## 3 Results and Discussions
### Optimization of Dispersive Solid-Phase Extraction Procedure
#### 3.2.1 Selection of the Adsorbent
During the GO modification, NaOH reacts with oxygen surface functional groups and replaces H\({}^{+}\). These phenolate forms can facilitate the replacement of Na\({}^{+}\) with metal ions, even at lower pH values increasing this way the metal adsorption onto NaGO surface. The mechanism for the adsorption of the metal ions was also confirmed by FTIR measurements, that were obtained for graphene oxide and NaGO prior and after adsorption of metals. The FTIR measurements are presented in Figure 3.  
For graphene oxide, the bands at 1060, 1240, 1380 and 1730 cm\({}^{-1}\) could be due to the stretching vibration of the C-O (alkoxy) C-O-C (epoxy) O-C=O (carboxyl) and C=O bonds, respectively [43; 44], while the band at 1625 cm\({}^{-1}\) can be attributed to the C=C skeletal vibrations that come from the aromatic ring [45; 46]. For NaGO, it is seen that these bands due to oxygen containing groups were eliminated, indicating that the oxygen functional groups reacted with NaOH. This was confirmed by EDS spectra where Na was found on the sample's surface (Figure 1c). Figure 3: FTIR spectra for GO (**a**) and NaGO (**b**) before and after metal ion adsorption (HM). From FTIR results after metal adsorption onto GO, it is seen that the band at 1740 cm\({}^{-1}\), attributed to the stretching vibration of C=O of carboxyl groups, the band at 1150 cm\({}^{-1}\), due to phenolic O-H bending mode and the band at 980 cm\({}^{-1}\), related to epoxy groups, were no longer visible. As a result, it was indicated that these groups were involved in the metals' adsorption. The following equations describe the interactions [40]:  
\[2\text{(COH)+M}^{2+}\rightarrow\text{(CO}^{-})_{2}\text{M}^{2+}+2\text{H}^{+} \tag{1}\]  
\[2\text{CO}^{-}+\text{M}^{2+}\rightarrow\text{(CO}^{-})_{2}\text{M}^{2+} \tag{2}\]  
where C represents the sorbent's surface. At strong acidic conditions (pH = 2), the interaction that is most likely to occur is the weak bonds between the (-OH) groups of graphene oxide and the metal ions (Equation (1)). When the pH of the sample increases from three to six, the surface functional groups that contain oxygen (-COOH, -OH) are deprotonated and strong electrostatic attraction between them and the divalent metal cations (M\({}^{2+}\)) can take place (Equation (2)). Additionally, for the GO spectrum after metal adsorption, the band at 1615 cm\({}^{-1}\) was shifted to 1630 cm\({}^{-1}\). This can be mainly attributed to an ion-exchange reaction among the metal ions and the delocalized protonated \(\pi\)-electrons of the graphene layer (-Cp-H\({}_{3}\)O\({}^{+}\)) [47]. This was also the case for the sorption of cadmium(II), mercury(II), chromium(III) and lead(II) on activated carbons [47; 48; 49; 50; 51]. In the spectrum of NaGO after metal ion adsorption, this shift was more pronounced (from 1580 to 1500 cm\({}^{-1}\)) indicating that the ion-exchange reaction among the metal ions and the delocalized protonated \(\pi\)-electrons of the graphene layer was an important mechanism of adsorption for this 2D sorbent, where metal ions could more easily approach and be sorbed on its graphene layers. Thus, FTIR results suggest that adsorption of metal ions onto graphene oxide and NaGO surface occurs through an ion-exchange reaction between the metal ions and the delocalized protonated \(\pi\)-electrons of the graphene layer, as well as through weak bonds and/or electrostatic forces among the oxygen containing groups of graphene oxide and NaGO and the metal ions. Based on these results, finally, NaGO material was chosen for further examination and an adsorption pH value of 5 was employed for the whole experimental process.