| The study on the interface behaviors during the extraction is a new area in the extraction chemistry.It provides a new direction to understand the micro-mechanism of salt effect in the extraction.In this thesis,the specific salt effect on the extraction of anions(vanadium,chromium)and cations(rare earths)were studied from the viewpoint of interfacial behavior of salt ions.Based on the knowledge obtained from the mechanistic study,the salt effect was then used to strengthen the extraction and separation of vanadium,chromium,separation of rare earth from transition metal and separation of adj acent rare earth praseodymium/neodymium.The main research content and results are given as follow:Total internal reflection UV-visible absorption spectroscopy(TIR-UV)was employed to investigate the competitive adsorption behavior of salt ions at the organic/aqueous solution interface.It was found that salt anions exhibited different affinity to the interface.In addition,the affinity of salt ions to the interface followed the order of SCN->ClO4->I->NO3->Br->Cl->SO42-,which was consistent with the Hofmeister series.It provided the first evidence for the Hofmeister series in the adsorption behavior of ion at the buried liquid/liquid interface.The molecular level knowledge on the competitive adsorption behavior of ions at the organic/aqueous solution interface was also obtained from molecular dynamic simulations.It provided a basis for the further understanding into the competitive adsorption behavior of salt ion with V(Ⅴ),Cr(Ⅵ)ions at the interface.The competitive adsorption behavior of SO42-ions with V(Ⅴ),Cr(Ⅵ)ions at the interface was studied by using attenuated total reflection infrared spectroscopy(ATR-IR).It was found that the intensity of ATR peak of Cr(Ⅵ)ions would decrease gradually during the increase of SO42-concentration.While the intensity of ATR peak of V(Ⅴ)ions would not be affected by the addition of SO42-ions.Furthermore,molecular dynamic simulations were employed to elucidate the different adsorption behaviors of V(Ⅴ)and Cr(Ⅵ)ions.The selective adsorption behaviors of V(Ⅴ)and Cr(Ⅵ)ions could be regulated by the introduction of SO42-ions.It provided a fundamental basis for the development of new methods for the separation of V(Ⅴ),Cr(Ⅵ).According to the competitive adsorption behavior of V(Ⅴ),Cr(Ⅵ)at the interface,Na2SO4 was used as additive to strengthen the extraction and separation of V(Ⅴ),Cr(Ⅵ)in the alkaline solutions.It was found that the extraction of Cr(Ⅵ)could be inhibited by regulating the concentration of SO42-in the aqueous solutions.In addition,the extraction mechanism of V(Ⅴ)in the organic phase was suggested to be anion exchange reaction between HVO42-in the aqueous solution and NO3-in the organic phase.Thermodynamic studies demonstrated that the extraction of V(Ⅴ)was an exothermic reaction.Besides,the results indicated that V(Ⅴ)could be stripped effectively by NaNO3 aqueous solution.The organic phase also exhibited excellent regeneration performance.The specific salt effect on the extraction of rare earth in the neutral phosphine extractant-organic phase was studied.Unlike the traditional salting-out effect theory,we found that salt effect was also closely related to the concentration of salts in the aqueous solution.When the concentration of salt was low,the ability of salt ions in enhancing the extraction of rare earth followed the order of SCN->ClO4->NO3->Br->Cl-,which was consistent with Hofmeister series.When the concentration of salt was high,the ability of salt ions in enhancing the extraction of rare earth followed the order of NO3->SCN->ClO4->Br->Cl-,which was not consistent with Hofmeister series.To obtain the fundamental mechanism behind the specific salt effect,molecular dynamic simulations was employed to study the interaction of TOPO molecules with rare earth ions at the interface.The result indicated that the interface propensity of salt anions dominated the specific salt effect on the extraction of rare earth in the TOPO organic phase when the concentration of salt was low.While the interface propensity and salting-out ability of salt anions would co-dominate the extraction of rare earth in the TOPO organic phase when the concentration of salt was high.The specific salt effect on the extraction of rare earth in the non-functional ionic liquids was also studied.The experimental results indicated that the ability of salt ions in enhancing the extraction of rare earth followed the order of SCN->NO3->Br->Cl->ClO4-.Furthermore,the results of molecular dynamic simulations demonstrated that salt anions act as the bridge to connect rare earth ion with ionic liquids.Combining the experiments and simulation,the specific salt effect on the extraction of rare earth in the non-functional ionic liquids could be attributed to the synergism of complexation and hydration behavior of salt anions.The experimental results of extraction behavior of rare earth in four kinds of typical non-functional ionic liquids further verified the mechanism proposed above.It provided a fundamental basis for regulating extraction behavior of metal ions by using their different complexation behaviors.According to the mechanism of salt effect on the extraction of rare earth in the non-functional ionic liquids,we constructed an ionic liquids-based aqueous two phase system to separate rare earth from transition metal based on the different complexation ability of NO3-with rare earth and transition metal ions.The results indicated that the efficient separation of Nd from Fe、Co、Ni could be achieved by regulating the experimental parameters.In addition,the structure of extraction complex in the ionic liquid-rich phase was elucidated by NMR and molecular dynamic simulations.The result indicated that NO3-ions were in the inner coordination shell of Nd3+ ions,while ionic liquids were in the outer coordination shell of Nd3+ ions.In addition,similar micro-environment was existed across the liquid/liquid interface in the ionic liquids-based aqueous two phase system,which resulting the extra-low viscosity and interfacial tension.These characters of ionic liquids-based aqueous two phase system was in favour of transfer kinetics of metal ions across the liquid/liquid interface,and then enhanced the extraction kinetic of rare earth ions.A three-liquids phase extraction system consisting of P507 organic phase,ionic liquid-rich phase and NaNO3 aqueous solution was constructed to strengthen the separation efficiency of adjacent rare earth Pr/Nd.It was found that the separation efficiency of Pr/Nd in the three-liquid-phase extraction system could be enhanced compared to that in the organic-aqueous two phase and ionic liquids-based aqueous two phase system.The enhanced separation efficiency of Pr/Nd could be attributed to the external push-pull effect on the extraction of Pr/Nd in the P507 organic phase and ionic liquid-rich phase.The extraction experimental result of total rare earths demonstrated that the organic phase preferred to extract heavy rare earths,while ionic liquid-rich phase preferred to extract light rare earths.Therefore,the separation and enrichment of light rare earths and heavy rare earths could be simultaneously achieved in the three-liquids phase extraction system. |
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