## 3 Results and Discussion
### Exposure to Metal Ions
However, the adsorption capacity of the GO/P(AA-co-AM) hydrogel for Zn(II) and Cd(II) metal ions was relatively weak; after the initial concentration of metal ions exceeded 300 mg\(\cdot\)L\({}^{-1}\), the adsorption capacity of the GO/P(AA-co-AM) hydrogel for Zn(II) and Cd(II) continued to increase with the initial concentration of metal ions. There were different cases of weakening, with the adsorption rate decreasing significantly with increasing initial concentrations of metal ions. Although the adsorption capacity of the hydrogel for both metal ions increased, when the concentration of the metal itself was increased to 1000 mg\(\cdot\)L\({}^{-1}\), the adsorption capacity of the hydrogel for both metal ions did not increase significantly when the initial concentration of the metal ions was increased. When the initial concentration of metal ions in the solution was 1000 mg\(\cdot\)L\({}^{-1}\), the adsorption of the GO/P(AA-co-AM) hydrogels for Zn(II) and Cd(II) was 723 mg\(\cdot\)g\({}^{-1}\) and 635 mg\(\cdot\)g\({}^{-1}\) with 53.87% and 54.83% adsorption, respectively. Figure 4: Adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) ions removed by GO/P(AA-co-AM) at initial concentrations of 0–2000 mg\(\cdot\)L\({}^{-1}\) (mGO/P(AA-co-AM) = 1 g\(\cdot\)L\({}^{-1}\), stirring speed = 200 r\(\cdot\)min\({}^{-1}\), \(t\) = 60 min). The effect of pH on the adsorption of metal ions by the GO/P(AA-co-AM) hydrogels is shown in Figure 5a. With the gradual increase in pH, the adsorption of the GO/P(AA-co-AM) hydrogel for Cu(II), Pb(II), and Zn(II) metal ions showed an increasing trend, which should be due to the high content of hydrogen ions in the solution at lower pHs. The high content of hydrogen ions may compete with metal ions for limited adsorption sites, thus leading to a decrease in the adsorption capacity of the hydrogel for metal ions or the hydrogen ions combined with the carboxyl group (-COO-) on the surface of the GO/P(AA-co-AM) hydrogel. This makes the negative charge on the surface of the hydrogel unable to attract the positive charge of the metal ions, which weakens the electrostatic adsorption between the hydrogel and the metal ions. The adsorption of all four metal ions on the GO/P(AA-co-AM) hydrogel at the lowest pH was greater than 550 mg\(\cdot\)g\({}^{-1}\), indicating that the adsorption of metal ions by the GO/P(AA-co-AM) hydrogels is not only electrostatic. However, when the pH is increased to a certain level, the metal ions combine with other anions in the solution, such as hydroxide, and precipitate metal compounds at higher pHs, which is why the adsorption effect of the GO/P(AA-co-AM) hydrogel on Cd(II) decreases with the increasing pH. The turnaround at pH = 4 and Pb(II) and Zn(II) metal ions are close to the maximum value of adsorption. There is an objective fact that the actual industrial wastewater generated has a generally low pH. Therefore, the solution at pH = 4 was chosen to analyze the adsorption mode of the four metal ions adsorbed by the GO/P(AA-co-AM) hydrogels. The effect of temperature on the adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) by the GO/P(AA-co-AM) hydrogels is shown in Figure 5b. In the graph, the adsorption of all four metal ions by the hydrogel increased to varying degrees with increasing temperature. This is because the increase in temperature improves the swelling properties of the hydrogel and the migration rate of metal ions in solution also accelerates at the same time, enhancing the exchange between the hydrogel and metal ions. It is concluded that the increase in temperature contributes to the adsorption reaction and indicates that the adsorption of metal ions by the GO/P(AA-co-AM) hydrogels is a heat absorption reaction.