## 4 Conclusions  
It was found that the graphene oxide could cross-link with acrylic acid and acrylamide, increasing the oxygen-containing functional groups and olefin double bonds on the hydrogel surface. The adsorption behavior of the GO/P(AA-co-AM) hydrogel for the adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) ions can be fitted using the quasi-secondary reaction kinetic model and the Langmuir isotherm model. The maximum adsorption capacities of the GO/P(AA-co-AM) hydrogels for the adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) were 1268.65 mg\(\cdot\)g\({}^{-1}\), 2026.87 mg\(\cdot\)g\({}^{-1}\), 704.24 mg\(\cdot\)g\({}^{-1}\), and 632.91 mg\(\cdot\)g\({}^{-1}\), respectively. It was shown that under different adsorption principles, the adsorption effect of the GO/P(AA-co-AM) hydrogels for the four metal ions was more balanced and stable, and the GO/P(AA-co-AM) hydrogels were suitable for the adsorption of multiple metal ions under coexistence conditions. The GO/P(AA-co-AM) hydrogel has a certain adsorption capacity even after three times of reuse. The present work shows that the GO/P(AA-co-AM) hydrogels can be used as an efficient and stable composite that can adsorb a wide range of metal ions. Experiment, data analysis, and paper writing: M.L., Y.W. ([PERSON]) and Y.W. ([PERSON]); supervision and paper revision: M.L., C.L. and X.L. ; writing--reviewing and  
Figure 11: Mechanistic diagram of the internal structure of GO/P(AA-AM) hydrogels. editing and correspondence: M.L. All authors have read and agreed to the published version of the manuscript. This work was supported by the Coal clean combustion technology for environmental protection equipment project, Item No. 3D516L911425, and the In situ extraction technology for remediation of heavy metal contaminated soil project, Item No. 3D5222559425. Not applicable. This work was supported by the Coal clean combustion technology for environmental protection equipment project, Item No. 3D516L911425, and the In situ extraction technology for remediation of heavy metal contaminated soil project, Item No. 3D5222559425. Not applicable. This work was supported by the Coal clean combustion technology for environmental protection equipment project, Item No. 3D516L911425, and the In situ extraction technology for remediation of heavy metal contaminated soil project, Item No. * _[PERSON] et al. (2015)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] Synthesis and characterization of copolymeric and terpolymeric hydrogel-siliver nanocomposites based on acrylic acid, acrylamide and itaconic acid: Investigation of their antibacterial activity against gram-negative bacteria. _Braz. J. Chem. Eng._**2015**, _32_, 509-518. [CrossRef]
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