Investigation On The Interaction Between Acidic Organic Phosphorus Extractant And Rare Earth Ion At Air-water Interface

The acidic organophosphorus extractants are widely employed in the industrial production of extracting and separating rare earth elements.The separation selectivity of extractant molecules for different rare earth ions depends on their ability to bind the target ions and the difference in the properties of rare earth complexes in the organic phase.While the properties of praseodymium and neodymium elements are so similar that the separation coefficient is generally about 1.5 through the conventional extraction method.Due to the solvent extraction is the result of the interaction of extractant molecules with metal ions at oil-water interface,exploring the difference in the interaction between Pr³⁺ ions and Nd³⁺ ions with extractant molecules at interface is helpful to the development of new separating methods.The extractant molecules,mainly in the form of dimers in the bulk phase,change their existing forms when they enter the interface.In order to understand the interaction between the extractant molecules and rare earth ions,Langmuir monolayer technique was used to imitate extractant molecules adsorbed at oil-water interface.The effect of extractant molecules existing forms on their interaction with Pr³⁺ and Nd³⁺ ions were investigated by infrared reflection absorption spectroscopy（IRRAS）and Brewster angle microscope（BAM）.The existing forms of extractant molecules at air-water interface were studied by spreading P507 molecules on the surface of water without metal ions.It was found that the π-A isotherms of P507 monolayers spread by n-hexane and dichloromethane are different.The characterization by IRRAS and BAM confirmed that the existing forms of P507 molecules in the monolayer changed with the spreading solvent polarity.And P507 molecules mainly exist in the form of monomers at interface as the spreading solvent polarity increased.Moreover,when the monolayer is completely protonated,P507 aggregates are more likely to form in the monolayer spread by n-hexane.This work investigated the existing forms of P507 molecules at air-water interface,which would help to understand their interfacial behavior at oil-water interface during solvent extraction.The interaction between P507 molecule and Nd³⁺ ions was analyzed by spreading P507 monolayers on the surface of NdCl₃ solution.The plateau region in the π-A isotherms appeared when P507 molecules bind to Nd³⁺ ions and the plateau gradually disappeared as NdCl₃ solution pH decreased.The characterization of BAM microscopy showed that the plateau region indicated the transition process of P507 monolayer from a two-dimensional monolayer to the three-dimensional structure.And when the plateau region disappeared,the aggregates of P507-Nd complexes formed in the monolayer.As the existing forms of P507 molecules at interface changed with n-hexane and dichloromethane,the IRRAS spectra confirmed the binding sites of P507 molecules with Nd³⁺ ions were changed with their existing forms.When P507 monolayer was spread by n-hexane,Nd³⁺ ions only bound to P=O groups.And as the dichloromethane was spread solvent,P=O and P-OH groups both bound to Nd³⁺ ions.Fourier transform infrared spectroscopy（FTIR）and IRRAS were used to compare the difference between Pr³⁺ ions and Nd³⁺ ions in the interaction with P507 molecules in bulk phase and at the interface,respectively.FTIR spectra showed that there was little difference between Nd³⁺ ion and Pr³⁺ ions in bulk phase.IRRAS spectra showed that the difference between Pr³⁺ ions and Nd³⁺ ions in the interaction with P507 molecules at air-water interface depended on spreading solvent and subphase pH.The Nd³⁺ ions had the stronger interaction with P507 molecules than Pr³⁺ ions in monolayer spread by dichloromethane,which was indicated by the band of vas PO2-at a lower wavenumber.The decrease in solution pH could enhance the coordination ability of Pr³⁺ ions with P507 molecules.But when the monolayer was spread by n-hexane,Pr³⁺ions were slighter stronger than Nd³⁺ ions in combination with P507 molecules.The difference in the interaction of P507 molecules with Pr³⁺ and Nd³⁺ ions at interface will provide a theoretical basis for the study of extraction behavior of them from the interface level.