Speciation Study Of Uranyl Complex With Organic Acids In Aqueous Phase By ESI-MS/MS

The specation of uranium determines the toxicity and geochemical migration properties of uranium-containing compounds.The study of uranium complex is a research hotspot in the direction of environmental protection,life safety and radioactive waste disposal.However,the lack of analytical methods is the bottleneck restricting the chemical research of uranium aqueous solution.In this paper,a method for rapid qualitative analysis of uranium-containing complexes in aqueous solution by electrospray ionization-tandem mass spectrometry has been established,which the effects of qualitative ions,collision capabilities and mass spectrometry conditions has been explored.By the ESI-MS/MS method without certified reference material,the structure of the uranium-containing ions was resolved to study the chemical structure of the uranium-containing compound in aqueous solution.The speciation of organic acids and uranyl were studied,then the existence of these organic acids uranyl complex and its mass spectrometry fragmentation pathway in aqueous phase were described.Based on the above studies,ESI-MS/MS was adopt to reveal the speciation of organic acids and uranyl of environmental samples.（1）The mechanism of the established method is as follows,the liquid sample is electrosprayed to form positive ions,and then they are bombarded and dissociated by high-energy neutral nitrogen molecules of 120 eV,160 eV or 200 eV in the collision cell of mass spectrometer.It is the key basis for the qualitative of uranium-containing ions that whether[UO₂]⁺signal emerges from precursor ions.The method has the advantages of less pre-treatment steps,higher recognition of uranium-containing complex,wider application range,stronger anti-interference ability and greater analysis efficiency.（2）Formic acid,acetic acid,lactic acid,oxalic acid,malic acid,tartaric acid and citric acid are the most common small molecular organic acids in the environment.The proposed method was used to study the complex of seven common organic acids and uranium in water and then to analyze their chemical structure.Thus,eleven kinds of uranyl formate complexes,fifteen kinds of uranyl acetate complexes,twelve kinds of uranyl lacetate complexes,eight uranyl oxalate complexes,twenty one kinds of uranyl malate complexes,thirteen kinds of uranyl tartrate complexes,seventeen kinds of uranyl citrate complexes were obtained.After study on the mass fragmentation regularities of the speciation of these organic acid uranyl complexes,the following conclusions can be obtained:1)For the seven kinds of organic acids,uranyl group can coordinate with up to three organic acids in solution,excluding oxalic acid.2)A uranyl group can only bind at most two oxalic acid groups,meanwhile,it may form tricoordinated complex ions with other organic acid groups.3)The speciation of uranyl complexs of malic acid,tartaric acid,citric acid and uranium are complicated,both mono-uranyl singly charged ions and double-charged ions or even tri-uranyl groups like[（UO₂）₃Tar（HTar）₂（H₂Tar）₂]²⁺were detected.Ions of poly-and multi-charged organic acid uranyl speciation are difficult to complex with H₂O.4)The chemical bond of monimeric organic acid uranyl ion is easily broken by dehydration or decarboxylation,and that of polyorganic acid uranyl ions are broken by lossing neutral organic acid molecules and dehydration or decarboxylation.5)The binding bond energy of uranyl and organic acid molecules or organic acid anions are significantly different,which means that organic acid molecules of electroneutral are most easily detached from the complex ions.6)Experiment results showed the following sequence of organic acid-uranyl complex bond energies of the complex speciation:organic acid poly-uranyl-binding bond>organic acid anion-uranyl bond energy>organic acid molecule-uranyl bond energy.（3）The speciation properties of the organic acid-uranyl complex have been investigated by ESI-MS/MS.Furthermore,speciation of five kinds of uranyl acetate,two kinds of uranyl oxalate and three kinds of uranyl citrate were identified.Multi-reaction monitoring（MRM）mode was adopted in order to improve the detection sensitivity of uranyl Acetate speciation.The sensitivity of uranyl Acetate characteristic ions in MRM mode is 10 times more than scanning mode.Electrospray-tandem mass spectrometry was attempted to study the complexes species of uranyl phytate in solution,since there were many applied research on uranium adsorption by phytic acid while lack of theoretical study relatedly.（4）It was found that one phytic acid molecule in the dissolved phytate uranyl complex binded to a uranyl group,in which the uranyl group was combined with two phosphate groups adjacent to the phytic acid,and could continue to form nine kinds of uranyl phytate ions with H₂O and phosphate groups.It was revealed that there are two forms of uranyl phytate which were soluble and precipitated forms.SEM,FTIR,XRD and other equipments were utilized to investigate the characterization of soluble and precipitated states of uranyl phytate.By investigation,we found that the uranyl-phosphate infrared characteristic peak signal of soluble uranyl phytate is more significant than the precipitated species.Besides,uranyl phytate of precipitated species has a typical semi-crystalline structure,while the soluble uranyl phytic acid is an amorphous solid.These conclusions provide an theoretical basis for furture treating research of phytic acid in uranium-containing wastewater and soil.In summary,this paper provides the method support for the in-depth exploration of the study on the coordination morphology of uranium nuclides and organic acids in aqueous solution.