Absorption of heavy metal ions using chitosangraphene nanocomposites

Master's Thesis in Polymer Industry Engineering

Abstract

With the advancement of human civilization, the development of technology and the ever-increasing population, the world is currently facing a problem called pollution, which threatens the lives of the inhabitants of the planet. Pollution caused by the accumulation of persistent toxic compounds in soil and water, such as chemicals, salts, heavy metals, and radioactive substances, are among the factors that cause many diseases, which greatly affect the health of humans, animals, and even plants. The release of heavy metals in the environment has caused many problems in the world due to the industrialization of society and the expansion of urbanization.

In this study, an attempt has been made to investigate the absorption of cadmium metal ions from aqueous solutions by using biocompatible and biodegradable polymer materials such as chitosan beads and using graphene functionalized with amine functional groups as one of the most widely used nanoparticles inside chitosan beads. For this purpose, chitosan/graphene nanocomposite seeds with weight percentages of 0.5%, 1%, 2% and 5% were prepared using the solution method. Graphene nanoparticles were first oxidized with a mixture of sulfuric acid and nitric acid, and then triethylenetetramine functional groups were grafted on them. The modified graphenes were identified using Fourier transform infrared analysis, thermogravimetry and scanning electron microscope EDX analysis, and the results indicated that the modification reaction was successful. In this regard, thermogravimetric curves showed that the operation of oxidizing and functionalizing graphene was done correctly. Also, the morphology of oxidized and functionalized nanoparticles was determined by scanning electron microscope, and EDX diagrams and images indicated the favorable distribution of oxygenated and nitrogenated functional groups grafted on the graphene surface. Next, the absorption of cadmium metal ions was done with an atomic absorption device, and the results indicated an increase in the amount of cadmium ion absorption by chitosan/graphene nanocomposites compared to chitosan grains by 20%. The atomic absorption device showed that the optimal amount of cadmium ion absorption occurs in a solution of 50ppm with a pH of 7, a contact time of 2 hours, and an adsorbent amount of 25mg. 1- Introduction Environmental pollution, including greenhouse gases and water-soluble toxic pollutants, has caused many concerns around the world. As such, these issues received a lot of attention at the recent United Nations World Summit on Sustainable Development in Rio +20. At the Rio+20 summit, world leaders, along with thousands of participants from the world's governments, the private sector, non-governmental organizations and other groups, came together to find solutions to reduce poverty, expand social justice and ensure the preservation of the environment on a planet that is ever more crowded [1]. Water is the most vital thing for all living things on the planet. Water can be treated in several ways such as desalination, filtration, osmosis, disinfection and sedimentation of pollutants. Among these methods, absorption method has many advantages over other methods. Adsorption method is a surface phenomenon during which pollutants are absorbed to the absorbent surface through physical forces. Adsorption depends on many factors such as temperature, pH, concentration of pollutants, contact time of absorbent phase with pollutants, particle size of absorbent phase and pollutant, temperature and nature of pollutant and absorbent material. The main pollutants in water are divided into two categories: Organic pollutants and ionic pollutants [2].

Ionic pollutants in water are divided into two categories:

A- heavy metal ions such as arsenic, mercury, cadmium, chromium, cobalt, copper, selenium and lead.

B- non-metallic ions such as fluoride, phosphate, nitrate and sulfide.

One ??of the most basic problems related to heavy metal ions is their non-decomposition in the body of living organisms.This means that these ions are not biodegradable and tend to accumulate in the tissues of living organisms. In fact, heavy metals are not excreted after entering the body, but are deposited and accumulated in tissues such as fat, muscles, bones and joints, which causes many diseases and complications in the body. In the last decade, the introduction of pollutants of human origin such as heavy metals into marine environments has increased to a great extent, which are considered as a serious threat to the life of aquatic environments. 

The accumulation of heavy metals in water, air and soil is a very important environmental problem. In the periodic table, those elements that have a high atomic weight and have metallic properties at room temperature are called heavy metals. Since there are different definitions for these elements and different elements are placed in this class, only the term metals or quasi-metals should be used. According to these definitions, metals from copper to bismuth in the periodic table that have a density greater than 4 are defined as heavy metals. In the periodic table, metals from groups 3 to 16 in periods 4 and 4 are called heavy metals. Many of these elements are not only not necessary for life, but also have toxic properties. Living organisms need very small amounts of heavy metals for growth and survival, which are called trace elements [1] such as iron, cobalt, copper, magnesium, molybdenum, vanadium, sternium and zinc. If the amount of these ions in the body increases from the minimum required and necessary, they cause growth disturbances. Other heavy metals such as mercury, lead and cadmium are not vital elements and do not have beneficial effects on the life of living organisms; So that their accumulation in the body of living organisms, especially mammals, causes dangerous diseases. The routes of entry of these ions into the body of mammals are usually through contaminated water and food[2].

Chitin and chitosan and their derivatives have attracted wide attention as an effective adsorbent for removing various pollutants from water due to their low cost and biodegradable nature, as well as having a high amount of nitrogen-bearing and carboxyl-bearing functional groups. Among these pollutants, we can mention metal cations and anions, radioactive materials, various pigments, phenol, as well as various anions and other pollutants. Chitin and chitosan have a very high potential to remove such pollutants from water. However, there is still a need to find practical tools such as surface adsorbents developed on a commercial scale [2].

For any absorption process, having a large cross-sectional area, high pore volume, and also having appropriate functional groups are among the key and primary requirements. To increase the absorption rate of polymers, nanoparticles are used due to having the mentioned characteristics. Currently, many nanoparticles including nanoclay and carbon nanotubes have been developed to remove pollutants from water. A nanoparticle that has recently attracted the attention of many scientists is called graphene. Graphene is a 1 (one atom) thick flat sheet made of carbon atoms arranged in a honeycomb crystal lattice. Graphene is the parent element of other carbon allotropes including graphite, carbon nanotubes and fullerene. Among the unique properties of graphene, it can be mentioned that it has high mechanical, thermal and chemical flexibility. Also, graphene has a very high specific cross-sectional area, which makes it a potential candidate as a high-performance adsorbent. However, graphene in its original form does not have much ability to absorb due to lack of suitable functional groups. Because of having 2Sp atoms, it can only absorb pollutants with van der Waals forces. Therefore, in this project, graphene is first modified and oxide groups are placed on it, and then amine groups are reacted with the surface of this material. At the end, this nanoparticle is used in chitosan hydrogel and the absorption rate of heavy metal ions is compared with pure chitosan hydrogel using this hydrogel nanocomposite.