###### Abstract  
Graphene oxide is a two-dimensional material that has been extensively studied in various fields due to its good mechanical properties, water dispersibility, and a large number of oxygen-containing functionalities on its surface. In this study, graphene oxide powder was prepared using graphite powder to take advantage of its large specific surface area and abundance of oxygen-containing functional groups. The graphene oxide powder was cross-linked with acrylic acid and acrylamide and polymerized to produce graphene oxide hydrogels, which were used to adsorb four metal ions including Cu(II), Pb(II), Zn(II), and Cd(II) from aqueous solutions. The adsorption performance of the graphene oxide hydrogels was investigated at different pHs, temperatures, initial metal ion concentrations, and competition principles, as well as their adsorption and desorption after three repeated adsorption-desorption experiments. It was found that the graphene oxide hydrogels exhibited good adsorption performance for all four metal ions under different conditions. The graphene oxide hydrogels for the adsorption of Cu(II), Pb(II), Zn(II), and Cd(II) ions were best fitted using the Langmuir monolayer adsorption model and the quasi-secondary reaction kinetic model. Good adsorption was achieved for all four metal ions under different competing adsorption principles. After three adsorption-desorption cycles, the adsorption capacity of the graphene oxide hydrogels for all four metal ions remained at 88% and above. These results indicate that graphene oxide hydrogels are a stable, efficient, low-cost, and reusable adsorbent material for the treatment of metal ions in solution. metal contamination; adsorption mechanism; graphene oxide; adsorption capacity; hydrogel +
Footnote †: journal: Applied Sciences  
Article  
Preparation of Graphene Oxide Hydrogels and Their Adsorption Applications toward Various Heavy Metal Ions in Aqueous Media  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], C.  
in wastewater have emerged at this stage; however, simply having a good wastewater treatment method implemented is no longer the center of research for researchers at home and abroad [6; 7]. The treatment of heavy metals in wastewater needs to increase efficiency while reducing costs, including economic and time costs, as environmentally friendly practices and recycling and other multi-dimensional advantages have become the current direction of research at home and abroad. There are many different metal ions in wastewater, among which Cu(II), Pb(II), Zn(II), and Cd(II) ions are often found together, which makes it more difficult to treat wastewater [8]. Compared with the common treatment methods for metal ions in wastewater, i.e., chemical addition, biofilm treatment, photodegradation, and other treatment methods, the adsorption method is the center of this experiment because of its short treatment cycle, low treatment cost, recyclability of adsorbent, and its environmental friendliness [9]. The adsorption effect can be improved by increasing the number of adsorbents [10]. To improve the adsorption effect, increasing the adsorption capacity of the adsorbent becomes a key issue to improve efficiency [11]. As the active site of the adsorbent is a certain size and directly affects the adsorption capacity of the adsorbent, increasing the active site of the adsorbent can effectively improve the adsorption effect [10; 12]. Therefore, it is important to adjust the internal structure of the adsorbent and study an adsorbent with high-adsorption efficiency for the treatment of metal ions in wastewater [13]. Graphene is a popular porous material used in research and is described as a two-dimensional carbon nanomaterial [14]. Graphene is a high-strength, high-toughness, and high-optical-absorption material widely investigated for battery applications, as well as for the thermal properties of graphene thin films.