result revealed that metal ions reacted with (S-H) groups on the surface of M2 xerogel. No obvious shift of the Si-O group after lead adsorption onto the two supports was observed.

To deepen the understanding of the mechanism of metal uptake, XPS analysis before and after metal ion adsorption were performed.

Figure 13. SEM micrographs of the two adsorbents after metal-ion adsorption.

exothermic nature of the adsorption process; besides, its magnitude revealed the type of adsorption mechanism (physisorption or chemisorption). Since the ΔH°

Thermodynamic parameters for heavy metal adsorption onto the two adsorbents M1 and M2.

<sup>Þ</sup> <sup>Δ</sup>H0 kJ:mol�<sup>1</sup>

M1 293 �22.51 �74.67 �162 .3

Determination of thermodynamic parameters for the adsorption of metal cations onto the two adsorbents:

M2 293 �23.92 �62.23 �143.31

Cd(II) T(K) ΔG0 kJ:mol�<sup>1</sup> ΔH<sup>0</sup> kJ:mol�<sup>1</sup> ΔS<sup>0</sup> J:mol�<sup>1</sup>

M1 293 �22.08 �69.08 �157.71

M2 293 �20.72 �59.86 �131.34

Zn(II) T(K) ΔG0 kJ:mol�<sup>1</sup> ΔH<sup>0</sup> kJ:mol�<sup>1</sup> ΔS<sup>0</sup> J:mol�<sup>1</sup>

M1 293 �23.16 �80.12 �172.35

M2 293 �21.87 �66.74 �152.27

<sup>Þ</sup> <sup>Δ</sup>S<sup>0</sup> <sup>J</sup>:mol�<sup>1</sup>

:K�<sup>1</sup> <sup>Þ</sup>

:K�<sup>1</sup>

:K�<sup>1</sup>

onto the xerogels occurred by means chemisorption [30]. The observed negative ΔS ° reflected a lessening in the randomness at the solid/solution interface during the

, this indicates that the adsorption process of metal ions

value was over 20 kJ.mol�<sup>1</sup>

Table 7.

142

Pb (II) T(K) ΔG0 kJ:mol�<sup>1</sup>

Figure 12.

(a) M1 and (b) M2.

Water Chemistry

303 �21.82 313 �20.94

303 �22 .89 313 �22.19

303 �21.72 313 �21.55

303 �20 .53 313 �20.39

303 �22.93 313 �22.75

303 �21 .64 313 �21.41

adsorption process [37].


#### Table 8.

Band positions before and after metal cation adsorption.

As displayed in Figure 14 a single peak was clearly observed at 398.2 eV, corresponding to the presence of N atom in primary and secondary amine groups. After metal ion adsorption, a new peak with higher binding energy was appeared at about 400.2 eV which may be attributed to the complexation between NH2 and metal ions (R-NH2—M2+). On the other hand, the S2p spectra exhibited a faint peak at 167.3 eV assigned to oxidized sulfur. Another peak was observed at 162.6 eV which corresponds to the unbounded S atom in thiol groups. After metal uptake, the peak ascribed to oxidized sulfur becomes much stronger as well as its ratio area, indicating the ion exchange reaction between (S-H) groups on the M2 xerogel surface and metal ions. Additionally, the XPS spectra of Pb4f, Cd 3d, and Zn2p were also obtained.

As portrayed in Figure 15, the binding energies for Pb 4f7/2, Cd 3d5/2, and Zn2p3/ <sup>2</sup> were 137.9 eV, 404.7 eV, and 1021.1 eV, respectively. This result is in agreement with the FTIR analysis which suggests that metal ions form a bidentate complex on amino functionalized xerogel.
