Effect Of Surface Modification Of Polyacrylonitrile On The Adsorption Properties Of Uranyl Ions In Simulated Seawater

With the continuous growth of the global economy,the demand for new energy in human survival and industrial development is urgently required.As a clean energy source,nuclear energy is the only reliable source,providing large-scale electricity without generating greenhouse gases,highly concerned by countries all over the world.However,the sustainable development of nuclear energy depends on the continued supply of uranium(nuclear fuel),which is currently derived from terrestrial uranium.Actually,the proven uranium deposits are exclusively centralized in certain areas of the earth.These deposits can only provide electricity supply within the next 100 years without regard for the increasing energy consumption rate.The ocean contains about 4.5 billion tons of uranium,which is thousands of times more than the total reserves of land uranium.The development of marine uranium resources can continuously supply sufficient nuclear fuel to nuclear power plants.In addition,the extraction of uranium from seawater can effectively reduce the negative impact of faults,which comes from land uranium mining.Accordingly,it is of great strategic significance to carry out research on uranyl extraction from seawater.In this thesis,for the adsorption of low-concentration uranyl in seawater,the easily modified polyacrylonitrile fiber as the matrix material,the small organic molecules or macromolecules,inorganic particles and organic-inorganic hybrid materials rich in functional groups were grafted onto the surface of polyacrylonitrile fibers by chemical modification,supported with inorganic particles and post-treatment electrospun nanofibers to develop four adsorbents with high adsorption capacity for uranyl extraction from seawater.The physicochemical properties of the adsorbents were well characterized by FT-IR,SEM,XRD and TEM.The effects of solution pH,initial uranyl concentrations and contact time on the adsorption performance were systematically investigated by batch adsorption experiments.The adsorption isotherm models and the kinetic models were established to study the adsorption process as well.The adsorption in competitive ion solution and simulated seawater were also carried out to further investigate the practicability of the four adsorbents.Moreover,the absorption mechanisms of the four materials were discussed through XPS and other analytical techniques.Functionalized with lysine,which has strong complexation for uranyl ions,onto the polyacrylonitrile fiber matrix,the fiber adsorbent(PAN-Lys)with high adsorption capacity was prepared.Since the conventional alkali hydrolysis process would destroy the mechanical strength of the fiber matrix,hydrazine hydrate was firstly used for the cross-linking pretreatment to make it cross-link during the process of hydrolysis,and then lysine was grafted onto the polyacrylonitrile fiber.The adsorption capacity of polyacrylonitrile fiber was increased from 96.2 mg g-1 to 390.3 mg g-1 with maintaining good mechanical properties.In the simulated seawater with concentrations of 3?30 ?g L-1,the removal ratios of PAN-Lys fiber toward uranyl ions were about 80%,showing good adsorption performance.The graft of lysine increases the adsorption capacity and selectivity of the fiber toward uranyl ions,which is primarily ascribed to the coordination of the uranyl ion by the lone pair of electrons on the oxygen atom in the carboxylic hydroxyl groups and the nitrogen atom in the amino groupsRich in imino nitrogen atoms with unique coordination to uranyl ions,the polyethyleneimine was modified onto polyacrylonitrile fiber to prepare uranyl adsorbent(PAN-PEI)used in seawater with high selectivity,and the adsorption properties of the organic macromolecule modified fiber adsorbent were further studied.Meanwhile,the intrinsic relationship between the grafting degrees of polyethyleneimine,mechanical properties and uranyl adsorption performance was explored.The results show that the higher the grafting degree of polyethyleneimine,the worse the mechanical strength of the adsorbent,while the selectivity toward uranyl ions and the removal ratio in simulated seawater show a trend of increasing at first and then decreasing.Considering mechanical strength and adsorption performance,PAN-PEI0.33 adsorbent has considerable application prospects in simulated seawater adsorption.PAN-PEI0.33 has a good desorption effect in HCl,and the desorption rate is as high as 88.1%.After 5 adsorption and desorption cycles,the adsorption capacity can still reach 435.7 mg g-1.PAN-PEI0.33 has high removal ratios of more than 94%under simulated seawater at low uranyl concentrations,showing excellent affinity and selectivity toward uranyl ions.With polyethyleneimine as the "anchor",a composite uranyl adsorbent with high adsorption capacity was prepared by grafting nickel-aluminum hydrotalcite with large specific surface area onto polyacrylonitrile fiber.Combining hydrotalcite with fiber materials can not only improve the defects of hydrotalcite powder materials in the practical application,but also enhance the adsorption capacity of polyacrylonitrile fiber by the rich functional groups in hydrotalcite,making the composite an excellent seawater uranyl adsorbent material.The experimental results show that the introduction of hydrotalcite further improves the selectivity of polyethylenimine-modified polyacrylonitrile fibers toward uranyl ions,and the distribution coefficient is as high as 4.8×105 mL g-1.Polyacrylonitrile and cobalt acetate composite nanofibers were prepared by electxospinning method,and then ZIF-67 was in situ grown onto the nanofibers with Co2+ as a site by the coordination of 2-methylimidazole with cobalt ions,followed with surface amidoxime functionalization of polyacrylonitrile nanofibers to prepare seawater uranyl adsorbent with high performance.The composite material exhibited a broad spectrum of pH for uranyl ions and the adsorption capacity can reach 498.4 mg g-1;it maintained excellent selectivity for uranyl ions in simulated seawater with the distribution coefficient as high as 1.8×105 mL g-1.