Organic Liquid Membrane Stability: A Possible Mechanism of Organic Loss during Extraction of Rare Earth Elements by P507-Span 80

Solvent extraction is a main method for separation and purification of rare earths, but the dissolution loss of organic extractant is a major problem in industrials. In this work, the Langmuir-Blodgett thin film technique was used to characterize the interface behavior in situ to explain the dissolution mechanism of the acidic phosphorus extractant P507 during rare earths extraction after the addition of Span80. The results showed that the π-A curve shrinking to the left with increasing P507 concentration is caused by the spread of excessive P507 molecules and formation of multimolecular aggregates. The viscoelastic ratio decreases with the increase of P507 concentration, and it is not conducive to the stability of the organic thin film. When the saponification degree of P507 reaches 30%, a sudden change occurred due to the formation of a three-dimensional network structure aggregate. When the degree of saponification is greater than 30%, a gel structure occurs at the interface, which leads to a serious organic loss. With an increase in the preloading amount of rare earth Er³⁺ in an organic thin film, further aggregation of extractant molecules would generate, causing force imbalance at the interface and dissolution loss of the organic phase. Increasing the preloading amount of Mg²⁺ in the organic phase enhanced the dehydration effect, improving interface stability to some extent. On the contrary, the loaded Al³⁺ in the organic phase can easily cause dissolution loss. FTIR spectrometry, ¹H NMR spectroscopy, DFT and Atomic Force Microscopy (AFM) characterization together confirmed the mechanism of organic loss.