Solubility of di-cyclohexylhydrogen phosphonate in supercritical carbon dioxide medium: Application towards the recovery of uranium from aqueous solution

Hydrogen phosphonates are di-esters of phosphonic acids and are widely employed as ligands for recovery of actinide species from different matrices. An alicyclic hydrogen phosphonate, namely, di-cyclohexylhydrogen phosphonate (DCyHeHP) is synthesized; its solubility in supercritical carbon dioxide medium is evaluated to-wards development of a supercritical fluid extraction (SFE) method for recovery of actinide species. The solubility experiments were carried out at three isotherms (313 K, 323 K, and 333 K) with pressure varying from 10 MPa -20 MPa, using a dynamic flow method. The mole fraction solubility ranges from 0.2 x 10-3 to 102 x 10-3 in the investigated region. The experimental solubility data was modelled using five semi-empirical equations; Mendez-Santiago, Chrastil, del Valle, Adachi Lu and Sparks equations. The experimental data was found to be self-consistence based on Mendez-Santiago equation. The prediction ability of these models was compared using absolute average relative deviation (AARD) and corrected Akaike Information Criterion (AICc). Among the models employed in the present study, Adachi Lu equation found to better with an AARD and AICc value of 4.2% and 87.2 respectively. Based on the solubility data, SFE method was developed for the selective recovery of uranium from representative fission product elements from an aqueous solution using SCCO2 containing DCyHeHP.