## 3 Results and Discussions
### Optimization of Dispersive Solid-Phase Extraction Procedure  
#### 3.2.1 Selection of the Adsorbent  
Initially, three graphene-oxide-based materials were evaluated for the extraction of metals from aqueous sample solutions. For the comparison of the materials, stable adsorption and desorption conditions were used. The examined pH ranges were between 2 and 6. The pH of the solution is a critical parameter for the toxic and nutrient element adsorption onto carbonaceous materials. The extraction recoveries that were obtained for the examined elements at preliminary experiments with GO, RGO and NaGO are shown at Figure 2.  
Figure 1: (**a**) XRD patterns for GO, RGO and NaGO. (**b**) SEM image for NaGO, (**c**) EDS results for NaGO and (**d**) TEM image for NaGO. With GO, the removal of the metal ions at low values of pH, for the examined metals (except from Pb), was found to be less than 20 %. However, with the increase in pH to the range of 4-6, the metal removal increased. As a result, that was the optimum pH range for GO, however, the removal of metal ions was still low (>50%). In order to explain the effect of pH on the adsorption of metal ions onto carbonaceous materials, both the metal speciation in solution and the surface acidity of these materials must be taken into account. In principle, from the results regarding metal speciation, the pH range where metals are present in their cationic form and no precipitation occurs can be determined. At low values of pH, the surface groups of graphene oxide that contain oxygen are protonated. As a result, suppression of adsorption might happen, due to the complexation among the positively charged metal ions and the functional groups of graphene oxide that carry equal charge, since they may repel each other. On the other hand, as the pH value of the solution increases, adsorption is enhanced, due to the interaction of metal ions with the surface of graphene oxide which is negatively charged. This has also been observed for the influence of the pH value on the adsorption of metal ions onto activated carbons [39; 40]. Similar results were observed for RGO since the pH range of 4-6 was also found to be the optimum range for metal removal with this material. Compared to the GO, with the RGO adsorbent, the metal removal increased and reached 50-90% for some metal ions. This could be attributed to the two-dimensional (2D) structure of RGO and the simpler adsorption of metal ions through Cp-cation interactions on this material, compared with the GO that exhibits a three-dimensional (3D) structure. These interactions are electrostatic based between the metal cations and the quadrupole moment of the aromatic rings of the carbonaceous material, which may be due to a bond between the positive charge (i.e., H\({}_{3}\)O\({}^{+}\) ions) and the delocalized \(\pi\) electrons of the aromatic rings [41]. If the delocalized \(\pi\) electrons are considered as a Lewis base, the interaction could be a cation exchange interaction among (-C\(\pi\)-H\({}_{3}\)O\({}^{+}\)) and metal ions and this would, thus, be a plausible mechanism [42]. \[C\pi\)-H\({}^{+}\)+M\({}^{+}\rightarrow\)C\(\pi\)-M\({}^{+}\)+ H  
Figure 2: Extraction recoveries at different pH values with graphene oxide (GO), reduced graphene oxide (RGO) and NaOH modified graphene oxide (NaGO).  
With (-C\(\pi\)-H\({}_{3}\)O\({}^{+}\)) to be after the equation:  
\[\text{-C}\pi+2\text{H}_{2}\text{O}\rightarrow\text{C}\pi\text{-H}_{3}\text{O}^{+}+ \text{OH}^{-}\]  
Different results were observed for NaGO. From Figure 2 it can be clearly observed that the use of NaGO led to higher recovery values and higher range of pH scale where extraction recoveries are considered robust. The optimum pH range, in which the metal removal was the highest, was found to be 3-6, while the metal removal increased and reached almost 100% for some metals. This could happen not only due to the 2D structure of NaGO but also due to the simpler adsorption through \(\pi\)-\(\pi\) stacking on this sorbent and due to ion exchange.