Formation of W/O microemulsions in the extraction of Nd(iii) by bis(2,4,4-trimethylpentyl)dithiophosphinic acid and its effects on Nd(iii) coordination

Abstract

The formation of water-in-oil (W/O) microemulsions during the extraction of Nd(III) by bis(2,4,4-trimethylpentyl)dithiophosphinic acid (also known as purified Cyanex 301, denoted as HC301) was studied. Results from the measurement of the concentration of Nd(III), Na⁺ and NO₃⁻ in the organic phase, IR spectroscopy, and dynamic light scattering (DLS) all indicated that W/O microemulsions could form as the ratio of the neutralized ligand to Nd(III) in the aqueous phase is over 3 : 1. The coordination environment of Nd(III) in the extracted complexes was monitored by absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) and was found to vary significantly with the formation of W/O microemulsions. When only 10% of HC301 was neutralized, with no formation of W/O microemulsions, the inner coordination shell of Nd(III) in the organic phase was occupied dominantly by sulfur atoms from HC301. As HC301 was further neutralized, the coordinated sulfur atoms around Nd(III) were replaced gradually by the oxygen atoms from water. This work provides further insights into the extraction mechanism in the extraction system using purified Cyanex 301 as an extractant.