Theoretical Investigations On Coordination Chemistry Of Pa(V) In Hydrous Solution

Pa is the first actinide element with f electron,which has unique properties.Pa(V)has a strong hydrolysis effect:it can easily to form colloid in water and adsorb on the wall of the vessel;it forms mono-oxoyl ion PaO3+ in noncomplexing strong acidic aqueous solution;it has strong radioactivity and biological toxicity.However,Pa is an important intermediate in the thorium uranium fuel cycle,and also has important applications in nuclear forensics and geological age measurement.Therefore,Pa chemistry has always been an attractive and challenging problem.This work starts with the chemistry of Pa(V)aqueous solution that was less involved by previous researchers.First,the coordination number of Pa(V)hydrate ions is studied,and the conversion to various ions in hydrolysis is analyzed.The selectivity of Pa(V)extraction by dialkylamides with different alkyl chains is investigated.All of these are innovative and basic work,which establish the foundation for the further study of Pa chemistry.Density functional theory(DFT)method was used in this work.Starting from the study of the initial state of Pa(V)in aqueous solution,the structure,thermodynamics and kinetics of Pa(V)ions with different number of water molecules were investigated to confirm the stable structure in water and analyze kinetic/thermodynamic relationship of the formation of Pa=O complex by initial hydration ions.The conversion of[Pa(H2O)9)]5+ ions in water was studied,the possible kinetic processes were calculated,and the stable structures of the initial and final states of each kinetic process were optimized.Taking[Pa(H2O)9]5+ion as the energy zero point,the relative energy of each structure was calculated.Factors such as the transition state,the stability of the final state structure and the change of thermodynamic data in the process were analyzed and discussed.Regarding to the extraction problem of Pa(V),the affinity between dialkylamide and Pa(V)was studied by using dialkylamide as extractant,which has been concerned by researchers in recent years.It has provided a theoretical basis for the selective extraction of Pa(V).The following conclusions are obtained:For the coordination number of Pa(V)hydrated ions,the two most stable initial ions are[Pa(H2O)9]5+ and[Pa(H2O)10]5+.The initial hydrated ions cannot exist stably in water,prone to transfer proton with the water ligand in the second coordination sphere,and form a stable Pa=O complex hydrated ion,which has high thermodynamic stability.The most stable Pa=O structure in aqueous solution is[PaO(H2O)6]3+.This structure is formed by the proton transfer and water dissociation mechanism starting from[Pa(H2O)9]5+ and[Pa(H2O)10]5+.[PaO(H2O)6]3+ adopts pentagonal biconical configuration.The relatively weak Pa-O bond(compared to the equatorial Pa-O bond)is in the trans position of Pa=O bond,which results in a easier substitution reaction in the ligand exchange and extraction,that is,N,N-dialkyamide preferentially replaces the trans water molecule of Pa=O bond.After[Pa(H2O)9]5+ ion is hydrolyzed,various ions form a hydrolysis reaction equilibrium in the aqueous solution.The transition state energy barrier of spontaneous hydrolysis process is very small,about 5kcal/mol or less.Spontaneous hydrolysis produces the most important balance in water:Pa(OH)23+(?)Pa(OH)4+(?)PaO3+(omitting water molecules and H3O+ions in ions).The reaction of Pa(OH)4+ to both sides is spontaneous,and there is a equilibrium relationship of mutual movement and transformation in aqueous solutions with different acidity and ions composition.The formation of PaO(OH)2+ and PaO(OH)2+requires lower acidity,further hydrolysis and Gibbs free energy increases significantly during the hydrolysis process.The formation of PaO2+ ion relative to PaO(OH)2+ ion,Gibbs free energy increased by 1.9kcal/mol.The key point is that there is a conflict between two Pa=O orbitals in PaO2+ions,which makes it more difficult to form PaO2+ ions,and even if PaO2+ ions exist in aqueous solution,they will be converted into PaO(OH)2+and PaO(OH)2-hydration ions.The extraction strength of 20 selected N,N-dialkylamides and Pa(V)([PaO(H2O)6]3+ in aqueous solution)in noncomplexing acids was investigated.It has been found that there are two possible coordination modes between Pa(V)and amide,one is the coordination of N,N-dialkylamide substituted water molecules of trans position of Pa=O in[PaO(H2O)6]3+ion,the other is the addition and coordination of N,N-dialkylamide with[PaO(H2O)6]3+ion first,and then the unstable binding body dissociates a molecule of water.According to the coordination ability of 20 N,N-dialkylamide and[PaO(H2O)6]3+,the choice of three alkyl groups in N,N-dialkylamide,the straight chain has higher extraction efficiency than the alkyl group with branch chain;and for the straight chain amide,no matter the alkyl group on the acyl group or the two alkyl groups connected by N atom,the long chain alkyl amide with even number of carbon chains has higher extraction efficiency.In N,N-dialkylamide,the two same alkyl groups connected to N atom are connected with ethyl branch chain.When the branch chain is in ?,?,? position in turn,the coordination between amide and Pa(V)is weakened,and the delta position further away has less effect.The methyl branched chain linked alkyl group of the acyl is transferred from ? to ?,and its coordination ability is also weakened.At present,it is considered that the extractant N,N-di-(2-ethyl-hexhl)isobutyramide(D2EHIBA),which is the most likely to replace tributyl phosphoric acid(TBP),another methyl group to the two same alkyl chains connected to N atom,and the methyl group moves from the ortho-position of D2EHIBA ethyl to the tail of the chain,and the coordination ability slightly increases,but the change is very small.Therefore,in the thorium uranium fuel cycle,dialkylamide with long-linear-chain alkyl groups should be selected to extract Pa(V)ions efficiently and selectively.