Understanding the effects of diluents on metal extraction efficiency of bis-2-ethylhexyl phosphoric acid using FTIR and two-dimensional correlation analysis.

The interaction of diluents with bis-2-ethylhexyl phosphoric acid (D2EHPA) during the saponification process for enhanced metal extraction efficiency was investigated. The Fourier Transform Infrared (FTIR) spectra of D2EHPA in different diluents and saponification degrees were obtained by attenuated total internal reflection spectroscopy to monitor the changes in its functional groups. The two-dimensional correlation spectroscopy (2DCOS), Pearson's correlation, and multiple linear regression model were used to elucidate the saponification process and the correlation between the functional groups and the extraction capacity of D2EHPA. The 2DCOS results revealed that diluents modulate the sequential evolution and percentages of D2EHPA functional groups in the FTIR spectra, thereby influencing its extraction efficiency toward Co and Ni. The information from the diagonal position of synchronous spectra was used to detect the main peaks, which are responsible for the metal extraction capacity of D2EHPA. Pearson's correlation confirmed that the -OH group strongly correlates positively with extraction capacity, whereas C-H, P[double bond, length as m-dash]O, and P-O-C groups exhibit negative correlations with Co and Ni extraction efficiency, highlighting the interplay between diluents and saponification degrees. Furthermore, the multiple linear regression models based on the key functional groups demonstrated strong predictive performance, with coefficient of determinations ranging from 0.8349 to 0.8769 and RMSE values of 6.14-11.60%. These findings provide new insights into the role of diluents in saponified extractants and offer a valuable tool for selecting and designing suitable diluents to optimise metal extraction.