Controllable Preparation Of Functional Chitosan-based Microspheres And Their Adsorption Of Iodide From Radioactive Wastewater

The 21st century continues to see an unrelenting increase in energy demand worldwide due to rapid population and industrial growth,resulting in significant strain on the energy supply from traditional fossil resources.As an alternative to traditional fossil fuels,nuclear energy is considered as an essential clean energy source for alleviating energy shortage,adjusting the energy structure and reducing greenhouse gas emissions.Among the various radionuclides produced during nuclear fission,the radioactive isotopes of iodine(¹²⁹I and ¹³¹I)were the main nuclear wastes with high radioactivity and solubility.Radioiodine wastes generated from the fission of uranium in a nuclear reactor may disperse rapidly in air and water in the event of nuclear leakage,causing long-term health risks.Therefore,the effective removal of radioactive iodide from water is essential to improve public health and promote the sustainable development of nuclear power.Aiming at the problems of poor reusability,difficult recovery and high economic cost of the developed adsorption materials,in this dissertation,a series of functional chitosan-based microsphere adsorbents were prepared for the adsorption of iodide（I^-）from simulated nuclear wastewater.The adsorption property and adsorption mechanisms were proposed.The main contents and conclusions are shown as follows:（1）To overcome the problem of acid dissolution of chitosan,chitosan microspheres（CMs）with smooth surface,good dispersibility and average particle size of 277μm were prepared by water-in-oil（W/O）emulsion cross-linking method.After five adsorption-desorption cycles,the I^-removal efficiency of CMs still maintained more than 92%,and the maximum adsorption capacity could reach 0.8186 mmol g^-1.The I^-adsorption behaviors of CMs were found to describable by the pseudo-second-order model and Langmuir model very well.The electrostatic attraction between I^-and protonated amino groups（-NH3⁺）was the main driving force for adsorption.（2）Magnetic chitosan microspheres（MCMs）,which coated Fe₃O₄ magnetic nanoparticles（MNPs）,were prepared successfully by the combined hydrothermal method and W/O emulsion polymerization for the I^-adsorption.The magnetic separation properties of MCMs were used to reduce the use costs of CMs.The prepared MCMs exhibited smooth spherical morphology with an average particle size of 306μm and good magnetic properties(8.81 emu g^-1).The chitosan component of MCMs played a primary role in the I^-adsorption process.From p H 3 to 9,the I^-removal efficiency of MCMs was more than 90%,and the maximum adsorption capacity could reach 0.8087 mmol g^-1.The I^-adsorption process of MCMs could be well fitted to the pseudo-second-order model and Sips model.（3）MCMs were modified by the amination and protonation to improve their I^-adsorption capacity.The protonated amination MCMs exhibited a typical spherical morphology with an average particle size of 318μm and good magnetic properties(4.10emu g^-1).The I^-removal efficiency of MCMs was more than 89%at p H=3～11,and the maximum adsorption capacity was 1.3482 mmol g^-1.The I^-adsorption of protonated amination MCMs followed the pseudo-first-order model,Langmuir model and Dubinin-Radushkevich model.The observed high adsorption capacity of protonated amination MCMs could be attributed to electrostatic attraction and ion exchange.（4）In order to further improve the adsorption selectivity of MCMs,silver chloride magnetic chitosan composite microspheres（Ag Cl-MCMs）were successfully prepared by a modified double-emulsion mixed cross-linking method.The prepared Ag Cl-MCMs presented a typical spherical morphology with an average particle size of 976μm and good magnetic properties(4.25 emu g^-1).From p H 2 to 11,the I^-removal efficiency of Ag Cl-MCMs was more than 89%,with the maximum adsorption capacity of 1.6965mmol g^-1,well-fitting with the pseudo-second-order model,Dubinin-Radushkevich model and Sips model.When the concentration of coexisting anions(Cl^-、NO₃^-和SO₄^2-)was 20 mmol L^-1,the I^-removal efficiency of Ag Cl-MCMs still maintained more than89%,performing excellent adsorption selectivity.In addition,the I^-adsorption process of Ag Cl-MCMs involved both physical adsorption and chemical adsorption:on the one hand,the positively charged-NH3⁺of Ag Cl-MCMs could adsorb I^-via electrostatic attraction;on the other hand,Ag Cl of Ag Cl-MCMs could capture I^-to generate Ag I.Compared with other I^-adsorption materials,the protonated amination MCMs and Ag Cl-MCMs developed in this dissertation possessed obvious advantages in adsorption capacity,p H range and phase separation.In addition,Ag Cl-MCMs also exhibited excellent adsorption selectivity.Therefore,this study is expected to prompt the functional chitosan-based microspheres as the efficient and recyclable biosorbents for I^-removal from radioactive wastewater.