Buffer Effect Of Formic Acid On The Extraction And Separation Of An(?) And Ln(?) By Cyanex 301 And Relevant Mechanism

Separation of Ln(?)and An(?)is one of the most challenging tasks in the field of nuclear industry.It is of great importance for the proper disposal of high-level liquid waste.The purified Cyanex 301 has shown good performance in the separation of trivalent lanthanides and actinides.However,this system also has a series of drawbacks.For example,the extraction and separation efficacy is very sensitive to the acidity and high separation performance can only be achieved under certain p H conditions.Therefore,aiming at overcoming this drawback and maintaining the separation performance of this system,buffer agents were screened and the formic acid was finally adopted to stabilize the p H during the extraction.The effect of formic acid system on the separation of trivalent lanthanides and actinides by Cyanex301 and its related mechanism have been studied.The main conclusions are as follows:(1)The pka of formic acid is right within the optimum p H range(3.5 ~4.0)in the extraction and separation of Am/Eu by Cyanex 301,thus affording strong buffer ability to this system.Moreover,the complexation ability of formic acid with Am or Eu is weak,which could help maintain sufficient extractionability to Am.(2)The extraction and separation ability of Am/Eu by purified Cyanex301 was further investigated under different total concentrations of formic acid and different p H conditions.The distribution ratio of Am and Eu decreases with the increase of the total concentration of formic acid,but the Am/Eu separation factor can be kept above 103 between p H 3.5~ 4.0.(3)At p H 3.75 and 4.00,four-stage cross-current extractions have been carried out for the separation of Am/Ln in test tubes.At p H 3.75,only a very small portion of lanthanides were extracted,e.g.,0.75% of Nd and 0.15% of Eu,while 99.93% of Am was extracted.At p H 4.00,2.3% of Nd and 0.4% of Eu were extracted,while more than 99.99% of Am was extracted.(4)The impact of ? irradiation on the buffered extraction system was also investigated.When the irradiation dose is below 50 k Gy,the Am/Eu separation factors are greater than 1000.When the irradiation dose is less than10 k Gy,the separation factors are greater than 3000 and the distribution ratio of Eu is kept below 0.01,which indicates that the buffered extraction system has excellent radiation resistance.(5)The complexation chemistry of formic acid with Nd3+ and Eu3+ was studied by UV/Vis titration,time-resolved luminescence spectroscopy and density functional theory(DFT).It was found that formic acid and Nd3+ couldform up to two complexes,i.e.,1:1 and 2:1 complexes.The stability constants decrease as the ionic strength increases from 1 mol/L to 3 mol/L(Na Cl O4 +HCOONa).The luminescence and DFT calculation results suggest formic acid tend to form monodentate complexes with trivalent lanthanides and one water molecule is replaced by each formic acid during the complexation.