Extraction kinetics of uranium(VI) and thorium(IV) with di(1-methyl-heptyl)methyl phosphonate from nitric acid medium using a Lewis cell

The extraction kinetics of uranium(VI) and thorium(IV) with di(1-methyl-heptyl)methyl phosphonate (DMHMP) dissolved in kerosene were investigated from HNO3 media. Experiments were performed in a constant interfacial area stirred Lewis cell. The results with the influence of stirring speed, specific interfacial area, and temperature indicated that the extraction process was governed by diffusion in the bulk phase or by chemical reaction at the liquid-liquid interface at the stirring speed below or above 140 rpm, respectively. The activation energies for the reactions were calculated as 28.5 kJ.mol(-1) and 26.8 kJ.mol(-1) for U(VI) and Th(IV), respectively. Meanwhile, the obtained thermodynamic parameters (Delta H, Delta S and Delta G) revealed that the extraction of both U(VI) and Th(IV) was an endothermic and non-spontaneous process and the structure of the extracted species was more ordered than that of the extractant itself. Furthermore, the influences of initial concentrations of HNO3, DMHMP and metal ions on the extraction rates were also examined. Based on the reaction orders with respect to the reactants, the rate equations and the extraction mechanism for U(VI) and Th(IV) were also presented.