نوع مقاله : مقاله پژوهشی
1 گروه پژوهشی فرآوری مواد معدنی پرتوزا، پژوهشکده چرخه سوخت هستهای، پژوهشگاه علوم و فنون هستهای، تهران، ایران
2 کارشناسی ارشد شیمی فیزیک، دانشکده فیزیک شیمی، دانشگاه الزهرا، تهران، ایران
عنوان مقاله [English]
Heavy metals such as uranium are among the most common pollutants in industrial effluents and water environments. Uranium is one of the most dangerous heavy metals in the environment due to its chemical toxicity and radioactivity. Removing toxic and radioactive pollutants from the environment for safe and efficient disposal of waste is a vital challenge that requires the development of selective and high-capacity separation materials. Since uranium contamination threatens surface and underground water, it is important to make more efforts for new materials and technologies to separate and remove uranium from aqueous solutions. The rapidly expanding field of nanotechnology is producing many exciting new materials with novel properties. Apart from all other applications, nanomaterials are expected to act as a new class of solid phase adsorbents for the separation of pollutants and heavy metals, including uranium, due to their unique surface and morphological characteristics. The present review has reported a summary of the types of nanosorbents for the purpose of separating and removing uranium ions. Adsorbent materials include magnetic nanoparticle, Fe3O4, hybrid nanomaterials, oxide and phosphate-based nanomaterials, and non-magnetic nanoparticles. According to reports, magnetic nanoparticles are used to remove elements such as: cadmium, cobalt, nickel, uranium, etc. This article has studied and reviewed various types of nanosorbents as very efficient structures for separating and removing uranium.
Uranium is one of the chemical elements whose atomic number is 92 and its symbol is U. Uranium is a silver-gray metal that is part of the actinide family. A uranium atom has 92 protons and 92 electrons. Because uranium isotopes are unstable, so that the half-life of its natural isotopes is between 159,200 years and 4.5 billion years, this element has weak radioactivity. The most common isotopes of natural uranium are uranium-238 (with 146 neutrons and constituting more than 99% of the uranium on earth) and uranium-235 (with 143 neutrons and constituting about 0.72% of the uranium on earth). The density of uranium is about 72% of the density of lead and a little less than gold or tungsten. Uranium exists naturally and in very small amounts, about a few parts per million, in soil, rocks and water, but it is commercially extracted from minerals such as uraninite. Uranium is one of the most important natural radionuclides in the earth's crust, which can cause surface and underground water pollution (Zhang et al., 1994). Uranium is present in the earth's crust in considerable amounts and its abundance is even greater than that of gold, so it can enter the food cycle of animals and humans in the form of a combination with other elements, even the waters of rivers, springs and wells. They contain measurable amounts of uranium, which, of course, have completely different concentrations in different geographical locations. Contaminated drinking water is the main way to enter the human food cycle and increase the rate of uranium adsorption into the human body. Figure 1 shows the routes of exposure to uranium, including through drinking water and how it is transferred to the food cycle. Humans enter some uranium into their body daily depending on the type of diet. This element can accumulate in the kidney and the first effect will be diabetic nephropathy. Short-term and long-term studies regarding the chemical toxicity of this element are not available, and therefore, a specified amount has not been obtained by the World Health Organization for uranium in drinking water. Also, remaining uranium in the body due to its radioactive nature can lead to an increase in the risk of cancer, including colon cancer and genetic problems (Anke et al., 2009, Ribera, 1996). With an average uranium concentration of 3.3 ng/ml, seawater is a source of about 4.5 billion tons of uranium (Singhal et al., 2017). Uranium is the heaviest and most abundant radioactive element that makes up 2.4 milligrams to 1 kilogram of the earth's layer. It can be easily dissolved, moved and settled in surface waters with little changes in the environment. With a half-life of millions to billions of years, uranium atoms slowly decay into a series of radioactive by-products: thorium-230, radium-226, radon-222. To use uranium as an energy source, the ore must be enriched to obtain a higher concentration of a certain isotope (uranium-235). Uranium-235 is fissile and produces a large amount of energy in the form of free heat, as well as a large amount of radioactive waste and enters the environment. Currently, spent uranium can only be stored, reprocessed or disposed of (Martins et al., 2010). New solid adsorbent materials are being investigated by many groups of scientists all over the world for the effective extraction of uranium from surface waters, including seas. Apart from the natural abundance of uranium in sea water, uranium contamination in groundwater caused by natural mineral rocks or its artificial activities has become a great concern for the health of living organisms (Chouyyok et al., 2016, Saha et al., 2019). The World Health Organization and the US Environmental Protection Agency have set 30 ng/ml as the maximum tolerable uranium concentration in drinking water (Saha et al., 2017). This substance has a very toxic and carcinogenic nature, therefore increasing efforts are made to prevent environmental pollution when dealing with sewage. Therefore, the development of more efficient solid adsorbent nanomaterials specific for metal ions may serve the dual purpose of uranium preconcentration from seawater and removal of uranium pollution from underground water. However, the extreme concentration of uranium in such samples and the interference of competing ions make this work challenging (Zheng et al., 2019). Magnetite nanoparticles have been studied by different scientists over the years to adsorb hexavalent uranium. The high magnetic response of magnetite nanoparticles makes them a unique choice compared to other solid phase extractants (Singhal et al., 2020). In some reports, the high adsorption capacity of hexavalent uranium was obtained while maintaining the good magnetic response of magnetite nanoparticles, the equilibrium time for batch extraction studies was from 2 to 24 hours (Zhou et al., 2019Tan et al., 2015 Li et al. , 2016). It should be mentioned that the selectivity of each nanoadsorbent towards 5 specific metal ions determines its application in complex environmental matrices. Therefore, there is still a lot of scope for the development of new magnetic nanoadsorbent materials. In this view, we will focus on the investigation of various types of nanosorbents for the selective extraction of uranium ions in water environments.