Study On The Coordination Of Amidoxime-based Ligands With Uranyl And Competing Metal Ions

Amidoxime based adsorbents are widely used in the extraction of uranium from seawater,the treatment of uranium containing waste liquid,and the treatment of waste water containing heavy metal ions,because the N and O atoms in the amidoxime functional group contain unlinked lone pairs of electrons,which are easy to form stable complexes with a variety of metal ions.Especially in the aspect of uranium extraction from seawater,the adsorption method using amidoxime based adsorbent is considered to be the promising technological method to achieve the scale up and industrialization of seawater uranium extraction.In order to further improve the adsorption capacity of amidoxime based adsorbent,it is necessary to systematically evaluate the binding ability of the different functional groups of amidoxime-based adsorbents to uranyl and competing metal ions,and then provide a theoretical basis to guide the design and preparation of new high-efficiency amidoxime-based adsorbents.In this paper,the coordination chemistry of four amidoxime based ligands with uranyl and competitive metal ions was investigated by analyzing the preparation process of amidoxime-based adsorbents and selecting water-soluble small molecular ligands with similar structure to the functional groups on the material surface and easy to prepare as analogues.The stability of the ligands in HCl solution and Na OH solution was studied by using NMR spectroscopy,and the results showed that the ligands with 5-carbon backbone（H₃^gA and H₂^gB）are more stable than the ligands with 4-carbon backbone（H₃^sA and H₂^sB）.The four ligands are more stable in alkaline solution than in acidic solution,and the stability of the ligands is ranked as H₂^gB>H₂^sB>H₃^gA>H₃^sA.The protonation constants of ligands H₃^sA,H₃^gA,H₂^sB and H₂^gB and their coordination stability constants with UO₂²⁺in aqueous solution were determined by potentiometric and spectrometric titrations.The results show that the coordination stability constants of H₃^gA and UO₂²⁺are about 1.5-2 orders of magnitude higher than that of H₃^sA and UO₂²⁺complexes with the same stoichiometric ratio;the coordination stability constants of H₂^sB and H₂^gB and UO₂²⁺are basically the same.The binding ability of ligands to UO₂²⁺is in the order of H₃^gA>H₃^sA>H₂^gB～H₂^sB.The infrared spectra,absorption spectra and quantum chemical calculations of UO₂²⁺complexes show that H₃^sA tends to form a 2:2 dimerized complex with UO₂²⁺,and H₃^gA tends to form a 2:1 complex with UO₂²⁺.When H₂^sB reacts with UO₂²⁺under heating conditions,H₂^sB will undergo functional group conversion reaction to form a complex with UO₂²⁺in the structure of H₃^sA.The coordination stability constants of ligands H₃^sA and H₂^sB with Fe³⁺,Cu²⁺,Pb²⁺,Ni²⁺were determined by potentiometric titration.The results show that the binding ability of H₃^sA and H₂^sB to metal ions is Fe³⁺>Cu²⁺>Pb²⁺>Ni²⁺.Several new single crystals containing copper and nickel complexes were obtained.By analyzing the changes of ligand structure before and after the formation of metal complexes,it was found that when H₂^sB and H₂^gB react with Cu²⁺and Ni²⁺,functional group conversion reaction will occur,in which H₂^sB can be changed into 2,5-diaminopyrrolidin-1-ol or its derivatives to form complexes with metal ions,and H₂^gB can be changed into 2,6-diaminopyrrolidin-1-ol to form complexes with metal ions.In addition,H₂^gB can be converted into ligand H₃^sA in HCl solution containing Ni²⁺.In this paper,the stability constants of four amidoxime based ligands forming complexes with UO₂²⁺and other metal ions have been determined,several new single crystal complexes have been cultured,and the functional group conversion reaction of double amidoxime ligands in the presence of different metal ions has been found.This paper provides basic thermodynamic data and theoretical basis for the study of the adsorption mechanism of amidoxime based adsorbent and the development of new adsorbent.