Efficient phenyl phosphate ester extractant synthesis and solvent extraction performance evaluation for transition metals

The extraction performance of traditional acidic phosphate ester extractants (P204 and P507) will decrease sharply as the aqueous-phase acidity increases. Although saponification pretreatment can maintain the stability of aqueous-phase acidity during solvent extraction, allowing for the efficient extraction of target metals, it also generates a large amount of salt-contaminated wastewater. Thus, an acid-resistant phenyl phosphate ester extractant was synthesized based on the conjugated effect of the benzene ring, and the impact of synthesis temperature on its structural composition and extraction performance was studied. The molecular structures of the synthesized products were determined as [(CH3)3CCH2C(CH3)2C6H4O]2P(=O)OH (phosphate di-ester, DAP) and (CH3)3CCH2C(CH3)2C6H4OP(=O)(OH)2 (phosphate mono-ester, MAP) through FT-IR, NMR, and HRMS characterization. The extraction rates of Ni2+, Co2+, and Mn2+ by DAP/MAP synthesized at 75 °C were 57.58 %, 69.42 %, and 75.70 %, respectively, without saponification pretreatment. Under the same conditions, the extraction rates of Ni2+, Co2+, and Mn2+ by P204 were 21.16 %, 16.11 %, and 19.31 %, while those by P507 were 22.79 %, 9.05 %, and 11.31 %, respectively. The density functional theory (DFT) calculations indicate that P = O and P-OH groups are the active centers of the DAP/MAP coordination reaction with metal ions. The Gibbs free energy changes during extraction reveal the performance difference between DAP and MAP (sequence: DAP > MAP). The order of coordination ability of the three transition metal ions with DAP and MAP is Mn2+ > Co2+ > Ni2+, consistent with the experimental data.