## 3 Results and Discussion
### Exposure to Metal Ions  
Figure 4 shows the adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) ions by the GO/P(AA-co-AM) hydrogels for different initial metal concentrations. While investigating whether the GO/P(AA-co-AM) hydrogels have the theoretical adsorption capacity, an attempt was made to investigate the limits of adsorption of the GO/P(AA-co-AM) hydrogels for the four metals. The graphs show the adsorption limits of GO/P(AA-co-AM) for the removal of Cu(II), Pb(II), Zn(II), and Cd(II) at initial concentrations of 0-2000 mg\(\cdot\)L\({}^{-1}\). The amount of GO/P(AA-co-AM) hydrogel used was 1.0 g\(\cdot\)L\({}^{-1}\), the adsorption conditions were 200 r\(\cdot\)min\({}^{-1}\), and the adsorption time was 60 min. It can be seen from Figure 4 that the adsorption of the GO/P(AA-co-AM) hydrogel for the four metals varied, but in the initial concentration range of 0-300 mg\(\cdot\)L\({}^{-1}\) of metal ions, the adsorption of the four metals on the GO/P(AA-co-AM) hydrogels was positively correlated at initial concentrations of 0-300 mg\(\cdot\)L\({}^{-1}\). Increasing the initial concentrations of the metal ions resulted in different deviations from the positive correlation line. This may be because the GO/P(AA-co-AM) hydrogel has a large number of folds and adsorption sites on the surface, and when the ion concentration is in a certain range, the metal ions can be adsorbed on the hydrogel surface without ion exchange or other forms of exchange. However, when the initial concentration of the metal continues to increase, the original adsorption site is close to being occupied, and then ion exchange or other forms of exchange are required to enable the metal ions  
Figure 3: FTIR spectra after adsorption of GO/P(AA-co-AM)-Cu, GO/P(AA-co-AM)-Pb, GO/P(AA-co-AM)-Zn, and GO/P(AA-co-AM)-Cd. to be adsorbed into the hydrogel. The adsorption obtained by this exchange is limited by many objective conditions and the competitive ability of different metal ions is different; thus, different adsorption effects can be produced. Among the different adsorption effects of the four metal ions shown in Figure 4, the best metal ion adsorbed by the GO/P(AA-co-AM) hydrogel was Pb(II), and the GO/P(AA-co-AM) hydrogel could still maintain a good adsorption effect when the initial concentration amount of Pb(II) ion in the solution was 2000 mg\(\cdot\)L\({}^{-1}\). The adsorption amount of Pb(II) adsorbed by the GO/P(AA-co-AM) hydrogel with an initial concentration of 2000 mg\(\cdot\)L\({}^{-1}\) was 1658.5 mg\(\cdot\)g\({}^{-1}\), and the adsorption rate could reach 82.93% and did not reach the adsorption limit. Continuing to increase the initial concentration of metal ions, the adsorption amount of the GO/P(AA-co-AM) hydrogels for Pb(II) would continue to increase, but the adsorption rate would decrease. This is because as the initial concentration increases, the active sites on the surface of the hydrogel decrease as the metal ions are bound; however, the middle of the hydrogel is also free of some metal ions and hydrogen ions (H\({}^{+}\)) and free hydrogen bonds (-H) that are combined with the active sites, and as the concentration of hydrogen ions and hydrogen bonds increases and the concentration of metal ions in the solution decreases, the free metal ions in the solution are prevented from adsorbing. As the concentration of hydrogen ions and hydrogen bonds increases and the concentration of metal ions in solution decreases, the free metal ions in solution are prevented from binding to the active sites on the GO/P(AA-co-AM) hydrogels. Compared with the adsorption capacity of the GO/P(AA-co-AM) hydrogel for Pb(II), the adsorption capacity of the GO/P(AA-co-AM) hydrogels for Cu(II) was relatively weak. The adsorption capacity of the GO/P(AA-co-AM) hydrogel for Cu(II) with an initial concentration of 2000 mg\(\cdot\)L\({}^{-1}\) was 1341.87 mg\(\cdot\)g\({}^{-1}\) and the adsorption rate could reach 67.09%.