Coordination chemistry of lanthanides in a AOT-CMPO solvent extraction system: UV-Vis and XAFS studies

The extractability and coordination chemistry for lanthanides in a binary extraction system containing aerosol OT (AOT) and a phosphine oxide (CMPO) have been investigated by means of UV-Vis and X-ray absorption fine structure (XAFS) techniques over a range of extractant concentrations. In addition to the Fourier transform EXAFS analysis, the application of wavelet transform (WT) to the Fourier transform was adapted to the Ho L-III edge to provide a better contrast between the heavier and lighter backscattering atoms at higher coordination shells in the mixed extractant system. Through the variation of the ratio of AOT and CMPO and the acid concentration of the aqueous phase, a synergistic effect appeared at a 1 : 1 ratio of AOT and CMPO using a moderate acid concentration (0.5 M HNO₃). It was also found that CMPO itself is directly coordinated with the metal ions by forming the bond with P═O and C═O, while AOT extracts the lanthanide as a hydrated form through reverse micelle formation. However, the complex of CMPO (C═O-M and P═O-M) to metal becomes weaker as it is mixed with AOT. Our results indicate that complexation from the CMPO is aiding the control of water extraction through reverse micelles from AOT, which results in the cooperative effect of certain mixtures of AOT-CMPO. The information from this coordination environment offers insight into the role of reverse micelle formation in a mixed-extractant solvent extraction system.