Preparation Of Amidoximated Polyacrylonitrile Fibers And Their Adsorption Of Uranyl Ions And Competing Ions

Due to the very limited uranium resources onland, the research aiming to recoveruranium from seawater has been studied since50s of the last century in order to meetthe growing demand for nuclear fuel. Through unremitting efforts and attempts, scholarshave found that the adsorption technique is the most promising industrial approach.Among all of the adsorbents, adsorbents consisting of the amidoxime (AO) group arewidely recognized as the best owing to their high capacity and selectivity. AO group isthe key functional group of AO-based adsorbents. The conversion ratio (CR) of nitrilegroup and density of the amidoxime group (AOGD) are two important indicators. Therelationship between the adsorption capacity and the content of AO group and themicroscopic changes during the process of amidoximation should be worthy of furtherexploration.Generally speaking, it should become a major way to improve the adsorptionproperties of the material by increasing the content of AO group. However, this paperargues that, when the amidoxime content of the material comes to a certain extent,morphology and mechanical properties of the material will change, and by then theincreasement of AO group will be no more favorable, even hindering the adsorption.Therefore, the purpose of this paper is to investigate the relationship between thecontent of the AO group and the adsorption capacity of the material and to discover thelaw of the adsorption, in order to provide a reference for the further development ofadsorption material for the extraction of uranium from seawater.The content of this paper mainly includes two parts. The first part is the research ofamidoximation of polyacrylonitrile (PAN) fibers, and the characterization ofmorphology and properties. In this paper, polyacrylonitrile (PAN) fibers were chosen tobe the base material, and amidoxime polyacrylonitrile (AO-PAN) fibers were preparedby directly reacting PAN fibers with hydroxylamine. The influences of the reaction time,temperature, concentration of hydroxylamine, solvent volume ratio and pH value on theCR and AOGD were investigated. The results showed that extending the reaction time,raising the temperature and increasing the hydroxylamine concentration to some extent,are conducive to the increasement of CR and AOGD. Then both of CR and AOGD tendto achieve equilibrium. Neutral condition is most conducive to amidoxime reaction. Thefiber began to shrink and cake with the CR beyond10.8%. The morphology of thefibers was observed with a scanning electron microscope (SEM) and an opticalmicroscope. The results illustrated that the surface of the fibers did not show any seriouscracks or degradation. The fibers become swollen in aqueous solutions owing to thepenetration of water molecules because of the hydrophilicity of the AO groups, so therewould be a layer of hydrogel formed on the fibers surface. The thickness of the hydrogelwould increase with the increasement of CR and AOGD. The mechanical property offibers was tested with an electronic single silk strength tester and Dynamic Mechanical Analysis (DMA). The results indicated that the mechanical property decreases afteramidoximation. The breaking strength of the raw fibers was7.1cN/dTex, and it beganto decrease significantly after CR exceeded10.8%. When CR reached25.4%, thebreaking strength decreased to5.5cN/dTex.The second part is the research on the adsorption properties of AO-PAN fibersunder different experimental conditions. The adsorption experiments were performed inuranyl ion solution. The results showed that the adsorption capacity leveled off after48h. With CR increasing from0to4.7%, the amount of adsorbed ions increased sharplyfrom0to2.3mg/g; however, when CR reached10.8%, the amount of adsorbed ionsdecreased to2.0mg/g and decreased further to1.2mg/g for AO-PAN fibers with CR of46.3%.There was a consistent trend in the conclusion from the adsorption experimentscarried out in the seawater and simulated seawater. It indicated that even though theamount of ions adsorbed on the fibers strongly depended on CR and AOGD of thematerial, however, the relationships were not linear. Increasing CR and AOGD of thefibers would benefit to the adsorption property only when they were at low values. AfterCR exceeded10.8or AOGD exceeded1.9mmol/g, the adsorption property of fiberswould be restricted. This was because with the increasing of the CR and AOGD, thethickness of hydrogel layer formed on the fibers surface would also increase. Whenthickness reached1.4μm, the hydrogel layer might be a serious obstacle for adsorption.Therefore, only increasing CR or AOGD of the fibers by means of high levels ofamidoximation might hinder the adsorption capacity. It is crucial to control the balancebetween CR or AOGD, mechanical property, and the thickness of hydrogel layer duringthe preparation of AO-PAN fibers with ideal adsorption capacity.