Thermodynamic modeling of Pu(IV) and nitric acid extraction by 1.1 M tri-iso-amyl phosphate in n-dodecane

A thermodynamic model for the liquid-liquid extraction of tetravalent plutonium (Pu(IV)) by 1.1 M Tri iso Amyl Phosphate (TiAP) in n-dodecane was developed. The activity coefficients of species in the aqueous phase were estimated using eUNIQUAC activity coefficient model. The plutonium speciation in the aqueous nitric acid solution of 1-6 M concentration region was modelled by taking account of mono nitrate (Pu(NO3)(3+)), di nitrate (Pu(NO3)(2+)(2)), tetra nitrate (Pu(NO3)(4)) and hexa nitrate (Pu(NO3)(2-)(6)) complexes. The eUNIQUAC activity coefficient model parameter and stability constant of tetra and hexa nitrate complexes were determined using the experimental water activity as well as extraction data. The distribution coefficient of nitric acid and Pu(IV) in the 1.1 M TiAP was estimated by accounting HNO3 center dot TiAP, HNO3 center dot 2TiAP and Pu(NO3)(4)center dot 2TiAP complexes in the organic phase. A good correlation between the estimated and experimental organic phase concentration of nitric acid and plutonium nitrate was obtained.