Adsorption of heavy metal ions use chitosan/graphene nanocomposites: A review study

The emission of heavy metals in the environment has caused many problems in the world due to the industrialization of society and the expansion of urbanization. Environmental pollution, including greenhouse gases and water-soluble toxic pollutants, is a major concern around the world. One of the most important issues in protecting the environment and the health of humans, animals and plants is the health and cleanliness of water. Water is the most important substance vital for all living things on the planet. Water can be treated in several ways, such as desalination, purification, osmosis, disinfection, and deposition of contaminants. Among these methods, the adsorption method has many advantages over other methods. The adsorption method is a surface phenomenon during which contaminants are adsorbed in the adsorbent surface with physical forces and adsorption depends on many factors such as temperature, pH, concentration of contaminants, time of adsorption phase contact with contaminants, particle size of adsorption phase and contaminants and temperature and nature of pollutants and adsorption. Chitosan has received widespread attention as an adsorbent for pollutants because of its low cost and great adsorption potentials. Chitosan has abundant hydroxyl and amino groups that can bind heavy metal ions. However, it has defects such as sensitivity to pH, low thermal stability, and low mechanical strength, which limit the application of chitosan in wastewater treatment. The functional groups of chitosan can be modified to improve its performance via crosslinking and graft modification. The porosity and specific surface area of chitosan in powder form are not ideal, therefore, physical modification has been attempted to generate chitosan nanoparticles and hydrogel. Chitosan has also been integrated with other materials (e.g. graphene, zeolite) resulting in composite materials with improved adsorption performance. This review mainly focuses on reports about the application of chitosan and its derivatives to remove different heavy metals. The preparation strategy, adsorption mechanism, and factors affecting the adsorption performance of adsorbents for each type of heavy metal are discussed in detail. Recent reports on important organic pollutants (dyes and phenol) removal by chitosan and its derivatives are also briefly discussed.