New Cationic Framework Materials For Efficient And Selective Removal Of Anionic Radionuclides And Study Of Its Sorption Mechanisms

With the development of nuclear energy,how to safely dispose a large amount of radioactive wastes generated during the nuclear fuel cycle has become an issue the government and public concern.These radioactive wastes contain long-lived actinides and fission products,which exist as cations or anions in radioactive liquid wastes.The long-lived anions(⁹⁹TcO₄^-,⁷⁹SeO₄^2-/⁷⁹SeO₃^2-,¹²⁹IO₃^-)are more likely to leak and migrate than cations in current storage form of geological repositories due to the anions are negatively charged whereas most of minerals and rocks are neutral or anionic structures that can't stop the migration of the negatively charged species.Therefore,it would be critical and urgent for developing new techniques and materials for efficient removal of anionic radionuclides from those contaminated natural water systems.In high level nuclear waste solutions,there is also a large excess of competing anions.Hence,how to selectively and effectively remove these anionic radionuclides becomes a technical problem in the field of environmental radiochemistry.Cationic framework materials are considered as one of the most effective sorbents to remove anionic radionuclides.The superior properties of structure endow cationic MOFs with many advantages compared to the traditional porous materials.Until now,examples of water-stable cationic MOFs used in real anion exchange applications are still quite scarce,and the exchange efficiency is limited.Design of new hydrolytically and radiolytically stable cationic MOFs for selectively capturing ⁹⁹TcO₄^-,⁷⁹SeO₃^2-and⁷⁹SeO₄^2-from nuclear waste solution or contaminated water system remains highly desirable.This dissertation mainly studied the use of cationic MOFs to remove radioactive⁹⁹Tc/⁷⁹Se and the sorption mechanism.The research content includes the removal of anionic radionuclides with cationic framework materials with different metal nodes,different dimensions and different pore sizes.The relationship of structure-properties is also investigated to find the rules to develop new materials that can be engaged in this field.With the help of the X-ray single crystal diffraction and EXAFS techniques,the sorption mechanism of anionic radionuclides onto cationic framework materials was demonstrated.Based on the results,a new strategy was proposed to construct cationic MOFs with high selectivity and high adsorption capacity.The details are as follows:Chapter 2:We report one of the most efficient scavenger materials,a cationic crystalline coordination polymer SBN for trapping ⁹⁹TcO₄^-.The one-dimensional cationic metal-organic framework?SBN?is synthesized through the self-assembly of Ag⁺cations and 4,4'-bipyridine.The adsorption capacity of ReO₄^-by SBN is noticeably higher than most of the reported sorbents up to date including anion exchange resins and other inorganic or hybrid anion sorbent.The local coordination environments of NO₃^-and ReO₄^-probed by the single crystal structures before and after ReO₄^-uptake clearly unravel the underlying mechanism.Each ReO₄^-in SBR binds to multiple Ag⁺sites forming strong Ag-O-Re bonds.These structural insights lead to a significant difference in the solubility product constant between SBN and SBR phases,which is further confirmed by first principle calculations.To the best of our knowledge,SBR is the least soluble perrhenate/pertechnetate salt reported,indicating this type of materials is promising in the immobilization of pertechnetate.Chapter 3:In order to improve the stability and selectivity of cationic MOFs,we designed a new type of three-dimensional cationic MOFs,SCU-100,which is assembled from a tetradentate neutral nitrogen-donor ligand and Ag⁺cations.SCU-100 is exhibiting impressively fast kinetics,high capacity,and good selectivity for ⁹⁹TcO₄^-uptake,showing a clear advance over all reported inorganic cationic materials.This work is the first reported practical case of ⁹⁹TcO₄^-removal by a cationic MOF material.The sorption mechanism is well elucidated by single crystal X-ray diffraction,showing that the sorbed ReO₄^-anion is able to selectively coordinate to the open Ag⁺sites forming Ag-O-Re bonds and a series of hydrogen bonds.This work demonstrates the promise of using this type of material as a scavenger for treating anionic radioactive contaminants during the nuclear waste partitioning and remediation processes,but one of the clear drawbacks for this material is that larger single crystals of SCU-100 disintegrate into microcrystalline materials after anion exchange,making chromatographic extraction inapplicable.Chapter 4:We document here a solution to the aforementioned demerits of SCU-100,based on a hydrolytically stable and radiation resistant cationic MOF.SCU-101,a three-dimensional cationic MOF,was constructed from Ni²⁺cations,oxalate,and a tetradentate nitrogen-containing neutrally charged ligand,exhibiting higher sorption capacity towards ⁹⁹TcO₄^-/ReO₄^-than that of other reported ion exchange materials.The ion exchange experiments of using a simulated Hanford LAW melter recycle stream also confirmed that SCU-101 can remove ⁹⁹TcO₄^-selectively and efficiently.Meanwhile,SCU-101-Tc is the first reported crystal structure revealing the detail coordination environment of ⁹⁹TcO₄^-immobilized in the crystal structure of porous materials.The high ⁹⁹TcO₄^-uptake capability is elucidated by the single crystal structure of ⁹⁹TcO₄^-incorporated materials and first principle theory analysis of electrostatic potential distribution and bonding.This work demonstrates that SCU-101 is an attractive sorbent material for ⁹⁹TcO₄^-removal from either nuclear waste solution with high ionic strengths or contaminated water systems with low ⁹⁹TcO₄^-concentrations.What's more,it is also beneficial for further designing of cationic MOF materials with improved sequestration capability towards anionic pollutants.Chapter 5:based on adsorption and separation of selenium pollutants from cation framework materials,a class of two-dimensional inorganic cation frameworks Y₂?OH?₅Cl·1.5H₂O was synthesized.Then,the sorption of SeO₃^2-/SeO₄^2-onto Y₂?OH?₅Cl·1.5H₂O were investigated.The experimental results show that this kind of cationic layered rare earth hydroxide material?Y₂?OH?₅Cl·1.5H₂O?is the most effective adsorbent for SeO₃^2-and SeO₄^2-with an adsorption capacity of 204 mg/g and 125 mg/g for Se?IV?and Se?VI?,respectively.The adsorption performance of the material at low selenium concentration in presence of competitive anions were also studied in this work.The results shows that the remnant selenium concentrations which treated with Y₂?OH?₅Cl·1.5H₂O can fully meet the Chinese National Standards for drinking water of selenium.Such excellent features make it possible for real application of cleaning up selenium pollutants in the environment.The sorption mechanism was unraveled by EXAFS techniques.The analysis reveals the adsorption mechanism of Y₂?OH?₅Cl·1.5H₂O for SeO₃^2-/SeO₄^2-is an ion exchange separation at a microscopic level,and further explains the intrinsic reasons for highly-efficient adsorption at the molecular level.The results show that SeO₄^2-forms a weakly outer-sphere complex on the surface of Y₂?OH?₅Cl·1.5H₂O during the anion exchange reaction,whilst SeO₃^2-forms a strong bidentate binuclear inner-sphere complex.