Liquid-liquid extraction of cerium(IV) from nitric acid media by di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP)

In this paper, the extraction of Ce(IV) from nitric acid solutions is investigated using di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP, B) in heptane as extractant. Ce(IV) can be extracted effectively from nitric acid solution, whereas it is poorly extracted from sulfuric acid solution. Compared with some other organophosphorus esters, DEHEHP has moderate extractablity for Ce(IV). The extraction efficiency varies with diluent in the order: aliphatic hydrocarbons > nitrobenzene > aromatic hydrocarbons > carbon tetrachloride > chloroform. Regeneration and loading capacities of DEHEHP have also been examined. Ce(IV) extraction in HNO3 solutions as well as extraction of HNO3 and H2O have been systematically studied. The Ce(IV) extraction increases with an increasing of HNO3 concentration and exhibits the maximum distribution ratio at 1-1.5 mol/L HNO3. Nitric acid, as a source of nitrate ion, enhances the extraction of metal ion. But it also competes with metal ions for extractant molecules by its own extraction under high acidities. The proposed extraction process is described by the following equilibrium equations: H+ + NO3- + H2O + B-(o) -->(KH) HNO3 . H2O . B-(o) Ce4+ + 4NO(3) + 2B((o)) > Ce(NO3)(4) . 2B((o)) The extraction equilibrium constants and infrared spectroscopies of extracted complexes are determined. The extraction thermodynamic parameters are also calculated and the extraction reaction is found to be exothermic. The extracted species are estimated to be HNO3 . H2O . B-(o) and Ce(NO3)(4).2B((o)), and then DEHEHP molecule seems to behave as a monodentate ligand while NO3- ion as a bidentate ligand. In consequence, it is presumed that DEHEHP can be used as a potential extractant for separating Ce(IV) from nitric acid leach liquors of bastnasite.