Preparation Of Amino-acid Modified Amidoxime Materials And The Adsorption Mechanism Of Uranium/Vanadium

Nuclear power is important source of electricity and heat with low carbon emission.It constitutes by a large role the energy source on way to realize the goal“carbon neutrality and peak carbon dioxide emissions”.Uranium is the main fuel for such clean energy.Since the uranium resource is limited on land,how to extract uranium from seawater is now facing the current researchers as one of the biggest difficulties.The primary concern in uranium extraction from seawater is about how to prepare uranium adsorbents with high affinity and selectivity.Much research and real practice in seawater proves that the uranium extraction from seawater is feasible by using amidoxime-based polymer materials.Nevertheless,because of the very complex seawater system,one of the challenges facing amidomixe-based polymer adsorbents is the difficulty in combining together high selectivity to uranium and kinetics of the functional groups.Therefore,the present urgent task is to design and prepare adsorbents with high selectivity to uranium and to evaluate their real performance.This dissertation bases the research on poly（acrylonitrile）and prepares three kinds of polymer adsorbents grafted with amidoxime and auxiliary amino acid functional groups by a cross-linking method.Modern instrumental characterization is used to analyze the adsorbent’s structure at the micro scale.The research studies by static experiments the effects of solution acidity,initial concentration,temperature and adsorption time on the material’s performance towards uranium and vanadium.The experimental data is analyzed with kinetic and thermodynamic models.Meanwhile,experiments in regard of competitive ions are conducted to further scrutinize the adsorbent’s practicality in real seawater.Finally,the adsorption mechanism for uranium and vanadium by the polymer adsorbents functionalized by amino acid and amidoxime groups is deeply investigated by combining the experimental finding and theoretical calculation.（1）Glutamic acid@poly（amidoxime）（Glu@PAO）material with large adsorption capacity and hydrophilicity is prepared by cross-linking poly（amidoxime）with carboxyl and amino functional groups showing strong capability in binding with uranium ions.The uranium’s adsorption capacity is increased from 29.15 mg·g^-1 to 49.73 mg·g^-1 and the equilibrium adsorption time is decreased to 200 min from the original 300 min.In the meantime,the material’s vanadium adsorption capacity reduces and the equilibrium vanadium adsorption time is lengthened to 400 min.These indicate that the introduction of Glu group increases the PAO’s uranium adsorption capacity and rate,while at the same time changing its performance towards vanadium ions.The experimental finding in a mixed solution of uranium and vanadium reveals that the introduction of Glu is very conducive to amidoxime material’s adsorption selectivity towards uranium and vanadium.When using simulated seawater,Glu@PAO fiber material has a larger uranium ion adsorption capacity than that of other competitive ions with comparable concentrations.The uranium ion adsorption capacity is 9 times the VANADIUM ION.Glu@PAO changes well the poor selectivity of poly（amidoxime）material in uranium extraction from seawater.（2）Trypsin@poly（amidoxime）（TB@PAO）and dry yeast@poly（amidoxime）（PAO-Y）membrane material are prepared by crosslinking PAO with the peptide ligand.The number increase of amino acids increases the number of carboxyl and amino groups showing strong uranium complexing ability and enhances the materials’hydrophilicity.TB@PAO membrane increases the uranium adsorption capacity to 89.27 mg·g^-1and the equilibrium adsorption time is 100 min,far exceeding its vanadium adsorption capacity and rate obtained under the same condition.In the experiments in regard of the competitive ions in simulated seawater,the adsorption capacity of UO₂²⁺afforded by TB@PAO is 11.24 mg·g^-1,which is about 6 times that of Vanadium ion.The distribution coefficient of UO₂²⁺is 1.97×10⁵ mg·L^-1,1 to 3 orders of magnitude higher than those of other metal ions.The results prove that introducing the long peptide enhances greatly the uranium adsorption capacity and rate afforded by poly（amidoxime）and decreases notably its vanadium adsorption.The experiment about competitive ions suggests that the modified adsorbent（TB@PAO）performs with large uranium adsorption capacity and selectivity in a medium of high salinity in seawater.（3）Dry yeast@poly（amidoxime）（PAO-Y）composite membrane is prepared by crosslinking PAO with the protein ligand having specific binding with uranium ion.By merit of the protein’s specific uranium binding,PAO-Y membrane has an uranium ion adsorption capacity of 131.05mg·g^-1and the equilibrium adsorption time further shortens to 50 min,while its vanadium ion adsorption capacity is 21.38 mg·g^-1with an equilibrium adsorption time of 400 min.This indicate that the introduction of dry yeast increases greatly the material’s uranium ion adsorption capacity and rate and meanwhile weakens its vanadium ion adsorption.The selectivity of PAO-Y towards uranium is higher than that towards vanadium in all the mixed solutions of uranium and vanadium having different concentrations.In simulated seawater,the uranium ion（U（VI））adsorption capacity of PAO-Y membrane is greatly larger than those of most metal ions.It is 5 times the adsorption capacity of VANADIUM ION and the distribution coefficient（K_d）of uranium ion is greater than 10⁵ mg·L^-1,meaning PAO-Y has a great affinity for uranium ion.（4）CAM-B3LYP/6-311++g（d,p）method is used to calculate the binding and free Gibbs energy in the association of carboxyl in amino acid and amidoxime groups with uranium and vandium ion after simplifying the ligand.The stable associated structure between UO₂²⁺and amidoxime and amino acid ligands is determined,together with the effects of the number of amino acid ligands on the binding energy between UO₂²⁺and amidoxime.The results show that the penta-coordinate uranium is most stable which is formed via the synergistic effect of amino acid and amidoxime.It is also the most probable binding form between this kind of polymer and uranium ion.The coordination of VO₂⁺by single amidoxime is found to be the most stable.