Preparation And Uranium Adsorption Mechanism Of Carboxyl/Amidoxime Functionalized Polymer

Uranium is an important strategic resource in nuclear industry,which restricts the sustainable development of nuclear power.The effective separation of uranium from seawater is a challenging international difficult scientific problem based on the large amount of uranium reserves in seawater,and the successful solution has important strategic significance and application prospect.The most important concern is the structure of functional materials for selective adsorption of uranium.Among all the adsorbents,the polymer materials with amidoxime groups have been widely recognized as the most promising one through marine field test.To improve the performance of such amidoxime functionalized materials,a large number of investigations have been carried out,typically in structural optimization,uranium adsorption assessment,and adsorption mechanism between uranium and amidoxime groups.Nonetheless there are some unsolved problems,besides the pollution-free disposal of highly hazardous organic contaminants(including acrylonitrile)produced in preparation process need to be considered,the adsorption capacity of uranium has no clear positive correlation with grafting degrees of functional polymers.Therefore it is necessary to design a new preparation method of the functional polymer and study how the external environment and the auxiliary groups besides amidoxime influence the coordination modes as well as the underlying adsorption mechanism.In the work of this thesis,we used irradiation grafting polymerization on polypropylene(PP)non-woven fabric and a new composite fiber(abbr.P-T)with skin-core structure where PP serves as the skin and poly(ethylene terephthalate)(PET)as the core.The functional polymers with amidoxime and different auxiliary groups have also been obtained successfully by controlling the reaction conditions.In order to find the key clues of the external environment and internal structures'influence on the adsorption behavior of uranium,different adsorption systems were designed and theoretical calculation was supplemented to find a possible explanation for the adsorption mechanism.The major research contents are as follows:(1)Amidoxime functionalized polymers with higher grafting degrees and small volumes of solid waste were prepared with irradiation emulsion grafting polymerization method.P-T displays both good irradiation stability(like PET)and good modification ability under irradiation(like PP),and the amidoxime functionalized polymers' grafting ratio can reach higher than 200%under the dose of 200 kGy with auxiliary groups including carboxyl,amide and imidazole.The structures and thermo performances of prepared materials were determined by Fourier transform infrared(FT-IR)and Differential scanning calorimetry(DSC).The wastes almost in solid state with less?1m3 produced by 1 ton functional polymers,which will bring less harmness to the surroundings.In order to impove the functional groups activation of amidoxime and carboxyl groups of P-T series functional polymers,all the adsorbents were pretreated with 2.5%KOH for 1 hours under the condition of 35? before employed in uranium extraction.The uranium adsorption capacity of P-T-g-AC(5/1)(degree of grafting?275%)was?132.4 mg/g equal to the same kind of functional polymers.(2)It was confirmed that a certain ratio exsiting between amidioxime groups and carboxyl groups when they coordinated with uranium.For investigating the applicability of P-T series functional polymers in real case,HCO3-/CO32-was chosen as the anionic species in solution at the seawater pH(?8).First of all,adsorption capacity tests of two P-T-g-AC fibers with different AO/CO ratios were carried out,it can be deduced the adsorption behavior is close to pseudo 2nd order kinetics and belongs to chemical adsorption.In the case of the mixture solution of V and U at 0.1mol/L HCO3-/CO32-,all the results demonstrated that V showed a positive relationship with amidoxime groups,but a poor relationship with carboxyl groups.When amide,imidazole and pyridine were used as the auxiliary groups,U ultimately lost competitive complex ability and once again the importance of carboxyl groups in extracting uranium from seawater was proved.(3)A novel penta-coordination U-AO2CO was first proposed that two amidoxime groups' amine-nitroges and oxime-oxygens with one carboxyl group's oxygen acting on one uranyl.Carboxyl/amidoxime functionalized polymer(PPAC)was obtained by means of peroxides grafting polymerization and hydroxylamine conversion for reducing interference from the trunk in analysis of the adsorption mechanism with uranium.The chemical composition and microstructure of PPAC were scrutinized by FT-IR,energy dispersive X-ray spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS),Scanning electron microscopy(SEM),Transmission electron microscopy(TEM)and specific surface area analysis.The performance study of over-dose PPAC in the adsorption of thorium and uranium was conducted in EDTA solution with lower concentrations under different pHs,it was found that the competing complex ability of uranium is higher than that of thorium on PPAC adsorbent.In the meantime,the best operating pH value for selective uranium adsorption is at?8 in the range of real seawater.By comparing the inductively coupled plasmamass spectroscopy(ICP-MS),FT-IR and X-ray diffraction(XRD)results,it was concluded that different mechanisms existing between PPAC and uranium at different pHs.A stable structure of five-member ring composed by one uranyl and one amidoxime group was dedcued by the amine possessing ion-exchange activities and the oxime with chelating function.On this basis and density functional theory(DFT)calculation,we proposed a novel penta-coordination form U-AO2CO.(4)Based on U-AO2CO complex,the major reason was explained firstly why the adsorption capacity of uranium had no clear positive correlation with grafting degrees of functional polymers and the ideal maximum adsorption capacity range of amidoxime funcationalized polymer with carboxyl groups was predicted.In order to study the stability of amidoxime and carboxyl groups coordinating with uranyl,DFT and Monte Carlo calculations were carried out to investigate the crucial configuration(linking activity,the probability of forming AO2CO and spatial hindrance)of polymer sorbents modified by such functional groups.The ideal maximum adsorption capacity was predicted to be 181.7-272.6 mg/g with molar ratio of amidoxime/carboxyl groups at 2:1 based on the coordination mechanism and formation probability of U-AO2CO complex.The uranium adsorption capacity of P-T-g-AC(5/1)was?132.4 mg/g similar with the depiction of U-AO2CO model.The discovery suggested that the grafting degree should be controlled at certain ranges which may make the preparation of functional polymers more economical.In this thesis,amidoxime functionalized polymers with superior performance have been investigated through a systematic study of matrix design,green preparation process and uranium adsorption properties,and the mechanism between functional materials and uranium has also been elucidated by theoretical calculations.The related work has a certain guiding significance on the development of material for uranium enrichment.