Graphene-based composites for heavy metal adsorption: a review on synthesis, mechanisms, and influencing factors

Heavy metals released into aquatic ecosystems have been a global concern due to their toxicity and persistence. Their eventual removal has turned into a necessary effort. Among other techniques, adsorption is the most reliable and prevalent method of water purification. The inherent advantages of graphene-based adsorbents, including their large specific surface area, small particle size, outstanding mechanical properties, abundance of functional groups, and excellent adsorption efficiency, have contributed to their appeal as effective adsorbents for the removal of heavy metals from aqueous solutions. Therefore, this article provided an overview of the latest advancements in graphene and its derivative composites in adsorbing heavy metals from the aqueous phase. A comprehensive and emphasized discussion on structural features, adsorption mechanisms, and synthesis techniques, along with a historical review of the composites carried out here. Furthermore, a summary of the factors affecting the adsorption capacity was provided, including temperature, solution pH, adsorbent dosages, contact time, initial metal concentration, presence of organic acid ligands, and coexisting ions. In addition, this article also emphasized the adsorption thermodynamics and the regeneration behavior of the graphene-based composite adsorbents. Lastly, a perspective and future direction for the use of graphene-based adsorbents in practice were given.